Purified Thermostable Nucleic Acid Polymerase and DNA Coding Sequences From Pyrodictium Species

The present invention relates to thermostable DNA polymerases from hyperthermophilic archael Pyrodictium species and means for isolating and producing the enzymes. Thermostable DNA polymerases are useful in many recombinant DNA techniques, especially nucleic acid amplification by the polymerase chain reaction (PCR).

Extensive research has been conducted on the isolation of DNA polymerases from mesophilic microorganisms such as E. coli. See, for example, Bessman et al., 1957, J. Biol. Chem. 223:171-177, and Buttin and Kornberg, 1966, J. Biol. Chem. 241:5419-5427.

Interest in DNA polymerases from thermophilic microbes increased with the invention of nucleic acid amplification processes. The use of thermostable enzymes, such as those described in U.S. Pat. No. 4,165,188, to amplify existing nucleic acid sequences in amounts that are large compared to the amount initially present was described U.S. Pat. Nos. 4,683,195 and 4,683,202, which describe the PCR process. These patents are incorporated herein by reference. The PCR process involves denaturation of a target nucleic acid, hybridization of primers, and synthesis of complementary strands catalyzed by a DNA polymerase. The extension product of each primer becomes a template for the production of the desired nucleic acid sequence. These patents disclose that, if the polymerase employed is a thermostable enzyme, then polymerase need not be added after every denaturation step, because heat will not destroy the polymerase activity.

The thermostable DNA polymerase from Thermus aquaticus (Taq) has been cloned, expressed, and purified from recombinant cells as described in Lawyer et al., 1989, J. Biol. Chem. 264:6427-6437, and U.S. Pat. Nos. 4,889,818 and 5,079,352, which are incorporated herein by reference. Crude preparations of a DNA polymerase activity isolated from T. aquaticus have been described by others (Chien et al., 1976, J. Bacteriol. 127:1550-1557, and Kaledin et al., 1980, Biokymiya 45:644-651).

U.S. Pat. No. 4,889,818, European Patent Publication No. 258,017, and PCT Publication No. 89/06691, the disclosures of which are incorporated herein by reference, all describe the isolation and recombinant expression of an .about.94 kDa thermostable DNA polymerase from Thermos aquaticus and the use of that polymerase in PCR. Although T. aquaticus DNA polymerase is especially preferred for use in PCR and other recombinant DNA techniques, a number of other thermophilic DNA polymerases have been purified, cloned, and expressed. (See co-pending, commonly assigned PCT Patent Publication Nos. WO 91/09950, WO 92/03556, WO 92/06200, and WO 92/06202, which are incorporated heroin by reference.)

Thermostable DNA polymerases are not irreversibly inactivated even when heated to 93.degree.-95.degree. C. for brief periods of time, as, for example, in the practice of DNA amplification by PCR. In contrast, at this elevated temperature E. coli DNA Pol I is inactivated.

Archaeal hyperthermophiles, such as Pyrodictium and Methanopyrus species, grow at temperatures up to about 110.degree. C. and are unable to grow below 80.degree. C. (see, Stetter et al., 1990, FEMS Microbiology Reviews 75:1170124, which is incorporated herein by reference). These sulfur reducing, strict anaerobes are isolated from submarine environments. For example, P. abyssi was isolated from a deep sea active "smoker" chimney off Guaymas Mexico at 2,000 meters depth and in 320.degree. C. of venting water (Pley et al., 1991, Systematic and Applied Microbiology 14:245). In contrast to the Pyrodictium species, other thermophilic microorganisms having an optimum growth temperature at or about 90.degree. C. and a maximum growth temperature at or about 100.degree. C. are not difficult to culture. For example, a gene encoding DNA polymerase has been cloned and sequenced from Thermococcus litoralis (EP No. 455,430).

In contrast, culture of the extreme hyperthermophilic microorganisms is made difficult by their inability to grow on agar solidified media. Individual cells of the Pyrodictium species are extremely fragile, and the organisms grow as fibrous networks. Standard bacterial fermentation techniques are extremely difficult for culturing Pyrodictium species due to the fragility of the cells and tendency of the cells to grow as networks clogging the steel parts of conventional fermentation apparatus. (See Staley, J. T. et al. eds., Bergey's Manual of Systematic Bacteriology, 1989, Williams and Wilkins, Baltimore, which is incorporated herein by reference.) These difficulties preclude laboratory culture for preparing large amounts of purified nucleic acid polymerase enzymes for characterization and amino acid sequence analysis. Those skilled in the art may be able to culture Pyrodictium to a cell density approaching 10.sup.6 -10.sup.7 cells/ml (see, for example, Phipps et al., 1991, EMBO J. 10(7):1711-1722). In contrast, E. coli is routinely grown to 0.3-1.0.times.10.sup.11 cells/ml.

Accordingly, there is a need for the characterization, amino acid sequence, DNA sequence, and expression in a non-native host, of hyperthermophile DNA polymerase enzymes to eliminate the prior difficulties associated with the native host. In addition there is a desire in the art to produce thermostable DNA polymerases having enhanced thermostability that may be used to improve the PCR process and to improve the results obtained when using a thermostable DNA polymerase in other recombinant techniques such as DNA sequencing, nick-translation, and reverse transcription.

The present invention meets these needs by providing DNA and amino acid sequence information, recombinant expression vectors and purification protocols for DNA polymerases from Pyrodictium species.

The present invention provides thermostable enzymes that catalyze the combination of nucleoside triphosphates to form a nucleic acid strand complementary to a nucleic acid template strand. The enzymes are DNA polymerases from Pyrodictium species. In a preferred embodiment, the enzyme is from P. occultum or P. abyssi. This material may be used in a temperature-cycling amplification reaction wherein nucleic acid sequences are produced from a given nucleic acid sequence in amounts that are large compared to the amount initially present so that the sequences can be manipulated and/or analyzed easily.

The genes encoding the P. occultum and P. abyssi DNA polymerase enzyme have also been identified and cloned and provide yet another means to prepare the thermostable enzyme of the present invention. In addition, DNA and amino acid sequences of the genes encoding the P. occultum and P. abyssi enzyme and derivatives of these genes encoding P. occultum and P. abyssi DNA polymerase activity are also provided. In addition, modified genes encoding and expressing 3'-5' exonucleasedeficient forms of Pyrodictium occultum and P. abyssi DNA polymerase activity are also provided.

The invention also encompasses stable enzyme compositions comprising a purified, thermostable P. occultum and/or P. abyssi enzyme as described above in a buffer containing one or more non-ionic polymeric detergents.

Finally, the invention provides a method of purification for the thermostable polymerase of the invention.

The present invention provides DNA sequences and expression vectors that encode Pyrodictium DNA polymerase. To facilitate understanding of the invention, a number of terms are defined below.

The terms "cell," "cell line," and "cell culture" can be used interchangeably and all such designations include progeny. Thus, the words "transformants" or "transformed cells" include the primary transformed cell and cultures derived from that cell without regard to the number of transfers. All progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations. Mutant progeny that have the same functionality as screened for in the originally transformed cell are included in the definition of transformants.

The term "control sequences" refers to DNA sequences necessary for the expression of an operably linked coding sequence in a particular host organism. The control sequences that are suitable for procaryotes, for example, include a promoter, optionally an operator sequence, a ribosome binding site, and possibly other sequences. Eucaryotic cells are known to utilize promoters, polyadenylation signals, and enhancers.

The term "expression system" refers to DNA sequences containing a desired coding sequence and control sequences in operable linkage, so that hosts transformed with these sequences are capable of producing the encoded proteins. To effect transformation, the expression system may be included on a vector; however, the relevant DNA may also be integrated into the host chromosome.

The term "gene" refers to a DNA sequence that comprises control and coding sequences necessary for the production of a recoverable bioactive polypeptide or precursor. The polypeptide can be encoded by a full length gene sequence or by any portion of the coding sequence so long as the enzymatic activity is retained.

The term "operably linked" refers to the positioning of the coding sequence such that control sequences will function to drive expression of the protein encoded by the coding sequence. Thus, a coding sequence "operably linked" to control sequences refers to a configuration wherein the coding sequences can be expressed under the direction of a control sequence.

The term "mixture" as it relates to mixtures containing Pyrodictiurn polymerase refers to a collection of materials which includes Pyrodictium polymerase but which can also include other proteins. If the Pyrodictium polymerase is derived from recombinant host cells, the other proteins will ordinarily be those associated with the host. Where the host is bacterial, the contaminating proteins will, of course, be bacterial proteins.

The term "non-ionic polymeric detergents" refers to surface-active agents that have no ionic charge and that are characterized for purposes of this invention, by an ability to stabilize the Pyrodictium enzyme at a pH range of from about 3.5 to about 9.5, preferably from 4 to 9.0.

The term "oligonucleotide" as used herein is defined as a molecule comprised of two or more deoxyribonucleotides or ribonucleotides, preferably more than three, and usually more than ten. The exact size of an oligonucleotide will depend on many factors, including the ultimate function or use of the oligonucleotide.

