Patent Publication Number: US-2022213463-A1

Title: Fe-s fusion protein acting as electron transfer chain, carbon monoxide formate redox enzyme mediated through fes fusion protein, strain bcf12 derived from thermococcus wherein enzyme is transformed, and use thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
     This application claims benefit under 35 U.S.C. 119(e), 120, 121, or 365(c), and is a National Stage entry from International Application No. PCT/KR2018/014807, filed Nov. 28, 2018, which claims priority to the benefit of Korean Patent Application Nos. 10-2018-0138475 filed on Nov. 12, 2018, 10-2018-0138467 filed on Nov. 12, 2018 and 10-2018-0138462 filed on Nov. 12, 2018 in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with Korean government support (i) under Project Grant No. PE99622 (Research Project Name: Institutional Purpose Project) support awarded by Ministry of Oceans and Fisheries (Research management institution: Korea Institute of Ocean Science and Technology; and Supervision Institution: Research Management Institution: Korea Institute of Ocean Science and Technology) for the Research subject of “Elucidation and application of carbon metabolic process in non-photosynthetic marine and extreme microorganisms”, and (ii) under Project Grant No. 1711066286 (Research Project Name: Development (R&amp;D) of technologies in response to climate change) support awarded by Ministry of Science and ICT (Research management institution: National Research Foundation of Korea; and Supervision Institution: Research Management Institution: Korea Institute of Ocean Science and Technology) for the Research subject of “Development of acetogen strain for producing 2,3-BDO using CO”. The Korean government has certain rights in the invention.” 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an Fe—S fusion protein acting as an electron transport chain, a novel carbon monoxide:formate oxidoreductase (CFOR) including the Fe—S fusion protein, novel  Thermococcus  strain BCF12 transformed with CFOR, and the use thereof. 
     2. Background Art 
     With the breakthrough development of genetic engineering technology, the production of biomaterials by biological processes has gradually replaced petrochemical processes. The production of biomaterials by methods of applying bioconversion processes has advantages in that cheap renewable resources may be used as raw materials, and the generation of greenhouse gases such as carbon dioxide may be minimized, thus solving environmental problems. Accordingly, in recent years, there have been increasing studies on the development of related strains and improvement in processes for the production of biomaterials by bioprocesses. 
     Specifically, as methods of generating regulatory mutant strains, there have been used various techniques of increasing the production efficiency of target substances by creating new metabolic pathways or manipulating/changing pathways included in metabolic processes. These techniques are based on the premise that the expression or activity of one or more target proteins (or enzymes) related to the synthesis of a specific target product is promoted or inhibited. For example, for a recombinant microorganism having an increased ability to produce butanol, Korean Patent Application Publication No. 10-2014-0064469 discloses a microorganism in which the expression or activity of a protein (or enzyme) involved in a pathway that converts acetyl-CoA to acetate is inhibited and the expression or activity of a protein (or enzyme) involved in a pathway that converts acetyl-CoA to butyryl-CoA is promoted. In addition, Japanese Patent No. 4760951 attempted to improve the production efficiency of butanol by introducing, into a microorganism, enzymes such as 3-hydroxybutyryl-CoA dehydrogenase (HBD), 3-hydroxybutyryl-CoA dehydratase (CRT), trans-enoyl-CoA reductase (TER), and aldehyde/alcohol dehydrogenase (AdhE2), which are involved in butanol synthesis. 
     However, in the above-described conventional arts, intermediate products necessary for the production pathway of the substance to be synthesized by reactions may be accumulated in a microorganism, but there are disadvantages in that, since various converting enzymes in addition to the enzymes involved in the synthesis pathway of the substance to be synthesized exist in cells, the production efficiency of the substance is low, and above all, it is not possible to directly create a new synthetic pathway that can directly produce the target substance to be produced and does not exist in nature, or to create a reaction that directly synthesizes the target substance. 
     In order to solve these problems, Korean Patent Application Publication No. 10-2017-0024466 and the like discloses a system in which the physical gap existing between enzymes related to the biosynthesis of the target substance is narrowed in order to increase the synthesis efficiency of the target substance, so that intermediate products may immediately participate in reactions, thus solving the problem that accumulation of the intermediate products is interfered with by other enzymes. However, the method of increasing the activities of enzymes through an indirect method of simply narrowing the physical gap existing between biosynthesis-related enzymes has a fundamental problem in that it does not affect the electron transport or redox reactions of complex enzymes, and thus an increase in enzyme activity beyond a certain level cannot be expected. 
     Accordingly, the present inventors have conducted on a system to which immediate and direct electron transport between enzymes related to the biosynthesis of a target substance may be coupled so that it is possible to increase the synthesis efficiency of the target substance and to create a reaction that can directly produce the target substance to be produced and does not exist in nature. As a result, the present inventors have developed an Fe—S fusion protein formed by covalently linking two or more Fe—S proteins through a flexible linker so that Fe—S protein may act as an electron transport chain functioning as a channel through which electrons, and have synthesized a novel carbon monoxide:formate oxidoreductase (CFOR), which is not found in nature, using the Fe—S fusion protein, and have found that, when the synthesized CFOR is introduced into a Thermococcus BCF12 strain, the strain may produce the target substance formate with high yield and in high yield, thereby completing the present invention. 
     SUMMARY 
     Therefore, an object of the present invention is to provide an Fe—S fusion protein obtained by linking two or more Fe—S proteins and having an electron transport function, a novel carbon monoxide:formate oxidoreductase including the same, and a method of producing formate using the same. 
     Other objects and advantages of the present invention will be more apparent from the following detailed description, the appended claims and the accompanying drawings. 
     In accordance with one aspect of the present invention, the present invention provides an Fe—S fusion protein comprised of: a flexible linker having any one amino acid sequence selected from among SEQ ID NOs: 1 to 6; and two or more Fe—S proteins, each including any one amino acid sequence selected from among SEQ ID NOs: 7 to 11, the Fe—S fusion protein being formed by covalently linking the two or more Fe—S proteins through the flexible linker and acting as an electron transport chain functioning as a channel through which electrons move. 
     The Fe—S fusion protein may be variants or fragments of amino acids having different sequences by deletion, insertion, substitution or a combination thereof of amino acid residues within a range that does not affect the function of the Fe—S fusion protein. In addition, amino acid exchange at protein or peptide level that does not change the activity of the Fe—S fusion protein as a whole is known in the art. In some cases, the Fe—S fusion protein may be modified by phosphorylation, sulfation, acrylation, glycosylation, methylation, farnesylation, or the like. 
     The term “peptide” as used herein refers to a chain-type polymer formed by joining 4 to 1,000 amino acid residues together by peptide bonds, and may be used interchangeably with the term “polypeptide”. 
     As used herein, the term “polynucleotide” refers to a polymer compound in which nucleotides, which are chemical monomers, each consisting of three parts (base, sugar, and phosphoric acid), are linked together in a chain form through a plurality of phosphate ester bonds. 
     In the present invention, the Fe—S fusion protein is one in which two different Fe—S proteins are linked together through the flexible linker. 
     As used herein, the term “Fe—S protein (iron-sulfur protein)” refers to a protein characterized by the presence of an “Fe—S cluster” containing sulfide-linked di-, tri-, or tetra-iron centers in variable oxidation states. 
     The “Fe—S cluster” appears in metalloproteins such as ferredoxins, and is found in various proteins such as NADH dehydrogenase, hydrogenases, coenzyme Q-cytochrome C reductase, succinate-coenzyme Q reductase, and nitrogenase. The “Fe—S protein (iron-sulfur protein)” comprised of the “Fe—S cluster” is best known for its role in the redox reaction of electron transport in mitochondria. 
     As used herein, the term “electron transport chain” refers to an electron transport channel in which two or more “Fe—S proteins” are covalently linked in tandem in a chain form through a flexible linker. As used herein, the term “Fe—S fusion protein” serving as an electron transport chain refers to a new fusion protein synthesized by the present inventors, which is comprised of two or more “Fe—S proteins” covalently linked through a flexible linker and serves as a unit element to form the electron transport chain. 
     In the Fe—S fusion protein of the present invention, the Fe—S fusion protein is one in which two or more Fe—S proteins are covalently linked together through a flexible linker having any one amino acid sequence selected from among SEQ ID NOs: 1 to 6 shown in Table 1 below. Examples of flexible linkers having a function similar to that of the above-described flexible linker are described in Chen X, et al. 2013. Adv Drug Deliv Rev 65:1357-69. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SEQ ID NO 
                 Flexible linker sequence 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 (GGGSG) 1 : GGGGS 
               
