BIOLOGICALLY ACTIVE FRAGMENT OF BACILLUS STEAROTHERMOPHILUS DNA POLYMERASE

Abstract: The present invention is directed to an isolated and purified DNA encoding a biologically active fragment of a thermostable, full length DNA polymerase I enzyme of Bacillus stearothermophilus. More particularly, the invention is directed to a DNA encoding an approximately 66,000 dalton DNA polymerase that lacks 273 amino acids from the N-terminus of the approximately 96,000 dalton B. stearothermophilus DNA polymerase I, and to the protein encoded thereby which has been designated the B. stearothermophilus DNA polymerase I exo-fragment. The enzyme fragments are useful in DNA sequencing, cDNA preparations, thermophilic Strand Displacement Amplification and other molecular biology applications.

Description: BIOLOGICALLY ACTIVE FRAGMENT OF BACILLUS STEAROTHERMOPHILUS DNA POLYMERASE BACKGROUND OF THE INVENTION A. Field of the Invention The present invention relates to a thermostable recombinant protein fragment having DNA polymerase activity, 3 '-5' exonuclease (proofreading) activity, reverse transcriptase activity and being substantially free of 5 '-3' exonuclease activity. The thermostable recombinant polypeptide of the present invention is useful because it is capable of providing enhanced polymerase activity in bio-applications, such as in cDNA production, Strand Displacement Amplification and DNA sequencing. B. Background The field of biotechnology was revolutionized by recombinant DNA technology, and DNA polymerase enzymes have become an indispensable tool in many in vitro recombinant DNA biological applications such as DNA sequencing; Polymerase Chain Reaction (PCR) and its many variations (see, e.g. , Erlich et al. , Current Communications in Molecular Biology: Polymerase Chain Reaction. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1989); Innis et al. , PCR protocols: A guide to methods and applications, Academic Press, San Diego, CA (1990)); Thermal Cycle Labeling (TCL) (Mead and Swaminathan, U.S. Patent App. Ser. No. 08/217,459, filed March 24, 1994; PCT App. No. US94\03246, filed March 24, 1994); Random Primer Labeling (RPL); Ligase Chain Reaction (LCR) (Wiedmann et al. , PCR Methods and Applications 3: S51 - S64 (1994)); Strand Displacement Amplification ("SDA") (Walker, T.G. Emperical Aspects of Strand Displacement Amplification, Becton Dickenson Research Center, Cold Spring Harbor Laboratory Press (1993)), and other applications. To date, scientists have reported more than 40 different DNA polymerases. Comparisons of amino acid sequences has resulted in the placement of reported polymerase genes into four major families: namely, A, B, C, and X. Family A contains E. coli DNA polymerase I, an enzyme that is involved in repair of DNA and in replication during fast growth. Family B includes E. coli DNA polymerase EL Family C includes E. coli DNA polymerase HI, the major replication enzyme. The fourth group, Family X, contains enzymes such as the eukaryotic DNA polymerase β and eukaryotic terminal transferases (Ito and Braithwaite, Nucleic Acids Res. 19: 4045-4057 (1991)). DNA polymerase I (pol I) (Family A) enzymes have proved to be very useful for DNA sequencing applications, PCR, SDA, and other applications known in the art. Structure-function relationship studies indicate that the known DNA pol I molecules share a similar modular organization. A 5 '- 3 ' exonuclease function is located in the N-terminal one-third of the enzyme. The remainder of the molecule forms one domain which is further classified into functional sub-domains. Adjacent to the 5 '-3 ' exonuclease domain lies a 3 '-5 ' exonuclease sub-domain, followed by a polymerase sub-domain (Blanco et al. , Gene 700: 27-38 (1991)). In addition to classifying DNA polymerase enzymes into the above families, it is also useful to classify such polymerases as mesophilic (purified from mesophilic organisms) or thermophilic (purified from thermophilic organisms). (See, e.g., Bessman et al , J. Biol. Chem. 233: 171-177 (1958); Buttin and Kornberg, J. Biol. Chem. 241: 5419-5427 (1966); Uemori et al. , Nucleic Acids Res. 21: 259-265 (1993); Lawyer et al. J. Biol. Chem. 264; 6427- 6437 (1989); and Kaledin et al. , Biokhimiya 45: 644-651 (1980)). The DNA polymerases of mesophilic origin are useful in many biological applications, such as in certain DNA sequencing applications. However, many important applications require thermal cycling to repeatedly denature template DNA and/or RNA and their extension products. Because the mesophilic DNA polymerases do not withstand the high temperatures or the thermal cycling of these applications, the thermostable DNA polymerases enjoy significant advantages over mesophilic DNA polymerases in such applications. Through deletion of the 5 ' one-third of DNA polymerase I genes, or by the proteolytic cleavage and subsequent removal of the portion of the holoenzyme encoded thereby, scientists have created DNA pol I fragments retaining polymerizing activity, but having reduced 5 '-3 ' exonuclease activity. (See, e.g. , Joyce and Grindley, Proc. Natl. Acad. Sci. 80. 1830-1834 (1983) (the Klenow-Fragment of the E. coli DNA polymerase enzyme); Lawyer et al., J. Biol. Chem. 264: 6427-6437 (1989); Gelfand et al. , U.S. Patent No. 5,079,352 (1992); Lawyer et al, PCR Methods and Applications 2 :275-287 (1993) (the Stoffel fragment of the T. aquaticus (Taq) DNA polymerase enzyme); and Barnes, Gene 112: 29-35 (1992) (the KlenTaq DNA polymerase).) The different reports of thermostable DNA polymerases and their derivatives suggest these enzymes possess different, unpredictable properties that may be advantageous or detrimental, depending on the biological application in which the DNA polymerase is to be employed (Myers and Gelfand, Biochemistry 30: 7661-7666 (1991)). The KlenTaq DNA polymerase is an example of an enzyme fragment with important properties differing from the Taq holoenzyme. The KlenTaq DNA polymerase reportedly has roughly a two-fold lower PCR- induced relative mutation rate than Taq polymerase holoenzyme. However, more units of KlenTaq are needed to obtain PCR products similar to those generated with Taq DNA pol I. Similarly, Lawyer et al. (1993) reported that T. aquaticus DNA polymerase I fragments possessed greater thermostability and were active over a broader Mg 2+ -range than the corresponding holoenzyme. Also, the deletion of the 5' to 3' exonuclease domain while maintaining an active 3' to 5' exonuclease (proofreading) domain in Ultma ™ DNA Polymerase (Perkin Elmer, Branchburg, N.J.) is reported to provide an increase in fidelity. (Sninsky, Gelfand and Erdman. Amplifications, Perkin Elmer, 1995). While each of the above described enzyme fragments produced a different but useful property, it is wholly unpredictable whether a fragment of a polymerase enzyme will have a particular property. However, there exists a need in the art for new, thermostable DNA polymerase enzymes for use in new molecular biology applications. More particularly, there exists a need for thermostable DNA polymerase enzymes having high purity, high DNA polymerase specific activity, high reverse transcriptase activity, low levels of exonuclease activity, and possessing high fidelity (low mutation frequencies) and high processivity. An object of the present invention is to provide a polymerase enzyme preparation of greater purity, quantity, and processivity than has heretofore been possible. A further object is to eliminate the need and expense of culturing of large volumes of thermophilic bacteria at high temperatures that is associated with preparing native thermostable polymerase enzyme preparations. Yet another object is to provide a recombinant polymerase possessing 3 '-5' 97/39113 PC17US97/06235 - 4 - exonuclease (proofreading) activity and higher reverse transcriptase activity as compared to the currently available enzymes (Bca polymerase, PanVera, Madison). 7/ 9113 - 5 - STJMMARY OF THE INVENTION The present invention has multiple aspects. In its simplest form, the present invention is directed to a thermostable and enzymatically active recombinant fragment of a bacterial polymerase enzyme (Bacillus stearothermophilus DNA polymerase I) wherein the desirable DNA polymerase activity and 3 '-5' exonuclease activity of the holoenzyme have been retained or enhanced and the undesirable 5 '-3' exonuclease activity of the holoenzyme has been substantially removed. This recombinant polypeptide fragment, which is referred to herein as "rBst exo-," has a molecular weight of about 66,000 daltons and corresponds to amino acid residues 274-876 of SEQ ID NO: 2. Unexpectedly, it has been discovered that rBst exo- has significant reverse transcriptional activity i.e. , a R.T. to polymerase ratio that is greater than 1. The other aspects of the present invention are directed to the various intermediates that are used in preparation of this recombinant polypeptide fragment. More specifically, this invention is also directed to isolated and purified polynucleotides (e.g. , cDNA, DNA sequences, complementary sequences and RNA transcripts thereof) encoding a thermostable polypeptide fragment ofthe DNA polymerase I enzyme of Bacillus stearothermophilus having DNA polymerase activity. Preferred DNA include: the isolated Bacillus stearothermophilus strain 10 DNA pol I gene comprising nucleotides described in Figure 2a; nucleotide 316-2943 of SEQ ID NO: 1 (exclusive of the stop codon) or nucleotides 316-2946 of SEQ ID NO: 1 (inclusive of the stop codon) and; a portion of the insert of plasmid pPEK 5 (ATCC Accession No.# ), said portion encoding a thermostable polypeptide having DNA polymerase activity. Also within the scope of this invention is rBst pol I (SEQ ID NO: 2) which is encoded by nucleotides 316-2946 of SEQ ID NO: 1 (inclusive of the stop codon). Additional DNA molecules within the scope ofthis invention are an isolated DNA or a cDNA having nucleotides 1135-2943 or 1135-2946 of (SEQ ED NO: 1) encoding the Bst exo- fragment (without and with the stop codon, respectively). The present invention is also directed to a plasmid encompassing the above mentioned DNA molecules, e.g., pPEK 5 and pPRB5, and to a host cell, such as a prokaryotic or eukaryotic host cell that has been stably transformed with DNA vectors, or plasmids of the invention. Another aspect of the invention is directed to such transformed host cells that are capable of expressing a thermostable polypeptide encoded by the DNA of the present invention, the peptide having DNA polymerase activity. In another aspect, this invention provides a purified thermostable polypeptide having DNA polymerase activity. Preferred peptides include a Bacillus stearothermophilus exo- DNA polymerase I fragment, which is substantially free of 5 '-3' endonuclease activity or other Bacillus stearothermophilus proteins with 3 '-5' exonuclease activity and substantially free of reduced 5'-3' exonuclease activity as compared to the holoenzyme. In another aspect, this invention provides methods for purifying a thermostable polypeptide having DNA polymerase activity including the steps of transforming a host cell with a DNA of the present invention to create a transformed host cell; cultivating the transformed host cell under conditions that promote expression of a thermostable polypeptide encoded by the DNA, the polypeptide having DNA polymerase activity; and purifying the thermostable polypeptide. In the preferred method, commercially available chromatography columns are used to purify the expressed polypeptide. In another aspect, this invention provides methods of using the DNA constructs of the invention to produce a recombinant thermostable polypeptide having DNA polymerase activity, 3 '-5' exonuclease activity and being free of 5 '-3' exonuclease activity. One such method involves using a DNA encoding a DNA polymerase enzyme to generate an active fragment of the DNA polymerase enzyme, including the steps of: deleting a portion of the DNA to create a modified DNA via restriction endonuclease cleavage; expressing the modified DNA to produce a DNA polymerase enzyme fragment; assaying the DNA polymerase enzyme fragment for DNA polymerase activity and selecting a DNA polymerase enzyme fragment having DNA polymerase activity wherein the DNA is selected from among the DNA described herein. In another aspect, this invention provides methods for using the proteins of the invention in biological applications, such as in DNA sequencing, cDNA preparation, Strand Displacement Amplification and other applications or processes that would be apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 graphically depict the cloning strategy for the DNA encoding the exo- fragment of B. stearothermophilus