WO2008074346A2

WO2008074346A2 - Chimeric dna polymerase

Info

Publication number: WO2008074346A2
Application number: PCT/EP2006/012242
Authority: WO
Inventors: Konstantin Ignatov; Vladimir Kramarov; Dimitrij Plachov
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-12-19
Filing date: 2006-12-19
Publication date: 2008-06-26
Anticipated expiration: 2009-06-19
Also published as: WO2008074346A3

Abstract

Chimeric thermostable DNA polymerase is provided, along with DNA sequence that encodes the enzyme. Also provided are methods for producing and using the enzyme.

Description

"Chimeric DNA polymerase"

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to thermostable DNA polymerases, polynucleotide sequences encoding them, methods for their synthesis and manufacture, and methods for their use. Thermostable DNA polymerases are well known, and are useful in a wide range of laboratory processes, especially in molecular biology. Primer extension techniques, nucleic acid sequencing and the polymerase chain reaction (PCR) all employ such enzymes.

2. Background Art

DNA polymerases, which catalyze the template-directed polymerization of deoxyribonucleoside triphosphates (dNTPs) to form DNA₁ are used in a variety of in vitro DNA synthesis applications, such as primer extension techniques, DNA sequencing and DNA amplification. Thermostable DNA polymerases are particularly useful in a number of these techniques, as thermostable enzymes are able to be used at relatively high temperatures. This has benefits with respect to fidelity of primer binding, for example, owing to the high stringency of the conditions employed. Of known enzymes, the DNA polymerases isolated from Thermus aquaticus (Taq) and Thermus thermophilus (Tth) are perhaps the best characterized.

These enzymes have a defined range of different properties and limitations. For example, Taq and Tth DNA polymerases differ from each other in the following practically significant properties:

1) Tth DNA polymerase is more effective than Taq DNA polymerase for amplification of long (over 2 kb) DNA sequences in PCR [Ohler L.D., and Rose E. A., PCR Methods Appl. V.2 (1992), P. 51-59; Ignatov K.B. et al., MoI. Biol. (Russ.) V.31 (1997), P. 956-961] which is seen as a larger quantity of DNA produced;

2) Taq DNA polymerase is more sensitive than Tth DNA polymerase to the presence of a mismatched (non-complementary to template) nucleotide at the 3'-end of the primer [Ignatov K.B. et al., Bioorg. Khim. (Russ.) V.23 (1997), P. 817-822], which allows to employ Taq DNA polymerase in allele-specific primer extension reactions;

3) Taq DNA polymerase is more specific than Tth DNA polymerase in DNA amplification in the course of PCR [ignatov K.B. et al., Bioorg. Khim. (Russ.) V.23 (1997), P. 817-822], and thus yields a higher ratio of target product to total synthesized DNA 2 HT- ^*R0/EP

For increasing the efficiency of laboratory processes employing the above-mentioned DNA polymerases, a creation of a novel DNA polymerase that would possess at least some advantages and lack at least some of the drawbacks of these enzymes is deemed very useful. For instance, primer extension techniques (such as allele-specific primer extension) and PCR-amplification of DNA would need a thermostable DNA polymerase combining the efficiency of DNA synthesis of Tth DNA polymerase and the specificity of PCR-based DNA amplification characterisitic of Taq DNA polymerase.

It has been shown earlier that combining in one polypeptide chain of portions of protein molecules from different DNA polymerases may lead to construction of chimeric DNA polymerases having a combination of properties possessed by the parental DNA polymerases [Ignatov K.B. et al., MoI. Biol. (Russ.) V.31 (1997), P. 956-961 ; Villbrandt B. et al., Protein Eng. V.13 (2000), P. 645-654; U.S. Patent No. 6,228,628; U.K. Patent No. GB2344591].

The N-terminal region of Taq DNA polymerase has been shown to exert a significant effect on the efficiency of PCR with DNA templates longer than 2 kb. For example, deletion of the first 235 amino acids of Taq DNA polymerase reduces the enzyme's ability to amplify long DNA sequences [Barnes W.M., Gene V.112 (1992), P. 29-35]. The ability of Taq and Tth DNA polymerases to amplify long DNA sequences has also been attributed to sequences between the corresponding amino acid positions 498 and 554 for Taq DNA polymerase and 500 and 556 for Tth DNA polymerase [Blanco L. et al., Gene V.100 (1991 ), P. 27-28; Ignatov K.B. et al., MoI. Biol. (Russ.) V.31 (1997), P. 956-961]. Differences in those regions have, however, no effect on the specificity of DNA synthesis and the sensitivity of the two DNA polymerases to the presence of a mismatch at the 3'-end of the primer [Ignatov K.B. et al., MoI. Biol. (Russ.) V.31 (1997), P. 956-961].

It has now been found that combining in one polypeptide chain the N-terminal region of Tth DNA polymerase (including the region spanning amino acids 500 - 556) with the C-terminal region of Taq polymerase (including the sequence corresponding to amino acids 600 - 832 of the Taq sequence) results in a chimeric thermostable DNA polymerase possessing the synthesis efficiency of Tth DNA polymerase and the specificity of Taq DNA polymerase. The chimeric DNA polymerase according to the invention with a combination of desirable properties that do not occur in nature is inter alia useful in a variety of in vitro DNA synthesis applications.

SUMMARY OF THE INVENTION

The present invention provides a chimeric thermostable DNA polymerase which has the properties of high efficiency of long (over 2 kb) DNA sequences amplification in PCR, high sensitivity to the presence of a mismatched (non-complementary to template) nucleotide at the 3'-end of the primer, and high specificity in DNA amplification in the course of PCR. Said properties being derived from at least two different sources, wherein the properties are preferably in combination.

The chimeric DNA polymerases of the present invention have the N-terminal region from the Tth DNA polymerase and a C-terminal region from the Taq DNA polymerase. The N-terminal region contains at least a region of amino acids 280-555 of Tth DNA polymerase. The C- terminal region contains at least a region of amino acids 600-832 of Taq DNA polymerase. The chimeric DNA polymerase of the present invention preferably comprises an N-terminal region and a C-terminal region, wherein said N-terminal region comprises an amino acid sequence consists of amino acids "k" through "n" of a Thermus thermophilic (Tth) DNA polymerase, wherein "k" is between amino acids 1 and 280 of a Tth DNA polymerase, wherein "n" is between amino acids 555 and 601 of a Tth DNA polymerase and corresponding to an amino acid "m" of a Thermus aquaticus (Taq) DNA polymerase, wherein "m" is m=n-2; wherein said C-terminal region comprises amino acids m+1 through 832 of said Taq DNA polymerase.

