US 5,650,308 AGrant

Recombinant Bacillus Thuringiensis Strain Construction Method

Issue Date:1997-07-22

•8 Claims

•27 Drawing Sheets

Abstract

A transposable element, or transposon, isolated from Bacillus thuringiensis (B.t.) and designated as transposon Tn5401. The invention also includes a method of using this transposon in a site-specific recombination system for construction of recombinant B.t. strains that contain insecticidal B.t. toxin protein genes and that are free of DNA not native to B.t.

Metadata

Assignee

Ecogen, Inc.

Inventor

James A. Baum

Application Information

Application Number:US 4785851

Filing Date:1995-06-07

Priority Date:1993-07-08

Art Unit:185

Classifications

IPC:

C12N 1500C12N 121C12N 1575

Field of Search:

435172.3;320.1;252.31

Patent Drawings (27 sheets)

Description

FIELD OF THE INVENTION

The present invention relates to a novel transposon isolated from Bacillus thuringiensis and its use in a site-specific recombination system for the construction of recombinant Bacillus thuringiensis strains that contain one or more insecticidal toxin genes introduced from other Bacillus thuringiensis strains and that are useful as insecticides.

BACKGROUND OF THE INVENTION

Bacillus thuringiensis ("B.t.") is a gram-positive soil bacterium that produces proteinaceous crystalline inclusions during sporulation. These B.t. crystal proteins are often highly toxic to specific insects. Insecticidal activities have been identified for crystal proteins from various B.t. strains against insect larvae from the insect orders Lepidoptera (caterpillars), Diptera (mosquitos, flies) and Coleoptera (beetles).

Recently certain B.t. strains and B.t. crystal proteins have been reported as having activity against non-insect species such as nematodes. The term "insecticidal," as used herein with reference to B.t. strains and their crystal proteins, is intended to include such pathogenic activities against non-insect species.

Individual B.t. crystal proteins, also called delta-endotoxins or parasporal crystals or toxin proteins, can differ extensively in their structure and insecticidal activity. These insecticidal proteins are encoded by genes typically located on large plasmids, greater than 30 megadaltons (mDa) in size, that are found in B.t. strains. A number of these B.t. toxin genes have been cloned and the insecticidal crystal protein products characterized for their specific insecticidal properties. A good review of cloned B.t. toxin genes and crystal proteins is given by Hofte et al., Microbiol. Rev. 53:242-255 (1989) (hereinafter Hofte and Whiteley, 1989), who also propose a useful nomenclature and classification scheme that has been adopted in this disclosure.

The insecticidal properties of B.t. have been long recognized, and B.t. strains were first commercially introduced in biological insecticide products in the 1960's. Commercialized B.t. insecticide formulations typically contain dried B.t. fermentation cultures whose crystal protein is toxic to various insect species and, in the past, were derived from "wild-type" B.t. strains, i.e., purified cultures of B.t. strains isolated from natural sources.

Several newly commercialized B.t. strains are genetically altered strains that have increased insecticidal potency as well as insecticidal activity against a broader spectrum of target insects, as compared with the parent B.t. strains. Such strains are exemplified in International Patent Publication No. WO 88/08877, published Nov. 17, 1988 by applicant Ecogen Inc. and in its counterpart U.S. Pat. No. 5,080,897 issued to Gonzalez, Jr. et al. on Jan. 14, 1992.

Development of these genetically altered B.t. strains did not involve recombinant DNA technology but was instead based on the techniques of plasmid conjugal transfer, which is a natural form of genetic exchange between bacteria, and of plasmid curing, in which certain nonessential plasmids are deleted from a bacterium.

Plasmid conjugal transfer, or conjugation, is limited by the fact that many plasmids carrying useful toxin genes are not amenable to transfer from their native host B.t. strain to another "recipient" B.t. strain. Furthermore, some plasmids which can be transferred by conjugation are inherently incompatible with other plasmids, so a stable "transconjugant" B.t. strain, containing the two desired, incompatible plasmids, cannot be constructed.

Another drawback to conjugation is that some mobilizable, or transferable, plasmids carry undesirable toxin genes in addition to the desired gene, so the quantity of the desired crystal protein produced is limited by concurrent production of an unwanted crystal protein.

Despite the demonstrated efficacy of commercialized transconjugant B.t. strains against certain target insects, there is a clear need for improved B.t. strains against other insect pests. Development of such B.t. strains will be facilitated by use of recombinant DNA technology in B.t. strain construction.

Recombinant DNA procedures provide great flexibility in the construction of novel plasmids containing one or more toxin genes, by permitting selection, manipulation and control of crystal protein type and production and of gene regulation and expression. Some techniques for utilizing the recombinant DNA approach in the production of transformed B.t. strains are described in European Patent Application Publication No. EP 0 342 633, published Nov. 23, 1989 by applicant Ciba-Geigy AG, and in European Patent Application Publication No. 0 537 105, published Apr. 14, 1993 by applicant Sandoz Ltd.

The recombinant B.t. strains disclosed in EP 0 342 633, EP 0 537 105 and other publications are generally characterized by the presence of one or more antibiotic resistance marker genes on the recombinant plasmid harboring the desired B.t. toxin gene(s). Such antibiotic resistance marker genes provide a means for the identification and selection of transformed B.t. strains containing the recombinant toxin-encoding plasmid but are undesirable in viable B.t. strains developed for use in commercial insecticide formulations. Since antibiotic resistance genes are not ordinarily present in native B.t. strains, pesticide and environmental regulatory agencies may be reluctant to approve antibiotic-resistant recombinant B.t. strains for unrestricted environmental release and for use in biological insecticide formulations.

A major reason for the presence of antibiotic resistance genes in recombinant B.t. strains described in the literature is the use of bifunctional cloning vectors containing such resistance marker genes. Portions of these cloning vectors are typically derived from plasmids not native to B.t., e.g., Escherichia coli, Bacillus cereus, Bacillus subtilis or Staphylococcus aureus plasmids, and contain, in addition to the antibiotic resistance marker gene, an origin of replication from a non-B.t. source that is also functional in B.t. and therefore permits the cloning vector to be replicated and maintained in B.t.

International Patent Publication No. WO 91/18102, published Nov. 28, 1990 by applicant Ecogen Inc., describes a plasmid shuttle vector for recombinant B.t. strain development that facilitates incorporation of recombinant plasmids into B.t. strain constructs that contain no DNA derived from E. coli or other non-B.t. biological sources. Using this shuttle vector, a cloned B.t. toxin gene and B.t. plasmid replication origin region are isolated as a single restriction fragment that, upon self-ligation, is introduced into B.t. by cotransformation, This plasmid shuttle vector utilizes a B.t. replication origin derived from large resident plasmids of B.t., a multiple cloning site and strategically placed restriction endonuclease cleavage sites to enable construction of B.t. strains that are free of antibiotic resistance marker genes and free of non-B.t. replication origins.

A second approach for constructing such B.t. strains is a multistep technique described by Lereclus et al., Bio/Technology 10:418-421 (1992) that relies on the presence of IS232 in a resident B.t. toxin plasmid to effect homologous recombination. A cloned B.t. toxin gene is inserted within a cloned fragment of IS232 (which is found on some naturally-occurring toxin-encoding B.t. plasmids) that is inserted into a shuttle plasmid thermosensitive for replication in B.t. The shuttle plasmid is then used to transform a B.t. strain containing the IS232 fragment on a resident B.t. plasmid, and transformants are selected at non-permissive temperature for clones in which the shuttle vector has integrated via homologous recombination into a copy of IS232 present on the resident plasmid. Subsequently, individual clones are screened for a second homologous recombination event that eliminates the shuttle vector and conserves the newly introduced toxin gene. This technique is limited by the laborious nature of its steps and its reliance on homologous recombination using IS232-containing resident B.t. plasmids, whose copy number cannot readily be altered to increase gene expression.

Removal of unwanted selectable marker genes or other unwanted DNA has been described for transgenic plants and eukaryotic cells via the so-called Cre/lox recombination system of bacteriophage P1, where the cre gene encoding the Cre recombinase enzyme is activated to delete the unwanted DNA, which is bracketed by Iox recombination site sequences. International Patent Publication No. WO 93/01283, published Jan. 21, 1993 by applicant U.S. Department of Agriculture, and Dale et al., "Gene transfer with subsequent removal of the selection gene from the host genome," Proc. Natl. Acad. Sci. USA 88:10558-10562 (1991), describe such a system for removal of a antibiotic resistance marker gene from transgenic tobacco plants.

U.S. Pat. No. 4,959,317 issued Sep. 25, 1990 to Sauer describes the application of the Cre/lox recombination system to yeast cells and to a mouse cell line to delete or invert selected DNA sequences.

Hofte and Whiteley, 1989, in discussing factors such as conjugative plasmid transfer that account for the observed mobility of crystal protein genes among B.t. strains, note past reports of some cryIA-type genes and the cryIVB gene being associated with insertion sequence (IS) elements on transposon-like structures (see paragraph bridging pages 245-246). Nevertheless, the role of repeat sequence and/or insertion sequence elements and transposon-like structures in the mobility of B.t. crystal protein genes still remains speculative.

Among known B.t. strains, only one transposon (transposable element) has been reported in the literature as having been isolated from B.t. Mahillon et al., EMBO J. 7:1515-1526 (1988) provide a detailed description of this transposon, originally reported in a 1983 publication and now named Tn4430. Murphy, "Transposable Elements in Gram-Positive Bacteria," Chapt. 9 in Mobile DNA, Berg et al., eds., Am. Soc. Microbiol., Washington, D.C. (1989) pp. 269-288, likewise discusses Tn4430, in the context of other transposable elements found in gram-positive bacteria.

