The atp6 coding region has been disrupted and a novel reading frame generated in the mitochondrial genome of cytoplasmic male-sterile radish

The gene atp6, encoding subunit 6 of the mitochondrial F0-ATPase complex, has been characterized from both the normal (fertile) and Ogura (male-sterile) radish cytoplasms in order to determine if previously identified atp6 transcriptional differences could play a role in cytoplasmic male sterility. Normal radish atp6 encodes a 262-amino acid polypeptide that exhibits approximately 80% sequence identity with other plant atp6 polypeptides. A tRNAfMet gene is located 150 base pairs 5′ to atp6, and the two genes may be co-transcribed. As a result of extensive rearrangement, sequences that comprise the normal atp6 locus are present in three widely separated regions of the Ogura mitochondrial genome. Both 5′ and 3′ rearrangement breakpoints have been identified and found to be associated with short repeated sequences. The normal and Ogura atp6 loci share a common 987-base pair region containing most of the atp6 coding region and 106 base pairs of the 3′-flanking region. A 105-codon open reading frame is transcribed as the first gene of an Ogura atp6 bicistronic mRNA. This sequence is not present in normal radish mitochondrial DNA. There are many nucleotide differences in the Ogura atp6 5′-flanking and coding regions that can be expected to eliminate normal translation of atp6. At least two possibilities exist for translation of Ogura atp6, both of which would result in an NH2-terminal amino acid sequence different from that of normal radish.