Title of Invention

MODIFIED YEASTS FOR THE PRODUCTION OF STEROIDAL DERIVATIVES

Abstract

The present invention relates to novel yeast strains, methods and genetic constructs for their preparation, and their use for the synthesising or modifying steroid compounds. More particularly, the invention describes strains having a reduced 20α-HSD type activity, in particular by modification of the GCY1 and/or YPR1 genes. The yeast strains of the invention make it possible to improve the efficacy of synthesis or to increase selectivity or yields of the method, as well as the quality of the final product. The strains, methods and compounds are useful in the search for, the development and the production of products with therapeutic or prophylactic activity, in a human or animal, in particular steroid derivatives.

Full Text

MODIFIED YEASTS AND USES, IN PARTICULAR FOR THE PRODUCTION OF STEROIDAL DERIVATIVES The present invention relates to the biological and pharmaceutical fields. It relates in particular to novel compositions and methods useful for the production of steroidal compounds, or for the (selective) conversion of steroidal compounds. It relates more particularly to novel yeast strains and genetic methods and constructs for their preparation, and to their use for the synthesis or modification of steroidal compounds. The yeast strains of the invention make it possible to improve the efficacy of synthesis and to increase the selectivity or the yields of the method, and the quality of the final product. The strains, methods and compounds of the invention are useful in research, development and production of products with therapeutic or prophylactic activity, in humans or animals, in particular of steroidal derivatives. The natural capacity of microorganisms to convert steroids has been widely described in the literature. In this regard, they represent an advantageous alternative for the production of steroidal derivatives which are difficult to obtain by chemical synthesis. Yeasts are moreover particularly suitable for the expression of cDNA encoding enzymes which are active in organelles. As a result, yeasts, such as S. cerevisiae, have been widely used to express cDNAs encoding steroidogenic enzymes such as microsomal or mitochondrial P450s. Furthermore, some studies intended to express enzymes involved in the pathway for the biosynthesis of hydrocortisone have made it possible to show that yeasts were capable of efficiently converting certain intermediates. Thus, the use of transformed yeasts allowing the expression of one or more mammalian enzymes involved in the pathway for the biosynthesis of steroids has been described for example in application EP340878, patent US 5,137,822 or in Dumas et al. Likewise, the applicants have found that A5-3β-hydroxysteroids such as pregnenolone, 17a-hydroxypregnenolone and DHEA were converted by yeasts to the corresponding acetate esters. The applicants have also demonstrated that this conversion was essentially performed by the product of the ATF2 gene (Cauet et al., 1999) . Yeasts therefore represent a particularly useful organism from the industrial point of view for the production of steroidal derivatives. However, it is also known that 17a-hydroxyprogesterone may, under certain conditions, be reduced by yeasts to 4-pregnene-17a,20β-diol-3-one (Dumas et al., 1994) and that the production of this by¬product affects the yields of synthesis and the quality of the final product. However, up until now, the enzymatic activity or activities responsible for this reaction, of the 20a-hydroxysteroid dehydrogenase (20aHSD) type, has/have not been identified. The present invention results precisely from the study of the endogenous activities of yeast acting on hydroxyprogesterone and describes the identification of two genes encoding enzymes endowed with 20βHSD type activity. More particularly, the present application shows that the GCYl and YPRl genes are carriers of the 20βHSD type activity in yeast, and that the product of these genes makes it possible, for example, to convert hydroxyprogesterone to by-products in vitro. The present application shows, moreover, that the suppression of the activity of these genes in yeast considerably reduces or suppresses the formation of by-products of 4-pregnene-17a,20β-diol-3-one type, and makes it possible to significantly improve the yields of synthesis of steroidal derivatives and/or to convert hydroxyprogesterone (or its precursors) to steroidal derivatives in a more selective manner. The present application therefore describes novel compositions and methods which can be used for "the synthesis of steroidal derivatives with better selectivity. The invention describes, in particular, novel yeast strains having a reduced 20αHSD type activity and essentially incapable of converting hydroxyprogesterone to 4-pregnene-17α,20α-diol-3-one type by-products. The invention may also be used to increase the production of such products, for their use or conversion to active compounds. A first subject of the invention consists more particularly in a method for modifying a steroid compound, comprising bringing this compound (or a precursor thereof) into contact with a yeast having a reduced 20(XHSD activity, in particular a yeast having a nonfunctional, in particular disrupted, GCYl and/or YPRl gene, more preferably a yeast of the genus Saccharomyces, or a preparation derived from such a yeast. The invention also relates to the use of a yeast having a reduced 20α(HSD activity, in particular a yeast having a nonfunctional, in particular disrupted, GCYl and/or YPRl gene, more preferably a yeast of the genus Saccharomyces, or a preparation derived from such a yeast, for the preparation, production, synthesis, modification and/or improvement of steroidal compounds in vitro or ex vivo. The invention also relates to any method for producing steroidal derivatives from hydroxysteroid compounds, in particular hydroxyprogesterone or its precursors, using a yeast having a reduced 20aHSD type activity, in particular a yeast having a nonfunctional, in particular disrupted, GCYl and/or YPRl gene, more preferably a yeast of the genus Saccharomyces, or a preparation derived from such a yeast. The invention also relates to a method for converting 17α-hydroxyprogesterone, in particular to 11-deoxycortisol, using a yeast having a reduced 20CXHSD type activity, in particular a yeast having a nonfunctional, in particular disrupted, GCYl and/or YPRl gene, more preferably a yeast of the genus Saccharomyces, or a preparation derived from such a yeast. The subject of the invention is also the use of a yeast having a reduced 20aHSD activity, in particular a yeast having a nonfunctional, in particular disrupted, GCYl and/or YPRl gene, more preferably a yeast of the genus Saccharomyces, or a preparation derived from such a yeast, for the conversion of 17a-hydroxyprogesterone to 11-deoxycortisol. Another subject of the invention further consists in a method for modifying the 20cxHSD type activity of a yeast, comprising the modification of the activity of the GCYl and/or YPRl gene of the said yeast. It involves more particularly a method for reducing or inhibiting the 20aHSD activity of a yeast, comprising the inactivation of the GCYl and/or YPRl gene of the said yeast, preferably by gene disruption, more preferably still on yeasts of the genus Saccharomyces. The subject of the present invention is also particular strains of yeast having a reduced 20aHSD type activity. More preferably, this involves yeasts possessing a nonfunctional YPRl gene, yeasts possessing a nonfunctional GCYl gene and YPRl gene, or certain yeasts possessing a nonfunctional GCYl gene. The invention also relates to any acellular preparation derived from a yeast as described above, in particular a cellular lysate, a cellular homogenate, a culture supernatant, or a