Abstract:
The present invention relates to the identification of alleles of the MET2 and SKP2 genes having the effect of reducing the production of sulphites, of hydrogen sulphide and of acetaldehyde by  Saccharomyces , and to the use of these alleles in methods for controlling the production of these compounds during alcoholic fermentation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application under 35 U.S.C. §371 of International Application No. PCT/IB2013/050623, filed Jan. 24, 2013, which claims the benefit of and priority to French Patent Application No. 1250717, filed Jan. 25, 2012. Each of these applications is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The present invention relates to the control of the production of sulfites, of hydrogen sulfide and of acetaldehyde during alcoholic fermentation by yeasts. 
     Sulfur dioxide (SO 2 ) and its various forms in equilibrium in solution (HSO 3   − , SO 3   − ), collectively denoted sulfites, are used as additives in enology, principally to improve the conservation of wines, owing to its antioxidant and antibacterial properties. However, an excessive amount of sulfites in wine can lead to intolerances and allergies in certain consumers; they may also be prejudicial to its organoleptic qualities, given that they give, if there in excess, drying sensations. Excessive amounts of sulfites at the end of alcoholic fermentation can thus be disadvantageous when the wine producer wants to carry out malolactic fermentation. Lactic acid bacteria, responsible for this fermentation, are inhibited by low sulfite contents, and an excess delays the initiation of said fermentation. Hydrogen sulfide is also a metabolite formed by yeasts in fermentation which is prejudicial to the quality of wines when it is present in excess owing to the “rotten egg” or “reduced” tastes that it imparts. 
     It is therefore important to be able to optimize the amount of sulfites and of hydrogen sulfide in wines and during winemaking. A major difficulty in this context comes from the fact that part of the sulfites and of the hydrogen sulfide present in the wine comes from the fermentative metabolism of yeasts, where they constitute intermediates in the synthesis of sulfur-containing amino acids. Inorganic sulfate enters the cell by means of a sulfate permease. It is activated to give adenosylphosphosulfate (APS) by ATP-sulfurylase, then the APS is phosphorylated by adenosylphosphosulfate kinase to produce phosphoadenosylphosphosulfate (PAPS). The PAPS is then reduced to SO 2  by PAPS reductase. The SO 2  is reduced to H 2 S by sulfite reductase. Homocysteine, which is the precursor of sulfur-containing amino acids, is synthesized by reaction of H 2 S with O-acetylhomoserine, catalyzed by O-acetylhomoserine sulfhydrylase. 
     Since the amount of sulfites produced by yeasts during fermentation varies from one yeast strain to another, this complicates the control of the overall sulfite content. The same is true for hydrogen sulfide, the amount of which formed depends greatly on the yeast strain. 
     Another compound, the presence of which in wine above certain amounts is considered to be undesirable, is acetaldehyde. Acetaldehyde at too high a concentration gives wines “musty” notes which are considered to be negative. It is produced by yeasts during fermentation, and its production appears to correlate with the SO 2  content, and like that of the SO 2 , varies from one yeast strain to another. 
     Various approaches have been proposed for obtaining yeast strains producing reduced amounts of sulfites and/or of hydrogen sulfide. 
     PCT application WO 2008/115759 and PCT application WO 2009/046485, and also the publications by Cordente et al. (FEMS Yeast Res, 9, 446-59, 2009) and Linderholm et al. (Appl Environ Microbiol, 76, 7699-707, 2010), describe various mutations in the METS or MET10 genes (encoding the 2 catalytic subunits of sulfite reductase) which have the effect of reducing hydrogen sulfide production. Application WO 2009/030863 and the publication by Marullo et al. (FEMS Yeast Res, 7, 1295-306, 2007) describe various markers associated with characteristics of interest in enological yeasts. One of these markers (YOL083w) located on chromosome XV is associated with a reduced H 2 S production. 
     SUMMARY 
     The inventors have now identified alleles of two genes involved in sulfur metabolism in  Saccharomyces , as being associated with a reduced production of SO 2 , of acetaldehyde and, in the case of one of these genes, of H 2 S. 
     The first of these genes is the SKP2 gene, located on chromosome XIV (nt 49397 to 51688 in the  Saccharomyces  genome database). The corresponding cDNA sequence and the corresponding polypeptide sequence (for the reference  Saccharomyces cerevisiae  S288C strain) are available in the GenBank database under the respective accession numbers NM_001183149.1 (GI:296147470) and NP_014088.1 (GI:6324018). SKP2 encodes a protein of F-box type which is involved in the stability of various sulfur metabolism proteins and in particular of adenosylphosphosulfate kinase responsible for the conversion of APS to PAPS. It has recently been shown (Yoshida et al., Yeast, 28, 109-21, 2011) that the inactivation of the SKP2 gene results in a stabilization of adenosylphosphosulfate kinase, and in an increase in the production of H 2 S and of SO 2 . 
