In the aerobic degradation of benzoate by bacteria, benzoate is first dihydroxylated by a ring-hydroxylating dioxygenase to form a cis-diol (1,2-dihydroxycyclohexa-3,4-diene carboxylate) which is subsequently transformed to a catechol by an NAD(+)-dependent cis-diol dehydrogenase. The structural gene for this dehydrogenase, encoded on TOL plasmid pWW0 of Pseudomonas putida (xylL) and that encoded on the chromosome of Acinetobacter calcoaceticus (benD), were sequenced. They encode polypeptides of about 28 kDa in size. These proteins are similar to each other, exhibiting 58% sequence identity. They are also similar to other proteins of at least 20 different functions, which are members of the short-chain alcohol dehydrogenase family. The alignment of these proteins suggest two amino acids, lysine and tyrosine, as catalytically important residues.