Oligonucleotides can be prepared by any suitable method, including, for example, cloning and restriction of appropriate sequences and direct chemical synthesis by a method such as the phosphotriester method of Narang et al., 1979, Meth. Enzymol. 68:90-99; the phosphodiester method of Brown et al., 1979, Meth. Enzymol. 68:109-151; the diethylphosphoramidite method of Beaucage etak, 1981, Tetrahedron Lett. 22:1859-1862; the triester method of Matteucci et al., 1981, J. Am. Chem. Soc. 103:3185-3191 or automated synthesis methods; and the solid support method of U.S. Pat. No. 4,458,066.

The term "primer" as used herein refers to an oligonucleotide, whether natural or synthetic, which is capable of acting as a point of initiation of synthesis when placed under conditions in which primer extension is initiated. Synthesis of a primer extension product which is complementary to a nucleic acid strand is initiated in the presence of nucleoside triphosphates and a DNA polymerase or reverse transcriptase enzyme in an appropriate buffer at a suitable temperature. A "buffer" includes cofactors (such as divalent metal ions) and salt (to provide the appropriate ionic strength), adjusted to the desired pH. For Pyrodictium polymerases, the buffer preferably contains 1 to 3 mM of a magnesium salt, preferably MgCl.sub.2, 50 to 200 .mu.M of each nucleotide, and 0.2 to 1 .mu.M of each primer, along with 10-100 mM KCl, 10 mM Tris buffer (pH 7.5-8.5), and 100 .mu.g/ml gelatin (although gelatin is not required, and should be avoided in some applications, such as DNA sequencing).

A primer is preferably a single-stranded oligodeoxyribonucleotide. The appropriate length of a primer depends on the intended use of the primer but typically ranges from 15 to 35 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template but must be sufficiently complementary to hybridize with a template.

The term "primer" may refer to more than one primer, particularly in the case where there is some ambiguity in the information regarding one or both ends of the target region to be amplified. For instance, if a nucleic acid sequence is inferred from a protein sequence, a "primer" is actually a collection of primer oligonucleotides containing sequences representing all possible codon variations based on the degeneracy of the genetic code. One of the primers in this collection will be homologous with the end of the target sequence. Likewise, if a "conserved" region shows significant levels of polymorphism in a population, mixtures of primers can be prepared that will amplify adjacent sequences.

A primer may be "substantially" complementary to a strand of specific sequence of the template. A primer must be sufficiently complementary to hybridize with a template strand for primer elongation to occur. A primer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5' end of the primer, with the remainder of the primer sequence being substantially complementary to the strand. Non-complementary bases or longer sequences can be interspersed into the primer, provided that the primer sequence has sufficient complementarity with the sequence of the template to hybridize and thereby form a template primer complex for synthesis of the extension product of the primer.

A primer can be labeled, if desired, by incorporating a label detectable by spectroscopic, photochemical, biochemical, immunochemical, or chemical means. For example, useful labels include .sup.32 P, fluorescent dyes, electron-dense reagents, enzymes (as commonly used in ELISAS), biotin, or haptens and proteins for which antisera or monoclonal antibodies are available. A label can also be used to "capture" the primer, so as to facilitate the immobilization of either the primer or a primer extension product, such as amplified DNA, on a solid support.

The terms "restriction endonucleases" and "restriction enzymes" refer to bacterial enzymes which cut double-stranded DNA at or near a specific nucleotide sequence.

The terms "thermostable polymerase" and "thermostable enzyme" refer to an enzyme which is stable to heat and is heat resistant and catalyzes combination of the nucleotides in the proper manner to form primer extension products that are complementary to a template nucleic acid strand. Generally, synthesis of a primer extension product begins at the 3' end of the primer and proceeds in the 5' direction along the template strand, until synthesis terminates.

The Pyrodictium thermostable enzymes of the present invention satisfy the requirements for effective use in the amplification reaction known as the polymerase chain reaction or PCR as described in U.S. Pat. No. 4,965, 188 (incorporated herein by reference). The Pyrodictium enzymes do not become irreversibly denatured (inactivated) when subjected to the elevated temperatures for the time necessary to effect denaturation of double-stranded nucleic acids, a key step in the PCR process. Irreversible denaturation for purposes herein refers to permanent and complete loss of enzymatic activity. The heating conditions necessary for nucleic acid denaturation will depend, e.g., on the buffer salt concentration and the composition and length of the nucleic acids being denatured, but typically range from about 90.degree. C. to about 105.degree. C. for a time depending mainly on the temperature and the nucleic acid length, typically from a few seconds up to four minutes.

Higher temperatures may be required as the buffer salt concentration and/or GC composition of the nucleic acid is increased. The Pyrodictium enzymes do not become irreversibly denatured from relatively short exposures to temperatures of about 95.degree. C.-100.degree. C. The extreme thermostability of the Pyrodictium DNA polymerase enzymes provides additional advantages over previously characterized thermostable enzymes. Prior to the present invention, efficient PCR at denaturation temperatures as high as 100.degree. C. had not been demonstrated. No thermostable DNA polymerases have been described for this purpose. However, as the G/C content of a target nucleic acid increases, the temperature necessary to denature (T.sub.den), the duplex also increases. For target sequences that require a T.sub.den step of over 95.degree. C., previous protocols require that solvents are included in the PCR for partially destabilizing the duplex, thus, lowering the effective T.sub.den. Agents such as glycerol, DMSO, or formamide have been used in this manner in PCR (Korge et al., 1992, Proc. Natl. Acad. Sci. U.S.A. 89:910-914, and Wong et al., 1991, Nuc. Acids Res. 19:225 1- 2259, incorporated herein by reference). These agents, in addition to destabilizing duplex DNA will affect primer stability, can inhibit enzyme activity, and varying concentrations of DMSO or formamide decrease the thermoresistance (i.e., half-life) of thermophilic DNA polymerases. Accordingly, a significant number of optimization experiments and reaction conditions need to be evaluated when utilizing these cosolvents. In contrast, simply raising the T.sub.den to 100.degree. C. with Pot or Pab DNA polymerase in an otherwise standard PCR can facilitate complete strand separation of PCR product eliminating the need for DNA helix destabilizing agents.

The extreme hyperthermophilic polymerases disclosed herein are stable at temperatures exceeding 100.degree. C., and even as high as 110.degree. C. However, at these temperatures depending on the pH and ionic strength, the integrity of the target DNA may be adversely affected (Ekert and Kunkel, 1992, In PCR: A Practical Approach, eds. McPherson, Quirke and Taylor, Oxford University Press, pages 225-244, incorporated herein by reference).

The Pyrodictium DNA polymerase has an optimum temperature at which it functions that is higher than about 45.degree. C. Temperatures below 45.degree. C. facilitate hybridization of primer to template, but depending on salt composition and concentration and primer composition and length, hybridization of primer to template can occur at higher temperatures (e.g., 45.degree.-70.degree. C.), which may promote specificity of the primer hybridization reaction. The enzymes of the invention exhibit activity over a broad temperature range up to 85.degree. C. The optimal activity is template dependent and generally in the range of 70.degree.-80.degree. C.

The present invention provides DNA sequences encoding the thermostable DNA polymerase activity of Pyrodictium species. The preferred embodiments of the invention provide the nucleic acid and amino acid sequences for P. abyssi and P. occultum DNA polymerase. The entire P. abyssi and P. occultum DNA polymerase coding sequences are depicted below as SEQ ID No. 1 (P. abyssi) and SEQ ID No. 3 (P. occultum). The deduced amino acid sequences are listed as SEQ ID No. 2 (P. abyssi) and SEQ ID No. 4 (P. occultum). For convenience, the nucleotide and amino acid sequences of these polymerases are numbered for reference.

The present invention provides nucleic acid sequences providing means for comparison of P. occultum and P. abyssi DNA polymerase sequences with other thermostable polymerase enzymes. Such a comparison demonstrates that these novel sequences are unrelated to previously described nucleic acid sequences encoding eubacterial thennostable DNA polymerases. Consequently, methods for identifying Pyrodictium DNA polymerase enzymes based on the published sequences of known eubacterial thermostable DNA polymerases are not suitable for isolating nucleic acid sequences encoding Pyrodictium DNA polymerase enzymes. ##STR1##

As a result of the present invention, Pyrodictium DNA polymerase amino acid sequences can be used to design novel degenerate primers to find new, previously undiscovered hyperthermic DNA polymerase genes. The generic utility of the degenerate primer process is exemplified in WO 92/06202, which is incorporated herein by reference. The publication describes the use of degenerate primers for cloning the gene encoding Thermosipho africanus DNA polymerase. Prior to the present invention, degenerate priming methods were demonstrated to be suitable for isolating genes encoding novel thermostable DNA polymerase enzymes. The success of these methods lies in part in the identification of conserved motifs among the thermostable DNA polymerases of, for example, Thermus aquaticus and Thermus thermophilus.

