Sequence and expression of a fused mitochondrial gene, associated with Petunia cytoplasmic male sterility, compared with normal mitochondrial genes in fertile and sterile plants

The mitochondrial genome of plants specifies the trait known as cytoplasmic male sterility (CMS). By comparing recombinant mitochondrial genomes present in Petunia CMS and fertile somatic hybrids, a mitochondrial DNA (mtDNA) region associated with the sterility phenotype was identified. This CMS-associated mtDNA region carries a gene fusion (termed Pcf ) which contains coding region homologous to an ATP synthase proteolipid gene ( atp 9), a cytochrome oxidase subunit II gene ( cox ll), and an unidentified reading frame, designated urf S. The 5' flanking sequences are identical to those of a normal atp 9 gene in the CMS mtDNA; the source of the 3' flanking sequences has not been identified. S1 nuclease protection experiments have identified three Pcf gene transcripts, whose termini map to the same 5' locations as three transcripts of two normal atp 9 genes in CMS and fertile plants. In a fertile Petunia line, an additional transcribed atp 9 gene with a divergent 5' flanking region was identified. Although transcript abundance of homologous atp9 genes did not vary significantly between leaves and anthers in CMS and fertile lines, Pcf gene transcripts were four to five times higher in anthers than in leaves of the CMS plants.