Patent Publication Number: US-2019194710-A1

Title: Method for Producing Recombinant Protein

Description:
TECHNICAL FIELD 
     The present invention relates to a method for production of a recombinant protein on an industrial scale using microorganisms. 
     BACKGROUND ART 
     Production of recombinant proteins on an industrial scale is carried out using methods such as fed batch culture, semi-continuous culture and continuous culture. 
     Fed batch culture is also known as semi-batch culture. Batch culture is a culturing method in which all of the substrates necessary for culturing (the nutrients, medium components, etc.) are added to the culture medium at the start of culturing. Fed batch culture, on the other hand, is a culturing method in which the culturing is initiated in culture medium prepared with a suitable substrate concentration, and supplemental culturing is then carried out with sequential addition of each of the consumed substrates. Batch culturing and fed batch culturing are both culturing methods in which the culture solution is not removed from the culturing tank (bioreactor) until culturing is complete. For culturing systems where growth is inhibited in the presence of a high substrate concentration, fed batch culture can avoid growth inhibition by regulating the substrate concentration to an appropriately low concentration. Fed batch culture often makes it possible to achieve high cell density, allowing production of recombinant proteins at high concentration. Known methods for adding feed substrates include a method of sequential addition of the feed substrate in separate portions, a constant flow addition method in which the feed substrate is added at a constant flow rate (constant speed feeding), and an exponential feed method in which the flow rate of the feed substrate is exponentially increased. 
     Continuous culture is a method in which feeding of fresh medium and discharge of culture solution are carried out continuously while maintaining a constant culture solution volume, so that culturing is in a steady state without time-related change in the medium composition of the culture solution. The cell density in continuous culturing is general low compared to the cell density in fed batch culturing. Continuous culture, however, allows continuous production of recombinant proteins. 
     Semi-continuous culture is a method in which, after having obtained a specific volume or biomass by batch culture or fed batch culture, a portion of the culture solution including the recombinant protein is removed from the bioreactor while fresh culture medium is added to the bioreactor to continue production of the recombinant protein, and the process is repeated (Patent Literature 1). 
     For structural proteins such as resilin and elastin that have very high elasticity and repulsion elasticity while also having rubber-like properties, keratin, as one of the major proteins in cashmere and wool, collagen which serves to provide dynamic strength to various connective tissues, silkworm silk which is light, strong and has characteristic gloss, and spider silk which has excellent strength and ductility, methods are being devised to produce such structural proteins as recombinant proteins on an industrial scale, but many issues currently remain in regard to their implementation. Even in the case of spider silk proteins, which are being avidly researched, the production levels are low and expression of natural spider silk proteins in bacterial hosts is considered to be inefficient (Non Patent Literature 1 and Patent Literature 2). 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] Japanese Patent Public Inspection No. 2010-527239 
         [Patent Literature 2] International Patent Application Publication No. WO2015/042164 
       
    
     Non Patent Literature 
     
         
         [Non Patent Literature 1] Appl Microbiol Biotechnol., 1998, 49(1), pp. 31-38. 
       