DNA polymerase I. The abbreviations used are: D: Dra I; H: Hpa I; N: Neo I; Pr: Promoter; Sa: Sal I. Jagged lines (II I III I III 11) represent vector DNA; dark shaded rectangles depict the Bst exo- fragment DNA sequences and light shaded rectangles depict the 5 '-3' exonuclease domain gene sequences. The graphical depictions are not drawn to scale, and not all available restriction sites are shown in all steps. FIGURES 2a and 2b depict DNA and amino acid sequences for compositions of the present invention. FIGURE 2a depicts the DNA sequence for the Bst DNA pol I coding sequence and for 5' and 3 ' untranslated sequences. Start codons for the holoenzyme and the exo- fragment are indicated by indentations and bold type. The sequence of the 757-830 probe fragment is underlined. The stop codon (TAA) is indicated by bold type. FIGURE 2b depicts the deduced amino acid sequence for the Bst DNA pol I coding region. The bold typed amino acid (M) is the first amino acid believed to be translated during translation of plasmid pPEK 5, encoding the Bst exo- fragment. An asterisk (*) indicates the stop codon TAG. FIGURE 3 depicts the relative DNA polymerase enzymatic activity, at different buffered pH levels, of native Bacillus stearothermophilus exo- (nBst exo-: solid line); recombinant Bacillus stearothermophilus exo- (rBst exo-: dashed line); and recombinant Bacillus caldotenax exo- fragments (rBca exo-: dotted line, PanVera, Madison, WI). FIGURE 4 depicts the relative DNA polymerase enzymatic activity, at different concentrations of MgCl 2 , of native Bacillus stearothermophilus exo- (nBst exo-: solid line); recombinant Bacillus stearothermophilus exo- (rBst exo-: dashed line); and recombinant Bacillus caldotenax exo- fragment (rBca exo-: dotted line, PanVera, Madison, WI). FIGURES 5a and 5b comparatively depict the various activities of the enzyme of the present invention as a function of Mn ++ concentration. FIGURE 5a depicts the relative DNA polymerase enzymatic activity, at different concentrations of MnCl 2 , of native Bacillus stearothermophilus exo- (nBst exo-: solid line); recombinant Bacillus stearothermophilus exo- (rBst exo-: dashed line); and recombinant Bacillus caldotenax exo- fragment (rBca exo-: dotted line, PanVera, Madison, WT). FIGURE 5b depicts the relative reverse transcriptase enzymatic activity at different concentrations of MnCl 2 , of native Bacillus stearothermophilus exo- (nBst exo-: solid line); recombinant Bacillus stearothermophilus exo- (rBst exo-: dashed line); and recombinant Bacillus caldotenax exo- fragment (rBca exo-: dotted line, PanVera, Madison, WI). FIGURE 6 depicts the relative DNA polymerase enzymatic activity, at different temperatures, of native Bacillus stearothermophilus exo- (nBst exo-: solid line); recombinant Bacillus stearothermophilus exo- (rBst exo-: dashed line); and recombinant Bacillus caldotenax exo- fragment (rBca exo-: dotted line, PanVera, Madison, WI). FIGURE 7 photographically depicts a portion of an autoradiograph of reverse transcriptase products from poly rA.dT or mRNA targets using native Bacillus stearothermophilus exo- fragment (nBst exo-), recombinant Bacillus stearothermophilus exo- fragment (rBst exo-), and recombinant Bacillus caldotenax exo- fragment (rBca exo-). In Figure 7, Lanes 1-6 contain the following: Lane 1, nBst exo- of mRNA; Lane 2, rBst exo- of mRNA; Lane 3, rBca exo- of mRNA; Lane 4, lkb DNA Ladder; Lane 5, nBst exo- of poly rA:dT; Lane 6, rBst exo- of poly rA:dT; Lane 7, rBca exo- of poly rA:dT. FIGURE 8 photographically depicts the purity of purified native Bacillus stearothermophilus holoenzyme (nBst holo), native Bacillus stearothermophilus exo- fragment (nBst exo-), recombinant Bacillus stearothermophilus exo- fragment (rBst exo-), and commerciaUy available recombinant Bacillus caldotenax exo- fragment (rBca exo-, PanVera, Madison, WI) on a 20.0% SDS-PAGE gel stained with silver. In Figure 8, Lanes 1-6 contain the following polypeptides: Lane 1, Low Molecular Weight Markers; Lane 2, nBst holo; Lane 3, nBst exo-; Lane 4, rBst exo-; Lane 5, rBca exo-; and Lane 6, Low Molecular Weight Markers. FIGURES 9a and 9b photographically depict portions of autoradiographs of sequencing gels showing DNA sequence obtained with the indicated polymerases. Abbreviations; native Bacillus stearothermophilus exo- fragment (nBst exo-); recombinant Bacillus stearothermophilus exo- fragment (rBst exo-); recombinant Bacillus caldotenax exo- fragment (rBca exo-). FIGURE 10 photographically depicts portions of autoradiographs of sequencing gels showing processivity differences obtained with the indicated polymerases. Abbreviations; native Bacillus stearothermophilus exo- fragment (nBst exo-); recombinant Bacillus stearothermophilus exo- fragment (rBst exo-); recombinant Bacillus caldotenax exo- fragment (rBca exo-). FIGURE 11 photographically depicts a portion of an autoradiograph of thermophilic SDA produαs with recombinant Bacillus stearothermophilus exo- fragment (rBst exo-). nFTATT ED DESCRIPTION OF THE INVENTION In its first aspect, the present invention is directed to an isolated and purified DNA (nucleotides 316-2946 of SEQ ID NO: 1) encoding a biologically active, thermostable, full length DNA polymerase I enzyme of Bacillus stearothermophilus, ("Bst pol I"). By the phrase "thermostable" as used herein is meant an enzyme that exhibits thermal stability at 65° - 80° C, preferably 65° - 70° C. The present invention is also directed to a cDNA having nucleotides 316-2943 (exclusive of the stop codon) of SEQ ID NO: 1 or a cDNA having nucleotides 316-2946 (inclusive of the stop codon) of SEQ ID NO: 1, which enable the Bst pol I enzyme. In another aspect, the invention is directed to a DNA (nucleotides 1135-2946 of SEQ ID NO: 1) encoding an approximately 66,000 dalton DNA polymerase that lacks 273 amino acids from the N-terminus of the approximately 96,000 dalton B. stearothermophilus DNA polymerase I (SEQ ID NO: 2), and to the protein encoded thereby which has been designated the B. stearothermophilus DNA polymerase I exo- fragment (i.e. , amino acid residues 274-876 of SEQ ID NO: 2). The enzyme fragments are useful in DNA sequencing, cDNA preparations, thermophilic Strand Displacement Amplification and other molecular biology applications. As a first step in the generation of the DNA and polypeptides of the present invention, native B. stearothermophilus DNA polymerase I was purified and isolated from B. stearothermophilus strain 10 cells (this strain was received from Bruce Roe, University of Oklahoma) and amino acid sequence information was determined for this 90-100 kilodalton (kD) native holoenzyme. (See Example 1.) Additionally , a Bacillus stearothermophilus genomic library was constructed in phage λ Dash π library and amplified. (See Example 2.) Published amino acid sequence information from various Thermus species and Bacillus caldotenax DNA pol I genes (Uemori et al , J. Biochem. 113: 401-410 (1993) was used to create a degenerate set of synthetic DNA primers of which primers 757 (SEQ. ID NO: 4) and 830 (SEQ. ID NO: 5) were found useful for isolating a portion of the Bacillus stearothermophilus DNA polymerase I gene. Also, N-terminal amino acid sequence information of native Bst DNA polymerase I was deteπnined (see Example 1) and used to create a synthetic primer, BCA (SEQ. ID NO: 3), to identify the 5' end of the Bacillus stearothermophilus DNA polymerase I gene (see Example 3). The 757 primer (SEQ. ID NO: 4) was synthesized to have a sequence which binds to the top strand of the B. stearothermophilus strain 10 DNA pol I gene. The 830 primer (SEQ. ID NO: 5) was synthesized to have a sequence that binds to the 3 -end of the B. stearothermophilus gene on the opposite strand. A DNA amplification reaction was performed with primer 757 (SEQ. ED NO: 4), primer 830 (SEQ. ED NO: 5), and B. stearothermophilus strain 10 genomic DNA. The amplification reaction yielded a single amplification product, designated the "757-830 fragment." This fragment was cloned into pTZ18U vector, amplified in E. coli, and sequenced. As explained in detail in Example 4, the 757-830 fragment (obtained by the procedures outlined in Example 3) was further amplified and used to generate probes via thermal cycle labeling (TCL). The probes were used to isolate the Bacillus stearothermophilus DNA pol I gene from the B. stearothermophilus genomic library that had been constructed (See Example 2). The amplified B. stearothermophilus genomic library was plated on 2XTY plates and grown until plaques formed. Duplicate plaque lifts were obtained from each plate onto Hybond N filters, and these filters were then screened using the above described TCL probes using hybridization methods well known in the art. Positive plaques were selected, purified by dilution and re-screened with the 757 (SEQ ID NO: 4), 830 (SEQ ID NO: 5), and BCA (SEQ ID NO: 3) probes, and then further characterized. In particular, two clones with inserts of 14-16 kb, which were designated λ41 1 and λ51 1. were chosen for further analyses. Clones λ41 1 and λ51 1 were used as a starting point from which the complete B. stearothermophilus DNA pol I gene was assembled and sequenced. As explained in detail in Example 5 and with reference to FIGURE 1 , restriction mapping, subcloning, and partial sequencing led to the determination that a subclone of λ511 designated pB5X3 contained about 1/2 of the Bst DNA pol I gene (3' end), whereas a subclone from X411 designated pB4S6 contained the remaining 5' portion of the gene that overlapped the coding sequence contained in clone pB5X3. A primer walking procedure was used to obtain the complete sequence ofthe gene. Specifically, primers homologous or complimentary to the ends of previously determined sequences were synthesized and used in additional sequencing reactions. By repeating this process, the entire length ofthe gene was sequentially sequenced. The foregoing results demonstrate that an aspect of the invention is directed to a purified and isolated DNA encoding a thermostable polypeptide having DNA polymerase activity, the DNA comprismg nucleotides described in Figure 2a. This DNA may be operatively linked to other DNA, such as expression vectors known in the art. The invention is also directed to a vector having at least one insert consisting essentially of nucleotides described in Figure 2a, the nucleotides encoding a thermostable polypeptide having DNA polymerase activity. With the gene sequence established, the deduced amino acid sequences of the B. stearothermophilus DNA pol I gene were aligned and compared to the deduced amino acid sequences of the purported B. caldotenax DNA pol I gene (Uemori et al. , J. Biochem. 113: 401-410 (1993). The following substitutions or deletions and their location were found in the rBst DNA pol I amino acid sequence when compared to the rBca DNA pol I sequence: Met m , Tφ 164 , Tip JJO , deletion m . To produce the recombinant B. stearothermophilus exo- DNA pol I protein of the present invention, a full-length B. stearothermophilus DNA pol I gene clone was constructed and expressed in E. coli. As detailed in Example 6 and FIGURE 1 , plasmids pB5X3 and pB4S6 were further restriction mapped and subsequently subcloned to generate plasmid pPRB5 , containing a 3.5 kb insert containing the entire Bst DNA pol I gene along with 5' and 3' non-coding regions. A Dra I, Hpa I restriction fragment isolated from pPRB5 was ligated to a similar vector to generate the 5 ' deleted exo- fragment designated pPEK5 E. coli DH5αF' were transformed with plasmid pPEK5 and grown in a fermentor to recombinantly produce B. stearothermophilus exo- fragment. As detailed in Example 6, this recombinant protein was purified from the lysed E. coli with a method that included a heat denaturation of E. coli proteins, Mono Q and Mono S chromatography. The calculated DNA polymerase specific activity of B. stearothermophilus DNA exo- fragment isolated by this procedure was determined to be approximately 150,000 U/mg protein. The foregoing description of methods and recombinant cells demonstrates that the present invention is directed to more than DNA and polypeptides. Another important aspect of the invention is directed to a host cell transformed with a DNA, vector, or plasmid of the present invention, including those specifically mentioned above. Preferably, the host cell transformed with the DNA is capable of expressing a thermostable polypeptide encoded by the DNA, wherein the polypeptide has DNA polymerase activity. By host cell is meant both prokaryotic host cells, including E. coli cells, and eukaryotic host cells. In addition to being directed to DNA transformed cells, and polypeptides, the present invention is directed