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 FMNH(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 (By similarity). FMNH2 + O2 = 5,6-dimethylbenzimidazole + D-erythrose 4-phosphate + dialurate + H(+) Homooctamer. Belongs to the BluB family.