Another aspect of the present invention relates to the DNA (chimeric gene) of the invention which encodes the chimeric thermostable DNA polymerase of the invention, recombinant DNA vector which contains the chimeric gene, and host cells transformed with the recombinant DNA vector.

Also provided are methods for producing and using the chimeric enzyme. The DNA polymerase can be easily and efficiently expressed to a high level in a recombinant expression system, thereby facilitating commercial production of the enzyme. The combination of properties possessed by the DNA polymerase of the present invention represents a significant advantage over thermostable DNA polymerases previously described in the literature.

The Taq-DNA-Polymerase preferably is Taq-DNA-Polymerase I. The Tth-DNA-Polymerase preferably is Tth-DNA-Polymerase I.

BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES.

FIG. 1 provides a scheme illustrating steps in construction of chimeric gene encoding the chimeric polymerase of the invention and an expression vector.

FIG. 2 provides a photograph of an agarose gel, which compares the yield of 2500-bp DNA fragment obtainable by PCR amplification with Taq DNA polymerase, Tth DNA polymerase and the chimeric DNA polymerase of this invention. RO/EP _{4 w}_

FIG. 3 provides a photograph of an agarose gel, which compares the specificity of PCR amplification reactions performed with Taq DNA polymerase, Tth DNA polymerase and the chimeric DNA polymerase of this invention.

TABLE 1 provides data of radioactive label incorporation into the 500-bp DNA fragment synthesized with Taq, or Tth, or the chimeric DNA polymerase by PCR with primers containing or not containing 3^'-mismatching nucleotides

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a chimeric thermostable DNA polymerase and means for producing the enzyme. To facilitate understanding of the invention, a number of terms are defined below. As used herein, "cell", "cell line", and "cell culture" can be used interchangeably and all such designations include progeny. Thus, the words "transformants" or "transformed cells" includes the primary subject cell and cultures derived therefrom without regard for the number of transfers. It is also understood that all progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations. Mutant progeny that have the same functionality as screened for in the originally transformed cell are included.

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

The term "gene" refers to a DNA sequence that comprises control and coding sequences necessary for the production of a recoverable bioactive polypeptide or precursor.

The term "oligonucleotide" as used herein is defined as a molecule comprised of two or more deoxyribonucleotides or ribonucleotides. The exact size will depend on many factors, which in turn depends on the ultimate function or use of the oligonucleotide. Oligonucleotides can be prepared by any suitable method, including, for example, cloning and restriction of appropriate sequences and direct chemical synthesis by a method such as the phosphotriester method, the diethylphosphoramidite method, and the solid support method. A review of synthesis methods is provided in [Goodchild J., Bioconjug. Chem. V.1 (1990), P. 165-187].

The term "primer" as used herein refers to an oligonucleotide, which is capable of acting as a point of initiation of synthesis when placed under conditions in which primer extension is RO/EP

5 H^-

initiated. Synthesis of a primer extension product, which is complementary to a nucleic acid strand, is initiated in the presence of the requisite four different nucleoside triphosphates and a thermostable DNA polymerase in an appropriate buffer at a suitable temperature. A "buffer" includes cofactors (such as divalent metal ions) and salt (to provide the appropriate ionic strength), adjusted to the desired pH.

A primer that hybridizes to the non-coding strand of a gene sequence (equivalently, is a subsequence of the coding strand) is referred to herein as an "upstream" primer. A primer that hybridizes to the coding strand of a gene sequence is referred to herein as a "downstream" primer.

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

The term "thermostable enzyme", as used herein, refers to an enzyme which is stable to heat and has an elevated temperature reaction optimum. The thermostable enzyme of the present invention catalyzes primer extension optimally at a temperature between 60 and 90. degree. C₁ and is usable under the temperature cycling conditions typically used in cycle sequence reactions and polymerase chain reaction amplifications (described in U.S. Pat. No. 4,965,188).

As used herein, a "chimeric" protein refers to a protein whose amino acid sequence represents a fusion product of subsequences of the amino acid sequences from at least two distinct proteins. A chimeric protein preferably is not produced by direct manipulation of amino acid sequences, but, rather, is expressed from a "chimeric" gene that encodes the chimeric amino acid sequence. The chimeric protein of the present invention consists of an amino-terminal (N-terminal) region derived from a Thermus thermophilus (Tth) DNA polymerase I and a carboxy-terminal (C-terminal) region derived from Thermus aquaticus (Taq) DNA polymerase I.

The N-terminal region refers to a region extending from the N-terminus (amino acid position 1 ) to an internal amino acid. Similarly, the C-terminal region refers to a region extending from an internal amino acid to the C-terminus. In the chimeric proteins of the present invention, the N-terminal region extends from the N-terminus (amino acid position 1 ) to the beginning of the C-terminal region, which extends to the C-terminus. Thus, taken together, the N-terminal and C-terminal regions encompass the entire amino acid sequence.

The Thermostable DNA Polymerase of the Invention

It will be appreciated that a chimeric protein may be constructed in a number of ways, most easily via the construction of a recombinant DNA molecule, followed by expression of the ^*RO/EP 6 [-F -

protein product. Manipulation at the DNA level allows DNA fragments from different genes to be joined together by ligation, to form DNA encoding a chimeric polymerase. DNA fragments from different DNA polymerase genes may be obtained by DNA purification, followed by restriction enzyme digestion, PCR, or even direct DNA synthesis, for example. The protein may then be expressed from the DNA, using expression vectors maintained within host cells. DNA cloning, manipulation and protein expression are all standard techniques in the art, and details of suitable techniques may be found in Sambrook et al, ^'Molecular cloning - A Laboratory Manual^', 1989.