to various methods for using DNA and polypeptides. For example, the purification protocols for rBst exo- fragments demonstrate that another aspect of the invention relates to methods of purifying a thermostable polypeptide having DNA polymerase activity. One such method includes the steps of expressing the thermostable polypeptide in a host cell, the polypeptide having an amino acid sequence encoded by a DNA of the present invention; lysing the cell to create a suspension containing the thermostable polypeptide and host cell proteins and cell debris. Preferably, such a method further includes the steps of heating the suspension to denature the host cell proteins; and centrifuging the suspension to remove the cell debris and denatured host cell proteins. Further purification is achieved by Mono Q and Mono S chromatography, respectively. As detailed in Example 9 and summarized in TABLE 3A, a number of experiments were conducted to characterize contaminating (non- polymerase related) exonuclease activities of purified B. stearothermophilus and B. caldotenax DNA pol I exo- fragments. The 5 '-3' exonuclease and endonuclease activity assayed was either very low or undetectable. However, the relatively higher release of labelled substrate, which is evident in the 3'-5' exonuclease assays of nBst exo- and rBst exo- fragments compared to that of rBca exo- fragment while maintaining a zero slope of %-release/unit enzyme, indicates an inherent 3 '-5' exonuclease (proofreading) activity in the purified Bst exo- fragment enzyme. As detailed in Example 8, a number of additional assays were performed to better characterize the recombinant Bst exo- fragment protein that had been purified and to compare these proteins to recombinant Bacillus caldotenax exo- fragment (rBca exo-, PanVera, Madison, WT). For example, the DNA polymerase activity of the nBst, rBst and rBca exo- fragment was analyzed at different pH values, and at different MgCl 2 and MnCl 2 concentrations. FIGURES 3 (pH optima); 4 (MgCl 2 optima); 5a and 5b (MnCl 2 optima) and 6 (temperature optima) summarize the results of some of these assays. The optima for the recombinant Bst and Bca exo- fragments are summarized in TABLE 1. To assay for thermostability, the enzymes were incubated for 10 minutes at different temperatures to define the temperature optimum. The highest activity (over 90%) was found at 70° C for nBst exo-, rBst exo-, and rBca exo- fragments with (approx. 20% remaining after 10 minutes at 80° C for all three. Reverse transcriptase (RT) (RNA dependant DNA polymerase) activity was detected in all three samples at 50° C, at a 1.0 mM MnCl 2 optimum concentration. Both the native and recombinant Bst exo- fragments yielded an RNA dependant DNA polymerase to DNA dependant DNA polymerase unit ratio equal to about 1.4 and 2.0, respectively, while the rBca fragment yielded a ratio equal to or less than 0.8. (FIGURE 5b). The purified rBst exo- fragment was found to possess inherent 3 '- 5.' exonuclease (proofreading) activity and no detectable 5 '-3 ' exonuclease activities. The preparation was more than 90% pure as judged by 20 % SDS-PAGE (FIGURE 8). The apparent molecular weight of 65 kD as judged by SDS-PAGE compares well with the calculated molecular weight of approximately 65 kD. The rBst exo- preparation was found to be free of detectable double- and single-stranded nucleases and endonuclease contamination activities. The isoelectric point was determined to be 5.6. The performances of nBst exo-, rBst exo-, and rBca fragments were tested in ssDNA sequencing (Example 9). The enzymes were useful in sequencing reactions utilizing internal labeling with [or 33 P]-dATP. In all the reactions tested, the exo- fragments provided readable DNA sequence information of more than 150 nucleotides. However, a stronger signal was evident with the native and recombinant Bst exo- fragments. Furthermore, the nBst and rBst exo- fragments were tested in dsDNA sequencing with results comparable to those seen with ssDNA target (Example IIB). The following examples are intended to describe various aspects of the invention in greater detail. More particularly, in Example 1, the purification and N-terminal amino acid sequencing of native Bacillus stearothermophilus DNA polymerase I is described. In Example 2, the construction and amplification of a Bacillus stearothermophilus genomic DNA library is described. In Example 3, the cloning and sequencing of a Bacillus stearothermophilus DNA polymerase I gene-specific probe fragment is described. Example 4 details the preparation of gene-specific probes and screening of the Bacillus stearothermophilus genomic library for clones containing the B. stearothermophilus DNA pol I gene. Example 5 details the sequencing of the B. stearothermophilus DNA polymerase I gene by primer walking. Example 6 details the cloning of the iS. stearothermophilus holoenzyme and the cloning and expression of the exo- fragment of B. stearothermophilus DNA polymerase I. In Example 7-13, the Applicants compared the activities of the rBst exo- enzyme of the present invention to native Bst exo- and the prior art enzyme rBca exo- (PanVera, Madison, WI). In Example 7, the characterization of recombinant B. stearothermophilus exo- DNA polymerase I exonuclease activities is detailed. TABLE 3A, which summarizes the results of Example 7, reflects that only the enzyme of the present invention (rBst exo-) is free from contamination with 3'-5' exonuclease activity, 5 '-3' exonuclease activity, ssDNAse activity, dsDNAse activity and endonuclease activity. In contrast, nBst exo- exhibited 0.06 % release /unit of 5 '-3' exonuclease activity, and rBca exo- exhibited 0.3% release/unit of enzyme of 5 '-3' exonuclease activity and 0.05 % release/unit of enzyme of dsDNAse activity. In Example 8, the effect of MgCl 2 , MnCl 2 , pH, and temperature on the activity of the native, recombinant B. stearothermophilus and recombinant B. caldotenax exo- DNA polymerases was compared. TABLE 4 in Example 8 reports that the pi for nBst exo-, rBst exo- and rBca exo- were determinable 5.4, 5.6 and 5.3, respectively. The optimal polymerase activity for each of these three enzymes was determined to be 1.0 mM MgCl 2 (See FIGURE 4). In contrast, the optimal activity for the nBst exo-, rBst exo- and rBca exo- fragments was observed to be at 0.5 mM MnCl 2 (FIGURE 5A). The temperature optima for all three enzymes, as depicted in FIGURE 6, is 70° C, dropping precipitously to about 20% activity remaining at 80° C. In Example 9, we demonstrated the utility of recombinant exo- fragment in reverse transcriptase (RT) procedures. Example 9 reflects that the ratio of reverse transcriptase (RT) units deteπnined at the optimal of about 1.0 mM MnCl 2 concentration to polymerase I (pol I) units vary significantly between the three enzymes (See FIGURE 5b). In particular, the ratio of RT:pol I obtained from the rBca exo- was 0.8 in contrast to about 1.4 and 2.0 for the nBst exo- and rBst exo- fragments, respectively. A ratio of RT:pol I that is greater than 1 is unexpected, and 2 or greater is very unexpected. It is also within the scope of the present invention that the Bst exo- fragment has an RT/pol I ratio of about 1 to 3, preferably 1.5-2.5. In Example 10, DNA sequencing with recombinant B. stearothermophilus DNA polymerases was performed. Example 10 reflects that the precipitable cDNA obtained when using Poly rA-.dT as template was greatest with rBst exo- at 2941 cpm's followed by nBst exo- at 2421 cpm's and rBca exo- at 2001 cpm's. Alternatively, the use of mRNA as template resulted in 837,037 cpm, 691,545 cpm and 430,418 cpm for nBst exo-, rBst exo- and rBca exo- enzymes respectively (See TABLE 6). Also, the cDNA was determined by autoradiograph to be superior in length and quantity when using nBst exo- or rBst exo- fragments versus the cDNA product obtained when using rBca exo- fragment enzyme. Example 11 compared the processivity (i.e. , the rate of DNA polymerization along a template) of rBst exo- fragment to native Bst exo- and rBca exo- fragment. In this assay, the enzyme with the greatest processivity of DNA polymerase activity would produce the largest DNA molecules which would move slowly and stain darkly on electrophoresis relative to smaller DNA molecules. When the polymerase products of the three polymerases were placed on a 6% polyacrylamide sequencing gel, and electrophoresed, the order of processivity in descending order was rBst exo-> nBst exo- > rBca exo-. Thus, the enzymes of the present invention, rBst exo- and nBst exo-, both demonstrated superior processivity over the prior art enzyme rBca exo-. (See FIGURE 10). Further, FIGURE 10 demonstrates the overall superior processivity of rBst exo- (as reflected by the dark and multiple lines at the top of the figure) to both nBst exo- and rBca exo-. In Example 12, the rBst exo- fragment was successfully used in thermophilic Strand Displacement Amplification. Accordingly, it is within the scope of the present invention that the Applicants' enzyme, particularly rBst exo-, be used in a thermophilic Strand Displacement Assay. Finally, Example 13 details the purification of Bst exo- fragment in accordance with one embodiment of Applicants' invention. These and other embodiments of the Applicants' invention would be obvious to those skilled in the art based upon the disclosure herein and the Examples that follow. EXAMPLE 1 Purification of Native Bst DNA Pol I and Native Exo- Fragment and N- Terminal Amino Acid Sequencing of Native Bst DNA Pol I Native B. stearothermophilus DNA polymerase I was isolated from B. stearothermophilus strain 10 cells and used to generate amino acid sequence information as described below. A. Bacillus stearothermophilus strain 10 (obtained from Bruce Roe University of Oklahoma) was cultured as follows: an isolated colony from an LB plate grown overnight at 55° C was used to inoculate 100 ml culture medium (0.1 g nitrilotriacetic acid, 3 g NZ Amine A, 3 g yeast extract, 5 g succinic acid [free acid], 0.001 g riboflavin, 0.522 g KjHP0 4 , 0.480 g MgS0 4 , 0.020 g NaCI, 2 ml Trace Metal Solution (0.5 ml H 2 SO„, 2.2 g MnS0 4 , 0.5 g ZnS0 4 , 0.5 g H 3 B0 3 , 0.016 g CuS0 4 , 0.025 g NaJvIo0 4 . 0.046 g cobalt nitrate per liter, adjusted to pH 8.0 with NaOH) and the culture was incubated overnight at 55° C with shaking. In the morning, 10 ml of the overnight culture was used to inoculate 1000 ml of medium. This culture was grown for about 8 hours at 55° C and then used as an inoculum for 170 liters of medium in a New Brunswick 250 liter fermentor equipped with a ML 4100 controller. The settings for a typical fermentation were 3 pounds back pressure, 60 liters per minutes (lpm) aeration, 100 rpm agitation, at 55° C. The fermentation was terminated when the cells reached a density of 2 -3 O.D., as measured at 600 nm. The cells were cooled down to room temperature and harvested by centrifugation at 17,000 φm in a CEPA type 61 continuous flow centrifuge with a flow rate of 2 lpm. The cell paste was stored at -70° C. B. stearothermophilus strain 10 cells (500 g) were thawed in 3 volumes of lysis buffer (20 mM Tris-HCl, pH 7.5, 0.5 mM ethylenediaminetetraacetate (EDTA), 1 mM Dithiothreitol (DTT), 10 mM MgCl 2 , 0.02% Brij 35) and homogenized. The suspension was then treated with 0.2 g/l of lysozyme (predissolved in lysis buffer) at 4° C for 45 minutes. Cells were homogenized twice at 9000 psi in a Manton Gaulin homogenizer with the suspension chilled to approximately 10° C between passes. The sample was mixed well and centrifuged at 13,500 x g for 1 hour. A sample of the supernatant was titrated with a 10% polyethylenimine (PEI) solution to determine optimum PEI precipitation. The full scale PEI precipitation was centrifuged at 13,500 for 1 hour. The recovered supernatant was AmS0 4 precipitated at 70% saturation and allowed to stir at 4° C for at least 60 minutes. After centrifugation at 9000 φm for 60 minutes the pellets were resuspended in 300 ml of P-11 buffer (20 mM Kpi, pH 6.5, 1.0 mM DTT). The sample was dialyzed overnight at 4° C. After clarification of the dialyzed sample by centrifugation at 9000 φm for 10 minutes, the sample was loaded onto a 400 ml pre-equilibrated P-ll column (4.4 X 27 cm) at a flow rate of 2.5 ml/ minutes The column was washed with 1 L of P-ll buffer. Elution of the enzyme was achieved by application of a 2800 ml linear gradient of P-11 buffer versus P-ll buffer at 300 mM KPi, pH 6.5. The fractions were assayed for Bst polymerase activity as described below and pooled. The P-11 pool was dialyzed for at least 3 hours versus 14 liters of Buffer B, (20 mM Tris-HCl, pH 7.5, 0.5 mM EDTA, 1.0 mM DTT, 10 mM The dialyzed P- 11 pool was harvested and adjusted to a