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

Summary 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.