In the Nicotiana sylvestris CMSII mutant, a recombination-mediated change 5 ' to the first exon of the mitochondrial nad1 gene is associated with lack of the NADH : ubiquinone oxidoreductase (complex I) NAD1 subunit

We previously reported that the Nicotiana sylvestris CMSII mutant mitochond rial DNA carried a large deletion. Several expressed sequences, most of whi ch are duplicated, and the unique copy of the nad7 gene encoding the NAD7 s ubunit of the NADH:ubiquinone oxidoreductase complex (complex I) are found in the deletion. Here, we show that the orf87-nad3-nad1/A cotranscription u nit transcribed from a unique promoter element in the wild-type, is disrupt ed in CMSII. Nad3, orf87 and the promoter element are part of the deleted s equence, whilst the nad1/A sequence is present and transcribed from a new p romoter brought by the recombination event, as indicated by Northern and pr imer extension experiments. However, Western analyses of mitochondrial prot ein fractions and of complex I purified using anti-NAD9 affinity columns, r evealed that NAD1 is lacking in CMSII mitochondria. Our results suggest tha t translation of nad1 transcripts rather than transcription itself could be altered in the mutant. Consequences of lack of this submit belonging the m embrane arm of complex I and thought to contain the ubiquinone-binding site , are discussed.