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.