               
                   
                   
               
               
                   
                 2 
                 (GGGSG) 2 : GGGSGGGGSG 
               
               
                   
                   
               
               
                   
                 3 
                 (GGGSG) 3 : GGGSGGGGSGGGGGS 
               
               
                   
                   
               
               
                   
                 4 
                 (GGGSG) 4 : GGGSGGGGSGGGGGSGGGGS 
               
               
                   
                   
               
               
                   
                 5 
                 (G) 5 : GGGGG 
               
               
                   
                   
               
               
                   
                 6 
                 (G) 6 : GGGGGG 
               
               
                   
                   
               
            
           
         
       
     
     In addition, contents related to peptide linkers including the flexible linkers are disclosed in detail in “Toon H. Evers et al., 2006, J. Christopher Anderson et al., 2010”, the disclosure of which is incorporated herein by reference. 
     Meanwhile, when Fe—S proteins enter a reduced state by accepting electrons, a hyperfine magnetic field of about 180 kG is formed. Fe—S proteins, which are linked together by the flexible linker and located within a short distance from each other, are tightly bound to the electron-accepted Fe—S proteins due to the effect of the formed hyperfine magnetic field. For this reason, the physical distance between the electron transport chains of the Fe—S proteins disappears, and as a result, the “Fe—S fusion protein” may be used as a single electron transport channel. 
     As used herein, the term “operatively linked” refers to a functional linkage between a nucleic acid expression control sequence (such as a promoter sequence, a signal sequence, or array of transcription factor binding sites) and another nucleic acid sequence, wherein the expression control sequence controls transcription and/or translation of the other nucleic acid sequence. 
     In addition, in the Fe—S fusion protein of the present invention, each of the Fe—S proteins may include any one selected from among the amino acid sequences of SEQ ID NOs: 7 to 11 or a combination of the amino acid sequences. The Fe—S proteins include the cysteine motifs of SEQ ID NOs: 7 to 11 shown in Table 2 below in order to form an Fe—S cluster. The present inventors compared 190 known Fe—S proteins and selected cysteine motifs having the amino acid sequences of SEQ ID NOs: 7 to 11. In addition, the present inventors confirmed through the Examples that an Fe—S fusion protein produced by covalently linking Fe—S proteins containing these motifs through a flexible linker acts as an electron transport chain. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 SEQ ID NO 
                 Fe-S protein 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 7 
                 CXXCXXXXCXXXC 
               
               
                   
               
               
                 8 
                 CXXCXXCXXXC 
               
               
                   
               
               
                 9 
                 CXXCXXCXXXXC 
               
               
                   
               
               
                 10 
                 CXXC 
               
               
                   
               
               
                 11 
                 CXXXC 
               
               
                   
               
            
           
         
       
     
