Modified thermostable DNA polymerase, and DNA polymerase composition for nucleic acid amplification

Abstract: A modified thermostable DNA polymerase having 5 % or less of the 3'-5' exonuclease activity of the enzyme before modification and a DNA polymerase composition for amplifying nucleic acid, which comprises the modified thermostable DNA polymerase having 0 to 5 % of the 3'-5' exonuclease activity of the enzyme before modification and an unmodified thermostable DNA polymerase having 3'-5' exonuclease activity or a modified thermostable DNA polymerase having 100 to 6 % of the 3'-5' exonuclease activity of a thermostable DNA polymerase before modification; a method for amplifying nucleic acid by use of said modified thermostable polymerase or said DNA polymerase composition; and a reagent therefor.

Description: The present invention relates to a modified thermostable DNA polymerase, a DNA polymerase composition for amplifying nucleic acid, and a reagent for amplifying nucleic acid containing said enzyme or composition, as well as a method for amplifying nucleic acid by use of said reagent. Conventionally, a large number of studies have been conducted on thermostable DNA polymerases for use in techniques for amplification of nucleic acid, such as polymerase chain reaction (PCR) etc. Examples of thermostable DNA polymerases used in PCR are DNA polymerase (Tth polymerase) mostly derived from Thermus thermophilus and DNA polymerase (Taq polymerase) derived from Thermus aquaticus . Other known examples are DNA polymerases derived from a hyperthermophilic archaeon strain, such as thermostable DNA polymerase derived from Pyrococcus furiosus (Pfu polymerase, WO92/09689, Unexamined Published Japanese Patent Application No. 328,969/1993) and thermostable DNA polymerase derived from Thermococcus litoralis (Tli polymerase, Unexamined Published Japanese Patent Application No. 7160/1994). The present inventors have previously found thermostable DNA polymerase excellent in thermostability and DNA extension rate derived from Pyrococcus sp. KOD1 (KOD polymerase, Unexamined Published Japanese Patent Application No. 298,879/1995). US-5 489 523 describes a recombinant Pyrocococcus furiosus DNA polymerase that is deficient in 3'-5'-exonuclease activity that is useful in DNA cycle sequencing reactions. EP-0 745 675 describes a thermostable DNA polymerase having at least 30 bases/second of DNA extension rate and a 3'-5'-exonuclease activity derived from an hyperthermophilic archaeon strain KOD1. However, these thermostable DNA polymerases have problems such as insufficient amplification of nucleic acid. Further problems with polymerase derived from a hyperthermophilic archaeon strain such as Pyrococcus sp. KOD1 are that it has a 3'-5' exonuclease activity and there is a limit to PCR conditions including reaction time, enzyme amount, primer concentration etc. Therefore, there is demand for novel thermostable DNA polymerase. As a result of their eager research, the present inventors have succeeded in creating a modified enzyme derived from Pyrococcus sp. KOD1, said enzyme having the 3'-5' exonuclease activity reduced to 5 % or less of the original polymerase before modification while maintaining the DNA extension rate and thermostability of the original polymerase. The present inventors have further found that the efficiency of amplification using a polymerase before modification is improved by use of its modified thermostable DNA polymerase having a DNA extension rate of at least 30 bases/second and capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °c because it was difficult to measure the pH at 95 °C) after treatment at 95 °C for 6 hours, said modified enzyme having the 3'-5' exonuclease activity reduced to 5 % or less of the polymerase before modification, and the present inventors thereby completed the present invention. That is, the present invention is a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 5 % or less of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; and thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours, molecular weight: 88 to 90 kDa; and amino acid sequence: as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 143-, 210- and 311-positions has been replaced by another amino acid. Further, the present invention is a method for amplifying nucleic acid, which comprises reacting DNA as a template, primers, dNTP and the thermostable DNA polymerase of claims 1 to 10, thus extending the primers to synthesize DNA primer extension products. Further, the present invention is a reagent for amplifying nucleic acid, which comprises 2 kinds of primer, one of which is complementary to a DNA extension product of another primer, dNTP, said thermostable DNA polymerase according to claims 1 to 10, and a buffer solution. As one of methods for amplifying long chain nucleic acid, there is a report on PCR making use of both Taq polymerase (KlenTaq-278) free of 3'-5'exonuclease activity and Pfu polymerase (or Tli polymerase) having 3'-5' exonuclease activity, or of a DNA polymerase composition containing a mixture of their mutant enzymes (Barns, W.M. (1994) Proc. Natl. Acad. Sci. USA 91, 2216-2220). There is another report on PCR making use of a polymerase composition containing a mixture of Tth polymerase free of 3'-5' exonuclease activity, Pfu polymerase (or Tli polymerase) with 3'-5' exonuclease activity, and thermostable DNA polymerase derived from Thermotoga maritima (Unexamined Published Japanese Patent Application No. 38198/1996). Higher efficiency of amplification can be attained by such a composition than by one kind of DNA polymerase but is still not sufficient because 2 kinds of DNA polymerase having different properties in thermostability and DNA extension rate are used. Hence, there has been demand for a method further excellent in efficiency of amplification. As a result of their eager research under these circumstances, the present inventors found that PCR excellent in efficiency of amplification can be effected using a DNA polymerase composition for nucleic acid amplification, consisting of a combination of first and second DNA polymerases being almost identical to each other with respect to thermostability and DNA extension rate, the activity of the second DNA polymerase being present at a lower level than that of the first DNA polymerase, specifically a DNA polymerase composition comprising DNA polymerases selected from the group consisting of a modified thermostable DNA polymerase (first polymerase) having 0 to 5 % of the 3'-5' exonuclease activity of the naturally occurring enzyme before modification and a modified thermostable DNA polymerase (second polymerase) having 100 to 6 % of the 3'-5' exonuclease activity of a naturally occurring DNA polymerase or of its original naturally occurring enzyme before modification. That is, the present invention is a DNA polymerase composition for amplifying nucleic acid, which comprises a modified thermostable DNA polymerase having 0 to 5 % of the 3'-5' exonuclease activity of the original enzyme before modification (first polymerase) and the original enzyme or a modified thermostable DNA polymerase having 100 to 6 % of the 3'-5' exonuclease activity of its original thermostable enzyme before modification (second polymerase), said first and second DNA polymerases having a DNA extension rate of at least 30 bases/second and capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours. Further, the present invention is a method for amplifying nucleic acid, which comprises reacting DNA as a template, primers, dNTP and said DNA polymerase composition, thus extending the primers to synthesize a DNA primer extension product. Further, the present invention is a reagent for amplifying nucleic acid, which comprises 2 kinds of primer, one of which is complementary to a DNA extension product of another primer, dNTP, said DNA polymerase composition, divalent ions, monovalent ions, and a buffer solution. FIG. 1 shows the polymerase activity of the modified DNA polymerase and degree of decomposition of DNA. FIG. 2 shows the thermostability of the modified DNA polymerase. FIG. 3 shows the result of PCR by use of the modified DNA polymerase (for plasmid). FIG. 4 shows the result of PCR by use of the modified DNA polymerase (for human genome). FIG. 5 shows the result of PCR by use of the DNA polymerase composition (for human genome). FIG. 6 shows the amino acid sequences of the exo regions of the thermostable DNA polymerase. FIG. 7 shows the polymerase activity of the modified DNA polymerase and degree of decomposition of DNA. FIG. 8 shows polymerase activity relative to the naturally occurring KOD polymerase. In the present invention, DNA polymerase activity refers to a catalytic activity to introduce deoxyribonucleoside-5'-monophosphate template-dependently into deoxyribonucleic acid by covalently binding the α-phosphate of deoxyribonucleoside-5'-triphosphate to the 3'-hydroxyl group of an oligonucleotide or polynucleotide annealed to a template DNA. If the enzyme activity in a sample is high, activity measurement shall be carried out after the sample is diluted with a preserving buffer solution. In the present invention, 25 