conductivity of less than 2.5 milli-mhos with cold H 2 0 and adjusted to a pH of 7.5 with 2N NaOH. The sample was applied to a 200 ml MBR Blue column (Cibacron Blue) and washed with 600 ml of Buffer B. The enzyme was eluted by application of a 2800 ml linear gradient of Buffer B versus Buffer B at 1.80 M NaCI. The fractions were assayed for Bst polymerase activity as described below and pooled. This MBR Blue pool was dialyzed against 14 L of Buffer B for at least 3 hours. The dialyzed MBR Blue pool was harvested and adjusted to a conductivity of less than 2.5 milli-mhos with cold H 2 0 and a pH of 7.5 with 2N NaOH. The sample was applied to a 100 ml Heparin- Agarose column and washed with 200 ml of Buffer B. The enzyme was eluted by application of a 1500 ml linear gradient of Buffer B versus Buffer B at 0.75 M NaCI. The fractions were assayed for Bst polymerase activity as described below and pooled. The Heparin- Agarose pool was dialyzed against 14 L of Buffer B for at least 3 hours. The dialyzed Heparin-agarose pool was harvested and adjusted to a conductivity of less than 2.5 milli-mhos with cold H 2 0 and a pH of 7.5 with 2N NaOH. The sample was filtered through a 0.8/0.2 μm unit and applied to a pre¬ packed Pharmacia 10x10 HP Q-Sepharose column. After a 100 ml wash with Buffer B, the enzyme was eluted with a linear 900 ml gradient of Buffer B versus Buffer B at 0.25 M NaCI. The fractions were assayed for Bst polymerase activity as described below and pooled. The HP Q-Sepharose pool was used to isolate native Bst DNA pol I for N-terminal amino acid sequencing as described below. An aliquot of the HP-Q Sepharose pool was digested with subtilisin (Karlsberg type, Sigma ) to determine optimum digestion time to yield a 65 kD product as seen by 12.5 % SDS-PAGE while retaining at least 80% activity. After treating the entire pool, the 65 kD fragment was purified from other fragments by application onto a pre-packed Pharmacia 5x5 Mono Q column. After a 10 ml wash with Buffer B, the enzyme was eluted with a 60 ml linear gradient of Buffer B versus Buffer B at 0.25 M NaCI. The fractions were assayed for activity as described below and for purity by 12.5 % SDS-PAGE before pooling. The pool was dialyzed against 2 L of final storage buffer (20 mM Kpi, pH 6.8, 1.0 mM DTT and 50% glycerol). To quantify DNA polymerase activity, a DNA polymerase activity assay was performed using a modification of a protocol described by Kaledin et al. , Biokhimiya 45: 644-651 (1980). Reactions were performed in a 50 μl reaction mixture of 50 mM Tris-HCl, pH 8.6 at 23° C; 100 mM KCl; 10 mM MgCl 2 ; 1 mM DTT; 0.2 mM each dCTP, dGTP, dTTP, pH 7.0; 0.2 mM [α 33 P]dATP, pH 7.0, 10 μCi/ml; 50 μg BSA; 15 μg activated DNA (Baril et al. Nucleic Acids Res. 5:2641-2653 (1977)); and 5 μl of diluted enzyme. Dilution Buffer consisted of: 50 mM Tris-HCl, pH 8.0 at 23° C, 1 mM DTT, 1 mM EDTA, 0.1 % Brij-35 and 10% (v/v) glycerol. For control purposes, native Bst exo- DNA polymerase with known activity was diluted to 20, 40 and 80 units/ml. Two reactions were run without enzyme as negative controls for background subtraction. A 45 μl reaction mixture, less enzyme, was prepared and the reaction was started by the addition of 5.0 μl of enzyme. After 10 minutes of incubation at 60° C, 40 μl was removed and added to 50 μl of yeast RNA co-precipitant (10 mg/ml in 0.1 M sodium acetate, pH 5.0). One ml of 10.0% trichloracetic acid (TCA), 2.0% sodium pyrophophate was added and the samples were allowed to precipitate on ice for at least 10 minutes. The mixture was filtered on a glass fiber filter disc and washed first with 5 % TCA/ 2 % sodium pyrophosphate, and then with 100% Reagent grade ethanol (Mallinkrodt). The dried filter disc was counted in 5.0 ml of scintillation fluid. One unit of activity is defined as the amount of enzyme required to incorporate 10 nmol of total nucleotide into acid insoluble form in 30 minutes at 60° C in this standard activity assay. The protein concentration was determined by Bradford Protein Assay (BioRad, Hercules, CA). The calculated DNA polymerase specific activity for the nBst exo- fragment was approximately 50,000 U/mg. B. To obtain amino acid sequence information from the isolated and purified native B. stearothermophilus DNA polymerase, about 50 μg of the native Bst DNA polymerase holoenzyme was separated on a preparative 7.5% SDS-polyacrylamide gel, blotted onto PVDF membrane and stained with amido black as described by Matsudaira, J. Biol. Chem. 262: 10035-10038 (1987). The major band at approximately 92 kD was excised and sequenced using an Applied Biosystems (Foster City, CA) 477A Protein Sequencer. The following sequence was found: Met,Lys,Lys,Lys,LeuNal,Leu,Ile,Asp,Gly,Ser,SerNal,Ala,Tyr,Arg. This sequence has been determined to map at positions 1 to 16 in the deduced amino acid sequence of the Bst DΝA pol I holoenzyme (SEQ ID NO: 2) as shown in FIGURE 2b. As explained in Example 3, knowledge of this amino acid sequence information was used to isolate the B. stearothermophilus DNA polymerase I gene. EXAMPLE 2 Construction and Amplification of a Bacillus stearothermophilus genomic DNA library A Bacillus stearothermophilus genomic library was constructed in phage λ Dash II and amplified in the following manner. Genomic DNA from the Bacillus stearothermophilus strain 10, cultured overnight as described above, was isolated according to the procedure described by Ausubel et al, Current Protocols in Molecular Biology, Greene Publishing Associates and John Wiley & Sons, New York (1990). In general, yields of genomic DNA between 100 and 900 μg were obtained from the cell pellet of about 1.5 ml of culture. One microgram of Bacillus stearothermophilus strain 10 genomic DNA was digested with 1.0 unit of Sau 3A I in a total volume of 100 μl. At 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 minutes, 5μl samples were removed and the enzyme was inactivated at 70° C for 10 minutes From each time point, aliquots were analyzed on a 1.2 % agarose/TBE gel. The 45 minutes time point displayed the desired size distribution or 3 kb to 20 kb fragments. A scale-up digestion was performed using 20 μg of Bacillus stearothermophilus strain 10 genomic DNA, 20.0 units of Sau 3A I and a total volume of 250 μl. Approximately 2600 pmoles of 5 '-ends of Sau 3 A I-digested B. stearothermophilus DNA were treated with 20 units of calf intestinal alkaline phosphatase for 30 minutes at 37° C, (CIP; Ausubel et al. (1990). The Sau 3A I digested, CIP treated B. stearothermophilus DNA was extracted with phenol/chloroform and chloroform, ethanol precipitated, pelleted, and washed in 70% ethanol. Two microliters of this DNA solution was visualized on a 1.2% agarose/TBE gel to check for amount and integrity. The pellet was stored at -20° C. This DNA is referred to as "CIP DNA". The B. stearothermophilus library was constructed as described in the manufacturers instructions using the phage λ DASH π / Bam HI Cloning Kit (Stratagene, LaJolla, CA). The pME/Bam HI test insert (0.3 μg) was run in parallel as a control. The ligation mixture was incubated overnight at 4° C. One microliter of the ligation samples were run on a 1.2% agarose/TBE gel to check ligation efficiency. The B. stearothermophilus DNA ligated to λ DASH II arms was packaged in vitro using the Gigapack JJ Gold Packaging Extract from Stratagene, according to the manufacturer's conditions. Control DNA provided by the manufacturer was also packaged. Following the protocol provided by Stratagene with the Lambda DASH π/BamHI Vector Kit, host bacteria were prepared: VCS 257 for wild type phage, SRB and SRB(P2) for the B. stearothermophilus library and the control. VCS 257 was grown in NZY+ maltose medium and SRB, and SRB(P2) were grown in NZY + maltose medium with 50 μg/ml kanamycin at 37° C for 6 hours. After centrifugation of the cells at 2800 x g for 10 minutes, the cells were resuspended with sterile 10 mM MgS0 4 to give an Aβoo,,,-, of 0.5. One microliter of the following dilutions were added to 200 μl of SRB cells. Two 1:10 serial dilutions were prepared from the control phage and the CIP Bst DNA library. Ten microliters of undiluted, 1 : 10, and 1 : 100 dilutions of phage were added to 200 μl of SRB cells. The cells were incubated with light shaking at 37° C for 15 minutes and after the addition of top agar, the mixture was poured onto LB/M/M plates. (15g agar, lOg tryptone, 5g yeast extract, lOg NaCI, 2.64g MgS0 4 , 2g maltose to 1000ml with deionized H 2 0, pH to 7.0 with 0.1N NaOH) The plates were incubated overnight at 37° C. After overnight incubation at 37° C, titer was determined by average plaque forming units per ml (pfu/ml). The B. stearothermophilus library was amplified using techniques described by Ausubel et al. , Current Protocols in Molecular Biology (1990). The primary and amplified libraries were titered on SRB cells. (TABLE 2). The amplified library was stored at 4° C. EXAMPLE 3 Cloning and Sequencing a Bst DNA Pol I Gene-Specific Probe Fragment The DNA sequences from Thermus species and Bacillus caldotenax were used to design the synthesis of two primers for the amplification of a B. stearothermophilus DNA polymerase gene fragment: primer 757 (25mer) (SEQ ID NO: 4) and primer 830 (23mer) (SEQ ID NO: 5) (synthesized by Synthetic Genetics, San Diego, CA). Primer 757 corresponds to nucleotides 2584-2609 of SEQ ID NO: 1 of the B. stearothermophilus DNA polymerase coding sequence, top strand (i.e., to a portion of the DNA sequence that encodes a portion of the Bst DNA pol I protein that is homologous to Peptide 1). Primer 830 hybridizes to the 3 '-end of the Bst DNA pol I gene at position 2794-2816 and has a sequence identical to the bottom strand. The specificity of the primers 757 (SEQ ID NO: 4) and 830 (SEQ ID NO: 5) for B. stearothermophilus genomic DNA was demonstrated by screening Southern blot transfers of genomic DNA from B. stearothermophilus, T. flavus, T. rubrum, and T. thermophilus. Both the biotin- 11 dUTP end labelled primers 757 (SEQ ID NO: 4) and 830 (SEQ ID NO: 5) hybridized strongly to B. stearothermophilus genomic DNA under high stringent conditions (52° C, 0.1 x SSC, 1.0% SDS). Additionally, the end labelled primer 830 (SEQ ID NO: 5) detected blots of digested B. stearothermophilus genomic DNA. Specifically, Bst genomic DNA was digested with different restriction enzymes, such as Eco RI and Sau 3A I for 10, 15, 30 and 60 minutes. 500 ng/lane of restricted DNA was electrophoresed on a 0.7% agarose gel. A Southern transfer of this gel onto Hybond-N was prepared. The denatured DNA on the Southern blots was UV- cross-linked to the filter for 3.0 minutes. Duplicate blots were prehybridized in 2 ml of hybridization buffer (50% deionized formamide, 7% SDS, 120 mM Na phosphate, pH 7.2, 250 mM NaCI, 1 mM EDTA and 1 mM cetyldimethylethylammonium bromide and 20μl of denatured salmon sperm DNA at 10 mg/ml) in a heat-sealed plastic bag at 52° C for 1 hour. Approximately 250 ng of the end labelled probe was added per blot and incubated overnight at 50° C. The Southern blots with the labelled probe were incubated with low stringency buffer (lx SSC, 1.0% SDS) for 1 hour at 52° C, washed with high stringency buffer (O.lx SSC, 1.0% SDS) for 1 hour at 52° C, dried, and then detected by avidin-alkaline phosphatase conjugates. An amplification reaction (100 μl) was performed containing 0.2 mM dNTPs, lx Cetus AmpliTaq Reaction Buffer (10 mM Tris-HCl, pH 8.4, 500 mM KCl, 15 mM MgCl 2 , 0.01 % gelatin), 30 pmoles of each primer, 200 ng of Eco RI digested Bst genomic DNA, 5 units of AmpliTaq* DNA polymerase (Perkin Elmer No. N801-0060), and a 50 μl light mineral oil overlay. The amplification was performed in thirty cycles of 91 ° C for 20 seconds, 55° C for 20 seconds and 72° C for 2 minutes. Under these conditions, primer set 757-830 gave a single amplification product from Bst genomic DNA. The calculated length of 232 base pairs for the amplification product coincided with the observed mobility in 0.7% agarose/TBE gels. This fragment was ligated into pTZ18U as described by the Pharmacia Sure Clone Kit. Clones, which were sequenced as described in the Sequal DNA Sequencing Kit (CHIMERx, Madison, WI), were shown to have homology to Bacillus caldotenax DNA polymerase. Hence the 232 bp fragment was used to make probes as described in the ZEPTO™ labelling kit manual (CHIMERx, Madison, WI). EXAMPLE 4 Preparation of Gene-Specific Probes and Screening of the Bacillus stearothermophilus Genomic Library for Clones Containing B. stearothermophilus DNA Pol I Gene The 757-830 fragment described in Example 3 was used to isolate the Bst DNA pol I gene from the Bst genomic library (See Example 2). Using the cloned 757-830 fragment as template and