RNA editing of the transcript coding for subunit 4 of NADH dehydrogenase in wheat mitochondria: uneven distribution of the editing sites among the four exons.

The wheat mitochondrial (mt) NADH dehydrogenase subunit 4 gene (nad4) has been localized and sequenced. This gene, about 8 kb long, is composed of four exons separated by three class II introns. The nad4 gene exists as a single copy in the wheat mitochondrial genome and it is transcribed into one abundant mRNA of 1.8 kb, whose extremities have been mapped. The complete cDNA sequence corresponding to the nad4 transcript has been determined by combining the direct sequencing of uncloned cDNA and a method involving cDNA synthesis and PCR amplification using specific oligonucleotides as primers, followed by cloning and sequencing of the amplification product. Comparison of the genomic sequence with that of the cDNA shows that all nad4 transcripts are fully edited at 23 positions, with an uneven distribution of the editing sites between the different exons: While exon 1 and exon 4 are extensively edited (with a change of 11% of the amino acid sequence), exon 2 is not edited at all and exon 3 is 0.5% edited. This uneven distribution is discussed.