A PROCESS FOR PRODUCING CHYMOSIN

Abstract: Chymosin may be produced in a filamentous fungus by transforming a filamentous fungus with a vector comprising a DNA sequence encoding prochymosin or a derivative thereof N-terminally fused to a DNA sequence encoding a signal peptide derived from the Aspergillus oryzae TAKA amylase gene or a derivative of said signal peptide, culturing the transformed filamentous fungus in a suitable culture medium to produce prochymosin and recovering prochymosin or chymosin from the medium.

Description: A PROCESS FOR PRODUCING CHYMOSIN FIELD OF INVENTION The present invention relates to a process for the production of chymosin in filamentous fungi. BACKGROUND OF THE INVENTION In recent years, procedures have been developed for the transformation of filamentous fungi, including Aspergillus niqer and Aspergillus nidulans. US 4,885,249 (Allelix) describes a general process for the transformation of ______ niger, exemplified by the introduction of plasmids carrying genes encoding selectable markers. EP 215 594 (Genencor) describes the expression and secretion of bovine chymosin in A_ Ϊ _ nidulans. using the signal sequence of ______ niger glucoamylase to provide secretion. It appears from EP 215 594, however, that chymosin expressed in this system is only secreted at extremely low levels. SUMMARY OF THE INVENTION It has surprisingly been found that when the gene encoding prochymosin is fused to the ______ niger TAKA amylase signal sequence, the levels of chymosin secreted from A . ;, niger are increased several fold compared to those apparent from EP 215 594. Accordingly, the present invention relates to a process for the production of chymosin or a derivative thereof in filamentous fungi, the process comprising (a) transforming a filamentous fungus host organism with a reco binant DNA vector which comprises a DNA sequence encoding prochymosin or a derivative thereof N-terminally fused to a DNA sequence encoding a signal peptide derived from the Aspergillus oryzae TAKA amylase gene or a derivative of said signal peptide, (b) culturing the transformed filamentous fungus host organism in a suitable culture medium under conditions conducive to the expression of prochymosin and secretion thereof to the medium, and (c) recovering the prochymosin or chymosin or derivative thereof from the medium. In the present context, the term "derivative 11 is intended to indicate a polypeptide which is derived from the native chymosin or signal peptide (as the case may be) by suitably modifying the DNA sequence coding for the native chymosin/signal peptide, resulting in the addition of one or more a ino acid at either or both the C- or N-terminal end, substitution of one or more amino acids at one or a number of different sites in the native amino acid sequence, deletion of one or more amino acids at either or both ends of the native amino acid sequence or at one or more sites within the native sequence, or insertion of one or more amino acids at one or more sites in the native amino acid sequence. Such modifications of the DNA sequence may be done by methods well known in the art. The term "filamentous fungus" is intended to include the groups Phyco ycetes, Zygomycetes, Ascomycetes, Basidiomycetes and fungi imperfecti, including Hyphomycetes such as the genera Aspergillus, Penicillium, Trichoderma, Fusarium and Humicola. The presence of the signal sequence serves to direct the expressed prochymosin or prochymosin derivative effectively into the secretory pathway of the host cell so that prochymosin or chymosin may readily be isolated from the culture medium (at least some of the product recovered will be mature chymosin as the prochymosin secreted from the cells is either subjected to automaturation or maturation by proteases produced by the host cell) . The amino acid sequence of the native TAKA-amylase signal peptide is as follows: Met Met Val Ala Trp Trp Ser Leu Phe Leu Tyr Gly Leu Gin Val Ala Ala Pro Ala Leu Ala It is, however, envisaged that derivatives (as defined above) of this sequence may also be employed for the present purpose. The chymosin to be produced by the process of the invention is primarily bovine chymosin. DETAILED DESCRIPTION OF THE INVENTION The vector may further comprise DNA sequences encoding