µl of Solution A below, 5 µl each of Solutions B and C below, and 10 µl of sterilized water are added to an Eppendorf tube, then stirred and mixed, and 5 µl of the above enzyme solution is added to it and reacted at 75 °C for 10 minutes. Thereafter, the sample is cooled on ice, and 50 µl of Solution E and 100 µl of Solution D below are added to it, then stirred, and cooled on ice for 10 minutes. The solution is filtered through a glass filter (Wattman GF/C Filter), and the filter is washed intensively with Solution D and ethanol, and the radioactivity of the filter is counted in a liquid scintillation counter (Packard) to determine the incorporation of the nucleotide into the template DNA. In the present invention, 1 unit of the enzyme activity shall be defined as the amount of the enzyme causing 10 nmol nucleotide per 30 minutes to be incorporated into the acid insoluble fragment under these conditions. A 40 mM Tris-HCl (pH 7.5) 16 mM magnesium chloride 15 mM dithiothreitol 100 µg/ml BSA B 2 µg/µl activated calf thymus DNA C 1.5 mM dNTP (250 cpm/pmol [ 3 H] dTTP) D 20 % trichloroacetic acid (2 mM sodium pyrrophosphate) E 1 µg/µl carrier DNA In the present invention, the 3'-5' exonuclease activity refers to the activity of deleting a 3'-terminal region of DNA to deliver 5'-mononucleotide to a template. The activity measurement method is as follows: 50 µl reaction solution (120 mM Tris-HCl (pH 8.8 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.1 % Triton X-100, 0.001 % BSA, 5 µg of E . coli DNA labeled with tritium) is pipetted into a 1.5 ml Eppendorf tube, followed by adding DNA polymerase to it. After the mixture is reacted at 75 °C for 10 minutes, the reaction is terminated by cooling on ice. Then, 50 µl of 0.1 % BSA is added to it as a carrier, and then 100 µl of a solution containing 10 % trichloroacetic acid and 2 % sodium pyrrophosphate is mixed with it. After the mixture is left on ice for 15 minutes, it is centrifuged at 12,000 r.p.m. for 10 minutes to separate precipitates. 100 µl of the supernatant is measured for radioactivity in a liquid scintillation counter (Packard) whereby the amount of the nucleotide delivered to the acid soluble fragment is determined. In the present invention, DNA extension rate refers to the number of DNAs extended per unit time. The measurement method is as follows: A reaction solution of DNA polymerase (20 mM Tris-HCl (pH 7.5), 8 mM magnesium chloride, 7.5 mM dithiothreitol, 100 µg/ml BSA, 0.1 mM dNTP, 0.2 µCi [α -32 P]dCTP) is reacted at 75 °C with a single-stranded chain of M13mp18 DNA to which a prier had been annealed. The reaction is terminated by adding an equal volume of a reaction terminating solution (50 mM sodium hydroxide, 10 mM EDTA, 5 % Ficoll, 0.05 Bromophenol Blue). The DNA extended by the reaction is fractionated by electrophoresis on alkali agarose gel, and the gel is dried and subjected to autoradiography. As the DNA size marker, labeled λ/HindIII is used. The DNA extension rate is determined on the basis of the DNA size as determined with a band of this marker as an indicator. In the present invention, thermostability means residual activity at pH 8.8 (the pH value determined at 25 °C) after treatment at 95 °C for 6 hours. One embodiment of the present invention is a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 5 % or less of the 3'-5' exonuclease activity of the original enzyme before modification, DNA extension rate: at least 30 bases/second, thermostability: it is capable of maintaining residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours, optimum temperature: about 75 °C, molecular weight: 88 to 90 kDa, and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 143-, 210- and 311-positions has been replaced by another amino acid. Another embodiment of the present invention is a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and is free of a 3'-5' exonuclease activity, DNA extension rate: at least 30 bases/second, thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours, optimum temperature: about 75 °C, molecular weight: 88 to 90 kDa, and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 143-, 210- and 311-positions has been replaced by another amino acid. The thermostable DNA polymerase of the present invention before modification is an enzyme derived from Pyrococcus sp. KOD as a hyperthermophilic archaeon strain isolated in Kodakara Island, Kagoshima prefecture, Japan. The microbial properties of KOD producing this enzyme is described in Unexamined Published Japanese Patent Application No 298,879/1995. This enzyme is produced by culturing this strain. This enzyme has the following physicochemical properties: action: it has a DNA polymerase activity and has a 3'-5' exonuclease activity, DNA extension rate: at least 120 bases/second, thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours, optimum temperature: about 75 °C, molecular weight: 88 to 90 kDa, and amino acid sequence: the amino acid sequence of SEQ ID NO: 2. The thermostable DNA polymerase of the present invention has an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 143-, 210- and 311-positions has been replaced by another amino acid. One example is the enzyme where in SEQ ID NO: 2 aspartic acid at the 141-position has been replaced by alanine; glutamic acid at the 143-position by alanine; aspartic acid at the 141-position and glutamic acid at the 143-position respectively by alanine; asparagine at the 210-position by aspartic acid; or tyrosine at the 311-position by phenylalanine. For production of these modified enzymes, there is a method in which a gene coding for naturally occurring KOD polymerase is mutated so that a novel enzyme having a lower 3'-5' exonuclease activity than the naturally occurring KOD polymerase is produced by protein engineering. Although the KOD polymerase-coding gene to be mutated is not particularly limited, a gene defined in SEQ ID NO: 3 in the Sequence Listing, derived from Pyrococcus sp. KOD, was used in one embodiment of the present invention. A DNA polymerase gene derived from the KOD1 strain contains 2 intervening sequences (1374 to 2453 bp and 2708 to 4316 bp), and therefore a modified DNA polymerase having 3'-5' exonuclease activity reduced can be obtained by e.g. preparing with a PCR fusion method a mature gene having a nucleotide sequence as shown in SEQ ID NO: 3 from a gene as shown in SEQ ID NO: 1 and using the thus prepared gene. In another embodiment of the present invention, a novel enzyme with a less 3'-5' activity than the naturally occurring KOD polymerase is produced by mutating a gene coding for the amino acid sequence of SEQ ID NO: 1. To mutate the naturally occurring KOD polymerase gene, any of the known methods can be used. For example, use can be made of methods which involve bringing a drug as a mutagen into contact with the naturally occurring KOD polymerase gene or irradiating the gene with UV ray, or of protein engineering means such as the PCR technique or site specific mutagenesis. E . coli , whose gene undergoes mutations frequently because its mismatch repair is destroyed, can also be used for in vivo mutation. The Chameleon site-directed mutagenesis kit (Stratagene) used in the present invention make use of the following steps: (1) denaturing a plasmid having a target gene inserted into it and then annealing a mutagenesis primer and a selective marker to said plasmid, (2) extending DNA by a DNA polymerase and then conducting ligation reaction using a ligase, (3) cleaving the plasmid with a restriction enzyme whose restriction site is not present in the selective primer but present in the plasmid serving as a template, whereby DNA which was not mutated is cleaved, (4) transforming E . coli with the remaining plasmid, (5) preparing the mutant plasmid from the transformant, followed by conducting (3) and (4) repeatedly so that the plasmid mutated as desired is obtained. The modified polymerase gene obtained as described above is transformed into e.g. E . coli and then plated on a agar medium containing a drug such as ampicillin to form a colony. The colony is inoculated onto a nutrient medium such as LB medium or 2 x YT medium, then cultured at 37 °C for 12 to 20 hours, and disrupted so that a crude enzyme solution is extracted from it. To disrupt the microorganism, any of the known means by physical disruption by ultrasonication or glass beads or with lytic enzyme such as lysozyme can be used. The crude enzyme is thermally treated e.g. at 80 °C for 30 minutes to inactivate the polymerases originating in the host. Then, its DNA polymerase activity is determined and its 3'-5' exonuclease activity is determined and their activity ratio is determined. Then, this ratio is compared with that of the naturally occurring KOD polymerase in order to screen the enzyme having a reduced 3'-5' exonuclease activity. From the strain selected in this manner, the DNA polymerase can be purified using any of the known means, for example as follows: The microorganism cultured in a nutrient medium is recovered and disrupted enzymatically or by physical means so that a crude enzyme is extracted. The crude enzyme extract is subjected to heat treatment e.g. at 80 °C for 30 