Fate of a larch unedited tRNA precursor expressed in potato mitochondria

In higher plant mitochondria, post-transcriptional C to U conversion known as editing mostly affects mRNAs. However, three tRNAs were also shown to be edited. Among them, three editing sites were identified in larch mitochondrial tRNAHis. We have previously shown that only the edited version can undergo maturation in vitro. In this paper, we introduced via direct DNA uptake the edited or unedited version of larch mitochondrial trnH into isolated potato mitochondria and expressed them under the control of potato mitochondrial 18 S rRNA promoter. As expected, the edited form of larch mitochondrial tRNAHis precursor was processed in the isolated organelles. By contrast, no mature tRNAHis was detected when using the unedited version of trnH. However, precursor molecules could be characterized by reverse transcription-PCR. These data demonstrate that the potato mitochondrial editing machinery is not able to recognize these "foreign" editing sites and confirm that these unedited tRNA precursor molecules are not correctly processed in organello. As a consequence, the fate of these RNA precursor molecules is likely to be degradation. Indeed, we detected by PCR two 3'-end truncated precursor RNAs. Interestingly, both RNA species exhibit poly( A) tails, a hallmark of degradation in plant mitochondria. Taken together, these data suggest that, in plant mitochondria, a defective unedited RNA precursor that cannot be processed to give a mature stable tRNA, is degraded through a polyadenylation-dependent pathway.