     In addition, in the Fe—S fusion protein of the present invention, the Fe—S fusion protein may be formed by covalently linking 2 to 5 Fe—S proteins together through a flexible linker. 
     Since the Fe—S fusion protein of the present invention acts as an electron transport chain functioning as a channel through which electrons move, two different enzymes may be linked to each other by the Fe—S fusion protein of the present invention. In this case, since electrons generated in any one enzyme may be transported to another enzyme to directly mediate a redox reaction, it is possible to activate the enzyme reaction in a manner not found in nature and to produce a target substance with high efficiency and in high yield. 
     In this case, the enzyme that generates the electrons to be transported may be operatively linked to the C-terminus of the Fe—S fusion protein, and the enzyme that accepts the electrons may be operatively linked to the N-terminus of the Fe—S fusion protein, so that the electrons may be transported through the Fe—S fusion protein. 
     For example, as shown in  FIG. 1 , when the enzyme that generates electrons to be transported is CO dehydrogenase (CODH) and the enzymes that accepts the electrons is formate dehydrogenase (Fdh), the electrons generated in the CO dehydrogenase may be transported to the formate dehydrogenase, thus producing formate with high efficiency and in high yield. 
     In addition, even when the enzyme that accepts electrons is replaced with succinate dehydrogenase, dimethyl sulfoxide (DMSO) reductase, hydrogenase or the like, the electron transport function may be maintained. In this case, depending on the types of enzymes (succinate dehydrogenase, dimethyl sulfoxide (DMSO) reductase, and hydrogenase), succinic acid production, DMSO reduction and hydrogen production may be performed with high efficiency and in high yield. 
     In addition, the Fe—S fusion protein of the present invention may further include a tag in order to facilitate isolation and purification. The fusion protein of the present invention may be tagged with various detectable tags. The tags include, but are not limited to, His(n), flag, c-Myc, HA, V5, VSV-G and HSV. The tag refers to a polypeptide sequence consisting of 3 to 40 amino acids, and imparts specific affinity to the fusion protein of the present invention, a peptide, a protein ligand (e.g., the fusion protein of the present invention), or a non-peptide ligand. In addition, tags that may be used in the present invention may include fluorescent tags, luminescent tags and chromogenic tags. 
     In the Fe—S fusion protein of the present invention, the Fe—S fusion protein may have the amino acid sequence of SEQ ID NO: 13. 
     The present invention also includes proteins and variants thereof or active fragments thereof, which have substantially the same amino acid sequence as the protein including the amino acid sequence of SEQ ID NO: 13. The term “substantially the same amino acid sequence refers to those having an amino acid sequence homology of at least 80%, preferably at least 90%, most preferably at least 95%, but is not limited thereto, and those having the same enzymatic activity while having an amino acid sequence homology of at least 80% are included within the scope of the present invention. 
     According to another aspect of the present invention, the present invention provides a nucleotide sequence encoding the Fe—S fusion protein of the present invention. Specifically, the nucleotide sequence may be the nucleotide sequence of SEQ ID NO: 17 encoding the amino acid sequence of SEQ ID NO: 13, but is not limited thereto, and it will be obvious to those skilled in the art that any nucleotide sequence encoding the Fe—S fusion protein may be used. 
     A gene encoding the Fe—S fusion protein of the present invention, a variant thereof, or an active fragment thereof may be variously modified in its coding region within a range that does not change the amino acid sequence of a protein expressed from the coding region, and may also be variously mutated by substitution, deletion, insertion or a combination thereof in regions other than the coding region within a range that does not affect the expression of the gene, and these mutated genes are also included within the scope of the present invention. Thus, the present invention includes nucleotide sequences and fragments thereof, which are substantially the same as the nucleotide sequence encoding the Fe—S fusion protein. The term “substantially the same nucleotide sequences” refers to those having a sequence homology of at least 80%, preferably at least 90%, most preferably at least 95%, but is not limited thereto, and those having a sequence homology of at least 85% are included within the scope of the present invention, as long as proteins encoded thereby have the same enzymatic activity. 
     According to still another aspect of the present invention, the present invention provides a recombinant vector including: (a) the nucleotide sequence encoding the Fe—S fusion protein; and (b) a promoter operatively linked to the nucleotide sequence. As used herein, the term “promoter” refers to a DNA sequence that controls the expression of a coding sequence or functional RNA. In the recombinant expression vector of the present invention, the nucleotide sequence encoding a substance to be expressed (i.e., Fe—S fusion protein) is operatively linked to the promoter. 
     The vector system of the present invention may be constructed by various methods known in the art, and a specific method therefor is disclosed in Sambrook et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press (2001), which is incorporated herein by reference. 
     When the expression vector of the present invention uses a prokaryotic cell as a host, it generally includes a strong promoter (e.g., tac promoter, lac promoter, lacUV5 promoter, lpp promoter, pL promoter, pR promoter, rac5 promoter, amp promoter, recA promoter, SP6 promoter, trp promoter, T7 promoter, etc.) capable of directing transcription, a ribosome binding site for initiation of translation, and a transcription/translation termination sequence. At this time, the bacterial replication origin may be selected from replication origins that are well known in the art to be useful in the stable bacterial replication of long DNA inserts, and examples thereof include, but are not limited to, ColE1, F-factor, and P1 replicon. As the bacterial selection marker of the present invention, a bacterial selection marker gene known in the art may be used. Examples of the bacterial selection marker gene include, but are not limited to, genes that confer resistance to antibiotics, such as ampicillin, kanamycin, tetracycline, Zeocin, neomycin, hygromycin, and chloramphenicol. Where  E. coli  is used as a host cell, the promoter and operator region for the tryptophan biosynthesis pathway (Yanofsky, C., J. Bacteriol., 158:1018-1024 (1984)) and the leftward promoter from phage λ (pLλ promoter, Herskowitz, I. and Hagen, D., Ann. Rev. Genet., 14:399-445 (1980)) may be used as regulating sequences. 
     In addition, where the recombinant vector of the present invention is applied to eukaryotic cells, promoters that may be used can control transcription of the substance to be expressed in the present invention, and include promoters derived from mammalian viruses and promoters derived from mammalian cell genomes. Examples of the promoter include, but are not limited to, cytomegalovirus (CMV) promoter, adenovirus late promoter, vaccinia virus 7.5K promoter, SV40 promoter, tk promoter of HSV, RSV promoter, EF1 alpha promoter, metallothionein promoter, beta-actin promoter, human IL-2 gene promoter, human IFN gene promoter, human IL-4 gene promoter, human lymphotoxin gene promoter, and human GM-CSF gene promoter. 
     Preferably, the recombinant vector that is used in the present invention includes a polyadenylation sequence (e.g., bovine growth hormone terminator or SV40-derived polyadenylation sequence). 
     The vector may include a selectable marker for selecting a host cell containing the vector. The selectable marker is used to select a cell transformed with the vector, and examples thereof include markers providing a selectable phenotype such as drug resistance, auxotrophy, resistance to a cytotoxic agent, or expression of a surface protein. Since only cells expressing the selectable marker survive in an environment treated with the selective agent, the transformed cell may be selected. In addition, where the vector is a replicable expression vector, the vector may include a replication origin which is a specific nucleic acid sequence initiating replication. 
     Expression vectors that may be used in bacterial hosts include bacterial plasmids obtained from  Escherichia coli,  such as pET, pRSET, pBluescript, pGEX2T, pUC vector, col E1, pCR1, pBR322, pMB9, and derivatives thereof, plasmids having a larger range of host, such as RP4, phage DNA exemplified by significantly various phage lambda derivatives, such as λgt10 and λgt11, NM989, and other DNA phages such as M13 and filamentous single stranded DNA phage. In particular, for expression in  E. coli,  a DNA sequence encoding an anthranilate synthase (TrpE) and a C-terminal polylinker may be included, and other expression vector systems are based on beta-galactosidase (pEX); lambda PL maltose binding protein (pMAL); and glutathione S-transferase (pGST) (Gene 67:31, 1988; Peptide Research 3:167, 1990). 
     According to yet another aspect of the present invention, the present invention provides a cell transformed with the recombinant vector of the present invention. The recombinant vector is inserted into a host cell to form a transformant or a recombinant microorganism. Suitable host cells for the vector may be prokaryotic cells such as  E. coli, Bacillus subtilis, Streptomyces  sp.,  Pseudomonas  sp.,  Proteus mirabilis  or  Staphylococcus  sp., and may also be Archaea such as  Thermococcus onnurineus.  In addition, the host cells may be fungi such as  Aspergillus  sp., yeasts such as  Pichia pastoris, Saccharomyces cerevisiae, Schizosaccharomyces  sp. and  Neurospora crassa,  and eukaryotic cells such as other lower eukaryotic cells, and cells of higher eukaryotes such as cells from insects. 
     Delivery of the vector of the present invention into the host cell may be performed using various methods known in the art. For example, where the host cells are eukaryotic cells, the delivery may be performed by a CaCl 2  method (Cohen, S. N. et al., Proc. Natl. Acac. Sci. USA, 69:2110-2114(1972)), a Hanahan&#39;s method (Cohen, S. N. et al., Proc. Natl. Acac. Sci. USA, 69:2110-2114(1972); and Hanahan, D., J. Mol. Biol., 166:557-580(1983)), and an electroporation method (Dower, W. J. et al., Nucleic. Acids Res., 16:6127-6145(1988)). Where the host cells are prokaryotic cells, the delivery may be performed using transduction, electroporation, lipofection, microinjection, particle bombardment, yeast spheroplast/cell fusion used in YAC,  Agrobacterium -mediated transformation used in plant cells, or the like. 
     In addition, the production of animal cells using the recombinant expression vector of the present invention may be performed by a gene transfer method that is commonly known in the art. Examples of the method include, but are not limited to, electroporation, liposome-mediated transfer (Wong, et al., 1980), and retrovirus-mediated transfer (Chen, H. Y., et al., (1990), J. Reprod. Fert. 41:173-182; Kopchick, J. J. et al., (1991) Methods for the introduction of recombinant DNA into chicken embryos. In Transgenic Animals, ed. N. L. First &amp; F. P. Haseltine, pp. 275-293, Boston; Butterworth-Heinemann; Lee, M.-R. and Shuman, R. (1990) Proc. 4th World Congr. Genet. Appl. Livestock Prod. 16, 107-110). 
     According to another aspect of the present invention, the present invention provides a method for producing an Fe—S fusion protein, the method including a step of expressing the Fe—S fusion protein by culturing the recombinant microorganism of the present invention. 
     According to another aspect of the present invention, the present invention provides a carbon monoxide:formate oxidoreductase (CFOR) wherein CO dehydrogenase (CODH) and formate dehydrogenase (Fdh) are linked together through an Fe—S fusion protein, the carbon monoxide:formate oxidoreductase (CFOR) including: an Fe—S fusion protein acting as an electron transport chain functioning as a channel through which electrons move, and formed by covalently linking two or more Fe—S proteins through a flexible linker having any one amino acid sequence selected from among SEQ ID NOs: 1 to 6; a CO dehydrogenase (CODH) operatively linked to the C-terminus of the Fe—S fusion protein; and a formate dehydrogenase (Fdh) operatively linked to the N-terminus of the Fe—S fusion protein, wherein electrons generated in the CO dehydrogenase are transported to the formate dehydrogenase (Fdh) through the Fe—S fusion protein. 
     In the carbon monoxide:formate oxidoreductase (CFOR), the CO dehydrogenase (CODH) and the formate dehydrogenase (Fdh) may be variants or fragments of amino acids having different sequences by deletion, insertion, substitution or a combination thereof of amino acid residues within a range that does not affect the protein functions of the CO dehydrogenase (CODH) and the formate dehydrogenase (Fdh). In addition, amino acid exchange at protein or peptide level that does not change the activities of the CO dehydrogenase (CODH) and the formate dehydrogenase (Fdh) as a whole is known in the art. In some cases, the CO dehydrogenase (CODH) and the formate dehydrogenase (Fdh) may be modified by phosphorylation, sulfation, acrylation, glycosylation, methylation, farnesylation, or the like. 
     In the present invention, the CO dehydrogenase (CODH) and the formate dehydrogenase (Fdh) are linked together through the Fe—S fusion protein, wherein the Fe—S fusion protein is one in which two different Fe—S proteins are linked together by the flexible linker. 
     In the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, the Fe—S fusion protein is one in which two or more Fe—S proteins are covalently linked through a flexible linker having any one amino acid sequence selected from among SEQ ID NOs: 1 to 6. 
     In addition, in the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, each of the Fe—S proteins may include any one selected from among the amino acid sequences of SEQ ID NOs: 7 to 11 or a combination of the amino acid sequences. 
     In addition, in the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, the Fe—S fusion protein may be formed by covalently linking 2 to 5 Fe—S proteins together through the flexible linker. 
