DNA Cycle Sequencing

This invention relates to methods for DNA sequencing in which a labelled nucleotide is incorporated into an extending DNA molecule during a chain termination sequencing method.

The sequence of nucleotide bases in a DNA molecule can be determined in a variety of ways. The chain termination method generally involves synthesizing DNA, complementary to the template strand to be sequenced, by extending a primer able to hybridize to a portion of that template strand, with a DNA polymerase. During the synthesis reaction, deoxynucleoside triphosphates (dNTPs) are incorporated to form a DNA fragment until a chain terminating agent, for example, a dideoxynucleotide triphosphate (ddNTP) is incorporated. Incorporation of a ddNTP prevents further DNA synthesis (a process called chain termination). The size of each DNA fragment synthesized in this procedure is then determined by gel electrophoresis, and this information used to determine the sequence of nucleotides in the original template DNA. For example, Tabor and Richardson, U.S. Pat. No. 4,795,699, describe a two step sequencing method in which an unlabelled primer is labelled in a labelling step, and then extended in the presence of excess dNTPs and a ddNTP in a chain termination step. In the labeling step a low concentration of dNTPs is provided (one being labelled) to allow a small amount of primer extension.

In the dideoxy sequencing method, the primer may be labelled, for example, with .sup.32 P, by a process using a polynucleotide kinase. Such labelling allows detection of extended primers after gel electrophoresis by autoradiography of the resulting gel. Alternatively, a labelled dNTP may be incorporated during the process of DNA synthesis, and the presence of such labelled dNTPs detected by autoradiography or other means. To this end, the dNTP may be labelled either radioactively with .sup.32 P or .sup.35 S. In another procedure, the primer can be labeled with one or more fluorescent moieties for detection by fluorescence. In yet another procedure, the ddNTP may be labelled, for example, with a fluorescent marker.

Examples of these procedures are provided in the following publications, none of which are admitted to be prior art to the present application:

Murray, 17 Nucleic Acid Research 8889, 1989 describes use of a .sup.32 P-labelled primer. The primer was labelled by use of a polynucleotide kinase and gamma-.sup.32 P ATP.

Higuchi and Ochman, 17 Nucleic Acid Research 5865, 1989, describe use of a kinase-labelled primer for sequencing DNA fragments produced by a polymerase chain reaction (PCR). PCR is a method for amplifying a desired nucleic acid by use of two primers which are extended by a DNA polymerase to produce a pair of DNA fragments complementary to the desired nucleic acid. These fragments then hybridize to provide the desired DNA. This process is repeated 15-30 times. If a thermostable DNA polymerase is used, the process is best cycled by maintaining the temperature of the reaction alternatively at a temperature suitable for primer extension (70.degree.-75.degree. C.), then at a temperature suitable for denaturation (90.degree.-100.degree. C.) to separate the DNA fragments and provide a template for the next cycle, and then at a temperature suitable for annealing (42.degree. C.-55.degree. C.) to bind primers to the template.

Straus and Zagursky, 10 BioTechniques 376, 1991, describe use of .alpha.-.sup.32 p dATP in a sequencing reaction.

Smith et al., 9 BioTechniques 52, 1990, and Adams-Blakesley, 13 Focus 56, 1991, describe sequencing of PCR products using a labelled primer.

Khrishnan et al., 19 Nucleic Acid Research 1153, 1991, describe use of a labelled primer for sequencing lambda DNA. The process of chain termination is repeated (or cycled) several times to allow production of a large number of products. This cycling is similar to a PCR since the reaction is allowed to proceed at 42.degree.-55.degree. C., then stopped at 95.degree. C., and started again at 42.degree.-55.degree. C. (the process is called thermal cycle sequencing). No new reagent need be added once the process is started. The cycling ensures that a large proportion of the labelled primer is extended in the chain termination steps.

Spurgeon and Burke, Abstract 125, Human Genome II, Oct. 22, 1990, describe use of a modified Taq DNA polymerase for thermal cycle sequencing in which an end-labelled primer, or an unlabelled primer is used. Fluorescent labelled ddNTPs and radioactive labelling techniques were also used.

Mead et al., Abstract No. 99, Human Genome II, Oct. 22, 1990, describe sequencing of lambda DNA in a polymerase chain-based reaction, using radioisotopes and fluorescent labels.