minutes and thereafter the KOD polymerase fraction is recovered by precipitation with sulfate ammonium. This crude enzyme fraction can be desalted by e.g. gel filtration on Sephadex G-25 (Pharmacia Biotech). After this procedure, a purified enzyme preparation can be obtained by chromatography such as Q-Sepharose, heparin-Sepharose etc. In this process, the enzyme preparation can be purified to such a degree that it shows an almost single band in SDS-PAGE. A DNA primer extension product can be produced using the modified thermostable DNA polymerase of the present invention by reacting primers and dNTP with DNA as a template to extend the primers. The primers are 2 kinds of oligonucleotide, one of which is preferably a primer complementary to a DNA extension product of another primer. Heating and cooling are carried out repeatedly. Magnesium ions and ammonium ions and/or potassium ions are preferably coexistent for the DNA polymerase of the present invention to maintain its activity. The PCR reaction solution may further contain a buffer solution and these ions along with BSA and a nonionic surface active agent such as Triton X-100 in the buffer solution. Because the 3'-5' exonuclease activity of the modified thermostable DNA polymerase of the present invention is reduced as compared with the enzyme before modification, PCR can be effected with higher efficiency of amplification by the modified thermostable DNA polymerase than by the enzyme before modification. Hereinafter, the composition of at least 2 kinds of thermostable DNA polymerase which are different in their 3'-5' exonuclease activity is described. A first DNA polymerase of the present invention is an enzyme having a 3'-5' exonuclease activity reduced to 0 to 5 % preferably 1 % or less of the 3'-5' exonuclease activity of the enzyme before modification. The first DNA polymerase includes an enzyme having an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 142-, 143-, 210- and 311-positions has been replaced by another amino acid. One example is an enzyme having an amino acid sequence as shown in SEQ ID NO: 2 in which aspartic acid at the 141-position has been replaced by alanine; glutamic acid at the 143-position by alanine; aspartic acid at the 141-position and glutamic acid at the 143-position respectively by alanine; asparagine at the 210-position by aspartic acid; or tyrosine at the 311-position by phenylalanine. Further, it includes the enzyme where isoleucine at the 142-position has been replaced by arginine. The first DNA polymerase of the present invention includes a modified thermostable DNA polymerase having the following physicochemical properties: action: it a DNA polymerase activity and has 0 to 5 % of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; thermostability: it is capable of maintaining residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; optimum temperature: about .75 °C; molecular weight: 88 to 90 kDa; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 142-, 143-, 210- and 311-positions has been replaced by another amino acid. The first DNA polymerase of the present invention further includes a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 0 to 5 % of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; optimum temperature: about 75 °C; molecular weight: 88 to 90 kDa; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which aspartic acid at the 141-position has been replaced by alanine; isoleucine at the 142-position by arginine; glutamic acid at the 143-position by alanine; aspartic acid at the 141-position and glutamic acid at the 143-position respectively by alanine; asparagine at the 210-position by aspartic acid; or tyrosine at the 311-position by phenylalanine. The second DNA polymerase of the present invention includes a modified thermostable polymerase having 100 to 6 % preferably 90 to 30 % of the 3'-5' exonuclease activity of a thermostable DNA polymerase having a 3'-5' exonuclease activity or the original unmodified thermostable DNA polymerase having a 3'-5' exonuclease activity. The second DNA polymerase includes e.g. the enzyme with the amino acid sequence of SEQ ID NO: 2 or with an amino acid sequence as shown in SEQ ID NO: 2 in which amino acids at the 140-, 142-, or 144-position have been replaced by other amino acids. One example is the enzyme with an amino acid sequence as shown in SEQ ID NO: 2 in which isoleucine at the 142-position has been replaced by aspartic acid, glutamic acid, asparagine, glutamine or lysin, or threonine at the 144-position by valine. The second DNA polymerase of the present invention includes a modified thermostable DNA polymerase having the following physicochemical properties: action: it has has a DNA polymerase activity and has a 3'-5' exonuclease activity; DNA extension rate: at least 30 bases/second; thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; optimum temperature: about 75 °C; molecular weight: 88 to 90 kDa; and amino acid sequence: the amino acid sequence of SEQ ID NO: 2. The second DNA polymerase of the present invention further includes a modified thermostable polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 100 to 6 % preferably 90 to 30 % of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; and thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids X 1 , X 2 and X 3 in an X 1 DX 2 EX 3 motif present in EXO 1 has been replaced by another amino acid. In the amino acid sequence of the DNA polymerase with a 3'-5' exonuclease activity, highly preserving amino acid regions for this exonuclease activity are known (EXO I, EXO II and EXO III, FIG 6). EXO I region contains an X 1 DX 2 EX 3 motif, and the amino acids D (aspartic acid) and E (glutamic acid) are known to be essential for the exonuclease activity. The second DNA polymerase of the present invention further includes a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 100 to 6 % preferably 90 to 30 % of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which amino acids at 140-, 142-, or 144-position have been replaced by other amino acids. The second DNA polymerase of the present invention further includes a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 100 to 6 % preferably 90 to 30 % of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; optimum temperature: about 75 °C; molecular weight: 88 to 90 KDa; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which is oleucine at the 142-position has been replaced by aspartic acid, glutamic acid, asparagine, glutamine or lysin, or threonine at the 144-position by valine. The DNA extension rate of the first and second DNA polymerases is at least at least 30 bases/second, preferably 100 to 120 bases/second and they are thermostable DNA polymerases capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours. The first and second DNA polymerases are preferably KOD polymerases or their mutants. In the present invention, the activity of the second DNA polymerase is preferably low than that of the first DNA polymerase, and the second DNA polymerase is preferably 0.02 to 0.1 unit every 2.5 units of the first DNA polymerase. To produce these modified enzymes, there is a method in which a gene coding for e.g. naturally occurring KOD polymerase is mutated so that the novel enzymes having reduced 3'-5' exonuclease activity as compared with the naturally occurring KOD polymerase are produced by protein engineering means. The KOD polymerase-coding gene to be mutated is not particularly limited. In one embodiment of the present invention, the gene shown in SEQ ID NO: 3 in the Sequence Listing, derived from Pyrococcus sp. KOD, was used. In another embodiment of the present invention, a gene coding for the amino acid sequence of SEQ ID NO: 1 is mutated to produce the novel enzyme having the 3'-5' exonuclease activity reduced as compared with that of the naturally occurring KOD polymerase. The thermostable DNA polymerase of the present invention before modification is an enzyme derived from Pyrococcus sp. KOD as 1 kind of hyperthermophilic archaeon strain isolated in Kodakara Island, Kagoshima prefecture, Japan. The microbial properties of KOD producing said enzyme are described in Unexamined Published Japanese Patent Application No 298,879/1995. Said enzyme is produced by culturing this strain. This enzyme has the following physicochemical properties: action: it has a DNA polymerase activity and has a 3'-5' exonuclease activity; DNA extension rate: at least 120 bases/second; thermostability: it is capable of maintaining 60 % or more residual activity at pH 8.8 (determined at 25 °C) after treatment at 95 °C for 6 hours; optimum temperature: about 75 °C; molecular weight: 88 to 90 kDa; and amino acid sequence: the amino acid sequence of SEQ ID NO: 2. The method of amplifying nucleic acid according to the present invention comprises reacting DNA as a template, primers, and 4 kinds of deoxyribonucleotide triphosphate (dNTP) by use of said DNA polymerase composition, thus extending the primers to synthesize a DNA primer extension product. In the PCR techniques as one method of amplifying nucleic acid according to the present invention, if a target nucleic acid in a sample is particularly long