Salmonella enterica synthesizes 5,6-dimethylbenzimidazolyl-(DMB)-α-riboside. Why some Firmicutes do not require the canonical DMB activation system to synthesize adenosylcobalamin

5,6-Dimethylbenzimidazolyl-(DMB)-alpha-ribotide [alpha-ribazole-5'-phosphate (alpha-RP)] is an intermediate in the biosynthesis of adenosylcobalamin (AdoCbl) in many prokaryotes. In such microbes, alpha-RP is synthesized by nicotinate mononucleotide (NaMN): DMB phosphoribosyltransferases (CobT in Salmonella enterica), in a reaction that is considered to be the canonical step for the activation of the base of the nucleotide present in adenosylcobamides. Some Firmicutes lack CobT-type enzymes but have a two-protein system comprised of a transporter (i.e., CblT) and a kinase (i.e., CblS) that can salvage exogenous alpha-ribazole (alpha-R) from the environment using CblT to take up alpha-R, followed by alpha-R phosphorylation by CblS. We report that Geobacillus kaustophilus CblT and CblS proteins restore alpha-RP synthesis in S. enterica lacking the CobT enzyme. We also show that a S. enterica cobT strain that synthesizes GkCblS ectopically makes only AdoCbl, even under growth conditions where the synthesis of pseudoCbl is favored. Our results indicate that S. enterica synthesizes alpha-R, a metabolite that had not been detected in this bacterium and that GkCblS has a strong preference for DMB-ribose over adenine-ribose as substrate. We propose that in some Firmicutes DMB is activated to alpha-RP via alpha-R using an as-yet-unknown route to convert DMB to alpha-R and CblS to convert alpha-R to alpha-RP.