This invention features a novel DNA sequencing method in which an unlabelled primer is extended in a labelling reaction performed in a cyclical manner. Such a method is advantageous over the prior use of .sup.32 P end-labeled primers (prepared using polynucleotide kinase and gamma-.sup.32 P ATP), since no additional reagents (e.g., polynucleotide kinase, and gamma-.sup.32 P ATP) are required over the usual sequencing reagents, the process of preparing labeled primer is not required, and, since the apparatus used in the method is identical to that which is used for thermal cycle sequencing, the two methods can be conveniently used together. This procedure allows the use of labels not otherwise usable with polynucleotide kinase, such as .alpha.-.sup.35 S dNTPs, fluorescent nucleotides, and nucleotides labeled with other useful moieties, such as biotin.

The critical feature of the present invention is the use of at least one labelled nucleotide in a cycled labelling step. This is generally followed by a cycled termination step. The repeated cycling of primed DNA synthesis results in production of many more newly synthesized molecules of DNA than molecules of template DNA used for sequencing. Thus, while sequencing using customary amounts of template (0.5-1.0 pmol) is possible when only a single round of replicative synthesis is performed, sequencing using less template DNA (e.q., 0.05 pmol) without a cycling step is difficult. Repeated cycling of the labelling step using limited amounts of template increases the amount of product to a level sufficient for normal detection procedures, such as autoradiography with overnight exposure.

In the method, at most three of the four required dNTPs for a sequencing reaction are present in the labelling step, with one or more of them being labelled. This effectively labels the primer and limits extension to a known length (i.e., to a length where the missing dNTP is required for further synthesis). In the method, the reaction is cycled between a temperature suitable for chain extension, and one suitable for denaturation of the extended primer and template molecules, generally 10 to 100 times over a period of one and one half to six hours. After this labelling step, the reaction is generally divided into four portions and an appropriate chain terminating mix, including the missing dNTP, is supplied to the reaction mixture and cycled between 10 and 200 times at appropriate temperatures for primer extension and denaturing. No additional DNA polymerase is required in this step, and the reaction can be completed in 1.5 to 16 hours.

The method of this invention allows sequencing of as little as 0.01 micrograms (0.005 picomoles) of M13 DNA with an 18 to 36 hour exposure for radioactively-labelled (.sup.35 S) material. With double-stranded plasmids, sequence information can be obtained from as little as 0.03 micrograms (0.015 pmol), without prior denaturation of the plasmid DNA with alkali. In addition, reliable sequence data can be obtained directly from a single M13 plaque, without further growth of the phage, by simply picking the plaque into a reaction vessel and adding the appropriate reagents. In addition, sequences can be obtained from single colonies of bacteria containing plasmids having the f1 replication origin (e.q., pTZ118R or pUC118) with a helper phage (M13 K07), or even for plasmids without helper phage.

Because the invention allows sequencing of small quantities of template, the need to grow clones and purify DNA from them is unnecessary, and the chance of adding impurities into the sequencing reaction along with the template DNA is thus reduced. In addition, because the DNA is effectively end-labelled by this procedure, there is little systematic variation in band intensity and no tendency for sequences to fade near the bottom of an autoradiogram of a sequencing gel.

Furthermore, the invention allows sequencing of plasmid and other double-stranded DNA templates without prior denaturation using alkali. These denaturation procedures are tedious, requiring an hour or more of skilled manipulation for denaturation and subsequent precipitation. Thus, the invention allows sequencing of DNA using existing apparatus with little modification to the procedure necessary to perform the claimed DNA sequencing reaction.

Thus, in a first aspect, the invention features a method for sequencing DNA which includes the following steps: providing a polynucleotide primer complementary to a region of the DNA, providing the DNA to be sequenced, and contacting that primer and DNA together in the presence of a DNA polymerase and between 1 and 3 dNTPs, at least one of the dNTPs being labelled. The primer and DNA are contacted under conditions which allow extension of the primer by addition of one or more of the dNTPs to the 3'-end of the primer to form an extended primer. The primer and DNA are then dissociated, or separated, generally by heating, and the contacting and separating steps repeated a plurality of times (usually 10-200 times). Finally, the extended primer is contacted with the DNA in the presence of a DNA polymerase (which is generally the same polymerase as used in the initial labelling step) all four dNTPs and a chain terminating agent.

