Genetic analysis of the metabolic pathways responsible for aroma metabolite production by Saccharomyces cerevisiae

During alcoholic fermentation, higher alcohols, esters, and acids are formed from amino acids via the Ehrlich pathway by yeast, but many of the genes encoding the enzymes have not yet been identified. When the BAT1/2 genes, encoding transaminases that deaminate amino acids in the first step of the Ehrlich pathway are deleted, higher metabolite formation is significantly decreased. Screening yeast strains with deletions of genes encoding decarboxylases, dehydrogenases, and reductases revealed nine genes whose absence had the most significant impact on higher alcohol production. The seven most promising genes (AAD6, BAT2, HOM2, PAD1, PRO2, SPE1, and THI3) were further investigated by constructing double- and triple-deletion mutants. All double-deletion strains showed a greater decrease in isobutanol, isoamyl alcohol, isobutyric, and isovaleric acid production than the corresponding single deletion strains with the double-deletion strains in combination with a dagger bat2 and the a dagger hom2-a dagger aad6 strain revealing the greatest impact. BAT2 is the dominant gene in these deletion strains and this suggests the initial transaminase step of the Ehrlich pathway is rate-limiting. The triple-deletion strains in combination with BAT2 (a dagger bat2-a dagger thi3-a dagger aad6 and a dagger bat2-a dagger thi3-a dagger hom2) had the greatest impact on the end metabolite production with the exception of isoamyl alcohol and isovaleric acid. The strain deleted for two dehydrogenases and a reductase (a dagger hom2-a dagger pro2-a dagger aad6) had a greater effect on the levels of these two compounds. This study contributes to the elucidation of the Ehrlich pathway and its significance for aroma production by fermenting yeast cells.