Regulatory subunit of acetohydroxy-acid synthase. Involved in the feed-back inhibition by branched-chain amino acids. Contains 2 repeats, each of them being able to activate partially the catalytic subunit. The enzyme reconstituted with the first repeat is inhibited by leucine, but not by valine or isoleucine and the enzyme reconstituted with the second repeat is not inhibited by any branched-chain amino acid. 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. The active enzyme may have an alpha(4)beta(4) structure (Probable). Belongs to the acetolactate synthase small subunit family.