     The inventors have identified, in the SKP2 gene, two mononucleotide polymorphisms which differentiate the JN10 strain from the JN17 strain: one in position 50 618 of chromosome XIV, where the JN10 strain has a G and the JN17 strain has an A, and the other in position 50 640 bp where the JN10 strain has a C, whereas the JN17 strain has a T. These polymorphisms are reflected by the changing of a valine for JN10, to isoleucine for JN17, at position 350 of the Skp2 protein (V350I), and also of a threonine in JN10 at position 357 of Skp2, to isoleucine in JN17 (T357I). 
     The SKP2 gene allele present in the JN17 strain had not been previously identified in any other strain of  Saccharomyces . The cDNA sequence of this allele is indicated in the appended sequence listing under the number SEQ ID NO: 1, and the deduced polypeptide sequence under the number SEQ ID NO: 2. 
     The second gene is the MET2 gene, also located on chromosome XIV (nt 117349 to 118809, coordinates indicated in the  Saccharomyces  genome database (http:www.yeastgenome.org) on Dec. 27, 2011). The corresponding cDNA sequence and the corresponding polypeptide sequence (for the reference  Saccharomyces cerevisiae  strain S288C) are available in the GenBank database under the respective accession numbers NM_001183115.1 (GI:296147504) and NP_014122.1 (GI:6324052). MET2 encodes homoserine-O-acetyl transferase which catalyzes the conversion of homoserine to O-acetyl homoserine, which is then condensed with H 2 S to form homocysteine. It has been shown (Hansen &amp; Kielland-Brandt, J Biotechnol, 50, 75-87, 1996) that the inactivation of the MET2 gene in  Saccharomyces  leads to an increase in the production of sulfites and of hydrogen sulfide. 
     The inventors have identified, in position 118 249 of chromosome XIV, a mononucleotide polymorphism which differentiates the MET2 genes of two  Saccharomyces cerevisiae  strains, one (JN10 strain) a strong producer of SO 2 , H 2 S and acetaldehyde under certain fermentation conditions, and the other (JN17 strain) a weak producer of these same compounds. The JN10 strain has a C whereas the JN17 strain has (like the reference strain S288C) a G, which leads to an amino acid change and the conversion of an arginine in the JN10 strain to glycine in the JN17 strain in position 301 of the Met2 protein (R301G). 
     A subject of the present invention is a method for obtaining a yeast strain of the  Saccharomyces  genus producing a lower amount of SO 2 , hydrogen sulfide and acetaldehyde than that produced by the parent strain from which it is derived, said method being characterized in that it comprises:
         the selection of a parent strain containing an allele of the SKP2 gene, hereinafter known as SKP2 (350/357)X , encoding an Skp2 protein in which the amino acid in position 350 and/or the amino acid in position 357 is (are) other than an isoleucine or isoleucines, and/or an allele of the MET2 gene, hereinafter known as MET2 301X , encoding a Met2 protein in which the amino acid in position 301 is other than a glycine;   the introduction, into said parent strain, of an allele of the SKP2 gene, hereinafter known as SKP2 (350/357)I , encoding an Skp2 protein in which the amino acid in position 350 and/or the amino acid in position 357 is (are) an isoleucine or isoleucines, and/or of an allele of the MET2 gene, hereinafter known as MET2 301G , encoding a Met2 protein in which the amino acid in position 301 is a glycine.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the impact of allelic change on the formation of SO 2  (A), of H 2   5  (B) and of acetaldehyde (C) for the JN10 MET2 JN17  and JN17 MET2 JN10  strains and the corresponding parent JN10 and JN17 strains. 
         FIG. 2  illustrates the formation of SO 2  (A), of H 2 S (B) and of acetaldehyde (C) for the diploid strains JN17/JN10skp2□::HPH and JN10/JN17skp2□::HPH, which have just one functional allele of SKP2 (respectively the SKP2 JN17  allele and the SKP2 JN10  allele). And the corresponding parent JN10 and JN17 strains. 
         FIG. 3  illustrates the formation of SO 2  (A), of H 2 S (B) and of acetaldehyde (C) for haploid derivatives ( 4 th backcross spores  1  to  4 ) having the following allele combinations: SKP 2   JN17 /MET 2   JN17 ; SKP 2   JN10 /MET2 JN17 ; SKP 2   JN17 /MET JN10 ; SKP 2   JN10 /MET 2   JN10  on virtually identical genetic backgrounds. 
       For example, if the parent strain contains an SKP2 (350/357)X  allele and a MET2 301G  allele, it will be possible to introduce herein an SKP2 (350/357)I  allele. Conversely, if the parent strain contains an SKP2 (350/357)I  allele and a MET2 301X  allele, it will be possible to introduce herein a MET2 301G  allele. If the parent strain contains an SKP2 (350/357)X  allele and a MET2 301X  allele, it is possible to introduce herein either an SKP2 (350/357)I  allele or a MET2 301G  allele. Preferably, it will be chosen to introduce herein both an SKP2 (350/357)I  allele and a MET2 301G  allele. 
     