    
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     It is an object of the present invention to provide a method for efficiently and stably producing recombinant proteins on an industrial scale. 
     Means for Solving the Problems 
     During the course of researching methods for producing recombinant proteins on an industrial scale by continuous culturing with transformed microorganisms, the present inventors have found that by not reusing the microorganisms whose expression has been already induced, it is possible to efficiently and stably achieve high production of desired recombinant proteins, and the invention has been completed upon this finding. 
     Specifically, the invention relates to the following respective inventions. 
     [1] A method for production of a recombinant protein using recombinant cells that express the recombinant protein under the control of an inducible promoter, 
     the method including continuous culturing with addition of fresh culture medium to a portion of the culture solution in which the recombinant cells have been grown, 
     wherein the recombinant cells whose expression of the recombinant protein has been induced are not reused. 
     [2] The method for production of a recombinant protein according to [1], wherein growth of the recombinant cells and induction of expression of the recombinant protein are carried out in different culturing tanks. 
     [3] The method for production of a recombinant protein according to [1] or [2], wherein growth of the recombinant cells is carried out by fed batch culture. 
     [4] A method for production of a recombinant protein using recombinant cells that express the recombinant protein under the control of an inducible promoter, 
     the method including repeating the following steps (A) to (E). 
     (A) A step of growing the recombinant cells in a culturing tank by batch culture or fed batch culture. 
     (B) A step of transferring a portion of the culture solution in the culturing tank to a receiving culturing tank, after the growth in step (A). 
     (C) A step of adding fresh culture medium to either of the two culturing tanks after the transfer in step (B), and advancing to step (A). 
     (D) A step of inducing expression of the recombinant protein in the other culturing tank that has not advanced to step (A) in step (C), and accumulating the recombinant protein. 
     (E) A step of separating and purifying the recombinant protein that has been accumulated in step (D) from the culture solution. 
     [5] The method for production of a recombinant protein according to any one of [1] to [4], wherein induction of expression of the recombinant protein is initiated when growth of the recombinant cells has reached the metaphase of the logarithmic growth stage. 
     [6] The method for production of a recombinant protein according to any one of [1] to [5], wherein induction of expression of the recombinant protein is carried out by adding an expression inducing agent to the culture solution or by varying the temperature of the culture solution. 
     [7] The method for production of a recombinant protein according to [4], wherein the amount of culture solution transferred to the receiving culturing tank in step (B) is 80 to 99 vol % based on the total amount of the culture solution. 
     [8] The method for production of a recombinant protein according to any one of [1] to [7], wherein the recombinant cells are cells in which a gene coding for the recombinant protein has been chromosomally integrated. 
     [9] The method for production of a recombinant protein according to any one of [1] to [8], wherein the recombinant protein is a structural protein. 
     [10] The method for production of a recombinant protein according to [9], wherein the structural protein is a protein derived from a protein selected from the group consisting of keratin, collagen, elastin, resilin, silkworm silk and spider silk. 
     [11] The method for production of a recombinant protein according to any one of [1] to [10], wherein the inducible promoter is an IPTG-inducible promoter selected from among T7 promoter, tac promoter, trc promoter, lac promoter and lacUV5 promoter, or a temperature-inducible promoter selected from among PR promoter and PL promoter. 
     Effects of the Invention 
     According to the invention it is possible to efficiently produce recombinant proteins on an industrial scale. 
     In expression-induced production of a recombinant protein by continuous culturing using a plasmid-introduced plasmid-type expressing strain, the issue of low productivity has been faced when using prior art technology, due to plasmid shedding, structural destabilization and reduced expression levels. According to the invention, an effect is further exhibited which allows efficient production of a recombinant protein in a stable manner without plasmid shedding, even with continuous culturing of a plasmid-type expressing strain, on a level at least comparable to a chromosomally integrated expressing strain that has the target protein integrated into its chromosomes. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically showing a culturing system to be used in the method for production of a recombinant protein according to an embodiment of the invention. 
         FIG. 2  is a graph showing production volume of recombinant modified fibroin by repeated fed batch culture in Example 1. 
         FIG. 3  is a graph showing production volume of recombinant modified fibroin by repeated fed batch culture in Comparative Example 1. 
         FIG. 4  is a diagram schematically showing a culturing system to be used in the method for production of a recombinant protein according to the prior art. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Embodiments for carrying out the invention will now be explained in further detail. However, the present invention is not limited to the embodiments described below. 
     [Method for Production of Recombinant Protein] 
     The method for production of a recombinant protein according to this embodiment uses recombinant cells that express a recombinant protein, and it includes continuous culturing with addition of fresh culture medium to a portion of the culture solution in which the recombinant cell have been grown, without reusing the recombinant cells in which expression of the recombinant protein has been induced. The recombinant cells to be used in the production method of this embodiment are preferably recombinant cells expressing a recombinant protein under the control of an inducible promoter. 
     (Recombinant Protein) 
     The recombinant protein to be produced by the production method of this embodiment (hereunder also referred to as “target protein”) may be any protein that is desired to be produced on an industrial scale, and for example, it may be an industrially useful protein, or a medically useful protein, or a structural protein. Specific examples of industrially useful or medically useful proteins include enzymes, regulatory proteins, receptors, peptide hormones, cytokines, membrane or transport proteins, antigens used for vaccination, vaccines, antigen-binding proteins, immunostimulatory proteins, allergens, and full length antibodies or antibody fragments or their derivatives. Specific examples of structural proteins include spider silk, silkworm silk, keratin, collagen, elastin and resilin, as well as proteins derived from them. 
     Examples of spider silk- or silkworm silk-like proteins which are fibroin-like proteins include proteins including a domain sequence represented by formula 1: [(A) n  motif-REP] m . (where (A) n  motif represents an amino acid sequence composed of 4 to 20 amino acid residues, and the number of alanine residues with respect to the total amino acid residues in the (A) n  motif is 80% or greater; REP represents an amino acid sequence composed of 10 to 200 amino acid residues; m represents an integer of 8 to 300; multiple (A) n  motifs may be identical amino acid sequences or different amino acid sequences; and multiple REP sequences may be identical amino acid sequences or different amino acid sequences). Specifically, these may be proteins including the amino acid sequence listed as SEQ ID NO: 1. 
     Examples of collagen-derived proteins include proteins including a domain sequence represented by formula 2: [REP2] o  (where o represents an integer of 5 to 300; REP2 represents an amino acid sequence comprising Gly-X-Y, with X and Y representing any amino acid residues other than Gly; and multiple REP2 sequences may be identical amino acid sequences or different amino acid sequences). Specifically, these may be proteins including the amino acid sequence listed as SEQ ID NO: 2. The amino acid sequence listed as SEQ ID NO: 2 has the amino acid sequence listed as SEQ ID NO: 6 (tag sequence and hinge sequence) added to the N-terminus of the amino acid sequence from the 301st residue to the 540th residue corresponding to the repeat portion and motif of the partial sequence of human collagen type 4 acquired from the NCBI database (NCBI Genbank Accession No.: CAA56335.1, GI:3702452). 
