Acetohydroxy acid synthase I is required for isoleucine and valine biosynthesis by Salmonella typhimurium LT2 during growth on acetate or long-chain fatty acids.

Salmonella typhimurium LT2 normally expresses two acetohydroxy acid synthases (AHAS I and AHAS II). The function of AHAS I in this organism was unclear, since AHAS I-deficient (ilvBN) mutants of LT2 grew well on glucose or succinate minimal media, whereas AHAS II-deficient (ilvGM) mutants requried isoleucine for normal growth on glucose minimal media. We report that AHAS I-deficient mutants of S. typhimurium required isoleucine and valine for growth on acetate or oleate minimal media, whereas AHAS II-deficient mutants were able to grow on these media without isoleucine supplementation.