The cobD gene of Salmonella typhimurium LT2 has been cloned, sequenced, and overexpressed. The overexpressed protein had a molecular mass of approximately 40 kDa, in agreement with the mass predicted by the deduced amino acid sequence (40.8 kDa). Computer analysis of the deduced amino acid sequence of CobD identified a consensus pyridoxal phosphate-binding motif. The role of CobD in cobalamin biosynthesis in this bacterium has been established. CobD was shown to decarboxylate L-threonine O-3-phosphate to yield (R)-1-amino-2-propanol O-2-phosphate. We propose that the latter is a substrate in the reaction catalyzed by the CbiB enzyme proposed to be responsible for the conversion of adenosylcobyric acid to adenosylcobinamide and that the product of the reaction is adenosylcobinamide phosphate, not adenosylcobinamide as previously thought. The implications of these findings are discussed in light of the demonstrated kinase activity of the CobU enzyme (O'Toole, G. A., and Escalante-Semerena, J. C. (1995) J. Biol. Chem. 270, 23560-23569) responsible for the conversion of adenosylcobinamide to adenosylcobinamide phosphate. These findings shed light on the strategy used by this bacterium for the assimilation of exogenous unphosphorylated cobinamide from its environment. To our knowledge, CobD is the first enzyme reported to have L-threonine-O-3-phosphate decarboxylase activity, and computer analysis of its amino acid sequence suggests that it may be a member of a new class of pyridoxal phosphate-dependent decarboxylases.