     Examples of resilin-derived proteins include proteins including a domain sequence represented by formula 3: [REP3] p  (where p represents an integer of 4 to 300; REP3 represents an amino acid sequence comprising Ser-J-J-Tyr-Gly-U-Pro; J represents any amino acid residue, and most preferably an amino acid residue selected from the group consisting of Asp, Ser and Thr; U represents any amino acid residue, and most preferably an amino acid residue selected from the group consisting of Pro, Ala, Thr and Ser; and multiple REP3 sequences may be identical amino acid sequences or different amino acid sequences). Specifically, these may be proteins including the amino acid sequence listed as SEQ ID NO: 3. The amino acid sequence listed as SEQ ID NO: 3 has the amino acid sequence listed as SEQ ID NO: 6 (tag sequence and hinge sequence) added to the N-terminus of the amino acid sequence from the 19th residue to the 321st residue of the amino acid sequence of resilin (NCBI Genbank Accession No. NP 611157, G1:24654243) wherein the 87th residue Thr is replaced by Ser, and the 95th residue Asn is replaced by Asp. 
     Examples of elastin-derived proteins include proteins having the amino acid sequences of NCBI Genbank Accession No. AAC98395 (human), 147076 (sheep) and NP786966 (cow). Specifically, these may be proteins including the amino acid sequence listed as SEQ ID NO: 4. The amino acid sequence listed as SEQ ID NO: 4 has the amino acid sequence listed as SEQ ID NO: 6 (tag sequence and hinge sequence) added to the N-terminus of the amino acid sequence from the 121st residue to the 390th residue of the amino acid sequence of NCBI Genbank Accession No. AAC98395. 
     Examples of keratin-derived proteins include  Capra hircus  type I keratin. Specifically, these may be proteins including the amino acid sequence listed as SEQ ID NO: 5 (the amino acid sequence of NCBI Genbank Accession No. ACY30466). 
     (Recombinant Cells Expressing Recombinant Protein) 
     The recombinant cells of this embodiment can be obtained, for example, by transforming a host with an expression vector having a nucleic acid sequence coding for the target protein and one or more regulatory sequences linked to the nucleic acid sequence in a functional manner. 
     The regulatory sequence is a sequence that regulates expression of the recombinant protein in the host (for example, a promoter, enhancer, ribosome binding sequence or transcription termination sequence), and it may be selected as appropriate depending on the type of host. The type of expression vector may be appropriately selected depending on the type of host, such as a plasmid vector, viral vector, cosmid vector, phasmid vector or artificial chromosome vector. 
     The host used may be any prokaryote, or any eukaryote such as yeast, filamentous fungi, insect cells, animal cells or plant cells. Preferred examples of prokaryotes include  E. coli, Bacillus subtilis, Pseudomonas, Corynebacterium  and  Lactococcus , with  E. coli  cells being more preferred. 
     The expression vector used may be any one capable of autoreplication in the host cells, or capable of integrating into the host chromosomes, and containing an inducible promoter at a location that allows transcription of nucleic acid coding for the target protein. 
     The inducible promoter may be an inducible promoter that functions in the host cells and is capable of inducing expression of the target protein. An inducible promoter is a promoter that can regulate transcription based on the presence of an inducer (expression inducing agent), the absence of a repressor molecule, or by physical factors such as increase or reduction in temperature, osmotic pressure or pH value. 
     Specific examples of inducible promoters for prokaryotes as the host include T7 promoter, tac and trc promoter and lac and lacUV5 promoter, which are induced by lactose or its analog IPTG (isopropylthiol-β-D-galactoside); araBAD promoter which is induced by arabinose; trp promoter which is induced by β-indoleacrylic acid addition or tryptophan starvation and inhibited by tryptophan addition; rhaBAD promoter which is induced by rhamnose; xylF promoter and xylA promoter which are induced by xylose; araBAD promoter which is induced by arabinose; λ phage PR promoter and PL promoter which are induced by temperature increase; phoA promoter which is induced by phosphate starvation; and cstA promoter and cstA-lacZ promoter which is induced by glucose starvation. 
     The prokaryote host such as a bacterium may be a microorganism belonging to  Escherichia, Brevibacillus, Serratia, Bacillus, Microbacterium, Brevibacterium, Corynebacterium  or  Pseudomonas.    
     Examples of microorganisms belonging to  Escherichia  include  Escherichia coli  BL21 (Novagen),  Escherichia coli  BL21 (DE3) (Life Technologies Corp.),  Escherichia coli  BLR (DE3) (Merck, Ltd.-Millipore),  Escherichia coli  DH1,  Escherichia coli  G1698 , Escherichia coli  HB101,  Escherichia coli  JM109,  Escherichia coli  K5 (ATCC 23506),  Escherichia coli  KY3276 , Escherichia coli  MC1000 , Escherichia coli  MG1655 (ATCC 47076),  Escherichia coli  No. 49 , Escherichia coli  Rosetta (DE3) (Novagen),  Escherichia coli  TB1 , Escherichia coli  Tuner (Novagen),  Escherichia coli  Tuner (DE3) (Novagen),  Escherichia coli  W1485 , Escherichia coli  W3110 (ATCC 27325),  Escherichia coli  XL1-Blue and  Escherichia coli  XL2-Blue. 
     Compounds of microorganisms belonging to  Brevibacillus  include  Brevibacillus agri, Brevibacillus borstenesis, Brevibacillus centrosporus, Brevibacillus formosus, Brevibacillus invocatus, Brevibacillus laterosporus, Brevibacillus limnophilus, Brevibacillus parabrevis, Brevibacillus reuszeri, Brevibacillus thermoruber, Brevibacillus brevis  47 (FERM BP-1223),  Brevibacillus brevis  47K (FERM BP-2308),  Brevibacillus brevis  47-5 (FERM BP-1664),  Brevibacillus brevis  47-5Q (JCM8975),  Brevibacillus choshinensis  HPD31 (FERM BP-1087),  Brevibacillus choshinensis  HPD31-S (FERM BP-6623),  Brevibacillus choshinensis  HPD31-OK (FERM BP-4573) and  Brevibacillus choshinensis  SP3 (Takara). 
     Examples of microorganisms belonging to  Serratia  include  Serratia  liquefacience ATCC14460 , Serratia entomophila, Serratia ficaria, Serratia fonticola, Serratia grimesii, Serratia proteamaculans, Serratia odorifera, Serratia plymuthica  and  Serratia rubidaea.    
     Examples of microorganisms belonging to  Bacillus  include  Bacillus subtilis  and  Bacillus amyloliquefaciens.    
     Examples of microorganisms belonging to  Microbacterium  include  Microbacterium ammoniaphilum  ATCC15354. 
     Examples of microorganisms belonging to  Brevibacterium  include  Brevibacterium divaricatum  ( Corynebacterium glutamicum ) ATCC14020 , Brevibacterium flavum  ( Corynebacterium glutamicum  ATCC14067) ATCC13826 and ATCC14067 , Brevibacterium immariophilum  ATCC14068 , Brevibacterium lactofermentum  ( Corynebacterium glutamicum  ATCC13869)ATCC13665 and ATCC13869 , Brevibacterium roseum  ATCC13825 , Brevibacterium saccharolyticum  ATCC14066 , Brevibacterium thiogenitalis  ATCC19240 , Brevibacterium  album ATCC15111 and  Brevibacterium cerinum  ATCC15112. 
     Examples of microorganisms belonging to  Corynebacterium  include  Corynebacterium ammoniagenes  ATCC6871 and ATCC6872 , Corynebacterium glutamicum  ATCC13032 , Corynebacterium glutamicum  ATCC14067 , Corynebacterium acetoacidophilum  ATCC13870 , Corynebacterium acetoglutamicum  ATCC15806 , Corynebacterium alkanolyticum  ATCC21511 , Corynebacterium callunae  ATCC15991 , Corynebacterium glutamicum  ATCC13020, ATCC13032 and ATCC13060 , Corynebacterium lilium  ATCC15990 , Corynebacterium melasecola  ATCC17965 , Corynebacterium thermoaminogenes  AJ12340 (FERMBP-1539) and  Corynebacterium herculis  ATCC13868. 
     Examples of microorganisms belonging to  Pseudomonas  include  Pseudomonas putida, Pseudomonas fluorescence, Pseudomonas brassicacearum, Pseudomonas fulva  and  Pseudomonas  sp. D-0110. 
     The method of introducing the expression vector into the host cells may be any method for introducing DNA into the host cells. For example, a method using calcium ion [Proc. Natl. Acad. Sci. USA, 69, 2110(1972)], a protoplast method (Japanese Unexamined Patent Application Publication SHO No. 63-248394) or the methods described in Gene, 17, 107(1982) or Molecular &amp; General Genetics, 168, 111(1979) may be used. 
     Transformation of a microorganism belonging to  Brevibacillus  may be carried out by the method of Takahashi et al. (J. Bacteriol., 1983, 156:1130-1134), the method of Takagi et al. (Agric. Biol. Chem., 1989, 53:3099-3100) or the method of Okamoto et al. (Biosci. Biotechnol. Biochem., 1997, 61:202-203), for example. 
     Examples for the vector for introduction of the nucleic acid coding for the target protein (hereunder referred to simply as “vector”) include pBTrp2, pBTac1 and pBTac2 (all commercially available from Boehringer Mannheim), pKK233-2 (Pharmacia), pSE280 (Invitrogen), pGEMEX-1 (Promega), pQE-8 (QIAGEN), pKYP10 (Japanese Unexamined Patent Application Publication SHO No. 58-110600), pKYP200 [Agric. Biol. Chem., 48, 669(1984)], pLSA1 [Agric. Biol. Chem., 53, 277(1989)], pGEL1 [Proc. Natl. Acad. Sci. USA, 82, 4306(1985)], pBluescript II SK(−) (Stratagene), pTrs30 [prepared from  Escherichia coli  JM109/pTrS30 (FERM BP-5407)], pTrs32 [prepared from  Escherichia coli  JM109/pTrS32 (FERM BP-5408)], pGHA2 [prepared from  Escherichia coli  IGHA2 (FERM B-400), Japanese Unexamined Patent Application Publication SHO No. 60-221091], pGKA2 [prepared from  Escherichia coli  IGKA2 (FERM BP-6798), Japanese Unexamined Patent Application Publication SHO No. 60-221091], pTerm2 (U.S. Pat. Nos. 4,686,191, 4,939,094, U55160735), pSupex, pUB110, pTP5, pC194, pEG400 [J. Bacteriol., 172, 2392(1990)], pGEX (Pharmacia) and pET system (Novagen). 
