RNA Editing of the Mitochondrial atpA/atp9 Co-Transcript of Triticale, Carrying the timopheevi Cytoplasmic Male Sterility Cytoplasm from Wheat

Subunits a and nine are components from two different portions of the mt H+-ATP synthase enzyme, namely the soluble catalytic (F,) and the membranous proton-conduction (F,-ATPase) complexes. In wheat and triticale the genes for both subunits are located adjacently in the mt genome, irrespective of whether these lines are carrying the aestivum, durum, or timopheevi cytoplasmic male sterility cytoplasm. However, in the timopheevi mt genome the atpA/ atp9 cistron is part of a large DNA repeat and thus present in two different genomic environments. The repeat comprises at least 9 kb that are delimited 941 bp downstream of the atp9 gene. The 2.7-kb triticale atpAlatp9 sequence shows 100% homology with a Triticum aestivum sequence (Schulte et al., 1989), containing both reading frames, the intergenic region, and at least the first 400 bp upstream and the first 46 bp downstream of the atpAlatp9 gene region. Transcription of the atpAlatp9 cistron in triticale gives rise to a co-transcript of 2.6 kb, which is posttranscriptionally modified by 5' processing. To investigate whether further RNA modifications occur, we have analyzed this triticale transcript for RNA editing (Table I). To date, editing of an atpA transcript has been described in only two dicotyledonous species (Schuster et al., 1991; Senda et al., 1993). Overlapping cDNA clones covering the completely transcribed gene region were generated by reverse transcription PCR, and 10 individual clones were subsequently sequenced. In total, 17 RNA-editing sites could be identified, a11 of which represent cytidine to uridine transitions. The open reading frame of the atpA transcript carries five nonsilent and two silent RNA-editing sites. Equivalent alterations have been detected in Oenothera (amino acid 497) and sugar beet (amino acids 393 and 431), while a11 remaining sites are pre-edited in these two dicotyledonous species. Modified codons are located preferentially in the carboxy-terminal region of the protein, while silent RNAediting sites are found exclusively at the amino terminus of the a subunit. Enzymatic studies with peptides derived by limited proteolysis from the a subunit suggest that the