In preferred embodiments, the DNA polymerase is Taq DNA polymerase (e.g., a form of Taq polymerase lacking a 5'-3' exonuclease activity), a low exonuclease form of T7 DNA polymerase (see Tabor and Richardson, U.S. Pat. No. 4,795,699), Klenow, AMV reverse transcriptase, Bst, Tth or Vent DNA polymerase, or exo-free forms of such polymerases; the dNTP is labelled with .sup.35 S or a fluorescent, chemiluminescent or other (e.g., are detectable as a ligand by indirect enzyme-linked assay, e.g., biotin) label; and the final contacting step in the presence of a chain terminating agent is followed by a separation step in which the extended primer is separated from the DNA, and those two steps repeated a plurality of times (e.g., between 30 and 200 times) until approximately 0.1-1.0 picomole (preferably 0.5 pmole) of labeled extended primer which is terminated by a dideoxynucleotide at its 3' end is produced.

In other preferred embodiments, the method is used to sequence:

Purified M13 DNA (single stranded, ss),

Purified Plasmid DNA (Double-stranded, ds),

Purified Lambda DNA (ds),

M13 DNA from a plaque or liquid culture (ss),

Plasmid DNA from a colony or liquid culture (ds),

plasmid-with-f1-ori, preferably, infected with helper phage, from a colony (ss),

PCR product, e.g., gel purified (ds),

PCR product treated w/ExoI & Alk. Phos. (ss),

Purified Cosmid DNA, and

DNA from other sources e.g., Yeast or bacterial genomic DNA or other phage DNAs.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.

The drawings will first briefly be described.

Drawings

FIG. 1 is a diagrammatic representation of a DNA sequencing procedure of this invention; and

FIGS. 2-7 are copies of autoradiograms obtained from DNA sequencing experiments using a variety of conditions described infra.

There follow examples of methods of the invention. These examples are not limiting in the invention, and those of ordinary skill in the art will recognize that the components used in the reactions may be readily substituted with equivalent reagents known in the art.

Methods

Referring to FIG. 1, the first step in the method is to examine the sequence of the template (usually vector sequence) and choose a combination of primer and labelling nucleotides. The primer is chosen by standard criteria, well known in the art, to prime DNA synthesis near the sequence of interest. The primer is typically a synthetic oligonucleotide 16-25 nucleotide bases in length.

A mixture of three nucleotides (of the possible four; dATP, dCTP, dGTP and dTTP) is chosen as follows: The first place where dAMP will be incorporated downstream from the 3' terminus of the primer is located. Similarly, the first sites for incorporation of dGMP, dCMP and dTTP are found. One of these sites will be farther from the primer than all the others. The nucleotide required at this site is omitted during the labeling phase of the protocol (dCTP in FIG. 1). One or more of the remaining three nucleotides must be labeled (e.g., .alpha.-.sup.32 p, .alpha.-.sup.35 S or a fluorescent molecule). A combination is chosen so that at least 2 labelled nucleotides will be incorporated prior to termination at the point where the missing nucleotide terminates extension. In FIG. 1, there are four dAMP nucleotides incorporated prior to the first site where dCMP would be required. Thus, a labelled dATP is a good choice in this example.

A suitable combination of primer and nucleotide mix can usually be found without difficulty, however, vectors can be specifically engineered for optimal labelling of sequences. If necessary the choice of which dNTP to leave out of the labelling mix can be determined empirically by running test labelings.

The following components were added to a microcentrifuge vial (0.4 ml) which was inserted into a thermocycler machine (e.g., Perkin-Elmer DNA Thermal Cycler): 0.005 pmol or more M13 DNA (e.g., M13mp18, 0.01 .mu.g), or 0.03 .mu.g double stranded plasmid DNA (e.g., pUC19); 2 .mu.l DELTA TAQ Reaction Buffer (U.S. Biochemical Corporation, Cleveland, Ohio, 260 mM Tris-HCl, pH 9.5 65 mM MgCl.sub.2); 1 .mu.l 3.0 .mu.M dGTP; 1 82 l 3.0 .mu.M dTTP; 0.5 .mu.l (5 .mu.Ci) of ›.alpha..sup.35 S!dATP (about 1200 Ci/mmol); 1 .mu.l -40 primer (0.5 .mu.M; 0.5 pmol/.mu.l 5' GTTTTCCCAGTCACGAC-3'); 2 .mu.l DELTA TAQ DNA polymerase 32U/.mu.l (from U.S. Biochemical Corporation, Cleveland, Ohio, in 20 mM Tris (pH8.5), 100 mM KCl, 0.1 mM EDTA, 1 mM DTT, 0.5% NP-40, 0.5% TWEEN-20 and 50% glycerol, diluted 8 fold in dilution buffer (10 mM Tris.HCl pH8.0, 1 mM 2-mercaptoethanol, 0.5% TWEEN-20, 0.5% NP-40)); and water to a total volume of 17.5 .mu.l.

