Sinorhizobium meliloti bluB Is Necessary for Production of 5,6-Dimethylbenzimidazole, the Lower Ligand of$B_{12}

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 103; no. 12; pp. 4634 - 4639
• Main Authors: Campbell, Gordon R. O., Taga, Michiko E., Mistry, Kavita, Lloret, Javier, Anderson, Peter J., Roth, John R., Walker, Graham C.
• Format: Journal Article
• Language: English
• Published: National Academy of Sciences 21.03.2006
• Subjects: Alfalfa; Bacteria; Biosynthesis; Enzymes; Fluorescence; Ligands; Nodules; Phenotypes; Plants; Symbiosis
• ISSN: 0027-8424; 1091-6490

An insight into a previously unknown step in$B_{12}$biosynthesis was unexpectedly obtained through our analysis of a mutant of the symbiotic nitrogen fixing bacterium Sinorhizobium meliloti. This mutant was identified based on its unusually bright fluorescence on plates containing the succinoglycan binding dye calcofluor. The mutant contains a Tn5 insertion in a gene that has not been characterized previously in S. meliloti. The closest known homolog is the bluB gene of Rhodobacter capsulatus, which is implicated in the biosynthesis of$B_{12}$(cobalamin). The S. meliloti bluB mutant is unable to grow in minimal media and fails to establish a symbiosis with alfalfa, and these defects can be rescued by the addition of vitamin$B_{12}$(cyanocobalamin) or the lower ligand of cobalamin, 5,6-dimethylbenzimidazole (DMB). Biochemical analysis demonstrated that the bluB mutant does not produce cobalamin unless DMB is supplied. Sequence comparison suggests that BluB is a member of the NADH/flavin mononucleotide (FMN)-dependent nitroreductase family, and we propose that it is involved in the conversion of FMN to DMB.