Trmt61B is a methyltransferase responsible for 1-methyladenosine at position 58 of human mitochondrial tRNAs

In human mitochondria, 1-methyladenosine (m(1)A) occurs at position 58 of tRNA(Leu(UUR)). In addition, partial m(1)A58 modifications have been found in human mitochondrial tRNA(Lys) and tRNA(Ser(UCN)). We identified human Trmt61B, which encodes a mitochondria-specific tRNA methyltransferase responsible for m(1)A58 in these three tRNAs. Trmt61B is dominantly localized to the mitochondria. m(1)A58 formation in human mitochondrial tRNA(Leu(UUR)) could be reconstituted in vitro using recombinant Trmt61B in the presence of Ado-Met as a methyl donor. Unlike the cytoplasmic tRNA m(1)A58 methyltransferase that consists of an alpha 2 beta 2 heterotetramer formed by Trmt61A and Trmt6, Trmt61B formed a homo-oligomer (presumably a homotetramer) that resembled the bacterial homotetrameric m(1)A58 methyltransferase. The bacterial origin of Trmt61B is supported by the results of the phylogenetic analysis.