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)‐ α ‐ribotide α ‐ribazole‐5′‐phosphate ( α ‐RP)] is an intermediate in the biosynthesis of adenosylcobalamin (AdoCbl) in many prokaryotes. In such microbes, α ‐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 α ‐ribazole ( α ‐R) from the environment using CblT to take up α ‐R, followed by α ‐R phosphorylation by CblS. We report that Geobacillus kaustophilus CblT and CblS proteins restore α ‐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 α ‐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 α ‐RP via α ‐R using an as‐yet‐unknown route to convert DMB to α ‐R and CblS to convert α ‐R to α ‐RP.