Relationship between RNA splicing and exon editing near intron junctions in wheat mitochondria

We have compared the RNA editing status of wheat mitochondrial unspliced precursor transcripts with spliced RNAs, focusing in particular on exon editing sites located very close to intron/exon junctions. Using direct sequencing of reverse transcriptase–PCR products to assess C‐to‐U editing in various RNA populations for the respiratory chain genes nad2 , nad4 , nad5 , nad7 , and cox2 , we observed that candidate exon sites immediately upstream or downstream of introns remained unedited in the presence of the adjacent intron, whereas sites further away were typically partially (or completely) edited in precursor molecules. The ‘late’ editing of exon sites adjacent to splice junctions is consistent with access of the editing machinery being sterically hindered by the intron or alternatively by an editing recognition element being created by the exon/exon structure. When we examined RNA isolated from the embryo‐to‐seedling stages of wheat development, we found that fully spliced messenger RNAs showed editing at the expected sites in all stages, including dormant seeds. In contrast, we observed that cox2 precursors were less completely edited in RNA populations from 0 to 24 h postimbibition embryos than in 6‐day‐old seedlings not only at splice junctions but also at other exon sites, consistent with more efficient coupling between editing and transcription during later stages of development.