A nuclear encoded tRNA of Trypanosoma brucei is imported into mitochondria

The mitochondrial genome of trypanosomes, unlike that of most other eukaryotes, does not appear to encode any tRNAs. Therefore, mitochondrial tRNAs must be either imported into the organelle or created through a novel mitochondrial process, such as RNA editing. Trypanosomal tRNA(Tyr), whose gene contains an 11-nucleotide intron, is present in both the cytosol and the mitochondrion and is encoded by a single-copy nuclear gene. By site-directed mutagenesis, point mutations were introduced into this tRNA gene, and the mutated gene was reintroduced into the trypanosomal nuclear genome by DNA transfection. Expression of the mutant tRNA led to the accumulation of unspliced tRNA(Tyr) (A. Schneider, K. P. McNally, and N. Agabian, J. Biol. Chem. 268:21868-21874, 1993). Cell fractionation revealed that a significant portion of the unspliced mutant tRNA(Tyr) was recovered in the mitochondrial fraction and was resistant to micrococcal nuclease treatment in the intact organelle. Expression of the nuclear integrated, mutated tRNA gene and recovery of its gene product in the mitochondrial fraction directly demonstrated import. In vitro experiments showed that the unspliced mutant tRNA(Tyr), in contrast to the spliced wild-type form, was no longer a substrate for the cognate aminoacyl synthetase. The presence of uncharged tRNA in the mitochondria demonstrated that aminoacylation was not coupled to import.