JP2009278871A - Method for isolating cyclic nucleic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To establish technology for selectively and preferentially isolating a cyclic nucleic acid from an organism-derived nucleic acid-containing sample with simply and quickly distinguishing the cyclic nucleic acid from nucleic acids of other forms. <P>SOLUTION: The method for isolating a cyclic nucleic acid from a nucleic acid-containing sample comprises a process for carrying out an isothermal amplification reaction of a nucleic acid using a chain-substituted polymerase against the nucleic acid-containing sample in the presence of a modified single-stranded DNA binding protein that has an amino acid sequence having a modification site in which modification composed of at least one of deletion, substitution, addition and insertion of one or more amino acids in a single-stranded DNA binding protein amino acid sequence occurs and expresses a function of contributing to the improvement in amplification efficiency of template nucleic acid in an isothermal amplification reaction system using a chain-substitution polymerase. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for isolating circular nucleic acid from a nucleic acid-containing sample. Specifically, a nucleic acid isothermal amplification reaction is performed in the presence of a given single-stranded DNA-binding protein, and circular nucleic acids are selectively and preferentially amplified, whereby circular nucleic acids are isolated from a nucleic acid-containing sample. Relates to the method of releasing.

  Many molecular biological techniques such as gene analysis and gene therapy require high-purity nucleic acids, particularly circular nucleic acids. Here, the circular nucleic acid is one in which the 5 ′ end and the 3 ′ end of the polynucleotide chain are linked by a phosphodiester bond and circularized. For example, many prokaryotic chromosomes, viruses, or plasmid DNA, as well as mitochondrial DNA and chloroplast DNA, have a double-stranded closed structure, and φX174 phage, M13 phage, etc., have a single-stranded closed circle. It is known to take a structure.

  Among them, plasmid DNA is one of bacterial extrachromosomal genes and has the property of being able to autonomously propagate independently of the host chromosome. That is, plasmid DNA can transmit a specific genetic trait independently of a host-derived chromosome in cells such as E. coli. Because of this property, it is used as a vector for amplification and expression of foreign genes in gene recombination experiments, and is applied to various biological techniques such as restriction enzyme treatment, sequencing, and transformation.

  Mitochondrial DNA is an extrachromosomal gene that exists in mitochondria, which are organelles. And mitochondrial DNA has large individual differences, suggesting the relationship with the onset, longevity, and motor ability of various diseases, and is expected to be used for genetic diagnosis. In addition, it is regarded as important as a research material for forensic appraisal, evolutionary biology, and anthropological analysis, as well as a research material for identification of food origin.

  However, biological samples include nucleic acids having various shapes such as genomic DNA and RNA, as well as biological proteins. In using a circular nucleic acid, it is necessary to isolate only the circular nucleic acid from these mixtures, and various attempts have been reported.

  In particular, many methods for isolating plasmid DNA have been reported. For example, there has been reported a method of isolating and purifying plasmid DNA by growing exponentially growing cells to a high level of organism mass and then lysing the cells (see Patent Document 1). In this method, cells are lysed with alkali. Upon lysis, chromosomal DNA is denatured, but plasmid DNA is carefully adjusted to a pH at which double-stranded helical structure can be reversibly reconstructed. It controls the rise.

  In addition, a method for isolating plasmid DNA from bacteria using the property that plasmid DNA binds to glass powder in the presence of sodium perchlorate has been reported (Non-patent Document 1). According to such a method, it is described that plasmid DNA can be separated from a substance that prevents the use of plasmid DNA in molecular biological procedures without taking measures such as enzymatic treatment, phenol extraction, dialysis, and centrifugation.

Furthermore, a method using density gradient centrifugation, liquid / liquid extraction, anion exchange chromatography, a method using a silica material and a chaotropic substance in combination, and the like are known.
A method utilizing density gradient centrifugation utilizes a cesium chloride concentration gradient to separate plasmid DNA from residual genomic DNA, RNA, proteins, and other cellular impurities. Subsequently, the plasmid DNA is isolated by removing salts by dialysis and finally concentrating by ethanol precipitation.
Liquid / liquid extraction is a method in which a clarified cell lysate is extracted with phenol, a mixture of phenol and chloroform, etc., and residual genomic DNA, RNA, protein, and other cellular impurities are transferred to the organic phase. On the other hand, plasmid DNA utilizes the fact that it remains in the aqueous phase. And finally, plasmid DNA is isolated by concentrating by ethanol precipitation.
In addition, the method using anion exchange chromatography, first, the plasmid DNA binds to the anion exchange material, but the salt concentration and the optimized concentration so that residual genomic DNA, RNA, protein, and other cellular impurities do not bind. Contact the clarified cell lysate under pH conditions. Thereafter, the plasmid DNA is eluted under high ionic strength conditions, and finally concentrated by ethanol precipitation to isolate the plasmid DNA.
The method of using the silica material and the chaotropic substance in combination utilizes the property of nucleic acid that binds to the silica material in the presence of the chaotropic substance. After the clarified cell lysate is mixed with the chaotropic substance, the silica material is used. Applies to Finally, the plasmid DNA is isolated by washing and removing the salts and then eluting into a buffer solution or water having a low salt concentration.

  These known plasmid DNA isolation methods are common in that they are performed by preparing a clarified cell lysate and purifying the plasmid from the clarified cell lysate. Therefore, since all are premised on the preparation of the clarified cell lysate, steps such as cell precipitation, centrifugation, filtration, and magnetic separation using magnetic beads are required for the preparation of the clarified cell lysate. Need to pass. However, for example, the filtration process requires a filter suitable for the purification protocol, and these processes are very complicated and time consuming. In addition, mitochondrial DNA has not been fully studied due to the difficulty of isolation. Therefore, establishment of a technique that can selectively and preferentially isolate a circular nucleic acid from a nucleic acid-containing sample derived from a living body has been demanded.

  As a method for isolating a nucleic acid, a method utilizing a nucleic acid amplification reaction is known. Recently, the present inventors have modified a specific amino acid in the amino acid sequence of a single-stranded DNA binding protein derived from Thermus thermophilus (hereinafter sometimes abbreviated as “SSB protein”). And found that the amplification efficiency of the template nucleic acid can be remarkably improved in a nucleic acid isothermal amplification system using a strand displacement polymerase (see Patent Document 2 and Non-Patent Document 2). Such a method is capable of specifically amplifying a target nucleic acid quickly and easily, thereby allowing specific detection and isolation of the target nucleic acid, and thus a significant contribution to the advancement of molecular biological technology. Is expected to fulfill

  However, the above-mentioned conventional method of the present inventors is not good at performing an amplification reaction using a circular nucleic acid and a linear nucleic acid alone as a template nucleic acid. Therefore, although it has been reported that any of them can be used as a template nucleic acid, the effect of using a sample in which a circular nucleic acid and a linear nucleic acid are mixed as a template nucleic acid has not been studied. There was room.

International Publication No. 95/01359 Pamphlet US Patent Application Publication No. 2007/092896 Marko MA. Et al., "A procedure for the large-scale isolation of highly purified plasmid DNA using alkaline extraction and binding to glass powder.", Anal Biochem., April 1982, Vol. 121, No. 2, 382 ~ 387 pages Inoue J., Shigemori Y., Mikawa T., "Improvements of rolling circle amplification (RCA) efficiency and accuracy using Thermus thermophilus SSB mutant protein.", Nucleic Acids Research, May 2006, Vol. 34, No. 9. , E69

  Therefore, an object of the present invention is to establish a technique capable of selectively and preferentially isolating a circular nucleic acid from a nucleic acid-containing sample derived from a living body in a simple and rapid manner while distinguishing it from other forms of nucleic acid.

  As a result of intensive studies by the inventor, by using the nucleic acid amplification method previously reported by the inventors, only circular nucleic acids are preferentially and selectively selected from nucleic acid-containing samples containing various forms of nucleic acids. It was found that it can be amplified. Furthermore, it has been found that by utilizing such a phenomenon, only a circular nucleic acid can be selectively isolated and purified from a nucleic acid-containing sample. The present invention has been completed based on these findings.

  In order to achieve the above object, an invention having the following configurations [1] to [9] is provided.

[1] A method for isolating circular nucleic acid from a nucleic acid-containing sample,
An amino acid sequence having a modified site in which a modification comprising at least one of deletion, substitution, addition and insertion of one or more amino acids has occurred relative to the amino acid sequence of the single-stranded DNA binding protein, and the chain In an isothermal amplification reaction system using a displacement polymerase, a strand displacement polymerase is used for the nucleic acid-containing sample in the presence of a modified single-stranded DNA binding protein that expresses a function that can contribute to an improvement in the amplification efficiency of the template nucleic acid. A method for isolating a circular nucleic acid from a nucleic acid-containing sample, comprising a step of amplifying the circular nucleic acid by performing an isothermal amplification reaction of the nucleic acid.
[2] The method of [1] above, wherein the circular nucleic acid is mitochondrial DNA.
[3] The method of [1] or [2] above, wherein the strand displacement polymerase is Phi29 DNA polymerase.
[4] Any of the above [1] to [3], wherein the single-stranded DNA binding protein is a single-stranded DNA binding protein derived from Thermus thermophilus or Thermus aquaticus That way.
[5] The 255th phenylalanine in the amino acid sequence of SEQ ID NO: 2 showing the amino acid sequence of the single-stranded DNA binding protein derived from Thermus thermophilus is the modified single-stranded DNA binding protein. Of the amino acid sequence of SEQ ID NO: 7 showing the amino acid sequence substituted with another amino acid or the amino acid sequence of a single-stranded DNA binding protein derived from Thermus aquaticus , the 256th phenylalanine is another amino acid. [4] The method according to [4] above, wherein the amino acid sequence is substituted.
[6] The method of the above-mentioned [5], wherein the other amino acid is proline.
[7] The amino acids 230 to 263 of the amino acid sequence of SEQ ID NO: 2 showing the amino acid sequence of the single-stranded DNA-binding protein derived from Thermus thermophilus , wherein the modified single-stranded DNA-binding protein is thermus thermophilus Of the amino acid sequence of SEQ ID NO: 7 showing the amino acid sequence of the single-stranded DNA binding protein derived from Thermus aquaticus The method according to [4] above, which has a sequence.

