Patent Publication Number: US-2023133071-A1

Title: Heat resistant mismatch endonuclease variant

Description:
TECHNICAL FIELD 
     The present invention relates to a heat-resistant mismatch endonuclease mutant which recognizes and cleaves a mismatched base pair (hereinafter, also merely referred to as a mismatch) in a double-stranded nucleic acid, a composition comprising the mismatch endonuclease mutant, and a method of using the mismatch endonuclease mutant. 
     BACKGROUND ART 
     In recent years, biotechnology has been remarkably developed. Particularly, in consequence of large-scale genomic analyses accompanying advances in genomic analysis techniques, enormous information of genome sequences has been accumulated. In addition, based on combinations of the above-mentioned information with analyses of various physiological functions, many functional genetic mutations have been found. Analyses of these mutations have been used for genetic diagnoses of human beings as well as improvement of agricultural crops and isolation or creation of useful microorganisms, and thus have greatly contributed to general living. 
     The mutation analyses have been performed by direct analyses of genomic sequences or by use of enzymes that recognize mismatched base pairs. A mutation analysis method comprises detection with a factor capable of binding specifically to a mismatched base pair formed by paring of a mutant-type DNA and a wild-type DNA. A representative example of the mutation analysis method includes detection of mutation sites by use of MutS, MutT, and MutL complexes from  Escherichia coli  (Patent Literature 1). 
     A mutation analysis method comprising use of a mismatch endonuclease which specifically cleaves mismatch sites is also known. In this method, a mismatch endonuclease is used to cleave a DNA in the vicinity of a mismatched base pair, and the DNA fragments thus obtained are analyzed to detect the presence or absence and the position of mutations. As a representative example, a method comprising use of a cell gene product from celery is known (Patent Literature 2), and the method is actually used for analyses of base mutations. However, the enzyme is not heat-resistant, and therefore cannot be used in techniques involving a high-temperature reaction process, such as PCR. Thus, in order to detect base mutations, the method requires four steps of amplification, formation of mismatches, cleavage of mismatches, and detection. 
     Other examples of mismatch endonuclease include mismatch endonucleases from  Pyrococcus furiosus, Methanocaldococcus jannaschii, Thermococcus barophylus,  and  Thermococcus kodakarensis  (Patent Literatures 3 and 4). However, these mismatch endonucleases recognize and cleave a plurality of mismatches, and therefore have a problem in recognition of mismatches (substrate specificity). 
     In some cases, the substrate specificity of a mismatch endonuclease is not suitable for a genetic mutation to be detected, and thus the genetic mutation cannot be analyzed. In such cases, there is no other choice than addition of an appropriately designed nucleic acid called a suppressive oligonucleotide to a reaction system to specifically detect the genetic mutation (Patent Literature 5). 
     In addition to mutation analyses, examples of biotechnological techniques that have a lot of influences include nucleic acid amplification techniques. A representative example of the nucleic acid amplification techniques is polymerase chain reaction (PCR). PCR is a technique for easily amplifying a desired nucleic acid fragment in vitro. PCR is an experimental technique which is essential in broad fields including the fields of biology, medicine, and agriculture, as well as research regarding genes. PCR is also applied to detection of mutated genes and analysis of methylation of DNA. Isothermal nucleic acid amplification methods such as a LAMP method and an ICAN method do not require special equipment, and therefore they are used as cheaper methods for detection of nucleic acids. For structural analyses of the whole genome which have been performed in recent years, a whole-genome amplification method is an important technique, in particular for analyses of scarce samples. 
     In these nucleic acid amplification methods, incorporation of incorrect bases occurs with a constant probability. The probability has been reduced through improvement of a polymerase or the like. However, the incorporation of incorrect bases still disturbs precise analyses. 
     The nucleic acid amplification techniques are used not only for amplification of a DNA having a specific nucleotide sequence but also for amplification of a mixture of DNAs having a common nucleotide sequence region at both ends. Specific examples of such nucleic acid amplification techniques include construction of genomic libraries or cDNA libraries. In constructing such libraries, however, a DNA molecule with a higher content is preferentially amplified, which may disturb analyses or screening of various kinds of DNAs. 
     To solve the above problem, the proportion of a DNA with a higher content is reduced by normalization utilizing self-hybridization (Non-patent Literature 1). SSH-PCR in which PCR and self-hybridization are combined is also used (Non-patent Literature 2). Using these methods, however, DNAs homologous to the DNA with a higher content may be also removed. 
     In detection of a DNA by a nucleic acid amplification method, a target DNA and a non-target DNA may compete for amplification. In other words, when a non-target DNA is amplified simultaneously with amplification of a target DNA, it is difficult to detect the target DNA. The above problem may be solved by use of real-time PCR in which probes such as cycling probes or TaqMan probes are used to detect only a target DNA. In the case where a non-target DNA exists in an excessively large amount relative to a target DNA, however, it is difficult to detect the target DNA because of false-positive reaction with the non-target DNA. 
     Such a problem may occur, for example, in detection of a small number of mutant alleles in the presence of normal alleles (for example, detection of circulating tumor genes), detection of a small number of methylated or non-methylated alleles by epigenetic assay, detection of a small amount of fetal DNA sequences circulating in the mother&#39;s blood, and the like. 
     To solve the above problem, a method called enrichment PCR or restriction endonuclease-mediated selective polymerase chain reaction (REMS PCR) has been developed (Non-patent Literature 3). This method involves use of a heat-resistant restriction enzyme. In this method, a DNA having a mutated nucleotide sequence is selectively detected using primers which, for example, are designed so that cleavage by the restriction enzyme occurs only when a template has a normal nucleotide sequence. Depending on a target DNA to be detected, however, there may be no heat-resistant restriction enzyme having a recognition sequence suitable to detection by REMS PCR. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: U.S. Pat. No. 5,922,539 B 
         Patent Literature 2: WO 01/062974 
         Patent Literature 3: WO 2014/142261 
         Patent Literature 4: WO 2016/039377 
         Patent Literature 5: WO 2016/152812 
       
    
     Non Patent Literature 
     
         
         Non-Patent Literature 1: “Nucleic Acids Research”, 2004 February, vol. 32, No. 3, e37 
         Non-Patent Literature 2: “Methods in Molecular Biology”, 2009, vol. 496, No. 2, pp. 223-243 
         Non-Patent Literature 3: “American Journal of Pathology”, 1998 August, vol. 153, No. 2, pp. 373-379 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     Objectives of the present invention include provision of a heat-resistant mismatch endonuclease mutant having improved specificity for a mismatch as compared with a conventional mismatch endonuclease, a composition comprising the mismatch endonuclease mutant, and a method of using the mismatch endonuclease mutant. 
     Solution to Problems 
     As a result of intensive efforts under the above circumstances, the present inventors succeeded in producing a mismatch endonuclease mutant capable of specifically cleaving a G-G mismatch and a mismatch endonuclease mutant capable of specifically cleaving a T-T mismatch. In addition, the present inventors succeeded in connecting two monomers respectively from the above-mentioned two mutants with a linker, though a mismatch endonuclease is originally a homodimer, to produce a mismatch endonuclease mutant capable of specifically cleaving a G-T/T-G mismatch. Thus, the present invention was completed. 
     Specifically, the first aspect of the present invention provides: 
     [1] A polypeptide represented by the following (A) or (B):
     (A) a polypeptide selected from the group consisting of the following (1) to (4), and having an activity of cleaving a nucleic acid chain having guanine (G) forming a mismatched base pair in a double-stranded nucleic acid:   

     (1) a polypeptide comprising an amino acid sequence comprising a substitution of a basic amino acid residue for serine at position 47 and a substitution of an acidic amino acid residue for asparagine at position 76 in an amino acid sequence set forth in SEQ ID NO: 1; 
     (2) a polypeptide comprising an amino acid sequence comprising mutations of further one to ten amino acid residues in the substituted amino acid sequence described in (1), wherein the mutations are substitution, deletion, insertion and/or addition and the amino acid substitutions in the substituted amino acid sequence described in (1) are retained; 
     (3) a polypeptide comprising an amino acid sequence having 50% or more homology to the substituted or mutated amino acid sequence described in (1) or (2), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (1) or (2) are retained; and 
     (4) a polypeptide comprising an amino acid sequence having 50% or more identity to the substituted or mutated amino acid sequence described in (1) or (2), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (1) or (2) are retained; or
     (B) a polypeptide selected from the group consisting of the following (5) to (8), and having an activity of cleaving a nucleic acid chain having thymine (T) forming a mismatched base pair in a double-stranded nucleic acid:   

     (5) a polypeptide comprising an amino acid sequence comprising a substitution of a basic amino acid residue for glutamine at position 78 and a substitution of a neutral (polar, uncharged) amino acid residue or a sulfur-containing amino acid residue for leucine at position 123 or of a hydrophobic amino acid residue or a sulfur-containing amino acid residue for leucine at position 125 in an amino acid sequence set forth in SEQ ID NO: 1; 
     (6) a polypeptide comprising an amino acid sequence comprising mutations of further one to ten amino acid residues in the substituted amino acid sequence described in (5), wherein the mutations are substitution, deletion, insertion and/or addition and the amino acid substitutions in the substituted amino acid sequence described in (5) are retained; 
     (7) a polypeptide comprising an amino acid sequence having 50% or more homology to the substituted or mutated amino acid sequence described in (5) or (6), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (5) or (6) are retained; and 
     (8) a polypeptide comprising an amino acid sequence having 50% or more identity to the substituted or mutated amino acid sequence described in (5) or (6), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (5) or (6) are retained; 
     [2] The polypeptide according to [1], wherein the basic amino acid residue is lysine, arginine, or histidine, 
     the acidic amino acid residue is aspartic acid or glutamic acid, 
     the neutral (polar, uncharged) amino acid is threonine, serine, asparagine, glutamine, tyrosine, or cysteine, 
     the sulfur-containing amino acid is cysteine or methionine, and 
     the hydrophobic (non-polar) amino acid is alanine, glycine, valine, leucine, isoleucine, proline, tryptophan, phenylalanine, or methionine; 
     [3] The polypeptide according to [1] or [2], which comprises an amino acid sequence selected from SEQ ID NOs: 6 to 15; 
     [4] A nucleic acid encoding the polypeptide according to any one of [1] to [3]; 
     [5] The nucleic acid according to [4], which comprises a nucleotide sequence selected from SEQ ID NOs: 21 to 30; 
     [6] An expression vector comprising the nucleic acid according to [4] or [5] and an expression regulatory sequence; 
     [7] A cell transformed with the expression vector according to [6], expressing a polypeptide having a mismatch endonuclease activity; 
     [8] A method for cleaving a double-stranded nucleic acid having a mismatch, the method comprising a step of treating the double-stranded nucleic acid with the polypeptide according to any one of [1] to [3]; 
     [9] A composition comprising the following (a) to (d): 
     (a) the polypeptide according to any one of [1] to [3]; 
     (b) an oligonucleotide that forms a double-stranded nucleic acid containing at least one mismatch when hybridized with a non-target nucleic acid, wherein the double-stranded nucleic acid is cleaved by the polypeptide of (a), and forms a double-stranded nucleic acid that is not cleaved by the polypeptide of (a) when hybridized with a target nucleic acid; 
     (c) at least one pair of oligonucleotide primers; and 
     (d) a polypeptide having a DNA polymerase activity; 
     [10] A method for amplifying a nucleic acid, the method comprising the following steps (1) and (2):
     (1) a step of preparing a composition comprising a nucleic acid molecule as a template and the following (a) to (d);   

