The hdc genes encoding the inducible pyridoxal-P-dependent histidine decarboxylase (HisDCase) of Klebsiella planticola and Enterobacter aerogenes were isolated, sequenced, and expressed in Escherichia coli under control of the lac promoter, and the overproduced enzymes were purified to homogeneity from the recombinant host. Formation of inclusion bodies during synthesis of the E. aerogenes enzyme was avoided by cooling the culture and inducing at 25 degrees C. The cloned enzymes were produced in amounts three to four times those present in the fully induced native hosts and were identical in properties to those isolated earlier (Guirard, B. M., and Snell, E. E. (1987) J. Bacteriol. 169, 3963-3968). The two enzymes showed 85% sequence identity and also showed 80% sequence identity with the previously sequenced (Vaaler, G. L., Brasch, M. A., and Snell, E. E. (1986) J. Biol. Chem. 261, 11010-11014) HisDCase of Morganella morganii. Nevertheless, antibodies to the M. morganii HisDCase do not cross-react with these enzymes suggesting that the regions of amino acid variations are located on the outer surface of the proteins. All three HisDCases are the same length (377 amino acid residues); encoded N-terminal methionine was completely removed in each case. These closely related pyridoxal-P enzymes show no sequence homology with the pyruvoyl-dependent HisDCases of Gram-positive bacteria.