Biosynthesis of branched-chain amino acids in yeast: regulation of synthesis of the enzymes of isoleucine and valine biosynthesis.

Regulation of the levels of the five enzymes required for the biosynthesis of isoleucine and valine was studied in a Saccharomyces sp. When a mixture of isoleucine, valine, and leucine was added to the medium, the enzymes in the wild-type strain were repressed from about 30% (transaminase B) to about 90% (acetohydroxy acid synthetase) relative to the level in minimal medium-grown cells. Repression was also observed when threonine replaced isoleucine in the mixture but not when it replaced the other two amino acids. Significant derepression relative to the level in minimal-grown cells was not obtained by growing suitably blocked auxotrophs on medium containing limiting amounts of valine, isoleucine, or leucine.