The T. brucei TRM5 methyltransferase plays an essential role in mitochondrial protein synthesis and function.

All tRNAs undergo post-transcriptional chemical modifications as part of their natural maturation pathway. Some modifications, especially those in the anticodon loop, play important functions in translational efficiency and fidelity. Among these, 1-methylguanosine, at position 37 (m(1)G37) of the anticodon loop in several tRNAs, is evolutionarily conserved and participates in translational reading frame maintenance. In eukaryotes, the tRNA methyltransferase TRM5 is responsible for m(1)G formation in nucleus-encoded as well as mitochondria-encoded tRNAs, reflecting the universal importance of this modification for protein synthesis. However, it is not clear what role, if any, mitochondrial TRM5 serves in organisms that do not encode tRNAs in their mitochondrial genomes. These organisms may easily satisfy the m(1)G37 requirement through their robust mitochondrial tRNA import mechanisms. We have explored this possibility in the parasitic protist Trypanosoma brucei and show that down-regulation of TRM5 by RNAi leads to the expected disappearance of m(1)G37, but with surprisingly little effect on cytoplasmic translation. On the contrary, lack of TRM5 causes a marked growth phenotype and a significant decrease in mitochondrial functions, including protein synthesis. These results suggest mitochondrial TRM5 may be needed to mature unmethylated tRNAs that reach the mitochondria and that could pose a problem for translational fidelity. This study also reveals an unexpected lack of import specificity between some fully matured and potentially defective tRNA species.