Mahillon et al., Plasmid 19:169-173 (1988), describe the cloning in E. coli and restriction mapping of three small cryptic plasmids from B.t. var. thuringiensis, one of the plasmids being pGI2 which was reported to contain the B.t. transposon Tn4430. The authors speculate (at page 173) that the cloned plasmids could serve as the starting point for the development of new shuttle vectors for E. coli and B.t. but offer no details concerning the construction and use of such hypothetical plasmid shuttle vectors. The complete nucleotide sequence of the small cryptic plasmid pGI2, including Tn4430, is reported by Mahillon et al. in Nucl. Acids Res. 16:11827-11828 (1988).

Earlier references cited by Mahillon et al. in EMBO J. 2:1515-1526 (1988) disclose that, although Tn4430 is widely distributed among B.t. species, the functional role of Tn4430 in B.t., if any, remains unclear. Despite occasional mention in investigative research publications concerning B.t., of Tn4430 and of homology of its elements with other known insertion sequence elements, this transposon has not been utilized to facilitate construction of insecticidal B.t. strains; see, e.g., Lereclus et al., FEMS Microbioi. Lett. 49:417-422 (1988).

The novel transposon of the present invention, designated Tn5401, is only the second transposon to be isolated from B.t. since the discovery of Tn4430 over ten years ago. Unlike Tn4430 which is widely distributed among B.t. species, transposon Tn5401 appears to be found in only a few relatively rare B.t. species.

The present invention also encompasses a site-specific recombination system for recombinant B.t. strain construction that preferably utilizes certain elements of transposon Tn5401, e.g., its internal resolution site and recombinase gene. The site-specific recombination system of this invention represents a significant advance over the approach described in International Patent Publication No. WO 91/18102 because it facilitates the rapid development and construction of recombinant B.t. strains whose recombinant plasmids possess highly desirable characteristics. They are completely free of foreign DNA from non-B.t. sources and can carry B.t. toxin genes that provide insecticidal properties superior to B.t. strains presently used in commercial bioinsecticides.

SUMMARY OF THE INVENTION

The transposable element of this invention is the isolated, purified transposon designated as Tn5401 and whose nucleotide base sequence (SEQ ID NO:1) is shown in FIG. 1, or a mutant thereof capable of functioning as a transposable element.

Several unique elements of Tn5401 are also within the scope of this invention. The locations of these elements are shown in the linear structural map of Tn5401 in FIG. 2. These elements include the isolated, purified DNA sequence containing the internal resolution site, "IRS", of Tn5401; the isolated, purified gene designated as the Tn5401 resolvase/recombinase gene, tpnI; and the isolated, purified gene designated as the Tn5401 transposase gene, tnpA.

The resolvase/recombinase gene product, the resolvase protein (SEQ ID NO:2), and the transposase gene product, the transposase protein (SEQ ID NO:3), are also within the scope of this invention. Recombinant plasmids containing either transposon Tn5401 or its internal resolution site, its resolvase/recombinase gene, or its transposase gene are also embodiments of the present invention, as are bacteria transformed with such recombinant plasmids and capable of expressing the applicable genes on such plasmids.

This invention also includes a plasmid shuttle vector useful for recombinant Bacillus thuringiensis (B.t.) strain development, which has (i) an origin of replication functional in B.t., preferably one native to a B.t. plasmid, such as B.t. origin of replication ori43, ori43.9, ori44 or ori60; (ii) DNA not native to B.t., preferably selected from selectable marker genes and origins of replication functional in E. coli or in a Bacillus host species other than B.t.; (iii) optionally and preferably, one or more insecticidal protein toxin genes; (iv) two identical internal resolution sites oriented in the same direction and flanking the DNA not native to B.t., thus enabling such non-B.t. DNA to be excised via a site-specific recombination event involving the two internal resolution sites. The internal resolution sites are preferably derived from a Tn3-type transposon and more preferably are identical to the internal resolution site of transposon Tn5401. Host B.t. strains or other bacterial strains transformed with this plasmid shuttle vector are also embodiments of this invention.

The method of constructing a recombinant B.t. strain containing no DNA elements foreign to B.t. is also within the scope of this invention, having the steps of (a) transforming a host B.t. strain with a plasmid shuttle vector containing (i) an origin of replication native to B.t., (ii) DNA not native to B.t. and useful in the construction of recombinant B.t. strains, selected from the group consisting of selectable marker genes, origins of replication functional in E. coli, and origins of replication functional in a Bacillus host species other than B.t., (iii) one or more insecticidal B.t. protein toxin genes, and (iv) two identical internal resolution sites oriented in the same direction and flanking the DNA not native to B.t., the sites being the same as an internal resolution site from a Tn3-type transposon native to B.t.; (b) introducing into the transformed B.t. strain resolvase protein to effect a site-specific recombination event involving the internal resolution sites, thereby excising from the plasmid shuttle vector the DNA not native to B.t.; and (c) recovering a recombinant B.t. strain containing a recombinant plasmid capable of replicating in the B.t. strain and containing (i) an origin of replication native to B.t., (ii) one or more insecticidal B.t. protein toxin genes, and (iii) a single internal resolution site, derived from the site-specific recombination event. Preferred Tn3-type transposon sources for the internal resolution site in the plasmid shuttle vector of this method are transposons Tn5401 and Tn4430.

The present invention also encompasses a recombinant plasmid capable of replicating in a Bacillus thuringiensis bacterium and having (i) at least one insecticidal protein toxin gene, (ii) an origin of replication functional in B.t., and (iii) a single internal resolution site, preferably derived from a Tn3-type transposon and more preferably identical to the internal resolution site of transposon Tn5401. Host B.t. strains or other bacterial strains containing such recombinant plasmids are also embodiments of this invention, as are insecticidal compositions with such transformed host B.t. strains, and as is the method of controlling insect pests utilizing such insecticidal compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 consists of FIGS. 1A through 1L and depicts the nucleotide sequence for Tn5401, the transposon of this invention. The deduced amino acid sequence for an open reading frame extending from nucleotide base positions 764 to 1681 (excluding the terminal nonsense codon) is also shown. The gene of this open reading frame, designated the resolvase gene, encodes a protein with 306 amino acids. The deduced amino acid sequence for another open reading frame, extending from nucleotide positions 1756 to 4770 (excluding the terminal nonsense codon), is also shown. The gene of this second open reading frame, designated the transposase gene, encodes a protein with 1005 amino acids. Certain restriction endonuclease cleavage sites (NsiI(2), NspI(2), ClaI, BssHII) are also shown.

FIG. 2 is a linear structural map of transposon Tn5401 whose 4837 basepair nucleotide sequence is shown in FIG. 1. Three open reading frames are shown: "orf1" (open arrow) which encodes a cryptic protein of 85 amino acids in the 3'-5' direction; "tnpI" (dark shaded arrow), the resolvase gene; and "tnpA" (light shaded arrow), the transposase gene. An internal resolution site is located within the bracketed DNA fragment, "IRS region", shown in FIG. 2. Inverted repeats of 53 basepairs at either end of the structural map are shown as black arrowheads. Certain restriction endonuclease cleavage sites (NsiI(2), NspI(2), ClaI, BssHII) are also shown.

FIG. 3 is a circular structural map of the recombinant plasmid pEG922, a 15.7 kilobase (kb) plasmid which contains the transposon Tn5401 of this invention. Transposon Tn5401 is contained on a .about.7 kb SstI-SstI fragment which comprises the following elements shown in FIG. 3: Tn5401 resolvase gene, tnpI (dark shaded arrow), and Tn5401 transposase gene, tnpA (open arrow). Tn5401 also contains an introduced tetracycline resistance gene, tet (light shaded arrow), a "tag" that serves as a selectable marker for the transposon. Plasmid pEG922 was constructed by inserting the .about.7 kb SstI fragment containing Tn5401 into the unique SstI site of plasmid shuttle vector pEG491 (see FIG. 8B). The components of shuttle vector pEG491 include: the thin solid black segment indicating the E. coli replicon pUC18, the thick open segment indicating the temperature-sensitive replicon, rep.sup.ts, from plasmid pE194ts which is functional in gram-positive bacteria but which cannot operate at temperatures at or above 37.degree. C., and the thick black segment indicating the chloramphenicol acetyl transferase gene, cat. At the top of the circular structural map of pEG922 is a multiple cloning site. Abbreviations for the restriction endonuclease cleavage sites in the multiple cloning site of FIG. 3 are as follows: B=BamHI, Xb=XbaI, S=SalI, P=PstI, Sp=SphI, H=HindIII.

FIG. 4 is a circular structural map of the recombinant plasmid shuttle vector pEG928.9 of this invention, which is 13.2 kb in size. The shuttle vector contains identical internal resolution sites, IRS, in the same orientation (open arrows), and these two sites flank the E. coli replicon pTZ19u (open segment in FIG. 4) and a tetracycline resistance gene, tet, from plasmid pBC16 (dark shaded arrow). The plasmid shuttle vector also contains, outside of the IRS sites flanking the DNA not native to B.t., the ori43.9 B.t. plasmid origin of replication (light shaded segment) and a cryI-type B.t. protein toxin gene (solid arrow). Letter abbreviations for the restriction endonuclease cleavage sites shown in FIG. 4 are as follows: A=Asp718, Bl=BlnI, ClaI=ClaI, H=HindIII, HpaI=HpaI, Nsi=NsiI, Nsp=NspI, P=PstI, SalI=SalI, SstI=SstI, Xba=XbaI.

