Control of alcoholic fermentation through modulation of nitrogen metabolism in Saccharomyces cerevisiae

Saccharomyces cerevisiae sake strains exhibit high alcoholic fermentation performance. Comparative transcriptomic analysis revealed that the expression of genes required for nitrogen sensing and metabolism, including amino acid biosynthesis and uptake, was markedly lower in the sake strain than in the laboratory strain. Thus, we hypothesized that changes in nitrogen metabolism affect the fermentation capability of S. cerevisiae. To evaluate the impact of altered nitrogen metabolism on alcoholic fermentation, we focused on the transcription activators Gcn4p, Gln3p, and Gat1p, and the protein kinase Npr1p, all of which are key regulators controlling expression of genes for amino acid biosynthesis and uptake responding to nitrogen availability. Fermentation tests demonstrated that laboratory strain-derived single-deletion mutants of the regulator genes exhibited higher fermentation performance than the parental strain, which was accompanied by decrease in intracellular amino acid levels in the mutants. Disruption of the genes encoding glutamate dehydrogenases, which play a central role in nitrogen assimilation, also enhanced the fermentation rate. A Greatwall family protein kinase Rim15p inhibits alcoholic fermentation by diverting carbon flux from glycolysis to the synthesis of 1,3-β-glucan, a major cell wall component. Since the content of 1,3-β-glucan was unaffected by disruption of the regulator genes, the elevated fermentation performance of the disruptants was accomplished independently of the signaling pathway governed by Rim15p. The high fermentation rate of the disruptants might be attributed to increased carbon entry into glycolysis caused by the compromised biosynthesis of amino acids, which are synthesized from intermediary metabolites of glycolysis and tricarboxylic acid cycle.