  According to the configuration of [1] to [7] above, strand displacement is performed using, as a template, a nucleic acid-containing sample in which nucleic acids of various shapes are mixed in the presence of the modified SSB protein previously found by the present inventors. By performing isothermal amplification reaction of nucleic acid with polymerase, circular nucleic acid can be selectively and preferentially amplified. That is, circular nucleic acids can be preferentially and selectively amplified while suppressing amplification of genomic DNA other than circular nucleic acids, RNA, and other nucleic acid breaks. This makes it possible to isolate circular nucleic acids from nucleic acid-containing samples, and is expected to further enhance the utility value of circular nucleic acids useful in the field of molecular biology of mitochondrial DNA and plasmid DNA. Since the circular nucleic acid isolation method and kit of the present invention are based on the preferential and selective amplification of circular nucleic acids, the formation of a clarified cell lysate that was necessary in the conventional isolation technique, And no time-consuming and costly steps such as purification of plasmid DNA from the clarified cell lysate. In other words, since it comprises a simple process of nucleic acid amplification, the loss of nucleic acid can be minimized, and circular nucleic acids can be isolated efficiently, simply and rapidly. In many cases, biological samples are mixed with nucleic acids of various shapes and types, and are known to have various forms, all of which are nucleic acid-containing samples of the present invention. It also has the advantage that it can be the subject of methods and kits. For example, it is possible to preferentially and selectively isolate only a circular nucleic acid from a nucleic acid-containing sample having a low mixture ratio of circular nucleic acids and an unpurified or roughly purified nucleic acid-containing sample containing a large amount of contaminants such as bacterial cells.

[8] A kit for isolating circular nucleic acid from a nucleic acid-containing sample,
An amino acid sequence having a modified site in which a modification comprising at least one of deletion, substitution, addition and insertion of one or more amino acids has occurred relative to the amino acid sequence of the single-stranded DNA binding protein, and the chain A kit for isolating a circular nucleic acid from a nucleic acid-containing sample, comprising a modified single-stranded DNA binding protein that expresses a function that can contribute to an improvement in amplification efficiency of a template nucleic acid in an isothermal amplification reaction system using a substituted polymerase.
[9] The kit according to [8], further including a polymerase having strand displacement ability.

  According to the configuration of [8] to [9] above, simple and rapid isolation of the circular nucleic acid is possible by configuring the components necessary for isolation of the circular nucleic acid as a kit.

  Hereinafter, specific embodiments of the present invention will be described. However, these are merely examples of the present invention, and do not limit the present invention.

  The present invention provides a method for selectively and preferentially isolating and purifying only a circular nucleic acid from a nucleic acid-containing sample. Specifically, a nucleic acid isothermal amplification reaction using a strand displacement polymerase is performed in the presence of a modified SSB protein in which a specific amino acid in the amino acid sequence of the wild-type SSB protein is modified. During amplification, circular nucleic acid amplification proceeds selectively and preferentially over linear DNA such as genomic DNA, RNA, and other nucleic acid impurities, and circular nucleic acid is isolated and purified from nucleic acid-containing samples. It is something that can be done. The principle of the present invention is schematically shown in FIG.

  In the present specification, when simply referred to as “SSB protein”, it can be the basis of the modified SSB protein used in the present invention, and does not have a modified site in which intentional or unintentional modification occurs. Point to. Therefore, the so-called “wild-type SSB protein” used to clearly distinguish it from the modified SSB protein is synonymous.

  In the present invention, the target circular nucleic acid is not particularly limited as long as it has a circular structure in which the 5 ′ end and 3 ′ end of the polynucleotide chain are linked by a phosphodiester bond. Therefore, there is no restriction on the number of nucleic acid strands, and it does not matter whether the strand is a single strand or multiple strands. Moreover, there is no restriction | limiting with respect to the origin, chain length, and a base sequence. Specific examples include mitochondrial DNA, chloroplast DNA, plasmid DNA, or prokaryotic or viral genomic DNA having a circular structure. However, the present invention is not limited to this, and any nucleic acid having a circular structure is a target. Examples of the viral genomic DNA include single-stranded circular nucleic acids such as φX174 phage and M13 phage genomic DNA, SV40, polyomavirus, and papillomavirus double-stranded circular nucleic acids. Furthermore, even DNA fragments inserted into plasmids, phages, etc., artificial products synthesized using an automatic nucleic acid synthesizer or the like commonly used in the art, have a circular structure. Any of these can be a target nucleic acid.

  Here, the nucleic acid-containing sample is not particularly limited as long as it is a sample containing the target circular nucleic acid or possibly containing the target circular nucleic acid. Therefore, in addition to biological samples, environmental samples, foods, etc., artificial nucleic acid product samples constructed by DNA synthesis technology and the like are also included. Examples of biological samples include animal-derived cell cultures, tissues, blood, urine, feces, saliva, etc., plant-derived roots, stems, leaves, flowers, fruits, etc., and samples derived from bacteria, viruses, etc. Can also be the subject of the present invention as a nucleic acid containing sample. Examples of environmental samples include soil, groundwater, river water, lake water, seawater, and examples of food include meat, eggs, and processed foods.

  The nucleic acid-containing sample may be directly applied to the method of the present invention, but may be applied after performing a known nucleic acid extraction or the like for the purpose of revealing the target circular nucleic acid. For example, after separating cells, tissues, bacteria, viruses, etc. from a specimen, treatment with a surfactant such as SDS, enzyme treatment such as lysozyme and proteinase, heat treatment, ultrasonic treatment, treatment with chaotropic salt, etc. Examples are those in which cells or the like are destroyed by combination to separate nucleic acids.

  The modified SSB protein used in the present invention includes a template nucleic acid in a nucleic acid isothermal amplification reaction system using a strand displacement polymerase by modifying a specific amino acid in the amino acid sequence of the wild-type SSB protein. It refers to a modified SSB protein provided with a function that can contribute to an improvement in amplification efficiency. That is, the modified SSB protein of the present invention has a modified site where a specific amino acid is modified in the amino acid sequence of the wild-type SSB protein, and uses a strand displacement polymerase as compared with the wild-type SSB protein. In the isothermal amplification reaction system of nucleic acid, the specificity to the template nucleic acid is improved. Therefore, all modified SSB proteins that express a function that can contribute to an improvement in the amplification efficiency of the template nucleic acid in a nucleic acid isothermal amplification reaction system using a strand displacement polymerase are included.

  “Function that can contribute to the improvement of template nucleic acid amplification efficiency” means that non-specific amplification unrelated to the template nucleic acid is hardly observed in the isothermal amplification reaction using the strand displacement polymerase, and the template nucleic acid is high. It means that it can be amplified in a yield. Preferably, it means a function capable of improving the amplification efficiency of the template nucleic acid by 5 to 10 times. For example, it contributes to the improvement of the amplification efficiency of the template nucleic acid in the isothermal amplification reaction system using the strand displacement polymerase possessed by the protein having the amino acid sequence represented by sequence recognition number 3, 5, 8, or 9 described in detail below. It means that it is substantially equivalent to the function to be obtained.

Furthermore, the modified SSB protein preferably has a modification that causes a change in the interaction of the wild-type SSB protein with the strand displacement polymerase in the amino acid sequence. Here, "interaction with strand displacement polymerase" means interaction with SSB protein-strand displacement polymerase, highly thermophilic bacterium SSB protein-DNA, or highly thermophilic bacterium SSB protein-strand displacement polymerase-DNA. To do. Here, DNA is single-stranded DNA, and means either one or both of the single-stranded portion of the primer and the template nucleic acid. Here, it has been reported that SSB proteins derived from Escherichia coli, Sulfolobus sulfataricus, and the like bind not only to single-stranded DNA but also to polymerases such as primase, RNA polymerase, and DNA polymerase II (for example, Richard DJ et al. , Nucleic Acids Research, February 2004, Volume 32, Number 3, pages 1065-1074, Sun W et al., J. Bacteriol., December 1996, Volume 178, Number 23, 6701 ˜6705, see Fradkin GE et al., Mol. Biol. (Mosk), January-February 1988, Vol. 22, No. 1, pp. 111-116, etc.). From this document, when the SSB protein is added to an isothermal amplification system using a strand displacement polymerase, the SSB protein assists in the strand displacement reaction, while having the undesirable effect of reducing the amplification efficiency due to a decrease in polymerase activity. Presumed. Therefore, the modified SSB protein is preferably exemplified by those in which the amino acid sequence in the region having the ability of binding to the polymerase inherent in the SSB protein is disrupted. The above-described “change in the interaction of the SSB protein with the strand displacement polymerase” includes such a change that the binding ability of the SSB protein to the strand displacement polymerase originally decreases or disappears. It is presumed that a change in the interaction with the strand displacement polymerase can contribute to an improvement in the amplification efficiency of the template nucleic acid.

  The modified SSB protein preferably has a modification that causes a change in the DNA binding ability of the wild-type SSB protein in the amino acid sequence. It is recognized that there is a correlation between the function that can contribute to the improvement of the amplification efficiency of the template nucleic acid in the isothermal amplification reaction system and the function that strongly binds to DNA, and the wild-type SSB protein binds strongly to DNA. It is presumed that undesirable bonds may be formed and the efficiency of nucleic acid amplification may be reduced. Therefore, the modified SSB protein is preferably exemplified by those in which the amino acid sequence in the region having the ability to bind to DNA originally possessed by the SSB protein is disrupted. The above-mentioned “change in the DNA binding ability of the SSB protein” includes such a change that the binding ability of the wild-type SSB protein to the DNA originally possessed decreases or disappears. It is presumed that it can contribute to improvement of the amplification efficiency of the template nucleic acid by appropriately controlling

The wild-type SSB protein that is the basis of the modified type has a modified site where the amino acid sequence of the SSB protein isolated from nature and the base sequence encoding the SSB protein have been modified intentionally or unintentionally. It is not. SSB protein of hyperthermophilic bacteria is preferred, SSB protein of thermophilic bacteria from the genus Thermus , Thermococcus , Pyrococcus , Thermotoga etc. is exemplified The In particular, SSB proteins of hyperthermophilic bacteria derived from Thermus thermophilus , Thermus aquaticus and the like are preferably exemplified. However, the present invention is not limited to this, and a known SSB protein can be used as a basis for modification.

  In addition, as sequence information of the SSB protein that can be a suitable basis for the construction of the modified SSB protein, the amino acid sequence of the SSB protein derived from Thermus thermophilus is sequence recognition number 2, and the base sequence of the gene encoding the SSB protein Is shown in SEQ ID NO: 1 (GenBank: AJ564626). Further, the amino acid sequence of the SSB protein derived from Thermus aquaticus is shown in SEQ ID NO: 7, and the base sequence of the gene encoding the SSB protein is shown in SEQ ID NO: 6 (GenBank: AF276705).