These components were mixed and overlayed with 10 .mu.l light mineral oil (United States Biochemical Corporation). The vial was placed in the thermocycler and 30-100 cycles (more than 60 cycles is unnecessary) from 45.degree. C. for 1 min. to 70.degree. C. for 0.5 min. performed. (Temperatures can be cycled from 55.degree. to 70.degree. C., if desired, or even a constant temperature of about 62.degree. C. maintained (depending on the melting temperature of the primer and template to be sequenced). Such isothermal labelling is possible, but takes as long to complete (about 4-6 hours) as cycled temperatures.) The temperatures may be adjusted if the melting temperature of the primer/template is significantly higher or lower, but these temperatures work well for most primer/templates. A temperature of 95.degree. C. can be used instead of 70.degree. C. with equivalent results. This step can be completed in about 3 min. per cycle.

Four vials were labelled A, C, G, and T, and filled with 4 82 l of the corresponding DELTA TAQ termination mix: ddA Termination Mix (15 .mu.M each dATP, dCTP, dGTP, dTTP, 300 .mu.M ddATP); ddT Termination Mix (15 .mu.M each dATP, dCTP, dGTP, dTTP, 450 .mu.M ddTTP); ddC Termination Mix (15 .mu.M each dATP, dCTP, dGTP, dTTP, 225 .mu.M ddCTP); ddG Termination Mix (15 .mu.M each dATP, dCTP, dGTP, dTTP, 22.5 .mu.M ddGTP) (U.S. Biochemical Corporation, Cleveland, Ohio). No additional enzyme is added to the termination vials. The enzyme carried in from the prior step is sufficient.

The cycled labelling reaction was divided equally among the four termination vials (4 .mu.l to each termination reaction vial), and overlayed with 10 .mu.l of light mineral oil.

The four vials were placed in the thermocycler and 30-200 cycles (more than 60 cycles is unnecessary) performed from 95.degree. C. for 15 seconds, 55.degree. C. for 30 seconds, and 72.degree. C. for 120 seconds. This step was conveniently completed overnight. Other times and temperatures are also effective.

6 .mu.l of reaction mixture was removed (avoiding oil), 3 .mu.l of Stop Solution (95% Formamide 20 mM EDTA, 0.05% Bromophenol Blue, 0.05% Xylene Cyanol FF) added, and heated briefly to 70.degree.-80.degree. C. immediately prior to loading on a sequencing gel. Autoradiograms required an 18-36 hour exposure using Kodak XAR-5 film.

Referring to FIG. 2, the indicated amount of purified M13mp18 DNA (0.001-0.1 .mu.g , in lanes E-A respectively) was sequenced using the -40 primer and nucleotides as described above. The labeling step was cycled 30 times (45.degree. C. to 95.degree. C.) and the termination step was cycled 35 times (95.degree. C., 55.degree. C., and 72.degree. C.). The result in lane F is an example where all four dNTPs were present during the cycled labeling step (3 pmol each).

When the above labelling procedure is performed without cycling (using the standard protocol for the TAQuence sequencing kit, U.S. Biochemical Corporation), 0.1 .mu.g M13mp18 template DNA yields an autoradiogram where bands representing the first 200 bases from the primer are absent. The use of less DNA yields even fainter sequence.

An M13 plaque (M13mp18) was picked from an 18-36 hour-incubated plate using a large opening pipette tip. More phage can be obtained using a tip with the tip cut back to enlarge the opening to about 1.5 mm diameter. A small amount of soft agar adheres to the tip. The soft agar was mixed well (repeated sucking up and pushing out the liquid in the pipette) with 2 .mu.l of reaction buffer and 10 .mu.l of water. The tube was capped and heated for 10 minutes at 95.degree.-100.degree. C., and quickly spun to collect any condensation. 12 .mu.l was then quickly (so the agar does not solidify) removed for sequencing. The procedure outlined in Example 1 was then used for sequencing.

Referring to FIG. 4, the sequence of DNA from a single plaque of M13mp18 picked from a lawn of JM101 is shown. The DNA was extracted from the plaque by heating in sequencing buffer at 95.degree.-100.degree. C. for 10 min. The DNA was sequenced using the -40 primer and nucleotides as described above. The labeling step was cycled 99 times (45.degree. C. to 95.degree. C.) and the termination step was cycled 198 times (95.degree. C., 55.degree. C., and 72.degree. C.).

