The plant mitochondrial transfer RNA population: a mosaic of species with different genetic origins

The plant mitochondrial genome, despite its large size, does not possess a complete set of transfer RNA genes. Plant mitochondria contain a number of "cytosolic-like" tRNAs hybridizing to the nuclear genome. These tRNAs are imported from the cytosol, as shown in the case of tRNA(Leu) (C*AA), a species present in both the cytosol and the mitochondria of bean plants: after introduction of the bean nuclear gene encoding this tRNA into potato protoplasts, the presence of the corresponding foreign tRNA could indeed be detected in the cytosol and inside the mitochondria of regenerated transgenic potato plants. On the other hand, among the tRNA genes present in plant mitochondria, some are part of chloroplast DNA sequences which have been inserted into the mitochondrial genome during evolution. A number of these genes are transcribed and produce "chloroplast-like" tRNAs. A detailed study of the potato (Solanum tuberosum) mitochondrial tRNA population showed that, out of 31 identified species, 15 are typically mitochondrial, 11 are nuclear-encoded and 5 are "chloroplast-like". Differences in the "chloroplast-like" and nuclear-encoded species appear between monocotyledonous and dicotyledonous plants. Moreover, although the sequences of tRNA genes of chloroplast origin expressed in mitochondria are highly conserved, comparison of their flanking regions between different plants suggests that several genetic information transfer events from the chloroplast have occurred during evolution. An involvement of the aminoacyl-tRNA synthetases in mitochondrial tRNA import has been suggested. Indeed, the plant cytosolic and mitochondrial leucyl-tRNA synthetases seem to be identical, as are identical (except for a Gm in position 18) the plant cytosolic and mitochondrial leucine tRNAs previously sequenced. The elucidation of the tRNA import mechanisms may suggest ways of targeting other nucleic acids into mitochondria. The fact that the bean tRNA(Leu) (C*AA) gene is still expressed in transgenic potato plants after introduction of a four base-pair insertion in the anticodon loop and that the corresponding transcript is transported into the mitochondria, suggests that tRNAs could serve as vectors to carry extra RNA sequences into plant mitochondria.