Identification of yeast genes involved in Sauvignon Blanc aroma ...

834 Identification of yeast genesinfluencing volatile thiol productionin Sauvignon Blanc4.1 IntroductionA candidate-gene approach was chosen to identify yeast genes responsible for volatilethiol production during fermentation of SB grape must. Three factors led to this decision.Firstly, preliminary experiments showed that the variation of 3-mercaptohexan-1-ol(3MH) and 3-mercaptohexyl acetate (3MHA) production amongst various commercialyeast strains fermented in SB grape juice was small (see Appendix A.3). Therefore thepursuit of natural genetic variants did not seem like a promising approach.Secondly, the formation of 3MH and 4-mercapto-4-methylpentan-2-one (4MMP) fromcysteinylated precursors found in SB grape juice was demonstrated by using a cell-freeenzyme extract of Eubacterium limosum and purified tryptophanase from Escherichia coli(Tominaga et al., 1995, 1998c). It was hypothesized that a similar mechanism might beresponsible for the formation of volatile thiols during fermentation of SB must by theaction of yeast. This hypothesis made it attractive to focus candidate gene selection onpossible genes for uptake and cleavage of cysteinylated precursors.Thirdly, after the fermentation performance of BY4743 in grape-juice-like medium hadbeen optimized (see Chapter 3), the deployment of the set of yeast deletion strains seemedlike a valuable resource to elucidate the genetic function of volatile thiol production inyeast. During the course of this work Howell et al. (2005) and Thibon et al. (2008)showed that yeast carrying single-gene knockouts in BNA3, CYS3, GLO1, IRC7, and thetranscriptional activators GLN3 and URE2 can influence the formation of volatile thiols;however, this work was carried out in synthetic medium supplemented with synthetic