FIG. 5 is a schematic illustration of a method in which the plasmid shuttle vector of this invention, pEG928.9, is manipulated to excise its DNA elements which are not native to B.t. Removal of the foreign DNA elements, which are bracketed by duplicate IRS sites of transposon Tn5401, is accomplished by catalysis with a Tn5401 transposon-encoded resolvase/recombinase protein. Plasmid pEG928.9, shown and described in more detail in FIG. 4, contains DNA not native to B.t., i.e., the E. coli replicon pTZ19u and a tetracycline antibiotic resistance gene, tet. This foreign DNA is flanked on either side by copies of an internal resolution site, IRS, from transposon Tn5401, oriented in the same direction. Plasmid pEG922, shown and described in more detail in FIG. 3, contains transposon Tn5401, whose resolvase gene, tnpI, is capable of expressing the resolvase/recombinase protein at temperatures below 37.degree. C. in this temperature-sensitive plasmid. Sequential transformation of a host B.t. strain (not shown in the FIG.) with both plasmid pEG928.9 and plasmid pEG922 and incubation of the transformed host B.t. strain at a temperature of 31.degree. C. cause expression of the tnpI gene and production of resolvase/recombinase protein, which catalyzes a site-specific recombination event as shown in FIG. 5. The resultant plasmid pEG928.9.DELTA., an 8.0 kb derivative of pEG928.9 from which non-B.t. DNA elements have been excised via the site-specific recombination event, contains a B.t.-derived origin of replication, ori43.9, a cryI B.t. protein toxin gene, and a single copy of the internal resolution site, IRS, of transposon Tn5401. Abbreviations for the restriction endonuclease cleavage sites shown in this Figure are summarized in the descriptions of FIGS. 3 and 4.

FIG. 6 is a circular structural map of plasmid pEG911, from which transposon Tn5401 was isolated after recovery of the Tn5401-containing pEG911 derivative from B.t. var. morrisoni strain EG2158. Plasmid pEG911, approximately 10.9 kb in size, contains the following elements. The open arrow indicates the cryIIIB2 B.t. protein toxin gene; the open segment indicates the E. coli replicon pTZ19u; the solid arrow indicates the chloramphenicol acetyl transferase gene, cat, from plasmid pC194; the shaded segment indicates the ori60 B.t. plasmid origin of replication region; and the solid box segment with accompanying arrowhead indicates a B.t. gene transcription terminator. Abbreviations are used for some restriction endonuclease cleavage sites shown in the Figure and these are as follows: Dra=DraI, Kpn=KpnI, Sma=SmaI, Ssp=SspI, Xba=XbaI.

FIG. 7 consists of FIGS. 7A and 7B, which respectively show linear structural maps for plasmids pEG911-1 and pEG911-3, both of which are derivatives of plasmid pEG911 (see FIG. 6) and both of which contain an insertion of transposon Tn5401 from B.t. var. morrisoni strain EG2158. The long solid black segment in both of these structural maps indicates the transposon Tn5401. As indicated by the location of the ClaI and BssHII sites within Tn5401, pEG911-1 and pEG911-3 contain Tn5401 in opposite orientations. Identification of the various elements within plasmids pEG911-1 and pEG911-3 and the abbreviations for restriction sites are as described for FIG. 6.

FIG. 8 consists of FIGS. 8A and 8B and is a schematic diagram showing the recombinant DNA procedures used to derive plasmid pEG922, which is also shown in FIG. 3 and is utilized in the method of FIG. 5. Plasmid pEG922 contains the isolated transposon Tn5401 of this invention. Details of the steps shown in this Figure for the derivation of plasmid pEG922 are explained in Example 2. Abbreviations are used for some of the restriction endonuclease cleavage sites shown in FIG. 8 and these are as described for FIG. 3, which also provides a description of the circular structural map of plasmid pEG922.

FIG. 9 consists of FIGS. 9A, 9B, 9C, 9D and 9E and is a schematic diagram showing the recombinant DNA procedures used to derive the plasmid shuttle vector pEG928.9 of this invention, which is also shown in FIG. 4 and is utilized in the method of FIG. 5. Details of the steps shown in this Figure for the derivation of plasmid shuttle vector pEG928.9 are explained in Example 3. Abbreviations are used for some of the restriction endonuclease cleavage sites shown in FIG. 9 and these are as follows. For the multiple cloning site in plasmid shuttle vector pEG854 and its derivatives, plasmid clones p76 and p83: P=PstI, A=Asp718, Sm=SmaI, Bl=BlnI, B=BamHI, X=XbaI, St=SstI, C=ClaI, Hp=HpaI, Sp=SphI. For the multiple cloning site in pTZ19u and its derivative, plasmid clone p84: E=EcoRI, St=SstI, A=Asp718, Sm=SmaI, B=BamHI, Xb=XbaI, S=SalI, P=PstI, Sp=SphI, H=HindIII. Other abbreviations for restriction sites shown on plasmid shuttle vector pEG928.9 and its precursor plasmid clones are as described for FIG. 4, which also provides a description of the circular structural map of pEG928.9.

FIG. 10 shows circular structural maps of the recombinant plasmid shuttle vector pEG930.9 of this invention, which is 13.3 kb in size and of its derivative plasmid from a site-specific recombination event, plasmid pEG930.9.DELTA. which is 8.1 kb in size. The plasmid shuttle vector pEG930.9 is similar to plasmid shuttle vector pEG928.9 shown in FIG. 4 except that the cryI-type gene of plasmid pEG928.9 has been replaced with a coleopteran toxin cryIIIB2 gene (solid long arrow) and a cryI transcription terminator, ter (short dark shaded arrow). Other symbols and abbreviations for both plasmid pEG930.9 and plasmid pEG930.9.DELTA. are as described for FIG. 4.

FIG. 11 shows circular structural maps of the recombinant plasmid shuttle vector pEG931 of this invention, which is 13.3 kb in size and of its derivative plasmid from a site-specific recombination event, plasmid pEG931.DELTA. which is 8.1 kb in size. The plasmid shuttle vector pEG931 is similar to plasmid shuttle vector pEG928.9 shown in FIG. 4 except that the cryI-type gene of plasmid pEG928.9 has been replaced with a lepidopteran toxin cryIC gene (solid long arrow) and a cryI transcription terminator, ter (short dark shaded arrow). Other symbols and abbreviations for both plasmid pEG931 and plasmid pEG931.DELTA. are as described for FIG. 4.

MICROORGANISM DEPOSITS

To assure the availability of materials to those interested members of the public upon issuance of a patent on the present application, deposits of the following microorganisms were made prior to the filing of present application with the ARS Patent Collection, Agricultural Research Culture Collection, Northern Regional Research Laboratory (NRRL), 1815 North University Street, Peoria, Ill. 61604:

These microorganism deposits were made under the provisions of the "Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purpose of Patent Procedure." All restrictions on the availability to the public of these deposited microorganisms will be irrevocably removed upon issuance of a United States patent based on this application.

Description of the Preferred Embodiments

The transposon, or transposable element, of this invention was isolated from Bacillus thuringiensis and has been designated as transposon Tn5401. Tn5401 has the nucleotide sequence (SEQ ID NO:1) shown in FIG. 1. Two open reading frames within transposon Tn5401 are also shown in FIG. 1, along with their respective deduced amino acid sequences, and these are discussed in more detail below.

A structural map of Tn5401 is shown in FIG. 2 and includes the location of open reading frames within this 4837 basepair (bp) transposon; these elements are indicated by segments with arrowheads. The genes of these open reading frames, orf1, tnpI and tnpA, are as follows:

orf1 (open arrow in FIG. 2) potentially encodes a cryptic protein, whose significance is not presently known, of 85 amino acids (10.1 kDa) in the 3'-5' direction. The deduced amino acid sequence of orf1 is shown in the Sequence Listing accompanying this specification and designated as SEQ ID NO:4. Although not shown in FIG. 1, it is derived from the complementary nucleotide sequence extending from nucleotide base positions 351 to 608.

tnpI (dark shaded arrow in FIG. 2) encodes a protein, designated the resolvase protein of Tn5401, of 306 amino acids (35,613 Da) in the 5'-3' direction. In FIG. 1, the resolvase gene encodes the resolvase protein having the amino acid sequence (SEQ ID NO:2) located between nucleotide base positions 763 to 1682. The nucleotide base sequence of the resolvase gene, as shown in FIG. 1, extends from nucleotide base positions 764 to 1681 (excluding the terminal nonsense codon).

tnpA (light shaded arrow in FIG. 2) encodes a protein, designated the transposase protein of Tn5401, of 1005 amino acids (116,250 Da) in the 5'-3' direction. In FIG. 1, the transposase gene encodes the transposase protein having the amino acid sequence (SEQ ID NO:3) located between nucleotide base positions 1755 to 4771. The nucleotide base sequence of the transposase gene, as shown in FIG. 1, extends from nucleotide base positions 1756 to 4770 (excluding the terminal nonsense codon).

Another important distinguishing characteristic of transposon Tn5401 is an internal resolution site, IRS, located 5' to the resolvase open reading frame, within a .about.550 bp ClaI-NsiI fragment. This location of the IRS is shown by brackets on the linear structural map of FIG. 2 and has been designated in the Figure as "IRS region." In FIG. 1, the internal resolution site is located within the DNA fragment extending from nucleotide positions 217 (the initial nucleotide of a ClaI restriction endonuclease cleavage site) to 764 (the initial nucleotide of a NsiI restriction endonuclease cleavage site). The IRS located on this ClaI-NsiI fragment is believed to be situated on a .about.150 bp fragment immediately upstream of (5' to) the resolvase open reading frame, i.e., upstream of the NsiI site that initiates the resolvase tnpI gene, in particular, within the DNA fragment extending from nucleotide base positions 608 to 763 shown in FIG. 1.

Transposon Tn5401 is also characterized by 53 bp inverted repeats at the termini, which are depicted by the solid black arrowheads in the structural map of FIG. 2.

Several restriction endonuclease cleavage sites, i.e., NsiI (two occurrences), NspI (two occurrences), ClaI (one occurrence), BssHII (one occurrence), are also shown on the linear structural map of Tn5401 in FIG. 2 and in the nucleotide sequence of FIG. 1, and these are useful for isolating the IRS, as well as the orf1, resolvase and transposase genes.

Transcriptional start sites within Tn5401 have been mapped by primer extension analysis. Overlapping divergent promoters are located 5' to the resolvase gene: one directs the transcription of both tnpI and tnpA. Both promoters are derepressed on recombinant plasmids when the tnpI and tnpA genes are deleted, suggesting that transcription within the transposon is autoregulated, presumably by the resolvase protein.