Thus, due to the dissimilarity in DNA polymerase amino acid sequences between the extreme hyperthermophiles, for example, Pyrodictium species, and nonhyperthermophiles such as Thermus species these degenerate priming methods were not previously suitable for isolating and expressing pyrodictium polymerase genes. Applicants' invention has enabled the use of degenerate priming methods for isolating genes encoding novel DNA polymerase enzymes from extreme hyperthermophilic microbes. The gene encoding the DNA polymerase of the hyperthermophilic T. litoralis (Tli) has been described. While Tli, Pab and Poc DNA polymerases contain the amino acid sequence motifs that reflect eucaryotic DNA polymerases, Pab and Poc DNA polymerases have only limited and spotty amino acid sequence identity with Tli DNA polymerase. Specifically, amino acid sequence alignments indicate only 37% to 39% sequence identity between Poc or Pab with Tli DNA polymerase. Significant regions of non-identity with Tli DNA polymerase occur in the 20 amino acids that precede and the 10 amino acids that follow Region 1 (position 438 through 458 in SEQ ID Nos. 2 and 4). In addition, significant regions on non-identity with Tli DNA polymerase occur in the 10 to 15 amino acids that precede, and the 10 to 15 amino acids that follow Region 4 (position 611 through 634 in SEQ ID Nos. 2 and 4). These regions as well as other portions of the polymerase active site are highly conserved in Poc and Pab DNA polymerases and contribute significantly to the extraordinary thermostability of these DNA polymerase enzymes.

The present invention, by providing DNA and amino acid sequences for two Pyrodictium polymerase enzymes, therefore, enables the isolation of other extremely thermophilic DNA polymerase enzymes and the coding sequences for those enzymes. Further alignment of P. occultum and P. abyssi sequences with known thermostable enzyme sequences allows the selective identification of additional novel enzymes suitable for efficient PCR at denaturation temperatures of 100.degree. C.

The DNA and amino acid sequences shown above and the DNA compounds that encode those sequences can be used to design and construct recombinant DNA expression vectors to drive expression of Pyrodictium DNA polymerase activity in a wide variety of host cells. A DNA compound encoding all or part of the DNA sequence shown above can also be used as a probe to identify thermostable polymerase-encoding DNA from other archaea, especially Pyrodictium species and the amino acid sequence shown above can be used to design peptides for use as immunogens to prepare antibodies that can be used to identify and purify a thermostable polymerase.

Recombinant vectors that encode an amino acid sequence encoding a Pyrodictium DNA polymerase will typically be purified prior to use in a recombinant DNA technique. The present invention provides such purification methodology.

The molecular weight of the DNA polymerase purified from recombinant E. coli host which express the P. occultum or P. abyssi polymerase genes are determined by the above method to be about 90 kDa. The molecular weight of this same DNA polymerase as determined by the predicted amino acid sequence is calculated to be approximately 92.6 kilodaltons.

An important aspect of the present invention is the production of recombinant Pyrodictium DNA polymerase. As noted above, the gene encoding this enzyme has been cloned from two exemplary Pyrodictium species, P. occultum and P. abyssi, genomic DNA. The complete coding sequence for the P. occultum (Poc) DNA polymerase can be easily obtained in an .about.2.52 kb NheI restriction fragment of plasmid pPoc 4. This plasmid was deposited with the American Type Culture Collection (ATCC) in host cell E. coli Sure.RTM. Cells (Stratagene) on May 11, 1993, under Accession No. 69309. The complete coding sequence for P. abyssi (Pab) DNA polymerase can be easily obtained in an .about.3.74 kb SalI restriction fragment of plasmid pPab 14. This plasmid was deposited with the ATCC in host cell E. coli Sure.RTM. Cells (Stratagene) on May 11, 1993, and under Accession No. 69310.

The complete coding sequence and deduced amino acid sequence of the thermostable Pab and Poc DNA polymerase enzymes are provided above. The entire coding sequence of the DNA polymerase gene is not required, however, to produce a biologically active gene product with DNA polymerase activity. The availability of DNA encoding the Pyrodictium DNA polymerase sequence provides the opportunity to modify the coding sequence so as to generate mutein (mutant protein) forms also having DNA polymerase activity. Amino(N)-terminal deletions of approximately one-third of the coding sequence can provide a gene product that is quite active in polymerase assays. Because certain N-terminal shortened forms of the polymerase are active, the gene constructs used for expression of these polymerases can include the corresponding shortened forms of the coding sequence.

In addition to the N-terminal deletions, individual amino acid residues in the peptide chain comprising Pyroclictium polymerase may be modified by oxidation, reduction, or other derivation, and the protein may be cleaved to obtain fragments that retain activity. Such alterations that do not destroy activity do not remove the protein from the definition of a protein with Poc or Pab polymerase activity and so are specifically included within the scope of the present invention. Modifications to the primary structure of the Poc or Pab DNA polymerase gene by deletion, addition, or alteration so as to change the amino acids incorporated into the DNA polymerase during translation can be made without destroying the high temperature DNA polymerase activity of the protein. Such substitutions or other alternations result in the production of proteins having an amino acid sequence encoded by DNA falling within the contemplated scope of the present invention. Likewise, the cloned genomic sequence, or homologous synthetic sequences, of the Poc and Pab DNA polymerase genes can be used to express fusion polypeptides with Pyrodictium DNA polymerase activity or to express a protein with an amino acid sequence identical to that of native Poc or Pab DNA polymerase.

Thus, the present invention provides the complete coding sequence for Pab and Poc DNA polymerase enzymes from which expression vectors applicable to a variety of host systems can be constructed and the coding sequence expressed. Portions of the present polymerase-encoding sequence are also useful as probes to retrieve other thermostable polymerase-encoding sequences in a variety of species. Accordingly, portions of the genomic DNA encoding at least four to six amino acids can be synthesized as oligodeoxyribonucleotide probes that encode at least four to six amino acids and used to retrieve additional DNAs encoding a thermostable polymerase. Because there may not be an exact match between the nucleotide sequence of the thermostable DNA polymerase gene of Pab and Poc and the corresponding gene of other species, oligomers containing approximately 12-18 nucleotides (encoding the four to six amino acid sequence) are usually necessary to obtain hybridization under conditions of sufficient stringency to eliminate false positives. Sequences encoding six amino acids supply ample information for such probes.

The present invention, by providing the coding and amino acid sequences for Pab and Poc DNA polymerases, therefore enables the isolation of other thermostable polymerase enzymes and the coding sequences for those enzymes. Specifically, the invention provides means for preparing primers and probes for identifying nucleic acids encoding DNA polymerase enzymes contained within DNA isolates from related archaebacteria such as extreme hyperthermophiles including additional Pyrodictium species, P. brockii, and Methanopyrus species such as M. kandleri.

Several such regions of similarity between the Pab and Poc DNA polymerase coding sequences exist. For regions nine codons in length, probes corresponding to these regions can be used to identify and isolate sequences encoding thermostable polymerase enzymes that are identical (and complementary) to the probe for a contiguous sequence of at least five codons. For the region six codons in length, a probe corresponding to this region can be used to identify and isolate thermostable polymerase-encoding DNA sequences that are identical to the probe for a contiguous sequence of at least four codons.

One property found in the Pyrodictium DNA polymerase enzymes, but lacking in native Taq DNA polymerase and native Tth DNA polymerase, is 3'.fwdarw.5' exonuclease activity. This 3'.fwdarw.5' exonuclease activity is generally considered to be desirable, because misincorporated or unmatched bases of the synthesized nucleic acid sequence are eliminated by this activity. Therefore, the fidelity of PCR utilizing a polymerase with 3'.fwdarw.5' exonuclease activity (e.g. Pyrodictium DNA polymerase enzymes) is increased. However, the 3'.fwdarw.5' exonuclease activity found in Pyrodictium DNA polymerase enzymes can also increase non-specific background amplification in PCR by modifying the 3' end of the primers. The 3'.fwdarw.5' exonuclease activity can eliminate single-stranded DNAs, such as primers or single-stranded template. In essence, every 3'-nucleotide of a single-stranded primer or template is treated by the enzyme as unmatched and is therefore degraded. To avoid primer degradation in PCR, one can add phosphorothioate to the 3' ends of the primers. Phosphorothioate modified nucleotides are more resistant to removal by 3'.fwdarw.5' exonucleases.

Whether one desires to produce an enzyme identical to native Pab or Poc DNA polymerase or a derivative or homologue of that enzyme, the production of a recombinant form of the polymerase typically involves the construction of an expression vector, the transformation of a host cell with the vector, and culture of the transformed host cell under conditions such that expression will occur. To construct the expression vector, a DNA is obtained that encodes the mature (used here to include all muteins) enzyme or a fusion of the polymerase to an additional sequence that does not destroy activity or to an additional sequence cleavable under controlled conditions (such as treatment with peptidase) to give an active protein. The coding sequence is then placed in operable linkage with suitable control sequences in an expression vector. The vector can be designed to replicate autonomously in the host cell or to integrate into the chromosomal DNA of the host cell. The vector is used to transform a suitable host, and the transformed host is cultured under conditions suitable for expression of recombinant Pyrodictium polymerase. The Pyrodictium polymerase is isolated from the medium or from the cells; recovery and purification of the protein may not be necessary in some instances, where some impurities may be tolerated.

Construction of suitable vectors containing the desired coding and control sequences employs standard ligation and restriction techniques that are well understood in the art (see, for example, Molecular Cloning Laboratory Manual 2nd ed., Sambrook et al., 1989, Cold Spring Harbor Press, New York, N.Y., which is incorporated herein by reference). Isolated plasmids, DNA sequences, or synthesized oligonucleotides are cleaved, modified, and religated in the form desired. Suitable restriction sites can, if not normally available, be added to the ends of the coding sequence so as to facilitate construction of an expression vector by methods well known in the art.

