a, Overlay of SAM (grey), SAH (aquamarine) and 5’dAH+Met (light violet) in their complexes with BuMiaB and RNA substrates (17-mer for SAM, and 13-mer for SAH or 5’-dAH+Met). The adenine ring of SAM, SAH and 5’dAH forms face-to-face π-stacking interactions with Phe321. This stacking is further supported by edge-to-face interactions with two tyrosines (177, 352) and Phe350. N3 of the adenine ring H-bonds with the conserved Arg66 (shown in Fig. 3a), and N6 forms three H-bonds with the carbonyl groups of Ile65 (MTTase domain), and Tyr177 and Ser353 (RS domain). The ribose moiety of SAM, SAH and 5’dAH H-bonds with Arg66, Gln281 and Asp319. Methionine in the 5’dAH+Met structure or the methionine moiety of SAM (with the 17-mer RNA) and SAH (with 13-mer RNA) in structures with those molecules bound shows the canonical bidentate binding to the unique iron of the [Fe4S4]RS cluster. b, Overlay of SAM (grey) or 5’dAH+Met (light violet) in complex with BuMiaB and the 17-mer RNA (SAM) or 13-mer RNA (5’-dAH+Met). The i6A37 base is in pink for the structure with 5’dAH+Met and maroon for the structure with SAM. All figures have the same colour for the domains and their associated residues: tan for MTTase, grey for radical SAM and green for TRAM, except for Gln215 in the structure with 5’dAH+Met (panel a), which rotates.