Conserved sequence elements within the above-noted promoter region in the intergenic region between orf1 and tnpI, apparently serve as recognition sites for the resolvase protein. Four copies of a conserved 12 bp sequence element are present within the above-noted promoter region and are believed to be the recognition/binding site for the recombinase protein. Two copies of the 12 bp sequence element form a dyad sequence (nucleotide positions 639-666 in SEQ ID NO:1) that may be the site at which site-specific recombination actually occurs during the transposition process. All four copies of the 12 bp sequence are believed to be essential for site-specific recombination to occur. The 12 bp sequence is also located within the terminal inverted repeats of transposon Tn5401, thus accounting for the unusual length of these repeats.

Transposon Tn5401 appears to belong to the class of transposons designated as Tn3-type transposons, described by Heffron in "Tn3 and Its Relatives" in Mobile Genetic Elements, Shapiro, ed., Academic Press, Orlando (1983), pp. 223-260. Transposons in the Tn3 family have the following characteristics:

(1) short inverted repeats at either end, which exhibit homology with other family members,

(2) a high molecular weight protein (transposase) encoded by the transposon and essential for transposition;

(3) a two stage transposition mechanism involving fusion of donor (with transposon) and recipient DNA molecules, including a duplication of the transposon to form a cointegrate molecule, followed by a resolution/recombination event at an internal resolution site within each transposon copy to yield donor and recipient DNA molecules each containing the transposon;

(4) a recombinase protein encoded by the transposon and required for resolution of the cointegrate molecule;

(5) an internal site-specific recombination site that enables the resolvase protein to effect resolution/recombination of the cointegrate molecule; and

(6) a 5-bp duplication of target DNA at the site of insertion, AT-rich target sites apparently being favored.

Members of the Tn3 family or class of transposons are predominantly derived from gram-negative bacteria, but one exception is Tn4430 originally isolated from a gram-positive organism and described by Mahillon et al., EMBO J. 7:1515-1526 (1983). Until the inventor's discovery of Tn5401, the prior art transposon Tn4430 was the only transposon reported to be originally isolated from a B.t. or Bacillus species.

Transposon Tn5401 is present in B.t. var. morrisoni strain EG2158 which produces a coleopteran-active protein encoded by a cryIIIA gene on an 88 megadalton (MDa) resident plasmid. Tn5401 is located on two resident plasmids of B.t. strain EG2158, a 35 MDa plasmid and a 72 MDa plasmid.

Subsequent to the discovery of Tn5401, the inventor has screened numerous other B.t. species and determined that Tn5401 is only rarely found among B.t. species. This finding contrasts with the prior art transposon Tn4430 which is widely distributed among B.t. species.

Transposon Tn5401 has been identified by the inventor as also being present in B.t. "tenebrionis" (DSM2803), also called B.t. "san diego", a morrisoni variety that is coleopteran toxic like B.t. strain EG2158.

The transposition mechanism of Tn5401 appears to be similar to that of other transposable elements in the Tn3 class. The transpositioning functionality of transposon Tn5401 is not limited to gram-positive bacteria such as B.t. but may likely also be demonstrated in gram-negative bacteria such as E. coli. For Tn5401 and other Tn3-type transposons, the net outcome of transposition is the insertion of a duplicate copy of the transposon into another (target) plasmid or chromosomal site. The first step of transposition involves the joinder, or cointegration, of a transposon-containing donor plasmid with a target plasmid to form a cointegrate DNA molecule that contains a duplicated copy of the transposon. The transposase gene in the transposon and its encoded protein transposase are essential for this initial step of transposition and apparently effect formation of the duplicate copy of the transposon in the cointegrate plasmid.

The second step of transposition involves a site-specific recombination event, in which the original transposon-containing donor plasmid and target plasmid, the latter now containing a copy of the transposon, are formed from the cointegrate plasmid. The site-specific recombination event occurs at a specific site in the transposon, the internal resolution site, referred to herein as the IRS. The resolvase protein encoded by the resolvase gene in the transposon apparently catalyzes a recombination event between the two internal resolution sites of the duplicate transposons, resulting in the formation of the transposon-containing donor plasmid and a target plasmid that has been modified by incorporation of a copy of the transposon.

The site-specific recombination event occurs between the duplicate internal resolution sites in the cointegrate molecule and has the effect of removing, or excising, the DNA located between the duplicate sites. The resolvase/recombinase protein encoded by the resolvase gene in the transposon can apparently catalyze this site-specific recombination event on any plasmid molecule containing two copies of the internal resolution site. These aspects of the transposition mechanism and of transposon Tn5401 are utilized in the site-specific recombination system of this invention, which is described in more detail below.

Transposon Tn5401 is also useful in transposon tagging to isolate genes of interest, e.g., in mutational studies of protein toxin genes in B.t. Transposons are known to be useful as molecular probes and genetic markers. In the event a transposon inserts itself into a gene, the gene is inactivated and a mutant phenotype is produced. Tn5401 is especially useful for such gene studies in B.t. since this transposon favors insertion into plasmid DNA rather than chromosomal DNA. The present invention, for this reason, includes use of recombinant plasmids containing Tn5401, preferably in conjunction with a selectable and/or screenable marker, e.g., an antibiotic resistance marker gene, functional in the host microorganism harboring the recombinant plasmid.

This invention also extends to mutants and derivatives (hereinafter referred to collectively as "mutants") of (i) transposon Tn5401 which are capable of functioning as transposable elements, (ii) the resolvase gene of Tn5401 whose resolvase/recombinase gene products have resolving functionality, and (iii) the transposase gene of Tn5401 whose transposase gene products have transpositioning functionality. Methods for creating or obtaining such mutants are well known to those skilled in the art of molecular cloning.

In another aspect of this invention, transposon Tn5401 has been isolated on a recombinant plasmid designated pEG922, whose circular restriction map is shown in FIG. 3. Transposon Tn5401 is located on a .about.7 kb SstI-SstI DNA fragment in plasmid pEG922 and the tnpI and tnpA genes of Tn5401 are shown in FIG. 3. As is evident from FIG. 3, the transposon Tn5401 has been tagged with a selectable marker gene, tet, for tetracycline antibiotic resistance. The derivation of recombinant plasmid pEG922 is described in more detail in Example 2. Plasmid pEG922 is useful for its ability to transform gram-positive bacteria such as B. thuringiensis, B. cereus, B. megaterium, and B. subtilis, as well as transform gram-negative bacteria such as E. coli, and such transformed bacteria are capable of expressing the tpnI and tnpA genes on Tn5401. Consequently, transformed bacteria containing pEG922 are able to produce the recombinase/resolvase protein gene product of tnpI. Likewise, expression of the transposase protein gene product of tnpA enables the transposon to exhibit transpositioning functionality. Besides plasmid pEG922, other recombinant plasmids containing the transposon Tn5401 are also within the scope of this invention.

Other embodiments of this invention include a bacterium, e.g., Bacillus thuringiensis or E. coli, transformed with a recombinant plasmid carrying transposon Tn5401. E. coli strain EG7669 harbors recombinant plasmid pEG922 and is one such strain.

Additional embodiments of this invention also include recombinant plasmids containing either the tnpA gene or tnpI gene, or both, of Tn5401, as well as bacteria transformed with such recombinant plasmids and capable of expressing the gene or genes in such plasmids. Preferred bacteria include Bacillus species, particularly B.t., and include E. coli.

The transposable element Tn5401 of this invention provides the basis for a site-specific recombination system for construction of stable recombinant plasmids in insecticidal recombinant B.t. strains that contain only DNA that is native to B.t. and that are free of foreign (non-B.t. derived) DNA. This site-specific recombination system utilizes the internal resolution site of Tn5401, two copies of the internal resolution site being incorporated into a recombinant B.t. plasmid in the same orientation. The two copies of the internal resolution site are used to bracket all DNA elements foreign to B.t., e.g., selectable marker genes, E. coli replicons and other DNA useful in the construction and characterization of such a recombinant plasmid. Elimination of the foreign DNA is accomplished by a site-specific recombination event involving the internal resolution sites. This event is effected enzymatically by using the site-specific resolvase/recombinase protein that is encoded by the tnpI gene of Tn5401. The resultant recombinant plasmids are completely free of foreign DNA. Such recombinant B.t. plasmids, harboring a desired B.t. toxin gene or combinations of toxin genes, may be used to construct B.t. strains for use as the active ingredient in commercial B.t.-based insecticides.

An essential aspect of the site-specific recombination system of this invention is a plasmid shuttle vector having the following elements: (i) an origin of replication functional in B.t.; (ii) DNA not native to B.t.; and (iii) two identical internal resolution sites oriented in the same direction and flanking the DNA not native to B.t. The two identical internal resolution sites thus segregate the DNA not native to B.t. from the DNA native to B.t. Use of this plasmid shuttle vector in a B.t. host strain facilitates removal or excision of the non-B.t. DNA via a site-specific recombination event involving, i.e., between, the two internal resolution sites. The site-specific recombination event is catalyzed by the introduction of resolvase/recombinase protein that recognizes the particular IRS site utilized in the plasmid shuttle vector.

The plasmid shuttle vector optionally and preferably contains at least one insecticidal protein toxin gene that is intended to be introduced into the recombinant B.t. strain construct. This gene (or genes) is situated on the plasmid shuttle vector in a location outside of the DNA not native to B.t. and outside of the internal resolution sites that flank the foreign DNA.

One preferred embodiment of the plasmid shuttle vector of this invention is plasmid pEG928.9, whose circular structural map is shown in FIG. 4. Details of the derivation of plasmid shuttle vector pEG928.9 are described in Example 3.

The duplicate copies of the internal resolution sites (IRS) utilized in this plasmid shuttle vector are desirably derived from, or identical to, an IRS of a Tn3-type transposon.