For portions of vectors or coding sequences that require sequence modifications, a variety site-specific primer-directed mutagenesis methods are available. For example, the polymerase chain reaction (PCR) can be used to perform site-specific mutagenesis. PCR Protocols, ed. by Innis et al., 1990, Academic Press, San Diego, Calif., and PCR Technology ed. by Henry Erlich, 1989, Stockton Press, New York, N.Y., describe methods for cloning, modifying, and sequencing DNA using PCR and are incorporated herein by reference.

Control sequences, expression vectors, and transformation methods are dependent on the type of host cell used to express the gene. Generally, procaryotic, yeast, insect, or mammalian cells are used as hosts. Procaryotic hosts are in general the most efficient and convenient for the production of recombinant proteins and are, therefore, preferred for the expression of Pyrodictium DNA polymerase enzymes.

The procaryote most frequently used to express recombinant proteins is E. coli. For cloning and sequencing, and for expression of constructions under control of most bacterial promoters, E. coli K12 strain MM294, obtained from the E. coli Genetic Stock Center under GCSC #6135, can be used as the host. For expression vectors with the P.sub.L N.sub.RBS control sequence, E. coli K12 strain MC1000 lambda lysogen, N.sub.7 N.sub.53 cI857 SusP.sub.80, ATCC 39531, may be used. E. coli DG116, which was deposited with the ATCC (ATCC 53606) on Apr. 7, 1987, and E. coli KB2, which was deposited with the ATCC (ATCC 53075) on Mar. 29, 1985, are also useful host cells. For M13 phage recombinants, E. coli strains susceptible to phage infection, such as E. coli K12 strain DG98, are employed. The DG98 strain was deposited with the ATCC (ATCC 39768 ) on Jul. 13, 1984.

However, microbial strains other than E. coli can also be used, such as bacilli, for example Bacillus subtilis, various species of Pseudomonas, and other bacterial strains, for recombinant expression of Pyrodictium DNA polymerase enzymes.

In addition to bacteria, eucaryotic microbes, such as yeast, can also be used as recombinant host cells. See, for example, Stinchcomb et al., 1979, Nature 282:39; Tschempe et al., 1980, Gene 10:157; and Clarke et al., 1983, Meth. Enz. 101:300.

The Pyrodictium gene can also be expressed in eucaryotic host cell cultures derived from multicellular organisms. See, for example, Tissue Culture, Academic Press, Cruz and Patterson, editors (1973). Useful host cell lines include COS-7, COS-A2, CV-1, murine cells such as murine myelomas N51 and VERO, HeLa cells, and Chinese hamster ovary (CHO) cells. Plant cells can also be used as hosts, and control sequences compatible with plant cells, such as the nopaline synthase promoter and polyadenylation signal sequences (Depicker et al., 1982, J. Mol. Appl. Gen. 1:561) are available.

Depending on the host cell used, transformation is done using standard techniques appropriate to such cells. The calcium treatment employing calcium chloride, as described by Cohen, 1972, Proc. Natl. Acad. Sci. U.S.A. 69:2110 is used for procaryotes or other cells that contain substantial cell wall barriers. For mammalian cells, the calcium phosphate precipitation method of Graham and van der Eb, 1978, Virology 52:546 is preferred. Transformations into yeast are carried out according to the method of Van Solingen et al., 1977, J. Bact. 130:946 and Hsiao et al., 1979, Proc. Natl. Acad. Sci. U.S.A. 76:3829.

Once the Pyrodictium DNA polymerase has been expressed in a recombinant host cell, purification of the protein may be desired. Although the purification procedures previously described can be used to purify the recombinant thermostable polymerase of the invention, hydrophobic interaction chromatography purification methods are preferred. Hydrophobic interaction chromatography is a separation technique in which substances are separated on the basis of differing strengths of hydrophobic interaction with an uncharged bed material containing hydrophobic groups. Typically, the column is first equilibrated under conditions favorable to hydrophobic binding, e.g., high ionic strength. A descending salt gradient may be used to elute the sample.

Detailed protocols for purifying recombinant thermostable DNA polymerases have been described in, for example, PCT Patent Publication Nos. WO 92/03556, published Mar. 5, 1992, and WO 91/09950, published Jul. 11, 1991. These publications are incorporated herein by reference. The methods described therein for Thermotoga maritima are suitable. Example 9 provides a preferred protocol for purifying recombinant Pyrodictium polymerase enzymes.

For long-term stability, the Pyrodictium DNA polymerase enzyme is preferably stored in a buffer that contains one or more non-ionic polymeric detergents. Such detergents are generally those that have a molecular weight in the range of approximately 100 to 250,00 preferably about 4,000 to 200,000 daltons and stabilize the enzyme at a pH of from about 3.5 to about 9.5, preferably from about 4 to 8.5. Examples of such detergents include those specified on pages 295-298 of McCutcheon's Emulsifiers & Detergents. North American edition (1983), published by the McCutcheon Division of MC Publishing Co., 175 Rock Road, Glen Rock, N.J. (U.S.A.), the entire disclosure of which is incorporated herein by reference. Preferably, the detergents are selected from the group comprising ethoxylated fatty alcohol ethers and lauryl ethers, ethoxylated alkyl phenols, octylphenoxy polyethoxy ethanol compounds, modified oxyethylated and/or oxypropylated straight-chain alcohols, polyethylene glycol monooleate compounds, polysorbate compounds, and phenolic fatty alcohol ethers. More particularly preferred are Tween 20, a polyoxyethylated (20) sorbitan monolaurate from ICI Americas Inc., Wilmington, Del., and Iconol.TM. NP-40, an ethoxylated alkyl phenol (nonyl) from BASF Wyandotte Corp. Parsippany, N.J.

The thermostable enzyme of this invention may be used for any purpose in which such enzyme activity is necessary or desired. In a particularly preferred embodiment, the enzyme catalyzes the nucleic acid amplification reaction known as PCR.

Although the PCR process is well known in the art (sec U.S. Pat. Nos. 4,683,195; 4,683,202; and 4,965,188, each of which is incorporated herein by reference) and although commercial vendors, such as Perkin Elmer, sell PCR reagents and publish PCR protocols, some general PCR information is provided below for purposes of clarity and full understanding of the invention to those unfamiliar with the PCR process.

To amplify a target nucleic acid sequence in a sample by PCR, the sequence must be accessible to the components of the amplification system. In general, this accessibility is ensured by isolating the nucleic acids from the sample. A variety of techniques for extracting nucleic acids from biological samples arc known in the art. For example, see those described in Higuchi et al., 1989 in PCR Technology (Erlich ed., Stockton Press, New York).

Because the nucleic acid in the sample is first denatured (assuming the sample nucleic acid is double-stranded) to begin the PCR process, and because simply heating some samples results in the disruption of cells, isolation of nucleic acid from the sample can sometimes be accomplished in conjunction with strand separation. Strand separation can be accomplished by any suitable denaturing method, however, including physical, chemical, or enzymatic means. Typical heat denaturation involves temperatures ranging from about 80.degree.-105.degree. C. for times ranging from seconds to about 1 to 10 minutes.

As noted above strand separation may be accomplished in conjunction with the isolation of the sample nucleic acid or as a separate step. In the preferred embodiment of the PCR process, strand separation is achieved by heating the reaction to a sufficiently high temperature for an effective time to cause the denaturation of the duplex, but not to cause an irreversible denaturation of the polymerase (see U.S. Pat. No. 4,965,188). No matter how strand separation is achieved, however, once the strands are separated, the next step in PCR involves hybridizing the separated strands with primers that flank the target sequence. The primers are then extended to form complementary copies of the target strands, and the cycle of denaturation, hybridization, and extension is repeated as many times as necessary to obtain the desired amount of amplified nucleic acid.

For successful PCR amplification, the primers are designed so that the position at which each primer hybridizes along a duplex sequence is such that an extension product synthesized from one primer, when separated from the template (complement), serves as a template for the extension of the other primer to yield an amplified segment of nucleic acid of defined length.

Template-dependent extension of primers in PCR is catalyzed by a polymerizing agent in the presence of adequate amounts of four deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, and dTrP) in a reaction medium comprised of the appropriate salts, metal cations, and pH buffering system.

The amplification method is useful not only for producing large amounts of a specific nucleic acid sequence of known sequence but also for producing nucleic acid sequences which are known to exist but are not completely specified. One need know only a sufficient number of bases at both ends of the sequence in sufficient detail so that two oligonucleotide primers can be prepared which will hybridize to different strands of the desired sequence at relative positions along the sequence such that an extension product synthesized from one primer, when separated from the template (complement), can serve as a template for extension of the other primer into a nucleic acid sequence of defined length. The greater the knowledge about the bases at both ends of the sequence, the greater can be the specificity of the primers for the target nucleic acid sequence and the efficiency of the process.