     In the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, the CO dehydrogenase (CODH) may be a gene derived from any one strain selected from the group consisting of  Thermococcus onnurineus  NA1,  Thermococcus  sp. CH5,  Thermococcus guaymasensis, Thermococcus profundus, T hermococcus radiotolerans, Thermococcus gammatolerans, Thermococcus barophilus, Thermococcus  AM4,  Methanothermobacter thermoautotrophicus, Archaeoglobus fulgidus, Clostridium autoethanogenum, Clostridium ljungdahlii, Clostridium carboxidivorans, Oxobacter pfennigii, Peptostrep tococcus productus, Acetobacterium woodii, Eubacterium limosum, Butyribacterium methylotrophicum, Rubrivivax gelatinosus, Rhodopseudomonas palustris, Rhodospirillum rubrum, Citrobacter  sp Y19,  Methanosarcina barkeri, Methanosarcina acetivorans, Moorella thermoacetica, Moorella thermoautotrophica, Moorella  strain AMP,  Carboxydothermus hydrogenoformans, Carboxydibrachium pacificus, Carboxydocella sporoproducens, Carboxydocella thermoautotrophica, Thermincola carboxydiphila, Thermincola ferriacetica, Thermolithobacter carboxydivorans, Thermosinus carboxydivorans, Desulfotomaculum kuznetsovii, Desulfotomaculum thermobenzoicum  sub sp.  thermosyntrophicum,  and  Desulfotomaculum carboxydivorans,  and the formate dehydrogenase (Fdh) may be a gene derived from any one strain selected from the group consisting of  Thermococcus onnurineus  NA1,  Thermococcus fumicolans, Thermococcus  sp. CH5,  Thermococcus celericrescens, Thermococcus litoralis, Thermococcus pacificus, Thermococcus profundus, Thermococcus radiotolerans, Thermococcus stetteri, Thermococcus waiotapuensis, Thermococcus  sp. AM4,  Thermococcus sibiricus, Thermococcus kodakarensis, Thermococcus gammatolerans, Thermococcus barophilus, Thermococcus  sp. 4557,  Pyrococcus furiosus, Pyrococcus abyssi, Pyrococcus yayanosii, Pyrococcus  sp. NA2,  Carboxydothermus hydrogenofomans, Rubrivivax gelatinosus, Escherichia coli, Rhodospirillum rubrum, Moorella thermoacetica, Clostridium autoethanogenum, Clostridium ljungdahlii, Acetobacterium woodii, Eubacterium limosum, Clostridium carboxidivorans,  and  Rhodopseudomonas palustris.    
     In addition, in the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, the Fe—S fusion protein acts as an electron transport chain functioning as an electron transport channel, and thus directly mediate a redox reaction. Thus, electrons generated in the CO dehydrogenase are transported directly to the formate dehydrogenase (Fdh). Even when the formate dehydrogenase (Fdh) is replaced with succinate dehydrogenase, dimethyl sulfoxide (DMSO) reductase, hydrogenase or the like, the electron transport function may be maintained. In this case, the functions of succinic acid production, DMSO reduction and hydrogen production are newly provided by succinate dehydrogenase, dimethyl sulfoxide (DMSO) reductase, and hydrogenase, which are used for replacement. 
     In addition the carbon monoxide:formate oxidoreductase (CFOR) of the present invention may further include a tag. A tag that may be included in the carbon monoxide:formate oxidoreductase (CFOR) is as described above with respect to a tag that may be included in the Fe—S fusion protein, description thereof will be omitted herein. 
     In the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, the carbon monoxide:formate oxidoreductase (CFOR) may have the amino acid sequence of SEQ ID NO: 15 in which the CO dehydrogenase (CODH) of SEQ ID NO: 12, the Fe—S fusion protein of SEQ ID NO: 13 and the formate dehydrogenase (Fdh) of SEQ ID NO: 14 are operatively linked to one another. 
     The present invention includes proteins, variants thereof, or active fragments thereof, which have substantially the same amino acid sequences as the protein including the amino acid sequence of SEQ ID NO: 15. The term “substantially the same amino acid sequences” refers to those having an amino acid sequence homology of at least 80%, preferably at least 90%, most preferably at least 95%, but is not limited thereto, and those having the same enzymatic activity while having an amino acid sequence homology of at least 80% are included within the scope of the present invention. 
     According to another aspect of the present invention, the present invention provides a nucleotide sequence encoding the carbon monoxide:formate oxidoreductase (CFOR) of the present invention. Specifically, the nucleotide sequence may be, for example, the nucleotide sequence of SEQ ID NO: 16 encoding the amino acid sequence of SEQ ID NO: 15, but is not limited thereto, and it will be obvious to those skilled in the art that any nucleotide sequence encoding the carbon monoxide:formate oxidoreductase may be used. 
     A gene encoding the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, a variant thereof, or an active fragment thereof may be variously modified in its coding region within a range that does not change the amino acid sequence of a protein expressed from the coding region, and may also be variously mutated by substitution, deletion, insertion or a combination thereof in regions other than the coding region within a range that does not affect the expression of the gene, and these mutated genes are also included within the scope of the present invention. Thus, the present invention includes nucleotide sequences and fragments thereof, which are substantially the same as the nucleotide sequence encoding the carbon monoxide:formate oxidoreductase (CFOR). The term “substantially the same nucleotide sequences” refers to those having a sequence homology of at least 80%, preferably at least 90%, most preferably at least 95%, but is not limited thereto, and those having a sequence homology of at least 85% are included within the scope of the present invention, as long as proteins encoded thereby have the same enzymatic activity. 
     According to another aspect of the present invention, the present invention provides a recombinant vector including: (a) a nucleotide sequence encoding the carbon monoxide:formate oxidoreductase (CFOR); and (b) a promoter operatively linked to the nucleotide sequence. As used herein, the term “promoter” refers to a DNA sequence that controls the expression of a coding sequence or functional RNA. In the recombinant expression vector of the present invention, the nucleotide sequence encoding a substance to be expressed (i.e., carbon monoxide:formate oxidoreductase is operatively linked to the promoter. 
     The vector system of the present invention may be constructed by various methods known in the art, and a specific method therefor is as described above with respect to the vector system into which the Fe—S fusion protein may be introduced, and thus description thereof will be omitted herein. 
     Another aspect of the present invention, the present invention provides a cell transformed with the recombinant vector of the present invention. The recombinant vector is inserted into a host cell to form a transformant or a recombinant microorganism. Suitable host cells for the vector and a method of delivering the vector of the present invention into the host cell are as described above with respect to the transformed cell into which the Fe—S fusion protein may be introduced, and thus description thereof will be omitted herein. 
     According to another aspect of the present invention, the present invention provides a method for producing a carbon monoxide:formate oxidoreductase (CFOR), the method including a step of expressing the carbon monoxide:formate oxidoreductase (CFOR) by culturing the recombinant microorganism of the present invention. 
     According to another aspect of the present invention, the present invention provides a method for producing formate, the method including steps of: (a) synthesizing formate from CO gas by supplying the CO gas in the presence of the carbon monoxide:formate oxidoreductase (CFOR) or the transformed cell; and (b) recovering the synthesized formate. 
     According to another aspect of the present invention, the present invention provides novel Thermococcus strain BCF12 (accession number: KCTC 13649BP). As demonstrated in an example of the present invention, novel  Thermococcus  strain BCF12 (accession number: KCTC 13649BP) is one transformed with the carbon monoxide:formate oxidoreductase (CFOR), and thus has an improved ability to produce formate. 
     The strain was deposited with the Korea Research Institute of Bioscience and Biotechnology on Sep. 21, 2018 under accession number KCTC 13649BP. 
     According to another aspect of the present invention, the present invention provides a method for producing a carbon monoxide:formate oxidoreductase (CFOR), the method including a step of expressing the carbon monoxide:formate oxidoreductase (CFOR) by culturing novel Thermococcus strain BCF12 (accession number: KCTC 13649BP) of the present invention. 
     According to another aspect of the present invention, the present invention provides a method for producing formate, the method including steps of: (a) synthesizing formate from CO gas by supplying the CO gas in the presence of the novel strain BCF12 (accession number: KCTC 13649BP) or the carbon monoxide:formate oxidoreductase (CFOR); and (b) recovering the synthesized formate. 
     As described above, two different enzymes may be physically linked directly to each other through the Fe—S fusion protein of the present invention, and thus electrons generated in any one enzyme may be transported directly to another enzyme through the Fe—S cluster of the Fe—S fusion protein. 
     In addition, the carbon monoxide:formate oxidoreductase (CFOR) of the present invention is produced by directly linking CO dehydrogenase (CODH) and formate dehydrogenase directly to each other through the Fe—S fusion protein consisting of Fe—S proteins, and thus electrons generated in the CO dehydrogenase may be transported directly to the formate dehydrogenase through the Fe—S cluster of the Fe—S fusion protein. 
     Accordingly, a reaction that produces formate with high efficiency through oxidation reaction of carbon monoxide by directly supplying electrons necessary for the formate dehydrogenase reaction in the conversion of carbon dioxide into formate is possible without leakage of generated electrons. 
     In addition, the present invention has an advantage in that the overall enzyme reaction rate and yield can be dramatically improved using a new carbon monoxide:formate oxidoreductase (CFOR) reaction. Furthermore, it is possible to ensure the stability of each enzyme by allowing the enzymes to exist in a physically fixed state in cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the structure of a carbon monoxide:formate oxidoreductase (CFOR) of the present invention, and shows a series of processes in which electrons generated from the oxidation of CO gas by CO dehydrogenase (CODH) move through an Fe—S cluster (⋄) present in the electron transport chain Fe—S fusion protein and is transported to the CO 2  reductase formate dehydrogenase (FDH), and CO 2  is converted into formate by an enzymatic reaction. 
         FIG. 2  is a schematic view illustrating the types of Fe—S clusters that are generally present in Fe—S protein (iron-sulfur protein). 
         FIG. 3  shows ferredoxin ((A) of  FIG. 3 ) containing an Fe—S cluster and the protein structure of formate dehydrogenase (FDH). 
         FIG. 4  shows a process of producing a carbon monoxide:formate oxidoreductase (CFOR) gene by recombination of CO dehydrogenase (CODH) and formate dehydrogenase. 
         FIGS. 5A to 5C  show the genetic structure of each transformant produced by inserting the carbon monoxide:formate oxidoreductase (CFOR) into the genome of a host cell [ FIG. 5A : a schematic view of fdh3 and codh gene clusters present in the  T. onnurineus  NA1 genome and target genes used for cloning;  FIG. 5B : construction of a fusion protein between TON_0541 and TON_1017. A schematic view showing cloning of strains to which a flexible linker consisting of one ‘GGGGS (SEQ ID NO: 1)’ amino acid moiety ( T. onnurineus  BCF01), a flexible linker consisting of two ‘GGGGS (SEQ ID NO: 1)’ amino acid moieties ( T. onnurineus  BCF02), and a flexible linker consisting of three ‘GGGGS (SEQ ID NO: 1)’ amino acid moieties ( T. onnurineus  BCF03) were applied;  FIG. 5C : construction of a fusion protein between TON_0540 and TON_1017. A schematic view showing cloning of a strain to which a flexible linker consisting of one ‘GGGGS (SEQ ID NO: 1)’ amino acid moiety ( T. onnurineus  BCF12) was applied]. 
         FIG. 6  depicts graphs showing the results of measuring changes in cell growth (A) and formate production (B) in transformants ( T. onnurineus  BCF01, BCF02, BCF03, and BCF12), into which the carbon monoxide:formate oxidoreductase (CFOR) has been introduced, under a CO condition. 
         FIG. 7  depicts graphs showing the results of analyzing the CO gas-to-formate bioconversion performance of a  T. onnurineus  BCF12 strain through a resting cell experiment. 
         FIG. 8  is a photograph showing the results of PAGE of the carbon monoxide:formate oxidoreductase (CFOR) synthesized by the  T. onnurineus  BCF12 strain. 
         FIG. 9  is a graph showing the results of analyzing the CO gas-to-formate bioconversion enzyme activity of the carbon monoxide:formate oxidoreductase (CFOR) in different types of buffer. 
         FIG. 10  is a graph showing the results of operating a bioreactor for the mutant strain  T. onnurineus  BCF12 strain into which the carbon monoxide:formate oxidoreductase (CFOR) has been introduced. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the present invention will be described in more detail with reference to examples. These examples serve merely to illustrate the present invention, and thus the scope of the present invention is not construed as being limited by these examples. 
     EXAMPLE 1 
     Cloning of Carbon Monoxide:Formate Oxidoreductase Fusion Protein 
     As an expression vector for a carbon monoxide:formate oxidoreductase (CFOR), pNA1comFosC1096 derived from the fosmid vector pCC1FOS was used. 
     pNA1comFosC1096 was constructed from pCC1FOS to have a 1-kb flanking region so that an insert DNA could be inserted between TON_1126 and TON_1127 of  Thermococcus onnurineus  NA1 used as a host cell. 
     The insert DNA was inserted together with the strong promoter P 0157  promoter and HMG-CoA reductase so as to have resistance to simvastatin. 
     The fdh3 region (TON_0539-0541) and codh region (TON_1017-1020) of  Thermococcus onnurineus  NA1 were amplified by PCR, and pFd3CoL1C1118, pFd3CoL2C1119 and pFd3CoL1C1120 recombinant plasmids were constructed so that the Fe—S proteins TON_0541 and TON_1017 were fused with each other by each of the linkers (GGGGS) 1  (SEQ ID NO: 1), (GGGGS) 2  (SEQ ID NO: 2) and (GGGGS) 3  (SEQ ID NO: 3). The recombinant plasmid pFd3NHisCoL1C1128 had the same structure as that of pFd3CoL1C1118, and His 6 -tag was inserted into the N-terminus of Fdh3 so that isolation was possible by His-tag affinity chromatography. 
     The recombinant plasmid pFd3NHisCoL1C1132 was constructed to simplify the structure of CFOR and increase the efficiency of conversion of CO into formate. TON_0540 and TON_0541 of the fdh3 region are all Fe—S proteins that are subunits involved in electron transport. While the above-described plasmid was constructed so that TON_0541 and TON_1017 were fused to each other by the linker, pFd3NHisCoL1C1132 was constructed so that TON_0540 and TON_1017 were linked directly to each other. In addition, His 6 -tag was inserted into the N-terminus of Fdh3 so that isolation of the CFOR protein was possible. 
     For construction of mutant strain D04, the fdh3 gene region was removed from the genome. To this end, the recombinant plasmid pldFdh3clusterA derived from a pUC118 vector was constructed. pldFdh3clusterA had the simvastatin-resistance gene HMG-CoA reductase as a selection marker, and a 1-kb region flanking the gene fdh3 region (TON_0539-0542) to be removed was inserted into each of the right arm (RA) and the left arm (LA). The primer sequences inserted into each recombinant plasmid are shown in Table 3 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Identifier 
                 Sequence 
                 Remarks 
               