Particularly suitable Tn3-type transposon sources for the IRS are transposons native to B.t. such as transposons Tn4430 and Tn5401. These IRS-source transposons are well suited for construction of insecticidal recombinant B.t. strains having no DNA that is not native to B.t. A disadvantage of Tn4430 as the IRS source is the widespread existence of this transposon in B.t. strains. The host B.t. strain selected for construction of the recombinant B.t. should be free of the transposon utilized as the IRS source in the plasmid shuttle vector, so as to avoid possible interference with the site-specific recombination event in the method of this invention.

For this reason, duplicate copies of the internal resolution site in the plasmid shuttle vector are most preferably derived from, or identical to, the internal resolution site of transposon Tn5401. As noted earlier in the discussion of Tn5401, this transposon is infrequently found in B.t. species, a fact that makes most B.t. strains suitable candidates as host strains for the site-specific recombination method of this invention.

It should be noted, however, that internal resolution sites or site-specific recombination sites from other sources are likewise usable in this plasmid shuttle vector and in the site-specific recombination system of this invention, if the fact of the IRS or the site-specific recombination site not being native to B.t. is not critical.

In the plasmid shuttle vector of this invention, the origin of replication functional in B.t. is preferably a replication origin that is native to B.t., i.e., is identical to or derived from a B.t. plasmid origin of replication. B.t. replication origins from large B.t. plasmids, i.e., plasmids larger than about 20-25 mDa in size, are preferred since such replicons are more likely to produce stable recombinant plasmids than replicons derived from small B.t. plasmids, which typically replicate by a different mechanism, i.e., rolling circle replication.

Preferred B.t. plasmid origins of replications are ori43 ori60 and ori44, described in PCT International Patent Publication No. WO 91/18102, published Nov. 28, 1990 by applicant Ecogen Inc. The ori43 replicon is present in plasmid shuttle vector pEG854, which is contained in E. coli strain EG7534 which is a deposited microorganism described in WO 91/18102. The preferred B.t. origin of replication also includes mutants of these three and other B.t. replicons, particularly those mutants exhibiting higher copy numbers than the progenitor replicon. One such replicon, ori43.9, is utilized in plasmid shuttle vector pEG928.9 of this invention and is preferred because its high copy number characteristic often promotes increased expression levels of insecticidal toxin protein genes located on the same plasmid.

The plasmid shuttle vector of this invention also contains DNA elements not native to B.t., and this foreign DNA is flanked, or segregated, by the duplicate copies of the internal resolution sites. The foreign DNA is excised from the plasmid shuttle vector by the site-specific recombination event between the two internal resolution sites, but this non-native DNA can serve many useful purposes prior to the recombination event. Examples of such useful foreign DNA are selectable and/or screenable marker genes, such as antibiotic resistance genes functional in B.t. or E. coli or other cloning hosts; origins of replication functional in E. coli; and origins of replication functional in gram-positive microorganisms other than B.t., e.g., in Bacillus species. Other DNA elements not native to B.t. may also be useful in the construction, development and characterization of insecticidal recombinant B.t. constructs, and these are also within the scope of the term "DNA not native to B.t. ", as used herein. The term "DNA not native to B.t.", as used herein, is not intended to cover short polynucleotide stretches that are derived from multiple cloning sites or that are other synthesized, non-biological DNA.

The choice of the insecticidal protein toxin gene that is optionally and preferably present in the plasmid shuttle vector is not critical. The insecticidal protein toxin gene is normally selected to enhance the insecticidal characteristics of the B.t. host strain transformed with the plasmid shuttle vector. The insecticidal toxin gene is preferably selected from among wild-type or recombinant B.t. toxin genes. Exemplary B.t. toxin genes are those described by Hofte and Whiteley, 1989, as well as more recently reported B.t. genes such as cryIF, cryIIIB2 and cryIIIB3.

Bacteria transformed with the plasmid shuttle vector and capable of expressing at least one of the genes in the plasmid shuttle vector are also within the scope of this invention and are desirably selected from the group consisting of Bacillus thuringiensis and E. coli. One such recombinant Bacillus thuringiensis strain is B.t. strain EG7684 which contains plasmid shuttle vector pEG928.9.

It should be evident that the site-specific recombination system of this invention is not strictly limited to B.t. but is equally applicable to the construction of other Bacillus species recombinant constructs, if suitable changes are made in the plasmid shuttle vector, e.g., selection of an origin of replication functional in the selected Bacillus host species, DNA not native to the selected host species and optional insecticidal protein toxin genes capable of being expressed by the selected replicon.

The site-specific recombination system of this invention is schematically exemplified in FIG. 5, which illustrates plasmid shuttle vector pEG928.9 undergoing a site-specific recombination event catalyzed with recombinase/resolvase protein produced by the tnpI gene of the Tn5401-containing plasmid pEG922. The resultant plasmid pEG928.9.DELTA. contains a single copy of the IRS, lacks DNA not native to B.t., and contains a B.t.-derived replicon and a B.t. cryI-type protein toxin gene. The method of this invention as exemplified in FIG. 5 is described in detail in Example 5.

A preferred method of this invention, for constructing a recombinant B.t. strain containing no DNA elements foreign to B.t., involves (a) transforming a host B.t. strain with a plasmid shuttle vector containing (i) an origin of replication native to B.t.; (ii) DNA not native to B.t. and useful in the construction of recombinant B.t. strains, selected from the group consisting of selectable marker genes, origins of replication functional in E. coli, and origins of replication functional in Bacillus host species other than B.t.; (iii) one or more insecticidal B.t. protein toxin genes; and (iv) two identical internal resolution sites oriented in the same direction and flanking the DNA not native to B.t., the sites being the same as an internal resolution site from a Tn3-type transposon native to B.t.; (b) introducing into the transformed B.t. strain a resolvase protein to effect a site-specific recombination event involving the internal resolution sites, thereby excising from the plasmid shuttle vector the DNA not native to B.t.; and (c) recovering a recombinant B.t. strain containing a recombinant plasmid capable of replicating in the B.t. strain and containing (i) an origin of replication native to B.t.; (ii) one or more insecticidal protein toxin genes; and (iii) a single internal resolution site, derived from the site-specific recombination event.

In this method, the resolvase/recombinase protein should correspond to that produced by the resolvase/recombinase gene in the Tn3-type transposon used as the IRS source. The only requirement is that the resolvase/recombinase protein recognize the particular IRS site utilized.

The elements of the recombinant plasmid present in the recovered recombinant B.t. strain correspond, of course, to the same elements in the plasmid shuttle vector originally introduced into the host B.t. strain. Selection of the elements of the plasmid shuttle vector used in this method is governed by the same considerations discussed earlier for the plasmid shuttle vector of this invention.

Preferred Tn3-type transposon sources for the duplicate IRS sites in the plasmid shuttle vector are Tn4430 and Tn5401.

Introduction of the resolvase protein into the B.t. transformant containing the plasmid shuttle vector serves to effect a site-specific recombination event between the IRS sites in the vector. This introduction of the protein catalyzing agent may be accomplished by transforming the B.t. transformant with a second recombinant plasmid containing a resolvase gene and capable of expressing the resolvase protein. To facilitate efficient removal of the resolvase gene containing plasmid from the B.t. host strain following site-specific recombination, this plasmid desirably contains a temperature-sensitive replicon or other means for effecting its deletion and an antibiotic selectable marker gene different from the selectable marker gene carried on the plasmid shuttle vector. This approach is utilized in the site-specific recombination method described in Example 5.

Alternative means exist for introducing the recombinase protein into the transformed B.t. host strain containing the plasmid shuttle vector. One technique involves the direct introduction of the protein into the transformed B.t. cells, via the transient introduction of the recombinase protein via electroporation, lipofection or the like.

A second approach involves insertion of the recombinase gene into the plasmid shuttle vector within the non-B.t. DNA region flanked by the IRS sites. For IRS sites the same as that of transposon Tn5401, a mutant of the corresponding resolvase gene, tnpI, should produce a recombinase protein that is thermosensitive, being inactive at .about.37.degree. C. but active at .about.30.degree. C. This tnpI.sup.ts variant could be obtained by a variety of well-known in vitro mutagenesis procedures, including chemical mutagenesis of the tnpI gene, followed by selection for tnpI variants that catalyze recombination at 30.degree. C. but not at 37.degree. C. Transformation of a suitable B.t. host strain with a tnpI.sup.ts -containing plasmid shuttle vector at a temperature of 37.degree. C. will prevent expression of the tnpI gene, but this will allow for selection of transformants containing the plasmid shuttle vector. Subsequently, the B.t. transformants are grown at a temperature of 30.degree. C., resulting in expression of a functional recombinase protein and excision of the foreign DNA elements, as well as excision of the tnpI.sup.ts gene, since both are contained within the non-B.t. DNA region flanked by the IRS sites.

Both of these alternative procedures for introducing the recombinase protein to effect site-specific recombination avoid the need to introduce a second recombinant plasmid, i.e., one containing an expressible recombinase gene, into the transformed B.t. strain and avoid the need to thereafter delete the same second recombinant plasmid following the recombination event.

The site-specific recombination system of this invention yields recombinant toxin plasmids that possess a unique combination of elements. The recombinant plasmids, capable of replicating in B.t. bacteria, contain at least one insecticidal protein toxin gene, an origin of replication functional in B.t., and a single internal resolution site (or other single site-specific recombination site).

In a preferred embodiment, the single internal resolution site of the recombinant plasmid is derived from a Tn3-type transposon or is identical to the IRS in such a transposon. The Tn3-type transposon IRS source is desirably one that is native to B.t. The internal resolution site is preferably identical to the IRS of transposon Tn4430 or, more preferably, transposon Tn5401.

The origin of replication in these recombinant toxin plasmids is preferably native to B.t. The B.t.-functional origin of replication is preferably derived from, or identical to, a replicon of a large B.t. plasmid, for the same reasons discussed previously for the plasmid shuttle vector of this invention.

The bacteria containing these recombinant toxin plasmids are preferably Bacillus thuringiensis but other bacterial hosts can be used, provided that the replicon in the plasmid is capable of functioning in such a non-B.t. host.

