Catalyzes the complex 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). 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. Homodimer. Belongs to the ThiC family. 5-hydroxybenzimidazole synthase subfamily.