  “Modification” means that one or more amino acids in the amino acid sequence of the protein that is the basis of the modification have been modified by at least one of deletion, substitution, insertion, and addition. “A modification comprising at least one of deletion, substitution, insertion and addition of one or more amino acids” means a known DNA recombination technique or a point mutation introduction method for a gene encoding a protein that is the basis of the modification. Etc. means that as many amino acids as can be deleted, substituted, inserted or added are deleted, substituted, inserted or added, including combinations thereof.

  Such modifications can be introduced artificially or can occur unintentionally in nature. The modified SSB protein in the present invention includes both modified forms.

  The modified site is not particularly limited as long as it has a function that can contribute to an improvement in the amplification efficiency of the template nucleic acid in a nucleic acid isothermal amplification reaction system using a strand displacement polymerase. Specifically, it is preferable that a modification occurs in the carboxyl terminal region (hereinafter sometimes abbreviated as “C terminal”) of the wild-type SSB protein. Here, the C-terminal region does not have a strict boundary. However, when an SSB protein derived from Thermus thermophilus is taken as an example, a region consisting of the 263th phenolalanine from the vicinity of the 230th proline of the protein is shown. Point to. In addition, when the SSB protein derived from Thermus aquaticus is taken as an example, it indicates a region consisting of 264th phenolalanine from the vicinity of the 231st proline of the protein.

  In addition, examples in which the amino acid sequence of the wild-type SSB protein is modified so as to have at least 3 consecutive prolines are also exemplified. That is, the modified SSB protein used in the present invention is preferably exemplified by a protein having the sequence Pro-Pro-Pro in the amino acid sequence, and one or several proline residues before and after the sequence. It may have.

  In particular, the 255th phenylalanine of SEQ ID NO: 2 representing the amino acid sequence of the SSB protein derived from wild-type Thermus thermophilus and the 256th phenylalanine of SEQ ID NO: 7 representing the amino acid sequence of the SSB protein derived from Thermus aquaticus Are preferably substituted for other amino acids for use in the present invention. As such other amino acids, proline is preferably exemplified. The amino acid sequence of the modified SSB protein obtained by substituting the 255th phenylalanine with proline in the amino acid sequence of the SSB protein derived from the wild type Thermus thermophilus is shown in SEQ ID NO: 3. In addition, the amino acid sequence of the modified SSB protein in which the 256th phenylalanine in the amino acid sequence of the SSB protein derived from Thermus aquaticus is replaced with proline is shown in SEQ ID NO: 8. However, the present invention is not limited to this. For example, phenylalanine may be substituted with other aromatic amino acids, or with amino acids other than aromatic amino acids, with other nonpolar amino acids, or with polar amino acids. In addition, substitution to acidic amino acids, basic amino acids, neutral amino acids, and the like are also included.

  Further, those having an amino acid sequence in which the C-terminal region of the protein is deleted from the amino acid sequence of the SSB protein are also preferably exemplified. For example, the amino acid sequence below the 230th proline in SEQ ID NO: 2 representing the amino acid sequence of the SSB protein derived from Thermus thermophilus can be mentioned. That is, it is a protein having an amino acid sequence consisting of amino acids 1 to 229 of SEQ ID NO: 2 representing the amino acid sequence of the SSB protein derived from Thermus thermophilus. The base sequence to be used is shown in SEQ ID NO: 4. The amino acid below the 231st proline of SEQ ID NO: 7 representing the SSB protein derived from Thermus aquaticus is mentioned. That is, it is a protein having an amino acid sequence consisting of amino acids 1 to 230 of SEQ ID NO: 2 representing an SSB protein derived from Thermus aquaticus, and the amino acid sequence is shown in SEQ ID NO: 9. Furthermore, as long as the function capable of improving the amplification efficiency of the template nucleic acid is expressed in the isothermal amplification reaction system of nucleic acid using a strand displacement polymerase, one or more amino acid sequences on the amino acid sequences shown in SEQ ID NO: 3, 5, 8, and 9 are used. A protein comprising an amino acid sequence having a modified site having a modified site in which a modification consisting of at least one of amino acid deletion, substitution, addition and insertion has occurred is also included.

  Here, the case of SSB protein derived from Thermus thermophilus and Thermus aquaticus was exemplified. However, when these other types of SSB proteins having high sequence similarity are used as the basis for modification, SSB proteins modified at the positions corresponding to the above substitution positions and cleavage positions are the same as these modified forms. Therefore, it is easily derived that it is suitable for use in the present invention.

  The modified SSB protein used in the present invention can be obtained by a known method. For example, by modifying the gene encoding the SSB protein that is the basis of the modification, transforming the host cell with the obtained modified gene, and collecting the SSB protein from the culture of the transformant Can be acquired.

  A gene encoding such an SSB protein can be obtained using a known gene cloning technique. It can also be obtained by synthesizing by a conventional method based on gene information that can be obtained by searching from a known database such as GenBank. For example, a DNA synthesis technique such as a phosphoramidite method can be used.

  There is no restriction | limiting in particular as a method to modify the gene which codes SSB protein, The mutagenesis technique for preparation of the modified protein well-known to those skilled in the art can be utilized. For example, a known mutagenesis technique such as a site-directed mutagenesis method, a PCR abrupt induction method that introduces a point mutation using a PCR method, or a transposon insertion mutagenesis method can be used. For example, refer to publicly known literature (for example, Chase JW et al., The Journal Biological Chemistry, 259 (2), pp. 805 to 814, January 25, 1984) related to mutation introduction into SSB protein derived from E. coli. it can. A commercially available mutation introduction kit (for example, QuikChange (registered trademark) Site-directed Mutagenesis Kit (manufactured by Stratagene)) may be used. Moreover, once the amino acid sequence of the target modified SSB protein is determined, an appropriate base sequence encoding it can be determined. Based on such sequence information, a gene encoding a modified SSB protein can be synthesized using a DNA synthesis method such as a conventional phosphoramidite method.

  Specific examples of the gene encoding such a modified SSB protein include DNA comprising a base sequence encoding the amino acid sequence shown in SEQ ID NOs: 3, 5, 8, and 9. As long as it encodes a protein that expresses a function capable of improving the amplification efficiency of the template nucleic acid in an isothermal amplification reaction system of nucleic acid using a strand displacement polymerase, the amino acid sequences of SEQ ID NOs: 3, 5, 8, and 9 are encoded. Also included is a gene comprising a DNA that hybridizes under stringent conditions with a DNA comprising a base sequence complementary to the DNA comprising the base sequence to be Here, the stringent condition means that DNAs having the identity of 60% or more, preferably 70%, more preferably 80% or more, particularly preferably 90% or more in the base sequence are preferentially hybridized. The condition to obtain. Stringency can be prepared by appropriately changing the salt concentration and temperature during the hybridization reaction and washing. For example, the conditions for Southern hybridization described in Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd edition, (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, and the like.

  More specifically, hybridization is exemplified in 50% (v / v) formamide, 5 × SSC at 42 ° C. for 16 hours. Here, 1 × SSC is 0.15 M NaCl, 0.015 M sodium citrate, pH 7.0. Further, sodium dodecyl sulfate (SDS) may be contained in an amount of 0.1 to 1.0% (v / v), and denatured non-specific DNA may be contained in an amount of 0 to 200 μl. The washing conditions include washing for 5 minutes at 5 ° C. in 2 × SSC, 0.1% SDS, and washing for 30 minutes to 4 hours at 65 ° C. in 0.1 × SSC, 0.1% SDS. . Also, those skilled in the art can easily understand these equivalent conditions.

  In order to transform a host cell using the obtained modified gene, a known host / expression vector system such as Escherichia coli can be used. For example, a modified SSB protein suitable for use in the present invention is linked to a DNA vector that can be stably amplified, and introduced into a host such as Escherichia coli that can efficiently express the modified SSB protein to produce a transformant.

  The vector that can be used is not particularly limited as long as it can incorporate foreign DNA and can replicate autonomously in a host cell. Thus, the vector comprises a sequence of at least one restriction enzyme site into which a nucleic acid molecule encoding a modified SSB protein suitable for use in the present invention can be inserted. For example, plasmid vectors (pET system, pEX system, pUC system, pBR system, etc.), phage vectors (λgt10, λgt11, λZAP, etc.), cosmid vectors, virus vectors (vaccinia virus, baculovirus, etc.) and the like are included. The

  The recombinant vector is incorporated so that the nucleic acid molecule encoding the modified SSB protein can express its function. Therefore, other known base sequences necessary for the functional expression of the nucleic acid molecule may be included. For example, a promoter sequence, a leader sequence, a signal sequence, a ribosome binding sequence and the like can be mentioned. As the promoter sequence, for example, when the host is Escherichia coli, lac promoter, trp promoter and the like are preferably exemplified. However, the present invention is not limited to this, and a known promoter sequence can be used. Furthermore, the recombinant vector of the present invention can also include a marking sequence that can confer phenotypic selection in the host. Examples of such marking sequences include sequences encoding genes such as drug resistance and auxotrophy. Specifically, kanamycin resistance gene, chloramphenicol resistance gene, ampicillin resistance gene and the like are exemplified.

  The insertion of the nucleic acid molecule encoding the SSB protein into the vector is, for example, a method in which the gene of the present invention is cleaved with an appropriate restriction enzyme and inserted into a restriction enzyme site or a multicloning site of an appropriate vector and ligated. However, it is not limited to this. For ligation, a known method such as a method using DNA ligase can be used. Commercially available ligation kits such as DNA Ligation Kit (Takara Bio Inc.) can also be used.

Here, the host cell is not particularly limited as long as it can efficiently express the modified SSB protein. Prokaryotes can be preferably used, and in particular, E. coli can be used. In addition, Bacillus subtilis, Bacillus bacteria, Pseudomonas bacteria, and the like can also be used. As E. coli, for example, E. coli BL21 (DE3) pLysS, E. coli BL21, E. coli DH5α, E. coli JM109 and the like can be used. Furthermore, eukaryotic cells can be used without being limited to prokaryotes. For example, yeasts such as Saccharomyces cerevisiae , insect cells such as Sf9 cells, animal cells such as CHO cells and COS-7 cells, and the like can be used. As a transformation method, a known method such as a calcium chloride method, an electroporation method, a liposome transfection method, a microinjection method, or the like can be used.

