New archaeal methyltransferases forming 1-methyladenosine or 1-methyladenosine and 1-methylguanosine at position 9 of tRNA

Two archaeal tRNA methyltransferases belonging to the SPOUT superfamily and displaying unexpected activities are identified. These enzymes are orthologous to the yeast Trm10p methyltransferase, which catalyses the formation of 1-methylguanosine at position 9 of tRNA. In contrast, the Trm10p orthologue from the crenarchaeon Sulfolobus acidocaldarius forms 1-methyladenosine at the same position. Even more surprisingly, the Trm10p orthologue from the euryarchaeon Thermococcus kodakaraensis methylates the N¹-atom of either adenosine or guanosine at position 9 in different tRNAs. This is to our knowledge the first example of a tRNA methyltransferase with a broadened nucleoside recognition capability. The evolution of tRNA methyltransferases methylating the N¹ atom of a purine residue is discussed.