The present invention, therefore, also provides DNA encoding the chimeric thermostable DNA polymerase, along with vector containing this DNA, host cells containing this vector, and cultures of such cells, as well as methods for making the enzyme. The invention also includes nucleic acid species, which hybridize to DNA encoding the protein of the invention, hybridization being carried out under standard conditions, preferably 60. degree. C. and 6X SSC. Additionally, the present invention includes kits containing the enzyme of the invention in combination with other reagents, suitable for use in laboratory experiments. DNA and vectors encoding all or part of an enzyme of the invention may suitably incorporate such control elements, such as start/stop codons, promoters etc. as are deemed necessary or useful, as the skilled person desires. Suitable constructs are illustrated in the accompanying Examples.

The thermostable DNA polymerase of the present invention is a chimeric DNA polymerase in which the N-terminal region comprises an N-terminal region of Tth DNA polymerase and the C-terminal region comprises a C-terminal region of Taq DNA polymerase. The N-terminal region from the Tth DNA polymerase encompasses a portion of, or all of, the 5'-nuclease domain and a portion of the DNA polymerase domain. The C-terminal region from Taq DNA polymerase encompasses a portion of the DNA polymerase domain. The portion of the DNA polymerase domain of Taq DNA polymerase encompassed by the C-terminal region of the chimeric protein will correspond functionally and/or structurally to that portion of the DNA polymerase domain of the Tth DNA polymerase not encompassed by the N-terminal region of the chimeric protein.

The chimeric DNA polymerase can preferably additionally contain substitution of Asp for GIu (amino acid position 2) and substitution of Leu for Ala (amino acid position 3) in the N- terminal region from the Tth DNA polymerase.

The DNA polymerase of the invention is a chimeric enzyme that comprises of a portion derived from Tth DNA polymerase and a portion derived from Taq DNA polymerase. The chimeric enzyme is preferably prepared from a chimeric gene, i.e., a DNA that encodes the chimeric enzyme and consists of a portion derived from the Tth DNA polymerase gene and a portion derived from the Taq DNA polymerase gene. The chimeric gene can be produced 7 t-φ- RO/EP

from the Tth DNA polymerase gene and the Taq DNA polymerase gene using standard gene manipulation techniques well known in the field of molecular biology, as described in Example 1. A gene encoding Tth DNA polymerase, the nucleotide sequence of the Tth DNA polymerase gene, as well as the full amino acid sequence of the encoded protein, is described in U.S. Pat. No. 5,618,711. The gene encoding Taq DNA polymerase, the nucleotide sequence of the Taq DNA polymerase gene, as well as the full amino acid sequence of the encoded protein, are described in [Lawyer, F. C. et al., J. Biol. Chem., 261 , 11 , 6427-6437] and U.S. Pat. No. 5,079,352.

Structure of the Chimeric Thermostable DNA Polymerase of the Invention.

An example of an amino acid sequence of the chimeric DNA polymerase of the invention is given in SEQ ID No. 1. A part of the amino acid sequence of the chimeric DNA polymerase consisting of amino acids 4 through 600 is identical to the sequence of amino acids 4-600 of Tth DNA polymerase. A part of the amino acid sequence of the chimeric DNA polymerase consisting of amino acids 556 through 834 is identical to the sequence of amino acids 554- 832 of Taq DNA polymerase. Thus, the sequence of amino acids 556-600 of the chimeric DNA polymerase is identical to both the sequence of amino acids 556-600 of Tth DNA polymerase and the sequence of amino acids 554-598 of Taq DNA polymerase. The sequence of amino acids 1-3 of the chimeric DNA polymerase arose from recombinant expression vector construction (described in example 1 ).

The chimeric polymerase of the invention can comprise deletions substitutions or additives of one or more amino acids, which do not significantly change the biological properties of the resulting polymerase. The invention also relates to nucleic acid sequences encoding such polymerase. These nucleic acids can comprise nucleic acid substitutions, deletions or additions compared to the sequence depicted in SEQ ID No. 2. Furthermore the invention comprises amino acid sequences and nucleic acid sequences which are at least 80%, favourably 90% or at least 95% identical to SEQ ID No. 1 or SEQ ID No. 2, respectively, without changing the biological properties of the chimeric protein of the invention, in particular its efficiency and specificity.

^*RO/EP 8 -[Q]*

SEQ ID No. 1.

Amino acid sequence of the chimeric DNA polymerase of the invention.

1 mdlmlplfep kgrvllvdgh hlayrtffal kglttsrgep vqavygfaks llkalkedgy 61 kavfvvfdak apsfrheaye aykagraptp edfprqtali kelvdllgft rlevpgyead 121 dvlatlakka ekegyevril tadrdlyqlv sdrvavlhpe ghlitpewlw ekyglrpeqw 181 vdfralvgdp sdnlpgvkgi gektaikllk ewgslenllk nldrvkpenv rekikahled 241 Irlslelsrv rtdlplevdl aqgrepdreg lraflerlef gsllhefgll eapapleeap 301 wpppegafvg fvlsrpepmw aelkalaacr dgrvhraadp laglkdlkev rgllakdlav 361 lasregldlv pgddpmllay lldpsnttpe gvarryggew tedaahrall serlhrnllk 421 rlegeekllw lyhevekpls rvlahmeatg vrrdvaylqa lslelaeeir rleeevfrla 481 ghpfnlnsrd qlervlfdel rlpalgktqk tgkrstsaav lealreahpi vekilqhrel 541 tklkntyvdp Ipslvhprtg rlhtrfnqta tatgrlsssd pnlqnipvrt plgqrirraf 601 iaeegwllva Idysqielrv lahlsgdenl irvfqegrdi htetaswmfg vpreavdplm 661 rraaktinfg vlygmsahrl sqelaipyee aqafieryfq sfpkvrawie ktleegrrrg 721 yvetlfgrrr yvpdlearvk svreaaerma fnmpvqgtaa dlmklamvkl fprleemgar 781 mllqvhdelv leapkeraea varlakevme gvyplavple vevgigedwl sake

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Nucleic Acid Encodes the Chimeric DNA polymerase of the Invention.

The nucleotide sequence of the nucleic acid encoding the chimeric DNA polymerase is given in SEQ ID No. 2. The nucleic acid encoding the chimeric DNA polymerase was obtained as described in Example 1.

The nucleotide sequence of the nucleic acid encoding the chimeric DNA polymerase consists of subsequences: the sequence of nucleotides 1 - 8, which arose from recombinant expression vector construction (described in Example 1 ); the sequence of nucleotides 9 - 1786, which was taken from gene of Tth DNA polymerase, and which is identical to the nucleotide sequence 9 - 1786 of Tth DNA polymerase gene; the sequence of nucleotides 1787 - 2505, which was taken from gene of Taq DNA polymerase, and which is identical to the nucleotide sequence 1781 - 2499 of Taq DNA polymerase gene.