     When  Escherichia coli  is used as the host, pUC18, pBluescriptII, pSupex, pET22b or pCold may be mentioned as suitable vectors. 
     Specific examples of suitable vectors for microorganisms belonging to  Brevibacillus  include pUB110, which is publicly known as a  Bacillus subtilis  vector, or pHY500 (Japanese Unexamined Patent Application Publication HEI No. 2-31682), pNY700 (Japanese Unexamined Patent Application Publication HEI No. 4-278091), pHY4831 (J. Bacteriol., 1987, 1239-1245), pNU200 (Udaka, S, Journal of Japan Society for Bioscience, Biotechnology and Agrochemistry 1987, 61:669-676), pNU100 (Appl. Microbiol. Biotechnol., 1989, 30:75-80), pNU211 (J. Biochem., 1992, 112:488-491), pNU211R2 L5 (Japanese Unexamined Patent Application Publication HEI No. 7-170984), pNH301 (Appl. Environ. Microbiol., 1992, 58:525-531), pNH326, pNH400 (J. Bacteriol., 1995, 177:745-749), pHT210 (Japanese Unexamined Patent Application Publication HEI No. 6-133782), pHT110R2L5 (Appl. Microbiol. Biotechnol., 1994, 42:358-363), or pNCO2, which is a shuttle vector between  E. coli  and a microorganism belonging to  Brevibacillus  (Japanese Unexamined Patent Application Publication No. 2002-238569). 
     Examples of eukaryote hosts include yeast and filamentous fungi (molds). 
     Examples of yeasts include yeast belonging to  Saccharomyces, Schizosaccharomyces, Kluyveromyces, Trichosporon, Schwanniomyces, Pichia, Candida, Yarrowia  and  Hansenula . More specifically, they include  Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces marxianus, Trichosporon pullulans, Schwanniomyces alluvius, Schwanniomyces occidentalis, Candida utilis, Pichia pastoris, Pichia angusta, Pichia methanolica, Pichia polymorpha, Pichia stipitis, Yarrowia lipolytica  and  Hansenula polymorpha.    
     When yeast is used as the host cells, the expression vector preferably includes a replication origin (when amplification in the host is necessary), and a selection marker for proliferation of the vector in  E. coli , an inducible promoter and terminator for expression of the recombinant protein in the yeast, and a selection marker for the yeast. 
     When the expression vector is a nonintegrated vector, it preferably also includes an autonomously replicating sequence (ARS). This can increase the stability of the expression vector in the cells (Myers, A. M., et al. (1986) Gene 45:299-310). 
     Examples of vectors when yeast are used as the host include YEP13 (ATCC37115), YEp24 (ATCC37051), YCp50 (ATCC37419), YIp, pHS19, pHS15, pA0804, pHIL3O1, pHIL-S1, pPIC9K, pPICZα, pGAPZα and pPICZ B. 
     Specific examples of inducible promoters, when yeast are used as the host, include galactose inducible gal 1 promoter and gal 10 promoter; copper inducible CUP 1 promoter; thiamine inducible nmt1 promoter; and methanol inducible AOX1 promoter, AOX2 promoter, DHAS promoter, DAS promoter, FDH promoter, FMDH promoter, MOX promoter and ZZA1, PEX5-, PEX8- and PEX14-promoters. 
     The method of introducing the expression vector into the yeast may be any method for introducing DNA into yeast, and examples include electroporation methods (Methods Enzymol., 194, 182(1990)), spheroplast methods (Proc. Natl. Acad. Sci., USA, 81, 4889(1984)), lithium acetate methods (J. Bacteriol., 153, 163(1983)), and the method described in Proc. Natl. Acad. Sci. USA, 75, 1929(1978). 
     Examples of filamentous fungi include fungi belonging to  Acremonium, Aspergillus, Ustilago, Trichoderma, Neurospora, Fusarium, Humicola, Penicillium, Myceliophtora, Botryts, Magnaporthe, Mucor, Metarhizium, Monascus, Rhizopus  and  Rhizomucor.    
     Specific examples of filamentous fungi include  Acremonium alabamense, Acremonium cellulolyticus, Aspergillus aculeatus, Aspergillus awamori, Aspergillus oryzae, Aspergillus sake, Aspergillus sojae, Aspergillus tubigensis, Aspergillus niger, Aspergillus nidulans, Aspergillus parasiticus, Aspergillus ficuum, Aspergillus phoeicus, Aspergillus foetidus, Aspergillus flavus, Aspergillus fumigatus, Aspergillus japonicus, Trichoderma viride, Trichoderma harzianum Trichoderma reseei, Chrysosporium lucknowense, Thermoascus, Sporotrichum, Sporotrichum cellulophilum, Talaromyces, Thielavia terrestris, Thielavia, Neurospora crassa, Fusarium oxysporus, Fusarium graminearum, Fusarium venenatum, Humicola insolens, Penicillium chrysogenum, Penicillium camemberti, Penicillium canescens, Penicillium emersonii, Penicillium funiculosum, Penicillium griseoroseum, Penicillium purpurogenum, Penicillium roqueforti, Myceliophtaora thermophilum, Mucor ambiguus, Mucor circinelloides, Mucor fragilis, Mucor hiemalis, Mucor inaequisporus, Mucor oblongiellipticus, Mucor racemosus, Mucor recurvus, Mocor saturninus, Mocor subtilissmus, Ogataea polymorpha, Phanerochaete chrysosporium, Rhizomucor miehei, Rhizomucor pusillus  and  Rhizopus arrhizus.    
     Specific examples of inducible promoters when filamentous fungi are used as the host include salicylic acid inducible PR1a promoter; cycloheximide inducible Place promoter; and quinic acid inducible Pqa-2 promoter. 
     Introduction of the expression vector into filamentous fungi may be carried out using a method known in the prior art. Such methods include the method of Cohen et al. (calcium chloride method) [Proc. Natl. Acad. Sci. USA, 69:2110(1972)], the protoplast method [Mol. Gen. Genet., 168:111(1979)], the competent method [J. Mol. Biol., 56:209(1971)] and electroporation. 
     The recombinant cells of this embodiment may have nucleic acid coding for the target protein integrated into the chromosomes (chromosomal DNA). The nucleic acid coding for the target protein is linked in a functional manner to one or more regulatory sequences. The regulatory sequences in this case may be exogenous or endogenous. 
     The method of integrating the nucleic acid coding for the target protein into the host chromosomes may be any publicly known method, and for example, it may be a λred method implementing a recombinant mechanism for double chain break repair of λ phage, a Red/ET homologous recombination method, or a transfer method using transposon activity with pUT-mini Tn5. For example, a “pUTmini-Tn5 Kit transposon gene transfer kit” by Biomedal may be used to integrate nucleic acid coding for the target protein into host chromosomes, following the method described in the kit manual. 
     (Culturing Method) 
     The production method of this embodiment includes continuous culturing with addition of fresh culture medium to a portion of the culture solution in which the recombinant cells have been grown, without reusing the recombinant cells in which expression of the recombinant protein has been induced. Here, “without reusing” means that the recombinant cells in which expression of the recombinant protein has been induced are not used in culturing for subsequent regrowth. The culturing may be carried out under aerobic conditions, such as deep aeration stirring culture. 
     The culturing method for this embodiment may be, for example, a method of growing recombinant cells by batch culture or fed batch culture, and then dividing the culture solution containing the grown recombinant cells into cells for inducing expression of the recombinant protein and cells for culturing for subsequent regrowth, and adding fresh culture medium to the culture solution for culturing for subsequent regrowth, and repeating the culturing procedure. The amount of culture solution for inducing expression of the recombinant protein may be 70 to 99 vol %, preferably 80 to 99 vol % and more preferably 90 to 99 vol %, for example, based on the total culture solution. The amount of culture solution divided out for culturing for subsequent regrowth may be 1 to 30 vol %, preferably 1 to 20 vol % and more preferably 1 to 10 vol %, for example, based on the total culture solution. 
     When the culturing for growth is to be fed batch culture, the feed substrate solution may include one or more nutrients of the medium component, for example. Feeding of the feed substrate solution may be carried out as a continuous system or a discontinuous system, according to a method known in the technical field. The feed amount is not particularly restricted, and feeding may be carried out by combining a linear constant coefficient system, a linear increment system, a stepwise increment system or an exponential feed system, with the proliferated cell count as the index. The amount of cells can be confirmed based on the dry cell weight, wet cell weight or colony formation units. Feeding allows the recombinant cells to be cultured to a high density. 
     The method of culturing according to another embodiment may be, for example, a method of continuously feeding fresh culture medium and discharging the culture solution, while maintaining approximately constant culture solution volume, and conducting culturing in a steady state such that the medium composition in the culture solution does not change with time (continuous culture), with the discharged culture solution being used as culture solution for inducing expression of the recombinant protein. 