    
    
     DETAILED DESCRIPTION 
     In the context of the disclosure of the present invention, the name “SKP2 (350/357)I  allele” encompasses: an allele (more specifically known as SKP2 350I/357X  allele) encoding an Skp2 protein in which the amino acid in position 350 is an isoleucine and the amino acid in position 357 is other than an isoleucine; an allele (more specifically known as SKP2 350X/357I  allele) encoding an Skp2 protein in which the amino acid in position 350 is other than an isoleucine; an allele (more specifically known as SKP2 350I/357I  allele) in which the amino acid in position 350 and the amino acid in position 357 are both isoleucines, the latter allele being particularly preferred. 
     According to one preferred embodiment of the present invention, said parent strain contains an allele of the SKP2 gene, hereinafter known as SKP2 350V/357T , encoding an Skp2 protein in which the amino acid in position 350 is a valine and/or the amino acid in position 357 is a threonine, and/or an allele of the MET2 gene, hereinafter known as MET2 301R , encoding a Met2 protein in which the amino acid in position 301 is an arginine. 
     Advantageously, said yeast strain belongs to the  Saccharomyces cerevisiae  species. 
     The SKP2 (350/357)I  allele and/or the MET2 301G  allele can be introduced into the parent strain by various methods, well known in themselves to those skilled in the art. They can be introduced, for example, by crossing with a strain which has the desired SKP2 (350/357)I  allele and/or MET2 301G  allele, and selection from the descendants of this cross, of those to which said allele has been transmitted. 
     The SKP2 (350/357)I  allele and/or the MET2 301G  allele can also be introduced by replacement of the initial allele (respectively SKP2 (350/357)X  and MET2 301X ) or in addition to said allele, using conventional genetic engineering techniques (cf. for example AMBERG et al., Methods in yeast genetics: a Cold Spring Harbor Laboratory course manual, Cold Spring Harbor Laboratory Press, 2005). 
     If the method in accordance with the invention is carried out using a haploid parent strain carrying the SKP2 (350/357)X  allele, the introduction, into said strain, of a copy of the SKP2 (350/357)I  allele by crossing produces a heterozygous SKP2 (350/357)X /SKP2 (350/357)I  strain, producing an amount of sulfites, hydrogen sulfide and acetaldehyde which is lower than that produced by the parent strain from which it is derived. It is also possible to obtain haploid descendants of this strain which have the SKP2 (350/357)I  allele and therefore produce low amounts of sulfites, of hydrogen sulfide and of acetaldehyde. By means of the series of backcrosses between descendants having the SKP2 (350/357)I  allele and the parent strain, it is thus possible to obtain a strain with a genome close to that of the parent strain, having acquired the SKP2 (350/357)I  allele and producing low amounts of sulfites, of hydrogen sulfide and of acetaldehyde. Likewise, if the method in accordance with the invention is carried out using a parent strain carrying the MET2 301X  allele, the crossing of said strain with a strain having the MET2 301G  allele produces a heterozygous MET2 301X /MET2 301G  strain, producing an amount of sulfites, of hydrogen sulfide and of acetaldehyde which is lower than that produced by the parent strain from which it is derived. It is also possible, as in the case of SKP2, to obtain haploid descendants of this strain having the MET2 301G  allele, and by means of backcrosses with the parent strain, to obtain a strain having the MET2 301G  allele on the genetic background of the parent strain. 
     The subject of the present invention is also an isolated polynucleotide encoding the Skp2 protein of sequence SEQ ID NO: 2, which corresponds to the SKP2 350I/357I  allele. 
     According to one preferred embodiment of the present invention, this polynucleotide is defined by the sequence SEQ ID NO: 1. 
     This polynucleotide can be used, in the context of the method in accordance with the invention described above, to introduce the SKP2 350I/357I  allele into a yeast strain. 
     A subject of the present invention is also a nucleic acid vector containing a polynucleotide of sequence SEQ ID NO: 1, or a fragment thereof containing at least the region 1045-1075 of SEQ ID NO: 1. 
     Said vector may be any type of vector usable in yeast, in particular in  Saccharomyces . Such vectors are well known in themselves. Use may, for example, be made of extrachromosomal replicating vectors, such as the Yep vectors or the Yrp vectors. Use may also be made of integrating vectors such as the Yip vectors. 
     In the context of an integrating vector, the polynucleotide of sequence SEQ ID NO: 1, or said fragment, is flanked upstream and downstream by sequences of at least 20 bp, preferably of 40 to 60 bp, which are homologues to those flanking the SKP2 gene or the region 1045-1075 of said gene in the strain into which it is desired to introduce the SKP2 350I/357I  allele. 
     The DNA fragment containing the sequence SEQ ID NO: 1, or at least the region 1045-1075 of SEQ ID NO: 1, will be optionally combined with a marker gene (gene encoding a protein which confers resistance to an inhibitor or gene which makes it possible to complement a mutation responsible for an auxotrophy of the recipient strain) facilitating the selection of the clones having acquired the fragment by transformation. 
     A subject of the present invention is also a method for evaluating the capacity of a strain of  Saccharomyces , preferably of  Saccharomyces cerevisiae , to produce SO 2 , hydrogen sulfide and acetaldehyde, characterized in that it comprises:
         genotyping of said strain for the SKP2 gene, and the detection of the presence of an SKP2 (350/357)X  allele and in particular of the SKP2 350V/357T  allele, and/or of an SKP2 (350/357)I  allele, and in particular of the SKP2 350I/357I  allele; and/or   the genotyping of said strain for the SKP2 gene, and the detection of the presence of an SKP2 (350/357)X  allele and in particular of the SKP2 350V/357T  allele, and/or of an SKP2 (350/357)I  allele, and in particular of the SKP2 350I/357I  allele.       

