mitochondrial rna editing truncates a chimeric open reading frame associated with s malesterility in maize adjacent mitochondrial open reading frames orf355 and orf77 are associated with s cytoplasmic male sterility cmss in maize but the mechanisms leading to collapse of developing cmss pollen are unknown sequence similarity between orf77 and the mitochondrial atp synthase subunit 9 atp9 locus led us to examine rna editing in orf77 and atp9 transcripts of precollapse cmss microspores editing of atp9 was not influenced by the presence of orf77 transcripts sequence analysis of cdna clones demonstrated that atp9 transcripts are fully edited in cmss microspores orf77 nucleotides corresponding to edited nucleotides in atp9 were either not edited or edited inefficiently within the context of orf77 perhaps due to limited conservation of flanking sequences between orf77 and atp9 however eight of ten orf77 cdna clones carried an unexpected terminating edit that truncated orf77 to predict a peptide of 17 amino acids orf17 sharing significant identity with the cterminal transmembrane domain of the atp9 protein