derived enriched or (pre-)purified solution, and the like. As indicated above, the present invention describes, for the first time, yeast strains (or cells or cultures), and derived preparations, having a reduced, or undetectable, 20α(HSD type activity. The invention indeed describes the identification of yeast genes carrying this activity, the GCYl and YPRl genes, and shows that these genes can be specifically modified, in particular by means of genetic recombination techniques, without harming the growth or survival capacity of the cells, or their ability to transform or convert steroidal compounds. The invention thus provides, for the first time, methods for the synthesis, production, modification and/or conversion of steroidal compounds using advantageous yeasts. A subject of the invention therefore consists, more particularly, in the use of a yeast strain (or cell or culture), characterized in that it possesses a genetic modification and in that it has a reduced 20αHSD activity, for the production of steroidal compounds. The present invention uses, more particularly, a yeast strain, characterized in that it possesses: a genetic modification of the GCYl gene, or a genetic modification of the YPRl gene, or a genetic modification of the GCYl and YPRl genes. More preferably, the genetic modification(s) present in the yeasts of the invention are inactivating modifications, that is to say which lead to the loss of activity of the gene and/or of the corresponding protein. A most particularly preferred type of inactivating genetic modification according to the invention is a gene disruption, as will be described in detail in the remainder of the text. More specifically, the invention therefore consists in the use of yeasts in which: the GCYl gene is nonfunctional, the YPRl gene is nonfunctional, or the GCYl and YPRl genes are nonfunctional, for the preparation of steroidal compounds. Such yeasts possess a reduced, or undetectable, 20αHSD activity, and are therefore particularly advantageous for the production or the modification or the conversion of steroidal compounds. According to a preferred embodiment of the invention, the yeasts belong more preferably to the genus Saccharomyces, in particular S. cerevisiae. Thus, in a more specific embodiment, the present invention consists in methods or uses of cells (or strains or cultures) of yeast of genus S. cerevisiae comprising a nonfunctional, preferably disrupted, GCYl gene and/or YPRl gene. However, although the examples relate more specifically to the yeast Saccharomyces cerevisiae, it is understood that the teaching of the invention is not limited to this particular type of yeasts and may be essentially extended to any yeast having a natural activity of the 20αHSD type or containing a GCYl or YPRl gene. In this context, there may be mentioned in particular the yeasts Saccharomyces, Kluyveromyces (in particular K. lactis), Schizosaccharomyces, Hansenula, Pichia (in particular P. pastoris), Candida (in particular C. maltosa) , and the like, whose culture in fermenters and genetic modification have been described in the prior art. Moreover, for the purposes of the invention, the expression GCYl gene is understood to mean the S. cerevisiae GCYl gene as described in GenBank under the reference X96740 (Bandlow et al., Gene 90(1), 1990, 105-114) , as well as any functional variant or homologue thereof present in yeast cells. Similarly, the YPRl gene denotes the S. cerevisiae YPRl (or YDR3 68w) gene as described in GenBank under the reference X80642, as well as any functional variant or homologue thereof present in yeast cells. The sequence of these genes may also be obtained from other banks in which the complete sequence of the genome of the yeast S. cerevisiae is described (Stanford University, MIPS, and the like). The functional homologues may be identified by a search for sequence homologies, or by hybridization cloning, using probes derived from S. cerevisiae GCYl and YPRl genes, according to conventional molecular biology techniques. As indicated, the present invention consists in methods or uses of yeasts exhibiting a genetic modification of one or more genes involved in the 20(xHSD activity, in particular the GCYl and/or YPRl genes, and preferably having a reduced, or even suppressed, 20aHSD activity. For the purposes of the invention, the term "genetic modification" denotes any alteration of the genome of a cell, obtained by any possible method, such as the use of mutagenic agents and/or the production of modification(s) by the genetic or recombinant route. Preferably, a genetic modification is a modification of the sequence of at least one gene, resulting in the modification of the activity" of the gene, and in particular in the stimulation or, preferably, the inactivation of the said gene. The inactivation of a gene, or the nonfunctional character of a gene, can manifest itself by the absence of expression of a protein, by the expression of a nonfunctional form of the protein, because of mutation(s), deletion(s), substitution(s), insertion(s), and the like, or by the expression of the protein in low levels, not allowing sufficient activity. As a result, the genetic modification of a gene may affect in particular all or part of the coding region of the said gene or of a regulatory region of the said gene (promoter and the like). Preferably, the genetic modification according to the invention comprises at least one mutation, substitution, deletion and/or insertion of one or more base pairs in the regulatory or coding region of the gene considered. More preferably still, it involves a modification via deletion of all or part of the gene considered, which may be replaced by foreign sequences, according to the gene disruption (or "gene replacement") technique. Genetic modifications via deletion and/or insertion are preferred for carrying out the present invention since they are selective for the gene considered and are stable over time. More preferably, the genetic modification therefore consists in the replacement of at least part of the gene considered with foreign sequences. This modification may be accomplished by known techniques consisting in preparing a modified gene in vitro, which may be introduced into the genome of yeasts by double homologous recombination, as described in the examples (also see Baudin et al., Nucleic Acids Res. 21(14) (1993) 3329). Thus, a preferred subject of the invention consists in methods or uses of yeasts in which all or part of the GCYl and/or YPRl gene has been replaced by foreign (or heterologous) sequences, for example by a marker gene (encoding resistance to an antibiotic). More particularly, for gene disruption, a recombinant nucleic acid is prepared in vitro, comprising a chosen foreign sequence bordered by sequences homologous to contiguous "or noncontiguous regions of the gene considered. The foreign sequence may be for example a marker gene, a gene complementing an auxotrophy, an expression unit, and the like. More particularly, the foreign sequence may be an auxotrophic selection gene complementing a nutritional requirement in the host yeast strain, such as the URA3 gene, LEU2 gene, TRPl gene, HIS3 gene or ADE2 gene for example; a dominant selection gene, such as a gene for resistance to an antibiotic (G418, phleomycin, hygromycin B, and the like) ; or a reporter gene (P-galactosidase, and the like). It may also be an expression interrupting unit, comprising, for example, a transcriptional terminator such as in particular a yeast terminator chosen from CYCl, TDH3, TEFl or PGK. It is understood that any other foreign sequence (i.e., not naturally present in this form in the gene considered) making it possible to alter the conditions for expression of the gene and/or the actual structure of the protein encoded may be used in the context of the present invention. The nucleic