Involved in the biosynthesis of cobalamin (vitamin B12). Catalyzes the oxidative fragmentation and contraction of the isoalloxazine heterocycle and the cleavage of the ribityl tail of FMN(2) to form 5,6-dimethylbenzimidazole (DMB) and D-erythrose 4-phosphate (E4P). NAD(P)H is only required initially to reduce FMN and oxygen drives the oxidative fragmentation. FMNH2 + O2 = 5,6-dimethylbenzimidazole + D-erythrose 4-phosphate + dialurate + H(+) Inhibited by high concentrations of FMN. kcat is 15 h(-1), 7.2 h(-1) and 7.6 h(-1) for FMN, NAD and NADP, respectively. Homooctamer. Cells lacking this gene are 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 B12 (cyanocobalamin) or the lower ligand of cobalamin, 5,6-dimethylbenzimidazole (DMB). Belongs to the BluB family.