Loss of RNA editing of rps1 sequences in Oenothera mitochondria.

We have analysed a region downstream from the atp9 gene in Oenothera mitochondrial DNA which contains an open reading frame of 224 codons. This open reading frame, designated orf224, is co-transcribed with the atp9 gene. In wheat mitochondria, a homologous reading frame (orf174) has been described which is considerably shorter at the N-terminus compared to the putative Oenothera gene. The deduced polypeptides from both species show high similarity to the N-terminal third of ribosomal protein S1 of bacteria. Transcripts of orf174 are edited in wheat mitochondria whereas the similarly conserved cytidine positions in orf224 mRNAs of Oenothera are silent. On the other hand, atp9 sequences, which are located upstream on the same co-transcript, are fully edited. Because of this, we conclude that editing sites are selected independently for mRNA sections. Our results suggest that orf224 represents a transcribed pseudogene in Oenothera. The active gene for a functional ribosomal protein S1 for mitochondria is therefore expected to be present in the nucleus. A nuclear localization for this gene is likewise suggested for Arabidopsis due to the fact that rps1 sequences are absent from the mitochondrial genome.