     (a) the polypeptide according to any one of [1] to [3]; 
     (b) an oligonucleotide that forms a double-stranded nucleic acid containing at least one mismatch when hybridized with a non-target nucleic acid, wherein the double-stranded nucleic acid is cleaved by the polypeptide of (a), and forms a double-stranded nucleic acid that is not cleaved by the polypeptide of (a) when hybridized with a target nucleic acid; 
     (c) at least one pair of oligonucleotide primers; and 
     (d) a polypeptide having a DNA polymerase activity; and
     (2) a step of reacting the composition obtained in step (1) under appropriate conditions to perform nucleic acid amplification;   

     [11] The method according to [10], wherein the nucleic acid amplification is performed by a polymerase chain reaction (PCR) method, an isothermal nucleic acid amplification method, or a multiple displacement amplification (MDA) method; 
     [12] A method for suppressing amplification of a nucleic acid comprising a specific nucleotide sequence, the method comprising a step of performing a nucleic acid amplification reaction in the presence of the following (a) to (d): 
     (a) an oligodeoxynucleotide designed to produce one or several mismatches when hybridized with the nucleic acid comprising a specific nucleotide sequence or a complementary strand thereof; 
     (b) a DNA polymerase; 
     (c) at least one pair of oligonucleotide primers; and 
     (d) the polypeptide according to any one of [1] to [3]; 
     [13] The method according to [12], wherein the nucleic acid amplification reaction is polymerase chain reaction (PCR) or isothermal nucleic acid amplification; 
     [14] A method for preferentially amplifying a target DNA, the method comprising suppressing amplification of a DNA having a nucleotide sequence that differs from the target DNA by one or several nucleotides by the method according to [12] or [13]; 
     [15] The method according to [14], which is used for amplifying one of a DNA having a wild-type nucleotide sequence and a DNA that differs from the wild-type DNA by a single nucleotide polymorphism mutation distinctively from the other; 
     [16] The method according to [15], wherein the single nucleotide polymorphism mutation is a single nucleotide polymorphism mutation that correlates with canceration or a therapeutic effect of a therapeutic agent for cancer; and 
     [17] A polypeptide in a dimer-form, selected from a group consisting of: 
     (1) a homodimer having the polypeptide according to claim  1  (A) as a subunit, 
     (2) a homodimer having the polypeptide according to claim  1  (B) as a subunit, and 
     (3) a heterodimer having the polypeptide according to [1] (A) and the polypeptide according to [1] (B) as subunits. 
     Effects of the Invention 
     According to the present invention, a heat-resistant mismatch endonuclease mutant which has great utility in biotechnology, a composition comprising the mismatch endonuclease mutant, and a method of using the mismatch endonuclease mutant are provided. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     As used herein, the term “mismatch” refers to base pairings different from Watson-Crick base pairs present in double-stranded nucleic acids, in other words, pairing of bases in combinations other than base pairings of G (guanine base)—C (cytosine base), and A (adenine base)—T (thymine base) or U (uracil base). 
     As used herein, the term “GG-specific” means having specificity for a G-G mismatch, the term “TT-specific” means having specificity for a T-T mismatch, and the term “GT/TG-specific” means having specificity for a G-T or T-G mismatch. In other words, these terms mean that substantially recognizing nothing other than the indicated mismatches. 
     As used herein, the term “a polypeptide having a mismatch endonuclease activity (sometimes, merely referred to as a mismatch endonuclease) means a nuclease having an activity of cleaving mismatch sites present in double-stranded nucleic acids. The mismatch endonuclease activity includes an activity of cleaving phosphodiester bonds adjacent to nucleotides forming mismatched base pairs, and an activity of cleaving phosphodiester bonds adjacent to nucleotides located 1 to 5, preferably 1 to 3 base pairs away from mismatched base pairs. In the present invention, the mismatch endonuclease is preferably a nuclease having an activity of specifically recognizing a specific mismatched base pair in a double-stranded nucleic acid to cleave the double-stranded nucleic acid. As used herein, the heat-resistant mismatch endonuclease means a nuclease having an activity of cleaving mismatch sites present in double-stranded nucleic acids at temperature of 50° C. or higher. In the present invention, a heat-resistant mismatch endonuclease is preferably used. 
     The mismatch endonuclease in the present invention is an enzyme that forms a dimer to exhibit the activity. It is known that when a monomer (also called a “subunit” which means a component of a dimer) of the protein is expressed in a host such as  Escherichia coli  or the like, the monomers naturally associate to form a dimer (homodimer). As used herein, the polypeptide “having an activity of cleaving a nucleic acid chain having guanine (G) forming a mismatched base pair in a double-stranded nucleic acid” and the polypeptide “having an activity of cleaving a nucleic acid chain having thymine (T) forming a mismatched base pair in a double-stranded nucleic acid” mean the monomers. 
     Amino acids are classified into hydrophilic (polar) amino acids and hydrophobic (nonpolar) amino acids based on their chemical properties. 
     Hydrophilic (polar) amino acids include acidic amino acids, basic amino acids, neutral (polar, uncharged) amino acids, tyrosine among aromatic ring-containing amino acids, and cysteine among sulfur-containing amino acids. 
     Hydrophobic (nonpolar) amino acids include amino acids having aliphatic side chains, tryptophan and phenylalanine among aromatic ring-containing amino acids, and methionine among sulfur-containing amino acids. 
     Acidic amino acids include aspartic acid (Asp, D) and glutamic acid (Glu, E). 
     Basic amino acids include lysine (Lys, K), arginine (Arg, R), and histidine (His, H). 
     Neutral (polar, uncharged) amino acids include threonine (Thr, T), serine (Ser, S), asparagine (Asn, N), glutamine (Gln, Q), tyrosine (Tyr, Y), and cysteine (Cys). 
     Aromatic ring-containing amino acids include tyrosine (Tyr, Y), tryptophan (Trp, W), and phenylalanine (Phe, F). 
     Sulfur-containing amino acids include cysteine (Cys, C) and methionine (Met, M). 
     Amino acids having aliphatic side chains include alanine (Ala, A), glycine (Gly, G), valine (Val, V), leucine (Leu, L), isoleucine (Ile, I), and proline (Pro, P). Proline is an imino acid with a cyclized side chain. 
     As used herein, “similar amino acids” refer to amino acids classified into the same group based on the chemical properties as mentioned above. For example, lysine, arginine, and histidine are classified as basic amino acids, and thus they are similar amino acids to one another. 
     As used herein, “homology of amino acid sequences” means homology or similarity between polypeptide sequences in consideration of the above-mentioned similar amino acids. Amino acids classified into the same group based on the chemical properties of amino acids as mentioned above are regarded as “homologous”. 
     As used herein, “identity of amino acid sequences” means identity between polypeptide sequences in no consideration of the above-mentioned similar amino acids. As used herein, “identity of nucleotide sequences” means identity between polynucleotide sequences. 
     Known programs can be used to calculate scores for the amino acid sequence homology, the amino acid sequence identity, and the nucleotide sequence identity. Examples of the programs include, but not limited to, BLAST, BLAT, FASTA, SSEARCH, and MPsrch. 
     Amino acid numbers (also referred to as amino acid positions) as used herein are based on the amino acid sequence of SEQ ID NO: 1. Therefore, positions of amino acids indicated by amino acid numbers as used herein may be different from positions indicated by the same amino acid numbers when counted from N-terminals in homologous proteins derived from other organisms or mutant proteins of the polypeptide of SEQ ID NO: 1. In other words, a “position corresponding to position 47 in an amino acid sequence of a wild-type mismatch endonuclease” as used herein refers to an amino acid residue regarded as being at the same position as position 47 in the wild-type amino acid sequence (SEQ ID NO: 1) when the wild-type amino acid sequence is compared to and aligned with mutant amino acid sequences or amino acid sequences of mismatch endonuclease derived from other organisms. 
     As used herein, the “target nucleic acid” or “target DNA” is a nucleic acid that is not cleaved by the mismatch endonuclease mutant of the present invention. In other words, the target nucleic acid or target DNA is a nucleic acid to be amplified (as a template) by the amplification method of the present invention. On the other hand, the “non-target nucleic acid” or “non-target DNA” is a nucleic acid that is cleaved by the mismatch endonuclease mutant of the present invention. In other words, the non-target nucleic acid or non-target DNA is a nucleic acid that is cleaved by the cleavage method of the present invention and thus not amplified by the amplification method of the present invention. The “target nucleic acid” or “target DNA” and the “non-target nucleic acid” or “non-target DNA” are not particularly limited and may be any nucleic acids that are desired to be distinguished from each other. 
     Hereinafter, the present invention will be described in detail. 
     1. Mismatch Endonuclease Mutant of the Present Invention 
     The first aspect of the present invention relates to mismatch endonuclease mutants, i.e., mismatch endonuclease mutants having altered substrate specificity. The mismatch endonuclease mutants are characterized by changes in an amino acid sequence of a site that contributes to substrate recognition and/or cleavage in a wild-type mismatch endonuclease from  Pyrococcus furiosus  (also referred to as PfuNucS) or a homolog thereof, or a mutant having a mismatch endonuclease activity of PfuNucS. 
     The mismatch endonuclease mutant of the present invention can be prepared, for example, by introducing amino acid substitutions into a polypeptide from  Pyrococcus furiosus,  PF_RS00065 (RefSeq ID: WP_11011124.1, SEQ ID NO: 1, former name: PF0012, former RefSeq ID: NP_577741), wherein the amino acid substitutions are (i) an amino acid substitution of a basic amino acid for serine at position 47 and an amino acid substitution of an acidic amino acid for asparagine at position 76, or (ii) an amino acid substitution of a basic amino acid for glutamine at position 78, and an amino acid substitution of a neutral (polar, uncharged) amino acid or a sulfur-containing amino acid for leucine at position 123 or of a hydrophobic amino acid or a sulfur-containing amino acid for leucine at position 125. 
     Further, the mismatch endonuclease mutant of the present invention can be also prepared by introducing mutations corresponding to the above-mentioned amino acid substitutions (i) or (ii) into polypeptide W77F (SEQ ID NO: 2) in which tryptophan at position 77 in PF_RS00065 is replaced by phenylalanine, polypeptide MJ RS01180 from  Methanocaldococcus jannaschii  (RefSeq ID: NP_247194, SEQ ID NO: 3, former name: MJ_0225) which is a homolog of PF_RS00065, polypeptide TERMP_01877 from  Thermococcus barophilus  (RefSeq ID: YP_004072075, SEQ ID NO: 4) which is a homolog of PF_RS00065, or a polypeptide (SEQ ID NO: 5) from  Thermococcus kodakarensis  strain KOD1 (JCM 12380T) which is a homolog of PF_RS00065. 
     The mismatch endonuclease mutant of the present invention is preferably a thermostable enzyme. For example, a polypeptide stably retaining the activity even in a thermal cycle such as PCR is preferably used in the method of the present invention. A polypeptide that is not inactivated even at 50° C. or higher, preferably 60° C. or higher, further preferably 70° C. or higher, and more preferably 70° C. or higher can be used in the method of the present invention. 
     For example, the mismatch endonuclease mutant of the present invention comprises a polypeptide shown in (A) and/or (B) as described below. Preferable examples of the mismatch endonuclease mutant of the present invention include mismatch endonuclease mutants consisting of the polypeptides shown in (A) and/or (B) as described below. 
     (A) A polypeptide selected from the group consisting of the following (1) to (4), and having an activity of cleaving a nucleic acid chain having guanine (G) forming a mismatched base pair in a double-stranded nucleic acid: 
     (1) a polypeptide comprising an amino acid sequence comprising a substitution of a basic amino acid residue for serine at position 47 and a substitution of an acidic amino acid residue for asparagine at position 76 in an amino acid sequence set forth in SEQ ID NO: 1; 
     (2) a polypeptide comprising an amino acid sequence comprising mutations of further one to ten amino acid residues in the substituted amino acid sequence described in (1), wherein the mutations are substitution, deletion, insertion and/or addition and the amino acid substitutions in the substituted amino acid sequence described in (1) are retained; 
     (3) a polypeptide comprising an amino acid sequence having 50% or more homology to the substituted or mutated amino acid sequence described in (1) or (2), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (1) or (2) are retained; and 
     (4) a polypeptide comprising an amino acid sequence having 50% or more identity to the substituted or mutated amino acid sequence described in (1) or (2), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (1) or (2) are retained. 
     (B) A polypeptide selected from the group consisting of the following (5) to (8), and having an activity of cleaving a nucleic acid chain having thymine (T) forming a mismatched base pair in a double-stranded nucleic acid: 
     (5) a polypeptide comprising an amino acid sequence comprising a substitution of a basic amino acid residue for glutamine at position 78, and a substitution of a neutral (polar, uncharged) amino acid residue or a sulfur-containing amino acid residue for leucine at position 123 or of a hydrophobic amino acid residue or a sulfur-containing amino acid residue for leucine at position 125 in an amino acid sequence set forth in SEQ ID NO: 1; 