A colony containing a cloning vector (e.q., pUC19) is picked from a 36 hour-incubated plate (L broth) using a sterile wire or loop, suspended in 2 .mu.l of reaction buffer and 10 .mu.l of water, and mixed well. The colony was heated at 95.degree.-100.degree. C. for 20 minutes, briefly spun to sediment the debris, and 12 .mu.l of liquid removed for sequencing. The procedure outlined in Example 1 was then used for sequencing.

Referring to FIG. 3, results of cycled-labeling sequencing of plasmid (pUC19) DNA are shown. The indicated amount of purified pUC19 DNA (0.01-3 .mu.g) was sequenced using the -40 primer and nucleotides as described above. The labeling step was cycled 85 times (45.degree. C. to 95.degree. C.) and the termination step was cycled 198 times (95.degree. C., 55.degree. C. and 72 .degree. C.).

Infection of a cell containing a plasmid with the f1 origin with helper phage results in the production of large amounts of phage-like particles containing one strand of the plasmid DNA. (The strand produced is determined by the orientation of the f1 origin). These are secreted from the cell. Helper phage particles are also produced in lesser quantity (due to an alteration in the helper phage origin) but these do not sequence with the primer used. Thus, when a colony is infected with helper phage more plasmid DNA (but only one strand) is available for sequencing. Plasmid sequencing from a single colony is marginal, probably because it is difficult to obtain sufficient template DNA routinely. Infection with a helper phage produces more template, making sequencing possible.

Host strain JM101 was transformed with a plasmid vector (pTZ18U; Mead et al., 13 Nucleic Acid Research 1103, 1985) which contains the replication origin from bacteriophage f1. After incubating the competent cells with the plasmid at 0.degree. C. (45 min.) and 42.degree. C. (2 min.), the cells were diluted in L Broth and incubated at 37.degree. C. for 30 min. M13 helper phage M13K07 (Vierra and Messing, 153 Meth. Enz. 3, 1987; 10 .mu.l of 10.sup.10 pfu/ml) was added, and incubation continued at 37.degree. C. for 20 min. Transformants were then plated on media containing ampicillin and kanamycin. After incubation for 18-36 hours, a single colony was picked.

A colony from an 18-36 hour-incubated plate was picked using a sterile wire or loop, suspended in 2 .mu.l of reaction buffer and 10 .mu.l of water, and mixed well. The mixture was heated at 95.degree.-100.degree. C. for 20 minutes; alternatively, it was heated at 37.degree. C., for 30 minutes centrifuged briefly, and the supernatant incubated at 95.degree.-100.degree. C. for 10 minutes. The supernatant was then briefly spun to sediment the debris, and 12 .mu.l removed for sequencing as above.

Referring to FIG. 5, the sequence of DNA from a single colony of JM101 containing pTZ18U infected with M13K07 is shown. The DNA was extracted from the colony by heating in sequencing buffer at 95.degree.-100.degree. C. for 10 min. The DNA was sequenced using the -40 primer and nucleotides as described above. The labeling step was cycled 99 times (45.degree. C. to 95.degree. C.) and the termination step was cycled 198 times (95.degree. C., 55.degree. C., and 72.degree. C.).

Purified DNA from bacteriophage lambda (1 .mu.g in 5 .mu.l H.sub.2 O) was mixed with 1 .mu.l (0.5 pmol) primer (SEQ. ID. NO. 1: 5'-GGTTATCGAAATCAGCCACAGCGCC; corresponding to bases 7630-7606 in bacteriophage lambda), 2 .mu.l DELTA TAQ buffer, 1 .mu.l 3 .mu.M dGTP, 1 .mu.l 3 .mu.M dTTP, 0.5 .mu.l .alpha.-.sup.35 S dCTP (1200 Ci/mmol, 10 .mu.Ci/.mu.l) and diluted DELTA TAQ DNA polymerase (2 .mu.l 4U/.mu.l) and 5 .mu.l H.sub.2 O for a total volume of 17.5 .mu.l . This mixture was incubated in the DNA Thermal Cycler at 50.degree. C. for 60 sec., then 90.degree. C. for 30 sec. for 99 cycles. Using this mixture of nucleotides, the primer was extended 7 bases with the sequence (SEQ. ID. NO. 2: TCCCGTT with the C's labeled.