acid thus prepared is then introduced into the yeasts, by conventional techniques (lithium, protoplasts, and the like) , leading to the insertion of the foreign sequence into the genome" of the yeast, within the sequence of the gene considered, optionally as a replacement for a region thereof, by double homologous recombination. It is understood that any other genetic modification technique may be used in the context of the present invention, such as for example site-directed mutagenesis, the use of transposons and the like. Specific examples of yeasts having a GCYl and/or YPRl gene inactivated by gene disruption are in particular: - the TGyi70 cells (gcyl::LEU2) : in the TGY170 cells, a portion of the GCYl gene has been replaced by a nucleic acid encoding the LEU2 protein allowing selection of the recombinants. - The TGY197 cells (gcyl::LEU2, ydr368w::URA3) : the TGY197 cells comprise, in relation to the TGY17 0 cells, an additional genetic modification affecting the YPRl (also designated YDR368w) gene, in which a portion has been replaced by the selection gene URA3. - The TGY195 cells (ydr368w::URA3) : the TGY195 cells comprise a genetic modification affecting the YPRl (also designated YDR368W) gene, in which a portion has been replaced by the selection gene URA3. - The TGY194 cells (gcyl::URA3) : in the TGY194 cells, a portion of the GCYl gene has been replaced by a nucleic acid encoding the URA3 protein allowing selection of the recombinants. Such cells also constitute a particular subject of the invention. In particular, the invention relates to any yeast cell (or strain or culture) comprising a genetic modification of the (or in the) YPRl gene, in particular a deletion and/or an insertion of the or in the YPRl gene. The invention" also relates to any yeast cell (or strain or culture) comprising a genetic modification of the (or in the) GCYl and YPRl genes, in particular a deletion and/or an insertion of the or in the GCYl and YPRl genes. The invention also relates to acellular preparations derived from such yeasts. The cells of the invention or used in the methods of the invention advantageously have a reduced 20α(XHSD type activity, that is to say reduced by at least 20%, preferably by at least 40%, more preferably by at least 60%, relative to the nongenetically-modified strain. As is shown in the examples, the invention demonstrates that the inactivation of the GCYl gene in yeast leads to a 95% reduction in the 20αHSD type activity in the supernatant of a cellular homogenate. The results obtained also show that the double genetic modification of the GCYl and YPRl genes leads to the suppression of the 20αHSD type activity, which is then undetectable in the supernatant of a cellular homogenate. These results provide the demonstration of the role of these genes, and illustrate the possibility of modifying them in order to improve the properties of the yeasts, for the applications of production of steroidal derivatives. The present invention can be used for the production of steroidal compounds, for various pharmaceutical applications. In this regard, the invention describes methods for producing steroidal compounds using the yeasts of the invention. The application also consists in improved methods for producing steroidal compounds using yeasts having a reduced 20αHSD activity. The methods of the invention are advantageously carried out by bringing a population of yeasts as described above into contact, in vitro, with a steroidal compound, followed by extraction of compounds synthesized. The initial steroidal compound may be any natural or modified or synthetic steroid, in particular any hydroxysteroid or precursor compound, in . particular cholesterol, progesterone, pregnenolone or 170H-progesterone. The methods of the invention can "be used for the production of steroidal derivatives such as ll-deoxycortisol, Cortisol, hydrocortisone, and the like, or derivatives thereof. Other aspects and advantages of the present invention will emerge on reading the examples which follow, which should be considered as illustrative and nonlimiting. LEGEND TO THE FIGURES Figure 1: SDS-PAGE analysis of a purified fraction of the 20aHSD activity by chromatography on Redl20-Agarose from a yeast homogenate. The top arrow indicates the band whose N-terminal sequence corresponds to GCYl, the bottom arrow, that corresponding to the N-terminal sequence of YPRl. Lane 1 corresponds to the molecular weight marker while lane 2 corresponds to the purified fraction of the 20aHSD activity. Figure 2: production of 4-pregnene-17a,20a-diol-3-one by yeasts S. cerevisiae cultured in galactose (YNB-gal) or glucose (YPD) medium in the presence of 0.1 mg/ml of 17a-hydroxyproges terone. Figure 3: production of 4-pregnene-17a,20a-diol-3-one by yeasts S. cerevisiae of the wild type (wt) or mutated in the sequence of the GCYl gene (gey-) cultured in galactose (YNB-gal) or glucose (YPD) medium in the presence of 0.1 mg/ml of 17a-hydroxyprogesterone. Figure 4: structure of the plasmid for disrupting the GCYl gene. The plasmid is linearized with the enzymes BamHI and HindIII and then transformed in S. cerevisiae according to the method described in the section Materials and Methods. The deletion of the sequence of the GCYl gene comprises the promoter and 3 06bp of coding sequence, including the start codon for translation. Figure 5: structure of the plasmid for disrupting the GCYl gene (plasmid pTG12010 clone 40) . The plasmid is linearized with the enzymes EcoRI and SphI and then transformed in S. cerevisiae according to the method described in the section Materials and Methods. The deletion of the protein sequence of GCYl comprises amino acids 47 to 268 inclusive. pTGl2010 clone 36 possesses the same structure, without the Clal site in 5" of the URA3 gene but with a HindIII site in 3" of the URA3 gene. Figure 6: structure of the plasmid for disrupting the YPRl gene (YDR3 68w) (plasmid pTGl2011). The plasmid is linearized with the enzyme Xhol and then transformed in S. cerevisiae according to the method described in the section Materials and Methods. The deletion of the protein sequence of YPRl comprises amino acids 5 to 198 inclusive. MATERIALS AND METHODS Chemical products: 17a-hydroxyprogesterone was obtained from Hoechst Marion Roussel (Romainville, France). Tergitol Nonidet P40 and Tyloxapol were obtained from Sigma. Enzymatic test: Conversion in vivo of 17a-hydroxyprogesterone: the yeast cells were cultured at 28°C in YPD medium (10 ml) inoculated at A600 = 0.1 from a 24 h preculture. 100 |ll of a 17a-hydroxyprogesterone solution (10 mg/ml) in a Tergitol and ethanol mixture (1/1; v:v) were then added to the culture. Aliquots of culture broth (250 μl) were collected at various intervals and the steroids were extracted with dichloromethane. The steroids were then separated on Ultrasphere ODS in the presence of 45% aqueous acetonitrile at a flow rate of 1 ml/min, at 45°C, These steroids were detected at 240 nm. Cells: The E. coli BJ5183 strain (Hanahan, 1983) was used for the recombinations in vivo and the strain C600, hsdR (Hubacek and Glover, 1970) for the conventional ligation reactions. The parental strain of yeast FYl679-28c (MATa ura3-52 trpl-63, leu2- Ifenl his3- 200GAL) (Thierry et al., 1995) was used. The strains TGY170, TGY197, TGY195, TGY194, TGY212, TGY245 and FY1679-2Bc/pTGl0497 were constructed as described in the examples. Conventional methods of molecular biology and of recombination in vivo in E. coli and in yeast were used, as described in Sambrook et al. (1989) or in Degryse et al. (1995, 1996) . Culture of the yeasts: The yeasts were generally cultured on synthetic minimum medium (Sherman, 1991) supplemented with nutritional supplies at 100 )μg/ml. For the transformations of S. cerevisiae, the cells were made competent according to the lithium acetate technique (Ito et al., 