     A subject of the present invention is also reagents for carrying out the genotyping method in accordance with the invention. 
     These reagents comprise in particular:
         allele-specific oligonucleotide probes for differentiating the SKP2 350V/357T  allele from an SKP2 (350/357)I  allele, and in particular from the SKP2 350I/357I  allele, or for differentiating the MET2 301R  allele from the MET2 301G  allele, by hybridizing selectively with one or other of the alleles to be differentiated;   specific primers for differentiating the SKP2 350V/357T  allele from an SKP2 (350/357)I  allele, and in particular from the SKP2 350I/357I  allele, or for differentiating the MET2 301R  allele from the MET2 301G  allele, and also kits of primers containing at least one specific primer in accordance with the invention. Generally, these kits of primers comprise a primer specific for each allele to be detected, and a common primer, capable of hybridizing, under the same amplification conditions, with all the alleles of the gene concerned.       

     Probes in accordance with the invention for differentiating the SKP2 350V/357T  allele from an SKP2 (350/357)I  allele, and in particular from the SKP2 350I/357I  allele, can for example be made up of fragments of 15 to 30 bp of the sequence: CTAGAAAATGTAACGRTAGACACCGAATCGCTAGATAYTCCAATGGAATTCTT (SEQ ID NO: 4, where A, T, C, G, R and Y have their usual meaning in the IUPAC code), said fragments containing at least the locus of the G/A polymorphism, or at least the locus of the C/T polymorphism, and where appropriate the 2 polymorphic loci of said sequence, or made up of the sequences complementary thereto. 
     The probes in which R=G, and also the probes in which Y═C, can hybridize selectively with the SKP2 350V/357T  allele, while the probes in which R=A and those in which Y=T can hybridize selectively with an SKP2 (350/357)I  allele, and in particular the SKP2 350I/357I  allele. 
     Probes in accordance with the invention for differentiating the MET2 301R  allele from the MET2 301G  allele can for example be made up of fragments of 15 to 30 bp of the sequence: ATTTCTGGGCAAAAASGTCAAAGCGTGGTGT (SEQ ID NO: 3, where A, T, C, G and S have their usual meaning in the IUPAC code), said fragments containing the locus of the CIG polymorphisms of said sequence, or made up of the sequences complementary thereto. The probes in which S═C can hybridize selectively with the MET2 301R  allele, while the probes in which S=G can hybridize selectively with the MET2 301G  allele. 
     Specific primers in accordance with the invention for differentiating SKP2 350V  from SKP2 350I  can for example be made up of fragments of 15 to 30 bp of the sequence SEQ ID NO: 4 containing at least the locus of the G/A polymorphism or the sequence complementary thereto. The primers in which R=G can be used for the selective amplification of SKP2 350V , while the primers in which R=A can be used for the selective amplification of SKP2 350I . 
     Specific primers in accordance with the invention for differentiating SKP2 357T  from SKP2 357I  can for example be made up of fragments of 15 to 30 bp of the sequence SEQ ID NO: 4 containing at least the locus of the C/T polymorphism, or the sequences complementary thereto. 
     The primers in which Y═C can be used for the selective amplification of SKP2 357T  and those in which Y=T can be used for the selective amplification of SKP2 357I . 
     According to one preferred embodiment of a kit of primers in accordance with the invention for differentiating the SKP2 350V/357T  allele from an SKP2 (350/357)I  allele, it comprises a pair of specific primers for differentiating SKP2 350V  from SKP2 350I , and a pair of specific primers for differentiating SKP2 357T  from SKP2 357I . 
     Specific primers in accordance with the invention for differentiating the MET2 301R  allele from the MET2 301G  allele can for example be made up of fragments of 15 to 30 bp of the sequence SEQ ID NO: 3 containing at least the locus of the C/G polymorphism in said sequence, or made up of the sequences complementary thereto. The primers in which S═C can be used for the selective amplification of the MET2 301R  allele, while the primers in which S=G can be used for the selective amplification of the MET2 301G  allele. 