functions facilitating gene expression, typically a promoter, transcription initiation sites, and transcription termination and polyadenylation functions. The promoter which may be preceded by upstream activating sequences and enhancer sequences as known in the art may be any DNA sequence exhibiting a strong transcriptional activity in A. niger, and may be derived from a gene encoding an extracellular or intracellular protein such as an amylase, a glucoamylase, a protease, a lipase, a cellulase or a glycolytic enzyme. Examples of suitable promoters are those derived from the gene encoding A__. oryzae TAKA amylase, Rhizomucor miehei aspartic proteinase, A___ niger neutral α-amylase, ______ niger acid stable α-amylase, A-. niger glucoamylase, Rhizomucor miehei lipase, or A. orvzae alkaline protease. Examples of promoters from genes encoding glycolytic enzymes are the A ^ _ orvzae triose phosphate isomerase, ADH and PGK promoters. The filamentous fungus used as the host organism is preferably selected from an Aspergillus sp. such as ______ niger or ______ oryzae. in particular A^. niger. When the host organism is .^ niger, a preferred promoter for use in the process of the present invention is the j_ niger glucoamylase promoter or the A. oryzae TAKA amylase promoter as they exhibit a strong transcriptional activity in A__ niger. Termination and polyadenylation sequences may suitably be derived from the same sources as the promoter. The techniques used to transform the host organism may suitably be adapted from the methods of transforming A___ nidulans described in, for instance, Yelton et al., Proc. Natl. Acad. Sci. USA 81. 1984, pp. 1470-1474, or EP 215 594, or from the methods of transforming ______ niger described in, for instance Buxton et al., Gene 37, 1985, pp. 207-215 or US 4,885,249. In the process of the present invention, _A__ niger may be transformed with a vector system comprising a DNA sequence coding for a selection marker which is capable of being incorporated in the genome of the host organism on transformation, but which is either not expressed by the host before transformation or not expressed in sufficient amounts to permit growth under selective conditions. Transformants can then be selected and isolated from non-transformants on the basis of the incorporated selection marker. Suitable selection markers may be derived from the ______ nidulans or _H__ niger argB gene, the ______ nidulans trpC gene, the iV_ . nidulans amdS gene, the Neurospora crassa pyr4 or DHFR genes, or the ______ niger or A.. . oryzae niaD gene. Preferred selection markers for use in the present invention are derived from the A ^ _ nidulans or ^ _ niger amdS or argB genes. Wild-type A ^ _ niger strains are usually ArgB * (which means that the argB gene is expressed in A__ niger) . Thus, if argB is chosen as the selection marker, an ArgB " mutant strain of Aj_ niger (which does not express the ArgB gene) must be used as the host organism. On the other hand, the amdS gene may be used as the selection marker in wild-type A^_ niger strains which do not express this gene in sufficient amounts to permit growth under selective conditions. The gene coding for prochymosin fused to the TAKA-amylase signal sequence as well as to promoter and terminator sequences may be inserted in a vector containing the selection marker, or it may be inserted in a separate vector for introduction into the host cell. The vector or vectors may be linear or closed circular molecules. In one embodiment of the process of the invention, two vectors, one carrying the DNA sequence coding for the selection marker, and the other carrying the DNA sequence encoding prochymosin or a derivative thereof N- terminally fused to the DNA sequence encoding a signal peptide derived from the Aspergillus oryzae TAKA amylase gene, may be introduced into the host cell. The medium used to culture the transformed host cells may be any conventional medium suitable for growing filamentous fungi. The transformants are usually stable and may be cultured in the absence of selection pressure. However, if the transformants are found to be unstable, the selection marker introduced into the cells may be used for selection. The prochymosin or chymosin produced by the host cells may conveniently be recovered