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10 -m*-

SEQ ID No. 2.

Nucleotide sequence encoding the chimeric DNA polymerase of the invention.

1 atggatctga tgcttccgct ctttgaaccc aaaggccggg tcctcctggt ggacggccac 61 cacctggcct accgcacctt cttcgccctg aagggcctca ccacgagccg gggcgaaccg 121 gtgcaggcgg tctacggctt cgccaagagc ctcctcaagg ccctgaagga ggacgggtac 181 aaggccgtct tcgtggtctt tgacgccaag gccccctcct tccgccacga ggcctacgag 241 gcctacaagg cggggagggc cccgaccccc gaggacttcc cccggcagct cgccctcatc 301 aaggagctgg tggacctcct ggggtttacc cgcctcgagg tccccggcta cgaggcggac 361 gacgttctcg ccaccctggc caagaaggcg gaaaaggagg ggtacgaggt gcgcatcctc 421 accgccgacc gcgacctcta ccaactcgtc tccgaccgcg tcgccgtcct ccaccccgag 481 ggccacctca tcaccccgga gtggctttgg gagaagtacg gcctcaggcc ggagcagtgg 541 gtggacttcc gcgccctcgt gggggacccc tccgacaacc tccccggggt caagggcatc 601 ggggagaaga ccgccctcaa gctcctcaag gagtggggaa gcctggaaaa cctcctcaag 661 aacctggacc gggtaaagcc agaaaacgtc cgggagaaga tcaaggccca cctggaagac 721 ctcaggctct ccttggagct ctcccgggtg cgcaccgacc tccccctgga ggtggacctc 781 gcccaggggc gggagcccga ccgggagggg cttagggcct tcctggagag gctggagttc 841 ggcagcctcc tccacgagtt cggcctcctg gaggcccccg cccccctgga ggaggccccc 901 tggcccccgc cggaaggggc cttcgtgggc ttcgtcctct cccgccccga gcccatgtgg 961 gcggagctta aagccctggc cgcctgcagg gacggccggg tgcaccgggc agcagacccc 1021 ttggcggggc taaaggacct caaggaggtc cggggcctcc tcgccaagga cctcgccgtc 1081 ttggcctcga gggaggggct agacctcgtg cccggggacg accccatgct cctcgcctac 1141 ctcctggacc cctccaacac cacccccgag ggggtggcgc ggcgctacgg gggggagtgg 1201 acggaggacg ccgcccaccg ggccctcctc tcggagaggc tccatcggaa cctccttaag 1261 cgcctcgagg gggaggagaa gctcctttgg ctctaccacg aggtggaaaa gcccctctcc 1321 cgggtcctgg cccacatgga ggccaccggg gtacggcggg acgtggccta ccttcaggcc 1381 ctttccctgg agcttgcgga ggagatccgc cgcctcgagg aggaggtctt ccgcttggcg 1441 ggccacccct tcaacctcaa ctcccgggac cagctggaaa gggtgctctt tgacgagctt 1501 aggcttcccg ccttggggaa gacgcaaaag acaggcaagc gctccaccag cgccgcggtg 1561 ctggaggccc tacgggaggc ccaccccatc gtggagaaga tcctccagca ccgggagctc 1621 accaagctca agaacaccta cgtggacccc ctcccaagcc tcgtccaccc gaggacgggc 1681 cgcctccaca cccgcttcaa ccagacggcc acggccacgg ggaggcttag tagctccgac 1741 cccaacctgc agaacatccc cgtccgcacc cccttgggcc agaggatccg ccgggccttc 1801 atcgccgagg aggggtggct attggtggcc ctggactata gccagataga gctcagggtg 1861 ctggcccacc tctccggcga cgagaacctg atccgggtct tccaggaggg gcgggacatc 1921 cacacggaga ccgccagctg gatgttcggc gtcccccggg aggccgtgga ccccctgatg 1981 cgccgggcgg ccaagaccat caacttcggg gtcctctacg gcatgtcggc ccaccgcctc 2041 tcccaggagc tagccatccc ttacgaggag gcccaggcct tcattgagcg ctactttcag 2101 agcttcccca aggtgcgggc ctggattgag aagaccctgg aggagggcag gaggcggggg 2161 tacgtggaga ccctcttcgg ccgccgccgc tacgtgccag acctagaggc ccgggtgaag 2221 agcgtgcggg aggcggccga gcgcatggcc ttcaacatgc ccgtccaggg caccgccgcc 11 H-ar- ^*R0/EP

2281 gacctcatga agctggctat ggtgaagctc ttccccaggc tggaggaaat gggggccagg 2341 atgctccttc aggtccacga cgagctggtc ctcgaggccc caaaagagag ggcggaggcc 2401 gtggcccggc tggccaagga ggtcatggag ggggtgtatc ccctggccgt gcccctggag 2461 gtggaggtgg ggatagggga ggactggctc tccgccaagg agtga

*RO/EP 12 [-1-E -

Advantages of the DNA Polymerase of the Invention

The chimeric thermostable DNA polymerase of the invention represents an improvement over thermostable DNA polymerases described in the literature. In particular, the DNA polymerase of the invention provides the following combination of properties:

1. High efficiency of amplification of long DNA sequences.

The efficiency of the chimeric enzyme is at least 5 times as high as that of Taq DNA polymerase and is no less than that of Tth DNA polymerase (Example 3).

2. High sensitivity to the presence of a mismatched nucleotide at the 3' primer end.

The chimeric enzyme is at least 6-fold more sensitive to the presence of a mismatch at the 3'-end of the primer than Tth DNA polymerase and is no less sensitive than Taq DNA polymerase (Example 4).

3. High specificity of DNA amplification in PCR.

The chimeric enzyme shows much higher specificity in PCR-based amplification of DNA than Tth DNA polymerase and no less specificity than Taq DNA polymerase (Example 5). Furthermore, (4) the DNA polymerase can be easily and efficiently expressed to a high level in a recombinant expression system, thereby facilitating commercial production of the enzyme (Example 2).