               
                   
                   
               
             
            
               
                   
                 priFCfosFdh3F 
                 TAAAATGCTT 
                 Forward primer 
               
               
                   
                   
                 GGGAGATGAC 
                 used for PCR 
               
               
                   
                   
                 CTAGGATGGC 
                 of fdh3 of 
               
               
                   
                   
                 ACAGAATAAT 
                 BCF01, BCF02, 
               
               
                   
                   
                 TCACTCG 
                 BCF03 orBCFI2 
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 18) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosFdh3R1 
                 GGCATGCTGC 
                 Linker-containing 
               
               
                   
                   
                 CTCCGCCGCC 
                 reverse primer 
               
               
                   
                   
                 CCCCAGGTAA 
                 used for PCR 
               
               
                   
                   
                 GCCTCATATT 
                 offdh3 of BCP01 
               
               
                   
                   
                 TG 
                   
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 19) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosFdh3R2 
                 GCTGCCTCCG 
                 Linker-containing 
               
               
                   
                   
                 CCGCCGCTTC 
                 reverse primer 
               
               
                   
                   
                 CGCCTCCTCC 
                 used for PCR 
               
               
                   
                   
                 CCCCAGGTAA 
                 of fdh3 of BCF02 
               
               
                   
                   
                 GCCTCATATT 
                   
               
               
                   
                   
                 TG 
                   
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 20) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosFdh3R3 
                 GCTGGGCTGC 
                 Linker-containing 
               
               
                   
                   
                 CTCCGCCGCC 
                 reverse primer 
               
               
                   
                   
                 CCCGAAGTAG 
                 used for PCR 
               
               
                   
                   
                 GCGAGCG 
                 of fdh3 of BCF12 
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 21) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosCodhFI 
                 TGGGGGGCGG 
                 Linker-containing 
               
               
                   
                   
                 CGGAGGCAGC 
                 forward primer 
               
               
                   
                   
                 ATGCCAGCTT 
                 used for PCR of 
               
               
                   
                   
                 TTTCCGGTTC 
                 codh of BCFOI 
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 22) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosCodhF2 
                 GGCGGCGGAG 
                 Linker-containing 
               