1983), after growth on YPD medium (Sherman, 1991). RESULTS Identification of the 20(XHSD activity in yeast, responsible for unwanted reactions on 17a- hydroxyprogesterone The NADPH-dependent reduction of 17a-hydroxyprogesterone on C20 to 4-pregnene-17a,20a-diol-3-one by the yeast S. cerevisiae has been previously described (Dumas et al., 1994). This activity is similar to the 20aHSD activity reported in various tissues. The enzymes characterized from these tissues are monomeric, with a molecular weight of about 3 5 kDa. With the aim of identifying the enzyme(s) responsible for the 20aHSD activity in yeast, searches" for homologies with the enzyme 20o(HSD from bovine testes in S. cerevisiae banks were carried out. These searches made it possible to identify 6 products of yeast genes exhibiting 44 to 32% amino acid sequence identity with the mammalian enzyme. These genes are assembled in Table I. With the aim of better characterizing the enzymes involved, the yeast 20(XHSD type activity was reconstituted in vitro using 17a-hydroxyprogesterone and NADPH as substrates. Various preparations derived from S. cerevisiae yeast culture were tested in this system, which allowed localization of the activity in the supernatant after centrifugation at 100,000 x g of a cellular homogenate. This result indicates that the enzymatic activity is soluble. Partial purification of the 20aHSD type activity by Red 120 chromatography was then carried out, which allowed the production of a doublet in the 35 kDa region, after SDS-PAGE (Fig. 1). The sequencing of these bands showed that they were mainly composed of the product of the GCYl and YPRl genes. These two enzymes form part of the bovine homologues of 20aHSD which are listed in Table I. The complete sequence of the GCYl and YPRl genes is accessible, for example, in GenBank under the references X96740 and X80642, respectively. It is advantageous to note that GCYl has been described as encoding aldo-keto-reductase (AKR) whose expression is significantly increased in the presence of galactose (Magdolen et al. , Gene 90(1), 1990, 105). The AKR enzymes have a broad substrate specificity. They metabolize various substrates, including the aliphatic aldehydes, monosaccharides, prostaglandins and steroids. Thus, GCYl constitutes a good potential candidate, and we decided to verify if this enzyme could be involved in the generation of 4-pregnene-17a,20a-diol-3-one from 17a-hydroxyprogesterone. Inducible character of the 20aHSD activity and expression in an acellular system The experiments carried out made it possible to demonstrate that the 20o(HSD activity is inducible by galactose in yeast. Thus, the conversion in vivo of 17a- hydroxyprogesterone to 4-pregnene-17a,20a-diol-3-one was determined in yeast cultures cultured on various carbon sources. The delay observed when the yeasts are cultured on glucose is not observed in the presence of galactose (Fig. 2). This observation is in agreement with a repression by glucose of the gene encoding 2 0aHSD. The conversion starts after 16 h when the glucose is depleted. The conversion of 17a-hydroxyprogesterone to 4- pregnene-17a,20a-diol-3-one is approximately 4 times higher after 48 h when the yeasts are cultured in the presence of galactose. These results were moreover confirmed in vitro by measuring the 2 0(XHSD activity on an acellular extract obtained from yeasts cultured in galactose - or glucose medium. The 20aHSD specific activities were respectively 0.05 and 0.75 )iM/min/mg in the homogenates of cells cultured in glucose or galactose medium. These results therefore show (i) that the 20aHSD activity is carried by the product of the yeast GCYl and YPRl genes, (ii) that these enzymes are soluble, and (iii) that their activity may be increased in the presence of galactose and repressed in the presence of glucose. Construction and properties of yeasts containing a nonfunctional GCYl and/or YPRl gene With the aim of confirming that Gcylp was responsible for the 20aHSD activity, the gene corresponding to ORF YOR120W was deleted (Knock Out) from the yeast genome. The results obtained show that the strain obtained has a highly reduced 20aHSD activity compared with the wild-type strain. Moreover, strains in which the YPRl gene alone or in combination with GCYl is deleted were also constructed and tested for their activity, as described below. Construction of deficient GCYl and/or YPRl yeasts: The yeasts deficient in GCYl and/or YPRl activity were prepared by gene disruption. More particularly: The strain TGY170 (FY1679-28c, gcyl::LEU2) was constructed by disrupting the GCYl gene by means of the plasmid Pgcyl::LEU2. The strain TGY197 (FY1679-28c, gcyl::LEU2 ydr3 68w::URA3) was generated by additional disruption of the YDR3 68W gene (YPRl), according to the method described for ATF2 (Cauet et al. , 1999) by means of the plasmid pTG12011. The strains TGY195 were generated by disruption of the gene YDR3 68w (YPRl), according to the method described for ATF2 (Cauet et al. , 1999) by means of the plasmid pTGl2011. The strains TGY194 (FYl679-28c, gcyl::URA3) were constructed by disrupting the GCYl gene by means of the plasmid pTG12010. The strains FY1679-28c/pTGl0497 and TGY245 were constructed by means of the plasmids pTG10497 and PTG12045. The following plasmids were used to disrupt the GCYl and YPRl genes: pgcyl::LEU2, pTG12010, pTG12011 (Figures 4-6), PTG12086 and pTGl2045. The single copy plasmid PTG10497 is used for the expression of P450c21. The plasmid pgcyl::LEU2, described by Magdolen et al., Gene 90 (1990) 105-114, contains the GCYl gene whose coding sequence and promoter have been interrupted by the sequence encoding the LEU2 gene. More precisely, the promoter and the coding portion corresponding to the EcoRV and Hindi restriction fragment were replaced by the Hpal fragment of 2.17 Kbases of the LEU2 gene. Thus, the GCYl gene was deleted for its promoter and for 306 base pairs of its codihCr sequence. The plasmid pgcyl::LEU2 was linearized by the restriction enzymes HindIII and BamHI, the HindIII BamHI fragment of 3.1 kb containing the disrupted gene was prepared in order to transform the strain Fy 1679-28c using the protocol described by Gietz RD et al. (Yeast 1995 Apr 15; 11(4): 355-60 Studies on the transformation of intact yeast cells by the LiAc/SS-DNA/PEG procedure). The colonies were selected on a leucine-free medium. The positive colonies in this screen are then cultured in rich meditim in order to carry out a bioconversion of 170H progesterone as described in Dumas et al. (Eur J Biochem 1996 Jun 1; 238(2): 495-504 11 beta-hydroxylase activity in recombinant yeast mitochondria. In vivo conversion of 11-deoxycortisol to hydrocortisone). The concentration of the substrate 170H progesterone is 100 mg/1, the carbon source is galactose and the initial optical density is 0.1. The volume of the culture is 10 ml, the incubation is 48 hours at 3 0°C. After incubating for 48 hours, the positive clones are evaluated by extracting one ml of medium (with the cells) with 2 ml of dichloromethane and then analysing the organic phase by reversed phase high-performance liquid chromatography as described above (Dumas et al., 1996). The chromatograms are analysed for the presence of 17,20-dihydroprogesterone in comparison with the purified product. During this incubation, a quantity of the order of 4 mg/1 of 17,20-dihydroprogesterone, that is 4% of the substrate, appears in the culture medium for the wild-type strain (not transformed by the disrupting fragment), whereas in some transformants, the presence of 17,20-dihydroprogesterone is now only 1 mg/1. A TGY170 strain converting a low level of 170H progesterone to 17,20OH progesterone and exhibiting identical growth to the wild-type strain is selected. Two new plasmids pTG12 010 and pTGl2011 were constructed to allow the disruption of the GCYl and YPRl genes associated with the selection marker URA3. The plasmid pTG12010 was constructed on a plasmid pUC19 base (Gene 1985; 33(1): 103-19 Improved M13 phage cloning vectors and host strains: nucleotide sequences of the Ml3mpl8 and pUC19 vectors. Yanisch-Perron C, Vieira J, Messing J) while the plasmid pTGl2011 was constructed on a plasmid pPOLYIII base (Lathe, R., Vilotte, J.