Converts acetolactate into acetoin. Regulates leucine and valine biosynthesis by diverting the flux of alpha-acetolactate towards acetoin when the branched-chain amino acids are present in high concentration. (2S)-2-acetolactate + H(+) = (R)-acetoin + CO2 The enzyme is active only in the presence of branched-chain amino acids. Valine results in much higher activation than leucine or isoleucine. Polyol metabolism; (R,R)-butane-2,3-diol biosynthesis; (R,R)-butane-2,3-diol from pyruvate: step 2/3. Slower growth in media containing valine, leucine plus isoleucine, or leucine plus valine. Belongs to the alpha-acetolactate decarboxylase family.