Any nucleic acid sequence, in purified or nonpurified form, can be utilized as the starting nucleic acid(s), provided it contains or is suspected to contain the specific nucleic acid sequence desired. Thus, the process may employ, for example, DNA or RNA, including messenger RNA, which DNA or RNA may be single-stranded or double-stranded. For example, if the template is RNA, a suitable polymerizing agent to convert the RNA into a complementary, copy-DNA (cDNA) sequence is reverse transcriptase (RT), such as avian myeloblastosis virus RT and Thermus thermophilus DNA polymerase, a thermostable DNA polymerase with reverse transcriptase activity developed and manufactured by Hoffmann-La Roche Inc. and marketed by Perkin Elmer.

Whether the nucleic acid is single- or double-stranded, the DNA polymerase from Pyrodictium may be added at the denaturation step or when the temperature is being reduced to or is in the range for promoting hybridization. Although the thermostability of Pyrodictium polymerase allows one to add the polymerase to the reaction mixture at any time, one can substantially inhibit non-specific amplification by adding the polymerase to the reaction mixture at a point in time when the mixture will not be cooled below the stringent hybridization temperature. After hybridization, the reaction mixture is then heated to or maintained at a temperature at which the activity of the enzyme is promoted or optimized, i.e., a temperature sufficient to increase the activity of the enzyme in facilitating synthesis of the primer extension products from the hybridized primer and template. The temperature must actually be sufficient to synthesize an extension product of each primer which is complementary to each nucleic acid template, but must not be so high as to denature each extension product from its complementary template (i.e., the temperature is generally less than about 80.degree.-90.degree. C.).

Depending on the nucleic acid(s) employed, the typical temperature effective for this synthesis reaction generally ranges from about 40.degree.-80.degree. C., preferably 50.degree.-75.degree. C. The temperature more preferably ranges from about 65.degree.-75.degree. C. for P. occultum and P. abyssi DNA polymerase enzymes. The period of time required for this synthesis may range from about 0.5 to 40 minutes or more, depending mainly on the temperature, the length of the nucleic acid and the enzyme. The extension time is usually about 30 seconds to three minutes. If the nucleic acid is longer, a longer time period is generally required for complementary strand synthesis.

Those skilled in the art will know that the PCR process is most usually carded out as an automated process with a thermostable enzyme. In this process, the temperature of the reaction mixture is cycled through a denaturing region, a primer annealing region, and a reaction region. A machine specifically adapted for use with a thermostable enzyme is commercially available from Perkin Elmer.

Those skilled in the art will also be aware of the problem of contamination of a PCR by the amplified nucleic acid from previous reactions. Methods to reduce this problem are provided in U.S. patent application Ser. No. 609,157, filed Nov. 2, 1990 now abandoned, incorporated herein by reference.

PCR amplification may yield primer dimers or oligomers, double-stranded side products containing the sequences of several primer molecule joined end-to-end, the yield of which correlates negatively with the yield of amplified target sequence. Nonspecific priming and primer dimer and oligomer formation can occur whenever all of the PCR reagents are mixed, even at ambient and sub-ambient temperatures in the absence of thermal cycling and in the presence or absence of target DNA. At 37.degree. C., for example, Taq retains approximate 1-2% activity, although the optimal temperature is about 75.degree.-80.degree. C. Methods for overcoming non-specific extension and primer dimer formation include segregation of at least one reagent from the others in a way such that all reagents do not come together before the first amplification cycle. PCT Patent Publication No. WO 91/12342, which is incorporated herein by reference, describes methods and compositions for minimizing non-specific extension and primer dimer.

Because of the extremely high optimum growth temperature of Pyrodictium species, the present invention provides compositions that may be useful for minimizing non-specific primer extension. Specifically, the optimal growth temperature for Pyrodictium occultum and P. abyssi is 100.degree.-105.degree. C., approximately 30.degree.-35.degree. C. higher than, for example, Thermus aquaticus. Consequently, the residual activity of Pyrodictium DNA polymerases at room temperature is expected to be minimal and may eliminate the need to segregate at least one reagent prior to the first cycle of PCR. Thus, the present invention offers the potential of reduced non-specific extension at non-stringent annealing temperatures in a PCR without the use of wax barriers or other means of reagent segregation.

Those of skill in the art will recognize that the present invention provides novel compositions for the practice of any methods for which a DNA polymerase has utility. In a preferred embodiment, the enzymes are useful for amplifying nucleic acid sequences by PCR. Other amplification methods, particularly those requiring a heat denaturation step such as PLCR (Barany, 1991, PCR Methods and Applications 1(1):5-13) or gap-LCR (see, for example, PCT Patent Publication No. 90/01069, published Feb. 8, 1990) will also benefit from the present invention. Cycle sequencing methods (Caruthers et al., 1989, BioTechniques 7:494-499, and Koop et al., 1992, BioTechniques 14:442-447, incorporated herein by reference) will particularly benefit from 3'-5' exonuclease deficient Pab and Poc DNA polymerase enzymes.

Pyrodictium DNA polymerase is very useful in carrying out the diverse processes in which amplification of a nucleic acid sequence by the polymerase chain reaction is useful. Such methods include cloning, DNA sequencing, reverse transcription and asymmetric PCR. Further, the enzymes of the invention are suitable for use in diagnostic, forensic, and research applications. The following examples are offered by way of illustration only and by no means intended to limit the scope of the claimed invention.

Pyrodictium abyssi cells were received from Dr. Karl O. Stetter, University Regensburg, Regensburg, Germany. The isolate, AVZ (DSM6158) is described in Pley et al., 1991, System Applied Microbiology 14:245-253, which is incorporated herein by reference. DNA was purified by the method described in Lawyer et al., 1989, J. Biological Chemistry 264(11):6427-6437, which is incorporated herein by reference. About 25 .mu.g of Pyrodictium abyssi DNA was partially digested with the restriction enzyme Sau3AI and size-fractionated by gel electrophoresis. Ten ng of fragments which were larger than 3.5 kb and smaller than 8.5 kb were used for cloning into the BamHI site of pUC19 vector (Clontech, Palo Alto, Calif.). The pUC19 plasmid vector has the lac promoter upstream from the BamHI cloning site. The promoter can induce heterologous expression of cloned open reading frames lacking promoter sequences. The recombinant plasmids were transformed into E. coli SURE cells (Strategene). Genotype of SURE.RTM. cells: mcrA, .DELTA. (mcrBC-hsdRMS-mrr) 171, endA1, supE44, thi-1, .lambda.-, gyrA96, relA1, lac, recB, recJ, sbcC, umuC::Tn5(kan.sup.R), urvC, (F', proAB, lacl Z.DELTA.M15, Tn10[tet.sup.R ]).

A rapid filter assay for the detection of thermoresistant and thermophilic DNA polymerase activity was used to screen the Pyrodictium abyssi genomic DNA library (Sagner et al., 1991, Gene 97:119-123, incorporated herein by reference). According to the method, recombinant colonies are bound to nitrocellulose membrane and are incubated at elevated temperature in a polymerization buffer containing .alpha.[.sup.32 P]-labeled dNTPs. By autoradiography of the dried filters, colonies which express thermophilic DNA polymerase activity can be directly identified. The membrane-bound colonies are heated to 95.degree. C. to irreversibly inactivate host DNA polymerases and are subsequently incubated at elevated temperatures to reveal the presence of thermophilic DNA polymerase activity.

Approximately 500 colonies were plated per petri dish and grown overnight at 37.degree. C. Subsequently, the colonies were replica-plated onto nitrocellulose membranes and grown for 4 hours. The membranes were placed upside down on agarose plates which were placed for 20 minutes at room temperature on filter papers soaked with a mixture of chloroform/toulene (1:1). The membranes containing the permeabilized colonies were then incubated at 95.degree. C. for 5 minutes in a polymerization buffer containing 50 mM Tris-HCl pH 8.8, 7 mM MgCl.sub.2, 3 mM .beta.Me to inactivate any nonthermoresistant (e.g., E. coli) DNA polymerase activity. Immediately after inactivation the membranes were transferred to the polymerization buffer containing 50 mM Tris-HCl pH 8.8, 7 mM MgCl.sub.2, 3 mM .beta.Me, 12 .mu.M dCTP, 12 .mu.M dGTP, 12 .mu.M dATP, 12 .mu.M dTTP, and 1 .mu.Ci per ml .alpha.[.sup.32 P]-dGTP. After incubation for 30 minutes at 65.degree. C. the membranes were washed twice for 5 minutes in a solution of 5% TCA and 1% pyrophosphate to remove unincorporated .alpha.[.sup.32 P]-dGTP. The membranes were analyzed by autoradiography at -70.degree. C. Seven clones were apparent on X-ray films of duplicated membranes after 3 days.

Plasmid DNAs were isolated from these 7 clones, restriction analysis was performed to determine the size and orientation of insert fragments relative to the pUC19 vector. DNA sequence analysis was performed on the largest clone, pPab 14. The "universal" forward and reverse sequencing primers, Nos. 1212 and 1233, respectively, purchased from New England BioLabs, Beverly, Mass., were used to obtain preliminary DNA sequences. From the preliminary DNA sequence, further sequencing primers were designed to obtain DNA sequence of more internal regions of the cloned insert. DNA sequence analysis has been performed for both strands.