primers 757 (SEQ ID NO: 4) and 830 (SEQ ID NO: 5), the 757-830 fragment was first amplified by PCR as described above to obtain larger quantities of the fragment for use in preparing probes to screen the Bst genomic library. The amplified 757-830 fragment, migrating at about 240 bp, was cut out of a preparative 0.7% agarose TBE gel, eluted, phenol-chloroform extracted and ethanol precipitated. Approximately 1 μg of the 757-830 fragment was digested with Cvi JI* (CHIMERx, Madison, WI) to generate sequence specific primers for labelling as described in the ZEPTO™ labelling manual. Each set of duplicate plaque lifts or targets was screened using the 757-830 fragment labelled with [ 33 P]dCTP. Digestion with Cvi JI*, as well as this method of labeling, is described in a co-owned, copending U.S. Patent Application Ser. No. 08/217,459, filed March 25, 1994, entitled "Methods and Materials for Restriction Endonuclease Applications, "incorporated herein by reference in its entirety. The PCT counteφart of this application, filed March 24, 1994, is PCT App. No. US94/03246. The 757-830 intact fragment was labelled with [ 33 P]dCTP as described in the ZEPTO™ labeling manual; a total of 6 x IO 7 cpm of [α 33 P]dCTP at 1 x IO 9 cpm/μg was incoφorated. For probes, 1-5 x 10 6 cpm of radio-labelled DNA was added to each plaque lift. To screen the amplified B. stearothermophilus genomic library (Example 2), the phage library was plated on two plates each at 10 s plaque- forming units (pfu)/100 mm 2XTY plates. Duplicate plaque lifts on Hybond N - 25 - from each plate were obtained and prepared for hybridization by methods well known in the art (Sambrook, Fritsch, and Maniatis, Molecular Cloning, A Laboratory Manual, 2nd ed. , Cold Spring Harbor Press, Cold Spring Harbor, NY (1989)). The DNA on the plaque lifts was UV-cross-linked to the Hybond N for 3 minutes and placed in a heat-sealed plastic bag for prehybridization as described above. 1-5 x 10* cpm of the [α PJdCTP 757-830 TCL probe were added to the duplicate filters. The filters were incubated overnight at 52° C and washed the next day with low and high stringency buffers as described above. Four positive plaques (hybridizing with the labelled probes) out of 10 3 pfu from the amplified CIP Bst DNA library were detected on the duplicate plaque lifts. These four stocks of phage designated XB211, XB411, XB511, and XB711 were grown to 5 x 10 s pfu/2XTY plate, 5 plates per stock. The phages were eluted from the plates by a standard protocol (Sambrook et al. (1989)). The eluant was treated with 20μg/ml DNAse and 50μg/ml RNase A for 1 hr. at 37° C and extracted with both phenol-chloroform and chloroform. The DNA was ethanol-precipitated, pelleted, washed with ethanol, resuspended in 1 ml of TE buffer (10 mM Tris pH 8.0, 1 mM EDTA) and purified as described in the Lambda DNA Purification Kit (CHIMERx, Madison, WT). Phage DNA XB211 was restriction-digested with EcoRI, XB411 with Sal I, λB511 and XB71 1 with Xbal. After digestion, each phage DNA was separately ligated into pTZ18U (Mead et al , Protein Engineering 1: 67-74 (1986)) that had been similarly digested and dephosphorylated to minimize self- ligation. The plasmid derivatives of these clones were designated pB2, pB4S, pB5X and pB7X respectively. After transformation, clones were verified by sequence homology to Bca sequence data and/or by hybridization with labelled 757 or 830 primers under the hybridization conditions described previously. This information confirmed that the clones contained authentic B. stearothermophilus strain 10 DNA polymerase gene sequence and their respective orientations. EXAMPLE 5 Sequencing the B. stearothermophilus DNA Polymerase I Gene and cloning of the B. stearothermophilus DNA Polymerase I and exo- Fragment A primer walking sequencing strategy was employed to obtain the sequence of the Bst DNA pol I gene (Sambrook et al. , Molecular Cloning. A Laboratory Manual. Cold Spring Harbor Press (1989)). To obtain sequence information from the clones by primer walking, primers homologous or complimentary to the ends of previously determined sequences were synthesized as described above and used in additional sequencing reactions (TABLE 2, Bseql- 25). By repeating this process, the entire length of the gene was sequentially sequenced. Clones λ411 and λ511 were confirmed to include both the 5' and 3' Bst DNA pol I coding sequences as described in Example 4. Sequence analysis of these clones provided advantageous restriction endonuclease sites which were used to join the 5 ' and 3 ' sequences to obtain a complete Bst DNA pol I encoding insert. In particular, a shared singular Sal I site and an available Neo I site were used. An expression vector designated pPR was digested with BspH I for subsequent ligation to the complete Bst DNA pol I encoding insert. BspH I restricted sites can serve as compatible cohesive ends for ligation with Neo I restricted sites due to identical overhang termini. After transformation and confirmation of the Bst pol I insert, expression of the encoded Bst DNA pol I polypeptide was successfully achieved utilizing the temperature dependant pR operator/promoter region. Sequence information determined that a singular Dra I site located at the junction of the nucleotide sequence encoding the 5 '-3' exonuclease domain and the 3 '-5' proofreading domain. Therefore, restriction with this enzyme and subsequent religation yielded an intact exo- fragment in frame with the pR operator/promoter. After transformation and confirmation of the exo- fragment insert, expression of the encoded Bst exo- fragment polypeptide was successfully achieved. The DNA sequence of the B. stearothermophilus DNA polymerase I gene, the exo- fragment and the flanking sequences (italicized) are given in FIGURE 2a. Also, the translated deduced amino acid sequence is given in FIGURE 2b for the holo and exo- fragments. The DNA sequence of 3317 b.p. (SEQ ID NO: 1) for the Bst DNA polymerase I gene has been determined, of which 2631 bases (i.e., nucleotides 316-2946 of (SEQ ID NO: 1)) are deduced to encode a nBst pol I polypeptide (SEQ ID NO: 2) of 877 amino acids (including the TAA stop codon), and of which nucleotides 1135-2946 encode the Bst exo- fragment (including the TAA stop codon). EXAMPLE 6 Cloning and Expression of the exo- Fragment of B. stearothermophilus DNA Polymerase I Expression studies using plasmid pPEK 5, which contains the 3' two-thirds of the DNA polymerase I gene fused to the pR operator/promoter, was performed. As deduced from the DNA sequence, the putative first amino acid encoded by the insert of plasmid pPEK 5 corresponds to Met in FIGURE 2b (bold type), i.e. , residue 285 of SEQ ID NO: 2. It was hypothesized that the insert in pPEK 5 would encode a fragment of DNA polymerase I lacking the 5 '-3' exonuclease domain (the Bst exo- fragment) due to the absence of the 5' one-third portion of this gene. E. coli DH5αF' [pPEK 5] was grown in a 4 liter Erhlenmeyer flask in LB medium (Sambrook et al. , Molecular Cloning, A Laboratory Manual, 2nd ed. (1989)) supplemented with 50 μg/ml ampicillin with vigorous aeration at 30° C. At O.D.βoo mn = 1.0, the culture was induced by transferring to a 42° C incubator and aUowed to grow an additional 2 hours. The culture was spun down in a Shaφles centrifuge and stored frozen at -70° C. One and one-quarter grams of E. coli [pPEK 5] were thawed in 10 ml of lysis buffer (20 mM Tris-HCl pH 8.0, 0.5 mM EDTA, 50 mM NaCI, 1 mM DTT, 0.02% Brij-35, 0.1 mg/ml lysozyme, and 1.0 mM PMSF). The cell suspension was allowed to stir at room temperature for 30 minutes. The lysate was heated to 65° C for 15 minutes then cooled on ice for 5 minutes. After centrifugation at 12000 φm for 30 minutes to remove cell debris and denatured proteins, the supernatant was filtered through a 0.8/0.2 μm filtration unit and applied to a prepacked Pharmacia 5x5 Mono Q column equilibrated in Buffer A: 20 mM Tris-HCl, pH 7.5, 1.0 mM DTT, 0.5 mM EDTA, 10 mM MgCl 2 , and 0.02% Brij-35. After washing with 20 ml of Buffer A, the enzyme was eluted with a 90 ml linear gradient of Buffer A versus Buffer A at 0.4 M NaCI. The 1 ml fractions were assayed for activity and pooled accordingly. The Mono Q pool was diluted with Buffer A at pH 6.0 to reduce the conductivity to 5 milli-mhos and lower the pH to 6.0. The pool was then applied to a prepacked Pharmacia 5x5 Mono S column equilibrated in Buffer A at pH 6.0. After washing with 20 ml of equilibration buffer, the enzyme was eluted with a 90 ml linear gradient of Buffer A, pH 6.0 to Buffer A, pH 6.0 at 0.75 M NaCI. The 1 ml fractions were assayed for activity and purity measured by 12.5 % SDS-PAGE and pooled accordingly. The Mono S pool was dialyzed overnight at 4° C versus 1 liter of Final Storage Buffer (20 mM Kpi, pH 6.5, 1.0 mM DTT, and 50% glycerol). The protein concentration was deteπnined by the Bradford Protein Assay (BioRad, Hercules, CA). The calculated DNA polymerase specific activity for the rBst exo- fragment was approximately 150,000 U/mg. Using conventional lyophilization techniques, a solution containing the rBst exo- fragment may be lyophilized for storage. Preferably, the lyophilization solution includes one or more stabilizing agents such as an albumin, trehalose, maltitol, sucrose, sorbitol and ficoll. Typically, the lyophilization solution is aqueous. However, for prolonged storage at low temperatures, all or part of the solvent in the lyophilization solution may be glycerol. EXAMPLE 7 Characterization of B. stearothermophilus DNA Polymerase I Exonuclease Activities The purity and molecular weight of the B. stearothermophilus DNA polymerase exo- fragments and the rBca exo- fragment was estimated by SDS-polyacrylamide gel electrophoresis (i.e. , SDS-PAGE) using the Pharmacia PhastSystem (Piscataway, NJ). FIGURE 8 photographically depicts the purity of purified native Bacillus stearothermophilus holoenzyme (nBst holo), native Bacillus stearothermophilus exo- fragment (nBst exo-), recombinant Bacillus stearothermophilus exo- fragment (rBst exo-), and commercially available recombinant Bacillus caldotenax exo- fragment (rBca exo-, PanVera, Madison, WI) on a 20.0% SDS-PAGE gel stained with silver. In FIGURE 8, Lanes 1-6 contain the following polypeptides: Lane 1 , Low Molecular Weight Markers; Lane 2, nBst holo; Lane 3, nBst exo-; Lane 4, rBst exo-; Lane 5, rBca exo-; and Lane 6, Low Molecular Weight Markers. Assays were performed to determine intrinsic/extrinsic exonuclease, endonuclease, and DNAse activities of the native Bst exo- fragment, recombinant Bst exo- fragment and for recombinant B. caldotenax exo- fragment (PanVera, Madison, WT). The protocols are described below and the results are summarized in TABLE 3A. A 3 '-5' exonuclease activity assay was performed in a final volume of 10 μl containing 50 mM Tris-HCl, pH 7.6, 10 mM MgCl 2 , 1 mM DTT, 0.15 μg of [3'- 3 H] dCTP and dGTP labelled λ DNA/Taq I fragments and 5, 10 and 20 units of enzyme. Each sample was overlaid with 10 μl of light mineral oil and incubated at 60° C for 1 hour. The reaction was terminated by the addition of 50 μl yeast t-RNA and 200 μl of 10% TCA. After incubation for 10 minutes on ice, the samples were centrifuged for 7 minutes in a microcentrifuge. Supernatant (200 μl) was removed and added to 6 ml of scintillation fluid and counted in a scintillation counter. The results are presented in TABLE 3A as the slope of the % -label released per unit of enzyme. An average of about 5800 cpm per sample was observed for the Bst exo- fragments compared to about 180 cpm per sample for the rBca exo- fragment. A 5 '-3' assay was performed in a manner identical to the 3 '-5' exonuclease assay, except for the use of [5'- 32 P] λ DNA/ Haem fragments as substrate. Double-stranded and single stranded DNAse assays were performed using the protocol for the 3 '-5' exonuclease assay, except for the use of f^P] λ DNA as substrate. The DNA was treated for 3 minutes at 100° C and immediately chilled on ice prior to assaying for single stranded DNAse activity. An assay for endonuclease activity was performed in a final volume of 10 μl containing: 50 mM Tris-HCl, pH 7.6, 10 mM MgCl 2 , 1 mM β- mercaptoethanol, 0.5 μg pBR322, and 5, 10 or 20 units of enzyme. Each sample was overlaid with 10 μl of light mineral oil and incubated at 60° C for 1 hour. Two microliters of 0.25 % bromophenol blue, 1 mM EDTA, and 40% sucrose was added to stop the reaction. After a short centrifugation, 6 μl of the bottom layer was removed and electrophoresed on 1.5% agarose gels in IX TBE. The mobility change from the supercoiled to the linear form of pBR322 was recorded. EXAMPLE 8 Comparison of the Polymerase Activities Of B. stearothermophilus and B. caldotenax DNA