     Common primers which can be used in combination with the specific primers for differentiating the MET2 301R  allele from the MET2 301G  allele in the kits of primers in accordance with the invention can for example be made up of fragments of 15 to 30 bp of the following sequence: ATGTTATGCCTGAGGTATGTGTGGTATCTA (SEQ ID NO: 5, where A, T, C and G have their usual meaning in the IUPAC code), or made up of the sequences complementary thereto. 
     Common primers which can be used in combination with the specific primers for differentiating SKP2 350V  from SKP2 350I  and/or with the specific primers for differentiating SKP2 357T  from SKP2 357I  in the kits of primers in accordance with the invention can for example be made up of fragments of 15 to 30 bp of the following sequence: AGTCCACTACAAAAAGTCATTTATTTTTGC (SEQ ID NO: 6, where A, T, C and G have their usual meaning in the IUPAC code), or made up of the sequences complementary thereto. 
     The present invention will be understood more clearly from the further description which follows, which refers to nonlimiting examples illustrating the effects of the alleles of the MET2 and SKP2 genes on the production of SO 2 , of hydrogen sulfide and of acetaldehyde. 
     THE EXAMPLES 
     EXAMPLE 1 
     Effect of the Alleles of the MET2 Gene on the Production of SO 2 , of Hydrogen Sulfide and of Acetaldehyde 
     The  Saccharomyces cerevisiae  JN10 strain (strong producer of SO 2 , H 2 S and acetaldehyde) has a MET2 gene allele which encodes a Met2 protein in which the amino acid in position 301 is an arginine, whereas the JN17 strain (weak producer of these same compounds) has a MET2 gene allele encoding a Met2 protein in which the amino acid in position 301 is a glycine. 
     The impact of the replacement of the MET2 allele of JN10 (MET2 JN10 ) with that of JN17 (MET2 JN17 ), or conversely that of the replacement of the MET2 allele of JN17 with that of JN10, were evaluated. 
     Firstly, the initial MET2 JN10  or MET2 JN17  allele was deleted and replaced with a cassette containing a geneticin-resistance gene (KANMX4), according to the method described by Wach et al. (Yeast, 10, 1793-808, 1994). The transformed cells are selected on the basis of their resistance to the antibiotic, and of their methionine auxotrophy. 
     The MET2 JN17  allele amplified from the genomic DNA of the JN17 strain was then introduced, as a replacement for the geneticin-resistance cassette, into the JN10 strain, and vice versa, the MET2 JN10  allele amplified from the genomic DNA of the JN10 strain was introduced, as a replacement for the geneticin-resistance cassette, into the JN17 strain. The transformed strains are selected on the basis of the restoration of their methionine prototrophy. 
     The impacts of the allelic change on the formation of SO 2 , of H 2 S and of acetaldehyde were evaluated during alcoholic fermentations under enological conditions. 
     The results are represented in  FIG. 1 . A: production of SO 2 ; B: production of H 2 S; C: production of acetaldehyde. 
     The replacement of the MET2 JN10  allele with the MET2 JN17  allele in the JN10 strain (JN10-MET2 JN17  strain) leads to a reduction in the concentration of SO 2  formed of approximately 40%. Likewise, the production of H 2 S is significantly reduced, 1 on a scale ranging from 0 to 2. The acetaldehyde level is also decreased by close to 40%. The reverse allelic replacement (MET2 JN10  allele on the genetic background of the JN17 strain: JN17-MET2 JN10  strain) has no impact on the production of SO 2 , nor on that of acetaldehyde; on the other hand, an increase in the production of H 2 S is observed compared with the JN17 parental strain. 
     EXAMPLE 2 
     Effect of the Alleles of the SKP2 Gene in the Production of SO 2 , of Acetaldehyde and of Hydrogen Sulfide 
     The SKP2 gene allele present in the  Saccharomyces cerevisiae  JN10 strain) (SKP2 JN10 ) encodes an Skp2 protein in which the amino acid in position 350 is a valine and the amino acid in position 357 is a threonine, whereas the allele present in the JN17 strain (SKP2 JN17 ) encodes an Skp2 protein in which the amino acids in positions 350 and 357 are isoleucines. 
