Transfer RNA molecules used in this study. Secondary structures of E. coli tRNAfMet1 (A) and tRNAMet (B) are shown, with mutations M1 and M3 of the anticodon stem of tRNAfMet indicated. Isoacceptor tRNAfMet2 is identical to tRNAfMet1 except that nucleotide 46 is A rather than m7G. D, dihydrouridine; Ψ, pseudouridine; s4U, 4-thiouridine; Cm, 2′-O-methylcytidine; Gm, 2′-O-methylguanosine; m7G, 7-methylguanosine; ac4C, N4-acetylcytidine; acp3U, 3-(3-amino-3-carboxypropyl)uridine; t6A, N6-threonylcarbamoyladenosine.