The combination of properties possessed by the DNA polymerase of the invention is particularly useful in polymerase chain reactions, and provides significantly improved results. Each of these properties is illustrated below in the accompanying Examples.

EXAMPLES

The Examples relate to the production and testing of chimeric polymerase of the invention. The Examples are illustrative of, but not binding on, the present invention. Any methods, preparations, solutions and such like, which are not specifically defined, may be found in Sambrook et al. All solutions are aqueous and made up in sterile, deionized water, unless otherwise specified. All enzymes were obtained from the Bioline Limited (London, GB)

EXAMPLE 1

Construction of a Chimeric Gene and an Expression System

A chimeric gene was constructed, comprising a portion of the Tth DNA polymerase gene and a portion of the Taq DNA polymerase gene. In more detail, the procedure was as follows, in this Example.

A fragment of Tth DNA polymerase gene [U.S. Pat. No. 5,618,711], representing amino acids 4 to 597, was obtained by PCR amplification of total Thermus thermophilus DNA, primed by ^*RO/EP 13 [-_~tø _

the two synthetic DNA primers PrTTHI and PrTTH2 (below). Total DNA from Thermus thermophilus was isolated by the phenol deproteinization method. The primers used were: PrTTHI 5¹- ATAGATCTGATGCTTCCGCTCTTTGA -3'[SEQ ID NO 3] PrTTH2 5'- GGCCCGGCGGATCCTCTGGCCCAA -3'[SEQ ID NO 4] Upstream primer PrTTHI is homologous to wild type DNA starting at codon 4; this primer is designed to incorporate a BgI Il site into the amplified DNA product. Downstream primer PrTTH2 is homologous to codons 592-599 on the non-coding strand of the wild-type gene encoding Tth DNA polymerase and includes a BamH I site.

PCR was performed using a DNA Thermal Cycler 480 (Perkin-Elmer-Cetus). The reaction mixture (50 mkl_) contained 67 mM Tris-HCI (pH 8.8), 16.6 mM (NH₄)₂ SO₄, 0.01% Tween- 20, 0.2 mM of each dNTP's, 1.5 mM MgCI₂, 10 pmol of each primer, 100 ng of DNA as a template, and 5 U of Taq DNA polymerase. The reaction included 25 cycles: 94. degree. C-- 30 s; 58. degree. C.--30 s; 72. degree. C.--100 s.

A DNA fragment of Taq DNA polymerase gene [Lawyer, F. C. et al., J. Biol. Chem., V.261 , P.

6427-6437], encoding amino acids 592 to 832 was obtained by PCR amplification of total

Thermus aquaticus YT 1 DNA, primed by the two synthetic DNA primers PrTAQI and

PrTAQ2 (below). Total DNA from Thernus aquaticus YT 1 was isolated by the phenol deproteinization method [Sambrook et al.]. The primers used were:

PrTAQI 5'- CAGAGGATCCGCCGGGCCTTCA -3'[SEQ ID NO 5]

PrTAQ2 5'- AAGTCGACTCACTCCTTGGCGGAGAGCCA -3'[SEQ ID NO 6]

Upstream primer PrTAQI is homologous to wild type Thermus aquaticus YT 1 DNA [Lawyer et al.] starting at codon 592 of the DNA polymerase gene and includes a BamH I site.

Downstream primer PrTAQ2 is homologous to codons 827-832 on¹ the other strand of the wild-type gene encoding Thermus aquaticus DNA polymerase and is designed to incorporate a SaIG I site and a stop codon into the amplified fragment.

PCR was performed using a DNA Thermal Cycler 480 (Perkin-Elmer-Cetus). The reaction mixture (50 mkL) contained 67 mM Tris-HCI (pH 8.8), 16.6 mM (NH₄)₂SO₄₎ 0.01 % v/v Tween-

20, 0.2 mM of each dNTP's, 1.5 mM MgCI₂, 10 pmol of each primer, 100 ng of DNA as a template, and 5 U of Taq DNA polymerase. The reaction included 25 cycles: 94.degree. C--

30 s; 58.degree. C.--30 s; 72. degree. C.--150 s.

The amplified fragments (from Tth and Taq genes) were purified by 2% w/v agarose-gel electrophoresis, phenol extraction and were precipitated by ethanol. They were then digested with restriction endonuclease BamH I and ligated. The chimeric DNA fragment consisting of the Tth and Taq DNA fragments was obtained as a result of the manipulations. The chimeric DNA fragment was purified by 1.5% w/v agarose-gel electrophoresis and phenol extraction, and was then precipitated by ethanoi. The fragment was digested with restriction endonucleases BgI Il and SaIG I and ligated into plasmid pCQV2 [Queen, C, J. MoI. Appl. Genet., V.2, P.1-10] which had been digested with the BamH I and SaIG I restriction enzymes and previously treated with calf intestinal alkaline phosphatase ¹⁴ Mg - *RO/EP

[Sambrook et al.]. As a result, the chimeric gene encoding the chimeric DNA polymerase was cloned into pCQV2 under the control of the P_R-promoter.

Ligation was conducted with T4 DNA ligase in a 50 mkl_ volume containing 200 ng vector (plasmid pCQV2) and 200 ng of the insert. E. coli JM 109 cells were transformed with the ligation mixture according to the method of Dower et al. [Dower et al., Nucl. Acid. Res., V.16 (1988), P. 1 127]. Transformed cells were grown on LB medium at 30. degree. C. Clones were selected from ampicillin resistant colonies and checked to determine which ones contained the chimeric DNA polymerase gene insert.

Selected positives clones were assayed for production of protein of the corresponding MW by 12% SDS-polyacrylamide gel electrophoreses [Laemmli U., Nature V.227 (1970), P.680- 685]. The cells were grown to an optical density of A₆₀₀=O.4 in 500 ml of LB medium containing ampicillin (75 mkg/ml) at 30. degree. C. Heating to 42. degree C. induced expression of the cloned gene. The cells were further incubated for 4 h at 42. degree. C. Cells were harvested by centrifugation and the enzyme was partially purified as follows. All samples were isolated at 4. degree. C. Cells (0.5 g) were suspended in 2 ml of buffer A (20 mM K-phosphate pH 7.0, 2 mM DTT, 0.5 mM EDTA) containing 0.2M NaCI and 0.1 mM phenylmethylsulphonylfluoride (PMSF). The cells were disrupted by ultrasonic disintegration (MSE, 150 wt) at maximum amplitude for 15 sec (3 impulses, each for 5 sec) with cooling on ice. The suspension was centrifuged at 20,000 g, the supernatant collected, and 5% v/v polyethylenimine was added to a final concentration of 0.1 % v/v. The resulting precipitate was separated by centrifugation, and the supernatant removed. The supernatant proteins were then precipitated by solid ammonium sulfate at 75% saturation. The polymerase- containing precipitate was collected by centrifugation at 20,000 g, dissolved in 3 ml of buffer A, containing 0.1 M NaCI and 0.2% Tween-20, then heated for 5 minutes at 75. degree. C. and centrifuged (10 min, 20,000 g). Denatured proteins were discarded and supernatant was assayed by its ability to perform PCR. A plasmid was isolated and purified from cells in which truncated chimeric polymerase was active in PCR.

