Expression of the wheat mitochondrial nad3-rps12 transcription unit: correlation between editing and mRNA maturation

In plant mitochondria, RNA editing involves the conversion of cytidines in the genomic DNA into uridines in the corresponding RNA. Analysis of cDNAs prepared by reverse transcription of mitochondrial RNAs has shown that partially edited RNAs are present in wheat mitochondria. The extent of this partial editing as well as its potential influence on the corresponding protein sequence were studied along with the expression of a wheat mitochondrial locus. The sequence, expression, and RNA editing of the wheat mitochondrial transcription unit containing four open reading frames (nad3, rps12, orf299, orf156), all cotranscribed into a same predominant precursor RNA, have been studied. The product of orf156 is an 18-kD mitochondrial membrane protein of unknown function, whereas the product of orf299 could not be detected and this sequence seems to be a pseudogene. Sequences of cDNA clones derived by the polymerase chain reaction technique show that nad3, rps12, and orf156 transcripts are edited, whereas orf299 is not edited, except for a sequence identical to part of the coxII gene. Analysis of cDNA clones obtained from the precursor RNA shows the presence of a large number of partially edited nad3-rps12 transcripts with no evident polarity for the editing process. This shows that RNA editing is a post-transcriptional event. In addition, study of partial editing at the level of precursor, mature, and polysomal transcripts shows that mainly mature, completely edited sequences are used for translation. Deletions of a nucleotide at editing sites were observed in a number of cDNA clones, suggesting that C----U RNA editing in plant mitochondria would be achieved by nucleotide replacement.