H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Sensitive to valine inhibition. Amino-acid biosynthesis; L-isoleucine biosynthesis; L-isoleucine from 2-oxobutanoate: step 1/4. Amino-acid biosynthesis; L-valine biosynthesis; L-valine from pyruvate: step 1/4. Dimer of large and small chains. S.typhimurium contains genes for 3 AHAS isozymes: ilvBN, ilvGM and ilvIH. Belongs to the acetolactate synthase small subunit family.