Isoleucine-valine requiring mutants of Salmonella typhimurium. 3. Valine-sensitive strains

Salmonella typhimurium mutants defective in acetohydroxy acid synthases I and II

An analysis of transposon-induced mutants shows that Salmonella typhimurium possesses two major isozymes of acetohydroxy acid synthase, the enzymes which mediate the first common step in isoleucine and valine biosynthesis. A third (minor) acetohydroxy acid synthase is present, but its significance in isoleucine and valine synthesis may be negligible. Mutants defective in acetohydroxy acid synthase II (ilvG::Tn10) require isoleucine, alpha-ketobutyrate, or threonine for growth, a mutant defective in acetohydroxy acid synthase I (ilvB::Tn5) is a prototroph, and a double mutant (ilvG::Tn10 ilvB::Tn5) requires isoleucine plus valine for growth.

Control of acetohydroxy acid synthetase in Escherichia coli 9723

A method by which three acetohydroxy acid synthetase activities are separated from extracts of Escherichia coli 9723 has been developed. Isoleucine specifically represses synthesis of one of the enzymes, which is not sensitive to valine inhibition, and isoleucine also simultaneously enhances the production of a second activity, which is valine inhibitable. The valine-inhibitable activity is repressed by leucine and valine, a combination of which is more effective than either alone. The third acetohydroxy acid synthetase, which is more active at pH 6 than at 8, is not controlled by the branched-chain amino acids. In a mutant of E. coli 9723 selected for the ability of valine to inhibit growth, the isoleucine-repressible acetohydroxy acid synthetase activity was no longer present, but isoleucine addition still resulted in enhanced production of the valine-inhibitable activity.

Temperature-sensitive growth inhibition by valine in Salmonella typhimurium: alteration of one form of acetohydroxy acid synthetase

A strain of Salmonella typhimurium, MF1680, was isolated that required isoleucine for normal growth at 37 C but not at 27 C. In minimal medium the growth of MF1680 was strongly blocked by valine. Growth inhibition was reversed by isoleucine. When this mutant was grown at 37 C, the form of acetohydroxy acid synthetase not inhibited by valine, (AHAS(R)), could not be detected. The site of this mutation in AHAS(R), termed ilvG, was found to map in the ilv gene cluster between ilvD and ilvE.