tRNA (5-methylaminomethyl-2-thiouridylate)-methyltransferase (trmU, asuE, or mnmA) is involved in the biosynthesis of the modified nucleoside 5-methylaminomethyl-2-thiouridine (mnm5s2U34) present in the wobble position of some tRNAs. This enzyme appears not to occur in the Archaea. [Protein synthesis, tRNA and rRNA base modification]