               
                   
                   
                 GCAGCGGAGG 
                 forward primer 
               
               
                   
                   
                 AGGCGGAAGC 
                 used for PCR of 
               
               
                   
                   
                 ATGCCAGCTT 
                 codh of BCF02 
               
               
                   
                   
                 TTTCCGGTTC 
                   
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 23) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosCodhF3 
                 TCGGGGGCGG 
                 Linker-containing 
               
               
                   
                   
                 CGGAGGCAGC 
                 forward primer 
               
               
                   
                   
                 CCAGCTTTTT 
                 used for PCR of 
               
               
                   
                   
                 CCGGTTCC 
                 codh of BCF12 
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 24) 
                   
               
               
                   
                   
               
               
                   
                 priFCfosCodhR 
                 TGGCCATCGT 
                 Reverse primer 
               
               
                   
                   
                 TGACGCCACG 
                 used for PCR 
               
               
                   
                   
                 CATGCGACGT 
                 of codh of 
               
               
                   
                   
                 CTCACCTCCT 
                 BCF01, BCF02, 
               
               
                   
                   
                 GAGTTTAAAC 
                 BCF03 or BCF12 
               
               
                   
                   
                 CTCAT 
                   
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 25) 
                   
               
               
                   
                   
               
               
                   
                 priFd3NhisCoLaR 
                 TCCTCGTGAT 
                 Left arm (LA) 
               
               
                   
                   
                 GGTGGTGATG 
                 reverse primer 
               
               
                   
                   
                 GTGCATCCGC 
                 used for insert 
               
               
                   
                   
                 ACCACCGCCC 
                 DNA PCR of BCF09 
               
               
                   
                   
                 T 
                 and containing 
               
               
                   
                   
                 (SEQ ID 
                 his-tag at 
               
               
                   
                   
                 NO: 26) 
                 N-terminus of 
               
               
                   
                   
                   
                 Fdh3 
               
               
                   
                   
               
               
                   
                 priFd3NhisCoRaF 
                 GGATGCACCA 
                 Right arm (RA) 
               
               
                   
                   
                 TCACCACCAT 
                 reverse primer 
               
               
                   
                   
                 CACGAGGAGT 
                 used for insert 
               
               
                   
                   
                 TTAAGATTGG 
                 DNA PCR of BCF09 
               
               
                   
                   
                 CCTG 
                 and containing 
               
               
                   
                   
                 (SEQ ID 
                 his-tag at 
               
               
                   
                   
                 NO: 27) 
                 N-terminus of 
               
               
                   
                   
                   
                 Fdh3 
               
               
                   
                   
               
               
                   
                 priFd3ChisCoLaR 
                 CTTCAGTGAT 
                 Left arm (LA) 
               
               
                   
                   
                 GGTGGTGATG 
                 reverse primer 
               
               
                   
                   
                 GTGGCACCCC 
                 containing his- 
               
               
                   
                   
                 CCAATCTTCT 
                 tag at C-terminus 
               
               
                   
                   
                 C(SEQ ID 
                 of Fdh3 
               
               
                   
                   
                 NO: 28) 
                   
               
               
                   
                   
               
               
                   
                 priFd3ChisCoRaF 
                 GGTGCCACCA 
                 Right arm (RA) 
               
               
                   
                   
                 TCACCACCAT 
                 reverse primer 
               
               
                   
                   
                 CACTGAAGAT 
                 containing 
               
               
                   
                   
                 GGAGAAAAAG 
                 his-tag at 
               
               
                   
                   
                 CTGTTC 
                 C-terminus 
               
               
                   
                   
                 (SEQ ID 
                 of Fdli3 
               
               
                   
                   
                 NO: 29) 
                   
               
               
                   
                   
               
               
                   
                 priFdSCoNhisLaR 
                 CCGGCGTGAT 
                 Left arm (LA) 
               
               
                   
                   
                 GGTGGTGATG 
                 reverse primer 
               
               
                   
                   
                 GTGCATTTTC 
                 containing his- 
               
               
                   
                   
                 ACCACCTCAA 
                 tag at N-terminus 
               
               
                   
                   
                 TACCAC 
                 of Codh 
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 30) 
                   
               
               
                   
                   
               
               
                   
                 priFd3CoNhisRaF 
                 AAATGCACCA 
                 Right arm (RA) 
               
               
                   
                   
                 TCACCACCAT 
                 reverse primer 
               
               
                   
                   
                 CACGCCGGAA 
                 containing his- 
               
               
                   
                   
                 AGAAGGTTCC 
                 tag at 
               
               
                   
                   
                 C 
                 N-terminus of 
               
               
                   
                   
                 (SEQ ID 
                 Codh 
               
               
                   
                   
                 NO: 31) 
                   
               
               
                   
                   
               
               
                   
                 priFd3CoChisLaR 
                 TATTAGTGAT 
                 Left arm (LA) 
               
               
                   
                   
                 GGTGGTGATG 
                 reverse primer 
               
               
                   
                   
                 GTGGATGGGC 
                 used for insert 
               
               
                   
                   
                 CATCCAAGTT 
                 DNA PCR of BCF07 
               
               
                   
                   
                 TTTTC 
                 and containing 
               
               
                   
                   
                 (SEQ ID 
                 his-tag at 
               
               
                   
                   
                 NO: 32) 
                 C-terminus of 
               
               
                   
                   
                   
                 Codh 
               
               
                   
                   
               
               
                   
                 priFd3CoChisRaF 
                 CCATCCACCA 
                 Right (RA) 
               
               
                   
                   
                 TCACCACCAT 
                 reverse primer 
               
               
                   
                   
                 CACTAATAGT 
                 used for insert 
               
               
                   
                   
                 TTCTATTATT 
                 DNA PCR of BCF07 
               
               
                   
                   
                 TTAACTTTG 
                 and containing 
               
               
                   
                   
                 (SEQ ID 
                 his-tag at 
               
               
                   
                   
                 NO: 33) 
                 C-terminus 
               
               
                   
                   
                   
                 of Codh 
               
               
                   
                   
               
            
           
         
       
     
     EXAMPLE 2 
     Construction of Mutant Strain 
     Mutant strain DO1 of  Thermococcus onnurineus  is a mutant strain constructed by removing fdh2C (TON1563-1564) and fdh3 (TON_0539) from the genome. Mutant strain D02 was constructed using DO1 as a parent strain by removing fdh1C (TON_0280-0281) from the genome, and mutant strain D04 was constructed using D02 as a parent strain by removing fdh3C (TON_0539-0542). 
     For transformation,  Thermococcus onnurineus  was pre-cultured in modified medium 1 (MM1) containing maltodextrin to obtain a culture of  Thermococcus onnurineus.  The culture was resuspended in 0.8×Artificial Sea Water (ASW), and then 5 μg of the recombinant plasmid of Example 1 was added thereto and introduced into the cells by heat shock at 80° C. Thereafter, a small amount of medium was added to the cells which were then stabilized at 80° C. for 2 hours. The stabilized transformed cells were inoculated and cultured in a medium containing 10 μM simvastatin, and passaged twice so as to be sufficiently enriched. Thereafter, a single colony was obtained and the genotype thereof was analyzed through PCR. 
     The mutant strains BCF01, BCF02 and BCF03 were constructed using D02 as a parent strain so that the fdh3 region (TON_0538-0541) and the codh region (TON_1017-1020) were fused with each other by each of the linkers (GGGGS) 1 (SEQ ID NO: 1), (GGGGS) 2 (SEQ ID NO: 2) and (GGGGS) 3 (SEQ ID NO: 3). 
     The mutant strains BCF01, BCF02 and BCF03 were transformed with pFd3CoL1C1118, pFd3CoL2C1119 and pFd3CoL3C1120 recombinant plasmids, respectively, and the carbon monoxide:formate oxidoreductase (CFOR) introduced into each of the recombinant plasmids was inserted between TON_1126 and TON_1127 of  Thermococcus onnurineus.    
     In addition, the mutant strain BCF09 was constructed using D02 as a parent strain and transformed with a pFd3NHisCoL1C1128 recombinant plasmid, and the same CFOR protein as in BCF01 was introduced therein. In addition, His 6 -tag was added to the N-terminus of Fdh3. 
     In addition, the mutant strain BCF12 was constructed using D04 as a parent strain and transformed with a pFd3NHisCoL1C1132 recombinant plasmid so that the fdh3 region (TON_0538-0540) and the codh region (TON_1017-1020) were fused with each other by the linker (GGGGS) 1 (SEQ ID NO: 1). Also, His 6 -tag was inserted into the N-terminus of Fdh3 (TON_0539). 
     The kinds of strains and plasmids used in the experiment are summarized in Table 4 below. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Table 1. Strains and fosmids used this study. 
               
