An open reading frame for a protein involved in cytochrome c biogenesis is split into two parts in Brassica mitochondria.

A plant mitochondrial gene with sequence similarity to ccl1, a bacterial gene involved in cytochrome c biogenesis, occurs as a single open reading frame in wheat, Oenothera, carrot and Marchantia mtDNAs. In Brassica napus "Polima" or Brassica campestris mtDNA, however, this open reading frame is split into two segments that are located 60 kb apart on the master-circle form of the genome. Although transcripts of the split ORF are edited in a manner similar to that of a functional gene, transcripts that span both portions of the ORF are not evident in gel-blot hybridization experiments. Low-abundance transcripts that span both portions of the split ORF can be detected by RT-PCR, but these contain an additional 54-bp sequence, inserted between the two segments, that is unrelated to the corresponding sequences of other plants. Since this additional sequence introduces an in-frame stop codon, no transcripts have been found that could be translated to yield a protein product of a size similar to that present in other plant species. An antiserum directed against the product of the corresponding wheat gene detects polypeptides of similar size in wheat and Brassica mitochondria. This antiserum, however, immunoprecipitates a labelled polypeptide from the products of wheat, but not Brassica, in organello protein synthesis. Gel-blot analysis of total Brassica DNA indicates that sequences capable of hybridizing with the ORF are present in the nuclear genome. We suggest that the functional form of the Brassica gene may reside in the nucleus.