The bipartite structure of the tRNA m1A58 methyltransferase from S. cerevisiae is conserved in humans

Among all types of RNA, tRNA is unique given that it possesses the largest assortment and abundance of modified nucleosides. The methylation at N(1) of adenosine 58 is a conserved modification, occurring in bacterial, archaeal, and eukaryotic tRNAs. In the yeast Saccharomyces cerevisiae, the tRNA 1-methyladenosine 58 (m(1)A58) methyltransferase (Mtase) is a two-subunit enzyme encoded by the essential genes TRM6 (GCD10) and TRM61 (GCD14). While the significance of many tRNA modifications is poorly understood, methylation of A58 is known to be critical for maintaining the stability of initiator tRNA(Met) in yeast. Furthermore, all retroviruses utilize m(1)A58-containing tRNAs to prime reverse transcription, and it has been shown that the presence of m(1)A58 in human tRNA(3) (Lys) is needed for accurate termination of plus-strand strong-stop DNA synthesis during HIV-1 replication. In this study we have identified the human homologs of the yeast m(1)A Mtase through amino acid sequence identity and complementation of trm6 and trm61 mutant phenotypes. When coexpressed in yeast, human Trm6p and Trm61p restored the formation of m(1)A in tRNA, modifying both yeast initiator tRNA(Met) and human tRNA(3) (Lys). Stable hTrm6p/hTrm61p complexes purified from yeast maintained tRNA m(1)A Mtase activity in vitro. The human m(1)A Mtase complex also exhibited substrate specificity--modifying wild-type yeast tRNA(i) (Met) but not an A58U mutant. Therefore, the human tRNA m(1)A Mtase shares both functional and structural homology with the yeast tRNA m(1)A Mtase, possessing similar enzymatic activity as well as a conserved binary composition.