Plasmid pDG 168 is a .lambda.P.sub.L cloning and expression vector that comprises the .lambda.P.sub.L promoter and gene N ribosome-binding site (see, U.S. Pat. No. 4,711,845, which is incorporated herein by reference), a restriction site polylinker positioned so that the sequences cloned in to the polylinker can be expressed under control of the .lambda.P.sub.L -N.sub.RBS, and a transcription terminator form the Bacillus thuringiensis delta-toxin gene (see, U.S. Pat. No. 4,666,848, which is incorporated herein by reference). Plasmid pDG 168 also carries a mutated RNA II gene which renders the plasmid temperature sensitive for copy number (see, U.S. Pat. No. 4,631,257, which is incorporated herein by reference) and an ampicillin resistance gene in E. coli K12 strain DG 116. The construction of pDG168 is described in PCT Patent Publication No. WO 91/09950, published Jul. 11, 1991, at Example 6, which is incorporated herein by reference.

These elements act in concert to provide a useful and powerful expression vector. At 30.degree.-32.degree. C., the copy number of the plasmid is low, and in a host cell that carries a temperature sensitive .lambda. repressor gene, such as cI857 the P.sub.L promoter does not function. At 37.degree.-41.degree. C., however, the copy number of the plasmid is 25-50 fold higher than at 30.degree.-32.degree. C., and the cI857 repressor is inactivated allowing the promoter to function. thus, pDG168 was selected for constructing expression vectors for Pab DNA polymerase.

The DNA sequence analysis of pPab14 revealed an open reading frame of 803 amino acids having an ATG start codon at nucleotide position 869 and a TGA stop codon at nucleotide position 3280. The 5' end of the Pab gene was mutagenized with oligonucleotide primers AW397 (SEQ ID No. 5) and AW398 (SEQ ID No. 6) by PCR amplification (as described below). AW397 (SEQ ID No. 5) is forward primer which was designed to alter the Pab DNA sequence at the ATG start to introduce an NdeI restriction site. Primer AW397 (SEQ ID No. 5) also introduced mutations in the fifth and sixth codons of the Pab polymerase gene sequence to be more compatible with the codon usage of E. coli, without changing the amino acid sequence of the encoded protein. The reverse primer, AW398 (SEQ ID No. 6), was chosen to include a SpeI site corresponding to amino acid position 174. In addition, a KpnI site was introduced after the SpeI site.

The PCR reaction mixture contained 10 ng of SalI linearized pPab14 DNA as the template; 10 pmol of primers AW397 (SEQ ID No. 5) and AW398 (SEQ ID No. 6); 50 .mu.M of each dATP, dCTP, dTTP, and dGTP; 2 mM MgCl.sub.2 ; 10 mM Tris-HCl, pH 8.3; 50 mM KCl and 1 unit Taq polymerase in 50 .mu.gl reaction volume. The reaction thermo-profile was 95.degree. C. for 30"; 65.degree. C. for 30' and 72.degree. C. for 30" and amplified for 12 cycles. The 500 bp amplified product was digested with NdeI and KpnI and loaded on an 1% Seakem agarose gel. The PCR product fragment was purified with Geneclean kit (Bio 101, San Diego, Calif.) and subcloned into expression vector pDG 168, which had been digested with NdeI and KpnI. The resulting clone was named pAW111. The desired mutations were confirmed via restriction enzyme analysis and DNA sequence analysis.

The 3' end of the Pab polymerase gene was modified via restriction enzyme digestion and use of a synthetic oligonucleotide duplex. AW399 (SEQ ID No. 7) was designed according to the 3' end of the Pab pol gene from AflII site at amino acid position 785-786. It changes the TGA stop codon to TAA as well. AW400 (SEQ ID No. 8) is the complementary strand of AW399 (SEQ ID No. 7) except that it has XmaI cohesive end at it's 5' end. When AW399 (SEQ ID No. 7) anneals to AW400 (SEQ ID No. 8), it produces a 60 bp synthetic duplex with 5' cohesive AflII/XmaI ends. The duplex was then cloned into plasmid pPab2 that have been digested with AlfII and XmaI. The resulting plasmid was designated pAW113. Plasmid Pab2 was one of the 7 clones isolated from the genomic library as described in Example 1. Plasmid Pab2 contains the entire Pab pol gene but is .about.250 bp shorter than Pab14 at the 5' end. Thus, it lacks a flanking 5'-end AlfII site which facilitated the cloning strategy of replacing the 3' end AflII-XmaI fragment with the synthetic duplex AW399 (SEQ ID No. 7)/AW400 (SEQ ID No. 8) as described above. The DNA sequence of the replaced fragment was confirmed by DNA sequence analysis.

Finally, the 1.89 kb fragment of the Pab polymerase gene region, SpeI through the stop codon was isolated from pAW113 by digestion with SpeI and XmaI, and purified via gel electrophoresis. The resulting fragment was ligated with plasmid pAW111 that had been digested with SpeI and XmaI.

The ligation condition was 20 .mu.g/ml DNA, 20 mM Tris-HCl, pH 7.4, 50 mM NaCl, 10 mM MgCl.sub.2, 40 .mu.M ATP and 0.2 Weiss unit T4 DNA ligase per 20 .mu.l reaction at 16.degree. C. overnight. Ligations were transformed into DG116 host cells. Candidates were screened for appropriate restriction enzyme sites. The desired plasmid was designated pAW115.

The oligonucleotides used in this example are shown below.

Pab and Poc genomic DNA (0.5 .mu.g each) were digested with HindIII, and were separated by gel electrophoresis through an 0.8% agarose gel. Pyrodictium occultum cells were received from Dr. Karl O. Stetter, University Regensburg, Regensburg, Germany. DNA was purified by the method described in Lawyer et al., 1989, J. Biological Chemistry 264(11):6427-6437, which is incorporated herein by reference. The DNA fragments in the gel were denatured in 1.5 M NaCl and 0.5 M NaOH solution for 30 minutes and were neutralized in a solution of 1 M Tris-HCl, pH 8.0 and 1.5 M NaCl for 30 minutes, and then were transferred to a Biodyne nylon membrane (Pall Biosupport, East Hills, N.Y.) using 20.times.SSPE (3.6 M NaCl, 200 mM NaPO.sub.4 /pH 7.4, 20 mM EDTA/pH 7.4). The DNA attached to the membrane was then hybridized to a .sup.32 P-labeled 240 bp PCR product which encoded amino acids 515-614 of the Pab polymerase gene. The prehybridization solution was 6.times.SSPE, 5.times. Denhardt's reagent, 0.5% SDS, 100 .mu.g/ml denatured, sheared, salmon sperm DNA. Hybridization solution was the same except that Denhardt's reagent was used at 2.times., and contained 10.sup.6 cpm .sup.32 P-labeled PCR-amplified probe. Prehybridization and hybridization were both at 55.degree. C. The blot was washed sequentially as follows: 2.times.SSPE, 0.5% SDS, 10 minutes at RT; 2.times.SSPE, 0.1% SDS, 15 minutes at RT; 0.1% SSPE, 0.1% SDS, 5 minutes at RT.

A strong signal was apparent at approximately 3.8 kb in the HindIII digest. This suggested that the Poc polymerase gene has homology with the Pab polymerase gene. Consequently, several PCR primers, designed from the Pab polymerase gene sequence, were evaluated for amplification of portions of the Poc polymerase gene. A specific PCR product, 295 bp in size resulted from a PCR using primer pair LS417 (SEQ ID No. 34) and LS396 (SEQ ID No. 35).

The PCR was performed at final concentration of 1.times. PCR buffer, 50 gM dNTPs, 0.1 .mu.M each primers, 1.25 units Taq in a total volume of 50 .mu.l. 1.times. PCR buffer contains 20 mM Tris pH 8.4, 50 mM KCl, 2 mM MgCl.sub.2. The reaction was amplified for 35 cycles.

The 295 bp PCR product was then subjected to DNA sequence analysis. The DNA sequence result showed that the Poc polymerase gene has 78% identity with the Pab polymerase gene in this region. A Poc polymerase specific oligonucleotide probe AW394 (SEQ ID No. 9) was designed using this DNA sequence data. The .sup.32 P-labeled AW394 (SEQ ID No. 9) was then used to screen a genomic Poc DNA bank to obtain Poc polymerase clones. The constriction of the genomic Poc DNA bank was as described in Example 1 for the genomic Pab DNA bank.

About 5,500 ampicillin-resistant colonies were selected on nitrocellulose filters and hybridized with .sup.32 P-labeled AW394 (SEQ ID No. 9). Plasmid DNA was isolated from 6 colonies that hybridized with the probe. Prehybridization and hybridization conditions were as described above. Wash conditions were 6.times.SSPE, 0.1% SDS for 5 minutes at room temperature and followed by 2.times.SSPE, 0.1% SDS for 15 minutes at 55.degree. C. Restriction enzyme analysis and PCR analysis were performed to determine the size and orientation of insert fragment relative to the pUC19 vector. The results revealed that pPoc3 and pPoc5 are identical clones. The sizes of the coding region, 5' end non-translated region and 3' end non-translated region of all identified POc polymerase clones are listed below.

DNA sequence analysis was performed on pPoc4. Universal and reverse sequencing primers were used to obtain preliminary DNA sequence information. From this DNA sequence additional sequencing primers were designed to obtain the DNA sequence of more internal regions of the insert. DNA sequence analysis has been performed for both strands.

