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Methyltransferases (MTases) form a major class of tRNA-modifying enzymes needed for the proper functioning of tRNA. Recently, RNA MTases from the TrmN/Trm14 family that are present in Archaea, Bacteria and Eukaryota have been shown to specifically modify tRNA ^Phe at guanosine 6 in the tRNA acceptor stem. Here, we report the first X-ray crystal structures of the tRNA m ² G6 ( N² -methylguanosine) MTase _TTC TrmN from Thermus thermophilus and its ortholog _Pf Trm14 from Pyrococcus furiosus . Structures of _Pf Trm14 were solved in complex with the methyl donor S -adenosyl- l -methionine (SAM or AdoMet), as well as the reaction product S -adenosyl-homocysteine (SAH or AdoHcy) and the inhibitor sinefungin. _TTC TrmN and …