and double-stranded, then it is denatured by heating to be separated into single-stranded chains. If separation of the long chain nucleic acid into single-stranded chains is inadequate, subsequent annealing and extension reaction of the primers will be prevented. Then, the single-stranded chains as a template, primers complementary to said template, preferably primers one of which is complementary to another DNA extension product, and dNTP are reacted in a PCR reaction solution using the DNA polymerase composition of the present invention. This reaction is carried out using a 2-stage temperature cycle, that is a high temperature stage for denaturing the nucleic acid to be amplified and a low temperature stage for annealing the primers to the denatured nucleic acid to initiate primer extension, and this cycle is repeated 25 to 40 times. Usually, 1 cycle consists of reaction at 94 °C for 0.5 to 1 minute and then at 68 °C for 0.5 to 10 minutes. The 2 primers are annealed to opposite strands of the template nucleic acid sequence, and an extension product starting at each primer is a copy complementary to the template nucleic acid, and the product is oriented so that it can hybridize to another primer when separated from the resulting double-stranded chain. The reaction time is conducted preferably for a sufficient period until the extension reaction completes chain extension. For amplification of 20 kb or more nucleic acid, an annealing and extension time of at least 10 to 20 minutes is preferable. A long chain nucleic acid is preferably protected from decomposition during amplification by using e.g. glycerol, dimethyl sulfoxide (DMSO) etc. The presence of a misincorporated nucleotide will finish chain extension earlier and the number of template chains for subsequent amplification will be decreased, resulting in reduction of efficiency of amplification of long chain nucleic acid. In the present invention, however, the nucleotide misincorporated during synthesis of a primer extension product will be removed because the 3'-5' exonuclease activity besides the DNA polymerase activity is present at a low level in the reaction solution, and the dominant polymerase activity enables complete chain extension. The pH and composition for a reaction buffer, salts (divalent and monovalent ions), and the design of primers are important for efficiency of amplification of long chain nucleic acid. Because the PCR reagent is prepared usually at room temperature before the denaturation step, the binding of primers to another primer or to a homologous part of a nucleic acid sequence may be caused. If an extension product is also formed by nonspecific binding of primers, the efficiency of amplification of the desired long chain product is reduced. To prevent nonspecific binding, "hot start method" such as addition of the enzyme after the reaction solution reaches a high temperature is preferably used. Divalent ions e.g. magnesium ions and monovalent ions e.g. ammonium and/or potassium ions are preferably allowed to coexist to maintain the activity of the DNA polymerase of the present invention. Further, a buffer solution, such ions, BSA, a nonionic surface active agent (e.g. Triton X-100) and buffer solution may be present in the reaction solution for nucleic acid amplification. The reagent for amplifying nucleic acid according to the present invention contains 2 primers, one of which is complementary to a DNA extension product of another primer, dNTP, said DNA polymerase composition, magnesium ions, ammonium ions and/or potassium ions, BSA, a nonionic surface active agent and a buffer solution. In the present invention, the activity of the second DNA polymerase is preferably lower than that of the first DNA polymerase, and it is preferable that the second DNA polymerase is present in 0.02 to 0.1 unit every 2.5 units of the first DNA polymerase. The reagent of the present invention may contain a solvent aid such as glycerin, DMSO, polyethylene glycol etc. The buffer solution used includes a tris buffer, tris(hydroxymethyl)methylglycine (tricine buffer), N-bis(hydroxyethyl)glycine (bicine buffer) etc. The optimum buffer solution and pH depend on the DNA polymerase used. If KOD polymerase or its mutant is used in the present invention, a buffer solution is used at pH 7.5 to 9.2 (at 25 °C) at concentration of 10 to 50 mM, preferably 20 to 120 mM. Divalent cations are preferably magnesium ions, and magnesium chloride etc. are used. Their concentration is preferably 1 to 2 mM. Monovalent cations are preferably ammonium ions or potassium ions, and ammonium sulfate, potassium glutamate, potassium acetate etc. are used. Their concentration is preferably 2 to 50 mM. The primers used are 2 kinds of oligonucleotide, one of which is a primer complementary to a DNA extension product of another primer. Their concentration is preferably 0.2 to 1 µM. Hereinafter, the present invention is described in detail with reference to the Examples. Reference Example 1. Cloning of DNA Polymerase Gene Derived from Hyperthermophilic Archaeon Strain KOD Hyperthermophilic archaeon strain KOD1 isolated in Kodakara Island, Kagoshima Prefecture, Japan, was cultured at 95 °C and then recovered. Genomic DNA of hyperthermophilic archaeon strain KOD was prepared in a usual manner from the microorganism. Two primers were synthesized on the basis of preserving regions in the amino acid sequence of DNA polymerase (Pfu polymerase) derived from Pyrococcus furiosus . PCR was conducted using the 2 primers and the genomic DNA as a template. The DNA fragment thus amplified by PCR was sequenced, and from the nucleotide sequence thus determined, its amino acid sequence was deduced. Then, the genomic DNA from the KOD1 strain was treated with restriction enzyme, and the di gest was subjected to Southern hybridization with the above amplification DNA fragment as a probe to determine the size of a fragment coding for the DNA polymerase (about 4 to 7 kbp). Further, the DNA fragment of this size was recovered from the corresponding agarose gel and inserted into plasmid pBS (Stratagene). The mixture thus obtained was transformed into E . coli JM109 to prepare a library. Colony hybridization with the same probe as in the Southern hybridization was conducted so that a clone strain ( E . coli JM109/pSBKOD1) considered to contain the DNA polymerase gene derived from the KOD1 strain was obtained from the library. Plasmid pSBKOD1 was recovered from the clone strain ( E . coli JM109/pSBKOD1) and sequenced in a usual manner. Its amino acid sequence was deduced from the nucleotide sequence thus determined. The DNA polymerase gene derived from the KOD1 strain consisted of 5010 bases and coded for 1670 amino acids (SEQ ID NO: 1). To prepare a complete polymerase gene, 2 intervening sequences (1374 to 2453 bp and 2708 to 4316 bp) were removed by a PCR fusion method. Three pairs of primers were used in the PCR fusion method and each pair was used in PCR where the plasmid recovered from the cloned strain was used as a template, so that 3 fragments free of the intervening sequences were amplified. The primers used in PCR were designed such that they have the same sequence as a sequence binding to the target site, and that they have different restriction enzyme sites at the terminals, that is, they have an EcoRV site at the N-terminal and a BamHI site at the C-terminal. Then, a fragment located in the middle of the PCR amplification fragment was mixed with a fragment located at the N-terminal side, and PCR was conducted using the respective fragments as primers. Further, a fragment located in the middle of the PCR amplification fragment was mixed with a fragment located at the C-terminal side, and PCR was conducted using the respective fragments as primers. PCR was conducted again using the 2 kinds of fragment thus obtained to give a complete gene fragment which is free of the intervening sequences, has an EcoRV site at the N-terminal and a BamHI site at the C-terminal, and codes for the DNA polymerase derived from the KOD1 strain. Further, this gene was subcloned in expression vector PET-8c capable of inducing expression of the gene under T7 promoter. For this subcloning, the NcoI/BamHI sites on PET-8c and the restriction enzyme sites created above were used. A recombinant expression vector (pET-pol) was thus obtained. E . coli BL21 (DE3)/pET-pol has been deposited as FERM BP-5513 with the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Japan. Example 1 . Subcloning of the KOD Polymerase Gene To modify thermostable DNA polymerase, the KOD polymerase gene was removed from plasmid pET-pol and subcloned in plasmid pBluescript as follows: The KOD polymerase gene, about 2.3 kb long, was removed by digesting plasmid pET-pol with restriction enzymes XbaI and BamHI (Toyobo Co., Ltd.). A ligation kit (Ligation high, a product of Toyobo Co., Ltd.) was then used for ligation of this DNA fragment into plasmid pBluescript SK(-) previously digested with XbaI and BamHI. Then, the resulting plasmid was transformed into commercially available competent cells (competent high JM109, available from Toyobo Co., Ltd.). The transformant was cultured at 35 °C for 16 hours in an LB agar medium containing 100 µg/ml ampicillin (1 % Bacto-trypton, 0.5 % yeast extract, 0.5 % sodium chloride, 1.5 % agar, produced by Gibco), and a plasmid was prepared from the resulting colonies. From its partial nucleotide sequence, this plasmid was confirmed to carry the KOD polymerase gene and it was designated plasmid pKOD1. Example 2. Preparation of Modified Gene (DA) and Purification of Modified Thermostable DNA Polymerase Plasmid pKOD1 obtained in Example 1 was used to prepare a plasmid (pKODDA) carrying a gene for a modified thermostable DNA polymerase of the KOD polymerase of SEQ ID NO: 2 in which aspartic acid at the 141-position had been replaced by alanine. For this preparation, a Chameleon site-directed mutagenesis kit (Stratagene) was used according to the manufacture's instructions. The selective primer used was a primer as shown in SEQ ID NO: 4. The mutagenesis primer used was a primer as shown in SEQ ID NO: 7. The mutant was confirmed by determining its nucleotide sequence. E . coli JM109 was transformed with the resulting plasmid to give JM109 (pKODDA). 