distribution of rna editing sites in oenothera mitochondrial mrnas and rrnas to investigate whether rna editing in plant mitochondria modifies structural rnas as well as proteincoding rnas we compared the genomicencoded information with the respective transcripts of several genes in oenothera the genes analysed are the 5s 18s and 26 s rrnas the alphasubunit of atpase atpa cytochrome b cytb orfb which is located upstream of cytochrome oxidase subunit iii and the respective leader trailer and spacer sequences all open reading frames were found to be edited to some degree the atpa coding region has the least edited mrna in oenothera mitochondria with only four nucleotides altered in the 1533 nucleotide open reading frame from this analysis we conclude that frequent rna editing is indicative of functional protein coding regions in plant mitochondria the extensive editing in orfb for example suggests that this orf codes for a mitochondrial protein no rna editing event was found in the 5s rrna or in the 1824 nucleotides analysed of the 18s rrna but two nucleotides were found to be altered in the 1970 nucleotides compared for the 26s rrna one nucleotide alteration has changed c to u the other in reverse u to c however only one of five cdna clones covering this region shows the modifications similar to many silent editing events in open reading frames rna editing in the structural rnas thus does not seem to be essential for their function in the mitochondrial ribosome