            
           
           
               
               
               
            
               
                 Strains or 
                   
                   
               
               
                 Plasmids 
                 Description 
                 Reference 
               
               
                   
               
               
                 Strains 
                   
                   
               
               
                   E. coli  DH5a 
                 Cloning host 
                 TaKaRa 
               
               
                 
                   T. onnurineus 
                 
               
               
                 NA1 
                 Wild-type strain 
                 Previous study 
               
               
                 D01 
                 NA1 derivative, Δfdh2C ΔTON_0539 
                 Previous study 
               
               
                 D02 
                 D01 derivative, Δfdh1C 
                 Previous study 
               
               
                 D04 
                 D02 derivative, Δfdh3C 
                 This study 
               
               
                 BCF01 
                 D02 derivative, P 0157 hmg pfu ::TON_0538-TON_0541:TON_1017- 
                 This study 
               
               
                   
                 TON_1020; fusion of TON_0541 and TON_1017 with linker 
               
               
                   
                 (GGGGS) 1  (SEQ ID NO: 1) 
               
               
                 BCF02 
                 D02 derivative, P 0157 hmg pfu ::TON_0538-TON_0541:TON_1017- 
                 This study 
               
               
                   
                 TON_1020; fusion of TON_0541 and TON_1017 with linker 
               
               
                   
                 (GGGGS) 2  (SEQ ID NO: 2) 
               
               
                 BCF03 
                 D02 derivative, P 0157 hmg pfu ::TON_0538-TON_0541:TON_1017- 
                 This study 
               
               
                   
                 TON_1020; fusion of TON_0541 and TON_1017 with linker 
               
               
                   
                 (GGGGS) 3  (SEQ ID NO: 3) 
               
               
                 BCF09 
                 D02 derivative, P 0157 hmg pfu ::TON_0538-TON_0541:TON_1017- 
                 This study 
               
               
                   
                 TON_1020; fusion of TON_0541 and TON_1017 with linker 
               
               
                   
                 (GGGGS) 1  (SEQ ID NO: 1); His 6 -tag inserted in N-terminus of 
               
               
                   
                 Fdh3 (TON_0539) 
               
               
                 BCF12 
                 D04 derivative, P 0157 hmgpfu::TON_0538-TON_0540:TON_1017- 
                 This study 
               
               
                   
                 TON_1020; fusion of TON_0540 and TON_1017 with linker 
               
               
                   
                 (GGGGS) 1  (SEQ ID NO: 1); His 6 -tag inserted in N-terminus of 
               
               
                   
                 Fdh3 (TON_0539) 
               
               
                 Fosmids 
               
               
                 pCC1FOS 
                 Backbone fosmid; Cm r   
                 EPICENTRE 
               
               
                 pNA1comFosC1096 
                 pCC1FOS carrying P 0157  promotor, HMG cassette, and 1 kbp Left- 
                 This study 
               
               
                   
                 arm (LA) and Right-arm (RA) for homologous recombination of  T.   
               
               
                   
                   onnurineus  NA1 
               
               
                   
                 genome; backbone fosmid for mutant construction; Sim r   
               
               
                 pFd3CoL1C1118 
                 pNA1comFosC1096 carrying fdh3 region (TON_0538-TON_0541) 
                 This study 
               
               
                   
                 and codh region (TON_1017-TON_1020) from  T. onnurineus  NA1; 
               
               
                   
                 fusion of TON_0541 
               
               
                   
                 and TON_1017 with linker (GGGGS) 1  (SEQ ID NO: 1) 
               
               
                 pFd3CoL2C1119 
                 pFd3CoL1C1118 carrying fusion of TON_0541 and TON_1017 
                 This study 
               
               
                   
                 with linker (GGGGS) 2  (SEQ ID NO: 2) 
               
               
                 pFd3CoL3C1120 
                 pFd3CoL1C1118 carrying fusion of TON_0541 and TON_1017 
                 This study 
               
               
                   
                 with linker (GGGGS) 3  (SEQ ID NO: 3) 
               
               
                 pFd3NHisCoL1C1128 
                 pFd3CoL1C1118 carrying fusion of TON_0541 and TON_1017 
                 This study 
               
               
                   
                 with linker (GGGGS) 1  (SEQ ID NO: 1); His 6 -tag inserted in N- 
               
               
                   
                 terminus of Fdh3 (TON_0539) 
               
               
                 pFd3NHisCoL1C1132 
                 pNA1comFosC1096 carrying fdh3 region (TON_0538-TON_0540) 
                 This study 
               
               
                   
                 and codh region (TON_1017-TON_1020) from  T. onnurineus  NA1; 
               
               
                   
                 fusion of TON_0540 
               
               
                   
                 and TON_1017 with linker (GGGGS) 1  (SEQ ID NO: 1); His 6 -tag 
               
               
                   
                 inserted in N-terminus of Fdh3 (TON_0539) 
               
               
                   
               
               
                 * Cm r , chloramphenicol resistance; 
               
               
                 Sim r , simvastatin resistance by HMG-CoA reductase; 
               
               
                 fdh1C, fdh1-mfh1-mnh1 gene cluster (TON_0280-0281); 
               
               
                 fdh2C, fdh2-mfh2-mnh2 gene cluster (TON_1563-1564); 
               
               
                 fdh3C, fdh3 gene cluster (TON_0539-0542); 
               
               
                 LA (left-arm) and RA (right-arm), 1 kbp DNA flanking region suitable for homologous recombination 
               
            
           
         
       