     The type of culture medium used for culturing is not particularly restricted. Any natural culture medium or synthetic culture medium may be used, so long as it contains a carbon source, nitrogen source and inorganic salts that can be assimilated by the recombinant cells, and allows efficient culturing of the recombinant cells. 
     The carbon source may be any one that can be assimilated by the recombinant cells, including carbohydrates such as glucose, fructose, sucrose and molasses containing them, starch and starch hydrolysate, organic acids such as acetic acid and propionic acid, and alcohols such as ethanol and propanol. 
     Examples of nitrogen sources include ammonia, ammonium salts of inorganic acids or organic acids such as ammonium chloride, ammonium sulfate, ammonium acetate and ammonium phosphate, and other nitrogen-containing compounds, as well as peptone, meat extract, yeast extract, corn steep liquor, casein hydrolysate, soybean meal and soybean meal hydrolysate, and various fermentative microbes and their digestion products. 
     Examples of inorganic salts that may be used include monopotassium phosphate, dipotassium phosphate, magnesium phosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, copper sulfate and calcium carbonate. 
     The culturing temperature is 15 to 40° C., for example. The pH of the culture solution during culturing is preferably kept at 3.0 to 9.0. The pH of the culture solution can be adjusted using an inorganic acid, organic acid, alkali solution, urea, calcium carbonate, ammonia or the like. 
     A production method according to the first embodiment will now be described in detail with reference to  FIG. 1 .  FIG. 1  is a diagram schematically showing a culturing system to be used in the method for production of a recombinant protein according to an embodiment of the invention. The culturing system  100  shown in  FIG. 1  comprises two tanks, a culturing tank  10  and a culturing tank  20 . The culturing tank  10  is a culturing tank for growth of recombinant cells. A feed substrate solution  31  is supplied to the culturing tank  10  from a feed substrate solution storage tank  30  connected through a pump  50 . 
     Upon reaching the anaphase from the metaphase during the logarithmic growth stage, the recombinant cells grown in the culturing tank  10  are transferred to the culturing tank  20  as culture solution  11 . The amount of transferred culture solution  11  may be 70 to 99 vol %, preferably 80 to 99 vol % and more preferably 90 to 99 vol %, for example, based on the total amount of the culture solution  11 . After transfer, a portion of the culture solution  11  remains in the culturing tank  10 , and fresh culture medium  41  is supplied from a culture medium storage tank  40 , after which growth of the recombinant cells is repeated. 
     The culturing tank  20  is a culturing tank for expression of a recombinant protein. In the culturing tank  20 , an inducible promoter is activated to induce expression of the recombinant protein. The activation of an inducible promoter referred to here means activation of transcription of a nucleic acid coding for a recombinant protein by an inducible promoter. For example, when an inducible promoter is activated by the presence of an inducing substance (expression inducing agent), addition of the inducing substance to the culturing tank  20  can induce expression of the recombinant protein. Also, when the inducible promoter is activated by increase or decrease in temperature, for example, expression of the recombinant protein can be induced by heating or cooling the culturing tank  20  to control the temperature in the culture solution  21 . 
     Expression of the recombinant protein is induced in the culturing tank  20  for 10 to 20 hours, for example. During this time, the feed substrate solution  31  may be supplied to the culturing tank  20  from the feed substrate solution storage tank  30  connected via a pump  51 . The feed substrate solution  31  includes one or more nutrients in the culture medium. Supplying the feed substrate solution  31  can increase the efficiency of inducing expression of the recombinant protein. 
     After conducting induction of expression of the recombinant protein for the prescribed time period, the culture solution  21  is transferred to a separation purification tank  60 . The amount of transferred culture solution  21  may be 50 to 100 vol %, preferably 80 to 100 vol % and more preferably 90 to 100 vol % and even more preferably 100 vol %, for example, based on the total amount of the culture solution  21 . The expressed recombinant protein is separated and purified at the separation purification tank  60 . 
     After the culture solution  21  has been transferred, the recombinant cells that have been grown in the culturing tank  10  are transferred to the culturing tank  20 , and expression of the recombinant protein is induced. Growth of the recombinant cells at the culturing tank  10  and induction of expression of the recombinant protein at the culturing tank  20  are then continuously repeated. 
     Since growth of the recombinant cells and induction of expression of the recombinant protein are carried out in different culturing tanks (the culturing tank  10  and culturing tank  20 ) in the culturing system  100 , the recombinant cells in which expression of the recombinant protein has been induced are not repeatedly reused. This construction allows efficient production of the recombinant protein in a stable manner, even when the cycle is repeated. When a plasmid-type expressing strain is used as the recombinant cells, plasmid shedding is minimized as well. 
       FIG. 4  is a diagram schematically showing a culturing system to be used in the method for production of a recombinant protein according to the prior art. The culturing system  200  shown in  FIG. 4  comprises only a single culturing tank  10 . Both growth of the recombinant cells and induction of expression of the recombinant protein are carried out in the culturing tank  10 . After conducting induction of expression of the recombinant protein for the prescribed time period in the culturing tank  10 , the culture solution  11  is transferred to a separation purification tank  60 . After transfer, a portion of the culture solution  11  remains in the culturing tank  10 , fresh culture medium  41  is supplied from a culture medium storage tank  40 , and growth of the recombinant cells and induction of expression of the recombinant protein are repeated. With the construction of the culturing system  200 , the production volume of the recombinant protein is markedly reduced when the cycle is repeated. Furthermore, plasmid shedding tends to occur when a plasmid-type expressing strain is used as the recombinant cells. 
     The production method according to one embodiment may include repetition of the following steps (A) to (E). 
     (A) A step of growing the recombinant cells in a culturing tank by batch culture or fed batch culture. 
     (B) A step of transferring a portion of the culture solution in the culturing tank to a receiving culturing tank, after the growth in step (A). 
     (C) A step of adding fresh culture medium to either of the two culturing tanks after the transfer in step (B), and advancing to step (A). 
     (D) A step of inducing expression of the recombinant protein in the other culturing tank that has not advanced to step (A) in step (C), and accumulating the recombinant protein. 
     (E) A step of separating and purifying the recombinant protein that has been accumulated in step (D) from the culture solution. 
     When step (D) is to be carried out in a receiving culturing tank, the amount of culture solution transferred to the receiving culturing tank in step (B) may be 70 to 99 vol %, preferably 80 to 99 vol % and more preferably 90 to 99 vol %, based on the total amount of the culture solution. When step (C) is to be carried out in a receiving culturing tank, the amount of culture solution transferred to the receiving culturing tank in step (B) may be 1 to 30 vol %, preferably 1 to 20 vol % and more preferably 1 to 10 vol %, based on the total amount of the culture solution. 
     (Inducing Expression of the Recombinant Protein) 
     Induction of expression of the recombinant protein is carried out by activating transcription by an inducible promoter (transcription of nucleic acid coding for the target protein). Activation of an inducible promoter may be carried out by a method known in the technical field, depending on the type of inducible promoter. 
     For example, when using an inducible promoter that is activated by the presence of an inducing substance (expression inducing agent), addition of the inducing substance to the culture solution can induce expression of the recombinant protein. The inducing substance may be added to the culture solution all at once or in several portions, or it may be added to the culture solution as a continuous feed. Feeding may also be by addition of the inducing substance to the feed substrate solution. The amount of inducing substance added may be set according to the type of inducing substance and inducible promoter, but as an example it may be in the range of 0.1 to 30 μg and preferably in the range of 0.5 to 20 μg, for 1 g of dry weight of the recombinant cells. 