-L. and Clark, J. A. Plasmid and bacteriophage vectors for excision of intact inserts JOURNAL Gene 57, 193-201 (1987)). Construction of the plasmids pTG12010 and pTG12011 The construction of the disruption of the GCYl gene by the URA3 gene in the plasmid pUCl9 in order to arrive at pTGl2010 was obtained by four successive PCR amplifications. On the one hand, three independent PCRs were carried out in order to obtain the 5" portion of the GCYl gene (PCRl) , the functional URA3 gene bordered by GCYl sequences (PCR2), the 3" portion of the GCYl gene (PCR3); the 5" and 3" portions of the GCYl gene with the aid of the pairs OTG11285, OTG11286 and OTG11287, OTG11289, on a genomic DNA template of the strain Fy 1679-28C. The sequence of the oligonucleotides is the following: OTG11285: GATTCGGTAATCTCCGAACAggtac cAATTATATCAGTTATTACCCGGGA (SEQ ID NO: 1); OTG11286: AGCCATCTTTCAAAGCGGTT (SEQ ID NO: 2); OTG11287: CCGATCGAATCAAAACGAACAG (SEQ ID NO: 3); OTG11289: TCTAATCAGCTAGTAAGAAC (SEQ ID NO: 4). The URA3 gene, flanked by the GCYl sequences (so as to obtain a deletion of a portion of the coding sequence of the GCYl gene) , is amplified with the aid of the oligonucleotides OTG113 05 (aaccgctttgaaagatggctATCGATTTTCAATTCAATTCATCATTTTTTTTTTAT TCTTTTTTTTG, (SEQ ID NO: 5) and OTG113 06 (CtgttegttttgattcgatcgggAAGCTTGGGTAATAACTGATATAATTAAATTGA ACTC (SEQ ID NO: 6) from a linearized plasmid pTG10054 template (Degryse et al., In vivo cloning by homologous recombination in yeast using a two-plasmid-based system. Yeast. 1995 Jun 15; 11(7): 629-40). The conditions with respect to buffer and concentration of template and primers for the amplification are described by the producer or manufacturer of the enzyme TAQ DNA polymerase, and in particular for the enzyme elongase developed by Life Technologies. The temperature cycles are the following: a first cycle of 6"30" to denature primer and template and then 30 cycles of 30 s at 93°C, 2 min at 54°C and 3 min at 68°C, the last cycle is of 5 min at 72°C. The products PCRl, PCR2 and PCR3 were mixed in equimolar quantities and again amplified with the aid of the oligonucleotides OTG11285 and OTG11289 (see above). The final product PCR4, having a size of 1.9 Kbases, is then subcloned between the Kpnl and BamHI restriction sites of the plasmid pUC19 in order to obtain the plasmid pTGl2010. The structure of the plasmid was checked by restriction profiling and nucleotide sequencing of the ends. The cloning of pTG12010 in fact made it possible to obtain two versions of this plasmid, the version pTG12010#40 (pTGl2010 clone 40) and pTG12010#3 6 (PTG12010 clone 36) . The initial desire was to obtain the GCYl gene interrupted by the URA3 gene bordered by the Clal and Hindlll sites respectively in 5" and in 3" of the gene. In fact, two different plasmids were obtained, pTG12010#36 and pTG12010#40. These two plasmids differ only in the presence or the absence of Clal and HindIII sites at the ends of the URA3 gene. The plasmid pTGl2010#40 possesses a Hindlll restriction site at the 3" end of the URA3 gene but no Clal site in 5". The plasmid pTG12 010#3 6 possesses no Hindlll site at the 3" end but a Clal site at the 5" end of the gene. This property is used to obtain the plasmid which possesses the URA3 gene bordered by the Hindlll and Clal sites interrupting the coding.sequence of GCYl. Construction of the plasmid pTG12036. The plasmid pTG12036 was constructed in 4 stages from PTG10802. The plasmid pTGlOSOl (which is responsible for the plasmid pTGl0802) is a plasmid of the pUC type into which a succession of restriction sites has been inserted between the Xhol and Xhol sites. This succession of sites comprises the HindIII, SnabI, Clal and Spel sites. Between the Hindlll and Clal sites, the Hindlll Clal cassette of pTGl0470 (as described below) comprising the promoter TEFl, the human cDNA p450c21 and the PGK terminator, was inserted between the Hindlll and Clal sites of pTGlOSOl to give pTG10802. This plasmid was then digested with Xhol and therefore the cassette introduced is deleted in order to introduce a PCR fragment bordered by Xhol sites. This fragment of 2.5 kb is obtained from amplification by the pair of oligonucleotides OTG11844 (tttgctcgaggttacagaagggc, SEQ ID NO: 13) and OTG11845 (gattctcgagcaattggctgacta, SEQ ID NO: 14) on the plasmid pTGl2010 (#40) in order to obtain a fragment bordered by Xhol" s containing the GCYl gene interrupted by the UPIA3 gene bordered in 5" by a Clal restriction site. This fragment was cloned between the Xhol sites of the plasmid PTG10802 in order to obtain the plasmid pTGl2035. With the aim of introducing the missing Hindlll site, the plasmid pTG12010 (#36) was used. This plasmid is essentially identical to pTGl2010 (#40) but possesses a Hindlll site in 3" of the URA3 gene at the limit with the GCYl gene and does not possess a Clal site in 5" of the URA3 gene at the junction with the GCYl gene. Recombination is carried out in vivo in E. coli, between the Ncol BamHI fragment of 2.2 kb of pTG12010 (#36) (which carries from 5" to 3" a fragment of the URA3 gene and in 3" a fragment of the GCYl gene) and a portion of the plasmid pTG12035, that is to say the large StuI, AfIII fragment of 4.45 kb. The plasmid obtained pTG1203 6 possesses the GCYl gene interrupted by the URA3 gene bordered by Clal and Hindlll sites in 5" and 3" respectively. Construction of the plasmid pTGl2086 This fragment is then replaced by the expression cassette of P450c21 carried by the Clal, HindIII 2.33 Kb fragment of the plasmid pTG10469 (see below) in order to obtain the plasmid pTGl2036. Construction of the expression plasmids for cytochrome P45Oc21. For overexpression of this protein in yeast, two types of promoter were used, TEFl ("transcription elongation factorl") and TDH3 ("glyceraldehyde-3-phosphate dehydrogenase 3"). In all cases, the transcription terminator is the PGK terminator. In these plasmids, the Sail, Mlul fragment carries the cDNA for human P450c21. Construction of the plasmids pTGl0470 and pTG10469. The plasmid pTG10298 was obtained by modifying pMAc21 (Expression and functional study of wild-type and mutant human cytochrome P450c21 in Saccharomyces cerevisiae. Wu DA, Hu MC, Chung BC DNA Cell Biol 1991 Apr; 10(3): 201-9)) Kpnl, Mlul digestion and introduction of the oligonucleotide OTG5868. The cDNA of this plasmid is obtained from the American Type Culture Collection under the name pc21/3c. It is the EcoRI-BamHI fragment of 1.6 Kb which served as base for the construction of the various plasmids. The modifications made are described in the above article and in the article (Expression of human 21-hydroxylase (P450c21) in bacterial and mammalian cells: A system to characterize normal and mutant enzyme Meng-Chun Hu and Bon-chu Chung DNA and Cell Biology 1991 Apr; 10 (3) 201-209) . In this procedure, the noncoding portion of P450c21 in the plasmid pMAc21 which contains the expression cassette for P450c21 was eliminated as well as the Kpnl site which is present therein. The plasmid pTGl0292 was obtained by transfer of the human c21 cDNA (Sail, Mlul fragment) from the plasmid •"pTG10298 into the plasmid PTG10031 with the aid of the Sail and Mlul sites. The plasmid pTGl0475 was obtained by PCR and recombination. Indeed, starting with the plasmid pTG10292, a fragment of the human P450c21 cDNA representing approximately 250 nucleotides was amplified with the aid of the oligonucleotides OTG7410 (GGAATTCCGTCGACAAAAATGCTGCTCCTGGGCCTGCTGC, SEQ ID NO: 15) and OTG5927 (CCTCAATGGTCCTCTTGGAGTTCAGCACC, SEQ ID NO: 16) . This fragment represents the coding sequence of the human P450c21 bordered by a Sail site and the sequence AAAA, as described in the oligonucleotide OTG7410. This fragment was digested with Sail and then ligated into the linear fragment of pTG10292 digested with Sail and then a recombination experiment was carried out in the strain BJ5183. The plasmid obtained pTGl0475 carries a cDNA for P450c21 with a coding sequence identical to that of the natural cDNA unlike the plasmid pMAc21 on a fragment which is compatible with the vectors which we use in the laboratory, that is to say a fragment bordered by the Sail and Mlul restriction sites. This fragment possesses the following environment around the ATG codon for initiation of translation GTCGACAAAAATGCTGCTCCTGGGCCTGCTGC (SEQ ID NO: 17). From this plasmid, the Sail, Mlul fragment carrying the human P450c21 cDNA was transferred into the plasmid pTG10158 (Degryse et al., 1996) by conventional cloning in order to obtain the plasmid pTGl0472. This same Sail Mlul fragment of the plasmid pTG10472 was then transferred by recombination into the plasmid pTG10085 (Degryse et al., 1996) to give the plasmid pTGl0469. This same fragment carrying the P450c21 cDNA on a Sail and Mlul restriction fragment was transferred into the plasmid pTG10092 to give the plasmid pTGl0470 (Degryse et al. , 1996). This plasmid therefore carries the cDNA for human P450c21 under the control of the TEFl promoter and a PGK terminator with a URA3-d selectable marker with an ATG initiation codon environment, as described above. construction of the plasmid PTG12086. This plasmid serves for the integration of an expression cassette for P450c21 and for the disruption of the GCYl gene at the same time. This plasmid was constructed from the plasmid PTG12036 and the plasmid pTG10614. The latter plasmid was constructed from pTG10212 (Degryse et al., Yeast 11: 629-640 (1995)) which is a yeast expression plasmid based on a TDH3 promoter, a PGK terminator and a URA3-d selectable marker. Through homologous recombination in E. coli, the selectable marker is replaced by the selectable marker of the plasmid pTG10054 (Degryse et al., 1995); to do this, the Mlul, Fspl fragment of pTG10054 of 2.1 kb containing the URA3 marker flanked by recombination sequences is recombined with the large HindIII fragment of pTG10212 to give the plasmid pTGl0610 which possesses the same characteristics as pTG10212 (Degryse et al., 1995) with a URA3 marker in the same orientation as pTGl0054. The Sail Mlul fragment carrying the cDNA for human cytochrome P450c21 of the plasmid pTG10472 (see above) is transferred into the plasmid pTG10610 to give the plasmid PTG10614. The Clal HindIII fragment of this plasmid containing from 5" to 3" the TDH3 promoter, the cDNA for human P450c21 bordered by Sail and Mlul sites and then the PGK terminator is transferred into the plasmid PTG12036 to give the plasmid pTG12086 which therefore contains the sequence of the GCYl gene interrupted by the TDH3 expression cassette for human cytochrome P450c21. Construction of the plasmid pTG12045. The unique SphI site of the plasmid pPolyIII is destroyed by inserting the pair of complementary oligonucleotides OTG11975 (AAATCGATAACATG, SEQ ID NO: 18) and OTG1197 6 (TTATCGATTTCATG, SEQ ID NO: 19) . The SphI site of pPOLYlII is destroyed,and replaced by a Clal site to give the plasmid pTG12040. Into the plasmid pTG12040, between the unique Clal and EcoRI sites, there is introduced a Clal EcoRI genomic DNA fragment corresponding to the 3" portion of 0.7 kb of the YPRl gene obtained by amplification with the oligonucleotides OTG11981 (ATTGATATCGATAAAAAGCACGGCGTTGAG, SEQ ID NO: 20) and OTG11982 (TCTCGGAATTCAGGTACTGCAGCCAG, SEQ ID NO: 21) to give the plasmid pTG12041. Into this plasmid pTGl2041 of 2.84 kb, the 5" portion of the YPRl gene (0.66 kb) amplified by the oligonucleotides OTG11314 (tacgctcgagACGTTGGTGTCATTGATATTCA, SEQ ID NO: 22) and OTG11980 (CAACTAAGCTTCATTCAAATAGATAGCCGC, SEQ ID NO: 23) from the genomic DNA of wild-type yeast is cloned in the form of an Xhol Hindi 11 fragment between the Sail and HindIII sites of the plasmid pTG12041. The plasmid PTG12042 of 3.5 kb is obtained. This plasmid carries the YPRl gene interrupted by the Clal and HindIII sites. Between these sites, the cytochrome P450c21 cassette is cloned in the form of a Clal Hindlll fragment of 2.33 kb obtained from the plasmid pTGl0469. The plasmid pTGl2045 is thus obtained. Construction of the plasmid pTGl0497. This plasmid is an expression plasmid with a low copy number (of the ARS CEN type) , which contains an expression cassette for human cytochrome P450c21 which is found under the control of the TDH3 promoter and the PGK terminator. This plasmid was constructed from the plasmid pTGl0434 which contains the URA3 selectable marker and the TEFl promoter, the PGK terminator and a replication origin in yeast of the ARSH4/CEN6 type (Degryse et al., 1995). This plasmid was modified so as to contain a marker LEU2 and promoter TDH3 in place of the markers URA3 and promoter TEFl, respectively. To do this, the Spel, Mlul fragment which contains the LEU2 marker bordered by recombination fragments which are the PGK terminator and a fragment of the replication origin is cloned by recombination in place of the URA3 region of the plasmid PTG10434 digested with HindiII to obtain the plasmid PTG10466. In this plasmid pTG10466, the TEFl promoter is replaced by the TDH3 promoter by recombining in E. coll the HindIII-EcoRI fragment of pTGl0212 (Degryse et al., 1995) (containing the replication origin of E. coli and the promoter and terminator TDH3 and PGK respectively) with the Mlul Fspl fragment of pTG10466 which contains the LEU2 marker and the ARSCEN replication origin which are bordered by the recombination sequences; the plasmid PTG10612 is thus obtained. Between the Sall and Mlul sites of this plasmid, the expression cassette TDH3::human P450c21 cytochrome and terminator is placed to give the plasmid pTGl0497. Construction of the deficient strains To obtain the strain lacking GCYl activity, the strain FY 167928c is transformed by the plasmid pTG121010 linearized with the enzymes SphI and EcoRI according to the lithium acetate method described above. The transformed clones are selected on a uracil-free medium and then screened by amplification on colony with the aid of the pairs of oligonucleotides OTG112 85, OTG112 89 and OTG11285, OTG11306 using, as template, an extract or a preparation of yeast genomic DNA according to the conditions described above. The colonies which show PCR DNA products of the expected size, 1.9 Kb and 1.4 Kb respectively, are then cultured in rich medium to carry out a bioconversion of 170H progesterone as described in Dumas et al. (Eur J Biochem 1996 Jun 1; 238(2): 495-504 11 beta-hydroxylase activity in recombinant yeast mitochondria. In vivo conversion of 11-deoxycortisol to hydrocortisone). The substrate concentration is 100 mg/1, the carbon source is galactose and the starting optical density is 0.1. The culture volume is 10 ml, the incubation is 24 hours at 30°C. After 24 hours of incubation, the positive clones are evaluated by extracting one ml of medium (with the cells) with 2 ml of dichloromethane and then analysing the organic phase by reversed phase high-performance liquid chromatography as described above (Dumas et al., 1996). The strains TGY194 #10 and TGY194 #11 lack 20 keto reductase activity on 170H progesterone and give a positive signal in PCR. The construction of the interruption of the YPRl (YDR3 68w) gene by the URA3 gene in the plasmid pPOLYIII to lead to pTG12011 was obtained by 4 successive PCRs. On the one hand, three independent PCRs were carried out in order to obtain the 5" portion of the YPRl gene (PCR 5), the functional URA3 gene bordered by YPRl sequences (PCR 