  The obtained transformant is inoculated into an appropriate medium and cultured according to a conventional method to express the modified SSB protein. The culture of the transformant may be selected in consideration of the nutritional physiological properties of the host cell. The medium to be used is not particularly limited as long as it contains nutrients that can be assimilated by the host cell and can efficiently express the protein in the transformant. Therefore, a medium containing a carbon source, a nitrogen source and other essential nutrients necessary for the growth of the host cell is preferable, regardless of whether it is a natural medium or a synthetic medium. Examples of the carbon source include glucose, dextran, starch, and the like, and examples of the nitrogen source include ammonium salts, nitrates, amino acids, peptone, and casein. As other nutrients, inorganic salts, vitamins, antibiotics, and the like can be included as desired. When the host cell is Escherichia coli, LB medium, M9 medium and the like can be suitably used. Moreover, although there is no restriction | limiting in particular also about a culture | cultivation form, A liquid medium can be utilized suitably from a viewpoint of mass culture.

  Selection of a host cell that expresses the modified SSB protein can be performed, for example, by the presence or absence of the expression of a marking sequence. For example, when a drug resistance gene is used as the marking sequence, it can be performed by culturing in a drug-containing medium corresponding to the drug resistance gene.

  The collection and purification of the modified SSB protein from the thus obtained transformant culture was performed in accordance with a general protein isolation and purification method depending on the fraction in which the modified SSB protein was present. A method may be applied. Specifically, when the SSB protein is produced outside the host cell, the culture solution is used as it is, or the host cell is removed by means such as centrifugation or filtration to obtain a culture supernatant. Subsequently, the enzyme of the present invention can be isolated and purified by appropriately selecting a known protein purification method for the culture supernatant. For example, known isolation and purification techniques such as ammonium sulfate precipitation, dialysis, SDS-PAGE electrophoresis, gel filtration, hydrophobic, anion, cation, affinity chromatography, etc. should be applied alone or in appropriate combination. Can do. In particular, when affinity chromatography is used, it is preferable to express the enzyme of the present invention as a fusion protein with a tag peptide such as His Tag and use the affinity for such tag peptide. When the SSB protein is produced in the host cell, the host cell is recovered by means such as centrifugation and filtration of the culture. Subsequently, the host cells are disrupted by an enzymatic disruption method such as lysozyme treatment or a physical disruption method such as ultrasonic treatment, freeze-thawing, and osmotic shock. After crushing, the solubilized fraction is collected by means such as centrifugation or filtration. The obtained solubilized fraction can be isolated and purified by treating in the same manner as in the case where it can be produced extracellularly.

  In addition, since the SSB protein suitable for use in the present invention is preferably based on the modification of an SSB protein derived from an extreme thermophile, heat stability is high, and heat treatment is used in the isolation and purification steps described above. It is useful and convenient to do. The host cell and culture supernatant obtained from the culture contain various proteins derived from the host cell. However, the host cell-derived contaminating protein is denatured and condensed and precipitated by heat treatment. Therefore, for example, when host cells are disrupted and subjected to heat treatment, proteins derived from host cells other than the modified SSB protein are thermally aggregated and can be separated and removed by centrifugation. Thus, the modified SSB protein that is not heat-denatured can be separated from the E. coli-derived protein as a soluble fraction and purified using affinity chromatography or the like. Furthermore, since the modified SSB protein is preferably derived from a highly thermophilic bacterium, the structure is stable at room temperature, and also has high stability against organic solvents. Therefore, the purification step can be performed at room temperature.

  Confirmation of whether or not the purified protein is a modified SSB protein in which a desired modification has occurred can be performed by a known amino acid analysis method. For example, an automatic amino acid determination method based on Edman degradation can be used. In addition, the purified SSB protein is subjected to an isothermal amplification reaction using a strand displacement polymerase, and compared with a wild-type SSB protein that does not have a modified site, whether or not the specificity to the template nucleic acid is improved is confirmed. Can be done.

  The “nucleic acid isothermal amplification reaction using a strand displacement polymerase” is a method of exponentially amplifying a nucleic acid by the strand displacement activity of a strand displacement polymerase under isothermal conditions that do not require heat denaturation by high heat. Examples of such isothermal amplification methods using strand displacement polymerase include rolling circle amplification (hereinafter may be abbreviated as “RCA”) method, Strand Displacement Amplification (hereinafter also abbreviated as SDA) method, etc. Are preferably exemplified, and in particular, the RCA method can be suitably used. The principle of the RCA method is as follows. In the isothermal amplification method of the template nucleic acid by the RCA method, for example, the strand displacement polymerase extends the complementary strand of the circular DNA based on a plurality of random primers annealed to the circular DNA that is the template nucleic acid under an isothermal condition. And even if the replication base point of another random primer is reached with the extension of the synthetic strand, the strand extension is continued while peeling off the other synthetic strand by the strand displacement activity of the strand displacement polymerase (branching). At this time, a site where the random primer can be annealed is exposed in the peeled synthetic strand. Since the peeled synthetic strand can also be used as a template nucleic acid to form a new DNA synthetic strand, exponential amplification occurs.

  The nucleic acid amplification can be performed based on a conventional method, and the reaction reagent and the like can be prepared based on a conventional method except that a modified SSB protein is added. A commercially available nucleic acid amplification kit (for example, TmpliPhi DNA Amplification kit: GE Healthcare) can also be used. Here, the amount of the modified SSB protein added is preferably 1.0 μg or more per 10 μl of the reaction solution.

  When the nucleic acid amplification is performed based on the RCA method, the primer is preferably a random primer, and for example, a random hexamer can be suitably used. As an example of another primer, a primer that specifically anneals to a target site of a template nucleic acid at a set temperature can be applied. This primer can be used alone or in a mixed state with the random primer.

  Random primers are designed to have a random sequence. Primers designed in this way can be chemically synthesized. For example, it can be chemically synthesized by solid phase synthesis using a known phosphoramidite method, and a primer having a desired base sequence can be automatically synthesized by a commercially available automatic nucleic acid synthesizer. The synthesized primer is purified by a known method such as HPLC as necessary.

  In the present invention, when an oligonucleotide primer containing a sequence complementary to a desired region of a general target nucleic acid is used, the primer can be prepared based on a conventional method. For example, a chemical synthesis method based on the phosphoramidite method or the like, or a restriction enzyme fragment or the like can be used when a target nucleic acid has already been obtained. When preparing a primer based on a chemical synthesis method, it is designed based on the sequence information of the target nucleic acid prior to synthesis. The primer can be designed using, for example, primer design support software so as to amplify a desired region. After synthesis, the primer is purified by means such as HPLC. Moreover, when performing chemical synthesis, it is also possible to use a commercially available automatic synthesizer. As such a primer, an oligonucleotide composed of 10 or more, preferably 15 or more, more preferably about 20 to 50 bases is designed with a desired amplification region sandwiched based on the base sequence of the target nucleic acid molecule. Illustrated.

  Here, the term “complementary” means that the primer and the target nucleic acid molecule can specifically bind according to the base pairing rule to form a stable duplex structure. Here, not only perfect complementarity, but only a few nucleobases fit along the base-pairing rules as long as the primer and target nucleic acid molecule are sufficient to form a stable duplex structure with each other Even partial complementarity is acceptable. The number of bases must be long enough to specifically recognize the target nucleic acid molecule. However, if the length is too long, a nonspecific reaction is induced, which is not preferable. Accordingly, the appropriate length is determined depending on many factors such as target nucleic acid sequence information such as GC content, and hybridization reaction conditions such as reaction temperature and salt concentration in the reaction solution.

  A preferred example of the strand displacement polymerase is bacteriophage-derived Phi29 DNA polymerase (US Pat. No. 5,198,543 and US Pat. No. 5,001,050, Blanco et al.). However, the present invention is not limited to this. For example, Bst large fragment DNA polymerase (Exo (-) Bst (Aliotta et al., Genet. Anal. (Netherlands), Vol. 12, pp. 185-195 (1996)) and Exo (-) Bca DNA polymerase ( Walker and Linn, Clinical Chemistry, 42, 1604-1608 (1996)), phage M2 DNA polymerase (Matsumoto et al., Gene, 84, 247 (1989)), phage φPRD1 DNA Polymerase (Jung et al., Proc. Natl. Acad. Sci., USA, 84, 8287 (1987)), VENT® DNA polymerase (Kong et al., J. Biol. Chem., Id. 268, 1965-1975 (1993)), Klenow fragment of DNA polymerase I (Jacobsen et al., Eur. J. Biochem., 45, 623-627 (1974)), T5 DNA polymerase (Chatterjee et al., Gene 97, 13-19 (1991)), Sequenase (registered trademark) (manufactured by Biochemicals, USA), PRD1 DNA polymerase (by Zhu and Ito, Biochem. Biophys. Acta., 1219, 267-276 (1994)) and T4 DNA polymerase holoenzyme (by Kaboord and Benkovic, Curr. Biol., 5, 149-157 (1995) )) And the like.

  Here, “isothermal” in isothermal amplification refers to performing an amplification reaction controlled at a constant temperature, whereas the PCR method changes the reaction temperature in each step of DNA denaturation, annealing, and chain extension. The constant temperature at which the amplification reaction is performed is preferably less than 60 ° C, more preferably less than 45 ° C, and even more preferably less than 37 ° C. This temperature is appropriately determined depending on the strand displacement polymerase to be applied. For example, when bacteriophage-derived Phi29 DNA polymerase described later is used, a suitable temperature range for performing the amplification reaction is 25 to 42 ° C, preferably 30 to 37 ° C, more preferably 30 to 34 ° C. In a constant temperature chamber such as an incubator set to the constant temperature, the sample is incubated for 4 to 24 hours, preferably 6 to 24 hours, more preferably about 15 to 24 hours, and the template nucleic acid is amplified.

  The present invention also provides a kit for isolating a circular nucleic acid from a nucleic acid-containing sample. The kit for isolating the circular nucleic acid of the present invention comprises the above modified SSB protein. Furthermore, it may be configured to appropriately contain components necessary for nucleic acid amplification such as DNA polymerase, an appropriate buffer, and dNTP. Moreover, in the case of a kit for detecting a pathogen or the like with a desired nucleic acid, an arbitrary primer or the like specific for desired nucleic acid amplification may be included. By configuring the necessary components as a kit in this way, it becomes possible to more easily and rapidly isolate a circular nucleic acid.

