Import of transfer RNAs into plant mitochondria

In higher plants, mitochondrial (mt) transfer RNAs (tRNAs) have 3 different genetic origins and can be divided into genuine (or native) mt tRNAs, chloroplast-like mt tRNAs (these 2 classes are encoded by mt DNA) and nuclear-encoded mt tRNAs. The existence of this third class of mt tRNAs has been demonstrated i) by the fact that they hybridize to nuclear DNA but not to mt DNA and ii) by the fact that upon transforming potato protoplasts with plasmid DNA containing a bean nuclear tRNA gene, the corresponding tRNA Leu is found inside the mitochondria of the regenerated transgenic potato plants. An Arabidopsis tRNA Ala , a normally imported tRNA species, whose G 3 :U 70 base-pair has been changed into a G:C base pair is no longer recognized by the alanyl-tRNA synthetase. Our first results obtained with transgenic tobacco plants transformed with the gene corresponding to this mutated tRNA suggest that recognition by the cognate aminoacyl-tRNA synthetase might be necessary for stable integration of a nucleus-encoded tRNA into plant mitochondria.