Regulation of branched-chain amino acid biosynthesis by alpha-acetolactate decarboxylase in Streptococcus thermophilus.

To demonstrate the presence of an active alpha-acetolactate decarboxylase in Streptococcus thermophilus and to investigate its physiological function. Streptococcus thermophilus CNRZ385 contains a gene encoding an alpha-acetolactate decarboxylase. Comparison of the production of alpha-acetolactate and its decarboxylation products, by the parent strain and an alpha-acetolactate decarboxylase-deficient mutant, demonstrated the presence of a control of the pool of alpha-acetolactate by valine, leucine and isoleucine. This control occurs via an allosteric activation of the alpha-acetolactate decarboxylase. Cell-free extracts of S. thermophilus were not able to decarboxylate the isoleucine precursor alpha-acetohydroxybutyrate. These results strongly suggest that one of the physiological functions of the alpha-acetolactate decarboxylase in S. thermophilus is to regulate leucine and valine biosynthesis by diverting the flux of alpha-acetolactate towards acetoin when the branched-chain amino acids are present at a high concentration. Regulation of branched-chain amino acid biosynthesis by alpha-acetolactate decarboxylase may occur in several other micro-organisms and explain some of their growth properties.