     When the inducible promoter is to be activated by increase or decrease in temperature, for example, expression of the recombinant protein may be induced by increasing or decreasing the temperature of the culture solution. For example, when using phage PR promoter or PL promoter which is activated by temperature increase, expression of the recombinant protein during growth may be suppressed when the temperature of the culture solution during growth is in the range of 20 to 37° C., and expression of the recombinant protein can then be induced by increasing the culture solution temperature to 38 to 44° C. In order to lessen the effect of heat shock protein during the process, the pH of the culture solution during growth may be adjusted to 6.5 to 7.5, as described in Japanese Unexamined Patent Application Publication HEI No. 6-292563, and the pH of the culture solution varied to 4.5 to 6.5 at the start of inducing expression of the recombinant protein, thereby allowing more stable induction of expression. 
     There is no particular restriction on the period from the stage of growth of the recombinant cells until the stage of inducing expression of the recombinant protein, and it may be appropriately set according to the culturing system configuration and the production process design. From the viewpoint of efficient production of the recombinant protein, it is preferred to initiate induction of expression of the recombinant protein when growth of the recombinant cells has reached the metaphase to the anaphase of the logarithmic growth stage. 
     Growth of the recombinant cells begins from the lag phase or induction phase (the period of delayed increase in the initial cell count), through the logarithmic growth stage (the period of logarithmic increase to twice the cell count per unit time), and reaches the stationary phase (the period where no net change is seen in the number of cells). The metaphase of the logarithmic growth stage is the period in which the cell count is midway between the cell count in the lag phase and the cell count in the stationary phase, and the anaphase of the logarithmic growth stage is the period from the metaphase until the stationary phase. As a specific example of the period for initiating induction of expression of the recombinant protein, for recombinant cells wherein the OD 600  value at the stationary phase is approximately 150, it is preferably the period in which the OD 600  value has reached 30 to 110, more preferably the period in which it has reached 40 to 90, and even more preferably the period in which it has reached 50 to 80. 
     The time for inducing expression of the recombinant protein may be a time length until the predetermined production volume has been obtained, which will depend on the type of host used and the target protein. Since the production rate varies depending on the culturing conditions such as the temperature of the culture solution, it is not necessary to absolutely specify the time for inducing expression of the recombinant protein. The time for inducing expression of the recombinant protein may also be set to match progression to separation and purification of the recombinant protein in the subsequent step. For industrial production, it is preferred to set the time for induction of expression of the recombinant protein so as not to affect growth of the recombinant cells being carried out in parallel, or transfer of the grown recombinant cells. 
     The culture solution in which expression of the recombinant protein has been induced is used for the following separation and purification of the target recombinant protein. 
     (Separation and Purification of Recombinant Protein) 
     Separation and purification of the recombinant protein may be carried out by a commonly used method. For example, if the recombinant protein is to be expressed in the cells in dissolved form, then after the culturing for inducing expression of the recombinant protein is completed, the host cells (recombinant cells) may be collected by centrifugal separation and suspended in an aqueous buffer, and then the host cells may be &gt;disrupted using an ultrasonic disruptor, French press, Manton Gaulin homogenizer or Dyno-Mill, to obtain a cell-free extract. The cell-free extract is centrifuged to obtain a supernatant, from which a purified preparation of the recombinant protein may be obtained using one or a combination of methods commonly used for isolation and purification of proteins, such as a solvent extraction method, a salting out method with ammonium sulfate or the like, a desalting method, a precipitation method with an organic solvent, an anion exchange chromatography method using a resin such as diethylaminoethyl (DEAE)-Sepharose or DIAION HPA-75 (product of Mitsubishi Chemical Corp.), a cation exchange chromatography method using a resin such as S-Sepharose FF (product of Pharmacia), a hydrophobic chromatography method using a resin such as Butyl Sepharose or Phenyl Sepharose, a gel filtration method using a molecular sieve, an affinity chromatography method, a chromatofocusing method, or an electrophoresis method such as isoelectric focusing. 
     When the recombinant protein is expressed in an insoluble form in the cells, the recombinant protein is collected in insoluble form as a precipitated fraction, by collecting and then disrupting and centrifuging the host cells in the same manner. The insoluble recombinant protein that is recovered may be solubilized using a protein denaturing agent. After the procedure, isolation and purification may be carried out in the same manner as described above to obtain a purified preparation of the recombinant protein. 
     When the recombinant protein has been secreted extracellularly, the recombinant protein may be recovered from the culture supernatant. That is, by treating the culture solution using a method such as centrifugal separation, it is possible to obtain the culture supernatant, from which a purified preparation of the recombinant protein may then be obtained by isolation and purification in the same manner as described above. 
     EXAMPLES 
     The present invention will now be explained in greater detail based on examples. However, the present invention is not limited to the examples described below. 
     Example 1 
     [(1) Preparation of Modified Fibroin-Expressing Strain] 
     (Preparation of Plasmid-Type Expressing Strain) 
     Modified fibroin (hereunder referred to as “SEC472”) having the amino acid sequence listed as SEQ ID NO: 1 was designed based on the nucleotide sequence and amino acid sequence for fibroin from  Nephila clavipes  (GenBank Accession No.: P46804.1, GI:1174415). 
     The amino acid sequence listed as SEQ ID NO: 1 is the amino acid sequence, having substitutions, insertions and deletions of amino acid residues as compared to  Nephila clavipes  fibroin, for increased productivity, and further having the amino acid sequence represented by SEQ ID NO: 6 (tag sequence and hinge sequence) added to the N-terminus. 
     Nucleic acid coding for SEC472 was then synthesized. The nucleic acid had an NdeI site added at the 5′-end and an EcoRI site added downstream from the stop codon. The nucleic acid was cloned in a cloning vector (pUC118). The nucleic acid was then subjected to restriction enzyme treatment with NdeI and EcoRI for cleavage, after which it was recombined with the protein expression vector pET-22b(+) to obtain an expression vector. 
       E. coli  BLR(DE3) was transformed with expression vector pET-22b(+), to obtain a plasmid-introduced plasmid-type expressing strain SEC659. 
     (Preparation of Chromosomally Integrated Expressing Strain) 
     A chromosomally integrated expressing strain was prepared using pUTmini-Tn5 Kit by Biomedal. 
     Nucleic acid coding for modified fibroin (SEC472) having the amino acid sequence listed as SEQ ID NO: 1 was synthesized. The nucleic acid had a NotI site added at the 5′-end and downstream from the stop codon. A plasmid was constructed having this nucleic acid inserted at the NotI site of pUTmini-Tn5 Km. Next, a strain obtained by transforming S17-1 λpir with this plasmid was mixed with  E. coli  BLR(DE3) at a 1:1 ratio, and cultured in LB- and Kin-containing plate culture medium. A chromosomally integrated expressing strain SEC714 integrating the nucleic acid in its chromosomes was obtained from the strains exhibiting Km resistance and Ap sensitivity. 
     [(2) Seed Culturing] 
     The plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714 prepared in (1) were each cultured for 15 hours in 2 mL of LB medium containing ampicillin. The same culture solution was added to 100 mL of seed culture medium containing ampicillin (Table 1) to an OD 600  of 0.005, and flask culturing was conducted to an OD 600  of 5 (approximately 15 hours) while keeping the culture solution temperature at 30° C., to obtain seed culture solutions. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Seed culture medium 
               
