5,6-dimethylbenzimidazole synthase (, BluB) is an enzyme with systematic name FMNH<sub>2</sub> oxidoreductase (5,6-dimethylbenzimidazole forming). This enzyme catalyses the following chemical reaction
The C-2 of 5,6-dimethylbenzimidazole is derived from C-1' of the ribityl group of FMNH<sub>2</sub> and 2-H from the ribityl 1'-pro-S hydrogen. This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in bacteria.