     The impact of the allelic form of the SKP2 gene (SKP2 JN10  or SKP2 JN17 ) was evaluated via the construction of hemizygotes. The allelic replacement was in fact a method that was more difficult to carry out than in the case of the MET2 gene since the inactivation of the SKP2 gene results only in a delay of growth on minimum medium (Yoshida et al., 2011, mentioned above), this being a phenotype which, contrary to the methionine auxotrophy observed in the case of the MET2 gene, is not easily usable as a selectable marker. 
     Firstly, the SKP2 gene was inactivated in each of the JN10 and JN17 parental strains, by insertion of the HPH cassette which confers resistance to hygromycin B, so as to obtain respectively the JN10skp2Δ::HPH strains and the JN17skp2Δ::HPH strain. The JN10skp2Δ::HPH strain was then crossed with the JN17 strain, and the JN17skp2Δ::HPH strain was crossed with the JN10 strain, so as to obtain respectively the diploid strains JN17JN10skp2Δ::HPH and JN10/JN17skp2Δ::HPH, which have just one functional allele of SKP2 (respectively the SKP2 JN17  allele and the SKP2 JN10  allele). The production of sulfites, of acetaldehyde and of hydrogen sulfide by these strains which are hemizygote for SKP2 was evaluated under enological alcoholic fermentation conditions. The results are shown in  FIG. 2 . A: production of SO 2 ; B: production of acetaldehyde; C: production of hydrogen sulfide. 
     It is noted that the production of SO 2  is lower in the hemizygote which has the SKP2 JN17  allele than in that which has the SKP2 JN10  allele. Likewise, the acetaldehyde content is lower when the SKP2 JN17  allele is active than when the allele is the one derived from the JN10 strain. Finally, the hydrogen sulfide content is lower when the SKP2 JN17  allele is active than when the allele is the one derived from the JN10 strain. The SKP2 JN17  allele therefore results in a reduction in SO 2 , acetaldehyde and hydrogen sulfide contents. 
     EXAMPLE 3 
     Combined Effect of the Alleles of the MET2 and SKP2 Gene of the Production of SO 2  and of Hydrogen Sulfide 
     The impact of a combination of the two allelic forms SKP2 JN17  and MET2 JN17  was evaluated by means of the construction of virtually isogenic strains having more than 93% of the genome of the JN10 strain, following cycles of backcrosses. The backcrosses consist of a series of successive crosses with the same strain (in this case JN10). The JN17 strain is first of all hybridized with the JN10 strain. The hybrid obtained, which has 50% of the genome of the JN10 strain and 50% of the genome of the JN17 strain and has the following genotype: SKP2 JN17 /SKP2 JN10  and MET2 JN17 /MET2 JN10 , is induced to sporulate. After sporulation, the haploid spores having the following genotype: SKP2 JN17  and MET2 JN17  are selected by allele-specific PCR for these two genes. These spores are then crossed again with the JN10 strain. A new hybrid is obtained, which has 75% of the genome of the JN10 strain and 25% of the genome of the JN17 strain and has the following genotype: SKP2 JN17 /SKP2 JN10  and MET2 JN17 /MET2 JN10 ; this hybrid is in turn induced to sporulate. The asci are dissected and a spore having the following genotype: SKP2 JN17  and MET2 JN17  is selected. The cycles of crossing/sporulation/selection of a spore are continued until derivatives having a very high percentage of the genome of the JN10 strain, in this case 93.25%, are obtained. 
     By sporulation of the diploid clones obtained during the final cycle, haploid derivatives (4th backcross spores 1 to 4) having the following allele combinations: SKP2 JN17 /MET2 JN17 ; SKP2 JN10 /MET2 JN17 ; SKP2 JN 17/MET2 JN10 ; SKP2 JN10 /MET2 JN10  on virtually identical genetic backgrounds are obtained. The production of SO 2 , of H 2 S and of acetaldehyde of these various derivatives was evaluated under enological alcoholic fermentation conditions. The results are shown in table I below, and by  FIG. 3 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                   
                   