            
           
           
               
               
               
            
               
                   
                 Reagent 
                 Concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Glucose 
                 5.0 
               
               
                   
                 KH 2 PO 4   
                 4.0 
               
               
                   
                 K 2 HPO 4   
                 9.3 
               
               
                   
                 Yeast Extract 
                 6.0 
               
               
                   
                 Ampicillin 
                 0.1 
               
               
                   
                   
               
            
           
         
       
     
     [(3) Preparation of Main Culture Medium] 
     The composition of the main culture medium is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Main culture medium 
               
            
           
           
               
               
               
            
               
                   
                 Reagent 
                 Concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Glucose 
                 10.0 
               
               
                   
                 KH 2 PO 4   
                 22.0 
               
               
                   
                 FeSO 4 •7H 2 O 
                 0.04 
               
               
                   
                 CaCl 2 •2H 2 O 
                 0.04 
               
               
                   
                 MgSO 4 •7H 2 O 
                 2.4 
               
               
                   
                 Yeast Extract 
                 11.25 
               
               
                   
                 Defoaming agent (Adeka, LG-295S) 
                 1.0 (mL/L) 
               
               
                   
                   
               
            
           
         
       
     
     A 500 mL portion of main culture medium (Table 2) was added to a culturing tank, and sterilization treatment was carried out for 20 minutes in an autoclave (TOMY LSX-500) at 121° C. After cooling to 37° C., 28% to 30% ammonia water (01266-88 by Kanto Kagaku Co., Ltd.) was used to adjust the pH to 6.1 to 6.3. 
     [(4) Preparation of Feed Substrate Solution] 
     The composition of the feed substrate solution is shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Feed substrate solution 
               
            
           
           
               
               
               
            
               
                   
                 Reagent 
                 Concentration (g/L) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Glucose 
                 600.0 
               
               
                   
                 MgSO 4 •7H 2 O 
                 10.0 
               
               
                   
                   
               
            
           
         
       