6 L sterilized TB medium (described in p. A. 2 in Molecular Cloning) containing 100 µg/ml ampicillin was introduced into a 10-L jar fermenter. Into this medium was inoculated E . coli JM109 (pKODDA) which had been cultured at 30 °C for 16 hours in 50 ml LB medium (1 % Bacto-trypton, 0.5 % yeast extract, 0.5 % sodium chloride, produced by Gibco) containing 100 µg/ml ampicillin in a 500-ml flask, and the microorganism was grown by shake culture at 35 °C for 12 hours under aeration. The microorganism was recovered from the culture by centrifugation, then suspended in 400 ml buffer (10 mM Tris-HCl (pH 8.0), 80 mM KCl, 5 mM 2-mercaptoethanol, 1 mM EDTA) and disrupted by ultrasonication to give a cell lysate. The cell lysate was heated at 85 °C for 30 minutes and centrifuged to remove insoluble solids. The supernatant was treated with polyethylene imine for removal of nucleic acids, then fractionated with sulfate ammonium and subjected to chromatography on heparin-Sepharose. Finally, the buffer solution was replaced by a preserving buffer solution (50 mM Tris-HCl (pH 8.0), 50 mM potassium chloride, 1 mM dithiothreitol, 0.1 % Tween 20, 0.1 % Nonidet™ P40, 50 % glycerin) so that the modified thermostable DNA polymerase (DA) was obtained. In the purification described above, the measurement of DNA polymerase activity was conducted in the following manner. When the enzyme activity was high, the sample was measured after dilution with the preserving buffer solution. (Reagent) A 40 mM Tris-HCl (pH 7.5) 16 mM magnesium chloride 15 mM dithiothreitol 100 µg/ml BSA B 2 µg/µl activated calf thymus DNA C 1.5 mM dNTP (250 cpm/pmol [ 3 H] dTTP) D 20 % trichloroacetic acid (2 mM sodium pyrrophosphate) E 1 µg/µl carrier DNA (Method) 25 µl of Solution A, 5 µl each of Solutions B and C, and 10 µl sterilized water are added to an Eppendorf tube and mixed by stirring. Then, 5 µl of the enzyme solution is added to the mixture and reacted at 75 °C for 10 minutes. Thereafter, it is cooled on ice for 10 minutes, followed by adding 50 µl of Solution E and 100 µl of Solution D. The mixture was stirred and cooled on ice for 10 minutes. This solution is filtered through a glass filter (Wattman GF/C filter), followed by extensive washing with Solution D and ethanol, and the radioactivity of the filter was counted in a liquid scintillation counter (Packard) to determine the incorporation of the nucleotide into the template DNA. 1 unit of the enzyme is assumed to be the amount of the enzyme causing incorporation, into the acid insoluble fragment, of 10 nmol nucleotide every 30 minutes under these conditions. Example 3. Preparation of Modified Gene (EA) and Purification of Modified Thermostable DNA Polymerase A plasmid (pKODEA) carrying a gene for a modified thermostable DNA polymerase of the KOD polymerase of SEQ ID NO: 2 in which glutamine at the 143-position had been replaced by alanine was prepared in the same manner as in Example 2. The selective primer used was a primer as shown in SEQ ID NO: 5. The mutagenesis primer used was a primer as shown in SEQ ID NO: 8. The modified thermostable DNA polymerase (EA) was obtained using the same purification method as in Example 2. Example 4. Preparation of Modified Gene (DEA) and Purification of Modified Thermostable DNA Polymerase A plasmid (pKODDEA) carrying a gene for a modified thermostable DNA polymerase of the KOD polymerase of SEQ ID NO: 2 in which aspartic acid at 141-position and glutamic acid at the 143-position had been replaced by alanine respectively was prepared in the same manner as in Example 2. The selective primer used was a primer as shown in SEQ ID NO: 4. The mutagenesis primer used was a primer as shown in SEQ ID. NO: 6. The modified thermostable DNA polymerase (DEA) was obtained using the same purification method as in Example 2. Example 5. Preparation of Modified Gene (ND) and Purification of Modified Thermostable DNA Polymerase A plasmid (pKODND) carrying a gene for a modified thermostable DNA polymerase of the KOD polymerase of SEQ ID NO: 2 in which asparagine at 210-position had been replaced by aspartic acid was prepared in the same manner as in Example 2. The selective primer used was a primer as shown in SEQ ID NO: 4. The mutagenesis primer used was a primer as shown in SEQ ID NO: 9. The modified thermostable DNA polymerase (ND) was obtained using the same purification method as in Example 2. Example 6. Preparation of Modified Gene (YF) and Purification of Modified Thermostable DNA Polymerase A plasmid (pKODYF) carrying a gene for a modified thermostable DNA polymerase of the KOD polymerase of SEQ ID NO: 2 in which tyrosine at 311-position had been replaced by phenylalanine was prepared in the same manner as in Example 2. The selective primer used was a primer as shown in SEQ ID NO: 4. The mutagenesis primer used was a primer as shown in SEQ ID NO: 10. he modified thermostable DNA polymerase (YF) was obtained using the same purification method as in Example 2. Example 7 . Confirmation of Exonuclease Activity of Modified Thermostable DNA Polymerase The exonuclease activities of the modified thermostable DNA polymerases (DA, EA, DEA, ND and YF) obtained in Examples 2 to 6 were determined in the following manner. As the control, the naturally occurring KOD polymerase (Toyobo Co., Ltd.) was used. 50 µl of a reaction solution (120 mM Tris-HCl (pH 8.8 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.1 % Triton X-100, 0.001 % BSA, 5 µg tritium-labeled E . coli DNA) was put to a 1.5 ml Eppendorf tube, and the DNA polymerase was added in amounts of 25, 50 and 100 units respectively. The naturally occurring KOD polymerase was used in amounts of 0.25, 0.5 and 1 unit respectively. After the mixture was reacted at 75 °C for 10 minutes, the reaction was terminated by cooling on ice. Then, 50 µl of 0.1 % BSA was added as a carrier to it, and then 100 µl of a solution containing 10 % trichloroacetic acid and 2 % sodium pyrrophosphate was mixed with it. After the mixture was left for 15 minutes on ice, it is centrifuged at 12,000 r.p.m. for 10 minutes to separate the precipitates present. 100 µl of the supernatant was measured for radioactivity in a liquid scintillation counter (Packard) whereby the amount of the nucleotide delivered into the acid soluble fragment was determined. FIG. 1 shows the polymerase activity of each DNA polymerase and the decomposition rate of DNA. In this result, the exonuclease activity of the 3 modified thermostable DNA polymerases (DEA, DA and EA) could not be detected. The modified thermostable DNA polymerase (ND) had about 0.1 %, and the modified thermostable DNA polymerase (YF) had about 0.01 % of the activity of the naturally occurring KOD polymerase. Example 8 . Confirmation of Thermostability The thermostability of the modified thermostable DNA polymerases obtained in Examples 2 to 6 (DA, EA, DEA, ND and YF) was determined in the following manner. 5 units of each purified modified DNA polymerase was mixed with 100 µl buffer solution (20 mM Tris-HCl pH 8.8 at 25 °C, 10 mM potassium chloride, 10 mM ammonium sulfate, 2 mM magnesium sulfate, 0.1 % Triton X-100, 0.1 mg/ml BSA, 5 mM 2-mercaptoethanol) and then pre-incubated at 95 °C. A sample was recovered from this mixture with time, and its polymerase activity was determined in the method described in Example 2. For comparison, Taq polymerase (Toyobo Co., Ltd.) and the naturally occurring KOD polymerase (Toyobo Co., Ltd.) were also subjected to the same procedure. As shown in FIG. 2, any of the modified thermostable DNA polymerases, similar to the naturally occurring KOD polymerase, had 60 % or more residual activity after treatment at 95 °C for 6 hours. On the other hand, Taq polymerase had 15 % or less residual activity. Example 9 . Measurement of DNA Extension Rate The modified thermostable DNA polymerases obtained in Examples 2 to 6 (DA, EA, DEA, ND and YF) was examined for DNA extension rate in the following manner. 