PCR assays were conducted using a DNA thermal cycler 480 (Perkin Elmer-Cetus). The reaction mixture (50 mkL) contained 67 mM Tris-HCI (pH 8.8 at 25. degree. C), 16.6 mM (NhU)₂SO₄, 0.01% Tween-20, 0.2 mM each dNTP, 1.5 mM MgCI₂, 10 pmol each primer (Pr.lambda.1 : 5'-GATGAGTTCGTGTCCGTACAACTGG-3'[SEQ ID NO 7] and Pr.lambda.2: 5'-GGTTATCGAAATCAGCCACAGCGCC-3'[SEQ ID NO 8]), 50 ng template lambda DNA and 2 .mkl of the above supernatant containing the enzyme. 30 cycles of the following cycle was carried out; 94. degree. C. for 30 seconds, 57. degree. C. for 40 seconds and 72 .degree. C. for 30 seconds.

Plasmid DNA was isolated from cells which produced a chimaeric enzyme that was active in PCR. The plasmid was purified, and designated pTTT. The nucleotide sequence encoding the chimaeric enzyme was verified by sequencing. The construction of pTTT is shown in FlG. 1. RO/EP 15 HP "

EXAMPLE 2

Preparation of Chimeric Polymerase Using an Expression Vector (Plasmid pTTT)

E. coli JM 109 cells were transformed with the plasmid pTTT according to the method of Dower et al. [1988, Nucl. Acid. Res., V.16, P.6127]. The transformed cells were grown to an optical density of A._6Oo = 0.4 in 7 L of LB medium containing ampicillin (75 mkg/ml) at 30. degree. C. Expression of the chimeric gene encoding the chimeric polymerase was induced by heating to 42. degree C. The cells were further incubated for 7 h at 42. degree. C. Cells were harvested by centrifugation

The cells (35g) were suspended in 70 ml of buffer A (20 mM K-phosphate pH 7.0, 2 mM DTT, 0.5 mM EDTA) containing 0.2M NaCI and 0.1 mM PMSF. The cellular walls were disrupted with an ultrasonic disintegrator (MSE, 150 wt) at maximum amplitude for 15 minutes (30 impulses, each for 30 sec) and with cooling on ice. The suspension was then centrifuged at 40,000 g, the pellet discarded, and 5% polyethylenimine was added to the supernatant to a final concentration of 0.1%. The precipitate was separated by centrifugation, and the remaining proteins precipitated with ammonium sulfate at 45% saturation. The resulting polymerase-containing precipitate was collected by centrifugation at 20,00Og and dissolved in buffer A (30 ml) containing 0.1 M NaCI and 0.2% Tween-20, heated for 15 minutes at 75. degree. C. in the presence of 10 mM MgCI₂, and centrifuged for 10 minutes at 40,000 g.

The supernatant was loaded on to a (2.5 X 20 cm) phosphocellulose P-1 1 column (Whatman) equilibrated in buffer A containing 0.1 M NaCI, and washed out with the same buffer. The proteins were eluted with a linear gradient of NaCI concentrations ranging from 100 to 500 mM in buffer A. The gradient volume was 800 ml, and the flow rate was 60 ml/h. Polymerase was eluted at NaCI concentrations ranging from 280 to 330 mM. The fractions were tested for Polymerase activity, assayed via inclusion of the radioactive- labeled nucleotide ³²P(dATP) into the acid-insoluble pellet [Myers T. W., Gelfand D. H., (1991 ) Biochemistry, v30, N31 , p7661-7666].

Specifically, the amount of the enzyme that incorporated 10 nmol of deoxynucleotide triphosphates into the acid-insoluble fraction within 30 minutes under conditions described below was taken as one unit of activity. The reaction mixture (50 mkL) contained 25 mM N- Tris [Hydroxymethyl] methyl-3-aminopropanesulphonic acid (TAPS), pH 9.3, 50 mM KCI, 2 mM MgCI₂; 1 mM β-mercaptoethanol; 0.2 mM of each dNTP's, 1 mkCi ³²P(dATP), and 12.5 mkg of activated salmon sperm DNA. The polymerase activity was determined at 73. degree. C. (Salmon sperm DNA (12.5 mg/ml) was activated in 10 mM Tris-HCI (pH 7.2) containing 5 mM MgCI. sub.2 with pancreatic DNase I (0.03 U/ml) at 4. degree. C. for 1 h and then heated at 95. degree. C. for 5 minutes.)

Fractions containing the polymerase activity were combined, dialyzed against buffer A containing 50 mM NaCI and loaded on to a column (0.6 X 6 cm) of DEAE-cellulose (Whatman) equilibrated with same buffer. The proteins were eluted with a linear gradient of NON-EXISTENT

^*R0/EP 16 _W-

NaCI concentrations ranging from 50 to 250 mM in buffer A. The gradient volume was 150 ml, and the flow rate was 15 ml/h. The polymerase was eluted at 150-200 mM NaCI.

Polymerase activity was assayed as described above. Yield of polymerase activity was

1 ,475,000 units.

The purified enzymes were stored at -2O.degree. C. in the following buffer: 100 mM NaCI; 10 mM Tris HCI pH 7.5; 1 mM DTT; 0.2% Tween 20 and 50% (v/v) glycerol.

Homogeneity of the polymerase preparations was not less than 95% according to SDS electrophoresis data on a 10% polyacrylamide gel.

EXAMPLE 3

Efficiency of PCR Amplification

The efficiency of PCR amplification by the Chimeric, Taq and Tth polymerases was estimatid by amplification of 2500-bp DNA fragment.

