EDITING AND IMPORT - STRATEGIES FOR PROVIDING PLANT-MITOCHONDRIA WITHA COMPLETE SET OF FUNCTIONAL TRANSFER-RNAS

The recombinations and mutations that plant mitochondrial DNA has unde rgone during evolution have led to the inactivation or complete loss o f a number of the 'native' transfer RNA genes deriving from the genome of the ancestral endosymbiont. Following sequence divergence in their genes, some native mitochondrial tRNAs are 'rescued' by editing a pos t-transcriptional process which changes the RNA primary sequence. Acco rding to in vitro studies with the native mitochondrial tRNA(Phe) from potato and tRNA(His) from larch, editing is required for efficient pr ocessing. Some of the native tRNA genes which have been inactivated or lost have been replaced by tRNA genes present in plastid DNA sequence s acquired by the mitochondrial genome during evolution, which raises the problem of the transcriptional regulation of tRNA genes in plant m itochondria. Finally, tRNAs for which no gene is present in the mitoch ondrial genome are imported from the cytosol. This process is highly s pecific for certain tRNAs, and it has been suggested that the cognate aminoacyl-tRNA synthetases may be responsible for this specificity. In deed, a mutation which blocks recognition of the cytosolic Arabidopsis thaliana tRNA(Ala) by the corresponding alanyl-tRNA synthetase also p revents mitochondrial import of this tRNA in transgenic plants. Conver sely, no significant mitochondrial co-import of the normally cytosol-s pecific tRNA(Asp) was detected in transgenic plants expressing the yea st cytosolic aspartyl-tRNA synthetase fused to a mitochondrial targeti ng sequence, suggesting that, although necessary, recognition by a cog nate aminoacyl-tRNA synthetase might not be sufficient to allow tRNA i mport into plant mitochondria.