0.2 µg of the primer (SEQ ID NO: 15) was annealed to a single-stranded chain of M13mp18 DNA, and then 1 unit of each purified modified DNA polymerase was reacted with the single-stranded chain at 75 °C for 20, 40, and 60 seconds respectively in 10 µl of a reaction solution (20 mM Tris-HCl (pH 7.5), 8 mM magnesium chloride, 7.5 mM dithiothreitol, 100 µg/ml BSA, 0.1 mM dNTP, 0.2 µCi [α -32 P]dCTP). The reaction was terminated by adding an equal volume of a reaction terminating solution (50 mM sodium hydroxide, 10 mM EDTA, 5 % Ficoll, 0.05 Bromophenol Blue). For comparison, Taq polymerase (Toyobo Co., Ltd.) and the naturally occurring KOD polymerase (Toyobo Co., Ltd.) were also subjected to the same procedure. The DNA extended by the reaction was fractionated by electrophoresis on alkali agarose gel, and the gel was dried and subjected to autoradiography. As a DNA size marker, labeled λ/HindIII was used. The DNA extension rate was determined using the size of the extended DNA determined with a band of this marker as an indicator. The result indicated that similar to the naturally occurring KOD polymerase, any of the modified polymerases had an extension rate of about 120 bases/second, while Taq polymerase had an extension rate of about 60 bases/second. Example 10 . Preparation of Mutant Gene (IN) and Purification of Modified Thermostable DNA Polymerase Plasmid pKOD1 obtained in Example 1 was used to prepare a plasmid (pKODIN) carrying a gene for modified thermostable DNA polymerase where in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by asparagine. This plasmid was prepared using a Chameleon site-directed mutagenesis kit (Stratagene) according to the manufacture's instructions. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 17. The mutant was confirmed by determining its nucleotide sequence. E . coli JM109 was transformed with the plasmid to give JM109 (pKODIN). Example 11 . Preparation of Mutant Gene (IE) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODIE) for KOD polymerase where in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by glutamic acid was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 18. The modified thermostable DNA polymerase (IE) was obtained using the same purification method as in Example 10. Example 12. Preparation of Mutant Gene (IQ) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODIQ) for KOD polymerase where, in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by glutamic acid was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 19. The modified thermostable DNA polymerase (IQ) was obtained using the same purification method as in Example 10. Example 13. Preparation of Mutant Gene (ID) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODID) for KOD polymerase where, in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by aspartic acid was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 20. The modified thermostable DNA polymerase (ID) was obtained using the same purification method as in Example 10. Example 14 . Preparation of Mutant Gene (TV) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODTV) for KOD polymerase where, in the X 1 DX 2 EX 3 motif located in the EXO1 region, threonine at X 3 had been replaced by valine was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 21. The modified thermostable DNA polymerase (TV) was obtained using the same purification method as in Example 10. Example 15 . Preparation of Mutant Gene (IK) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODIK) for KOD polymerase where, in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by lysin was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 23. The modified thermostable DNA polymerase (IK) was obtained using the same purification method as in Example 10. Example 16. Preparation of Mutant Gene (IR) and Purification of Modified Thermostable DNA Polymerase A thermostable polymerase gene (pKODIR) for KOD polymerase where, in the X 1 DX 2 EX 3 motif located in the EXO1 region, isoleucine at X 2 had been replaced by arginine was prepared in the same manner as in Example 10. The selective primer used was a primer as shown in SEQ ID NO: 16. The mutagenesis primer used was a primer as shown in SEQ ID NO: 22. The modified thermostable DNA polymerase (IR) was obtained using the same purification method as in Example 10. Example 17. Confirmation of Exonuclease Activity of Modified Thermostable DNA Polymerase The modified thermostable DNA polymerases obtained in Examples 10 to 16 (IN, IE, IQ, ID, YV, IK and IR) were examined for exonuclease activity in the following manner. As the control, the naturally occurring KOD polymerase (Toyobo Co. Ltd.) was used. 50 µl of a reaction solution (120 mM Tris-HCl (pH 8.8 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.1 % Triton X-100, 0.001 % BSA, 5 µg of tritium-labeled E . coli DNA) was pipetted into a 1.5-ml Eppendorf tube, followed by adding each DNA polymerase in amounts of 0.5, 1, and 1.5 units respectively. After the mixture was reacted at 75 °C for 10 minutes, the reaction was terminated on cooling on ice. Then, 50 ml of 0.1 % BSA was added to it as a carrier, and then 100 µl of a solution containing 10 % trichloroacetic acid and 2 % sodium pyrrophosphate was mixed with it. After the mixture was left on ice for 15 minutes, it was centrifuged at 12,000 r.p.m. for 10 minutes to separate the precipitates present. 100 µl of the supernatant was measured for radioactivity in a liquid scintillation counter (Packard) whereby the amount of the nucleotide delivered into the acid soluble fragment was determined. FIG. 7 shows the polymerase activity of each DNA polymerase and the decomposition rate of DNA. FIG. 8 shows their exonuclease activities relative to that of the naturally occurring KOD polymerase. As shown therein, the thermostable DNA polymerases with the 3'-5' exonuclease activity at different levels can be obtained according to the present invention. As compared with the naturally occurring KOD polymerase, the modified thermostable DNA polymerases had the 3'-5' exonuclease activity at the following levels: IN had about 95 %; IE, about 76 %; IQ, about 64 %; ID, about 52 %; TV, about 48 %; IK, about 30 %; and IR, about 0 %. Example 18. Measurement of Fidelity of DNA Extension in PCR by Modified DNA Polymerase The naturally occurring KOD polymerase, the modified thermostable DNA polymerases IE, ID, IK and IR, and Taq polymerase were examined for fidelity of DNA extension in PCR, as follows: Plasmid pUR288 (described in Current Protocols in Molecular Biology 1.5.6) was cleaved with restriction enzyme Scal. PCR was conducted using 1 ng of this plasmid and the primers of SEQ ID NOS: 13 and 14. After the reaction was finished, 5 µl of the reaction solution was subjected to agarose gel electrophoresis, and amplification of the about 5.3 kb target was confirmed. The remainder of the reaction solution was treated with phenol/chloroform and then precipitated with ethanol. The precipitate was dried and dissolved in 50 µl High buffer (50 mM Tris-HCl (pH 7.5), 100 mM NaCl, 10 mM MgCl 2 , 1 mM DTT). Further, 10 units of restriction enzyme ScaI (Toyobo Co., Ltd.) were added to it and the mixture was reacted at 37 °C for 16 hours. The target amplification product was separated by agarose gel electrophoresis and its corresponding agarose gel was cut off from the gel. From the agarose, the target DNA was purified using Gene Clean 2 (BIO101). 10 ng of the DNA thus purified was diluted to 10 µl with TE buffer (10 mM Tris-HCl (pH 8.0), 1 mM EDTA). To this solution was added 10 µl of a reaction solution from a ligation kit (Ligation high, Toyobo Co., Ltd.), and the mixture was reacted at 16 °C for 30 minutes. Then, the resulting DNA was transformed into commercially available competent cells (competent high JM109, Toyobo K.K.). The transformant was cultured at 35 °C for 16 hours in an LB agar medium (1 % Bacto-trypton, 0.5 % yeast extract, 0.5 % sodium chloride, 1.5 % agar, produced by Gibco) containing 100 µg/ml ampicillin, 1mM isopropylthio-β-galactoside (IPTG, Nakarai Tesque), 0.7 % 5-bromo-4-chloro-3-indolyl-β-D-galactoside (X-gal (Nakarai Tesque)) and then their colonies were counted. pUR288 carries the lacZ gene (β-D-galactosidase). Therefore, if DNA extension has proceeded with fidelity during PCR, blue colonies are formed on the agar medium. On the contrary, if misincorporation has occurred during DNA extension, the activity of β-galactosidase encoded by the lacZ gene is reduced or lost, resulting in occurrence of pale blue or white colonies. Assuming these plate blue colonies and white colonies are mutant colonies, mutant frequency (%) was determined when each enzyme was used, and the results are shown in Table 1 below. KOD IE ID IK IE rTaq Colonies in Total 2394 3267 4869 2826 1197 2831 Mutant Colonies 19 63 148 362 299 795 Mutant Frequency (%) 0.79 1.9 3.0 12.8 25.0 28.1 As is evident from Table 1, the modified thermostable DNA polymerases IE, ID, IK and IR obtained in the present invention were inferior to the naturally occurring KOD polymerase, but they showed lower degrees of mutation than that of Taq polymerase, that is, they demonstrated higher fidelity in DNA extension. Example 19. PCR by Use of Modified DNA Polymerase (for Plasmid) PCR was carried out using naturally occurring KOD polymerase (described in Unexamined Published Japanese Patent Application