PCR reactions were performed using a DNA thermal cycler 480 (Perkin Elmer-Cetus). The reaction mixture (50 mkl_) contained 67 mM Tris-HCI (pH 8.8 at 25. degree. C), 16.6 mM (NhU)₂SO₄, 0.01% Tween-20, 0.2 mM each dNTP, 1.5 mM MgCI₂, 10 pmol each primer (Pr.lambda.1: 5'-GATGAGTTCGTGTCCGTACAACTGG-3'[SEQ ID NO 7] and Pr.lambda.3: 5¹- TGTTGACCTTGCCTGCAGCAACGC -3'[SEQ ID NO 9]), 5 ng template lambda DNA. The reactions were performed with 0.5 U of Tth polymerase; or 0.5 U of the Chimeric polymerase; or 0.5 U, 1.5 U, 2.5 U of Taq polymerase. 26 cycles of the following cycle was carried out: 94. degree. C. for 30 seconds, 57. degree. C. for 40 seconds and 72 .degree. C. for 100 seconds.

The results are shown in FIG. 2, and indicate that 0.5 U of the chimeric enzyme of the invention has the efficiency of PCR amplification similar to 0.5 U of Tth polymerase and 2.5 U of Taq polymerase. Thus, the chimeric enzyme has at least 5 times higher efficiency in PCR than Taq polymerase.

EXAMPLE 4

Sensitivity to the Presence of a Mismatched Nucleotide at the 3' Primer End

Enzyme sensitivity of the Chimeric, Taq and Tth DNA polymerases to the presence of a mismatch at the 3'-end of a primer was estimatid by comparing the amounts of DNA synthesized in PCR with the primers either containing or not the 3 ^'-mismatching nucleotide. PCR amplification of the 500-bp phage lambda DNA fragment was performed with the primer pairs: Pr.lambda.1 [SEQ ID NO 7] / Pr.lambda.2 [SEQ ID NO 8]; Pr.lambda.12 (5^'- GATGAGTTCGTGTCCGTACAACTGC) [SEQ ID NO 10] / Pr.lambda.2 [SEQ ID NO 8]; Pr.lambda.13 (δ^'-GATGAGTTCGTGTCCGTACAACTGA) [SEQ ID NO 1 1] / Pr.lambda.2 [SEQ ID NO 8]; Pr.lambda.14 (δ^'-GATGAGTTCGTGTCCGTACAACTGT) [SEQ ID NO 12] / Pr.lambda.2 [SEQ ID NO 8]. The primers Pr.lambda.1 and Pr.lambda.2 were complementary to the corresponding fragment of phage lambda DNA; the primers Pr.lambda12, Pr.lambda13 ^*RO/EP 17 HP -

and Pr.lambda14 were identical to Pr.lambdal, except the 3^'-terminal nucleotide. The reaction mixture (50 μl) contained 67 mM Tris-HCI (pH 8.8), 16.6 mM (NhU)₂SO₄, 0.01 % Tween-20, 0.2 mM of each dNTPs, 1.5 mM MgCI₂, 17 pmol of each primer, 15 ng of phage lambda DNA as a template, and 1.5 U of the Chimeric, or Taq, or Tth DNA polymerase. The reaction proceeded in 25 cycles: 94°C for 45 s; 59°C for 30 s; 72°C for 30 s. To estimate the amount of the synthesized DNA, [alpha-³²P]dATP was added to the reaction mixture (2 μCi/50 μl reaction mixture), and radioactivity of the acid-insoluble fraction was then determined. For this purpose, the reaction was performed, and 20 μl of the resulting mixture was applied on a GF/B filter (Whatman). The filter was washed with 10% trichloroacetic acid and dried. The radioactivity was determined with a Beckman LS 9800 scintillation counter using Ready-Solv HP scintillation liquid (Beckman).

The results are shown in Table 1 , and indicate that the presence of mismatching nucleotide at the 3^'-end of the elongated DNA strand decreased to the same extent the PCR amplification efficiency by both Taq DNA polymerase and Chimeric DNA polymerase. Thus, the chimeric enzyme is no less sensitive to the presence of a mismatch than Taq DNA polymerase and is at least 6-fold more sensitive than Tth DNA polymerase (Table 1 ).

EXAMPLE 5

Specificity of DNA Amplification in PCR.

Specificity of PCR DNA amplification is a ratio of target product of amplification to total synthesized DNA. Enzyme specificity of the Chimeric, Taq and Tth DNA polymerases was estimatid by amplification of 2500-bp phage lambda DNA fragment in the presense of considerable quantity of E. coli DNA.

PCR reactions were performed using a DNA thermal cycler 480 (Perkin Elmer-Cetus). The reaction mixture (50 mkL) contained 67 mM Tris-HCI (pH 8.8 at 25. degree. C), 16.6 mM (NH^)₂SO₄, 0.01% Tween-20, 0.2 mM each dNTP, 1.5 mM MgCI₂, 20 pmol each primer (Pr.lambda.1 [SEQ ID NO 7] and Pr.lambda.3 [SEQ ID NO 9]), 5 ng template lambda DNA, and 300 ng of E. coli DNA. The reactions were performed with 3.5 U of Tth, Chimeric or Taq DNA polymerase. 30 cycles of the following cycle was carried out: 94. degree. C. for 30 seconds, 57. degree. C. for 40 seconds and 72 .degree. C. for 100 seconds. The results are shown in FIG. 3, and indicate that chimeric enzyme shows much higher specificity in PCR-based amplification of DNA than Tth DNA polymerase and no less specificity than Taq DNA polymerase. ^*RO/EP

18 [-H9 -

Table

TABLE 1. Radioactive label incorporation into the 500-bp DNA fragment synthesized with Taq, Tth, or the chimeric DNA polymerase by PCR with primers containing or not containing 3^*-mismatchin nucleotides described in detail in Exam le 4 .

O/EP

19 - PPi*-

Figures

The figures referred to in the Examples above are described more fully below.

FIG. 1. Scheme illustrating steps in construction of plasmid pTTT, which contains the chimeric gene of the chimeric polymerase of the invention (described in detail in Example 1).