The 5' end of the Poc polymerase gene in plasmid pPoc4 was mutagenized with oligonucleotide primers AW408 (SEQ ID No. 10) and AW409A (SEQ ID No. 11) via PCR amplification. AW408 (SEQ ID No. 10) is a forward primer designed to alter the DNA sequence of the Poc gene at the ATG start codon to introduce an NsiI restriction site. AW408 (SEQ ID No. 10) also was designed to introduce alterations in the second, third, fifth, and sixth codons of the Poc gene to provide a sequence more compatible with the codon usage of E. coli without changing the amino acid sequence of the encoded protein. The reverse primer AW409A (SEQ ID No. 11) was chosen to include an XbaI site at amino acid position 38. In addition, a KpnI site was introduced after the XbaI site for subsequent subcloning.

Plasmid pPoc4, linearized with KpnI, was used as the PCR template for amplification using the AW408 (SEQ ID No. 10)/AW409A (SEQ ID No. 11) primer pair, yielding a 138 bp PCR product. The PCR amplification procedure was as described above at Example 2. The amplified fragment was digested with NsiI, then treated with Klenow to cream a blunt end at the NsiI-cleaved end, and finally digested with KpnI. The resulting fragment was ligated with expression vector pDG 164 (which is described in detail in PCT Patent Publication No. WO 91/09950, at Example 6b, and incorporated herein by reference) that has been digested with NdeI, repaired with Klenow, to fill in the overhang and provide a blunt end for ligation, and then digested with KpnI. The ligation yielded an in-frame coding sequence of the 5' end of the Poc polymerase gene under control of the .lambda.P.sub.L promoter and bacteriophage T.sub.7 gene 10 ribosome binding site. The resulting construct was designated pAW118.

To effect subcloning of the 3' end of the Poc polymerase gene, a KpnI site was introduced after the stop codon. This was done by a PCR process as follows. The forward primer was chosen to include an EspI site at amino acid position 698-699, and the reverse primer was designed to incorporate a KpnI site immediately following an altered stop codon (TAA). The amplified 335 bp fragment was digested with EspI and KpnI, and cloned into plasmid pPoc4 digested with EspI and KpnI. The resulting construct was designated pAW120.

Finally, the Poc pol gene region XbaI through the stop codon was isolated from pAW120 by digestion with XbaI and KpnI. The resulting 2.3 kb fragment was ligated with pAW118 that had been digested with Xba and KpnI. The ligation product was transformed into DG116 host cells for expression and designated pAW121.

The oligonucleotides used in this example are given below.

Both the Pab pol gene and the Poc pol gene can be over-expressed under the control of the E. coli Trp promoter. Construction of the expression clones was performed as follows: The .lambda.P.sub.L promoter in expression clone, pAW115, was replaced by a Trp promoter sequences which was generated by PCR amplification using plasmid pLSG10 (plasmid pLSG10 is described in U.S. Pat. No. 5,079,352, which is incorporated herein by reference), as template and AW500 (SEQ ID No. 14) and AW501 (SEQ ID No. 15) as primers. The resulting PCR product was digested with NspV and NdeI and cloned into NspV and NdeI digested pAWl15 to give rise to a Pab pol expression clone, pAW118, under control of the E. coli Trp promoter.

An internal NdeI site in the Poc pol gene of pAW121, complicates of the exchange NspV-NdeI .lambda.P.sub.L promoter fragment and the Trp promoter fragment. Therefore, primers AW500 (SEQ ID No. 14) and AW502 (SEQ ID No. 16) were designed to amplify the Trp promoter sequence fragment from pLSG 10 and primers AW503 (SEQ ID No. 17) and AW504 (SEQ ID No. 18) were designed to amplify the 5' end 110 bp NdeI-XbaI fragment from pAW121. AW502 (SEQ ID No. 16) and AW503 (SEQ ID No. 17) overlap by 9 nucleotides. Using overlap extension PCR, the Trp promoter fragment and the 5' end 110 bp fragments were fused. The resulting PCR product was digested with NspV and XbaI and cloned into pAW121 which had been was digested with NspV and XbaI. The resulting Poc pol expression clone was named pAW123.

Because of the dramatic levels of amplification provided by the PCR process (up to 10.sup.11 to 6.times.10.sup.12 -fold), for certain applications the accuracy of replication (fidelity) is important. PCR fidelity is based on a two step process: misinsertion and misextension. If the DNA polymerase inserts an incorrect base and the resulting 3'-mismatched terminus is not extended, this truncated extension product cannot be amplified since the binding site for the downstream primer is not present. DNA polymerases extend a mismatched 3'-terminus more slowly than a matched 3'-terminus. In addition, different mismatches extend at disparate rates. See Kwok et al., 1990, Nuc. Acids Res. 18:999-1005, and Huang et al., 1992, Nuc. Acids Res. 20:4567-4573.

DNA polymerases with inherent 3' to 5' exonuclease or proofreading activity are able to improve fidelity by removing misinserted bases before extension. A convenient PCR and restriction endonuclease digestion assay has been developed to assess the ability of DNA polymerases with 3' to 5' exonuclease activity to remove 3'-terminal mismatched nucleotides prior to misextension. Several primers were designed which were either perfectly matched or 3'-mismatched (with every possible combination) to the first nucleotide of the BamHI restriction enzyme recognition sequence in the Thermus aquaticus DNA polymerase gene (Lawyer et al., 1989, J. Biol. Chem. 264:6427-6437 and U.S. Pat. No. 5,079,352). The perfect match primers, FR434 (SEQ ID No. 29) and FR438 (SEQ ID No. 33), amplify a 151 bp product that is completely digested with BamHI restriction enzyme to generate 132 bp and 19 bp DNA fragments. The 3'-terminal nucleotide of forward primer FR434 (SEQ ID No. 29) corresponds to nucleotide 1778 of the Taq DNA pol gene. Forward primers FR435 (SEQ ID No. 30), FR436 (SEQ ID No. 31), and FR437 (SEQ ID No. 32) contain a single 3'-terminal mismatch with respect to the wild-type Taq DNA pol gene and wild-type primer FR438 (SEQ ID No. 33) extension products, corresponding to A:C, T:C, and C:C mismatches, respectively. Any incorrect or misextension from primers FR435 (SEQ ID No. 30), FR436 (SEQ ID No. 31), or FR437 (SEQ ID No. 32) eliminates the BarnHi recognition site corresponding to nucleotides 1778-1783 of the Taq DNA pol gene. Alternatively, exonucleolytic proofreading removes the 3'-terminal mismatched nucleotides and permits incorporation of the correct dG residue, resulting in the accumulation of PCR products that now contain the diagnostic BamHI restriction enzyme site. Since all of the FR435 (SEQ ID No. 30), FR436 (SEQ ID No. 31), or FR437 (SEQ ID No. 32) primers are mismatched to the original target, this PCR/endonuclease digestion assay requires exonucleolytic proofreading in every cycle to correct the "mutant" primers and generate a PCR product that contains the diagnostic BamHI cleavage site. Misextension at any cycle will generate an efficiently copied (now mutant) template in the succeeding cycle (from primer FR438 [SEQ ID No. 33] extension) that is perfectly matched to all of the primers in the assay.

Primer FR434 (SEQ ID No. 29) corresponds identically to nucleotides 1760 through 1778 of the Taq DNA polymerase gene, and primer FR438 (SEQ ID No. 33) is complementary to nucleotides 1891 through 1910 of the Taq DNA polymerase gene. Primers FR435 (SEQ ID No. 30), FR436 (SEQ ID No. 31), and FR437 (SEQ ID No. 32) correspond identically to nucleotides 1760 through 1777 of the Taq DNA polymerase gene and contain the indicated (by *, underlined) 3'-terminal mismatched nucleotide at position 1778.

Recombinant Pab and Poc DNA polymerases were purified from E. coli K12 strain DG116 harboring plasmids pAW115 or pAW121, respectively. The purification involved cell lysis, heat treatment at 75.degree.-85.degree. C., polymin P precipitation of bulk nucleic acids, phenyl sepharose chromatography and heparin sepharose chromatography, according to Example 9.

Using this fidelity assay, wild-type recombinant Pab and Poc DNA polymerases arc able to correct mismatch primers FR435 (SEQ ID No. 30), FR436 (SEQ ID No. 31) and FR437 (SEQ ID No. 32) to generate PCR product that contains the requisite BamHI cleavage site, demonstrating the presence of 3' to 5' exonucleolytic proofreading activity.

Pab and Poc pol genes lacking 3'-5' exonuclease activity were constructed using site-directed mutagenesis by overlap extension PCR to alter the codons for Asp187 and Glu189 to code for alanine. Briefly, mutagenesis by overlap extension PCR involves the generation of DNA fragments that, by virtue of having incorporated complementary oligo primers in independent PCR reactions (see, Higuchi et al., 1988, Nuc. Acids Res. 16:7351-7367, and Ho et al., 1989, Gene 77:51-59, which are incorporated herein by reference, for a detailed description of this method). According to the method, these fragments are combined in a subsequent "fusion" reaction in which the overlapping ends anneal, allowing the 3' overlap of each strand to serve as a primer for the 3' extension of the complementary strand. The resulting fusion product is amplified further by PCR. Specific alterations in the nucleotide sequence can be introduced by incorporating nucleotide changes into the overlapping oligo primers.