     (6) a polypeptide comprising an amino acid sequence comprising mutations of further one to ten amino acid residues in the substituted amino acid sequence described in (5), wherein the mutations are substitution, deletion, insertion and/or addition and the amino acid substitutions in the substituted amino acid sequence described in (5) are retained; 
     (7) a polypeptide comprising an amino acid sequence having 50% or more homology to the substituted or mutated amino acid sequence described in (5) or (6), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (5) or (6) are retained; and 
     (8) a polypeptide comprising an amino acid sequence having 50% or more identity to the substituted or mutated amino acid sequence described in (5) or (6), wherein the amino acid substitutions or mutations in the substituted or mutated amino acid sequence described in (5) or (6) are retained. 
     The present invention provides polypeptides having GG-specific mismatch endonuclease activity. Examples of the polypeptides having GG-specific mismatch endonuclease activity include, but not limited to, polypeptides shown in (A) as described above. Preferable examples of the polypeptides having GG-specific mismatch endonuclease activity include, but not limited to, polypeptides comprising an amino acid sequence comprising a substitution of a basic amino acid residue for serine at position 47 and a substitution of an acidic amino acid residue for asparagine at position 76 in an amino acid sequence set forth in SEQ ID NO: 1, as described in above (A) (1). Further examples of the polypeptides having GG-specific mismatch endonuclease activity include polypeptides comprising an amino acid sequence comprising a substitution of an amino acid having an aliphatic side chain for leucine at position 123 as well as the above-mentioned amino acid substitutions at position 47 and position 76 in SEQ ID NO: 1. Further preferable examples of the polypeptides having GG-specific mismatch endonuclease activity include polypeptides comprising an amino acid sequence comprising a substitution of lysine, arginine or histidine for serine at position 47 and a substitution of aspartic acid or glutamic acid for arginine at position 76 in an amino acid sequence of SEQ ID NO: 1. Further preferable examples of the polypeptides having GG-specific mismatch endonuclease activity also include polypeptides comprising an amino acid sequence comprising a substitution of alanine, glycine, valine, leucine, isoleucine or proline for leucine at position 123 as well as the above-mentioned amino acid substitutions at position 47 and position 76 in SEQ ID NO: 1. 
     Further, the present invention provides polypeptides having TT-specific mismatched endonuclease activity. Examples of the polypeptides having TT-specific mismatch endonuclease activity include, but not limited to, polypeptides shown in (B) as described above. Preferable examples of the polypeptides having TT-specific mismatch endonuclease activity include, but not limited to, polypeptides comprising an amino acid sequence comprising a substitution of a basic amino acid residue for glutamine at position 78, and a substitution of a neutral (polar, uncharged) amino acid residue or a sulfur-containing amino acid residue for leucine at position 123 or of a hydrophobic amino acid residue or a sulfur-containing amino acid residue for leucine at position 125 in an amino acid sequence set forth in SEQ ID NO: 1, as described in above (B) (5). Further preferable examples of the polypeptides having TT-specific mismatch endonuclease activity include polypeptides comprising an amino acid sequence comprising a substitution of lysine, arginine or histidine for glutamine at position 78, and a substitution of threonine, serine, asparagine, glutamine, tyrosine, cysteine or methionine for leucine at position 123 or of alanine, glycine, valine, isoleucine, proline, tryptophan, phenylalanine, cysteine or methionine for leucine at position 125 in an amino acid sequence of SEQ ID NO: 1. In a further preferable example of the polypeptide having TT-specific mismatch endonuclease activity, leucine at position 125 may be replaced by an amino acid having an aliphatic side chain other than leucine. 
     The polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity may be in the form of a homodimer. When the polypeptide having GG-specific mismatch endonuclease activity is expressed as monomers in a host such as  Escherichia coli  or the like, the monomers naturally associate to form dimers (homodimers) and thus exhibit the activity. The same holds for the polypeptide having TT-specific mismatch endonuclease activity. 
     Further, the present invention also provides polypeptides having GT/TG-specific mismatch endonuclease activity. Examples of the polypeptides having GT/TG-specific mismatch endonuclease activity include, but not limited to, heterodimer proteins comprising the polypeptides shown in above (A) and the polypeptides shown in above (B). Preferable examples of the polypeptides having GT/TG-specific mismatch endonuclease activity include, but not limited to, heterodimer proteins comprising the polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity. The heterodimer protein can be produced by expressing the polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity in a host such as  Escherichia coli  or the like so as to form a dimer (heterodimer). However, a method for producing the heterodimer protein is not limited. After the polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity are expressed separately and then dissociated into monomers by any method, the monomers of both polypeptides may be mixed to allow to be reassociated. In a preferred method for producing the heterodimer protein, the polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity are expressed as a single polypeptide via a linker. An arrangement of the polypeptide having GG-specific mismatch endonuclease activity and the polypeptide having TT-specific mismatch endonuclease activity within the single polypeptide is not limited. They may be arranged in the order of [the polypeptide having GG-specific mismatch endonuclease activity]-[linker]-[the polypeptide having TT-specific mismatch endonuclease activity] or [the polypeptide having TT-specific mismatch endonuclease activity]-[linker]-[the polypeptide having GG-specific mismatch endonuclease activity]. The linker is preferably a peptide linker. The amino acid sequence and the number of residues (length) of the peptide linker are not limited, as long as the linker does not inhibit the heterodimerization and the cleavage of the double-stranded nucleic acid. Furthermore, a recognition sequence for protease for cleavage of fusion polypeptide, such as Factor Xa, PreScission protease, thrombin, enterokinase, TEV protease (tobacco etch virus protease) or the like may be present between the polypeptide having mismatch endonuclease activity and the linker. 
     The mismatch endonuclease mutant of the present invention may comprise, in addition to the above-mentioned amino acid substitutions described in (A) (1) and (B) (5), a mutation at another amino acid position as long as the mismatch endonuclease mutant does not lose the substrate specificity (the specificity for a GG, TT, or GT/TG mismatch). Examples of the mutation at another amino acid position include, but not limited to, a mutation that increases heat resistance, a mutation that increases resistance to an inhibitor, and a mutation that improves the property of mismatch endonuclease, as long as the substrate specificity of the mismatch endonuclease mutant of the present invention is maintained. As used herein, the “mutation” may be any of substitution, insertion, deletion, or addition of an amino acid. 
     For example, the mismatch endonuclease mutant of the present invention comprising a mutation at another amino acid position may comprise an amino acid sequence comprising mutations of further 1 to 10 amino acid residues in addition to the amino acid substitutions described above in (A) (1) and (B) (5). 
     Further, for example, the mismatch endonuclease mutant of the present invention comprising a mutation at another amino acid position may comprise an amino acid sequence having 50% or more homology to an amino acid sequence of SEQ ID NO: 1 comprising the amino acid substitutions described above in (A) (1) and (B) (5) or to an amino acid sequence of SEQ ID NO: 1 comprising the amino acid substitutions described above in (A) (1) and (B) (5) and mutations of further one to ten amino acid residues. The 50% or more homology includes for example 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, or 75% or more, preferably 80% or more, more preferably 85% or more, further more preferably 90% or more, and even more preferably 95% or more, 96% or more, 97% or. more, 98% or more or 99% or more amino acid sequence homology. 
     Further, for example, the mismatch endonuclease mutant of the present invention comprising a mutation at another amino acid position may comprise an amino acid sequence having 50% or more identity to an amino acid sequence of SEQ ID NO: 1 comprising the amino acid substitutions described above in (A) (1) and (B) (5) or to an amino acid sequence of SEQ ID NO: 1 comprising the amino acid substitutions described above in (A) (1) and (B) (5) and mutations of further one to ten amino acid residues. The 50% or more identity includes for example 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, or 75% or more, preferably 80% or more, more preferably 85% or more, further preferably comprises 90% or more, and even more preferably 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more amino acid sequence identity. 
     Further, the present invention includes the mismatch endonuclease mutant comprising an affinity tag added to the N-terminal or C-terminal for the purpose of facilitating purification. The affinity tag may be a known affinity tag, and examples thereof include a histidine (His) tag consisting of 4 to 8 consecutive His residues, a Flag tag, an HA tag, a c-myc tag, a glutathione S-transferase (GST) tag, a maltose binding protein (MBP) tag, and a Strep (II) tag consisting of 8 amino acid residues (Trp-Ser-His-Pro-Gln-Phe-Glu-Lys). The tag may be linked to the mutant of the present invention optionally via a linker comprising 1 to 15 amino acids. Furthermore, a recognition sequence of a protease for fusion polypeptide cleavage may be present between the polypeptide having a mismatch endonuclease activity and the linker. Examples of the protease include Factor Xa, PreScission protease, thrombin, enterokinase, and TEV protease (tobacco etch virus protease). Thus, such a polypeptide comprising the affinity tag, the linker, and/or the recognition sequence of a protease for fusion polypeptide cleavage is also an example of the mismatch endonuclease mutant of the present invention comprising a mutation at another amino acid position. For example, the mismatch endonuclease mutant of the present invention comprising a mutation at another amino acid position may comprise the affinity tag, the linker, and/or the recognition sequence of a protease for fusion polypeptide cleavage within the range of the mutated amino acid residue number of one to ten, the amino acid homology, or the amino acid identity as described above. 
     The mismatch endonuclease mutant of the present invention may comprise an artificial amino acid (also referred to as an unnatural amino acid). Examples of the artificial amino acid include halogenated (chloro, bromo, iodo) tyrosine, tyrosine sulfate, azidotyrosine, acetyllysine, azidophenylalanine, and fluorophenylalanine. A method for replacing a natural amino acid with an artificial amino acid is not particularly limited, and a known method can be used. 
     Examples of the mismatch endonuclease mutant of the present invention include, but not limited to, a mutant consisting of any of amino acid sequences set forth in SEQ ID NOs: 6 to 15. 
     Such a mismatch endonuclease mutant of the present invention is suitable for various uses as described later, for example, use in a method of excluding a DNA comprising a specific DNA sequence and amplifying and detecting other DNAs. 
     The activity of mismatch endonuclease can be measured using a double-stranded nucleic acid containing a mismatch as a substrate. Specifically, the activity is measured by reacting a mismatch endonuclease with an excess amount of a double-stranded nucleic acid containing a mismatch and then measuring the amount of cleaved nucleic acid per unit time. The cleaved double-stranded nucleic acid can be quantified separately from the nucleic acid that has not been cleaved by electrophoresis or the like. The double-stranded nucleic acid may be double-labeled with a fluorescent substance and a quencher so that an increase in fluorescence intensity can be detected only when cleaved. The activity measurement is facilitated by determining fluorescence intensity in a reaction solution at appropriate time intervals using the double-labeled double-stranded nucleic acid. It is also possible to investigate the cleavage activity targeting a specific mismatch by changing a mismatched base pair in the double-stranded nucleic acid as a substrate. 
     2. Nucleic Acid Encoding Mismatch Endonuclease Mutant of the Present Invention 
     The present invention provides a nucleic acid encoding a mismatch endonuclease mutant. Specifically, a nucleic acid encoding the above-mentioned mismatch endonuclease mutant of the present invention is provided. 
     Examples of the nucleic acid encoding the mismatch endonuclease mutant of the present invention include, but not limited to, nucleic acids comprising nucleotide sequences encoding the amino acid sequences set forth in SEQ ID NOs: 6 to 15. Preferred examples of the nucleic acid encoding the mismatch endonuclease mutant of the present invention include nucleic acids comprising nucleotide sequences encoding the amino acid sequences set forth in SEQ ID NOs: 21 to 30. 
     As examples of the mismatch endonuclease mutant of the present invention, which the present invention is not limited to, the amino acid sequences of the polypeptides prepared in Examples and nucleic acid sequences encoding them are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Mutant name 
                 Amino acid SEQ No. 
                 Nucleotide SEQ No. 
               
