The exon d of nad1 is located 993 bp upstream of the nad6 gene in the wheat mitochondrial genome. Transcription analyses of both sequences (nad1 exon d and the nad6 gene) were done by Northern hybridization using RNA from wheat seedlings and tissue cultures derived from immature embryos. A complicated pattern was generated with a probe including exon d of nad1 and the whole nad6 gene. An 0.71-kb transcript is specific to nad1 exon d whereas a 1.2-kb transcript is specific to the nad6 gene. Three larger transcripts hybridize to both probes suggesting that nad1 exon d and nad6 are co-transcribed. This co-transcription has been directly demonstrated by cDNA synthesis on mtRNAs and sequencing of the PCR amplification product.
A region of the mitochondrial (mt) DNA of wheat was studied because of its homology with other plant mtDNAs. Sequence analysis revealed an open reading frame encoding a polypeptide of 247 amino acids. Comparison of the sequence of the putative polypeptide with the protein sequence data of the Swiss-Prot library reveals homology with subunit 6 of the NADH-ubiquinone complex of mitochondria from Marchantia polymorpha, Podospora anserina, Chlamydomonas reinhardtii and of chloroplasts from M. polymorpha and Oryza sativa. No similarity was detected when compared with the subunit 6 of animal mitochondria, probably due to the rapid evolution of the sequence. A single 1.2 kb transcript appears in northern RNA blots. We found 15 edited sites of which only 13 give amino acid changes. This is the first report of a mt nad6 gene in higher plants.
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