The construction of a 3'-5' exonuclease minus mutant of Pab was accomplished as follows. The two overlapped primers AW493 (SEQ ID No. 20) and AW494 (SEQ ID No. 21) were designed to span Asp187 and Glu189, in which both Asp187 and Glu189 are replaced by alanine. The two external primers, AW492 (SEQ ID No. 19) and AW495 (SEQ ID No. 22), were chosen to locate at the unique SpeI and NsiI restriction sites at amino acid position 174-175 and amino acid position 304-305, respectively, thus making it possible to ligate the fusion product back into the expression vector. The products from the PCR using primer sets AW492 (SEQ ID No. 19)/AW493 (SEQ ID No. 20) and AW494 (SEQ ID No. 21)/AW495 (SEQ ID No. 22) were 70 bp and 373 bp fragments, respectively. The resulting two fragments (27 nucleotide 3' overlap) were fused by denaturing and annealing them in a subsequent primer extension reaction. The 416 bp fusion product was amplified further by PCR using the two external primers AW492 (SEQ ID No. 19) and AW495 (SEQ ID No. 22). The mutagenized 416 bp fragment was then cut with SpeI and NsiI and ligated back into the parent clone pAW115 which had also been digested with SpeI and NsiI. The resulting mutant clone was named pexo-Pab, and the desired mutations were confirmed by sequence analysis.

Similarly, the 3'-5' exonuclease minus mutant of Poc was constructed using the same approach. The overlapping primer pair used to introduce the mutation are AW489 (SEQ ID No. 24) and AW490 (SEQ ID No. 25). The two external primers, AW488 (SEQ ID No. 23) and AW491 (SEQ ID No. 26) are located at the unique XbaI and BssHII restriction sites at amino acid positions 37-39 and 260-262, respectively. The products from PCR using primer sets AW488 (SEQ ID No. 23)/AW489 (SEQ ID No. 24) and AW490 (SEQ ID No. 25)/AW491 (SEQ ID No. 26) were 476 bp and 243 bp fragments, respectively. These two fragments were fused and subjected to PCR amplification using the external primers AW488 (SEQ ID No. 23) and AW491 (SEQ ID No. 26). The mutagenized fragment was then cut with XbaI and BssHII and ligated back into the parent clone pAW121. The resulting mutant clone was named pexo-Poc.

The exonuclease activities of the exo-Pab DNA polymerase and exo-Poc DNA polymerase were determined using the mismatch incorporation proofreading assay. The results showed that both the exo-Pab pol and exo-Poc pol lacked the 3'-5' exonuclease activity.

The upper growth temperature of hyperthermophilic genus Pyrodictium is 110.degree. C. To test the thermostability of purified recombinant Pab pol, Poc pol and Taq pol in the PCR process, the following experiment was performed: 0.1 pg, 1 pg, and 10 pg of M13 DNA (New England Biolabs, Beverly, Mass.) were used as templates for PCR analysis by Pab, Poc and Taq. The reactions were subjected to 25, 30, 35 and 40 cycles at denaturing temperatures of 95.degree. C. or 100.degree. C. A PCR product of 350 bp was generated by using BW36 (SEQ ID No. 27) and BW42 (SEQ ID No. 28) as primers. BW36 SEQ ID No. 27 5'CCGATAGTTTGAGTTCTTCTACTCAGGC BW42 SEQ ID No. 28 5'GAAGAAAGCGAAAGGAGCGGGCGCTAGGGC PCR was performed at a final concentration of 1.times.PCR buffer, 50 .mu.gM dNTPs, 0.1 .mu.M each primers, 0.25 units Pab or 0.1 units Poc or 1.25 units Taq in a total reaction volume of 50 .mu.l.

A unit of Pab DNA polymerase and a unit of PoC DNA polymerase is defined, like for Taq DNA polymerase, as the amount of enzyme that will incorporate 10 nmoles total dNTPs into acid insoluble material per 30 minutes at 74.degree. C. Poc and Pab DNA polymerases are assayed as described in U.S. Pat. No. 4,889,818, which is incorporated herein by reference, for Taq DNA polymerase with the following changes in reaction components. Pab DNA polymerase: Tris-HCl pH 8.3 (25.degree. C.), 100 mM KCl, 5 mM MgCl.sub.2. Poc DNA polymerase: Tris-HCl pH 8.0 (25.degree. C.), 10 mM KCl, 5 mM MgCl.sub.2. 1.times.PCR buffer for Pab contains: 20 mM Tris-HCl, pH 8.4, 100 mM KCl, 1.5 mM MgCl.sub.2. 1.times.PCR buffer for Poc contains: 20 mM Tris-HCl, pH 8.4, 10 mM KCl, 1.0 mM MgCl.sub.2. 1.times.PCR buffer for Taq contains: 20 mM Tris, pH8.4, 50 mM KCl, 1.5 mM MgCl.sub.2. The amplification profile involved denaturation at 95.degree. C. or 100.degree. C. for 30 seconds, primer annealing and extension at 55.degree. C. for 30 seconds. The results showed that both Pab pol and Poc pol were extremely thermoresistant, functioning effectively in the PCR with denaturing temperature up to 100.degree. C. In contrast, Taq pol produced no product under these conditions at 100.degree. C.

Recombinant Pyrodictium DNA polymerase is purified as follows. Briefly, cells are thawed in 1 volume of TE buffer (50 mM Tris-Cl, pH 7.5, and 1.0 mM EDTA with 1 mM DTT), and protease inhibitors are added (PMSF to 2.4 mM, leupeptin to 1 .mu.g/ml, and TLCK to 0.2 mM). The cells are lysed in an Aminco french pressure cell at 20,000 psi and sonicated to reduce viscosity. The sonicate is diluted with TE buffer and protease inhibitors to 5.5.times. wet weight cell mass (Fraction I), adjusted to 0.2M ammonium sulfate, and brought rapidly to 85.degree. C. and maintained at 85.degree. C. for 15 minutes. The heat-treated supernatant is chilled rapidly to 0.degree. C., and the E. coli cell membranes and denatured proteins are removed following centrifugation at 20,000.times.G for 30 minutes. The supernatant containing Pyrodictium DNA polymerase (Fraction II) is saved. The level of Polymin P necessary to precipitate >95% of the nucleic acids is determined by trial precipitation (usually in the range of 0.6 to 1% w/v). The desired amount of Polymin P is added slowly with rapid stirring at 0.degree. C. for 30 minutes and the suspension centrifuged at 20,000.times.G for 30 min. to remove the precipitated nucleic acids. The supernatant (Fraction III) containing the Pyrodictium DNA polymerase is saved.

Fraction III is adjusted to 0.3 M ammonium sulfate and applied to a phenyl separose column that has been equilibrated in 50 mM Tris-Cl, pH 7.5, 0.3 M ammonium sulfate, 10 mM EDTA, and 1 mM DTT. The column is washed with 2 to 4 column volumes of the same buffer (A.sub.280 to baseline), and then 1 to 2 column volumes of TE buffer containing 50 mM KCl to remove most contaminating E. coli proteins. Pyrodictium DNA polymerase is then eluted from the column with buffer containing 50 mM Tris-Cl, pH 7.5, 2 M urea, 20% (w/v) ethylene glycol, 10 mM EDTA, and 1 mM DTT, and fractions containing DNA polymerase activity arc pooled (Fraction IV).

Final purification of recombinant Pyrodictium DNA polymerase is achieved using heparin sepharose chromatography, anion exchange chromatography, or affixed blue chromatography. Recombinant Pyrodictium DNA polymerase may be diafiltered into 2.5.times. storage buffer (50 mM Tris-HCl pH 8.0, 250 mM KCl, 2.5 mM DTT, 0.25 mM EDTA, 0.5% [w/v] Tween20), combined with 1.5 volumes of sterile 80% (w/v) glycerol, and stored at -20.degree. C.

The following bacteriophage and bacterial strains were deposited with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md., U.S.A. (ATCC). These deposits were made under the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for purposes of Patent Procedure and the Regulations thereunder (Budapest Treaty). This assures maintenance of a viable culture for 30 years from the date of deposit. The organisms will be made available by ATCC under the terms of the Budapest Treaty, and subject to an agreement between Applicants and ATCC that assures unrestricted availability upon issuance of the pertinent U.S. patent. Availability of the deposited strains is not to be construed as a license to practice the invention in contravention of the rights granted under the authority of any government in accordance with its patent laws.

The foregoing written specification is considered to be sufficient to enable one skilled in the an to practice the invention. The present invention is not to be limited in scope by the cell lines deposited, since the deposited embodiment is intended as a single illustration of one aspect of the invention and any cell lines that are functionally equivalent are within the scope of this invention. The deposit of materials therein does not constitute an admission that the written description herein contained is inadequate to enable the practice of any aspect of the invention, including the best mode thereof, nor are the deposits to be construed as limiting the scope of the claims to the specific illustrations that they represent. Indeed, various modifications of the invention in addition to those shown are described herein will become apparent to those skilled in the art from the foregoing description and fall within the scope of the appended claims.