  According to the method and kit for isolating circular nucleic acids of the present invention, circular nucleic acids are selectively and preferentially performed by performing an isothermal amplification reaction of nucleic acids using a strand displacement polymerase in the presence of the above-described modified SSB protein. Can be amplified. That is, circular nucleic acids can be preferentially and selectively amplified while suppressing amplification of linear nucleic acids such as genomic DNA, RNA and other nucleic acid fragments other than circular nucleic acids. This makes it possible to isolate circular nucleic acids from nucleic acid-containing samples, and is expected to further enhance the utility value of circular nucleic acids useful in the field of molecular biology of mitochondrial DNA and plasmid DNA. Since the circular nucleic acid isolation method and kit of the present invention are based on the preferential and selective amplification of circular nucleic acids, the clarified cell lysate required in the known plasmid DNA isolation technique And costly steps such as purification of plasmid DNA from the clarified cell lysate are not required. That is, since it is composed of a simple process of nucleic acid amplification, the loss of nucleic acid can be minimized, and thus a circular nucleic acid can be isolated efficiently, simply and rapidly. In many cases, biological samples are mixed with nucleic acids of various shapes and types, and are known to have various forms, all of which are nucleic acid-containing samples of the present invention. It also has the advantage that it can be the subject of methods and kits. For example, it is possible to preferentially and selectively isolate only a circular nucleic acid from a nucleic acid-containing sample having a low mixture ratio of circular nucleic acids and an unpurified or roughly purified nucleic acid-containing sample containing a large amount of contaminants such as bacterial cells.

  The circular nucleic acid isolation method and kit of the present invention having the advantages as described above make a great contribution in this technical field. Therefore, it can be used for a wide variety of applications such as the medical field, biochemical field, environmental field, and food field. In particular, mitochondrial DNA is particularly important as a research material for forensic appraisal, evolutionary biological and human biological analysis, and further as a research material sample for identification of food origin. These methods and kits can be used. However, the present invention is not limited to this, and the method and kit of the present invention can be applied without limitation as long as the circular nucleic acid isolation technique is applicable.

  The principle of the method and kit of the present invention can also be used for detection and quantification of circular nucleic acids, and such circular nucleic acid detection and quantification methods and kits also form part of the present invention.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

  In the following examples, unless otherwise specified, nucleic acid amplification reactions were performed using TempliPhi DNA Amplification Kit (GE Healthcare) according to the manufacturer's instructions. Details of the process are shown below.

1 μl of template nucleic acid (1 ng or more) and 9 μl of sample buffer (including random primer as a primer) were added and mixed in a test tube. The resulting reaction solution was heat denatured by incubating at 95 ° C. for 3 minutes, and then cooled to 0 ° C. in ice. After cooling, 10 μl of Reaction Buffer (including dNTPs), 1.0 μg of modified SSB protein, and 1 μl of Phi29 DNA polymerase were sequentially added and mixed. The obtained reaction solution was incubated at 30 ° C. for 4 to 8 hours to carry out amplification reaction. After amplification, the reaction was terminated by inactivating DNA polymerase by heating at 65 ° C for 3 minutes.
It was.

The sample buffer, reaction buffer, and Phi29 DNA polymerase used were those attached to the kit. Sample buffer can be replaced with 25 mM Tris-HCl (pH 7.5), 50 mM KCl, 10 mM MgCl ₂ , and 100 μM exonuclease resistant hexamer, and Reaction buffer is 37 mM Tris-HCl (pH 8.0). ), 50 mM KCl, 10 mM MgCl ₂ , 5 mM (NH ₄ ) ₂ SO ₄ , 1.0 mM dNTPs, 1 unit / ml yeast pyrophosphatase, 50 μM exonuclease-resistant hexamer, and 800 unit / ml phi29 DNA polymerase You can substitute it with what you include.

  The modified SSB protein includes the 255th phenylalanine of SEQ ID NO: 2 representing the amino acid sequence of an SSB protein derived from a wild-type Thermus thermophilus (hereinafter sometimes referred to as “wild-type Tth SSB protein”). Was substituted with proline. Its amino acid sequence is shown in SEQ ID NO: 3. Hereinafter, this protein is referred to as “Tth SSB-F255P protein”. This Tth SSB-F255P protein is obtained by the method described in U.S. Patent Application Publication No. 2007/092896 and Inoue J. et al., Nucleic Acids Research, May 2006, Vol. 34, No. 9, e69. What was prepared similarly was used.

Specifically, the gene encoding the Tth SSB-F255P protein is cloned by PCR using thermus thermophilus genomic DNA ( Thermus thermophilus genomic DNA; Takara Bio Inc.) as a template DNA and using primers Nde - SSB and primer Bam -SSBm255. did. PCR used Kod DNA polymerase as a primer. Primers were constructed containing Nde I and Bam HI restriction sites, the sequences of which are shown below.
Primer Nde - SSB
5´‐GCGGATCCAT ATGGCTCGAG GCCTGAACCG ‐3´
(Sequence recognition number 10)
Primer Bam - SSBm255
5´-ACAGGATCCT TATTAAAACG GCAACTCCTC CTCCGGCGGA GGGTCTTCCA AGCCTTCGTC -3´ (Sequence recognition number 11)

The obtained amplification product was cleaved with restriction enzymes Nde I and Bam HI, and then ligated to plasmid pET14 previously treated with Nde I and Bam HI. Subsequently, E. coli BL21 (DE3) pLysS strain (Novagen) was transformed and cultured until reaching OD ₆₀₀ = 0.5, and further 0.5 mM IPTG was added and cultured for 4 hours. After 1 g of cultured cells were disrupted by ultrasonic treatment, cell tissue fragments were removed by centrifugation, and the supernatant was heat treated at 70 ° C. for 20 minutes. After the heat treatment, further centrifugation was performed to remove the precipitate and obtain a supernatant. Subsequently, the supernatant was applied to a Heparin Sepharose 6 Fast Flow column (GE Healthcare), and the protein was eluted with a linear gradient of NaCl from 0.1 to 1.0 M. The fraction containing SSB protein was collected, (NH ₄ ) ₂ SO ₄ was added to a final concentration of 1 M, and then applied to a BUTYL1-TOYOPEARL 650S column (Tosoh). Then, elution was performed with a linear gradient of (NH ₄ ) ₂ SO ₄ from 1 M to 0 M, and the fraction containing the SSB protein was collected, and the obtained fraction was dialyzed. After dialysis, it was subjected to RESOURCEQ (GE Healthcare), and protein was eluted with a linear gradient of NaCl from 0.1 to 1.0 M. The fraction containing SSB protein was collected and purified with Centriprep Ym-10 (Amicon; Cosmo Bio) to obtain Tth SSB-F255P protein.

Example 1 Isolation of Circular Nucleic Acid-1
Examinations for isolating circular nucleic acids from nucleic acid-containing samples by the method of the present invention were conducted.

Examination for isolating a circular nucleic acid contained in a nucleic acid-containing sample at a ratio of 0.4% by the method of the present invention was conducted. A sample containing 10 ng of genomic DNA (Human genomic DNA, Promega) and 40 pg of pUC19 DNA (GE Healthcare) as a target circular nucleic acid was prepared as a nucleic acid-containing sample. Using this as a template nucleic acid, a nucleic acid amplification reaction was performed as described above using TmpliPhi DNA Amplification kit (GE Healthcare) in the presence of Tth SSB-F255P protein. The amplification reaction was performed for 8 hours. 2 μl of the reaction solution after the amplification reaction was collected, treated with restriction enzyme Hind- III, and subjected to 1.2% agarose gel electrophoresis. The gel after electrophoresis was stained with ethidium bromide to visualize the DNA band (Example 1).

Furthermore, as a comparative example, in place of Tth SSB-F255P protein, when using the conventional RCA method without adding any kind of SSB protein (Comparative Example 1), when adding wild type Tth SSB protein (Comparative Example 2) , And the case of adding SSB protein derived from wild-type E. coli (hereinafter sometimes referred to as “Eco SSB protein”) (Comparative Example 3) was also examined. In either case, the nucleic acid amplification reaction was performed in the same procedure as described above except that Tth SSB-F255P protein was added.

The wild type Tth SSB protein was prepared by the same procedure as the preparation of Tth SSB-F255P protein except that the following primers were used as primers.
Primer Nde - SSB
5'-GCGGATCCAT ATGGCTCGAG GCCTGAACCG-3 '(sequence recognition number 10)
Primer Bam - SSB
5'- TAAGGATCCT TATTAAAACG GCAACTCCTC CTCCGGCGGA AA -3 '(SEQ ID NO: 12)

The result of electrophoresis is shown in FIG.
Lane 1 shows the result of nucleic acid amplification in the case of the conventional RCA method without adding any kind of SSB protein (Comparative Example 1).
Lane 2 shows the result of nucleic acid amplification when Tth SSB-F255P protein of the method of the present invention was added (Example 1).
Lane 3 shows the result of nucleic acid amplification when wild type Tth SSB protein was added (Comparative Example 2).
Lane 4 shows the result of nucleic acid amplification when wild-type Eco SSB protein was added (Comparative Example 3).

  From FIG. 2 (A), it was confirmed that circular nucleic acids present only at a ratio of 0.4% in the nucleic acid-containing sample can be preferentially and selectively amplified by isothermal amplification in the presence of Tth SSB-F255P protein. (Lane 2: Example 1). On the other hand, in the absence of Tth SSB-F255P protein, such a phenomenon could not be confirmed (lanes 1, 3, and 4: Comparative Examples 1, 2, and 3). In particular, even in the presence of Tth SSB protein based on Tth SSB-F255P protein, preferential and selective amplification of circular nucleic acid was not observed.

  Further, in order to confirm the preferential and selective amplification of the circular nucleic acid in the presence of the Tth SSB-F255P protein in detail, the amplification amount of the circular nucleic acid was determined. Samples subjected to isothermal amplification in the presence of Tth SSB-F255P protein after electrophoresis (lane 2: Example 1) and samples subjected to conventional RCA method without adding any kind of SSB protein (lane 1: comparison) From the signal band of the circular nucleic acid obtained in Example 1), the amount of the amplified product of the circular nucleic acid was quantified using a fluorescence image analyzer and a nucleic acid concentration measurement method. As a method for measuring the nucleic acid concentration, a pico green measurement method was used, and the determination was performed based on the conventional method.