from the culture medium by well-known procedures including separating the cells from the medium by centrifugation or filtration, and precipitating proteinaceous components of the medium by means of a salt such as ammonium sulphate, followed by chromatographic procedures such as ion exchange chro atography, affinity chromatography, or the like. The invention is further illustrated in the following examples which are not in any way to be construed as limiting to the scope of the invention as claimed. EXAMPLE 1 Construction of a prochymosin expression vector Unless otherwise indicated, the procedures used in the construction of the expression vector are standard procedures, e.g. as described in Sambrook, et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, 1989. Plasmid pToC51 (the construction of which is described in Example 12 of EP 238 023) carrying the 5 1 portion of the prochymosin gene fused to the signal sequence from the A. oryzae TAKA amylase gene is used as the starting material. A Bglll restriction site is introduced immediately upstream of the ATG start codon of the signal sequence by polymerase chain reaction (PCR) (R.K. Saiki et al., Science 239, 1988, pp. 487- 491) using the Gene Amp kit as recommended by the manufacturer ( Perkin-Elmer ) . The following synthetic oligodeoxyribonucleotides are used primers for the reaction: I: GAAGATCTGAAGGCATTTATGATGGTC II: CCCAAAGTACTGACTATC 100 μl reaction mixture contains 25 pmol of each primer and 0.01 μg of pToC51. The approximately 290 bp PCR product is cleaved with the restriction enzymes Bglll and Pstl, and a 190 bp fragment is isolated on an agarose gel. This fragment is cloned into Bglll/Pstl digested pIC19R (described in J.L. Marsh et al., Gene 32, 1984, pp. 481-485). Plasmid DNA is recovered from the resulting transformants, and the insert is sequenced according to the method of Sanger (cf. Sanger et al., Proc. Natl. Acad. Sci. USA 74., 1977, pp. 5463-5467). The fragment comprises a DNA sequence coding for the TAKA amylase signal sequence and the 5 1 portion of the prochymosin gene. The 190 bp BGIII-Pstl fragment was then excised from pIC19R, and ligated with an approximately 350 bp Pstl-Xmal fragment of pToC56 (described in Example 12 of EP 238023) containing part of the prochymosin coding sequence, and an approximately 1.4 kb Xmal-Clal fragment of pToC56 containing the 3' half of the prochymosin gene and the A. niger glucoamylase gene transcription terminator and an approximately 3.7 kb Bgllll Clal fragment of p408.3 (described in Example 6 of EP 238023) containing the A. niger glucoamylase promoter cloned into the plasmid pIC19R. After ligation, the ligation mixture was transformed into competent E. coli MC1000 (M.J. Casadaban and S.N. Cohen, J.Mol.Biol. 138, pp. 179-209) cells. Transformants were selected for ampicillin resistance and the plasmid construction verified by several restriction enzyme digestions analysed on agarose gels. The resulting plasmid pToC195 thus contains the A. niger glucoamylase promoter followed by the A. oryzae TAKA- amylase signal sequence fused in frame to the coding sequence of prochymosin and the transcription terminator from the A. niger glucoamylase gene. EXAMPLE 2 Expression of prochymosin from the TAKA amylase signal sequence The expression vector pToC195 was co-transformed into A___ niger by the general procedure described in Example 9 of EP 238 023, using the plasmid p3SR2 which contains the A.;, nidulans amdS gene as the selection marker. After two reisolations of conidiospores, transformants were fermented in 10 ml of YPD medium (Sherman et al., Methods in Yeast Genetics, Cold Spring Harbor, 1981) . The content of prochymosin in the fermentation broth was determined by ELISA on a Western blot after SDS-PAGE, using an antibody raised against chymosin. The transformant producing the highest yield (about 100 mg/ml)of prochymosin of the correct size (as determined by Western blot) was then fermented in a 2 litre Kieler fermentor in a synthetic medium containing maltodextrin and urea. The fermentation is a batch fermentation where the pH is kept at 6 by means of NaOH. The chymosin content in the supernatant is determined by Western blotting. Several smaller immunoreactive bands are detected. These were presumed to be degradation