No. 298,879/1995) and the modified the rmostable DNA polymerase (described in Example 5), as follows: 2.5 units of each enzyme were added to 50 µl of a reaction solution (120 mM Tris-HCl (pH 8.0 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.2 mM dNTP, 0.1 % Triton X-100, 0.001 % BSA, 1 ng plasmid pBR322 rendered linear with restriction enzyme ScaI, and 10 pmol primers shown in SEQ ID NOS: 13 and 14), and PCR was carried out. The thermal cycler used was Model PJ2000 (Perkin Elmer). The reaction conditions were 94 °C, 30 seconds → 68 °C, 2.5 minutes, and this cycle was repeated 25 times. Taq polymerase (Toyobo K.K.) was subjected to PCR in the same manner except that the reaction solution was 10 mM Tris-HCl (pH 8.8 at 25 °C) containing 50 mM KCl, 1.5 mM MgCl 2 , 0.2 mM dNTP, 0.1 % Triton X-100, 1 ng plasmid pBR322 rendered linear with restriction enzyme ScaI, and 10 pmol primers shown in SEQ ID NOS: 13 and 14. After the reaction was finished, 5 µl of the reaction solution was subjected to agarose gel electrophoresis, and amplification of the about 4.3 kb target was confirmed. FIG. 3 shows the result of agarose gel electrophoresis. This result indicated that PCR amplification by the modified DNA polymerase was better than that by the naturally occurring KOD polymerase. Further, this amplification was better than that by Taq polymerase. Example 20. PCR by Use of Modified DNA Polymerase (for Human Genome) PCR was carried out using the modified thermostable DNA polymerase (described in Example 5) as follows: 2.5 units of the enzyme were added to 50 µl of a reaction solution (120 mM Tris-HCl (pH 8.0 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.2 mM dNTP, 0.1 % Triton X-100, 0.001 % BSA, 100 ng genomic DNA (Clontech) derived from human placenta, and 10 pmol primers shown in SEQ ID NOS: 11 and 12), and PCR was carried out. The thermal cycler used was Model PJ2000 (Perkin Elmer). The reaction conditions were 94 °C, 30 seconds → 68 °C, 3 minutes, and this cycle was repeated 25 times. For comparison, Taq polymerase (Toyobo K.K.) was also subjected to PCR in the same manner except that the reaction solution was 10 mM Tris-HCl (pH 8.8 at 25 °C) containing 50 mM KCl, 1.5 mM MgCl 2 , 0.2 mM dNTP, 0.1 % Triton X-100, 100 ng genomic DNA (Clontech) derived from human placenta, and 10 pmol primers shown in SEQ ID NOS: 11 and 12. After the reaction was finished, 5 µl of the reaction solution was subjected to agarose gel electrophoresis, and amplification of the about 4 kb target was confirmed. FIG. 4 shows the result of agarose gel electrophoresis. This result indicated that PCR amplification by the modified DNA polymerase was better than that by Taq polymerase. Example 21. PCR by Use of DNA Polymerase Composition (for Human Genome) PCR was carried out using a mixture of the modified thermostable DNA polymerase (DA, EA, DEA, ND or YF) and naturally occurring KOD polymerase, as follows: 2.5 units of ND and 0.05 unit of KOD polymerase were added to 50 µl of a reaction solution (120 mM Tris-HCl (pH 8.8 at 25 °C), 10 mM KCl, 6 mM ammonium sulfate, 1 mM MgCl 2 , 0.2 mM dNTP, 0.1 % Triton X-100, 0.001 % BSA, 30 ng genomic DNA (Clontech) derived from human placenta, and 10 pmol primers shown in SEQ ID NOS: 11 and 12). The thermal cycler used was Model PJ2000 (Perkin Elmer). The reaction conditions were 94 °C, 30 seconds → 68 °C, 3 minutes, and this cycle was repeated 30 times. For comparison, the modified thermostable DNA polymerase (ND), Taq polymerase (Toyobo Co., Ltd.), a commercial DNA polymerase mixture (ExTaq (Takara Shuzo Co., Ltd.), and Advantage Tth (Clontech) were subjected respectively to PCR using the same amounts of the genomic DNA and primers in the same manner except that the reaction solution was the buffer attached to the commercial product. After the reaction was finished, 5 µl of the reaction solution was subjected to agarose gel electrophoresis, and amplification of the about 4 kb target was confirmed. FIG. 5 shows the result of agarose gel electrophoresis. This result indicated that PCR amplification by a mixture of the modified DNA polymerase (ND) and the naturally occurring KOD polymerase was better than that by the commercial polymerase mixture. Nucleic acid amplification excellent in efficiency of amplification can be effected by a mixture of 2 or more DNA polymerases which are almost identical to each other with respect to thermostability and DNA extension rate but are different in their 3'-5' exonuclease activity. SEQUENCE LISTING SEQ. ID NO: 1 LENGTH: 5342 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: double TOPOLOGY: linear MOLECULAR TYPE: genomic DNA SOURCE: hyperthermophilic archaeon STRAIN NAME: KOD1 CHARACTERISTICS: 156-5165 P CDS 1374-2453 intervening sequence 2708-4316 intervening sequence SEQUENCE DESCRIPTION: SEQ. ID NO: 2 LENGTH: 774 amino acids TYPE: amino acid TOPOLOGY: linear MOLECULAR TYPE: protein SEQUENCE DESCRIPTION: SEQ. ID NO: 3 LENGTH: 2325 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: double TOPOLOGY: linear MOLECULAR TYPE: genomic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 4 LENGTH: 24 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 5 LENGTH: 24 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 6 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 7 LENGTH: 33 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 8 LENGTH: 30 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 9 LENGTH: 32 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 10 LENGTH: 33 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 11 LENGTH: 35 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 12 LENGTH: 35 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 13 LENGTH: 35 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 14 LENGTH: 34 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 15 LENGTH: 24 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 16 LENGTH: 24 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 17 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 18 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 19 LENGTH: 33 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 20 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 21 LENGTH: 30 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 22 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION: SEQ. ID NO: 23 LENGTH: 36 base pairs TYPE: nucleic acid (DNA) STRANDEDNESS: single TOPOLOGY: linear MOLECULAR TYPE: synthetic DNA SEQUENCE DESCRIPTION:

Claims: A modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 5% or less of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second; and thermostability: it is capable of maintaining 60% or more residual activity at pH 8.8 (determined at 25°C) after treatment at 95°C for 6 hours; optimum temperature: about 75°C; molecular weight: 88 to 90 kDa; and amino acid sequence: as shown in SEQ ID NO: 2 in which at least one of amino acids at the 141-, 143-, 210- and 311-positions has been replaced by another amino acid. The modified thermostable DNA polymerase according to claim 1, which is free of a 3'-5' exonuclease activity. The thermostable DNA polymerase according to any one of claims 1 and 2, wherein the DNA extension rate is not less than 60 bases/second. The thermostable DNA polymerase according to claim 1, wherein the 3'-5' exonuclease activity is reduced to about 1% or less. The thermostable DNA polymerase according claim 2, wherein in SEQ ID NO: 2 aspartic acid at the 141-position and/or glutamic acid at the 143-position have been replaced by other amino acids. The thermostable DNA polymerase according to claim 5, wherein aspartic acid at the 141-position and/or glutamic acid at the 143-position have been replaced by alanine. The thermostable DNA polymerase according claim 2, wherein in SEQ ID NO: 2 asparagine at the 210-position has been replaced by another amino acid. The thermostable DNA polymerase according to claim 7, wherein asparagine at the 210-position has been replaced by aspartic acid. The thermostable DNA polymerase according to claim 2, wherein in SEQ ID NO: 2 tyrosine at the 311-position has been replaced by another amino acid. The thermostable DNA polymerase according to claim 9, wherein tyrosine at the 311-position has been replaced by phenylalanine. A method for amplifying nucleic acid, which comprises reacting DNA as a template, primers, dNTP and the thermostable DNA polymerase according to any one of claims 1 to 10, thus extending the primers to synthesize a DNA primer extension product The method according to claim 11, wherein the primers are 2 kinds of oligonucleotide, one of which is complementary to a DNA extension product of another primer. The method according to claim 11 or 12, wherein heating and cooling are repeatedly carried out. A reagent for amplifying nucleic acid, which comprises 2 kinds of primer, one of which is complementary to a DNA extension product of another primer, dNTP, the thermostable DNA polymerase according to any one of claims 1 to 10, and a buffer solution. The reagent according to claim 14, which further comprises magnesium ions, ammonium ions and/or potassium ions, BSA, and a nonionic surface active agent. A DNA