6), the 3" portion of the YPRl gene (PCR 7). The PCR 5 DNA was obtained by amplification on a genomic DNA template with the oligonucleotides OTG11314 (tacgctcgagACGTTGGTGTCATTGATATTCA, (SEQ ID NO: 7) and OTG11315 (CTTCATTCAAATAGATAGCCG, (SEQ ID NO: 8), likewise the PCR 7 DNA is obtained by amplification with the aid of the oligonucleotides OTG11316 (TATGGCTAAAAAGCACGGCTT, (SEQ ID NO: 9) and OTG11317 (cgatctcgagTTTCTCGTTGTTCAGGTACTG, (SEQ ID NO: 10) on the same template. The URA3 gene flanked by the 5" and 3" YPRl region is amplified with the aid of the oligonucleotides OTG11463 (CGGCTATCTATTTGAATGAAGatcgattttcaattcaattcatcatttttttttta ttcttttttttg, (SEQ ID NO: 11) and OTG11464 (AACGCCGTGCTTTTTAGCCATAAGCTTgggtaataactgatataattaaattgaac tc, (SEQ ID NO: 12) on a linearized pTG10054 template as described above. The PCR 5, PCR 6 and PCR 7 products were mixed in equimolar quantities then amplified by PCR with the aid of the oligonucleotides OTG11314 and OTG11317 to give a l.X kb product of PCR 8 as described above. This PCR 8 product was digested with the enzyme Xhol and then subcloned into the plasmid pPOLYIII digested with Xhol. The orientation of the insertion in the plasmid pPOLYIII was determined by digestion with the enzymes Ncol and EcoRI. Strangely, the absence of a Clal site and of an HindIII site is noted for . the plasmid pTGl2010 and pTGl2011, respectively. The cloning junctions were checked by nucleotide sequencing. Construction of the TGY195 strain. The plasmid pTG12011 is digested with the enzyme Xhol, the product of digestion is then used to transform the strain Fy 1679-28c using the lithium chloride method cited above. The transformants are selected on a uracil-free medium. The transformants are analysed by PCR amplification using the oligonucleotides which served for the construction of the plasmid pTG12011. The positive clones in this test are then screened by the 170H progesterone bio conversion method described above in the presence of glucose as carbon source. A clone TGY195#4 is selected for new characterizations. Construction of the TGY197 strain A clone TGY195#4 exhibiting reduced 20 keto reductase activity under these conditions is selected for a new transformation with the aid of the plasmid pgcyl::LEU2 as described above for the strain Fyl679-28c. The clones capable of growing in the absence of leucine are then selected for a new bioconversion on 170H progesterone as described above in the presence of glucose or galactose. A clone TGYl97#a exhibiting reduced activity under the two bioconversion conditions is selected. Thus, the 20 keto reductase activity which was originally 12% {at 100 mg/1 of substrate) is reduced to about 0.2%, that is a reduction of more than 60 fold. Construction of the TGY245 strain The TGY245 strain is constructed from the TGY195#4 strain. From the TGY195#4 strain, the strain TGY212#1 is first obtained followed by the strain TGY243#1 and finally the strain TGY245. The strain TGY195#4 is transformed using both the plasmid YRP7 (1 p,g) and 5 ^g of plasmid pTG12045 digested with Notl. The transformed .strains are selected on a tryptophan-free medium. Colonies (678 colonies) are subcultured on a tryptophan-Containing medium (so as to eliminate the plasmid YRP7) and on a medium containing tryptophan and 5-fluoroorotate (5F0) so as to select the colonies which have lost the URA3 gene interrupting the GCYl gene. One colony is selected from this screen, TGY212#1. This colony is subjected to a bioconversion experiment as described above with 100 μg/ml of 170H progesterone substrate, the strain is grown in a minimum medium supplemented with necessary amino acids and uracil. This strain is capable of converting 170H progesterone to 11-deoxycortisol with an efficiency of the order of 47% over 24 hours with a low production of 4-pregnene-17α,20α-diol-3-one under these conditions ( To do this, the TGY212#1 strain was transformed with 3 ng of the plasmid pTGl2010#36 linearized with the restriction enzymes SphI and EcoRI. Twenty-seven transformants were selected on a minimum medium supplemented for the auxotrophies of TGY212#1 but containing no uracil. These colonies were subjected to bioconversion tests in a minimum medium supplemented with galactose as carbon source because the latter is a known inducer of GCYl. All the TGY243 clones exhibited a capacity to convert 170H progesterone to 11-deoxycortisol without, as a result, producing detectable quantities of 4-pregnene-17α,20α-diol-3-one. A TGY243#1 clone was selected in order to introduce in the place of the URA3 gene an expression cassette for human P450c21. This TGY243#1 strain is transformed with the plasmid YRP7 (2 μg) and with the plasmid pTG12086 linearized with the enzyme Xhol (5 μg) . The pTG12086 transforming fragment contains the coding sequence of GCYl interrupted by an expression cassette for human P450c21 (TDH3::human CDNAP450c21::PGK°terminator). The colonies growing in the absence of tryptophan are selected. These 381 colonies are then transferred onto a medium containing tryptophan and 5-fluoroorotate. About ten colonies were then tested in a rich medium of the YPG type supplemented with tryptophan, histidine, leucine and uracil at a concentration of 50 μg/ml. The strains are then allowed to convert 170H progesterone at a concentration of 100 μg/ml starting with an OD600 nm of 0.1 for 16 hours. Among these 10 clones, a clone TGY245#1D is chosen based on two criteria, in particular the capacity to convert 170H progesterone to 11-deoxycortisol and secondly the absence of formation of 4-pregnene-17a, 20a-diol-3-one indicating the disruption of GCYl. Property of the deficient GCYl strains: The results obtained show that the "Knock Out" of CGYl eliminates the 20(XHSD activity inducible by galactose. Thus, the production in vivo of 4-pregnene-17a,20a-diol-3-one from 17a-hydroxyprogesterone (100 )μg/ml) was tested in a culture of wild-type strains and of TGY170 strains (gcyl-A::LEU2) , cultured either in glucose medium or in galactose medium (Fig. 3). In the case of a culture in galactose medium, a reduction of about 95% in the production of 4-pregnene-17a, 20a-diol-3-one is observed for the mutant strain compared with the wild-type strain. In glucose medium, the reduction is lower, which appears to indicate that the product of the GCYl gene comprises a 20aHSD activity which is inducible by galactose. Whatever the source of carbon used, a residual activity is present in the mutant gcyll-A, which is substantially identical. Properties of the deficient GCYl, YRPl double mutant: Given the fact that Gcylp and Yprlp were found associated with the 20(XHSD activity (see above) , and that Yprlp is the closest homologue of Gcylp (65% amino acid identity) , a double disruption of GCYl and of YPRl was performed and tested for its 20aHSD type activity. The results obtained show that the strain deficient for the two genes (TGY197) is essentially free of 20aHSD activity. More particularly, the TGY197 yeasts (gcyl::LEU2,yprl::URA3) were generated and tested in vivo for their 20ocHSD activity. The cells were cultured with either glucose or galactose as carbon source, in the presence of 100 μg/ml of 17a-hydroxyprogesterone. After 72 h, 4-pregnene-17a,20a-diol-3-one is undetectable in the fermentation liquid, demonstrating that the inactivation of the two genes leads to a suppression of the detectable 20aHSD activity. Substrate specificity of the wild-type and mutant strains A series of components already described as substrates for various classes of reductases (aldose-, aldehyde- and carbonyl-reductases) was tested on wild-type homogenates and on mutant strains under various culture conditions (Table II) . It is observed that Gcylp and YprlP are the only aldo-keto yeast reductases which accept 17a-hydroxyprogesterone as substrate. Gcylp appears to use all the substrates tested since in