Organisation and expression of mitochondrial atp9 genes from CMS and fertile carrots

The F-0-F-1 ATPase subunit 9 gene of carrot mitochondria has been isolated from both petaloid male-sterile (atp9-l) and normal fertile cytoplasms (atp9-3). The position occupied in atp9-3 by the TAA stop codon is, in the case of atp9-1 replaced by the CAA tripler coding fnr glutamine which makes this latter ORF 11 amino acid residues longer than that of the atp9-3. The 3' end of atp9-3 gene is flanked by a direct repeat of 42 bp. The sequence of the repeat unit is also present at the 3' end of atp9-1 but without reiteration. A truncated and presumably inactive version of atp9 (atp9-2) was found to be present in cytoplasms regardless of the fertility phenotype which they condition. The atp9-1 gene from petaloid cytoplasm appeared to be co-transcribed with the gene coding for 5S rRNA, and nuclear background influenced the accumulation of the respective transcript. The results are discussed with respect to a potential role of atp9-1 in generating the petaloid form of CMS.