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

Abstract

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 G 37 ) 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 G 37 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 G 37 , 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.

The Catalytic Domain of Topological Knot tRNA Methyltransferase (TrmH) Discriminates between Substrate tRNA and Nonsubstrate tRNA via an Induced-fit Process

“…TrmD catalyzes methyl transfer to the N1 atom of G37 in eubacterial tRNA (24). The m 1 G37 modification in archaea (54 -56), eukaryotes (57,58), and mitochondrial (59,60) tRNA is conferred by Trm5, which belongs to the class I enzymes. Thus, the same modification at the same position in tRNA is conferred by different enzymes, namely TrmD (class IV) and Trm5 (class I), according to the organism.…”

Arabidopsis TRM5 encodes a nuclear-localised bifunctional tRNA guanine and inosine-N1-methyltransferase that is important for growth

“…We localized Arabidopsis TRM5 to the nucleus which is in contrast to the cytoplasmic and mitochondrial localisation in other eukaryotes: Trypanosoma brucei, Homo sapiens (HeLa cells) and Saccharomyces cerevisiae (11,23,29), although in one study S. cerevisiae Trm5 has also been localized exclusively to the nucleus (53).…”

“…The discovery of m 1 G and m 1 I at position 37 in tRNAs of a wide range of eukaryotic and prokaryotic organisms underscores its importance as a key regulator of tRNA function and, presumably, translation (11,20,23). Previous studies in bacteria have shown that m 1 G37 is required for translational fidelity (20,57,58) and that mutations in the enzymes catalyzing m 1 G37 severely impact growth or cause lethal phenotypes (11,23,29). In eukaryotes, m 1 G37 modification requires the methyltransferase TRM5.…”

“…The yeast Trm5p protein has been shown to be localised to the cytoplasm and mitochondria (23) and it is thought that Trm5p protein present in the mitochondria is required to prevent unmodified tRNA affecting translational frameshifting (22). In the unicellular parasite Trypanosome brucei, Trm5 was located in both the nucleus and mitochondria and reducing Trm5 expression led to reduced mitochondria biogenesis and impaired growth (29). Interestingly, Trm5 and m 1 G37 were shown to be essential for mitochondrial protein synthesis but not cytosolic translation (29).…”

“…In the unicellular parasite Trypanosome brucei, Trm5 was located in both the nucleus and mitochondria and reducing Trm5 expression led to reduced mitochondria biogenesis and impaired growth (29). Interestingly, Trm5 and m 1 G37 were shown to be essential for mitochondrial protein synthesis but not cytosolic translation (29).…”