A 60 kDa COX1 protein in mitochondria of carrot irrespective of the presence of C-terminal extensions in the cox1 reading frames

An inverted repeat in the mitochondrial DNA of carrot mitochondrial type (mitotype) SW3 has been sequenced in the context of its up- and downstream genomic environments. The 3,159 bp repeat encodes rps7 and cox1, but the cox1 reading frame does not encode a stop codon at a position predicted by alignment with other plant cox1 genes; cox1 extends into both of the downstream flanking sequences as unique C-terminal extensions of 224 (cox1-1) or 284 (cox1-2) amino acids. Northern and RT-PCR analyses revealed that rps7 and cox1 are co-transcribed and RNA-edited. In three additional mitotypes of carrot, rps7 and cox1 are co-transcribed, but are present as single copies within the genome. In mitotype FG17, the cox1 gene has a C-terminal extension like cox1-1, but in mitotypes FW2 and FW4, the cox1 genes lack C-terminal extensions and have stop codons and 3′ UTRs like those of other plant cox1 genes. The C-terminal extensions are transcribed in SW3 and FG17 and result in longer primary transcripts than those from FW2 and FW4. Western immunoblotting of mitochondrial proteins from all mitotypes detected COX1 products of the predicted unextended size in all mitotypes, irrespective of the presence of a C-terminal extension in the reading frame.