LYSINE INHIBITION OF SACCHAROMYCES-CEREVISIAE - ROLE OF REPRESSIBLE L-LYSINE EPSILON-AMINOTRANSFERASE

Lysine added to grain mashes under nitrogen-limiting conditions (as in most industrial fermentations) inhibited growth of Saccharomyces cerevisiae. This inhibition was relieved by raising the assimilable nitrogen content. Lysine-induced inhibition is not mediated through accumulation of alpha-oxoadipic acid, an intermediate of lysine metabolism which accumulates by a back up of intermediates in de novo synthesis. Lysine degradation is regulated by the synthesis of L-lysine epsilon-aminotransferase, an enzyme that catalyses the first step in one of three possible routes of lysine degradation (not previously reported in S. cerevisiae). Synthesis is repressed under nitrogen-limiting conditions, but derepressed when excess assimilable nitrogen is available. Derepression results in degradation of lysine and decreases inhibitory effects on growth. The toxic compound appears to be lysine itself.