DUAL ROLE OF ALPHA-ACETOLACTATE DECARBOXYLASE IN LACTOCOCCUS-LACTIS SUBSP LACTIS

The alpha-acetolactate decarboxylase gene aldB is clustered with the g enes for the branched-chain amino acids (BCAA) in Lactococcus lactis s ubsp. lactis. It can be transcribed with BCAA genes under isoleucine r egulation or independently of BCAA synthesis under the control of its own promoter. The product of aldB is responsible for leucine sensibili ty under valine starvation. In the presence of more than 10 mu M leuci ne, the alpha-acetolactate produced by the biosynthetic acetohydroxy a cid synthase IIvBN is transformed to acetoin by AldB and, consequently , is not available for valine synthesis. AldB is also involved in acet oin formation in the 2,3-butanediol pathway, initiated by the cataboli c acetolactate synthase, AlsS. The differences in the genetic organiza tion, the expression, and the kinetics parameters of these enzymes bet ween L. lactis and Klebsiella terrigena, Bacillus subtilis, or Leucono stoc oenos suggest that this pathway plays a different role in the met abolism in these bacteria. Thus, the alpha-acetolactate decarboxylase from L. lactis plays a dual role in the cell: (i) as key regulator of valine and leucine biosynthesis, by controlling the acetolactate flux by a shift to catabolism; and (ii) as an enzyme catalyzing the second step of the 2,3-butanediol pathway.