               
                   
               
             
            
               
                 S47K + N76D 
                 SEQ ID NO: 6 
                 SEQ ID NO: 21 
               
               
                 S47R + N76D + L123A 
                 SEQ ID NO: 7 
                 SEQ ID NO: 22 
               
               
                 S47R + N76D + L123G 
                 SEQ ID NO: 8 
                 SEQ ID NO: 23 
               
               
                 Q78R + L123S 
                 SEQ ID NO: 9 
                 SEQ ID NO: 24 
               
               
                 Q78R + L123T 
                 SEQ ID NO: 10 
                 SEQ ID NO: 25 
               
               
                 Q78R + L123M 
                 SEQ ID NO: 11 
                 SEQ ID NO: 26 
               
               
                 Q78R + L123C 
                 SEQ ID NO: 12 
                 SEQ ID NO: 27 
               
               
                 Q78R + L125V 
                 SEQ ID NO: 13 
                 SEQ ID NO: 28 
               
               
                 Q78R + L125A 
                 SEQ ID NO: 14 
                 SEQ ID NO: 29 
               
               
                 GT/TG specific 
                 SEQ ID NO: 15 
                 SEQ ID NO: 30 
               
               
                 heterodimer 
               
               
                   
               
            
           
         
       
     
     The nucleic acid encoding the mismatch endonuclease mutant of the present invention is not particularly limited as long as it is composed of codons encoding a protein that can be expressed in a host used and has a reverse transcriptase activity. The nucleic acid may be subjected to codon optimization to allow the expression in the host or to increase the expression level. The codon optimization is preferably performed by a method commonly used in the art. 
     3. Expression Vector Containing Nucleic Acid Encoding Mismatch Endonuclease Mutant of the Present Invention 
     The expression vector of the present invention contains a nucleic acid encoding the mismatch endonuclease mutant of the present invention, and an expression regulatory sequence operably linked to the nucleic acid. 
     The expression vector used in the present invention may be an expression vector usually used in the art, and is not particularly limited. A vector capable of autonomously replicating in a host cell, or a vector capable of being integrated into a host chromosome can be used. A vector compatible with the host may be used. 
     Examples of the expression vector into which a nucleic acid encoding the mismatch endonuclease mutant of the present invention is inserted include a plasmid vector, a phage vector, and a viral vector. The plasmid vector may be a plasmid suitable for a host to be used, and examples of such a plasmid include a plasmid derived from  Escherichia coli,  a plasmid derived from a bacterium belonging to the genus  Bacillus,  and a plasmid derived from yeast. Such plasmid vectors are well known to those skilled in the art, and there are many commercially available plasmid vectors. Such known plasmids and altered plasmids thereof can be used in the present invention. The phage vector may be, for example, λ phage or the like. The viral vector may be, for example, an animal virus such as a retrovirus or a vaccinia virus, or an insect virus such as a baculovirus. Furthermore, many heterologous protein expression systems using yeast, insect cells and mammalian cells as hosts have been constructed, and are already commercially available. Such expression systems may be used in production of the mismatch endonuclease mutant of the present invention. 
     A promoter to be contained in the expression vector of the present invention can be selected depending on the host. For example, for  Escherichia coli,  a promoter derived from  Escherichia coli  or phage or an altered promoter thereof including, but not limited to, trp promoter, lac promoter, PL promoter and PR promoter, can be used. Further, an expression system containing a phage-derived promoter and a phage-derived RNA polymerase gene in combination (for example, a pET expression system, etc.) may be used. 
     To facilitate purification of the expressed polypeptide, the expression vector of the present invention may further contain a nucleic acid encoding an affinity tag. The nucleic acid encoding an affinity tag is inserted into the vector so that a fusion protein of the mismatch endonuclease mutant of the present invention and the affinity tag can be expressed. Examples of the affinity tag include, but not limited to, a histidine (His) tag, a glutathione S-transferase (GST) tag, a maltose binding protein (MBP) tag, and a Strep (II) tag consisting of 8 amino acid residues (Trp-Ser-His-Pro-Gln-Phe-Glu-Lys). The tag may be added to either the 5′end side and/or the 3′end side of the nucleic acid encoding the mismatch endonuclease mutant of the present invention. The tag may be appropriately added to a position that does not interfere with the expression and the tag function. The tag is preferably a tag that can be cleaved during or after purification of the expressed polypeptide. Examples of such a tag that can be cleaved include, but not limited to, tags comprising nucleic acids encoding recognition sequences of proteases for cleavage of fusion polypeptides, such as Factor Xa, PreScission protease, thrombin, enterokinase, and TEV protease (tobacco etch virus protease). 
     The expression vector of the present invention further contains one or more expression regulatory sequences. Examples of the expression regulatory sequence include, but not limited to, a promoter, a gene involved in regulation of a promoter, a ribosome binding sequence, a polyadenylation signal, a transcription termination sequence (transcription terminator), and an enhancer. The expression vector of the present invention may further contain a gene encoding an origin of replication or a marker (drug resistance gene, fluorescence marker, or luminescence marker) used for selection of transformants, and a nucleotide sequence for enhancing translation efficiency. 
     4. Cells Transformed with Expression Vector of the Present Invention 
     Cells (hosts) to be transformed with the vector for expressing the mismatch endonuclease mutant of the present invention may be hosts commonly used in the art, and are not particularly limited. Examples thereof include bacteria ( E. coli, Bacillus subtilis,  etc.), yeast, filamentous fungi, insect cells, eukaryotic cells, and animal cells (mammalian cells including human cells, etc.). 
     When a prokaryotic cell is used as a host cell, examples of the host cell include bacteria belonging to the genus  Escherichia  such as  Escherichia coli,  the genus  Bacillus  such as  Bacillus subtilis,  the genus  Pseudomonas  such as  Pseudomonas putida,  and the genus  Rhizobium  such as  Rhizobium meliloti. Escherichia coli  that can be used for production of heterologous protein is well known to those skilled in the art, and there are many commercially available  E. coli  strains (for example,  Escherichia coli  BL21T1R,  Escherichia coli  BL21,  E. coli  XL1-Blue,  E. coli  XL2-Blue,  E. coli  DH1,  E. coli  JM109,  E. coli  HB101, etc.). Further,  Bacillus subtilis  MI114,  B. subtilis  207-21 and the like belonging to the genus  Bacillus, Brevibacillus choshinensis  and the like belonging to the genus  Brevibacillus  are known as hosts for production of heterologous proteins. The above-mentioned host cells and suitable expression vectors can be used in combination for production of the mismatch endonuclease mutant of the present invention. Preferably,  E. coli  BL21T1R or BL21DE3, which is strain BL21 of  E. coli,  can be used. 
     A method for introducing the expression vector into the host is not particularly limited as long as it can introduce a nucleic acid into the host. Examples of the method include a method using calcium ions, an electroporation method, a spheroplast method, and a lithium acetate method. A method for introducing the expression vector into insect cells is not particularly limited as long as it can introduce a DNA into insect cells, and examples thereof include a calcium phosphate method, a lipofection method, and an electroporation method. When a phage vector or a viral vector is used, host cells can be infected by a suitable method for the vector and thereby a transformant expressing the mismatch endonuclease mutant of the present invention can be obtained. 
     The transformant is cultured. The mismatch endonuclease mutant of the present invention can be collected from the culture of the transformant. The culture conditions are not particularly limited as long as they are suitable for the expression vector used, the host used and the like. For example,  Escherichia coli  BL21DE3 is transformed with a pET vector, and the transformant is inoculated into an LB medium and cultured with shaking at 37° C. When an OD value of the culture reaches 0.2 to 0.8, IPTG is added to the medium, and then, shaking culture is continued to induce the expression of a desired protein, for example, at 15 to 30° C. for 2 to 5 hours, preferably 25° C. for 4 to 5 hours. Then, the culture is centrifuged, and bacterial cells thus obtained are washed, and then subjected to ultrasonication or lysis with lysozyme to obtain a total cell extract containing the mutant of the present invention. Since the extract contains a large amount of impurities, the mutant of the present invention is preferably purified by appropriately combining purification methods used in the art, for example, ammonium sulfate precipitation, anion exchange column, cation exchange column, gel filtration column, affinity chromatograph column, dialysis, and the like. The mutant with an affinity tag can be easily purified by using an affinity carrier appropriate for the property of: the affinity tag. Further, in addition to IPTG, other necessary inducers such as L-arabinose may be added at an appropriate timing, depending on the type of host or expression vector used. 
     5. Method for Producing Nucleic Acid Encoding Mismatch Endonuclease Mutant of the Present Invention 
     The method for producing a nucleic acid of the present invention comprises a step of, for example, replacing a codon encoding serine at a position corresponding to position 47 in the amino acid sequence of SEQ ID NO: 1 by a codon encoding a basic amino acid, in a nucleic acid encoding a mismatch endonuclease that is polypeptide PF_RS00065 or a mutant polypeptide thereof W77F, or a homologue of RS_00065. The codon for serine is preferably replaced by a codon for arginine or lysine, more preferably by a codon for arginine. Similarly, codons for the amino acid residues at the other positions can be also replaced. 
     As described above in section 1, the mismatch endonuclease mutant of the present invention may comprise a mutation at another amino acid position. Even in that case, the codon can be replaced similarly. 
     Preferred examples of the nucleic acid encoding the mismatch endonuclease mutant comprising the codon replacement as described above include nucleic acids of SEQ ID NOs: 16 to 30. 
     The codon replacement may be performed by a known method, including, but not limited to, introduction of mutation by a known method such as site-specific mutagenesis using a primer for introduction of mutation, and artificial synthesis of a nucleic acid having a mutated sequence (or a part of the sequence). Further, codon optimization may be performed for the purpose of allowing the expression in the host used or increasing the expression level. The codon optimization can be performed by a method commonly used in the art. 
     The method for producing a nucleic acid of the present invention may further comprise replacing a codon for the purpose of stabilization and increase of protein production in the host, in the nucleic acid encoding the mismatch endonuclease mutant. 
     For the method for producing a nucleic acid of the present invention, the nucleic acid as explained above in section 2 can be applied. 
     6. Method for Cleaving Double-Stranded Nucleic Acid of the Present Invention 
     The method for cleaving a double-stranded nucleic acid of the present invention is performed by treating a double-stranded nucleic acid having a mismatch with the mismatch endonuclease mutant of the present invention as described above in section 1, in an appropriate buffer containing a divalent metal ion (for example, magnesium ion). According to the method of the present invention, a double-stranded nucleic acid containing a G-G mismatch, a T-T mismatch, a G-T mismatch or a T-G mismatch can be cleaved. 