Polymerases Biological properties of native B. stearothermophilus DNA pol I exo- fragment (nBst exo-, lot #30419; CHIMERx, Inc., Cat. No. 1112-01, Madison, WI); recombinant B. stearothermophilus exo- fragment (rBst exo-) purified from E. coli [pPEK 5]; and recombinant B. caldotenax exo- fragment (rBca exo-, PanVera Inc., Madison, WT) were compared using a number of protocols described below. The molecular weights and purities of the preparations of the various enzymes were estimated by acrylamide gel electrophoresis utilizing the Pharmacia PhastSystem (Piscataway, NJ) for electrophoresis and silver staining. A comparison of the apparent molecular weights estimated from 12.5% and 20.0% acrylamide gels and the calculated molecular weights derived from available sequence data is given in TABLE 3B. A purity of greater than 90% was estimated for the native and recombinant Bst exo- fragments and less than 80% estimated for the rBca exo- fragment analyzed. Using the Pharmacia PhastSystem, the polymerases and standards were subjected to isoelectric focusing. The experimentally derived pi values of the samples were compared to values calculated from derived amino acid sequence information. The results are given in TABLE 4. The relative DNA polymerase activities of the enzymes were assayed at 60° C at different pH values. The pH of selected buffers were adjusted at 23° C, to permit direct comparison to published results. TABLE 5A shows the measured pH values at 60° C for IX buffers which were titrated at 23° C. Unless otherwise indicated, pH values reported herein were adjusted at about 23° C. The activity assays were performed in a 100 μl (final volume) reaction mixture, containing 0.1 mM dCTP, dTTP, dGTP, [α 33 P]dATP, 0.3 mg/ml activated calf thymus DNA and 0.5 mg/ml BSA in a set of buffers containing: 50 mM KCl, 1 mM DTT, 10 mM MgCl 2 and 50 mM of one of three buffering compounds: PIPES, Tris or Triethylamine. A dilution to 0.1 units/μl of each polymerase enzyme were prepared, and 5 μl of these dilutions was added to the reaction mixture, followed by incubation at 60° C for 10 minutes Independent experiments were performed and curves constructed. FIGURE 3 graphically depicts the relative activities of the enzymes studied, calculated as the ratio of counts per minute (corrected for background and enzyme dilution) at a given pH to counts per minute at the maximum value for that enzyme. The optimal ranges (>90% activity) for the three enzymes tested are provided in TABLE 5B. These values are about 1 pH unit higher than for buffers measured at 60° C (see TABLE 5 A). The pH protocol described above was modified to determine the influence of MgCl 2 concentration on the activities of the DNA polymerases. The reaction buffers included 50 mM Tris-HCl pH 8.3 (23° C) and MgCl 2 concentrations from 0.0 to 20.0 mM. Independent experiments were performed and curves were constructed (FIGURE 4) showing the relative activity of nBst exo- fragment, rBst exo- fragment, and rBca exo- fragment. The optimal activity for all three enzymes is at a 1.0 mM MgCl 2 final concentration. The above protocol was modified to determine the influence of MnCl 2 concentration on the activities of the DNA polymerases (in the absence of magnesium ions). The reaction buffers included MnCl 2 concentrations from 0.0 to 5.0 mM. Due to the precipitation of oxidation products (Mn0 2 ) of MnCl 2 , the MnCl 2 solution was prepared just prior to the assay. Independent experiments were performed and a curve was constructed (FIGURE 5a) showing relative activity of the enzymes. The optimal activities for the native Bst, recombinant Bst exo- fragment and recombinant Bca exo- fragments were at 0.5 mM MnCl 2 . The temperature optima of the polymerase enzymes (nBst exo-, rBst exo- and rBca exo-) were determined by incubating 1.0 unit of enzyme for 10 minutes at 37, 50, 60, 65, 70, 75, and 80° C, in a 100 μl DNA polymerase activity assay as described previously. FIGURE 6 depicts the percent relative polymerase activity, calculated as described above, as a function of temperature. As reflected in FIGURE 6, the temperature optima was 70° C for all three enzymes; whereas at 80° C there was approximately 20% of activity remaining. EXAMPLE 9 Comparison of Reverse Transcriptase Activity of the DNA Polymerase Enzymes The RNA dependant DNA polymerase (reverse transcriptase, R.T.) activities of native and recombinant Bst exo- fragments and the recombinant Bca exo- fragment were compared. After a determination of the MnClj optima for each enzyme (FIGURE 5b), 1.0 unit of enzyme was compared in R.T. assays with either a Poly rA:dT 30 or mRNA substrate. In this assay, product quantity was determined by glass filter precipitation as previously described, and product quality by autoradiograph of a 1.2% TBE agarose gel containing reaction products. A. Using a modification of a procedure described by Meyers, T.W. and Gelfand, D.H., Biochemistry 30: 7661-7666 (1991), the reverse transcriptase activity of the native and recombinant Bst exo- fragments and the recombinant Bca exo- fragments were compared in the presence of 0 mM to 5.0 mM MnCl 2 . The reaction (50μl) contained lx Reaction Buffer (50 mM Tris-HCl, pH 8.6, 100 mM KCl), 1.0 mM DTT, 0.2 mM PolyA:dT S0 , 0.5 mM [3'- 3 H] dTTP (80 μCi/ml), 0.5 units enzyme, and the balance H 2 0. The reaction mix without enzyme was preincubated at 50° C for 1 minute prior to enzyme addition. The reaction was incubated at 50° C for 10 minutes after which 40 μl was removed and filter precipitated as previously described. As seen in FIGURE 5b, the MnCl 2 optima for all three enzymes is 1.0 mM. Also, the ratio of RNA dependant RNA polymerase to DNA dependant DNA polymerase was about 1.4 and 2.0 for native and recombinant Bst exo- fragments respectively, and about 0.8 for the recombinant Bca exo- fragment. B. A variation of the assay described above at 1.0 mM MnCl 2 was used to compare quantity and quality of R.T. activities using 1.0 unit of each enzyme by filter precipitation and autoradiography respectively. A 15 μl sample was removed after 1 hour of incubation at 50° C for filter precipitation as previously described. Another 15μl sample was remove and mixed with 5 μl of stop solution (95% deionized formamide, 10 mM EDTA, 0.05% xylene cyanole FF, 0.05% bromopbenol blue ) and loaded onto a 1.2 % TBE agarose gel along with 0.5 μg of [γ 33 P] labelled 1 kb Perfect Ladder (CHIMERx) and electrophoresed at 100 volts for approximately 2 hours. The gel was then dried in a LabConco gel drier for 30 minutes. The dried gel was then autoradiographed at -70° C for 3 days and developed to visualize bands. Messenger RNA was isolated from approximately 1.0 gram of hybridoma cells (Ausubel et al. , Current Protocols in Molecular Biology, Greene Publishing Associates and John Wiley & Sons, New York (1990)). The following substitutions in the above assay described in Example 9 A were made; 1.0 μg of mRNA primed with 0.5 mM Oligo dT J0 (SuperTech, Bethesda, MD) for PolyrA:dT J0 and 0.5 mM mixed dNTPs with 0.05 mCi/ml [a 33 ?] dATP for [3'- 3 H] dTTP. When using polyrA:dT j0 as substrate the only substitution was 5 mM [or^P] dTTP for p'-Η] dTTP. Precipitable counts minus background for each enzyme with each substrate are shown in TABLE 6. As seen in FIGURE 7 both the native and recombinant Bst exo- fragments show an increase in product quantity and length as compared to products made by the recombinant Bca exo- fragment. EXAMPLE IO DNA Sequencing Using nBst, rBst and rBca Exo- Fragments Native and recombinant Bst exo- fragment and recombinant Bca exo- fragment (PanVera, Madison) were tested for their performance in DNA sequencing with internal labelling using ssDNA and dsDNA template by substituting nBst exo-, rBst exo- or rBca exo- in the Bst DNA Sequencing Kit (BioRad, Hercules, CA). A. The ssDNA sequencing reactions for the native and recombinant Bst exo- fragments and the recombinant Bca exo-fragment were performed using 5.0 units of enzyme. Briefly, a reaction cocktail (12 μl) was prepared containing 2.0 μl ssM13mpl8 DNA (approx. 0.4 μg), 5.0 μl 5x Bst Sequencing Reaction buffer (100 mM Tris-HCl, pH 8.6, 100 mM MgCl 2 ), l.Oμl [α^PJ-dATP (10 mCi/ml), 5.0 units of enzyme (2.0 μl of a 2.5 unit/μl solution) and balance H 2 0. Four d/ddNTP mixtures were also prepared (A mix: 0.62 μM dATP, 62 μM dCTP, 62 μM dGTP, 62 μM dTTP, 25 μM ddATP; C mix: 0.8 μM dATP, 8 μM dCTP, 80 μM dGTP, 80 μM dTTP, 50 μM ddCTP; G mix: 0.8 μM dATP, 80 μM dCTP, 4 μM dGTP, 80 μM dTTP, 75 μM ddGTP; T mix: 0.8 μM dATP, 80 μM dCTP, 80 μM dGTP, 8 μM dTTP, 150 μM ddTTP). The sequencing reactions were performed by mixing 2.5 μl of reaction cocktail with 2 μl of the appropriate pre-incubated d/ddNTP mixture and incubating at 65° C for 2 minutes. After this incubation, 2 μl of IX Chase Solution (0.5 mM each dNTP) was added, gently mixed and the reaction allowed to incubate for an additional 2.0 minutes. The reaction was terminated by adding 4.0 μl of stop solution (95 % deionized formamide, 10 mM EDTA, 0.05% xylene cyanole FF, 0.05% bromophenol blue) and placing the reaction on ice. The reactions were heated at 90° C for 3 minutes just prior to loading onto a 6% sequencing gel. Two microliters of each sample was loaded and electrophoresed at 2000 volts for 1.5 hours. The gel was autoradiographed and analyzed. FIGURE 9A photographically depicts a portion of a sequencing gel showing the same DNA sequence for all enzymes used. Very little background was observed with all three enzymes and an increase in intensity was seen with the Bst enzymes. B. The utility of recombinant Bst exo-fragment and native Bst exo- fragment for sequencing with internal labeling using double-stranded DNA template was demonstrated in a sequencing reaction in which a [α 33 P]-dATP labeling protocol and double stranded pUC19 template were used. To promote efficient priming, 2 ug (18 μl) of pUC19 double- stranded DNA template was denatured by adding 2 μl of 2 M NaOH, and incubating for 5 minutes at room temperature. The reaction was neutralized by adding 2 μl of 2 M sodium acetate, pH 4.6, precipitated, 70% EtOH washed, air- dried, and resuspended in 7 μl deionized water. In addition, reaction cocktails without enzyme were heated to 70° C for 10 minutes then allowed to cool at room temperature for 10 minutes. For each enzyme, a 12 μl extension/labeling sequencing reaction was performed in the same manner as described in EXAMPLE 10A except for 2 μg denatured pUC18 dsDNA in substitution for M13mpl8 ssDNA. Each reaction was heated at 90° C for 3 minutes immediately prior to loading 2 μl onto a 6.0% sequencing gel. Results are depicted in FIGURE 9b. Greater than 150 bases of readable pUC18 sequence with little background is seen. EXAMPLE 11 Comparison of the Processivity of DNA Polymerase Enzymes Using a modification of a procedure described by Tabor et al. , J. Biol Chem. 262: 16212-16223 (1987), the processivity of the native and the recombinant Bst exo- fragments, and recombinant Bca exo- fragment were compared. The "processivity" of a DNA polymerase enzyme is a measure of the rate at which the enzyme moves forward along a template while catalyzing DNA synthesis, i.e., a measure of the speed at which DNA polymerization takes place in the presence of the enzyme. To prepare the assay, a 50 μl reaction cocktail was prepared with 2.5 μg M13 mplδ ssDNA, 10 μl ddATP mix (20 μM dATP; 60 μM each of dCTP, dGTP, and dTTP; 300 μM ddATP), 2.5 μl -y∞P labelled forward sequencing primer (3 μg/μl), 10μl 5x reaction buffer (100 mM Tris-HCl, pH 8.6; 100 mM MgCl 2 ), balance H 2 0. Additionally, dilutions of the native and recombinant Bst exo- fragments, and the rBca exo- fragment were prepared with dilution buffer described in EXAMPLE 1 to create enzyme solutions of 0.25 and 0.025 units/ml. To perform the assay, 7.0 μl of the reaction cocktail were mixed with 2.0 μl of diluted DNA polymerase enzyme. The addition of 0.5 or 0.05 units of exo- fragment per reaction yields approximately 1:100 and 1:1000 enzyme molecule: template molecule ratios respectively. The use of such low polymerase concentrations minimizes the "bumping off" from template by competing polymerase molecules. Reaction mixtures were incubated at 65° C and 2μl samples were removed after 1.0, 3.0 and 6.0 minutes. Reactions were stopped by adding 1.0 μl stop buffer (EDTA/DTT/BromoPhenol Blue/xylene cyanol), heated at 90° C for 3 minutes and loaded onto a 6% polyacrylamide sequencing gels. The gel was electrophoresed for approximately 2 hours and autoradiographed at -70° C. In this assay, a highly processive DNA polymerase enzyme produces strong, slow-mobility (larger) labelled bands on an autoradiograph, whereas a less processive DNA polymerase