Together with BzaA, catalyzes the conversion of aminoimidazole ribotide (AIR) to 5-hydroxybenzimidazole (5-HBI) in a radical S-adenosyl-L-methionine (SAM)-dependent reaction. Is thus involved in the anaerobic biosynthesis of dimethylbenzimidazole (DMB), the lower axial ligand of vitamin B12 (cobalamin). Requires BzaA for catalytic activity, as BzaB alone displays no activity. 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole + AH2 + S-adenosyl-L-methionine = 5'-deoxyadenosine + 5-hydroxybenzimidazole + A + formate + 2 H(+) + L-methionine + NH4(+) + phosphate Binds 1 [4Fe-4S] cluster per subunit. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Cofactor biosynthesis; adenosylcobalamin biosynthesis. Belongs to the ThiC family. 5-hydroxybenzimidazole synthase subfamily.