     For the method for cleaving a double-stranded nucleic acid of the present invention, the double-stranded nucleic acid having a mismatch may be a double-stranded nucleic acid containing a mismatch inside (between two base pairs that are normally pairing). The double-stranded nucleic acid having a mismatch may be not only a double-stranded nucleic acid containing one mismatch, but also a double-stranded nucleic acid containing two or more mismatches at intervals or a double-stranded nucleic acid containing two or more consecutive mismatches. Examples of the mismatch for the method for cleaving a double-stranded nucleic acid of the present invention include preferably 1 to 8 consecutive mismatches, more preferably 1 to 4 consecutive mismatches, further preferably two consecutive mismatches, and one mismatch, which exist in the double-stranded nucleic acid. In the method for cleaving a double-stranded nucleic acid of the present invention, when two or more mismatches are present in the double-stranded nucleic acid, the two or more mismatches may be the same type of mismatches or may be different types of mismatches. 
     Further, an oligodeoxynucleotide as disclosed in WO2014/142261 and the like may be used. As used herein, the oligodeoxynucleotide is an oligodeoxynucleotide designed to generate one or several mismatches when hybridized with a nucleic acid having a specific nucleotide sequence (for example, having a genetic mutation of interest). For use as a probe for mutation detection, the oligodeoxynucleotide can be labeled with a fluorescent substance and a quenching substance at both ends. The length of the oligodeoxynucleotide can be appropriately determined so that the oligodeoxynucleotide can be hybridized with the nucleic acid having a specific nucleotide sequence under reaction conditions to be carried out. When the oligodeoxynucleotide is hybridized with the nucleic acid having a specific nucleotide sequence, a mismatch is preferably generated at a position at least 3 nucleotides away from both the 5′ end and 3′ end of the oligodeoxynucleotide. 
     Further, when the genetic mutation cannot be analyzed because the genetic mutation to be detected does not match the substrate specificity of the mismatch endonuclease, a suppressive oligonucleotide as disclosed in WO2016/152812 and the like may be used. As used herein, the suppressive oligonucleotide is an oligonucleotide that forms at least one mismatch when the oligonucleotide is hybridized with a non-target nucleic acid, and forms more mismatches when the oligonucleotide is hybridized with a target nucleic acid than when the oligonucleotide is hybridized with the non-target nucleic acid. The number of the at least one mismatch formed when the suppressive oligonucleotide is hybridized with a non-target nucleic acid is not particularly limited as long as selective cleavage of the non-target nucleic acid occurs, and examples thereof include 1 to 7, 1 to 5, or 1 to 3 mismatches depending on the length of the suppressive oligonucleotide. When the number of the at least one mismatch is expressed as a percentage of the length of the suppressive oligonucleotide, for example, the at least one mismatch accounts for 1-20%, 3-15%, or 4-8% of the length of the suppressive oligonucleotide. As an example of this aspect, a combination of the target nucleic acid and the non-target nucleic acid which differ by only one base in their nucleotide sequences is explained. When the suppressive oligonucleotide is hybridized with the target nucleic acid, a mismatch is formed between the suppressive oligonucleotide and the base that differs between the target nucleic acid and the non-target nucleic acid (hereinafter, sometimes, referred to as a first mismatch), while another mismatch is formed (hereinafter, sometimes, referred to as a second mismatch). 
     The base pair of the first mismatch relates to the base differing between the target nucleic acid and the non-target nucleic acid, regardless of whether the mismatched base pair is recognized and cleaved by the co-existing polypeptide having a mismatch endonuclease activity. 
     The second mismatch is a mismatched base pair that is recognized and cleaved by the co-existing polypeptide having a mismatch endonuclease activity. For example, when a polypeptide having a mismatch endonuclease activity to recognize and cleave guanine base-guanine base, guanine base-thymine base, or thymine base-thymine base is used, the suppressive oligonucleotide is designed to form a mismatch selected from the above-mentioned mismatched base pairs. 
     When the suppressive oligonucleotide is hybridized with the non-target nucleic acid, the second mismatch is formed while the first mismatch is not formed. 
     Designing and using the suppressive oligonucleotide having the above-mentioned properties enable selective cleavage of a non-target nucleic acid by a polypeptide having a mismatch endonuclease activity. The present invention is not limited to use of the suppressive oligonucleotide capable of forming one or two mismatches as mentioned above. As long as selective cleavage of the desired nucleic acid occurs, a suppressive oligonucleotide capable of forming 3 or more mismatches may be designed and used. In such a case, at least one mismatch formed in a target nucleic acid and a non-target nucleic acid may be the second mismatch, and the other mismatches may or may not be recognized and cleaved by a polypeptide having a mismatch endonuclease activity. The 3 or more mismatches are preferably recognized and cleaved by a polypeptide having a mismatch endonuclease activity. 
     The oligodeoxynucleotide as disclosed in WO2014/142261 etc. and the suppressive oligonucleotide as disclosed in WO2016/152812 etc. are composed of DNA, or may partially comprise nucleotide analogs or RNA. In other words, the oligodeoxynucleotide and the suppressive oligonucleotide are not particularly limited as long as they have a structure that forms a double-stranded nucleic acid having at least one mismatch when hybridized with a non-target nucleic acid, and the mismatch is recognized and cleaved by a co-existing polypeptide having a mismatch endonuclease activity. 
     Examples of a double-stranded nucleic acid having the mismatch cleaved by the method of the present invention include a PCR product, a nucleic acid derived from a biological sample such as a genomic DNA or a fragment thereof, and a synthetic nucleic acid. The double-stranded nucleic acid having the mismatch may be a mixture of nucleic acids from biological samples, or a melting-reannealing mixture of a nucleic acid from a biological sample and a synthetic nucleic acid. For example, when a nucleic acid comprising a mutation and a wild-type nucleic acid are mixed, melted and reannealed, a mismatch is formed and cleavage by a mismatch endonuclease occurs at a position of the mismatch. The size of a nucleic acid fragment thus obtained after cleavage by mismatch endonuclease can be observed to estimate the presence or absence and the position of a mutation. Use of the mismatch endonuclease of the present invention allows mutation analysis by simply adding this mismatch endonuclease to a reaction solution for nucleic acid amplification such as PCR. For PCR, it is known that an increase of amplification effect is not found when the number of cycles is increased to above a certain number. This is mainly because of depletion of added primers or substrate dNTPs or competition between the primers and reaction products for annealing. At this time, annealing between reaction products occurs. If both a template having a mutation and a wild-type template are present, a mismatch is formed at the mutated position by annealing between a reaction product amplified from the mutated template and a reaction product amplified from the wild-type template. Therefore, mutation analysis becomes possible by simply performing PCR in the presence of the mismatch endonuclease of the present invention for a larger number of cycles than usual. In other words, the present invention provides a method for analyzing a mutation, the method comprising treating a double-stranded nucleic acid with the mismatch endonuclease described above in section 1. 
     The method for cleaving a double-stranded nucleic acid of the present invention can be performed in the presence of an acidic high molecular substance. The acidic high molecular substance has been found to have effect of controlling the mismatch recognition and cleavage activity of the polypeptide by coexistence of the acidic high molecular substance. The acidic high molecular substance exerts more effect in a sample containing a small amount of nucleic acid. Preferable examples of the acidic high molecular substance include polyanions. Preferable examples of the acidic high molecular substance also include acidic polysaccharides having a sugar backbone and acidic polysaccharides having a liner carbon chain. As the acidic high molecular substance, one or more substances selected from the group consisting of fucose sulfate-containing polysaccharide, dextran sulfate, carrageenan, heparin, rhamnan sulfate, dermatan sulfate (chondroitin sulfate B), heparan sulfate, hyaluronic acid, alginic acid, pectin, polyglutamic acid, polyacrylic acid, polyvinyl sulfate, polystyrene sulfate, and their salts, and different nucleic acids from a target nucleic acid and a non-target nucleic acid can be used. 
     The method for cleaving a double-stranded nucleic acid of the present invention may be performed further in combination with use of a proliferating cell nuclear antigen (PCNA). 
     7. Amplification Method of Double-Stranded Nucleic Acid of the Present Invention 
     The method for cleaving a double-stranded nucleic acid of the present invention can be performed even in a process of nucleic acid amplification reaction. A double-stranded nucleic acid having a mismatch formed by incorporation of an incorrect nucleotide during the amplification process is cleaved by adding a mismatch endonuclease to a reaction mixture for nucleic acid amplification. As a result, amplification of a nucleic acid having a different sequence from that of a template nucleic acid before the start of reaction is suppressed. Thus, nucleic acid amplification with a reduced error rate becomes possible. 
     That is, the present invention provides a nucleic acid amplification method comprising a step of cleaving a double-stranded nucleic acid having a mismatch by using the mismatch endonuclease mutant of the present invention as described above in section 1. The step of cleaving a double-stranded nucleic acid having a mismatch may be performed simultaneously with a step of nucleic acid amplification. Further, as an aspect of the present invention, a composition comprising (a) a DNA polymerase, (b) at least one pair of oligonucleotide primers, and (c) the mismatch endonuclease described above in section 1 is provided. 
     Similarly, as an aspect of the present invention, a nucleic acid amplification method comprising a step of preparing a composition comprising the above-described composition for nucleic acid amplification reaction and a nucleic acid molecule as a template, and a step of performing nucleic acid amplification by reacting the obtained composition under appropriate conditions is also provided. 
     The above-described composition may further comprise at least one selected from the group consisting of a reaction buffer, a divalent metal ion, a deoxyribonucleotide, an oligonucleotide probe, and an intercalating dye. When the composition is used for nucleic acid amplification reaction, the composition may further comprise a nucleic acid as a template for the nucleic acid amplification reaction. In the composition, the deoxyribonucleotide may contain a nucleotide analog. The reaction buffer means a compound or a mixture having an ability to mitigate change of hydrogen ion concentration (pH) in a reaction solution. Since a mixed solution of a weak acid and its salt, or a weak base and its salt generally has a strong buffering function, the mixed solution is widely used as a reaction buffer for the purpose of pH control. Examples of the reaction buffer used in the present invention include Good&#39;s buffers such as Tris-HCl, and HEPES-KOH, and phosphate buffers such as a sodium phosphate buffer. Examples of the divalent metal ion include a magnesium ion, a manganese ion, a zinc ion, and a cobalt ion. The divalent metal ion may be provided as a salt form such as a chloride, a sulfate, or an acetate. 
