REGULATION OF ISOLEUCINE AND VALINE BIOSYNTHESIS IN SALMONELLA-TYPHIMURIUM - EFFECT OF HISU ON REPRESSION CONTROL

The control of isoleucine and valine biosynthesis was examined in a hisU mutant of Salmonella typhimurium. It was found that the levels of expression of the ilvEDA operon and the ilvC gene were significantly reduced relative to an isogenic normal strain when grown in unsupplemented medium. In contrast, this hisU mutant exhibited only a slight reduction in total acetohydroxy acid synthase activity relative to that of the wild type. The hisU and hisU+ strains were examined to determine their derepressibility upon either leucine, valine or isoleucine limitation. Only during leucine limitation did the hisU strain exhibit impaired derepressibility relative to the hisU+ strain. In addition, repression control of threonine deaminase (the ilvA product of the ilvEDA operon) in this hisU mutant was refractory to exogenous supplementation with either leucine or valine. This response is in distinct contrast to that of the normal strain, in which the single addition of leucine or valine results in a significant reduction in the level of threonine deaminase. © 1979.