produces higher-mobility (smaller) fragments and/or bands with less intensity. As seen in FIGURE 10, the lanes designated with an arrow, corresponding to 6.0 minutes of incubation at a 1 : 1000 enzyme to substrate ratio were used to determine processivity. FIGURE 10 shows that rBca exo- produced no visible high molecular weight species in the slowest half of the electrophoretogram. In contrast, the nBst exo- fragment produced more species of both higher and lower molecular weight than did rBca exo-. Further, FIGURE 10 reflects that the rBst exo- fragment exhibited the greatest production of the slower moving high molecular weight DNA molecules. The rBst exo- fragment also produced more of the faster moving lower molecular weight DNA molecules than did rBca exo-. In terms of processivity, rBst exo- > > rBca exo- and rBst exo- > nBst exo-. Based upon FIGURE 10, the comparative processivity from increasing to decreasing order was as follows: recombinant Bst exo- > nBst exo- > rBca exo-. Thus, the rBst exo- fragment (amino acid residues 285-876 of SEQ ID NO: 2) of the present invention exhibited the greatest processivity of the three exo- fragments tested. EXAMPLE 12 Thermophilic Strand Displacement Amplification Using Recombinant B. stearothermophilus exo- Fragment Recombinant Bst exo- fragment was tested for its functional ability in thermophilic strand displacement amplification (SDA) essentially as described by Walker, T.G. Emperical Aspects of Strand Displacement Amplification. Becton Dickenson Research Center, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY (1993). The SDA methodology is also disclosed in U.S. Patent 5,270,184. Improved SDA methods are disclosed in U.S. Patents 5,270,252 and 5,455,166. The disclosures in each of these three patents, as related to SDA methodology, is incorporated herein by reference. Target plasmid, pSK4.3, containing the IS6110 region of Mycobacterium tuberculosis obtained from G.T. Walker, Becton Dickenson, Research Triangle Park, N.C.) was digested with Xba I (MBR, Milwaukee, WI) and serially diluted in human placenta! DNA (Sigma). SDA was performed in 50μl reactions containing 1x10° copies of target plasmid DNA, 500 ng of human placental DNA, 160 units BsoB I (New England Biolabs, ), 8 units of recombinant Bst exo- DNA polymerase, 1.4 mM each dCTPαS, dTTP dGTP, dATP (Pharmacia, Milwaukee, WI), 35 mM K 2 P0 4 pH 7.6, 0.1 mg/ml non- acetylated BSA, 3 mM Tris-HCl, 10 mM MgCl 2 , 11 mM NaCI, 0.3 mM DTT, 4 mM KCl, 4% glycerol, 0.008 mM EDTA, 500 nM of primers SI and S2 and 50 nM of primers Bl and B2 (the KCl, glycerol and EDTA are contributed by the BsoB I storage buffer). Prior to the addition of BsoB I and the rBst exo- DNA polymerase, incomplete samples (35 μl) were denatured by heating for 3 minutes at 100° C followed by 3 minutes at 60° C for primer annealing. BsoB I and Bst DNA polymerase were diluted together to 10.7 units/μl and 0.53 units/μl, respectively, in 15 μl of New England Biolabs Buffer (10 mM Tris-HCl pH 7.9, 10 mM MgCl 2 , 50 mM NaCI, 1 mM DTT). SDA proceeded for 20 minutes at 60° C. Amplification was terminated by heating for 5 minutes at 100" C. A non- SDA control was created by heating a sample in a boiling water bath immediately after enzyme addition. Following SDA, amplified products were detected by DNA polymerase extension of [γ- 32 P]-labelled detector probe hybridized to the central region of the amplified IS6110 target sequence. Five microliters from each completed SDA reaction were added to 5 μl of a reaction mixture containing 47 mM K 2 P0 4 pH 7.6, 0.2 mM each dATP, dCTP, dGTP and dTTP, 7 mM MgCl 2 , 0.1 mg/ml BSA and 0.1 μM [γ- 3I P]-labelled detector probe. The samples were heated for 2 minutes at 100° C followed by 2 minutes at 37° C. The detector probe was extended to a diagnostic length by the addition of 1.0 μl (9 units) of exo-Klenow polymerase (MBR, Milwaukee, WI) and incubation at 37° C for 15 minutes. Eleven microliters of a denaturing stop solution (95% deionized formamide, 10 mM EDTA, 1 mM DTT, 0.05% bromophenol blue/xylene cyanol) was added and 11 μl of each resultant sample was analyzed by denaturing gel electrophoresis and autoradiograph on Kodak X-ARS film for 3 hours. (See FIGURE 11). As depicted in FIGURE 11, amplified product is seen at 59 and 40 bp in the presence of active rBst exo- fragment ("rBst exo-") with undetectable background in the negative control C-C"). Thus, the rBst exo- fragment of the present invention exhibited utility in thermophilic SDA. EXAMPLE 13 Large-Scale Purification of Recombinant Bst exo- Fragment Thirty-five grams of £. coli [pPEK 5] were thawed in 150 ml of lysis buffer (20 mM Tris-HCl pH 8.0, 0.5 mM EDTA, 50 mM NaCI, 1 mM DTT, 0.02% Brij-35, 10 mM MgCl 2 , 0.1 mg/ml lysozyme, and 1.0 mM pheny Imethylsulfonylflouride). The cell suspension was allowed to stir at 4° C for 60 minutes. The homogenous suspension was sonicated for sbc, 30 second bursts with 30 seconds of cooling between each burst. The lysate was then heated to 65° C for 15 minutes to denature the non-thermostable proteins and then cooled on ice for 15 minutes. After centrifugation at 9000 φm for 60 minutes to remove cell debris and denatured proteins, the supernatant was filtered through a 0.8/0.2 μm filtration unit and applied to a Pharmacia 10x10 prepacked Mono Q column equilibrated in Buffer A: 20 mM Tris-HCl, pH 8.0, 1.0 mM DTT, O.lmM EDTA, 10 mM MgCl 2 , and 0.02% Brij-35. After washing with 100 ml of Buffer A, the enzyme was eluted with a 90 ml linear gradient of Buffer A versus Buffer A at 0.4 M NaCI. The 1 ml fractions were assayed for activity and pooled accordingly. The Mono Q pool was dia-filtrated using a 30 kD (cutoff) membrane with Buffer A at pH 6.0 to concentrate and exchange the Buffer to Mono S loading conditions (Buffer A at pH 6.0 and conductivity at less than 10 mmhos). The pool was then applied to a Pharmacia 5x5 prepacked Mono S column equilibrated in Buffer A at pH 6.0. After washing with 20 ml of equilibration buffer, the enzyme was eluted with a 90 ml linear gradient of Buffer A, pH 6.0 to Buffer A, pH 6.0 at 0.3 M NaCI. The 1 ml fractions were assayed for activity and purity was measured by 12.5% SDS-PAGE and pooled accordingly. The Mono S pool was dialyzed overnight a 4° C versus 20 volumes of Final Storage Buffer (20 mM potassium phosphate, pH 6.5, 1.0 mM DTT, and 50% glycerol). The protein concentration was deteπnined by the Bradford Protein Assay (BioRad, Hercules, CA). The calculated DNA polymerase specific activity for the rBst exo- fragment was approximately 150,000 U/mg. The biological activities of the recombinant enzyme purified by the above-described protocol were analyzed using the assays described in preceding Examples. In the endonuclease activity assay described in Example 9, five, ten, and twenty unit challenges resulted in less than 5.0% conversion of supercoiled pBR322 to the linear form. The results of other assays described in EXAMPLE 9 are summarized in TABLE 8: Deposit of Biological Materials: The following plasmids have been deposited with the American Type Culture Collection (ATCC), 12301 Parklawn Dr., Rockville MD 20852 (USA) pursuant to the provisions of the Budapest Treaty: Availability of the deposited materials 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 present invention has been described with reference to specific examples and embodiments. However, this application is intended to cover those changes and substitutions which, based on Applicants disclosure, are apparent and may be made by those skilled in the art without departing from the spirit and scope of the claims. SEQUENCE LISTING ( 1 ) GENERAL INFORMATION : (i) APPLICANT: Swammathan, Neela Wilkosz, Richard K. Muller, Reinhold (ll) TITLE OF INVENTION: BIOLOGICALLY ACTIVE FRAGMENT OF BACILLUS STEAROTHERMOPHILUS DNA POLYMERASE (iii) NUMBER OF SEQUENCES: 5 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Marshall, O'Toole, Gerstein, Murray & Borun (B) STREET: 6300 Sears Tower, 233 South Wacker Drive (C) CITY: Chicago (D) STATE: Illinois (E) COUNTRY: USA (F) ZIP: 60606-6402 (v) COMPUTER READABLE FORM- (A) MEDIUM TYPE Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM. PC-DOS/MS-DOS (D) SOFTWARE- Patentln Release #1.0, Version #1.30 (vi) CURRENT APPLICATION DATA- (A) APPLICATION NUMBER (B) FILING DATE (C) CLASSIFICATION (viii) ATTORNEY/AGENT INFORMATION (A) NAME: Pochopien, Donald J (B) REGISTRATION NUMBER 32,167 (C) REFERENCE/DOCKET NUMBER 28003/32630 (ix) TELECOMMUNICATION INFORMATION (A) TELEPHONE 312/474-6300 (B) TELEFAX 312/474-0448 (C) TELEX- 25-3856 (2) INFORMATION FOR SEQ ID NO 1 d) SEQUENCE CHARACTERISTICS (A) LENGTH- 3317 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS single (D) TOPOLOGY linear (ii) MOLECULE TYPE protein (ix) FEATURE: (A) NAME/KEY CDS (B) LOCATION. 316 .2943 (xi) SEQUENCE DESCRIPTION SEQ ID NO 1 : GTCGACAAGG CGCGCAGCCG CGATTCCGGC GGAACGGGGT TGGGCCTGGC GATTGTGAAA 60 CATTTGGTTG AGGCTCACCA TGGATATATT ACCGTAGCGA GCAAAGTGGG GCGCGGCACC 120 GTGTTCACGA TCCATTTTCC AAAGCCGGGG CGGTAGCCGG CTTCTTTTGA TCATCTCCAA 180 CTGAGAAGCC TCCCATTTTT CAGCGTGAGC GTAAGCAGGG GATGAATCGG CGCCTCCCAT 240 CATGGTGGGA GAGCGTTCAA GGCAAGCCGC AGGCATGGTA CAATAGGACA AGGAAGCATC 300 CGAGGAGGGA TGAGA TTG AAA AAA AAG CTT GTT TTA ATC GAC GGC AGC AGC 351 Leu Lys Lys Lys Leu Val Leu Ile Asp Gly Ser Ser 1 5 10 GTG GCG TAC CGC GCC TTT TTT GCC TTG CCG CTT TTG CAT AAC GAC AAA 399 Val Ala Tyr Arg Ala Phe Phe Ala Leu Pro Leu Leu His Asn Asp Lys 15 20 25 GGC ATC CAT ACG AAC GCC GTC TAC GGG TTT ACG ATG ATG TTG AAT AAA 447 Gly Ile His Thr Asn Ala Val Tyr Gly Phe Thr Met Met Leu Asn Lys 30 35 40 ATT TTG GCG GAA GAA GAG CCA ACT CAT ATG CTT GTC GCG TTT GAC GCC 495 Ile Leu Ala Glu Glu Glu Pro Thr His Met Leu Val Ala Phe Asp Ala 45 50 55 60 GGG AAA ACG ACG TTC CGG CAT GAA GCG TTT CAA GAG TAT AAA GGT GGG 543 Gly Lys Thr Thr Phe Arg His Glu Ala Phe Gin Glu Tyr Lys Gly Gly 65 70 75 CGC CAG CAG ACG CCA CCG GAG CTG TCG GAG CAG TTT CCG CTG TTG CGC 591 Arg Gin Gin Thr Pro Pro Glu Leu Ser Glu Gin Phe Pro Leu Leu Arg 80 85 90 GAG CTG CTG AGG GCG TAT CGC ATC CCC GCC TAT GAA CTC GAG AAC TAC 639 Glu Leu Leu Arg Ala Tyr Arg Ile Pro Ala Tyr Glu Leu Glu Asn Tyr 95 100 105 GAA GCG GAC GAT ATT ATC GGA ACG CTT GCC GCC CGC GCT GAG CAG GAA 687 Glu Ala Asp Asp Ile Ile Gly Thr Leu Ala Ala Arg Ala Glu Gin Glu 110 115 120 GGG TTT GAG ATG AAA GTC ATT TCC GGC GAC CGC GAT CTG ACC CAG CTC 735 Gly Phe Glu Met Lys Val Ile Ser Gly Asp Arg Asp Leu Thr Gin Leu 125 130 135 140 GCC TCC CCC CAT GTG ACG GTG GAC ATT ACG AAA AAA GGG ATT ACC GAT 783 Ala Ser Pro His Val Thr Val Asp Ile Thr Lys Lys Gly Ile Thr Asp 145 150 155 ATC GAA CCA TAC ACG CCG GAG ACG GTC CGC GAA AAA TAC GGC TTA ACT 831 Ile Glu Pro Tyr Thr Pro Glu Thr Val Arg Glu Lys Tyr Gly Leu Thr 160 165 170 CCG GAA CAA ATC GTT GAT TTG AAA GGA TTG ATG GGC GAC AAA TCG GAC 879 Pro Glu Gin Ile Val Asp Leu Lys Gly Leu Mec Gly Asp Lys Ser Asp 175 180 185 AAC ATC CCC GGA GTG CCG GGC ATC GGG GAA AAG ACG GCG GTC AAG CTG 927 Asn Ile Pro Gly Val Pro Gly Ile Gly Glu Lys Thr Ala Val Lys Leu 190 195 200 CTC AGG CAA TTC GGC ACG GTC GAA AAT GTG CTT GCC TCC ATT GAC GAG 975 Leu Arg Gin Phe Gly Thr Val Glu Asn Val Leu Ala Ser Ile Asp Glu 205 210 215 220 ATC AAA GGC GAA AAG TTG AAA GAA ACG CTG CGC CAA CAC CGG GAG ATG 1023 Ile Lys Gly Glu Lys Leu Lys Glu Thr Leu Arg Gin His Arg Glu Met 225 230 235 GCG CTG TTA AGC AAA AAG CTC GCC GCC ATT CGC CGC GAC GCC CCG GTC 1071 Ala Leu Leu Ser Lys Lys Leu Ala Ala Ile Arg Arg Asp Ala Pro Val 240 245 250 GAG CTC TCG CTT GAT GAC ATC GCC TAT CAA GGG GAA GAC CGG GAG AAA 1119 Glu Leu Ser Leu Asp Asp lie Ala Tyr Gin Gly Glu Asp Arg Glu Lys 255 260 265 GTG GTC GCT TTA TTT AAA GAG CTT GGG TTT CAA TCG TTT TTA GAG AAA 1167 Val Val Ala Leu Phe Lys Glu Leu Gly Phe Gin Ser Phe Leu Glu Lys 270 275 280 ATG GAA TCG CCG TCA TCA GAA GAG GAA AAA CCG CTT GCC AAG ATG GCA 1215 Met Glu Ser Pro Ser Ser Glu Glu Glu Lys Pro Leu Ala Lys Met Ala 285 290 295 300 TTT ACG CTT GCT GAC CGC GTG ACG GAG GAG ATG CTT GCC GAC AAG GCG 1263 Phe Thr Leu Ala Asp Arg Val Thr Glu Glu Met Leu Ala Asp Lys Ala 305 310 315 GCG CTT GTC GTT GAA GTG GTC GAG GAA AAT TAT CAT GAT GCG CCG ATC 1311 Ala Leu Val Val Glu Val Val Glu Glu Asn Tyr Hiε Asp Ala Pro Ile 320 325 330 GTC GGC ATC GCT GTG GTC AAC GAA CAT GGA CGQ TTT TTC CTG CGC CCG 1359 Val Gly Ile Ala Val Val Asn Glu His Gly Arg Phe Phe Leu Arg Pro 335 340 345 GAG ACG GCG CTT GCC GAT CCG CAG TTT GTC GCC TGG CTT GGT GAT GAA 1407 Glu Thr Ala Leu Ala Asp Pro Gin Phe Val Ala Trp Leu Gly Asp Glu 350 355 360 ACG AAG AAA AAA AGC ATG TTT GAC TCA AAG CGC GCG GCA GTC GCC TTG 1455 Thr Lys Lys Lys Ser Met Phe Asp Ser Lys Arg Ala Ala Val Ala Leu 365 370 375 380 AAA TGG AAA GGA ATT GAG CTA TGC GGC GTT TCC TTT GAT TTA TTG CTG 1503 Lys Trp Lys Gly Ile Glu Leu Cys