all cases, induction by galactose increases activity. In glucose medium, a basal activity was observed for all the components with the exception of 17a-hydroxyprogesterone, independently of the presence of GcylP and Yprlp. In galactose medium, a higher activity was observed in the mutant strains for the two aldehydes tested, although less pronounced than in the wild-type strains. This indicates that an enzyme specific for aldehydes other than GcylP is"inducible by galactose. In a recent report on the physiology of yeasts under osmotic stress, GCYl was identified as being reactive. The sequencing of a peptide isolated from an Aspergillus niger glycerol dehydrogenase has shown homology with two yeast proteins: Gcylp and Yprlp. The induction of GCYl under osmotic stress (in addition to its induction by galactose) could indicate that GCYl comprises a glycerol dehydrogenase activity, as suggested in Norbeck and Blomberg. However, such an activity has not been demonstrated up until now. The reduction of 17a-hydroxyprogesterone to 4-pregnene-17a,20a-diol-3-one in S. cerevisiae is mainly due to the product of the GCY gene and, to a lesser degree, to the product of the YPRl gene. According to the nomenclature proposed by Jez et al. (1997), these enzymes ought to be classified in the AKRIC subfamily. In mammals, HSDs, belonging to the AKR family, have been proposed for regulating the availability of steroid hormones. The physiological role of these enzymes in yeast remains unknown, since yeasts are not supposed to be capable to being exposed to steroids in a natural environment. Whatever the biological significance of such an activity in yeast, its elimination contributes to improving the production of corticosteroids from yeasts, in particular from genetically modified yeasts according to the invention. Example of bioconversion with human P450c21 in yeast in the presence and in the absence of GCYl and YPRl. With the aim of showing that the disruption of GCYl and YPRl is essential in order to obtain a specific bioconversion in the yeast S. cerevisiae, we compared the capacity for bioconversion of 170H-progesterone of the strains Fyl679-28c/pTG10497 (Fy/pTG10497), TGY212 #1 and TGY245#2D. The strain Fy/pTGl0497 carries the two wild-type genes GCYl and YPRl and the single copy plasmid of ARSCEN type ("Autonomously Replicating Sequence Centromer") for expression of human P450c21. The cDNA for human P450c21 is under the control of the TEFl promoter in this plasmid. The TG212#1 strain carries a copy of the expression cassette for the human P450c21 gene (TEFl::human P450c21::PGK terminator) integrated at the YPRl locus and possesses a wild-type copy of the GCYl gene. TGY245#2D does not possess copies of the YPRl and GCYl genes: instead, a copy of the cassette TEFl::human P450c21 and a copy of TDH3:P450c21 are integrated into each of the loci, respectively. These strains were cultured in minimum medium with a supplement of casamino acids for 48 hours. The strains are resuspended in fresh Kappeli medium with a supplement of uracil, histidine and tryptophan in the presence of 200 mg/1 of 170H progesterone. After an incubation of 72 hours, the medium in the presence of the yeast cells is extracted and analysed as above by reversed phase high-performance liquid chromatography. The presence of 170H progesterone, 11-deoxycortisol and 4-pregnene-17a, 2 0a-diol-3-one is measured. The results are presented in Table III. Each product is expressed as a percentage of the sum of all the products. According to this experiment, it appears clearly that the disruption of GCYl and YPRl significantly reduces the quantity of 4-pregnene-17a,20a-diol-3-one from 7-11% to a level which is not detectable by our techniques (sensitivity of 0.5 to 1 mg/1). REFERENCES Amberg et al. (1995), Yeast 11, 1275-1280. Cauet et al. (1999), Eur. J. Biochem, 261, 317-324. Degryse et al. (1995), J. Biotech. 39, 181-187. Degryse, E. (1996), Gene 170, 45-50. Degryse et al. (1995), Yeast 11, 629-640. Dumas et al. (1994), Cytochrome P450, 8""*" International Conference, Ed. M. C. Lechner, John Libbey Eurotext, Paris, pp. 527-530. Dumas et al. -(1996), Eur. J. Biochem. 238, 495-504. Duport et al. (1998), Nat. Biotech. 16, 1-6. Hanahan, D. (1983) J. Mol. Biol. 166, 557-580. Hubacek et al. (1970), J. Mol. Biol. 50, 111-127. Ito et al. (1983), J. Bact. 153, 163-168. Miller et al. (1988), Endocrine Revs 9, 295-318. Sakaki et al. (1989), DNA 8, 409-418. Sakaki et al. (1991), Pharmacogen. 1, 86-93. Sambrook et al. (1989), Cold Spring Harbor University Press, 2"^ edition. Cold Spring Harbor. Sherman, F. (1991), Methods Enzymol. 194, 3-21. Thierry et al. (1995), Yeast 11, 121-135. WU et al. (1991), Saccharomyces cerevisiae DNA Cell Biol. 10, 201-209. Artificial sequence description: oligonucleotide 12 aacgccgtgc tccttagcca taagcttggg taataactga"tataactaaa ctgaactc 58 13 23 DNA Artificial sequence Artificial sequence description: oligonucleotide 13 tttgcLcgag gttdcagaag ggc 23 14 24 DNA Artificial sequence Artificial sequence description: oligonucleotide 14 gattctcgag caattggctg acta 24 15 40 DNA Artificial sequence We Claim: 1. A method for producing a steroidal derivatives from hydroxysteroid compounds comprising bringing the said hydroxysteroid compounds or a precursor thereof into contact with a yeast having a reduced 20αHSD activity, or a preparation derived from such a yeast. 2. The method according to claim 1, in which the hydroxysteroid is hydroxyprogesterone or its precursors. 3. The method according to claim 2, wherein the hydroxyprogesterone is 17α - hydroxyprogesterone. 4. The method according to claim 3, wherein the 17a- hydroxyprogesterone is converted into 11-deoxyCortisol. 5. The method according to any one of the preceding claims, wherein the yeast has one or more nonfunctional GCYl and /or YPRl genes. 6. The method according to any one of the preceding claims, wherein the yeast is a Saccharomyces yeast. 7. The method according to one of the preceding claims, wherein the nonfiinctional gene(s) exhibit a deletion and/or an insertion. 8. The method to claim 7, wherein the nonfiinctional gene(s) is (are) disrupted. Yeast strain, wherein it possesses a genetic modification of the YPRl and/or GCYl gene(s). The Yeast strain according to claim 9, wherein the gene(s) is (are) replaced, completely or in part by the URA3 marker gene. The Yeast strain according to claim 9 or 10, wherein the genetic modification is an inactivating modification, preferably a gene disruption. The Yeast strain according to one of claims 9 to 11, wherein it is a Saccharomyces cerevisiae yeast. The Yeast strain according to one of claims 9 to 12, comprising a nonfunctional GCYl gene and YPRl gene, which are preferably disrupted. Acellular preparation such as cellular lysate, a cellular homogenate, a culture supernatant, or a derived enriched or purified solution derived from a yeast according to any one of claims 9 to 13. Method of modifying the 20aHSD activity of a yeast, comprising modifying the activity of the GCYl and/or YPRl gene of the said yeast. 16. The method according to claim 15, intended to reduce or inhibit the 20aHSD activity of a yeast, comprising the inactivation, preferably by disruption, of the GCYl and/or YPRl gene of the said yeast. 17. The yeast strain according to one of claims 9 to 13, for the preparation of steroidal compounds. 18. The yeast strain according to one of claims 9 to 13, having reduced 20aHSD activity, or a preparation derived from such a yeast, for the preparation, production, synthesis, modification and/or improvement of steroidal compounds in vitro or ex vivo. 19. The yeast strain according to one of claims 17 or 18, having reduced 20aHSD activity, or a preparation derived from such a yeast, for the conversion of 17a-hydroxyprogesterone to 11-deoxyCortisol.

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