     An example of the method for amplifying a double-stranded nucleic acid of the present invention is a method for amplifying DNA, which the present invention is not particularly limited to. Examples of the method for amplifying DNA include polymerase chain reaction (PCR), MDA (multiple displacement amplification), and isothermal nucleic acid amplification such as ICAN and LAMP. 
     The concentration of a polypeptide having a mismatch endonuclease activity in the above-described composition for nucleic acid amplification reaction may be determined by determining a concentration that does not inhibit a DNA amplification reaction or a concentration effective for cleavage of a double-stranded nucleic acid having a mismatch in each reaction system as appropriate. 
     As at least one pair of primers used for the above-described composition for nucleic acid amplification reaction, two or more primers suitable for each nucleic acid amplification method are selected. The primers may be DNA primers, RNA primers, or chimeric primers in which a part of a DNA molecule is replaced by RNA, as long as the desired amplification is attained. The primers may also be primers containing a known nucleic acid analog, and labeled primers, for example, with a fluorescent dye for the purpose of detection. 
     Further, as another aspect of the composition of the present invention, the composition may comprise a component for preventing carryover. Examples of such a component include dUTP, and uracil N-glycosidase. 
     The method for amplifying a double-stranded nucleic acid of the present invention can be performed in the presence of the above-described acidic high molecular substance. The acidic high molecular substance exerts more effect in a sample containing a small amount of nucleic acid. Preferable examples of the acidic high molecular substance are as described above for the method for selectively cleaving a non-target nucleic acid of the present invention. The method of the present invention may be performed further in combination with the above-described proliferating cell nuclear antigen (PCNA) or a mutant thereof. 
     8. Method for Suppressing Nucleic Acid Amplification of the Present Invention 
     The method for suppressing nucleic acid amplification of the present invention comprises utilizing the mismatch endonuclease mutant of the present invention described above in section 1 and the appropriately designed oligodeoxynucleotide or suppressive oligonucleotide to suppress amplification of a nucleic acid having a specific nucleotide sequence in a nucleic acid amplification reaction. 
     Thus, an aspect of the present invention provides a method for suppressing amplification of a nucleic acid having a specific nucleotide sequence in a nucleic acid amplification reaction, the method comprising a step of performing the nucleic acid amplification reaction in the presence of (a) an oligodeoxynucleotide that is designed to produce one or several mismatches when hybridized with the nucleic acid having a specific nucleotide sequence or a suppressive oligonucleotide, (b) a DNA polymerase, (c) at least one pair of oligonucleotide primers, and (d) a polypeptide having a mismatch endonuclease activity. A further aspect of the present invention provides a method for preferentially amplifying a target nucleic acid, the method comprising suppressing amplification of a nucleic acid having a specific nucleotide sequence containing one or several different nucleotides from the nucleotide sequence of the target nucleic acid by using the above-described method. 
     The oligodeoxynucleotide or suppressive oligonucleotide of (a) is not particularly limited as long as it is a single-stranded DNA having a nucleotide sequence designed to form one or several mismatches when hybridized with a nucleic acid having a specific nucleotide sequence. The oligodeoxynucleotide or suppressive oligonucleotide may be a so-called chimeric oligodeoxynucleotide in which a part of a DNA molecule is replaced by RNA. The 3′-end of the oligodeoxynucleotide or suppressive oligonucleotide may be modified so as to inhibit an extension reaction from the single-stranded DNA by a DNA polymerase, to which the present invention is not particularly limited. Examples of the modification include amination. The oligodeoxynucleotide or suppressive oligonucleotide may be protected from cleavage with a deoxyribonuclease by phosphorothioation or other modifications, as long as the nucleic acid to which the oligodeoxynucleotide or suppressive oligonucleotide is bound undergoes cleavage with the polypeptide having a mismatch endonuclease activity. The oligodeoxynucleotide or suppressive oligonucleotide may be labeled with a fluorescent dye, a quencher or the like for the purpose of detection. 
     The length of the oligodeoxynucleotide or suppressive oligonucleotide can be appropriately determined so that the oligodeoxynucleotide or suppressive oligonucleotide can be hybridized with the nucleic acid having a specific nucleotide sequence under conditions of the reaction performed. The position of a mismatch generated when the oligodeoxynucleotide or suppressive oligonucleotide is hybridized with the nucleic acid having a specific nucleotide sequence is preferably at least 3 nucleotides away from both the 5′ end and 3′ end of the oligodeoxynucleotide or suppressive oligonucleotide. 
     For example, when the method of the present invention is performed for suppressing amplification of a nucleic acid having a specific nucleotide sequence by a method including a reaction at high temperature such as PCR, a heat-resistant mismatch endonuclease is preferably used. The mismatch endonuclease described above in section 1 includes thermostable enzymes that are not inactivated in PCR, and such enzymes are suitable for the above use. 
     Therefore, the present invention further provides a composition for a nucleic acid amplification reaction, comprising the following (a) to (d): 
     (a) an oligodeoxynucleotide that is designed to produce one or several mismatches when hybridized with the nucleic acid comprising a specific nucleotide sequence or a suppressive oligonucleotide or a complementary strand thereof; 
     (b) a DNA polymerase; 
     (c) at least one pair of oligonucleotide primers; and 
     (d) the mismatch endonuclease mutant of the present invention described above in section 1. 
     The above-described composition may further comprise at least one selected from the group consisting of a reaction buffer, a divalent metal ion, a deoxyribonucleotide, an oligonucleotide probe, and an intercalating dye. The above-described composition may further comprise a nucleic acid as a template for nucleic acid amplification reaction. The above-described composition may further comprise dUTP or uracil N-glycosidase. 
     The method for suppressing amplification of a nucleic acid having a specific nucleotide sequence in a nucleic acid amplification reaction of the present invention can be performed by any nucleic acid amplification method. A preferred example of such a nucleic acid amplification method is, but not limited to, a method of amplifying DNA. Examples of the method of amplifying DNA include PCR, MDA, and isothermal nucleic acid amplification such as ICAN and LAMP. 
     The method for suppressing amplification of a nucleic acid having a specific nucleotide sequence in a nucleic acid amplification reaction of the present invention can be applied to any nucleic acid. When the method is applied to DNA amplification, such a DNA may be a DNA present in an artificially prepared DNA mixture, an environment-derived sample, a biological sample, or a DNA mixture prepared from the above-mentioned sample. Examples of the biological sample include, but not limited to, samples derived from mammals such as human. Examples of the DNA mixture include, but not limited to, a mixture of fragments of a genomic DNA, a mixture of cDNAs produced from mRNA by a reverse transcription reaction, and a mixture of a plurality of PCR products. Examples of a DNA having a specific nucleotide sequence whose amplification is suppressed include a reverse transcription product from rRNA which remains unseparated, and a small molecule DNA produced by pairing of primers. In the case of amplifying a gene library which is followed by functional screening, a library capable of more efficiently searching for unknown genes can be produced by suppressing amplification of DNAs having sequences of positive known genes. 
     The concentration of the polypeptide having a mismatch endonuclease activity in the method of the present invention may be determined by determining a concentration that does not inhibit a DNA amplification reaction or a concentration effective for cleavage of a double-stranded nucleic acid having a mismatch in each reaction system as appropriate. The concentration of the oligonucleotide or the suppressive oligodeoxynucleotide may be determined by optimizing the concentration used in consideration of the amount of a template and the amplification efficiency of a target DNA. For example, the concentration of the oligodeoxynucleotide or the suppressive oligonucleotide can be 0.1 to 10 times the concentration of a primer used for amplification reaction. 
     As described above, the present invention provides a method for preferentially amplifying a target nucleic acid. The method may further comprise a step of detecting the amplified target DNA. Hereinafter, this aspect of the present invention may be referred to as “the method for detecting a nucleic acid of the present invention”. For example, according to the detection method of the present invention in which a DNA is used as a target to be detected, even when a DNA that is not a target to be detected (a DNA having a specific nucleotide sequence) exists in an excessively large amount relative to a DNA that is a target to be detected (a target DNA), amplification of the non-target DNA as a template is suppressed by virtue of the oligonucleotide or suppressive oligonucleotide and the polypeptide having a mismatch endonuclease activity in the method for suppressing amplification of a nucleic acid having a specific nucleotide sequence in a nucleic acid amplification reaction of the present invention, and therefore the target DNA to be detected can be detected. 
     The method for suppressing amplification of a nucleic acid of the present invention can be performed in the presence of the above-described acidic high molecular substance. The acidic high molecular substance exerts more effect in a sample containing a small amount of nucleic acid. Preferable examples of the acidic high molecular substance are as described above for the method for selectively cleaving a non-target nucleic acid of the present invention. The method of the present invention may be performed further in combination with the above-described proliferating cell nuclear antigen (PCNA) or a mutant thereof. 
     9. Method for Detecting Nucleic Acid of the Present Invention 
     The method for detecting a nucleic acid of the present invention comprises a step of suppressing amplification of a non-target nucleic acid by the method for suppressing amplification of a nucleic acid of the present invention as described above in section 8 and thereby preferentially amplifying a target nucleic acid, and a step of detecting the amplified target nucleic acid. The method for detecting a nucleic acid of the present invention enables to distinctively detect a wild-type and a mutant-type, for example, of a nucleic acid corresponding to a gene wherein a mutation in the gene is known to be present. When the detection method of the present invention is performed using a DNA comprising a wild-type nucleotide sequence as the nucleic acid having a specific nucleotide sequence (non-target nucleic acid), a small number of a mutant allele can be detected in the presence of an excessively large amount of the normal allele (i.e., a DNA having the wild-type nucleotide sequence). For example, the method of the present invention is useful for detection of a circulating tumor DNA, or detection of a small amount of a fetal DNA sequence contained in the mother&#39;s blood. Examples of the mutation include microdeletion and point mutation. Polymorphisms generated by point mutation are called single nucleotide polymorphisms (SNPs). As used herein, a DNA having a mutant nucleotide sequence among SNPs is sometimes referred to as a DNA having a single nucleotide polymorphism mutation. 