Particularly preferred recombinant B.t. constructs containing the recombinant plasmids of this invention are described in Example 5. It should be evident from the discussion in Example 5 that this invention provides the means to construct a wide variety of insecticidal recombinant B.t. strains containing no DNA elements not native to B.t. The site-specific recombination system of this invention facilitates construction of insecticidal recombinant B.t. strains with good stability characteristics, exhibiting limited horizontal transfer of their recombinant plasmids. The invention also permits the rapid construction and evaluation of recombinant B.t. constructs with unique complements of B.t. toxin genes that previously could not be quickly and easily realized with the prior art techniques.

The basic methods employed in the construction and evaluation of the recombinant plasmids described in this specification are generally well-known to those proficient in the art of molecular cloning. Descriptions of these general laboratory procedures and definitions of nomenclature may be found in Maniatus et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1982) and in a subsequent edition by Sambrook et al. (1989).

The following Examples provide further explanation of the invention and methods of its use.

EXAMPLE 1

Isolation and DNA Sequence Analysis of Tn5401

The transposon Tn5401 of this invention was initially isolated from copies of a recombinant plasmid which had been introduced into B.t. var. morrisoni strain EG2158 by electroporation. Transposon Tn5401 was subsequently shown to be located on two resident plasmids (35 and 72 MDa in size) of B.t. strain EG2158 from which it had apparently "jumped", or transposed itself, into the recombinant plasmids.

The recombinant plasmid pEG911, shown in FIG. 6, was used as the donor plasmid in transformation studies with B.t. strain EG2158, employing the conventional electroporation protocol described by Mettus et al., Applied and Environ. Microbiol. 56:1128-1134 (1990). Restriction enzyme analysis of DNA from recombinant plasmids isolated from several B.t. strain EG2158 transformants indicated that a subpopulation of the pEG911 plasmids contained a common .about.5 kilobase (kb) DNA insert.

Recombinant plasmid DNAs from two different, independent B.t. strain EG2158 transformants, each containing the 5 kb DNA insert, were recovered in Escherichia coli by selecting for E. coli transformants resistant to 50 .mu.g/ml ampicillin. Linear restriction maps of two independent plasmid clones, designated pEG911-1 and pEG911-3, are shown in FIGS. 7A and 7B, respectively. Each of these plasmids contains a .about.5 kb insertion (shown as a solid black box and designated Tn5401) upstream of the cryIIIB2 B.t. protein toxin gene (shown as a white arrow) but in opposite orientations as indicated by the placement of the unique BssHII and ClaI restriction endonuclease cleavage sites.

Plasmid pEG911-1 was chosen as the template for DNA sequence analysis of the .about.5 kb DNA insert. Double-stranded DNA was sequenced according to the well-known dideoxy chain termination method using [alpha-.sup.35 S)dATP. Synthetic oligonucleotides were generated to serve as primers for DNA sequence analysis. Sequence analysis was initiated using primers complementary to DNA flanking the DNA insert. Subsequently, sequencing primers were synthesized as needed based on the derived DNA sequences.

The complete nucleotide sequence of the .about.5 kb insert of plasmid pEG911-1 is shown in FIG. 1 and has been identified as a transposable element based on an analysis of its characteristics. This transposable element, having a size of 4837 basepairs, has been designated as transposon Tn5401.

For microorganism deposit purposes, plasmid pEG911-1, containing transposon Tn5401 on the .about.5 kb insert, was introduced into an E. coli host strain, E. coli JM110, to yield E. coli strain EG7683.

A linear structural map of Tn5401 is shown in FIG. 2, which includes the location of open reading frames denoted by arrows: orf1 (open arrow) potentially encodes a cryptic protein of unknown significance, of 85 amino acids in the 3'-5' direction; tnpI (dark shaded arrow) encodes a protein in the 5'-3' direction which has been designated the resolvase protein of Tn5401 and which contains 306 amino acids; and tnpA (light shaded arrow) encodes a protein in the 5'-3' direction which has been designated the transposase protein of Tn5401 and which contains 1005 amino acids. Not shown on FIG. 2 is a second open reading frame, orf2, which, like orf1, is oriented in the 3'-5' direction and potentially encodes a small cryptic protein which is of unknown significance and which contains 74 amino acids. Although not shown in FIG. 1, this deduced second small cryptic protein is derived from the complementary nucleotide sequence extending from nucleotide base positions 122 to 343.

Another distinguishing characteristic of Tn5401 shown in FIG. 2 is the location of its internal resolution site, located within the bracketed DNA fragment designated as "IRS region" and is likely located near the promoter region for the tnpI gene. Yet another characteristic of Tn5401 is the inverted repeats of 53 basepairs at either end of the transposon, shown as black arrowheads at both ends of the structural map in FIG. 2.

Analysis of Tn5401 and its elements was carried out using available computer databases to determine homologies with other DNA in the databases. These analyses indicate that Tn5401 belongs in the Tn3 family of transposons and, based on comparisons of its resolvase and transposase proteins, is related to Tn4430, the prior art transposon which is also present in B.t. species.

The 36 kDa resolvase protein of Tn5401 shows 25% amino acid sequence identity to the resolvase/recombinase protein of the Tn4430, but no significant identity to the corresponding resolvase protein of Tn3.

The 116 kDa transposase protein of Tn5401 shows 29% amino acid sequence identity to the transposase protein of Tn4430 and 44% identity to the transposase protein of Tn3.

The respective sequences of the 10.1 kDa cryptic protein of orf1 and of the 8.6 kDa cryptic protein of orf2 in Tn5401 showed no apparent homology to sequences in the databases.

EXAMPLE 2

Construction of Transposon Vector pEG922 Containing Tn5401

The ability of the isolated transposon Tn5401 to transpose in a B.t. strain is shown in this Example. As described in more detail below and as shown in FIG. 8, the cloned transposon Tn5401 was tagged with an antibiotic resistance marker gene, then inserted into a temperature-sensitive recombinant shuttle vector functional in B.t. The resultant Tn5401-containing shuttle vector, designated pEG922 and a circular structural map of which is shown in FIG. 3, was used to transform a plasmid-free B.t. strain, and measurements of its transposition efficiency were obtained.

The recombinant DNA manipulations involved in the construction of pEG922 were carried out using conventional techniques, familiar to those skilled in the art of cloning DNA, and are shown in schematic form in FIG. 8.

A .about.3 kb BssHII-SstI DNA fragment from plasmid pEG911-1, shown in FIG. 7A and containing a portion of Tn5401, and a .about.2.3 kb BssHII-SstI DNA fragment from pEG911-3, also shown in FIG. 7B and containing the remaining portion of Tn5401, were cloned together as shown in FIG. 8A into the SstI site of the well-known E. coli cloning vector pTZ19u. The resulting cloned DNA fragment contained a reconstructed, complete Tn5401transposon with flanking SstI sites as shown in FIG. 8A. The Tn5401 transposon in this cloning vector was next tagged with a tetracycline antibiotic resistance marker gene (tet) from the well-known Bacillus cereus plasmid pBC16, contained on a .about.1.7 kb AccI-AccI DNA fragment, by insertion of this fragment into the unique ClaI site on transposon Tn5401, whose structural map is shown in FIG. 2. It should be noted that an AccI cleavage site is compatible with a ClaI site. The tagged transposon Tn5401 was subsequently recovered on a .about.7 kb SstI fragment, as shown in FIG. 8A.

As shown in FIG. 8B, the tet-tagged transposon Tn5401 on the SstI fragment was inserted into the unique SstI site of a temperature-sensitive shuttle vector, pEG491, whose circular structural map is shown in FIG. 8B. Plasmid pEG491 is a shuttle vector derived in part from the well-known cloning vector pUC18, which also contains a chloramphenicol acetyl transferase antibiotic resistance gene (cat) and a temperature-sensitive replicon (rep.sup.ts) functional in gram-positive bacteria, the latter element being derived from plasmid pE194ts. The temperature-sensitive replicon rep.sup.ts cannot operate at temperatures at or above 37.degree. C., in contrast to most B.t.-derived replicons which operate at higher temperatures.

Plasmid pE194ts is described by Villafane et al. in J. Bacteriol. 169:4822-4829 (1987), and its use with a transposon (Tn917) is described by Youngman et al. at p.101-102 in Plasmids: a practical approach, Hardy, ed., IRL Press, Oxford, England (1987) pp. 79-103.

The resulting transposon vector, designated pEG922, contains the tet-tagged transposon Tn5401 on a .about.7 kb fragment inserted into the SstI site of plasmid pEG491, as shown in FIG. 8B which contains a circular structural map of plasmid pEG922. Plasmid pEG491, as shown in FIG. 8B, contains the E. coli replicon pUC18 (thin black segment), a chloramphenicol acetyl transferase gene, cat (thick black segment), and a temperature-sensitive replicon, rep.sup.ts, from plasmid pE194ts which cannot operate at temperatures at or above 37.degree. C. At the top of plasmid pEG491 is a multiple cloning site. Abbreviations for the restriction endonuclease cleavage sites in the multiple cloning site are explained in the discussion of FIG. 3 (which also shows the structural map of pEG922) appearing in the Brief Description of the Drawings.

For microorganism deposit purposes, plasmid pEG922 was also used to transform a host strain, E. coli strain GM2163, to yield E. coli strain EG7669.

The recombinant shuttle plasmid pEG922 was introduced into a plasmid-free B.t. var. kurstaki strain EG7566 by electroporation following the procedures of Mettus et al. (1990). Transposition of Tn5401 within the resulting transformed B.t. strain was measured using the procedure described by Youngman in Plasmids: a practical approach, Hardy, ed., IRL Press, Oxford, England (1987) pp 79-103. Transposition frequencies were measured as the quotient of the number of tetracycline resistant colonies observed at a temperature of 41.degree. C divided by the number of chloramphenicol resistant colonies at a temperature of 30.degree. C. Transposition frequencies of 10.sup.-4 were routinely obtained in transformed B.t. strain EG7566, indicating that transposon Tn5401 contained on plasmid pEG922 was functional in B.t.