     
     A prescribed amount of feed substrate solution was added to the feeding pot, and sterilization treatment was carried out for 20 minutes in an autoclave (TOMY LSX-500) at 121° C. 
     [(5) Repeated Fed Batch Culture and Expression Induction] 
     Recombinant modified fibroin was produced with plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714, using the culturing system  100  illustrated in  FIG. 1 . A TSC-A1L-5 culturing tank (product of Takasugi Seisakusho, 1 L capacity) was used for the culturing tank  10  and the culturing tank  20 . The recombinant modified fibroin was produced by repeated fed batch culture in which culture solution (˜95%) fed up to the prescribed cell density was used for expression induction, while fresh culture medium was added to the remainder of the culture solution (˜5%) and fed batch culture was repeated. 
     The main culture medium (0.5 L initial medium volume) was loaded into the culturing tank  10 , and the obtained seed culture solution was added to an OD 600  of 0.05. Main culturing was conducted with the temperature of the culture solution kept at 37° C., and using 30% ammonia water and a 4 M phosphoric acid solution (Wako Pure Chemical Industries, Ltd.) for constant control to pH 6.9. Aerated stirring was carried out so that the dissolved oxygen concentration in the culture solution was maintained at 30-40% dissolved oxygen saturated concentration. A massflow controller (MPC0005BBRN0100D0 by Azbil Corp.) was used for control. 
     For the main culturing, feeding of the feed substrate solution  31  from the feed substrate solution storage tank  30  into the culturing tank  10  was initiated at the point where the dissolved oxygen saturated concentration exceeded about 55% after having fallen below about 30%. The feed rate of the feed substrate solution  31  was constant rate feeding at 6 g/hr. 
     After continuing fed batch culture in the culturing tank  10  until the OD 600  of the culture solution  11  reached about 60, approximately 95% of the culture solution  11  was transferred aseptically to the culturing tank  20  through a sampling line (not shown). After transfer, IPTG (expression inducing agent) was added to the culturing tank  20  to 0.2 mM, and expression induction of the recombinant modified fibroin was initiated. 
     After initiating expression induction, culturing was continued in the culturing tank  10  under the same conditions as the main culture, except that the feed substrate solution  31  was fed from the feed substrate solution storage tank  30  to the culturing tank  20  at a feed rate of 9 g/hr. After expression induction in the culturing tank  20  for approximately 16 hours, the total amount of culture solution  21  was transferred to the separation purification tank  60  and used for separation and purification of recombinant modified fibroin. 
     Fresh culture medium  41  was added from the culture medium storage tank  40  to the culturing tank  10  after transfer of the culture solution  11  to the culturing tank  20  (with approximately 5% of the culture solution  11  remaining), and main culturing and fed batch culturing were carried out in the culturing tank  10  under the same conditions as described above. The culture solution  11  in the culturing tank  10  that had been cultured by fed batch culture to an OD 600  of about 60 in the same manner as described above was transferred to the culturing tank  20  after the total amount of culture solution  21  had been transferred into the separation purification tank  60 , and expression induction of the recombinant modified fibroin was carried out. The repeated fed batch culturing was repeated 8 times, including the main culturing. 
       FIG. 2  shows the results of analyzing the production volume of recombinant modified fibroin in the 8 repeated fed batch cultures with plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714. The production volumes shown in  FIG. 2  are represented as relative values with the production volume of recombinant modified fibroin in the first repeated fed batch culture with SEC714 as 100%. 
     As shown in  FIG. 2 , both the plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714 maintained the production volume of recombinant modified fibroin in the first repeated fed batch culture even up through the 8th repeated fed batch culture. 
     The plasmid retention in plasmid-type expressing strain SEC659 was confirmed by the following method. The culture solution after each repeated fed batch culturing was diluted with LB medium and seeded on LB agar medium (culture medium A). After culturing for 18 to 20 hours in a thermostatic bath at 37° C., the 50 to 100 grown colonies were transferred onto ampicillin-added LB agar medium (culture medium B) using a sterilized toothpick. The strains transplanted in culture medium B were cultured for 12 to 18 hours in a thermostatic bath at 37° C., and then the number of colonies transferred from culture medium A to culture medium B and the number of colonies foamed on culture medium B were counted and calculation was performed by the following formula. 
     Plasmid retention=(number of colonies formed on culture medium B)/(number of colonies transferred from culture medium A to culture medium B) 
     As a result, the plasmid-type expressing strain SEC659 maintained a plasmid retention of 100% up through addition of IPTG in the 8th repeated fed batch culture. 
     Incidentally, stable continuous production was also possible by the method described above for recombinant proteins derived from keratin, collagen, elastin and resilin, including the amino acid sequences listed as SEQ ID NO: 2 to 5. 
     Comparative Example 1 
     [Repeated Fed Batch Culture and Expression Induction] 
     Recombinant modified fibroin was produced with plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714, using a conventional culturing system  200  as illustrated in  FIG. 4 . A TSC-A1L-5 culturing tank (product of Takasugi Seisakusho, 1 L capacity) was used for the culturing tank  10  and culturing tank  20 . The recombinant modified fibroin was produced by repeated fed batch culture in which fresh culture medium was added to a portion of the culture solution in which expression had been induced (˜5%) and fed batch culture was repeated. 
     Repeated fed batch culturing and expression induction were carried out under essentially the same conditions as Example 1, except that expression induction of the recombinant modified fibroin was carried out continuously not in the culturing tank  20  but in the culturing tank  10  in which the main culturing and fed batch culturing had been carried out. 
     That is, after fed batch culture had been continued in the culturing tank  10  until the OD 600  of the culture solution  11  reached about 60, IPTG (expression inducing agent) was subsequently added to the culturing tank  10  to 0.2 mM, and expression induction of the recombinant modified fibroin was initiated. After initiating expression induction, the feed substrate solution  31  was fed from the feed substrate solution storage tank  30  to the culturing tank  10  at a feed rate of 9 g/hr. After expression induction in the culturing tank  10  for approximately 16 hours, approximately 95% of the culture solution  11  was transferred to the separation purification tank  60  and used for separation and purification of recombinant modified fibroin. After the transfer, fresh culture medium  41  was added from the culture medium storage tank  40  to the culturing tank  10  (in which approximately 5% of the culture solution  11  remained), and main culturing, fed batch culturing and expression induction of recombinant modified fibroin were carried out under the same conditions. The repeated fed batch culturing was repeated 5 times. 
       FIG. 3  shows the results of analyzing the production volume of recombinant modified fibroin in the 5 repeated fed batch cultures with plasmid-type expressing strain SEC659 and chromosomally integrated expressing strain SEC714. The production volumes shown in  FIG. 3  are represented as relative values with the production volume of recombinant modified fibroin in the first repeated fed batch culture with SEC714 as 100%. 
     As shown in  FIG. 3 , when repeated fed batch culture was repeated only in the culturing tank  10  (that is, when the recombinant cells in which recombinant modified fibroin expression had been induced were reused), the production volume of recombinant modified fibroin after the 2nd culture was extremely reduced, with plasmid-type expressing strain SEC659. The plasmid retention measured in the same manner as Example 1 was 0%, at the point of addition of IPTG in the second repeated fed batch culture. With chromosomally integrated expressing strain SEC714, on the other hand, the production volume of recombinant modified fibroin was reduced with each repeated fed batch culture, although a more stable production volume was exhibited than with plasmid-type expressing strain SEC659. 
     REFERENCE SIGNS LIST 
       10 ,  20 : Culturing tank,  11 ,  21 : culture solution,  30 : feed substrate solution storage tank,  31 : feed substrate solution,  40 : culture medium storage tank,  41 : culture medium,  50 ,  51 : pump,  60 : separation purification tank,  100 ,  200 : culturing system.