SEQUENCING, PROCESSING, AND LOCALIZATION OF THE PETUNIA CMS-ASSOCIATED MITOCHONDRIAL PROTEIN

The petunia mitochondrial fused gene (pcf), which is associated with c ytoplasmic male sterility (CMS), is composed of sequences derived from atp9, coxII, and an unidentified reading frame termed urfS. Pcf trans cripts are modified by editing at 11 sites. Codon usage and nearest ne ighbor analysis suggest that the urfS region is not derived originally from a plant mitochondrial coding region. Although the gene contains an open reading frame coding for a 43 kDa protein, a 25 kDa gene produ ct has previously been identified (Nivison and Hanson, 1989). N-termin al sequencing revealed that the 25 kDa protein is encoded within the u rfS portion of pcf and that its actual molecular mass is 19.5 kDa. Thr ough pulse-chase labeling of protein in isolated mitochondria, the 25 kDa protein was found to be processed from a 43 kDa precursor protein representing the entire pcf gene sequence. Antibodies to synthetic pep tides encoded by the atp9 and coxII portions of pcf recognized petunia ATP9 or COXII but no other mitochondrial proteins on immunoblots. Con trolled proteolysis experiments showed that both the 43 kDa precursor and the 25 kDa protein are soluble or loosely associated with membrane s. Thus, the 25 kDa protein appears to be the only pcf-encoded protein that accumulates in mitochondria.