Subsequent studies by the inventor with plasmid pEG922 in plasmid-containing B.t. strains have shown that transposon Tn5401 favors transposition into other plasmids, rather than into chromosomal DNA, and exhibits an apparent preference for AT-rich regions of DNA. Transposition frequencies observed for one plasmid-containing B.t. strain were generally two to three orders of magnitude higher than those obtained with the plasmid-free B.t. strain EG7566, typically being about 10.sup.-1 -10.sup.-2.

EXAMPLE 3

Construction of Plasmid Shuttle Vector pEG928.9

The plasmid shuttle vector pEG928.9 of this invention is useful for insecticidal recombinant B.t. strain development and is illustrated in FIG. 4. Example 3 describes the construction of plasmid shuttle vector pEG928.9, and the recombinant DNA procedures involved in this construction are schematically illustrated in FIG. 9.

The plasmid shuttle vector pEG854 was used as a starting point for the derivation of plasmid pEG928.9, as shown in FIG. 9A. Plasmid shuttle vector pEG854 is contained in E. coli strain EG7534, which is a deposited microorganism described in PCT International Patent Publication WO 91/18102.

Plasmid shuttle vector pEG854 contains a B.t. plasmid origin of replication, ori43 (light shaded segment), a multiple cloning site (shown at the top of the plasmid), the E. coli replicon pTZ19u, and a chloramphenicol acetyl transferase gene, cat (black arrow). The internal resolution site, IRS, of transposon Tn5401 (see FIG. 2) contained on a .about.650 bp NsiI-NsiI fragment from plasmid pEG911-1 (see FIG. 7A) was inserted into the unique PstI site in the multiple cloning site of pEG854 to yield the cloned plasmid p76.

A second copy of the IRS from transposon Tn5401, contained on a 2.5 kb NspI-NspI fragment from plasmid pEG922 (see FIG. 3), was inserted into the unique SphI site in the multiple cloning site of p76 to yield the cloned plasmid p83, as shown in FIG. 9B. Both IRS copies were in the same orientation (clockwise) in plasmid p83, as shown by the open arrows. The IRS copy in the NspI-NspI fragment also contained the tetracycline antibiotic resistance gene, tet (dark shaded arrow), that had been introduced into pEG922 (see FIGS. 3 and 8) as a selectable marker tag for transposon Tn5401.

As shown in FIG. 9B, plasmid p83 was digested with SalI, BlnI and SstI, and fragments containing the IRS sites were isolated: a 3.5 kb SalI-BlnI fragment containing the IRS and B.t. origin of replication ori43, and a 2.5 kb SstI-SalI fragment containing the IRS and the tet selectable marker gene. As shown in FIG. 9C, these two IRS-containing fragments were inserted together into the SstI and XbaI sites in the multiple cloning site of the well-known E. coli 2.86 kb phagemid vector pTZ19u to generate the cloned plasmid p84. The two copies of the IRS were in the same orientation, as shown in FIG. 9C, with the IRS sites segregating the B.t. origin of replication ori43 from the tet selectable marker gene and pTZ19u replicon.

The multiple cloning site in the 8.9 kb plasmid p84 was removed, as shown in FIG. 9D, by digesting with Asp718 and HindIII, blunting the protruding ends with Klenow polymerase and religating to generate the cloned plasmid p85. Plasmid p85, containing the B.t. origin of replication ori43, was manipulated to replace ori43 with a cryI-type B.t. protein toxin gene, specifically a cryIC-cryIA(c) fusion gene. The choice of the specific B.t. toxin gene for insertion into p85 is not critical; any insecticidal protein toxin gene could be utilized, e.g., a B.t. cryI, cryII, cryIII or cryIV toxin gene could be utilized.

Plasmid p85 was cleaved with SalI and XbaI and the vector fragment lacking ori43 was ligated to a SalI-BlnI fragment containing a cryI-type gene as shown in FIGS. 9D and 9E; note that a BlnI cleavage site is compatible with that of XbaI. The resultant plasmid clone was designated plasmid p85, shown in FIG. 9E.

Plasmid pEG928.9, the shuttle vector plasmid of this invention, was obtained from plasmid p86 by insertion of a B.t. plasmid origin of replication into p86, as shown in FIG. 9E. A 2.8 kb SalI fragment, containing B.t. plasmid origin of replication ori43.9, was inserted into the unique SalI site of p86 to yield pEG928.9. The ori43.9 B.t. origin of replication gene and the cryI-type protein toxin gene are transcribed in the same direction. As shown in FIG. 9E, the duplicate copies of the Tn5401 internal resolution site segregate the DNA not native to B.t., i.e., the E. coli replicon pTZ19u and the tet selectable marker gene from the B.t. origin of replication and adjacent cryI-type protein toxin gene.

For microorganism deposit purposes, plasmid shuttle vector pEG928.9 was used to transform an acrystalliferous B.t. host strain, B.t. var. kurstaki strain EG10368 which is a derivative of B.t. var. kurstaki strain HD73-26 described in U.S. Pat. No. 5,080,897 issued to Gonzalez, Jr. et al. on Jan. 14, 1992, to yield B.t.var. kurstaki strain EG7684.

In an analogous manner, other plasmid shuttle vectors were also constructed and two of these, plasmid shuttle vectors pEG930.9 and pEG931, are illustrated in FIGS. 10 and 11. These plasmid shuttle vectors differ from plasmid pEG928.9 primarily in the insecticidal protein toxin gene carried on the plasmids: plasmid pEG930.9 carries a coleopteran toxin cryIIIB2 gene (described in U.S. Pat. No. 5,187,091 issued to Donovan et al. on Feb. 16, 1993) and plasmid pEG931 carries a lepidopteran toxin cryIC gene, whose gene product exhibits good activity against Spodoptera insect species. As is evident from the circular structural maps in FIGS. 10 and 11, plasmid shuttle vectors pEG930.9 and pEG931 contain a cryI transcription terminator located downstream of their respective cryIIIB2 and cryIC genes.

Use of plasmid shuttle vectors pEG928.9, pEG930.9 and pEG931 in a site-specific recombination system for constructing insecticidal recombinant B.t. strains is described in Example 5.

EXAMPLE 4

Site-Specific Recombination Catalyzed by Recombinase Protein from Tn5401

The ability of recombinase/resolvase protein from transposon Tn5401 to catalyze, in trans, a site-specific recombination event in a transformed, recombinant B.t. strain was demonstrated in this Example 4. The recombinant plasmid used to transform the host B.t. strains was plasmid p83, described in Example 3 and a circular structural map of which is illustrated in FIG. 9B. Plasmid p83 contains two identical copies of the Tn5401-derived internal resolution site, IRS, oriented in the same direction and flanking a tetracycline antibiotic resistance gene, tet, as shown in FIG. 9B. Plasmid p83 also contains an origin of replication functional in B.t., i.e., B.t.-derived ori43, and another selectable marker gene, a chloramphenicol acetyl transferase gene, cat, as shown in FIG. 9B.

A site-specific recombination event involving plasmid p83 was demonstrated by showing that the cat gene encoding resistance to chloramphenicol would be maintained after a site-specific recombination event between the two IRS regions but that tetracycline resistance would be lost because of excision of the tet gene during such recombination. The source of recombinase protein for catalyzing the site-specific recombination was B.t. var. morrisoni strain EG2158, which harbors transposon Tn5401 which contains the recombinase gene, tnpI.

Plasmid p83 was first introduced by a conventional electroporation technique into the transposon-free B.t. var. kurstaki strain EG7566, a plasmid-free derivative of B.t. var. kurstaki strain HD73-26 described in U.S. Pat. No. 5,080,897 issued to Gonzalez, Jr. et al. on Jan. 14, 1992, and also into the Tn5401-containing B.t. strain EG2158. Transformed B.t. colonies were selected separately for tetracycline resistance (Tet.sup.R) and for chloramphenicol resistance (cm.sup.R) and results are shown in the following table:

Both transformed B.t. strains exhibited chloramphenicol resistance, apparently due to the presence of the cat gene in the introduced plasmid p83. For the transposon-free B.t. strain EG7566 transformants, the existence of tetracycline resistance indicated that plasmid p83 was likely present as an intact plasmid, i.e., no site-specific recombination event had occurred. Restriction enzyme analysis of recombinant plasmids isolated from representative B.t. strain EG7566 transformants indicated that the structural integrity of plasmid p83 had been maintained.

The Tn5401-containing B.t. strain EG2158 transformants, on the other hand, exhibited no tetracycline resistance, indicating the likely loss of the tet selectable marker gene from site-specific recombination between the two IRS regions in p83. Restriction enzyme analysis of recombinant plasmids recovered from representative chloramphenicol-resistant B.t. strain EG2158 transformants confirmed that recombination had occurred between the two IRS regions, resulting in excision of the tet gene from this location in plasmid p83.

EXAMPLE 5

Construction of Recombinant B.t. Strains via Site-Specific Recombination Event Using Plasmid Shuttle Vector pEG928.9

Example 5 illustrates a method of constructing insecticidal recombinant B.t. strains containing no DNA foreign to B.t., utilizing the plasmid shuttle vector pEG928.9 and the Tn5401 transposon-containing recombinant plasmid pEG922 to effect a site-specific recombination event that produces the desired B.t. strain construct. The schematic steps of this method are shown in FIG. 5, and detailed circular structural maps of plasmid pEG928.9 and plasmid 922 are shown in FIGS. 4 and 3, respectively, and explained in the Brief Description of the Drawings for these two Figures.