Gly Val Ser Phe Asp Leu Leu Leu 385 390 395 GCC GCC TAT TTG CTT GAT CCG GCG CAA GGT GTT GAT GAT GTG GCT GCC 1551 Ala Ala Tyr Leu Leu Asp Pro Ala Gin Gly Val Asp Asp Val Ala Ala 400 405 410 GCA GCA AAA ATG AAG CAA TAC GAA GCG GTG CGC CCG GAT GAA GCG GTG 1599 Ala Ala Lys Met Lys Gin Tyr Glu Ala Val Arg Pro Asp Glu Ala Val 415 420 425 TAT GGC AAA GGG GCG AAG CGG GCC GTG CCG GAT GAG CCA GTG CTC GCC 1647 Tyr Gly Lys Gly Ala Lys Arg Ala Val Pro Asp Glu Pro Val Leu Ala 430 435 440 GAG CAT TTG GTC CGC AAG GCG GCG GCG ATT TGG GCG CTC GAA CGG CCG 1695 Glu His Leu Val Arg Lys Ala Ala Ala Ile Trp Ala Leu Glu Arg Pro 445 450 455 460 TTT TTG GAT GAG CTG CGC CGC AAC GAA CAA GAT CGG TTG CTC GTC GAG 1743 Phe Leu Asp Glu Leu Arg Arg Asn Glu Gin Asp Arg Leu Leu Val Glu 465 470 475 CTC GAG CAG CCG TTG TCT TCG ATT TTG GCG GAA ATG GAA TTT GCC GGA 1791 Leu Glu Gin Pro Leu Ser Ser Ile Leu Ala Glu Met Glu Phe Ala Gly 480 485 490 GTG AAA GTG GAT ACG AAG CGG CTC GAA CAG ATG GGC GAA GAG CTC GCC 1839 Val Lys Val Asp Thr Lys Arg Leu Glu Gin Met Gly Glu Glu Leu Ala 495 500 505 GAG CAG CTG CGC ACG GTC GAG CAG CGC ATT TAT GAG CTC GCC GGC CAA 1887 Glu Gin Leu Arg Thr Val Glu Gin Arg Ile Tyr Glu Leu Ala Gly Gin 510 515 520 GAA TTC AAC ATC AAT TCA CCG AAA CAG CTC GGC GTC ATT TTA TTT GAA 1935 Glu Phe Asn Ile Asn Ser Pro Lys Gin Leu Gly Val Ile Leu Phe Glu 525 530 535 540 AAA CTG CAG CTG CCC GTC TTG AAA AAA ACG AAA ACC GGC TAC TCC ACT 1983 Lys Leu Gin Leu Pro Val Leu Lys Lys Thr Lys Thr Gly Tyr Ser Thr 545 550 555 TCG GCG GAT GTG CTT GAA AAA CTT GCG CCT TAT CAC GAG ATC GTG GAA 2031 Ser Ala Asp Val Leu Glu Lys Leu Ala Pro Tyr His Glu Ile Val Glu 560 565 570 AAC ATT TTG CAT TAC CGC CAG CTT GGC AAG TTG CAG TCG ACG TAT ATT 2079 Asn Ile Leu His Tyr Arg Gin Leu Gly Lys Leu Gin Ser Thr Tyr Ile 575 580 585 GAA GGA TTG CTG AAA GTC GTG CGA CCC GAT ACA AAG AAG GTG CAT ACG 2127 Glu Gly Leu Leu Lys Val Val Arg Pro Asp Thr Lys Lys Val His Thr 590 595 600 ATT TTC AAT CAG GCG TTG ACG CAA ACC GGA CGG CTC AGC TCG ACG GAG 2175 Ile Phe Asn Gin Ala Leu Thr Gin Thr Gly Arg Leu Ser Ser Thr Glu 605 610 615 620 CCG AAC TTG CAA AAC ATT CCG ATT CGG CTT GAG GAA GGA CGG AAA ATC 2223 Pro Asn Leu Gin Asn Ile Pro Ile Arg Leu Glu Glu Gly Arg Lys Ile 625 630 635 CGC CAA GCG TTC GTG CCA TCG GAG TCT GAT TGG CTC ATT TTC GCC GCC 2271 Arg Gin Ala Phe Val Pro Ser Glu Ser Asp Trp Leu Ile Phe Ala Ala 640 645 650 GAC TAC TCG CAA ATT GAG TTG CGC GTC CTC GCC CAT ATT GCG GAA GAT 2319 Asp Tyr Ser Gin Ile Glu Leu Arg Val Leu Ala His Ile Ala Glu Asp 655 660 665 GAC AAT TTA ATG GAA GCG TTC CGC CGC GAT TTG GAT ATC CAT ACG AAA 2367 Asp Asn Leu Met Glu Ala Phe Arg Arg Asp Leu Asp Ile Hiε Thr Lys 670 675 680 ACA GCG ATG GAC ATT TTC CAA GTG AGC GAG GAC GAA GTG ACG CCC AAC 2415 Thr Ala Met Asp Ile Phe Gin Val Ser Glu Asp Glu Val Thr Pro Asn 685 690 695 700 ATG CGC CGT CAG GCG AAG GCG GTC AAC TTT GGG ATC GTT TAC GGG ATC 2463 Met Arg Arg Gin Ala Lys Ala Val Asn Phe Gly Ile Val Tyr Gly lie 705 710 715 AGT GAT TAC GGC TTG GCG CAA AAC TTA AAT ATT TCG CGC AAA GAG GCG 2511 Ser Asp Tyr Gly Leu Ala Gin Asn Leu Asn Ile Ser Arg Lys Glu Ala 720 725 730 GCT GAA TTC ATC GAG CGC TAC TTC GAA AGC TTC CCT GGC GTG AAG CGG 2559 Ala Glu Phe Ile Glu Arg Tyr Phe Glu Ser Phe Pro Gly Val Lys Arg 735 740 745 TAT ATG GAA AAC ATT GTG CAA GAA GCA AAA CAG AAA GGG TAT GTG ACG 2607 Tyr Met Glu Asn Ile Val Gin Glu Ala Lys Gin Lys Gly Tyr Val Thr 750 755 760 ACG CTG CTG CAT CGG CGC CGC TAT TTG CCG GAT ATC ACG AGC CGC AAC 2655 Thr Leu Leu His Arg Arg Arg Tyr Leu Pro Asp lie Thr Ser Arg Asn 765 770 775 780 TTC AAC GTC CGC AGC TTT GCT GAA CGG ATG GCG ATG AAC ACG CCG ATT 2703 Phe Asn Val Arg Ser Phe Ala Glu Arg Met Ala Met Asn Thr Pro Ile 785 790 795 CAA GGG AGC GCC GCT GAC ATT ATT AAA AAG GCG ATG ATC GAT CTG AAC 2751 Gin Gly Ser Ala Ala Asp Ile Ile Lys Lys Ala Met Ile Asp Leu Asn 800 805 810 GCC AGA CTG AAG GAA GAG CGG CTG CAA GCG CGC CTT TTG CTG CAG GTG 2799 Ala Arg Leu Lys Glu Glu Arg Leu Gin Ala Arg Leu Leu Leu Gin Val 815 820 825 CAT GAC GAG CTC ATT TTG GAG GCG CCG AAA GAA GAG ATG GAG CGG CTG 2847 His Asp Glu Leu Ile Leu Glu Ala Pro Lys Glu Glu Met Glu Arg Leu 830 835 840 TGC CGG CTC GTT CCG GAA GTG ATG GAG CAA GCG GTC ACA CTT CGC GTG 2895 Cys Arg Leu Val Pro Glu Val Met Glu Gin Ala Val Thr Leu Arg Val 845 850 855 860 CCG CTC AAA GTC GAT TAC CAT TAT GGC TCG ACG TGG TAT GAT GCG AAA 2943 Pro Leu Lys Val Asp Tyr His Tyr Gly Ser Thr Trp Tyr Asp Ala Lys 865 870 875 TAAAGAGAAG TCTTGGTGTG GAGCGCCGGC ATCCCTAAGA AGGCCTGTGA TGGAATGAAA 3003 AAGCAGTTTC ACAACGACTC TTCTCCAGTT GGGAAGCCTT GAACATCGAG CCGTCCTTCT 3063 CAACCAACAT GACCGATTTT GTGAAAATCA GCGTTTCTCA CCGGCCTTTT AGGCAGAATC 3123 TTTCGGTGCG ACGATTCTCG GCTGCGGGTC GATGAATTGG AGCGAAACAG CTGCCGCCCC 3183 ATGGAGAATC TTTCTCTCGG CGGATGAACC GGCGTCAATG TGAAAGCGTC GGCGGGAACG 3243 ATGCAGGAAA ACGGAGGAAA GGGGGGATCC GAATTCGTTC CCTTTAGTGA GGGTTAATTC 3303 CCGGCCGCGT CGAC 3317 (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 876 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: Leu Lys Lys Lys Leu Val Leu Ile Asp Gly Ser Ser Val Ala Tyr Arg 1 5 10 15 Ala Phe Phe Ala Leu Pro Leu Leu Hiε Asn Asp Lys Gly Ile His Thr 20 25 30 Asn Ala Val Tyr Gly Phe Thr Met Met Leu Asn Lys Ile Leu Ala Glu 35 40 45 Glu Glu Pro Thr His Met Leu Val Ala Phe Asp Ala Gly Lys Thr Thr 50 55 60 Phe Arg His Glu Ala Phe Gin Glu Tyr Lys Gly Gly Arg Gin Gin Thr 65 70 75 80 Pro Pro Glu Leu Ser Glu Gin Phe Pro Leu Leu Arg Glu Leu Leu Arg 85 90 95 Ala Tyr Arg Ile Pro Ala Tyr Glu Leu Glu Asn Tyr Glu Ala Asp Asp 100 105 110 Ile Ile Gly Thr Leu Ala Ala Arg Ala Glu Gin Glu Gly Phe Glu Met 115 120 125 Lys Val Ile Ser Gly Asp Arg Asp Leu Thr Gin Leu Ala Ser Pro Hiε 130 135 140 Val Thr Val Asp Ile Thr Lys Lys Gly Ile Thr Asp Ile Glu Pro Tyr 145 150 155 160 Thr Pro Glu Thr Val Arg Glu Lys Tyr Gly Leu Thr Pro Glu Gin Ile 165 170 175 Val Asp Leu Lys Gly Leu Met Gly Asp Lys Ser Asp Asn Ile Pro Gly 180 185 190 Val Pro Gly Ile Gly Glu Lys Thr Ala Val Lys Leu Leu Arg Gin Phe 195 200 205 Gly Thr Val Glu Asn Val Leu Ala Ser Ile Asp Glu Ile Lys Gly Glu 210 215 220 Lys Leu Lys Glu Thr Leu Arg Gin His Arg Glu Met Ala Leu Leu Ser 225 230 235 240 Lys Lys Leu Ala Ala Ile Arg Arg Asp Ala Pro Val Glu Leu Ser Leu 245 250 255 Asp Asp lie Ala Tyr Gin Gly Glu Asp Arg Glu Lys Val Val Ala Leu 260 265 270 Phe Lyε Glu Leu Gly Phe Gin Ser Phe Leu Glu Lys Met Glu Ser Pro 275 280 285 Ser Ser Glu Glu Glu Lys Pro Leu Ala Lys Met Ala Phe Thr Leu Ala 290 295 300 Asp Arg Val Thr Glu Glu Met Leu Ala Asp Lys Ala Ala Leu Val Val 305 310 315 320 Glu Val Val Glu Glu Asn Tyr His Asp Ala Pro lie Val Gly Ile Ala 325 330 335 Val Val Asn Glu Hiε Gly Arg Phe Phe Leu Arg Pro Glu Thr Ala Leu 340 345 350 Ala Asp Pro Gin Phe Val Ala Trp Leu Gly Asp Glu Thr Lys Lys Lys 355 360 365 Ser Met Phe Asp Ser Lys Arg Ala Ala Val Ala Leu Lys Trp Lys Gly 370 375 380 Ile Glu Leu Cys Gly Val Ser Phe Asp Leu Leu Leu Ala Ala Tyr Leu 385 390 395 400 Leu Asp Pro Ala Gin Gly Val Asp Asp Val Ala Ala Ala Ala Lys Met 405 410 415 Lys Gin Tyr Glu Ala Val Arg Pro Asp Glu Ala Val Tyr Gly Lys Gly 420 425 430 Ala Lys Arg Ala Val Pro Asp Glu Pro Val Leu Ala Glu Hiε Leu Val 435 440 445 Arg Lys Ala Ala Ala Ile Trp Ala Leu Glu Arg Pro Phe Leu Asp Glu 450 455 460 Leu Arg Arg Asn Glu Gin Asp Arg Leu Leu Val Glu Leu Glu Gin Pro 465 470 475 480 Leu Ser Ser Ile Leu Ala Glu Met Glu Phe Ala Gly Val Lys Val Asp 485 490 495 Thr Lys Arg Leu Glu Gin Met Gly Glu Glu Leu Ala Glu Gin Leu Arg 500 505 510 Thr Val Glu Gin Arg Ile Tyr Glu Leu Ala Gly Gin Glu Phe Asn Ile 515 520 525 Asn Ser Pro Lys Gin Leu Gly Val Ile Leu Phe Glu Lys Leu Gin Leu 530 535 540 Pro Val Leu Lys Lys Thr Lys Thr Gly Tyr Ser Thr Ser Ala Asp Val 545 550 555 560 Leu Glu Lys Leu Ala Pro Tyr His Glu Ile Val Glu Asn Ile Leu His 565 570 575 Tyr Arg Gin Leu Gly Lys Leu Gin Ser Thr Tyr Ile Glu Gly Leu Leu 580 585 590 Lys Val Val Arg Pro Asp Thr Lys Lys Val His Thr Ile Phe Asn Gin 595 600 605 Ala Leu Thr Gin Thr Gly Arg Leu Ser Ser Thr Glu Pro Asn Leu Gin 610 615 620 Asn Ile Pro Ile Arg Leu Glu Glu Gly Arg Lys lie Arg Gin Ala Phe 625 630 635 640 Val Pro Ser Glu Ser Asp Trp Leu Ile Phe Ala Ala Asp Tyr Ser Gin 645 650 655 Ile Glu Leu Arg Val Leu Ala His Ile Ala Glu Asp Asp Asn Leu Met 660 665 670 Glu Ala Phe Arg Arg Asp Leu Asp Ile Hiε Thr Lys Thr Ala Met Asp 675 680 685 Ile Phe Gin Val Ser Glu Asp Glu Val Thr Pro Asn Met Arg Arg Gin 690 695 700 Ala Lys Ala Val Asn Phe Gly Ile Val Tyr Gly Ile Ser Asp Tyr Gly 705 710 715 720 Leu Ala Gin Asn Leu Asn Ile Ser Arg Lys Glu Ala Ala Glu Phe Ile 725 730 735 Glu Arg Tyr Phe Glu Ser Phe Pro Gly Val Lys Arg Tyr Met Glu Asn 740 745 750 Ile Val Gin Glu Ala Lys Gin Lys Gly Tyr Val Thr Thr Leu Leu Hiε 755 760 765 Arg Arg Arg Tyr Leu Pro Asp Ile Thr Ser Arg Asn Phe Asn Val Arg 770 775 780 Ser Phe Ala Glu Arg Met Ala Met Asn Thr Pro Ile Gin Gly Ser Ala 785 790 795 800 Ala Asp Ile Ile Lys Lys Ala Met Ile Asp Leu Asn Ala Arg Leu Lys 805 810 815 Glu Glu Arg Leu Gin Ala Arg Leu Leu Leu Gin Val His Asp Glu Leu 820 825 830 Ile Leu Glu Ala Pro Lys Glu Glu Met Glu Arg Leu Cys Arg Leu Val 835 840 845 Pro Glu Val Met Glu Gin Ala Val Thr Leu Arg Val Pro Leu Lys Val 850 855 860 Asp Tyr Hiε Tyr Gly Ser Thr Trp Tyr Asp Ala Lys 865 870 875 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 42 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (il) MOLECULE TYPE. DNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: ATGAAAAAAA AGCTTGTTTT AATCGACGGC AGCAGCGTGG CG 42 (2) INFORMATION FOR SEQ ID NO:4 (i) SEQUENCE CHARACTERISTICS. (A) LENGTH: 26 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY, linear (ii) MOLECULE TYPE: DNA (xi) SEQUENCE DESCRIPTION SEQ ID NO 4 GCAAAACAGA AAGGGTATGT GACGAC 26 (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 23 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (xi) SEQUENCE DESCRIPTION: SEQ ID Nθ:5: AAAATGAGCT CGTCATGCAC CTG 23

Claims: What Is Claimed Is: 1. A purified and isolated DNA in accordance with SEQ ID NO: 1. 2. The DNA of Claim 1 consisting of nucleotides 316 to 2946 of SEQ ID NO: 1. 3. The DNA of Claim 1 consisting of nucleotides 1135 to 2946 of SEQ ID NO: 1. 4. A vector wherein the DNA of Claim 3 is operably linked to a promoter. 5. A cDNA consisting of nucleotides 1135 to 2946. 6. Plasmid pPEK 5. 7. A host cell transformed with a DNA having an expressible portion selected from nucleotides 316 to 2946 of SEQ ID NO: 1 and nucleotides 1135 to 2946 of SEQ ID NO: 1. 8. The host cell of Claim 7, wherein said host cell is capable of expressing a thermostable polypeptide encoded by said DNA, said polypeptide having DNA polymerase activity. 9. The host cell of Claim 8, wherein said host cell is a prokaryotic cell. 10. The host cell of Claim 9, wherein said host cell is an E. coli cell. 11. An expression vector comprising a promoter operably linked to nucleotides 1135 to 2946 of SEQ ID NO: 1. 12. The expression vector of Claim 10 having at least one insert consisting essentially of nucleotides 1135 to 2946 of SEQ ID NO: 1. 13. A purified fragment of Bacillus stearothermophilus DNA polymerase I protein in accordance with SEQ ID NO: 2, said fragment having thermostable DNA polymerase activity, 3 '-5' exonuclease activity, substantially no 5 '-3' exonuclease activity, and a reverse transcriptase to polymerase ratio between 1 and 3. 14. The fragment of Claim 13 being rBst exo-. 15. A purified fragment of Bacillus stearothermophilus DNA polymerase I protein encoded by the insert of plasmid pPEK 5. 16. The purified fragment of Claim 15 wherein the fragment has a DNA polymerase activity between 50,000 U/mg protein and 500,000 U/mg protein. 17. A isolated and purified thermostable and recombinant polypeptide having DNA polymerase activity, said polypeptide comprising the amino acid sequence of SEQ ID NO: 2. 18. A recombinant polypeptide consisting essentially of amino acid residues 274 to 876 of SEQ ID NO: 2. 19. The polypeptide of Claim 18 in lyophilized form. 20. The polypeptide of Claim 18 in solution form. 21. In a method for thermocyclic amplification of cDNA, the method comprising, employing a thermophilic Taq DNA polymerase or a fragment thereof to amplify said DNA in an aqueous sample, the improvement comprising: substituting for said Taq DNA polymerase or a fragment thereof, a thermostable recombinant polypeptide having amino acid residues 274 to 876 of SEQ ID NO: 2. 22. The method of Claim 21 wherein the thermocyclic amplification of DNA is performed by Strand Displacement Amplification. 23. The method of Claim 21 wherein thermocyclic amplification of DNA is performed by the Polymerase Chain Reaction.

Inventors: SWAMINATHAN NEELA, WILKOSZ RICHARD K, SWAMINATHAN, NEELA, WILKOSZ, RICHARD, K.
Applicants: MOLECULAR BIOLOGY RESOURCES, MOLECULAR BIOLOGY RESOURCES, INC.
Application year: 1997
Main classification: C12N9
IPC codes: C12N1, C12N9, C12N15, C12Q1, C12R1