FIG. 2. Electrophoretic analysis of PCR products, which compares the yield of 2500-bp DNA fragment obtainable by PCR amplification with Taq DNA polymerase, Tth DNA polymerase and the chimeric DNA polymerase of this invention and indicates that 0.5 U of the chimeric enzyme has the efficiency of PCR amplification similar to 0.5 U of Tth polymerase and 2.5 U of Taq polymerase. 2500-bp DNA fragment was amplified with 0.5 U of Tth (lane 1 ); 0.5 U of the chimeric enzyme (lane 2); 0.5 U, 1.5 U, 2.5 U of Taq polymerase (lanes 3, 4, 5 correspondingly) (described in detail in Example 3).

FIG. 3. Electrophoretic analysis of PCR products obtained in the presense of considerable quantity of E. coli DNA, which compares the specificity of PCR amplification reactions performed with Taq DNA polymerase, Tth DNA polymerase and the chimeric DNA polymerase and indicates that chimeric enzyme shows much higher specificity in PCR than Tth and no less specificity than Taq polymerase. The reactions were performed with 3.5 U of Tth (lane 1 ), 3.5 U of the chimeric enzyme (lane 2) and 3.5 U of Taq DNA polymerase (lane 3) (described in detail in Example 5).

/EP

20 f-2^-

REFERENCES

Patent Documents

United States Patent 6,228,628 // Mutant chimeric DNA polymerase / Gelfand D. H.,

Reichert F.L.

United Kingdom Patent GB2, 344,591 // Thermostable DNA polymerase / Kramarov V.M.,

Ignatov K.B., Hallinan J. P.

United States Patent 4,965,188 // Process for amplifying, detecting, and/or cloning nucleic acid sequences using a thermostable enzyme / Mullis K.B., Erlich H. A., Gelfand

D.H., Horn G., Saiki R.K.

United States Patent 5,618,71 1 // Recombinant expression vectors and purification methods for Thermus thermophilus DNA polymerase / Gelfand D. H., Lawyer F. C, Stoffel

S.

United States Patent 5,079,352 // Purified thermostable enzyme / Gelfand D. H., Stoffel S.,

Lawyer F.C., Saiki R.K.

Other References

Ohler L D. , and Rose E A. Optimization of long-distance PCR using a transposon-based model system. // PCR Methods Appl. V.2 (1992), P. 51-59

Ignatov K. B., Kramarov V.M., Chostyakova L. G. and Miroshnikov A.I. Factors determining different processivity of Tth and Taq DNA polymerases in amplification of phage λ

DNA. // MoI. Biol. (Russ.) V.31 (1997), P. 956-961

Ignatov K.B., Kramarov V.M., Uznadze O. L. and Miroshnikov A.I. Tth DNA polymerase - mediated amplification of DNA fragments using primers with mismatches in the 3'- region. // Bioorg. Khim. (Russ.) V.23 (1997), P. 817-822

Villbrandt B., Sobek H., Frey B. and Schomburg D. Domain exchange: chimeras of

Thermus aquaticus DNA polymerase, Escherichia coli DNA polymerase I and

Thermotoga neapolitana DNA polymerase // Protein Eng. V.13 (2000), P. 645-654.

Barnes W M. The fidelity of Taq polymerase catalyzing PCR is improved by an N- terminal deletion // Gene V.112 (1992), P. 29-35.

Blanco L., Bernad A., Blasco M. A., and Salas M. A general structure for DNA- dependent

DNA polymerases // Gene V.100 (1991 ), P. 27-28.

Goodchild J., Conjugates of oligonucleotides and modified oligonucleotides: a review of their synthesis and properties. // Bioconjugate Chemistry V.1 (1990), P. 165-187.

Sambrook et al, "Molecular cloning - A Laboratory Manual", Cold Spring Harbor

Laboratory Press, 1989.

Lawyer F. C₁ Stoffel S., Saiki R. K., Myambo K., Drummond R., and Gelfand D. H. Isolation, characterization and expression in E. coli of the DNA polymerase gene from Thermus aquaticus. Il J. Biol. Chem. V.264 (1989), P. 6427-6437

Queen C. A vector that uses phage signals for efficient synthesis of proteins in E.coli Il

J. MoI. Appl. Genet. V.2 (1983), P. 1-10.

Dower WJ. , Miller J. F. and Ragsdale CW. High efficiency transformation of E. coli by high voltage electroporation. // Nucleic Acids Res., V.16 (1988), P. 6127-6145 Laemmli U.K. Cleavage of structural proteins assembly of the head of bacteriophage

T4. // Nature, V.227 (1970), P. 680-685.

UH3O2O06013527

Claims

- 26 -"Claims:"We claim:

1. A chimeric thermostable DNA polymerase comprising an N-terminal region and a C- terminal region, wherein said N-terminal region comprises an amino acid sequence comprising amino acids in positions "k" through "n" of a Thermus thermophilus (Tth) DNA polymerase, wherein "k" is between amino acids 1 and 280 of a Tth DNA polymerase, wherein "n" is between amino acids 555 and 601 of a Tth DNA polymerase and corresponding to an amino acid in position "m" of a Thermus aquaticus (Taq) DNA polymerase, wherein "m" is m=n-2; wherein said C-terminal region comprises amino acids m+1 through 832 of said Taq DNA polymerase.

2. A chimeric thermostable DNA polymerase of claim 1 , wherein said enzyme comprises amino acids 4-595 of Thermus thermophilus DNA polymerase and amino acids 594- 832 of Thermus aquaticus DNA polymerase.

3. A chimeric thermostable DNA polymerase of claim 1 or 2 comprising the sequence given in SEQ ID No. 1 , or a sequence which is at least 80, 90 or 95% identical thereto.

4. An isolated nucleic acid that encodes the chimeric DNA polymerase of claim 2 or 3.

5. An isolated nucleic acid of claim 4 comprising the nucleotide sequence that is Seq ID No. 2, or a sequence which is 80, 90 or 95% identical thereto.

6. A recombinant DNA vector that comprises the nucleic acid of claim 4 or 5.

7. A laboratory kit containing the chimeric enzyme of any of the claims 1 to 3.

8. A laboratory kit of claim 7, wherein said chimeric enzyme is the enzyme of claim 1 to 3.

9. A chimeric thermostable DNA-Polymerase, which has the synthesis efficiency of Tth- DNA-Polymerase and the specificity of Taq-DNA-Polymerase.