RNA EDITING STATUS OF NAD7 INTRON DOMAINS IN WHEAT MITOCHONDRIA

The most highly conserved structures of group II introns are the helic al domains V and VI near the 3' splice site. Within this region of eac h of the four introns in the wheat mitochondrial nad7 gene encoding NA DH dehydrogenase subunit 7, there are A-C mispairs. To determine wheth er C-to-U type RNA editing restores conventional A-U pairing, we seque nced RT-PCR products from partially-spliced nad7template RNA and gel-f ractionated, excised intron RNA. We examined transcripts from germinat ing wheat embryos and seedlings because these two stages of developmen t show pronounced differences in steady state levels of nad7 intronic RNAs. We observed editing at only two of the six predicted sites, and they were located at homologous positions within domain V of the third and fourth introns. A third site was found to be edited within the un modelled domain VI loop of the fourth intron. Similar patterns of RNA editing were seen in wheat embryos and seedlings. These observations, and the presence of other non-conventional base pairs particularly wit hin domain V of plant mitochondrial introns, indicate weaker helical c ore structure than in ribozymic group II introns. Moreover, the incomp leteness or absence of editing in wheat nad7 excised intron RNA sugges ts that, although editing may contribute to splicing efficiency, it is not essential for splicing.