     
     EXAMPLE 3 
     Culture of Transformant and Measurement of Formate Production 
     The transformant produced in Example 2 was cultured in modified medium 1 containing 4 g/L yeast extract, 35 g/L NaCl, 0.7 g/L KCl, 3.9 g/L MgSO 4 , 0.4 g/L CaCl 2 H 2 O, 0.3 g/L NH 4 Cl, 0.15 g/L Na 2 HPO 4 , 0.03 g/L NaSiO 3 , 0.5 g/L NaHCO 3 , 0.5 g/L cysteine-HCl, 1 ml/L Holden&#39;s trace element, 2 ml/L Fe-EDTA solution, 1 ml/L Balch&#39;s vitamin solution, and 0.05 g/L Na 2 S.9H 2 O. The Fe-EDTA solution contained 1.54 g/L FeSO 4 .9H 2 O and 2.06 g/L Na 2 ·EDTA. The prepared medium was sterilized and then stored in an anaerobic chamber under anaerobic conditions. Each of the mutant strains D02, D04, BCF01, BCF02, BCF03 and BCF12 was cultured at 80° C. in a 160-ml serum vial containing 80 ml of medium and a head space filled with CO at 3 bar. 
     To measure the growth curve of each strain, the optical density was measured using a UV-Vis spectrophotometer (Shimadzu, UV-2600). The concentration of formate was analyzed using high-performance liquid chromatography (YL instrument, YL9100) with an ion exclusion chromatography column (Shodex, RSpak, KC-811) and measured using a UV detector. As a mobile phase, a 0.1% phosphoric acid aqueous solution was used. To analyze the gas composition of the final head space, gas chromatography (YL instrument, YL6100) with a Molsieve 5A column (Supelco, Bellefonte, Pa.) and a Porapack N column (Supelco) was used, and argon gas was used as a mobile phase. 
     As a result, referring to  FIG. 6 , it was observed that the peak growth of the transformant including the recombinant plasmid introduced therein was inhibited at a relatively low level compared to the host strain  Thermococcus onnurineus  D02 strain ((A) of  FIG. 6 ). In addition, it was confirmed that there was no formate production in the host strain  Thermococcus onnurineus  D02 strain, whereas formate was produced in all the BCF01, BCF02, BCF03 and BCF12 strains into which the recombinant plasmid was introduced ((B) of  FIG. 6 ). From these results, it could be confirmed that electron transport was possible in the carbon monoxide:formate oxidoreductase (CFOR) of the present invention, synthesized to include the Fe—S fusion protein obtained by covalently linking two or more Fe—S proteins together through one to three ‘GGGGS (SEQ ID NO: 1)’ flexible linkers. In addition, it was confirmed that formate production in the BCF12 strain constructed using the (GGGGS) 1 (SEQ ID NO: 1) flexible linker was the highest (19 mM concentration). 
     EXAMPLE 4 
     Analysis of CO Gas-to-Formate Bioconversion Performance through Resting Cell Experiment 
     A cell suspension to be used in a resting cell experiment was subjected to 5-L cell culture in a bioreactor. CO was continuously supplied, and the cells were harvested at an OD of 0.9 and centrifuged at 6,000 rpm for 30 minutes to isolate and harvest only the cells. A wash step of washing the obtained cells with an MM1 base (excluding yeast extract) free of nutrient components to remove components other than the cells was repeated three times. Finally, a resting cell experiment was performed using the cells with OD 600  of 0.5, suspended in an MM1 base. 
     In the resting cell experiment, 6 ml of the cell suspension with an OD 600  of 0.5 was placed in a 20-ml serum vial and then sealed, and the headspace was filled with 100% CO gas at a pressure of 2 bar, and then culture at 80° C. was performed. Formate production, CO consumption and hydrogen production were analyzed over time. 
     As a result, it could be confirmed that formate production continuously increased up to 48 hours, and CO gas consumption and hydrogen production were continuously maintained. As a result of stoichiometry at a time point of 48 hours, it could be confirmed that about 10% of CO consumption was converted to formate and about 90% was converted to bio-hydrogen. 
     EXAMPLE 5 
     Isolation and Purification of Protein 
     All protein isolation and purification procedures were performed under anaerobic conditions. In order to determine at protein level whether CO was converted to formate, the isolation of the corresponding fusion protein from the nadFd3CoHisL1C1127 strain that is the CODH C-terminal his-tag strain including the carbon monoxide:formate oxidoreductase (CFOR) was performed using an affinity column purification method. The mutant strain nadFd3CoHisL1C1127 strain including the fusion protein was cultured 350 ml of MMC (bis-Tris pH 6.5) medium and inoculated into a bioreactor. 
     5-L cell culture was performed with the bioreactor. CO was continuously supplied, and the cells were harvested at an OD of 0.9 and centrifuged at 6,000 rpm for 30 minutes to harvest only the cells separately. The obtained cells were suspended well in talon buffer [50 mM Tris-HCl (pH 8.0), 0.1 M KCl, 10% glycerol], and then uniformly disrupted using a sonicator. Then, the carbon monoxide:formate oxidoreductase (CFOR) was isolated using a Talon affinity column. 
     Protein was isolated from talon resin using a talon buffer containing 300 mM imidazole, and the protein concentration was quantified by Bradford assay. The isolated and purified protein was analyzed by 12% SDS-PAGE. 
     As a result, it could be observed that CO dehydrogenase (CODH) and formate dehydrogenase (Fdh) were isolated together with the TON_0540-TON_1017 protein which is the carbon monoxide:formate oxidoreductase (CFOR). From this result, it could be confirmed that CO dehydrogenase (CODH) and formate dehydrogenase (FDH3) were linked together by the Fe—S fusion protein to form a single new fusion protein (see  FIG. 8 ). 
     EXAMPLE 6 
     Measurement of Enzymatic Activities 
     The CODH enzyme activity, Fdh enzyme activity and CO gas-to-formate conversion ability of the carbon monoxide:formate oxidoreductase (CFOR) isolated in Example 5 were measured, and all experiments for measurement of the protein activities were measured under anaerobic conditions. 
     The CODH enzyme activity was measured using the methyl viologen method that quantifies the concentration of methyl viologen (MV) reduced when CO is given as an electron donor. 
     For activity measurement, 2 mM DTT, 10 mM MV and 0.5 μg CFOR protein were added to 1 ml of 50 mM Tris-HCl (pH 8.0) buffer in a cuvette sealed with a screw-cap, and then the cuvette was sealed by closing the lid. Then, the headspace was purged with CO gas and finally filled with CO gas at 1 bar, thus preparing a reaction. 
     The cuvette containing the mixture solution was placed on a heat block at 80° C., and the reaction was performed for 1 minute. Then, the reaction was terminated by placing the cuvette on ice, and the absorbance at a wavelength of 578 nm was measured using a spectrophotometer. 
     Measurement of the activity of formate dehydrogenase (FDH) was performed in the same manner as measurement of the CODH enzyme activity, except that 50 mM potassium phosphate (pH 7.6) buffer was used, formate was used instead of CO as an electron donor, and the enzymatic reaction was performed on a heat block at 80° C. for 5 minutes. 
     The activity of each of the CODH enzyme and the Fdh enzyme was calculated as the amount of enzyme catalyzing the reduction of 2 mmol methyl viologen, which is equivalent to the amount that catalyzes the oxidation of 1 mmol CO or formate. At this time, the extinction coefficient value is ε 578 =9.7 mM −1 ·cm −1 . 
     For measurement of the CO conversion/formate production activity of the isolated/purified fusion protein, 2 ml of a mixture solution obtained by adding the isolated fusion protein to each of five types of buffer (50 mM Bis-Tris pH 6.5, 150 mM HEPES pH 7.5, 50 mM potassium phosphate pH 7.6, 100 mM Tris pH 8.0, and 200 mM Bicine-KOH pH 8.5) at a final concentration of 100 μg/ml was tested in a 25-ml serum vial. A CO-CO 2  mixture gas (CO:CO 2 =53.5:46.5, vol./vol.) was injected into a 23-ml head space, and then reacted by incubation at 80° C. After 5 hours of the reaction, the concentration of formate produced was measured by LC. 
     As a result of measuring formate, formate production was found in all the buffer conditions, and the highest formate concentration (about 5 mmol/L) was measured in , 150 mM HEPES (pH 7.5). It was finally confirmed that formate was produced only from CO and CO 2  gases under in vitro conditions (see  FIG. 9 ). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Specific activities 
               
               
                   
                 Species 
                 (mmol min −1  mg −1 ) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 CO dehydrogenase 
                 1.8 
               
               
                   
                 Formate dehydrogenase 
                 120 
               
               
                   
                 Formate production 
                 1.67 × 10 −4   
               
               
                   
                   
               
            
           
         
       
     
     From the above-described results, it could be finally confirmed that a new type of fusion protein consisting of a complex of CO dehydrogenase (CODH) and formate dehydrogenase (FDH) linked to each other through the Fe—S fusion protein acting as an electron transport chain was constructed, and the function of converting CO 2  to formate by the enzymatic reaction induced by electron transfer through the Fe—S proteins was actually achieved. 
     EXAMPLE 7 
     Measurement of Formate Production Activity 
     Cells were inoculated into 1.5 L of MMC medium and then purged with 100% CO gas, and a batch culture bioreactor was operated at 80° C., and cell culture was performed under anaerobic conditions. A check valve was provided in a gas outlet to pressurize and regulate the gas pressure in the reactor. 
     As a result of operating the bioreactor for 9 hours, it was confirmed that formate production was 150 mmol/L and specific formate production rate was 22 mmol/g-DCW/hr (see  FIG. 10 ). 
     Although the present invention has been described in detail with reference to the specific features, it will be apparent to those skilled in the art that this description is only of a preferred embodiment thereof, and does not limit the scope of the present invention. Thus, the substantial scope of the present invention will be defined by the appended claims and equivalents thereto. 
       Thermococcus  strain BCF12 strain was deposited in the Korea Research Institute of Bioscience and Biotechnology (having the address of 181, Ipsin-gil, Jeongeup-si, Jeolllabuk-do 56212, Republic of Korea) under the Access number of KCTC 13649BP on Sep. 21, 2018. The deposit has been made under the terms of the Budapest Treaty and all restrictions imposed by the depositor on the availability to the public of the biological material will be irrevocably removed upon the granting of a patent. 
     A sequence listing electronically submitted with the present application on Oct. 6, 2021 as an ASCII text file named 20211006_Q49021YGO1_TU_SEQ, created on Oct. 6, 2021 and having a size of 47,000 bytes, is incorporated herein by reference in its entirety.