Regulation of synthesis of the acetohydroxy acid synthase I isoenzyme in Escherichia coli K-12

Abstract

We studied derivatives of Escherichia coli K-12 expressing only one of the three acetohydroxy acid synthases, acetohydroxy acid synthase I (the ilvB gene product), to gain insight into the mechanism of synthesis of this isoenzyme. We found that acetohydroxy acid synthase I is regulated by a multivalent control requiring leucine and valine. In fact, a starvation of each of these two amino acids causes a derepressed synthesis of acetohydroxy acid synthase I, while a starvation of isoleucine has no effect. Derepressed synthesis of this enzyme also occurs when cells are grown in the presence of α-ketobutyrate, unless the three branched-chain amino acids are also present. Additional studies with α-aminobutyric acid, an inhibitor of valyl-tRNA Val synthesis, show that valine must be charged to tRNA to participate in this regulatory system.