A single base change prevents import of cytosolic tRNA(Ala) into mitochondria in transgenic plants

Plant mitochondria do not contain a full set of tRNA genes, and the additional tRNAs needed for protein synthesis (including tRNA(Ala)) are imported from the cytosol. The import process appears to be highly specific for certain tRNAs, and it has been suggested that the cognate aminoacyl-tRNA synthetases may be responsible for this specificity. In order to test this, we have grown transgenic tobacco plants expressing Arabidopsis thaliana tRNA(Ala) carrying a U-70 to C-70 mutation, which we have previously shown blocks aminoacylation by the plant alanyl-tRNA synthetase. Unlike the wild-type tRNA(Ala), the mutant tRNA is not present in the mitochondrial tRNA fraction. This is the first report of a tRNA mutation which prevents mitochondrial import and strongly supports the hypothesis that aminoacyl-tRNA synthetases are involved in this process in plants. Insertion of four bases into the anticodon loop of tRNA(Ala) does not prevent mitochondrial import, implying that the tRNA might not need to participate in translation to be imported.