Plasmid shuttle vector pEG928.9, containing a cryI-type gene (a cryIC-cryIA(c) fusion gene), a B.t. origin of replication region (ori43.9, a high copy number mutant of ori43, derived from a 43-MDa B.t. toxin plasmid), and two identical internal resolution site (IRS) regions oriented in the same direction, was used to transform a B.t. host strain that served as the basis for the recombinant B.t. construct. As is discussed in Example 3, plasmid pEG928.9 also contains DNA not native to B.t. that is useful in the construction (particularly, development and characterization) of recombinant B.t. strains. This foreign DNA consists of an E. coli replicon pTZ19u and a tetracycline resistance gene, tet, useful as a selectable marker. The DNA not native to B.t. is desirably absent from the insecticidal recombinant B.t. construct produced by this method and for this reason is flanked by the duplicate IRS regions. The site-specific recombination event that occurs between the two IRS regions effects excision of the foreign DNA from the plasmid, and this was accomplished in this Example 5 as follows.

B.t. var. kurstaki strain EG10324 served as the host strain in this Example. B.t. strain EG10324 is a phage resistant mutant of B.t. var. kurstaki strain EG2348, described in U.S. Pat. No. 5,080,897 issued to Gonzalez, Jr. et al. on Jan. 14, 1992. This transconjugant B.t. strain exhibits insecticidal activity against lepidopteran insects. The addition of a recombinant toxin plasmid via the method of this Example was intended to broaden the insecticidal spectrum of the host strain. The cryIC-type B.t. toxin gene carried by plasmid shuttle vector pEG928.9 produces a toxin protein with good activity against Spodoptera species.

B.t. strain EG10324 was transformed with plasmid shuttle vector pEG928.9 using conventional electroporation techniques, e.g., similar to those described in Example 6 of WO 91/18102. B.t. strain EG10324 transformants that were selected for tetracycline resistance were analyzed via restriction enzyme digests, and this analysis confirmed the structural integrity of plasmid pEG928.9 in these tet.sup.R colonies.

These B.t. strain EG10324 transformants were next transformed with the Tn5401 transposon-containing plasmid pEG922, selecting this time for chloramphenicol resistance. Plasmid pEG922, described in Example 2 and shown in FIG. 3, contains the Tn5401 transposon of this invention, tagged with a tetracycline antibiotic resistance gene, tet. As noted previously in description of the construction of this plasmid in Example 2, plasmid pEG922 contains a thermosensitive replicon, rep.sup.ts, that is functional in gram-positive bacteria but that only operates at temperatures below 37.degree. C., in contrast to most B.t. replicons which operate at higher temperatures. This transposon-containing plasmid also contains another selectable marker gene, cat, for chloramphenicol acetyl transferase resistance.

B.t. strain EG10324 double transformants, i.e., containing both plasmid shuttle vector pE928.9 and the Tn5401-containing plasmid pE922, were selected for colonies exhibiting chloramphenicol resistance. In the double recombinant derivative of B.t. strain EG10324, plasmid pEG928.9 underwent the site-specific recombination event between its IRS regions, and this event was catalyzed by the introduction of recombinase/resolvase protein produced by expression of the tnpI gene in the Tn5401-containing plasmid pEG922. Production of the recombinase protein was ensured by culturing the double recombinant B.t. strain colonies overnight at a temperature of about 30.degree. C., at which the temperature-sensitive replicon in plasmid pEG922 operates.

The site-specific recombination event for plasmid pEG928.9 is schematically shown in FIG. 5, and this resulted in the formation of plasmid pEG928.9.DELTA.. Plasmid pEG928.9.DELTA. is a 8.0 mDa recombinant plasmid that contains the ori43.9 origin of replication functional in B.t., the cryIC-cryIA(c) B.t. protein toxin fusion gene, and a single copy of the internal resolution site, derived from the site-specific recombination event.

After the site-specific recombination had been effected, removal of plasmid pEG922 from the double recombinant B.t. strain EG10324 transformants also containing plasmid pEG928.9.DELTA. a was accomplished by culturing these B.t. colonies overnight at a temperature of 37.degree. C., a growth procedure effective to cure temperature-sensitive plasmid pEG922 from the resulting B.t. colonies.

The desired insecticidal recombinant B.t. construct, containing only a single recombinant plasmid, pEG928.9.DELTA., was recovered and was designated as B.t. strain EG7674.

B.t. strain EG7674 lacks the selectable marker genes utilized during its construction and is therefore chloramphenicol-and tetracycline-sensitive. B.t. strain EG7674 also lacks the E. coli replicon that was originally present in plasmid pEG928.9 but that was subsequently excised during the site-specific recombination event.

Plasmid assay studies of B.t. strain EG10324 and its recombinant derivatives described in this Example 10 confirmed the absence of plasmid pEG922 from B.t. strain EG7674. Hybridization with the ori43.9 plasmid origin of replication in a Southern blot study of the plasmid assay gel established the presence of pEG928.9.DELTA. as the only recombinant plasmid harbored by B.t. strain EG7674.

B.t. strain EG7674, containing no DNA not native to B.t., is insecticidal to a wide spectrum of lepidopteran insects and, because of the additional cryIC-cryIA(c) fusion gene on its recombinant plasmid pEG928.9.DELTA., is designed to exhibit improved insecticidal activity against Spodoptera exigua (beet armyworm) and Spodoptera littoralis (Egyptian leaf roller), as compared with the host B.t. strain EG10324.

In a similar manner, two other insecticidal recombinant B.t. constructs were prepared via the site-specific recombination method described above. Both of these B.t. constructs were similar to B.t. strain EG7674 in that their respective recombinant plasmids contained insecticidal B.t. protein toxin genes but no DNA not native to B.t.

The first construct was a coleopteran-toxic B.t. construct which used, as the host strain, transconjugant B.t. var. kurstaki strain EG2424 (described in U.S. Pat. No. 5,024,837 issued to Donovan et al. on Jun. 18, 1991) and plasmid shuttle vector pEG930.9 whose circular structural map is shown in FIG. 10. Plasmid shuttle vector pEG930.9 is similar to plasmid pEG928.9 except that, in lieu of the cryI-type gene of pEG928.9, it contains the coleopteran toxin cryIIIB2 gene (described in U.S. Pat. No. 5,187,091 issued to Donovan et al. on Feb. 16, 1993) and it contains a transcription terminator downstream of the cryIIIB2 gene. The resulting recombinant B.t. construct contained plasmid pEG930.9.DELTA., whose circular structural map is also shown in FIG. 10, and was designated B.t. strain EG7673. The presence of the cryIIIB2 gene in this recombinant B.t. construct, complementing the cryIIIA coleopteran toxin gene present on an 88 mDa plasmid of host B.t. strain EG2424, is designed to provide a wider spectrum of insecticidal activity against coleopteran insects, as compared with host B.t. strain EG2424.

The second B.t. construct was a lepidopteran-toxic B.t. construct which used a novel B.t. strain, designated EG10367, as the host strain and plasmid shuttle vector pEG931 whose circular structural map is shown in FIG. 11. Plasmid shuttle vector pEG931 is similar to plasmid pEG928.9 except that (i) a cryIC gene replaces the cryIC-cryIA(c) fusion gene of pEG928.9, (ii) it contains a transcription terminator downstream of the cryIC gene, and (iii) the B.t. origin of replication is ori43 rather than the high copy number mutant ori43.9 used in pEG928.9. The resulting recombinant B.t. construct contained plasmid pEG930.DELTA., whose circular structural map is also shown in FIG. 11, and was designated B.t. strain EG7681. The presence of the cryIC gene in this recombinant B.t. construct, complementing the cryIA(c) genes of host B.t. strain EG10367, is designed to provide a wider spectrum of insecticidal activity against lepidopteran insects, as compared with host B.t. strain EG10367.

Claims

What is claimed is:

1. A method of constructing a recombinant Bacillus thuringiensis (B.t.) strain containing no DNA elements foreign to B.t. which comprises: (a) transforming a host B.t. strain with a plasmid shuttle vector containing: (i) an origin of replication native to B.t.; (ii) DNA not native to B.t. selected from the group consisting of selectable marker genes, origins or replication functional in E. coli, and origins of replication functional in a Bacillus host species other than B.t.; (iii) one or more insecticidal B.t. protein toxin genes; and (iv) two identical internal resolution sites oriented in the same direction and flanking the DNA not native to B.t., the sites being the same as an internal resolution site from a Tn3-type transposon native to B.t.; (b) introducing into the transformed B.t. strain a resolvase protein to effect a site-specific recombination event involving the internal resolution sites, thereby excising from the plasmid shuttle vector the DNA not native to B.t.; and (c) recovering a recombinant B.t. strain containing a recombinant plasmid adapted replicating in the B.t. strain and containing (i) an origin of replication native to B.t.; (ii) one or more insecticidal B.t. protein toxin genes; and (iii) a single internal resolution site, derived from the site-specific recombination event.

2. The method of claim 1 wherein the internal resolution sites in the plasmid shuttle vector are identical to the internal resolution site of transposon Tn4430.

3. The method of claim 1 wherein the internal resolution sites in the plasmid shuttle vector are identical to the internal resolution site of transposon Tn540.

4. The method of claim 1 wherein the resolvase protein is directly introduced into the host B.t. strain.

5. The method of claim 1 wherein the resolvase protein is produced by expression of a corresponding resolvase gene carried by a recombinant plasmid, the plasmid having been introduced into the host B.t. strain.

6. The method of claim 5 wherein the recombinant plasmid carrying the resolvase gene is a plasmid with a thermosensitive replicon, to facilitate recovery of a recombinant B.t. strain free of the resolvase-containing recombinant plasmid.

7. The method of claim 6 wherein the thermosensitive plasmid is removed from the host B.t. strain following the site-specific recombination event effected by the resolvase protein.

8. The method of claim 3 wherein the resolvase protein is produced by expression of the res gene of transposon Tn5401.

Patent Citations (10)

Patent	Date	Inventor
US4959317	1990-09-01	Sauer
US5024837	1991-06-01	Donovan et al.
US5080897	1992-01-01	Gonzalez, Jr. et al.
US5102797	1992-04-01	Tucker et al.
US5187091	1993-02-01	Donovan et al.
EPX342633	1989-05-01
EPX537105	1993-04-01
WOX91-18102	1991-11-01
WOX93-01283	1993-01-01
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Abstract

Description