  The obtained result is shown in the graph of FIG. In the case of the conventional RCA method (Comparative Example 1), the amount of circular nucleic acid amplified is 0.2 μg, whereas according to isothermal amplification in the presence of Tth SSB-F255P protein (Example 1), 5.1 μg circular Nucleic acid amplification was confirmed.

  From the above results, it was found that according to the method of the present invention, preferential and selective nucleic acid amplification of circular nucleic acids, which was difficult with the conventional RCA method, can be achieved. It was also found that this effect is a unique effect depending on the modified SSB protein.

Example 2 Isolation of Circular Nucleic Acid-2
Subsequent to Example 1, studies for isolating circular nucleic acids from nucleic acid-containing samples were performed by the method of the present invention. In this example, studies were made on those having a lower ratio of circular nucleic acid in the nucleic acid-containing sample.

  Examination for isolating a circular nucleic acid contained in a nucleic acid-containing sample at a ratio of 0.02% or less by the method of the present invention was conducted. A nucleic acid-containing sample containing 10 ng of human genomic DNA and 1.6 pg of pUC19 DNA as a target circular nucleic acid was prepared in the same manner as in Example 1. Using this as a template nucleic acid, a nucleic acid amplification reaction was performed in the presence of Tth SSB-F255P protein in the same procedure as in Example 1 above, and subjected to electrophoresis (Example 2).

  Furthermore, as a comparative example, the case of the conventional RCA method in which any kind of SSB protein was not added in place of the Tth SSB-F255P protein (Comparative Example 4) was also examined. A nucleic acid amplification reaction was performed in the same procedure as described above except that Tth SSB-F255P protein was added.

The result of electrophoresis is shown in FIG.
Lane 1 shows the result of nucleic acid amplification in the case of the conventional RCA method without adding any kind of SSB protein (Comparative Example 4).
Lane 2 shows the results of nucleic acid amplification when Tth SSB-F255P protein of the method of the present invention was added (Example 2).

  From FIG. 3A, it is possible to preferentially and selectively amplify a circular nucleic acid that is present at a very low ratio of 0.02% in a nucleic acid-containing sample by isothermal amplification in the presence of Tth SSB-F255P protein. It was confirmed (lane 2: Example 2). On the other hand, with the conventional RCA method in the absence of Tth SSB-F255P protein, almost no amplification product derived from circular nucleic acid could be confirmed (lane 1: comparative example 4).

  Further, in order to confirm the preferential and selective amplification of the circular nucleic acid in the presence of the Tth SSB-F255P protein in detail, the amplification amount of the circular nucleic acid was determined according to the procedure of Example 1 above.

  The obtained result is shown in the graph of FIG. In the case of the conventional RCA method (Comparative Example 4), the amount of circular nucleic acid amplified is 0 μg, which is below the detection limit, whereas according to isothermal amplification in the presence of Tth SSB-F255P protein (Example 2) Amplification of 3.6 μg of circular nucleic acid was confirmed.

  From the above results, according to the method of the present invention, even a circular nucleic acid that is present only in a very low ratio in a nucleic acid-containing sample that is difficult to amplify by the conventional RCA method is preferential and selective. It has been found that efficient nucleic acid amplification can be achieved. Therefore, it can be said that this is an extremely reliable isolation method. This result was confirmed by the result of Example 1.

(Example 3) Isolation of circular nucleic acids contained in nucleic acid-containing samples at various ratios According to the method of the present invention, circular nucleic acids are isolated from nucleic acid-containing samples in which circular nucleic acids are mixed at various ratios. Was examined.

  Examination for isolating circular nucleic acids contained in various ratios in a nucleic acid-containing sample by the method of the present invention was conducted. A sample containing 10 ng of human genomic DNA and 200, 40, 8, 1.6, 0.32, 0.06 and 0.01 pg of pUC19 DNA as a target circular nucleic acid was prepared in the same manner as in Example 1 as a nucleic acid-containing sample. The ratio of the circular nucleic acid to the total nucleic acids was 2.0, 0.4, 0.08, 0.02, 0.003, 0.0006, and 0.0001 = 0%, respectively. Using each nucleic acid-containing sample as a template nucleic acid, a nucleic acid amplification reaction was performed in the same procedure as in Example 1 above, and subjected to electrophoresis (Example 3).

  Furthermore, as a comparative example, the case of the conventional RCA method in which any kind of SSB protein was not added in place of the Tth SSB-F255P protein (Comparative Example 5) was also examined. A nucleic acid amplification reaction was performed in the same procedure as described above except that Tth SSB-F255P protein was added.

The electrophoresis results are shown in FIGS. 4 (A) and (B).
FIG. 4 (A) shows the result of nucleic acid amplification in the case of the conventional RCA method in which any kind of SSB protein is not added (Comparative Example 5). 0.08, 0.02, 0.003, 0.0006, and 0% nucleic acid-containing samples.
FIG. 4 (B) shows the results of nucleic acid amplification when the Tth SSB-F255P protein of the method of the present invention was added (Example 3). Lanes 1 to 7 have circular nucleic acid ratios of 2.0, 0.4, respectively. For samples containing 0.08, 0.02, 0.003, 0.0006, 0% nucleic acid.

  By isothermal amplification in the presence of the Tth SSB-F255P protein, an amplification product derived from the circular nucleic acid could be confirmed until the ratio of the circular nucleic acid in the nucleic acid-containing sample was as extremely low as 0.0006% (FIG. 4 (A ): Example 3). On the other hand, in the conventional RCA method, when the ratio of the circular nucleic acid in the nucleic acid-containing sample is not 0.08% or more, the amplification product derived from the circular nucleic acid could not be confirmed. (FIG. 4B: Comparative Example 5).

  Further, in order to confirm the preferential and selective amplification of the circular nucleic acid in the presence of the Tth SSB-F255P protein in detail, the amplification amount of the circular nucleic acid was determined according to the procedure of Example 1 above.

  The obtained result is shown in the graph of FIG. In FIG. 5 (A), the open bar indicates the amount of nucleic acid amplification product in the case of the conventional RCA method (Comparative Example 5) without adding any kind of SSB protein, and the shaded bar indicates the Tth SSB-F255P protein. The amount of nucleic acid amplification product by nucleic acid amplification in the presence (Example 3) is shown.

  From FIG. 5 (A), the amount of circular nucleic acid amplified in the conventional RCA method (Comparative Example 5) for any nucleic acid-containing sample is the case of isothermal amplification in the presence of Tth SSB-F255P protein ( It was found to be low compared to Example 3). In addition, regarding the detection limit, depending on the isothermal amplification in the presence of the Tth SSB-F255P protein, the amplification product derived from the circular nucleic acid is a trace amount up to a very low ratio of the circular nucleic acid in the nucleic acid-containing sample of 0.0006%. (Example 3). On the other hand, in the conventional RCA method, when the ratio of the circular nucleic acid in the nucleic acid-containing sample was not 0.08% or more, the amplification product derived from the circular nucleic acid was below the detection limit (Comparative Example 5).

  Furthermore, in order to deepen the verification of preferential and selective amplification of circular nucleic acid in the presence of Tth SSB-F255P protein, the total amount of amplified nucleic acid was determined. From the signal band of the nucleic acid obtained from the sample (FIG. 4 (A) lanes 1 to 7: Example 3) obtained by isothermal amplification in the presence of the Tth SSB-F255P protein after electrophoresis, It was determined according to the procedure of Example 1 above. At this time, a sample obtained by performing nucleic acid amplification in the same procedure as described above without adding the template nucleic acid was used as a control.

The obtained result is shown in the graph of FIG.
FIG. 5B confirmed that the total nucleic acid amplification amount by isothermal amplification in the presence of the Tth SSB-F255P protein was constant regardless of the circular nucleic acid ratio in the nucleic acid-containing sample.

  From the above results, it was found that circular nucleic acids can be preferentially and selectively amplified by the method of the present invention. Such a result is a logical confirmation of the results of Examples 1 and 2. Therefore, it is understood that only the circular nucleic acid can be isolated preferentially and selectively from the nucleic acid-containing sample by the method of the present invention.

Example 4 Isolation of Circular Nucleic Acid from Bacterial Cells-1
Investigations were carried out to isolate circular nucleic acids directly from bacterial cells by the method of the present invention.

From the results of Examples 1 to 3, it was found that by the method of the present invention, only circular nucleic acids can be preferentially and selectively isolated from nucleic acid-containing samples. The method was examined for directly isolating circular nucleic acids from bacterial cells. As a nucleic acid-containing sample, E. coli DH5α (Takara Bio Inc.) retaining pUC18 DNA, which is a high copy plasmid, was used for the examination. Using this as a template nucleic acid, a nucleic acid amplification reaction was performed in the same manner as in Example 1 in the presence of Tth SSB-F255P protein to obtain an amplification product. Subsequently, a part of the reaction solution after amplification was separated and treated with the restriction enzyme Hind- III. The amplification products before and after the restriction enzyme treatment were subjected to electrophoresis in the same procedure as in Example 1 (Example 4).

  Furthermore, as a comparative example, the case of the conventional RCA method (Comparative Example 6) in which any type of SSB protein was not added in place of the Tth SSB-F255P protein was also examined. Except for the addition of Tth SSB-F255P protein, nucleic acid amplification reaction, restriction enzyme treatment, and electrophoresis were performed in the same procedure as above.

Electrophoresis results are shown in FIGS. 6 (A) and (B).
FIG. 6 (A) shows the results when subjected to electrophoresis immediately after nucleic acid amplification without performing restriction enzyme treatment. Lanes 1 to 5 show the results of nucleic acid amplification when the Tth SSB-F255P protein of the method of the present invention was added (Example 4), and lanes 6 to 10 are conventional ones in which any kind of SSB protein is not added. The result of nucleic acid amplification in the case of the RCA method is shown (Comparative Example 6).
FIG. 6B shows the results when the amplification product is subjected to electrophoresis after restriction enzyme treatment. Lanes 1 to 5 show the results of nucleic acid amplification when the Tth SSB-F255P protein of the method of the present invention was added (Example 4), and lanes 6 to 10 are conventional ones in which any kind of SSB protein is not added. The result of nucleic acid amplification in the case of the RCA method is shown (Comparative Example 6).

