a, Cartoon representation of the active site of BuMiaB with a pentasulfide bridge spanning the two [Fe4S4] clusters. MTTase domain, tan; radical SAM domain, grey; TRAM domain, green. b, Cartoon of BuMiaB crystallized in the presence of the 13-mer RNA substrate and 5’dAH+Met and showing the binding of the 13-mer (purple) at the interface of the three domains. c, Electrostatic surface potential (blue is positive, red is negative, and grey is neutral) indicates a positively charged active-site region promoting binding of the 13-mer. d, Conservation of amino acids in the active-site region as deduced from the CONSURF server42.