     Preferred examples of the small number of the mutant allele include, but not limited to, nucleic acids containing at least one single nucleotide polymorphism selected from the group consisting of a single nucleotide polymorphism mutation used as a tumor marker, a single nucleotide polymorphism mutation correlating with a therapeutic effect of an agent for the treatment of cancer, and a single nucleotide polymorphism mutation known to correlate with canceration of cells. Examples of SNPs include those frequently found in tumor cells, and those known to correlate with a therapeutic effect of an agent for the treatment of cancer or carcinogenesis. Examples of such SNPs include SNPs of K-ras genes, B-raf genes, and epidermal growth factor receptor (EGFR) genes. Somatic mutations in the K-ras gene are frequently found in colorectal cancer, lung adenocarcinoma, thyroid cancer, and the like. Somatic mutations in the B-raf gene are frequently found in colorectal cancer, malignant melanoma, papillary thyroid cancer, non-small cell lung cancer, lung adenocarcinoma, and the like. Somatic mutations in the EGFR gene are frequently found in various solid tumors. It is known that the treatment of a cancer with an EGFR inhibitor such as gefitinib or erlotinib is likely to be effective when the EGFR gene in the cancer tissue has a specific single nucleotide polymorphism mutation. In contrast, it is known that a cancer is likely to be resistant to an EGFR inhibitor when the K-ras gene in the cancer tissue has a single nucleotide polymorphism mutation. 
     The detection method of the present invention may be performed using, as the material, a DNA obtained after treatment of a composition containing a methylated DNA extracted from a sample from an organism with bisulfite. According to the detection method of the present invention, detection of a small number of a methylated allele in the presence of an excessively large amount of a non-methylated allele, or detection of a small number of a non-methylated allele in the presence of an excessively large amount of a methylated allele can be performed. 
     As the treatment with bisulfite, a known bisulfite method, which is used for detection of a methylated DNA can be used. By the treatment, non-methylated cytosine is changed into uracil, whereas methylated cytosine is not changed. When a reaction solution treated with bisulfite is subjected to amplification by PCR, uracil is changed into thymine and methylated cytosine is changed into cytosine. In other words, detection of a small number of a methylated allele in the presence of an excessively large amount of a non-methylated allele at a specific site, and detection of a small number of a non-methylated allele in the presence of an excessively large amount of a methylated allele respectively correspond to examination of the presence of cytosine in the presence of an excessively large amount of thymine, and examination of the presence of thymine in the presence of an excessively large amount of cytosine. When amplification of an excessively large amount of DNA containing thymine or cytosine is suppressed, the presence of a small number of a methylated allele or non-methylated allele is easily examined. 
     For the step of detecting the target nucleic acid in the detection method of the present invention, electrophoresis, nucleotide sequence analysis, or real-time PCR using a probe such as a cycling probe or a TaqMan probe can be used. For these detection methods, conventional techniques can be directly used. In particular, use of a high resolution melting (HRM) analysis method allows amplification and detection of a DNA of interest by one step, and thus rapid and simple examination of the DNA of interest is attained. 
     The method fox detecting a nucleic acid of the present invention can be performed in the presence of the above-described acidic high molecular substance. The acidic high molecular substance exerts more effect in a sample containing a small amount of nucleic acid. Preferable examples of the acidic high molecular substance are as described above for the method for selectively cleaving a non-target nucleic acid of the present invention. The method of the present invention may be performed further in combination with the above-described proliferating cell nuclear antigen (PCNA) or a mutant thereof. 
     EXAMPLES 
     Hereinafter, the present invention will be specifically described with reference to Examples, which the scope of the present invention is not limited to. 
     Experimental Method 1: 
     (1-1) Preparation of Mismatch Endonuclease Mutant 
     A nucleotide sequence of a gene encoding polypeptide PF_R00065 from  Pyrococcus furiosus  (RefSeq ID: WP_11011124.1, SEQ ID NO: 1) is shown in SEQ ID NO: 16. Based on the above-mentioned nucleotide sequence, a mutation was introduced into a specific position to prepare an artificial gene by a conventional method. The artificial gene thus obtained was introduced into plasmid pET6xHN-C (manufactured by Takara Bio USA) using In-Fusion (registered trademark) HD Cloning Kit (manufactured by Takara Bio USA). The plasmid thus obtained has a nucleotide sequence encoding a mismatch endonuclease mutant with a histidine tag added to the C-terminal side. 
     Next,  Escherichia coil  BL21 DE3 strain (manufactured by Takara Bio Inc.) was transformed with the plasmid, and cultured overnight at 37° C. on a 1.5% agarose LB plate containing 100 μg/ml of ampicillin. Three single colonies were selected from the plate, inoculated into an LB medium containing 100 μg/ml of ampicillin (hereinafter referred to as an LB-AP medium), and cultured overnight at 37° C. with shaking. Then, 300 μl of the culture was inoculated into 6 ml of the LB-AP medium and cultured overnight at 37° C. with shaking. When an OD600 value reached 0.6, IPTG was added at a final concentration of 1 mM to the culture, followed by further incubation at 25° C. for 4 hours to induce the expression of the target gene. Then, when the OD600 value reached 4, bacterial cells were collected. 
     The bacterial cells thus obtained were suspended in 400 μl of a solution containing 50 mM Tris-HCl (pH 7.5), 300 mM NaCl, 5% glycerol and 0.15% Triton X-100 (hereinafter referred to as Buffer S), and subjected to ultrasonication at 4° C. for 30 seconds using a sonicator (manufactured by Sonic &amp; Materials) three times. Thus a suspension became transparent. After ultrasonication, the suspension was centrifuged at 11000×g at 4° C. for 10 minutes, and a supernatant was collected. A crude extract thus obtained was subjected to Ni resin purification. 
     The Ni resin purification was carried out as follows. Specifically, 50 μl of Ni-NTA agarose (manufactured by Qiagen) was placed in a 1.5 ml tube, washed twice with 250 μl of sterile distilled water, and then equilibrated twice with 250 μl of Buffer A (50 mM Tris-HCl pH 7.5, 300 mM NaCl, 5% glycerol, and 5 mM imidazole). The equilibrated Ni-NTA agarose was suspended in 400 μl of the crude extract and allowed to stand for 30 minutes. Then, a suspension was centrifuged at 12000 rpm at 4° C. for 10 minutes. After removing a supernatant, the Ni-NTA agarose was washed 3 times with 100 μl of Buffer A. Then, an adsorbate was eluted from the Ni-NTA agarose with 100 μl of Buffer B (50 mM Tris-HCl pH 7.5, 300 mM NaCl, 5% glycerol and 300 mM imidazole). The eluate thus obtained was used in experiments as below as a mismatch endonuclease mutant solution. 
     (1-2) Preparation of Heterodimer-Type Mismatch Endonuclease Mutant 
     A gene encoding a heterodimer-type mismatch endonuclease mutant was prepared based on the gene encoding the mutant prepared in (1-1). An artificial gene was prepared by serially connecting a gene encoding a GG-specific mismatch endonuclease mutant and a gene encoding a TT-specific mismatch endonuclease mutant via a linker (SEQ ID NO: 35) in the same manner as in (1-1). 
     (2) Substrate Specificity Evaluation of Mismatch Endonuclease Mutant 
     (2-1) Nucleic Acid 
     NucS-Template-T (SEQ ID NO: 31), NucS-Template-G (SEQ ID NO: 32), NucS-Probe-T (SEQ ID NO: 33), and NucS-Probe-G (SEQ ID NO: 34) as nucleic acids forming substrate double-stranded DNAs were chemically synthesized by a known method. Double-stranded DNAs composed of combinations of the Templates and Probes were used as a substrate. FAM and Eclipse (registered trademark) Dark Quencher were added respectively to the 5′ end and the 3′ end of the two Probes. 
     (2-2) Cleavage Specificity 
     Total 25 μl of a mixture was prepared by mixing 2.5 μl of 10× buffer, 1 μl of 25 μM Template, 1.5 μl of 5 μM Probe, 5 μl of the mismatch endonuclease mutant solution, and 15 μl of sterile water. Using a thermal cycler, one cycle of 55° C. for 60 seconds was repeated for 60 cycles. Alternatively, using a thermal cycler, one cycle of 55° C. for 15 seconds was repeated for 60 cycles. The composition of the 10× buffer is 500 mM Tris-HCl (pH 9.2), 140 mM (NH 4 ) 2 SO 4 , 100 mM KCl, 25 mM MgCl 2 , and 0.1% BSA. The composition of a buffer for dilution of enzyme is 25 mM Tris-HCl (pH 8.0), 50 mM KCl, 0.1 mM DTT, 0.01% Gelatin, 0.5 Nonidet P-40, 0.5% Tween-20, And 0.09% BSA. 
     Example 1 
     Preparation of Mismatch Endonuclease Mutant 
     (1) Mismatch Endonuclease Mutant S47K+N76D 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which serine at position 47 and asparagine at position 76 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by lysine and aspartic acid respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 6 and SEQ ID NO: 21, respectively. 
     (2) Mismatch Endonuclease Mutant S47R+N76D+L123A 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which serine at position 47, asparagine at position 76, and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by lysine, aspartic acid, and alanine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 7 and SEQ ID NO: 22, respectively. 
     (3) Mismatch Endonuclease Mutant S47R+N76D+L123G 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which serine at position 47, asparagine at position 76, and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by lysine, aspartic acid, and glycine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 8 and SEQ ID NO: 23, respectively. 
     (4) Mismatch Endonuclease Mutant Q78R+L123S 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and serine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 9 and SEQ ID NO: 24, respectively. 
     (5) Mismatch Endonuclease Mutant Q78R+L123T 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and threonine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 10 and SEQ ID NO: 25, respectively. 
     (6) Mismatch Endonuclease Mutant Q78R+L123M 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and methionine respectively was produced to prepare a mismatch endonuclease mutant. solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 11 and SEQ ID NO: 26, respectively. 
     (7) Mismatch Endonuclease Mutant Q78R+L123C 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 123 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and cysteine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 12 and SEQ ID NO: 27, respectively. 
     (8) Mismatch Endonuclease Mutant Q78R+L125V 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 125 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and valine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 13 and SEQ ID NO: 28, respectively. 
     (9) Mismatch Endonuclease Mutant Q78R+L125A 
     According to Experimental Method 1 (1-1), an artificial gene encoding a mutant protein in which glutamine at position 78 and leucine at position 125 in polypeptide PF_RS00065 from  Pyrococcus furiosus  were replaced by arginine and alanine respectively was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 14 and SEQ ID NO: 29, respectively. 
     (10) Heterodimer-Type Mismatch Endonuclease Mutant 
     According to Experimental Method 1 (1-2), an artificial gene capable of expressing mutant 1 (S47R+N76D+L123A; SEQ ID NO: 7) and mutant 2 (Q78R+L123M; SEQ ID NO: 11) as single polypeptide via the linker (SEQ ID NO: 35) was produced to prepare a mismatch endonuclease mutant solution. An amino acid sequence and a nucleic acid sequence of the mutant protein are shown in SEQ ID NO: 15 and SEQ ID NO: 30, respectively. 
     Example 2 
     Substrate Specificity Evaluation of Mismatch Endonuclease Mutant 
     Using the mismatch endonuclease mutants prepared in Examples 1 (1) to (9), and the wild-type mismatch endonuclease (PfuNucS) and a mutant thereof W77F, substrate specificity evaluation was performed according to Experimental Method 1 (2). Results are shown in Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Substrate 
                 GG-specific/ 
               