                 Acetaldehyde 
               
               
                 SKP2 allele 
                 MET2 allele 
                 SO 2  (mg/l) 
                 H 2 S 
                 (mg/l) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 JN10 
                 JN10 
                 46 
                 2 
                 43 
               
               
                 JN10 
                 JN17 
                 28 
                 1 
                 20 
               
               
                 JN17 
                 JN10 
                 5 
                 1 
                 6 
               
               
                 JN17 
                 JN17 
                 5 
                 0 
                 6 
               
               
                   
               
               
                 H 2 S scale: 
               
               
                 0 = production not detected, 
               
               
                 1 = medium production, 
               
               
                 2 = strong production 
               
             
          
         
       
     
     It is noted that the SO 2  production of a derivative which has the two alleles SKP2 JN10 /MET2 JN10  is identical to that of the initial JN10 strain, whereas a derivative which has a combination of alleles of SKP2 JN10 /MET2 JN17  type produces intermediate amounts of SO 2 . Moreover, derivatives which have either the SKP2 JN17 /MET2 JN10  allele combination or the two alleles of the JN17 strain, SKP2 JN17 /MET2 JN17 , both produce very low amounts of SO 2  which are identical to those of the initial JN17 strain. 
     The effect of the various allele combinations on the production of acetaldehyde is identical to that observed on the production of SO 2 . 
     Furthermore, the derivatives which have the two alleles of the JN10 strain produce high amounts of H 2 S, identical to the JN10 parental strain, while the derivatives which have one of the two alleles of the JN17 strain, and therefore have the following genotypes: SKP2 JN17 /MET2 JN10  or SKP2 JN10 /MET2 JN17 , produce H 2 S in intermediate amounts and only the derivative which has the two alleles SKP2 JN17 /MET2 JN17  does not produce detectable H 2 S, in the same way as the JN17 parental strain.