The genes involved in the 2,3-butanediol pathway coding for α- acetolactate decarboxylase, α-acetolactate synthase (α-ALS), and acetoin (diacetyl) reductase were isolated from Klebsiella terrigena and shown to be located in one operon. This operon was also shown to exist in Enterobacter aerogenes. The budA gene, coding for α-acetolactate decarboxylase, gives in both organisms a protein of 259 amino acids. The amino acid similarity between these proteins is 87%. The K. terrigena genes budB and budC, coding for α-ALS and acetoin reductase, respectively, were sequenced. The 559- amino-acid-long α-ALS enzyme shows similarities to the large subunits of the Escherichia coli anabolic α-ALS enzymes encoded by the genes ilvB, ilvG, and ilvI. The K. terrigena α-ALS is also shown to complement an anabolic α- ALS-deficient E. coli strain for valine synthesis. The 243-amino-acid-long acetoin reductase has the consensus amino acid sequence for the insect-type alcohol dehydrogenase/ribitol dehydrogenase family and has extensive similarities with the N-terminal and internal regions of three known dehydrogenases and one oxidoreductase.