               
                 Mutant names 
                 specificity 
                 TT-specific 
               
               
                   
               
             
            
               
                 No mutation 
                 TT, GG, TG 
                 — 
               
               
                 (Wild-type) 
                   
                   
               
               
                   
               
               
                 W77F 
                 GG ≥ TG &gt;&gt; TT 
                 — 
               
               
                   
               
               
                 S47K + N76D 
                 GG 
                 GG-specific 
               
               
                   
               
               
                 S47R + N76D + 
                 GG 
                 GG-specific 
               
               
                 L123A 
                   
                   
               
               
                   
               
               
                 S47R + N76D + 
                 GG 
                 GG-specific 
               
               
                 L123G 
                   
                   
               
               
                   
               
               
                 Q78R + L123S 
                 TT 
                 TT-specific 
               
               
                   
               
               
                 Q78R + L123T 
                 TT 
                 TT-specific 
               
               
                   
               
               
                 Q78R + L123M 
                 TT 
                 TT-specific 
               
               
                   
               
               
                 Q78R + L123C 
                 TT 
                 TT-specific 
               
               
                   
               
               
                 Q78R + L125V 
                 TT 
                 TT-specific 
               
               
                   
               
               
                 Q78R + L125A 
                 TT 
                 TT-specific 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, GG-specific mismatch endonuclease mutants and TT-specific mismatch endonuclease mutants having substrate specificity altered from that of the wild-type mismatch endonucleases were obtained. 
     Example 3 
     Substrate Specificity Evaluation of Heterodimer-Type Mismatch Endonuclease Mutant 
     Using the heterodimer-type mismatch endonuclease mutant prepared in Example 1 (10), substrate specificity evaluation was performed according to Experimental Method 1 (2). 
     As a result, the heterodimer-type mismatch endonuclease mutant showed GT/TG mismatch-specific substrate specificity that was altered from that of the wild-type mismatch endonuclease and different from those of the mismatch endonuclease mutants prepared in Examples 1 (1) to (9). 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful in broad fields including the fields of genetic technology, biology, medicine, and agriculture. 
     Sequence Listing Free Text 
     
         
         SEQ ID NO: 1: Amino acid sequence of mismatch endonuclease PF0012 from  Pyrococcus furiosus    
         SEQ ID NO: 2; Amino acid sequence of mismatch endonuclease PF0012 W77F variant 
         SEQ ID NO: 3: Amino acid sequence of mismatch endonuclease TERMP_01877 from  Thermococcus barophilus    
         SEQ ID NO: 4: Amino acid sequence of mismatch endonuclease MJ_0225 from  Methanocaldococcus jannaschii    
         SEQ ID NO: 5: Amino acid sequence of mismatch endonuclease TKO NucS from  Thermococcus kodakarensis    
         SEQ ID NO: 6: Amino acid sequence of mismatch endonuclease variant S47K+N76D from PF0012 
         SEQ ID NO: 7: Amino acid sequence of mismatch endonuclease variant S47R+N76D+L123A from PF0012 
         SEQ ID NO: 8: Amino acid sequence of mismatch endonuclease variant S47R+N76D+L123G from PF0012 
         SEQ ID NO: 9: Amino acid sequence of mismatch endonuclease variant Q78R+L123S from PF0012 
         SEQ ID NO: 10: Amino acid sequence of mismatch endonuclease variant Q78R+L123T from PF0012 
         SEQ ID NO: 11: Amino acid sequence of mismatch endonuclease variant Q78R+L123M from PF0012 
         SEQ ID NO: 12: Amino acid sequence of mismatch endonuclease variant Q78R+L123C from PF0012 
         SEQ ID NO: 13: Amino acid sequence of mismatch endonuclease variant Q78R+L125V from PF0012 
         SEQ ID NO: 14: Amino acid sequence of mismatch endonuclease variant Q78R+L125A from PF0012 
         SEQ ID NO: 15: Amino acid sequence of mismatch endonuclease variant hetero dimer from variant S47R+N76D+L123A and variant Q78R+L123M 
         SEQ ID NO: 16: Nucleic acid sequence encoding gene of mismatch endonuclease PF0012 from  Pyrococcus furiosus    
         SEQ ID NO: 17: Nucleic acid sequence encoding gene of mismatch endonuclease PF0012 W77F variant 
         SEQ ID NO: 18: Nucleic acid sequence encoding gene of mismatch endonuclease TERMP_01877 from  Thermococcus barophilus    
         SEQ ID NO: 19: Nucleic acid sequence encoding gene of mismatch endonuclease MJ_0225 from  Methanocaldococcus jannaschii    
         SEQ ID NO: 20: Nucleic acid sequence encoding gene of mismatch endonuclease TKO NucS from  Thermococcus kodakarensis    
         SEQ ID NO: 21: Nucleic acid sequence encoding gene of mismatch endonuclease variant S47K+N76D from PF0012 
         SEQ ID NO: 22: Nucleic acid sequence encoding gene of mismatch endonuclease variant S47R+N76D+L123A from PF0012 
         SEQ ID NO: 23: Nucleic acid sequence encoding gene of mismatch endonuclease variant S47R+N76D+L123G from PF0012 
         SEQ ID NO: 24: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L123S from PF0012 
         SEQ ID NO: 25: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L123T from PF0012 
         SEQ ID NO: 26: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L123M from PF0012 
         SEQ ID NO: 27: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L123C from PF0012 
         SEQ ID NO: 28: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L125V from PF0012 
         SEQ ID NO: 29: Nucleic acid sequence encoding gene of mismatch endonuclease variant Q78R+L125A from PF0012 
         SEQ ID NO: 30: Nucleic acid sequence encoding gene of mismatch endonuclease variant hetero dimer from variant S47R+N76D+L123A and variant Q78R+L123M 
         SEQ ID NO: 31: Synthetic oligo nucleotide “NucS-Template-T” 
         SEQ ID NO: 32: Synthetic oligo nucleotide “NucS-Template-G” 
         SEQ ID NO: 33: Synthetic oligo nucleotide “NucS-Probe-T”. 5′-end is labeled with FAM and 3′-end is labeled with Eclipse. 
         SEQ ID NO: 34: Synthetic oligo nucleotide “NucS-Probe-G”. 5′-end is labeled with FAM and 3′-end is labeled with Eclipse. 
         SEQ ID NO: 35: Amino acid sequence of Linker 
         SEQ ID NO: 36: Nucleic acid sequence encoding Linker