Regulatory subunit of acetohydroxy-acid synthase. Probably involved in feedback inhibition by branched-chain amino acids. Not involved in herbicide tolerance. 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. The acetolactate synthase complex contains both large catalytic subunits and small regulatory subunits. Belongs to the acetolactate synthase small subunit family.