polymerase composition for amplifying nucleic acid, which comprises a modified thermostable DNA polymerase having 0 to 5 % of the 3'-5' exonuclease activity of the enzyme before modification (first polymerase) and a thermostable DNA polymerase having the 3'-5' exonuclease activity or a modified thermostable DNA polymerase having 100 to 6 % of the 3'-5' exonuclease activity of a thermostable DNA polymerase before modification (second polymerase), said first and second DNA polymerases having a DNA extension rate and a thermostability as defined in claim 1. The DNA polymerase composition according to claim 16, wherein the activity of the second DNA polymerase is lower than the activity of the first DNA polymerase. The DNA polymerase composition according to claim 16 or 17, wherein the second DNA polymerase is present in 0.02 to 0.1 unit every 2.5 units of the first DNA polymerase. The DNA polymerase composition according to any one of claims 16 to 18, wherein the 3'-5' exonuclease activity of the first DNA polymerase is reduced to about 1% or less of the 3'-5' exonuclease activity of the thermostable DNA polymerase before modification. The DNA polymerase composition according to claim 16, wherein the first DNA polymerase is a modified thermostable DNA polymerase further having the following physicochemical properties: optimum temperature and molecular weight are as defined in claim 1 and an amino acid sequence: as shown in SEQ ID NO: 2 in which at least one of the amino acids at the 141-, 142-, 143-, 210- and 311-positions has been replaced by another amino acid. The DNA polymerase composition according to claim 20, wherein in the amino acid sequence aspartic acid at the 141-position has been replaced by alanine, isoleucine at the 142-position by arginine, glutamic acid at the 143-position by alanine, aspartic acid at the 141-position and glutamic acid at the 143-position respectively by alanine, asparagine at the 210-position by aspartic acid, or tyrosine at the 311-position by phenylalanine. The DNA polymerase composition according to claim 16, wherein the first DNA polymerase is a thermostable DNA polymerase as shown in SEQ ID NO: 2 in which aspartic acid at the 141-position has been replaced by alanine and/or isoleucine at the 142-position has been replaced by arginine, and/or glutamic acid at the 143-position has been replaced by alanine and/or asparagine at the 210-position has been replaced by aspartic acid and/or tyrosine at the 311-position has been replaced by phenylalanine. The DNA polymerase composition according to claim 16, wherein the second polymerase is a thermostable DNA polymerase further having the following physicochemical properties: action: it has a DNA polymerase activity and has a 3'-5' exonuclease activity; and optimum temperature, and molecular weight are as defined in claim 1, and the amino acid sequence of SEQ ID NO: 2. The DNA polymerase composition according to claim 16, wherein the second polymerase is a modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 100 to 6% of the 3'-5' exonuclease activity of the enzyme before modification; and an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids X 1 , X 2 and X 3 in an X 1 D 2 X 2 EX 3 motif present in EXO 1 has been replaced by another amino acid. The DNA polymerase composition according to claim 16, wherein the second DNA polymerase is a modified thermostable DNA polymerase having the following physicochemical properties: action, DNA extension rate, and thermostability are as defined in claim 24, and amino acid sequence: as shown in SEQ ID NO: 2 in which amino acids at the 140-, 142- and 144-positions have been replaced by other amino acids. The DNA polymerase composition according to claim 16, wherein the second DNA polymerase is a modified thermostable DNA polymerase having the following physicochemical properties: action, DNA extension rate, and thermostability are as defined in claim 25, optimum temperature and molecular weight are as defined in claim 1, and amino acid sequence: as shown in SEQ ID NO: 2 in which iso-leucine at the 142-position has been replaced by aspartic acid, glutamic acid, asparagine, glutamine or lysin, or threonine at the 144-position by valine. The DNA polymerase composition according to claim 16, wherein the second DNA polymerase is a thermostable DNA polymerase as shown in SEQ ID NO: 2 in which isoleucine at the 142-position has been replaced by aspartic acid or by glutamic acid or by asparagine or by glutamine or by lysin . The DNA polymerase composition according to claim 16, wherein the second DNA polymerase is a thermostable DNA polymerase as shown in SEQ ID NO: 2 in which threonine at the 144-position has been replaced by valine. A DNA polymerase composition for amplifying nucleic acid, which comprises the following first and second DNA polymerases: the first polymerase is as defined in claim 22; the second polymerase; action, thermostability, optimum temperature, molecular weight, and amino acid sequence are as defined in claim 23, and DNA extension rate: at least 120 bases/second; or action, thermostability, optimum temperature, molecular weight, and amino acid sequence are as defined in claim 26, and DNA extension rate: at least 120 bases/second. A DNA polymerase composition for amplifying nucleic acid, which comprises the following first and second DNA polymerases: the first polymerase: action, DNA extension rate, thermostability, optimum temperature, and molecular weight are as defined in claim 20; and amino acid sequence: as shown in SEQ ID NO: 2 in which asparagine at the 210-position is replaced by aspartic acid, the second polymerase; action, thermostability, optimum temperature, molecular weight and amino acid sequence are as defined in claim 23, and DNA extension rate: at least 120 bases/second. A method for amplifying nucleic acid, which comprises reacting DNA as a template, primers, dNTP and the DNA polymerase composition of any one of claims 16 to 30, thus extending the primers to synthesize a DNA primer extension product. The method according to claim 31, wherein the primers are 2 kinds of oligonucleotide, one of which is complementary to a DNA extension product of another primer. The method according to claim 31 or 32, wherein heating and cooling are repeatedly carried out. A reagent for amplifying nucleic acid, which comprises the DNA polymerase composition of any one of claims 16 to 30, divalent ions, monovalent ions, primers, dNTP, and a buffer solution. The reagent according to claim 34, which comprises magnesium ions, ammonium ions and/or potassium ions, 2 kinds of primer, one of which is complementary to a DNA extension product of another primer, BSA, and a nonionic surface active agent. A modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 100 to 6% of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/second: thermostability: it is capable of maintaining 60% or more residual activity at pH 8.8 (determined at 25°C) after treatment at 95°C for 6 hours; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO: 2 in which at least one of amino acids at the 140-, 142- and 144-positions has been replaced by another amino acid. The modified thermostable DNA polymerase according to claim 36, further having the following physicochemical properties: action, DNA extension rate, and thermostability are as defined in claim 36, optimum temperature and molecular weight are as defined in claim 1, and amino acid sequence: as shown in SEQ ID NO:2 in which iso-leucine at the 142-position has been replaced by aspartic acid, glutamic acid, asparagin, glutamine or lysin, or threonine at the 144-position by valine. A modified thermostable DNA polymerase having the following physicochemical properties: action: it has a DNA polymerase activity and has 5% or less of the 3'-5' exonuclease activity of the enzyme before modification; DNA extension rate: at least 30 bases/ second; thermostability: it is capable of maintaining 60% or more residual activity at pH 8.8 (determined at 25°C) after treatment at 95°C for 6 hours; and amino acid sequence: an amino acid sequence as shown in SEQ ID NO:2 in which amino acid at the 142-position has been replaced by another-amino acid. The modified thermostable DNA polymerase according to claim 38, further having the following physicochemical properties: action, DNA extension rate, and thermostability are as defined in claim 38, optimum temperature and molecular weight are as defined in claim 1, and amino acid sequence: as shown in SEQ ID NO:2 in which iso-leucine at the 142-position has been replaced by arginine.

Inventors: IMANAKA TADAYUKI, KAMIMURA HIDEKI, KAWAKAMI BUNSEI, KAWAMURA YOSHIHISA, KITABAYASHI MASAO, KOMATSUBARA HIDEYUKI, TAKAGI MASAHIRO, IMANAKA, TADAYUKI, KAMIMURA, HIDEKI, KAWAKAMI, BUNSEI, KAWAMURA, YOSHIHISA, KITABAYASHI, MASAO, KOMATSUBARA, HIDEYUKI, TAKAGI, MASAHIRO, Kamimura, Hideki c/o Toyo Boseki K.K., Kawakami, Bunsei c/o Toyo Boseki K.K., Kawamura, Yoshihisa c/o Toyo Boseki K.K., Kitabayashi, Masao c/o Toyo Boseki K.K., Komatsubara, Hideyuki c/o Toyo Boseki k.k.
Applicants: TOYO BOSEKI, TOYO BOSEKI KABUSHIKI KAISHA
Application year: 1997
Main classification: C12N15
IPC codes: C12N15, C12Q1, C12N9