  In FIG. 6B, the band indicated by the arrow was presumed to be the band of the amplification product derived from the circular nucleic acid. When isothermal amplification reaction was performed in the presence of Tth SSB-F255P protein (FIG. 6B, lanes 1 to 5: Example 4), amplification products derived from circular nucleic acids were confirmed. In the case of (Figure 6 (B) lanes 6 to 10: Comparative Example 6), an amplification product derived from a circular nucleic acid could not be confirmed, but in Fig. 6 (A), the amplification product was treated with a restriction enzyme. 1 shows the result of electrophoresis without amplification, and it is confirmed that the amplification product becomes a large mass and the amplification product is aggregated on the polymer side.

  From the above results, according to the method of the present invention, it is possible to selectively and preferentially amplify circular nucleic acids even from a very crude nucleic acid-containing sample such as E. coli colonies, thereby selectively and preferentially amplifying circular nucleic acids. It was found that it could be isolated.

  Here, plasmid vectors include a high copy plasmid and a low copy plasmid. The amount of plasmid increased in one cell varies depending on the origin of replication in the vector and the surrounding base sequence. As a high-copy plasmid, in addition to the pUC system examined in this Example, the pBR system, etc. are known, and these origins of replication include a sequence arranged with ColE1, etc. The number of copies is about 500. On the other hand, the pACYC system is known as a low-copy plasmid, and since the origin of replication is strictly defined like p15, the number of copies is small, and the number of copies in one cell is about 20-30 It becomes.

  In Example 5 below, a low copy plasmid was also examined.

(Example 5) Isolation of circular nucleic acid from bacterial cells-2
Following Example 4, studies were carried out to isolate circular nucleic acids directly from bacterial cells by the method of the present invention. In the present example, isolation from E. coli cells carrying a low copy plasmid was attempted.

From the results of Example 4, it was confirmed that the high-copy plasmid can be selectively and preferentially isolated from a very crude nucleic acid-containing sample such as a bacterial colony. Attempts were made to isolate circular nucleic acids directly from bacterial cells carrying As a nucleic acid-containing sample, E. coli DH5α (Novagen) carrying pET22b DNA, which is a low-copy plasmid, was used for the examination. Using this as a template nucleic acid, a nucleic acid amplification reaction was performed in the same manner as in Example 1 in the presence of Tth SSB-F255P protein to obtain an amplification product. Subsequently, a part of the reaction solution after amplification was separated and treated with the restriction enzyme Hind- III. The amplification products before and after the restriction enzyme treatment were subjected to electrophoresis in the same procedure as in Example 1 (Example 5).

  Furthermore, as a comparative example, the case of the conventional RCA method (Comparative Example 7) in which any kind of SSB protein was not added in place of the Tth SSB-F255P protein was also examined. Except for the addition of Tth SSB-F255P protein, nucleic acid amplification reaction, restriction enzyme treatment, and electrophoresis were performed in the same procedure as above.

The electrophoresis results are shown in FIGS. 7 (A) and (B).
FIG. 7 (A) shows the results when subjected to electrophoresis immediately after nucleic acid amplification without performing restriction enzyme treatment. Lanes 1 to 5 show the results of nucleic acid amplification when the Tth SSB-F255P protein of the method of the present invention was added (Example 5), and lanes 6 to 10 are the conventional ones in which any kind of SSB protein is not added. The result of nucleic acid amplification in the case of the RCA method is shown (Comparative Example 7).
FIG. 7 (B) shows the results when the amplification product is subjected to electrophoresis after restriction enzyme treatment. Lanes 1 to 5 show the results of nucleic acid amplification when the Tth SSB-F255P protein of the method of the present invention was added (Example 5), and lanes 6 to 10 are conventional ones in which any kind of SSB protein is not added. The result of nucleic acid amplification in the case of the RCA method is shown (Comparative Example 7).

  In FIG. 7B, the band indicated by the arrow was presumed to be the band of the amplification product derived from the circular nucleic acid. Although it is not certain because the shape of the RCA reaction product is unknown, other bands are probably expected to be derived from circular nucleic acids. When an isothermal amplification reaction was performed in the presence of Tth SSB-F255P protein (FIG. 7B, lanes 1 to 4: Example 5), an amplification product derived from a circular nucleic acid was confirmed. However, in FIG. 7B, the amplification product derived from the circular nucleic acid could not be confirmed for the sample in lane 5, but this was due to the fact that the plasmid was not retained in the E. coli used as the sample. It is clear from the result of the corresponding FIG. 7 (A) lane 5. On the other hand, in the case of the RCA method (FIG. 7B, lanes 6 to 10: Comparative Example 7), an amplification product derived from a circular nucleic acid could not be confirmed.

  From the above results, the method of the present invention can be used to selectively and preferentially amplify not only high copy plasmids but also low copy plasmids from very crude nucleic acid-containing samples such as E. coli colonies. Thus, it has been found that circular nucleic acids can be selectively and preferentially isolated.

(Example 6) Examination of addition amount of Tth SSB-F255P protein Regarding the isolation of circular nucleic acid from a nucleic acid-containing sample, the addition amount of Tth SSB-F255P protein in the method of the present invention was examined.

The amount of Tth SSB-F255P protein added when carrying out the method of the present invention was examined. A nucleic acid amplification reaction was carried out in the same manner as in Example 1 except that 0.2 μg, 1.0 μg and 2.0 μg of Tth SSB-F255P protein were contained in 10 μl of the reaction solution. Each template nucleic acid containing 10 ng of human genomic DNA and 200, 40, 8, 1.6, 0.3, 0.06, and 0.003 pg of pUC19 DNA as a target circular nucleic acid was prepared in the same manner as in Example 1. A sample was used. The ratio of circular nucleic acid to total nucleic acids was 2.0, 0.4, 0.08, 0.02, 0.003, 0.0006, and 0.00003%, respectively. A part of the amplified reaction solution was collected, treated with restriction enzyme Hind- III, and subjected to electrophoresis in the same procedure as in Example 1 (Example 6).

  Furthermore, as a comparative example, the case of the conventional RCA method in which any kind of SSB protein was not added instead of Tth SSB-F255P protein (Comparative Example 8) was also examined. Except for the addition of Tth SSB-F255P protein, nucleic acid amplification reaction, restriction enzyme treatment, and electrophoresis were performed in the same procedure as above.

The electrophoresis results are shown in FIGS.
FIG. 8 (A) shows the result of nucleic acid amplification in the case of the conventional RCA method in which any kind of SSB protein is not added (Comparative Example 8). 0.08, 0.02, 0.003, 0.0006, and 0.00003% of nucleic acid containing samples.
FIG. 8 (B) shows the results of nucleic acid amplification when 0.2 μg / 10 μl of Tth SSB-F255P protein of the method of the present invention was added (Example 6). Lanes 1 to 7 each have a ratio of circular nucleic acids. For 2.0, 0.4, 0.08, 0.02, 0.003, 0.0006, and 0.00003% nucleic acid containing samples.
FIG. 8C shows nucleic acid amplification results when Tth SSB-F255P protein of the method of the present invention was added at 1.0 μg / 10 μl (Example 6). Lanes 1 to 7 each have a ratio of circular nucleic acids. 2.0, 0.4, 0.08, 0.02, 0.003, 0.0006, and 0.00003% nucleic acid-containing samples.
FIG. 8 (D) shows the results of nucleic acid amplification when Tth SSB-F255P protein of 2.0 μg / 10 μl of the method of the present invention was added (Example 6). Lanes 1 to 7 each have a ratio of circular nucleic acids. For 2.0, 0.4, 0.08, 0.02, 0.003, 0.0006, and 0.00003% nucleic acid containing samples.

  From the results shown in FIGS. 8 (A) to (D), when 1.0 μg / 10 μl or more of Tth SSB-F255P protein is added to the amplification reaction, the presence ratio of the circular nucleic acid in the nucleic acid-containing sample is low. A clear band derived from a circular nucleic acid was also confirmed. That is, it was confirmed that good results were obtained when isothermal amplification reaction was performed by adding 1.0 μg / 10 μl or more of Tth SSB-F255P protein.

  From the above results, it was found that it is desirable to prepare the reaction solution so that 1.0 μg / 10 μl or more of Tth SSB-F255P protein is contained in the practice of the present invention.

Example 7 Isolation of Circular Nucleic Acid-3
For the isolation of circular nucleic acids from nucleic acid-containing samples, the method of the present invention was compared with known methods that are suitably available for isolation of mitochondrial DNA.

  For the isolation of circular nucleic acids from nucleic acid-containing samples, the method of the present invention was compared with the REPLI-g Mitochondrial DNA Kit (Qiagen) based on known methods that are preferably available for isolation of mitochondrial DNA.

A sample containing 100 ng of human genomic DNA and 100 pg of pUC19 DNA as a target circular nucleic acid was prepared as a nucleic acid-containing sample. Using this as a template nucleic acid, a nucleic acid amplification reaction was performed in the same procedure as in Example 1 using TmpliPhi DNA Amplification kit (GE Healthcare) in the presence of 3.0 μg of Tth SSB-F255P protein to obtain an amplification product. The amplification reaction was performed for 12 hours. Subsequently, 5 μl of the reaction solution after the amplification reaction was collected and treated with restriction enzyme Hind- III. The amplification products before and after the restriction enzyme treatment were subjected to electrophoresis in the same procedure as in Example 1 (Example 7).

  As a comparative example, we examined the case where nucleic acid amplification was performed using REPLI-g Mitochondrial DNA Kit in the presence of 1.0 μg of Tth SSB-F255P protein using the same nucleic acid-containing sample as above as the template nucleic acid (Comparative Example) 9). At this time, the detailed procedure was performed according to the manufacturer's instructions, and the amplification reaction was performed for 12 hours. After amplification, restriction enzyme treatment and electrophoresis were performed in the same procedure as described above.

The result of electrophoresis is shown in FIG.
Lanes 1 and 2 show the results of performing the isothermal amplification reaction with the addition of the Tth SSB-F255P protein, which is the method of the present invention (Example 7), and Lane 1 shows the amplification product treated with a restriction enzyme. This was subjected to electrophoresis later, and lane 2 is the result of subjecting it to electrophoresis immediately after nucleic acid amplification without performing restriction enzyme treatment.
Lanes 3 and 4 show the results when isothermal amplification reaction was performed using the known method REPLI-g Mitochondrial DNA Kit (Comparative Example 9), and Lane 3 was subjected to electrophoresis after the amplification product was treated with a restriction enzyme. Lane 4 shows the result of the sample subjected to electrophoresis immediately after nucleic acid amplification without performing restriction enzyme treatment.

