Sinorhizobium meliloti bluB is necessary for production of 5,6-dimethylbenzimidazole, the lower ligand of B sub(12)

• Published in: Proceedings of the National Academy of Sciences - PNAS 2006-03, Vol.103 (12), p.4634-4639
• Main Authors: Campbell, Gordon RO, Taga, Michiko E, Mistry, Kavita, Lloret, Javier, Anderson, Peter J, Roth, John R, Walker, Graham C
• Format: Article
• Language: English
• Subjects: Rhodobacter capsulatus; Sinorhizobium meliloti
• ISSN: 0027-8424; 1091-6490

An insight into a previously unknown step in B sub(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 sub(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 sub(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.