A SALMONELLA-TYPHIMURIUM COBALAMIN-DEFICIENT MUTANT BLOCKED IN 1-AMINO-2-PROPANOL SYNTHESIS

Salmonella typhimurium synthesizes cobalamin (vitamin B-12) when grown under anaerobic conditions. All but one of the biosynthetic genes (co b) are located in a single operon which includes genes required for th e production of cobinamide and dimethylbenzimidazole, as well as the g enes needed to form cobalamin from these precursors. We isolated strai ns carrying mutations (cobD) which are unlinked to any of the previous ly described B-12 biosynthetic genes. Mutations in cobD are recessive and map at minute 14 of the linkage map, far from the major cluster of B-12 genes at minute 41. The cobD mutants appear to be defective in t he synthesis of 1-amino-2-propanol, because they can synthesize B-12 w hen this compound is provided exogenously. Labeling studies in other o rganisms have shown that aminopropanol, derived from threonine, is the precursor of the chain linking dimethylbenzimidazole to the corrinoid ring of B-12. Previously, a three-step pathway has been proposed for the synthesis of aminopropanol from threonine, including two enzymatic steps and a spontaneous nonenzymatic decarboxylation. We assayed the two enzymatic steps of the hypothetical pathway; cobD mutants are not defective in either. Furthermore, mutants blocked in one step of the p roposed pathway continue to make B-12. We conclude that the aminopropa nol for B-12 synthesis is not made by this pathway. Expression of a la c operon fused to the cobD promoter is unaffected by vitamin B-12 or o xygen, both of which are known to repress the main cob operon, suggest ing that the cobD gene is not regulated.