products of prochymosin, i.e. the prochymosin was partly subjected either to automaturation or by A. niger proteases present in the fermentation broth. REFERENCE EXAMPLE A. Construction of the prochymosin expression plasmid PMT837 The construction of pMT837 is illustrated in Figs. 1-3. The sequence encoding the C-terminal part of chymosin was isolated as a 0.6 kb EcoRI-Xbal fragment and subcloned into pUC13 to give pMT622 (Fig. 1) . Likewise a fragment from the AMG promoter region including the sequence encoding the glucoamylase prepropeptide was isolated from pCAMG91. This Smal-BssH2 fragment of approximately 0.4 Kb was cloned into Smal-BamHl cut pUC13 together with the BssH2-BamHl adaptor/linker KFN37 and 38 (Fig. 1) . The sequence of KFN37 is: CGCGCTGAGATCACCAG and of KFN38: GATCCTGGTGATCTCAG. The linker serves to obtain a sequence encoding a perfect fusion from the glucoamylase prepropeptide into prochymosin. The 3.0 kb EcoRl-Ndel fragment from pMT622 was ligated to the 0.8 kb Ndel- BamHl fragment from pMT626 and the 0.5 kb BamHl-EcoRl fragment from p285 (Fig. 1) to give pMT648. pMT648 therefore contains a short AMG promoter fragment in front of the sequence encoding the glucoamylase prepropeptide fused to the entire prochymosin. The argB gene of A. nidulans was subcloned from pSal43 (Gene 25 (1983) 109-117) as a 2.8 kb BamHl-Nrul into Bgl2-EcoRV cut plC19R to give pMT813 (Fig. 2). In order to include the argB marker in the expression plasmid and also to introduce a longer AMG promoter fragment the following steps were undertaken (Fig. 3) , pMT648 was cut with Xbal and blunt ended by Klenow polymerase and dNTP's, subsequently cut with BssH2 and a 1.1 kb fragment isolated. This fragment contains the prochymosin encoding sequence. Secondly, the argB gene was isolated from pMT813 as 2.8 kb Clal-blunted EcoRl fragment (EcoRl filled in with Klenow and dNTP's) . Finally a fragment containing the long AMG promoter was isolated as a 3.7 b Clal-BssH2 fragment from pCAMG91. These three fragments were ligated to give pMT830 (Fig. 3) . In a final step, pMT830 was cut with Xbal (regenerated by the ligation and blunted EcoRl and Xbal) dephosphorylated by alkaline phosphatase and ligated to an approximately 0.6 kb Xbal-Xbal fragment from pAMG/Term containing the AMG transcription terminator (Fig. 3) . The correct orientation of the terminator fragment was verified in the resulting plasmid PMT837. B. Expression of phochymosin from the AMG signal sequence pMT837 was transformed into A. niger argB " by the general procedure described in Example 9 of EP 238023 using selection for argB present on the plasmid. Transformants were reisolated twice through conidiospores. The transformants were fermented in 10 ml of YPD medium. The content of chymosin in the fermentation broth was determined by ELISA on a Western blot after SDS-PAGE using an antibody raised against chymosin. The best transformant produced approximately 0.5 mg/1 of immunoreactive material.

Claims: CLAIMS 1. A process for the production of chymosin or a derivative thereof in a filamentous fungus, the process comprising (a) transforming a filamentous fungus host organism with a recombinant DNA vector which comprises a DNA sequence encoding prochymosin or a derivative thereof N-terminally fused to a DNA sequence encoding a signal peptide derived from the Aspergillus orvzae TAKA amylase gene or a derivative of said signal peptide, (b) culturing the transformed filamentous fungus host organism in a suitable culture medium under conditions conducive to the expression of prochymosin and secretion thereof to the medium, and (c) recovering the prochymosin or chymosin or derivative thereof from the medium. 2. A process according to claim 1, wherein the filamentous fungus is an Aspergillus sp. 3. A process according to claim 2, wherein the Aspergillus sp. is ______ niger or ______ oryzae, in particular ______ niger. 4. A process according to claim 3, wherein the DNA vector further comprises a promoter, selected from the group consisting of the A___ niger glucoamylase promoter and the A. oryzae TAKA amylase promoter.

Inventors: CHRISTENSEN TOVE, CHRISTENSEN, TOVE
Applicants: NOVONORDISK AS, NOVO NORDISK A/S
Application year: 1992
Main classification: C12N15
IPC codes: C12N9, C12N15, C12R1