The mixture was then divided into four parts, combining one part with 4 .mu.l ddGTP termination mix (15 .mu.M each dATP, dCTP, dGTP, dTTP; 22.5 .mu.M ddGTP), a second with 4 .mu.l ddATP termination mix (15 .mu.M each dATP, dCTP, dGTP, dTTP; 300 .mu.M ddATP), the third with 4 .mu.l ddTTP termination mix (15 .mu.M each dATP, dCTP, dGTP, dTTP; 450 .mu.M ddTTP) and the fourth with 4 .mu.l ddCTP termination mix (15 .mu.M each dATP, dCTP, dGTP, dTTP; 225 .mu.M ddCTP). The mixtures were overlaid with 10 .mu.l mineral oil and the vials capped. These four vials were then placed in the thermal cycler and subjected to 198 cycles at 95.degree. C., 15 sec., 55.degree. C., 30 sec. and 72.degree. C., 120 SeC.

Upon completion of the cycles overnight, 6 .mu.l of the aqueous phase was removed from each vial, and 3 .mu.l of stop solution (95% Formamide, 20 mM EDTA, 0.05% Bromophenol Blue, 0.05% Xylene Cyanol FF) added. The mixtures were heated to 75.degree. C. for 2 min. and applied to the sequencing gel. Sequencing required only 1-2 .mu.g (0.03-0.05 pmol).

Referring to FIG. 6, the sequence of DNA from bacteriophage lambda DNA using a cycled labeling step protocol is shown. 0.5-5 .mu.g of lambda DNA was used as indicated in the Figure. The autoradiogram shown is the result of a 60 hr. exposure.

Upon completion of a polymerase chain reaction, the reaction mixture still contains excess primer, dNTPs and non-specifically amplified single-stranded DNAs. These are typically removed from the desired double-stranded product by precipitation, gel purification or other physical technique. These procedures are usually time-consuming and require manual manipulations for centrifugations or gel electrophoresis. I have discovered that the undesirable single-stranded DNA and primer can be removed by brief incubation with exonuclease I. Similarly, the excess dNTPs (which would interfere with the labeling portion of the sequencing process) can be removed by simultaneous treatment with alkaline phosphatase. Both enzymes can be inactivated by heating briefly. Subsequently the PCR product can be sequenced using the methods outlined in Example 1.

Specifically, PCR was performed as outlined in the Perkin-Elmer Cetus GENEAMP kit using 200 .mu.M dNTPs, 100 pmol each primer, 1 ng template DNA and buffer in a total volume of 100 .mu.l. Template DNA was pT7L-21 (a clone of a modified Tetrahymena ribozyme in pUC18; Zaug et al., 27 Biochemistry 8924-8931, 1988) and the primers were -40 universal sequencing primer (SEQ. ID. NO. 3: 5'-GTTTTCCCAGTCACGAC) and reverse sequencing primer (SEQ. ID. NO. 4: 5'-TTCACACAGGAAACAG). These primers yield a product 518 nucleotides in length. The polymerase chain reaction was performed using 30 cycles of 94.degree. C. for 1 min., 37.degree. C. for 1 min. and 72.degree. C. for 2 min.

An aliquot (10 .mu.l) of the PCR mixture was removed. Exonuclease I (USB Product No. 70013; 1 .mu.l of 10U/.mu.l) and shrimp alkaline phosphatase (USB Product No. 70092, 1 .mu.l of 1U .mu.l) were added. This mixture was incubated at 37.degree. C. for 15 min., followed by heat inactivation at 70.degree. C. for 15 min. This mixture was diluted to 200-2000 .mu.l with TE buffer (10 mM TRIS. HCl , pH7.5, 1 mM EDTA), and 10 .mu.l used for cycle-sequencing according to the method of example 1 using the -40 primer. Control experiments in which the treatments with either exonuclease I or shrimp alkaline phosphatase are omitted failed to provide good sequence data.

Referring to FIG. 7, the DNA sequence of the product of a polymerase chain reaction using a cycled labeling step protocol is shown. The labeling step was cycled 65 times (45.degree. C. to 70.degree. C.) and the termination step was cycled 198 times (95.degree. C., 55.degree. C. and 72.degree. C.). Aliquots of the PCR reaction mixture were treated with exonuclease I (lanes A and C) and/or shrimp alkaline phosphatase (lanes B and C) as described above, or untreated (lane D). The enzymes were then heat-inactivated and the DNA diluted 20-fold. This was used as template (10 .mu.l) using -40 primer and nucleotides as described above. Pre-treatment with both exonuclease and alkaline phosphatase yields the best sequence (C).

Other embodiments are within the following claims.