  From FIG. 9, when isothermal amplification reaction was performed with the addition of Tth SSB-F255P protein, an amplification product derived from circular nucleic acid was observed (lane 1: Example 7). On the other hand, when the isothermal amplification reaction was performed with the REPLI-g Mitochondrial DNA Kit, no amplification product derived from the circular nucleic acid could be observed (lane 3: comparative example 9).

  Based on the above results, in the REPLI-g Mitochondrial DNA Kit, which is marketed as a kit that can amplify the entire mitochondrial genome from mitochondria such as humans in the total DNA sample, amplification of plasmid DNA, which is the same circular nucleic acid as mitochondrial DNA, It turned out not to be possible.

(Reference Example 1) Effect of modified SSB protein used in the present invention on isothermal amplification reaction system by differential substitution polymerase Effect of modified SSB protein used in the present invention on nucleic acid amplification reaction system by differential substitution polymerase Was examined.

  A protein from which amino acids below the 229th phenylalanine of SEQ ID NO: representing the amino acid sequences of the Tth SSB-F255P protein and the wild type Tth SSB protein are deleted (hereinafter referred to as “Tth SSB-229 protein”). The nucleic acid amplification reaction was carried out in the presence of), and the influence of these proteins on the nucleic acid amplification system was examined.

Here, Tth SSB-229 protein was prepared by the same procedure as the preparation of Tth SSB-F255P protein except that the following primers were used as primers.
Primer Nde - SSB
5'-GCGGATCCAT ATGGCTCGAG GCCTGAACCG-3 '(sequence recognition number 10)
Primer Bam - SSBm229
5'-TAAGGATCCT CATCCACGGG TGGGGCGCT-3 '(SEQ ID NO: 13)

Specifically, the nucleic acid amplification reaction was performed under the following conditions.
1. 1. Add 2 μg of Tth SSB-229 protein 2. Add 2 μg of Tth SSB-F255P protein 3. Add only 0.2 μl of protein buffer. None of the proteins and protein buffer were added. Proteins dissolved in protein buffer were used. As the protein buffer, 1.5 M KCl, 50 mM Tris-HCl (pH 7.5), 1.0 mM EDTA, 0.5 mM DTT, and 50% glycerol solution were used.

  The nucleic acid amplification reaction is carried out using REPLI-g DNA Amplification kit (manufactured by QIAGEN) according to the manufacturer's protocol and reacting 2 ng of the template nucleic acid in a reaction solution volume of 20 μl for 18 hours under the conditions 1 to 4 above. Was done by. As a template nucleic acid to be amplified, human genomic DNA (Promega: catalog number G3041) was used. After amplification, 2 μl of the amplification reaction solution was collected and subjected to electrophoresis in the same procedure as in Example 1.

  Furthermore, the same amplification reaction was performed under the conditions 1 to 4 except that no template nucleic acid was added to the reaction solution. The isothermal amplification system has a problem in that primer dimers are formed even in the absence of template nucleic acid, and non-specific nucleic acid is amplified. Therefore, the influence of the amplification in the absence of the template nucleic acid on the erroneous amplification of the protein can be examined.

The electrophoresis results are shown in FIG.
Lanes 1 to 4 show the results of the nucleic acid amplification reaction performed in the presence of the template nucleic acid. And as for the lane 1, as a result of performing nucleic acid amplification without adding any SSB protein and the protein buffer, the lane 2 is under the condition where only the protein buffer is added without adding any SSB protein. As a result of amplification, lane 3 shows the amplification result under the condition where Tth SSB-F255P protein was added, and lane 4 shows the amplification result under the condition where Tth SSB-229 protein was added.
Lanes 5 to 8 show the results of performing a nucleic acid amplification reaction in the absence of the template nucleic acid. Lane 5 shows the result of nucleic acid amplification without adding any SSB protein and protein buffer. Lane 6 shows the result of amplification under the condition that only the protein buffer is added without adding SSB protein. Lane 7 shows the result of amplification under the condition where the Tth SSB-F255P protein was added, and Lane 8 shows the result of amplification under the condition where the Tth SSB-229 protein was added.

When Tth SSB-229 protein and Tth SSB-F255P protein were added and nucleic acid amplification was performed, amplification of DNA fragments specific to the template nucleic acid was confirmed (lanes 3 and 4), respectively. The purity levels of the Tth SSB-229 protein and Tth SSB-F255P protein used here are not constant, and it is assumed that the purity level of the Tth SSB-229 protein is particularly lower than that of the Tth SSB-F255P protein. When comparison was made taking into account differences in the degree of protein purification, etc., when Tth SSB-229 protein was added, the production of the same level of amplification product as when Tth SSB-F255P was added was confirmed (lane 3). 4 comparison).
In addition, in the absence of the template nucleic acid, no amplification product was produced when either Tth SSB-229 protein or Tth SSB-F255P protein was added (lanes 7 and 8). That is, it is understood that the addition of Tth SSB-229 protein and Tth SSB-F255P protein can suppress non-specific amplification to the template nucleic acid and achieve nucleic acid amplification specific to the template nucleic acid. Therefore, it was found that the Tth SSB-229 protein can contribute to the effect of improving the amplification efficiency of the template nucleic acid in the nucleic acid amplification reaction system, like the Tth SSB-F255P protein.
On the other hand, when the amplification reaction was performed without adding any of the Tth SSB-229 protein and the Tth SSB-F255P protein, amplification products were observed even in the absence of the template nucleic acid (lanes 5 and 6). The amplification pattern (lanes 1 and 2) obtained in the presence of the template nucleic acid had a portion similar to the amplification pattern (lanes 5 and 6) obtained in the absence of the template nucleic acid. Therefore, it was found that the amplification product obtained in the presence of the template nucleic acid confirmed in lanes 1 and 2 has a high probability of containing many non-specific amplification products not related to the template nucleic acid.

  From the above results, it was found that the effect of improving the amplification efficiency of the template nucleic acid confirmed under the addition of Tth SSB-229 protein and Tth SSB-F255P protein is an effect specific to these proteins. Further, considering the results in Example 2 together, the expression of the function of improving the amplification efficiency of the template nucleic acids exhibited by the Tth SSB-229 protein and the Tth SSB-F255P protein is a decrease in the DNA binding power of these proteins. Finding that it is relevant. Therefore, the modified SSB protein obtained by artificially reducing the binding force to DNA obtained here exhibits an appropriate DNA binding force required as a replication cofactor in the nucleic acid amplification reaction system. This is understood to improve the reaction efficiency in the nucleic acid amplification reaction system and suppress erroneous amplification.

  Provided is a method for isolating a circular nucleic acid from a nucleic acid-containing sample, which is useful in the medical field, biochemical field, environmental field, food field and the like.

The figure which shows the principle of the method of this invention typically The figure which shows the result of Example 1 which examined isolation of the circular nucleic acid by the method of this invention The figure which shows the result of Example 2 which examined isolation of circular nucleic acid by the method of this invention The figure which shows the result of Example 3 which examined isolation of the circular nucleic acid contained in a nucleic acid containing sample in various ratios by the method of this invention. The figure which shows the result of Example 3 which examined isolation of the circular nucleic acid contained in a nucleic acid containing sample in various ratios by the method of this invention. The figure which shows the result of Example 4 which examined isolation of the circular nucleic acid from a bacterial cell by the method of this invention. The figure which shows the result of Example 5 which examined isolation of the circular nucleic acid from a bacterial cell by the method of this invention. The figure which shows the result of Example 6 which examined the addition amount of modified SSB protein in implementation of the method of this invention The figure which shows the result of Example 7 which examined isolation of circular nucleic acid by the method of this invention The figure which shows the result of the reference example 1 which examined the influence which it has on the isothermal amplification reaction system by the differential substitution polymerase of a modified SSB protein

Claims (9)

A method for isolating circular nucleic acid from a nucleic acid-containing sample, comprising:
An amino acid sequence having a modified site in which a modification comprising at least one of deletion, substitution, addition and insertion of one or more amino acids has occurred relative to the amino acid sequence of the single-stranded DNA binding protein, and the chain In an isothermal amplification reaction system using a displacement polymerase, a strand displacement polymerase is used for the nucleic acid-containing sample in the presence of a modified single-stranded DNA binding protein that expresses a function that can contribute to an improvement in the amplification efficiency of the template nucleic acid. A method for isolating a circular nucleic acid from a nucleic acid-containing sample, comprising a step of amplifying the circular nucleic acid by performing an isothermal amplification reaction of the nucleic acid.   The method according to claim 1, wherein the circular nucleic acid is mitochondrial DNA.   The method according to claim 1 or 2, wherein the strand displacement polymerase is Phi29 DNA polymerase. 4. The single-stranded DNA binding protein according to claim 1, wherein the single-stranded DNA binding protein is a single-stranded DNA binding protein derived from Thermus thermophilus or Thermus aquaticus . Method. In the modified single-stranded DNA binding protein, the 255th phenylalanine is another amino acid in the amino acid sequence of SEQ ID NO: 2 showing the amino acid sequence of the single-stranded DNA binding protein derived from Thermus thermophilus. Of the amino acid sequence of SEQ ID NO: 7 showing the amino acid sequence substituted with or the amino acid sequence of the single-stranded DNA binding protein derived from Thermus aquaticus , the 256th phenylalanine was substituted with another amino acid. The method of claim 4 having an amino acid sequence.   The method according to claim 5, wherein the other amino acid is proline. In the modified single-stranded DNA binding protein, the amino acids 230 to 263 are deleted from the amino acid sequence of SEQ ID NO: 2 indicating the amino acid sequence of the single-stranded DNA binding protein derived from Thermus thermophilus. Of the amino acid sequence of SEQ ID NO: 7 showing the amino acid sequence of the single-stranded DNA binding protein derived from Thermus aquaticus or the amino acid sequence from amino acid sequence 231 to 264 is deleted. The method of claim 4. A kit for isolating circular nucleic acid from a nucleic acid-containing sample,
An amino acid sequence having a modified site in which a modification comprising at least one of deletion, substitution, addition and insertion of one or more amino acids has occurred relative to the amino acid sequence of the single-stranded DNA binding protein, and the chain A kit for isolating a circular nucleic acid from a nucleic acid-containing sample, comprising a modified single-stranded DNA binding protein that expresses a function that can contribute to an improvement in amplification efficiency of a template nucleic acid in an isothermal amplification reaction system using a substituted polymerase.   The kit according to claim 8, further comprising a polymerase having strand displacement ability.