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 the benzimidazole lower axial ligand of the cobamide produced by G.sulfurreducens. 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. Homodimer. Belongs to the ThiC family. 5-hydroxybenzimidazole synthase subfamily.