RNA editing is a post transcriptional process that alters the sequence of individual RNAs prior to translation. In plants, C to U editing is common in both mitochondrial and chloroplast sequences, where editing can create altered start or stop codons and often results in proteins whose sequence is different than that predicted by the DNA sequence alone. Editing in plants is widely interpreted as a correction mechanism for T to C mutations, and may have additional significance in regulation of gene and protein function. We have experimentally determined RNA editing sites for the cox1 gene from 18 species of angiosperms and inferred the status of editing sites for over 200 additional species. 1) Editing status for editable sites can be accurately inferred for cox1 gene from a careful inspection of DNA sequences, 2) Phylogenetic analysis of cox1 sequences is strongly affected by the inclusion of these sites. When editing sites are removed, phylogenies are consistent with hypotheses inferred from chloroplast and/or nuclear genes. When editing sites are included in the analysis, the effects depend on the choice of taxa but in some cases phylogenies are serious disrupted. 3) Abrupt reversals of the editing status ("Riveting") of all edited sites to create "processed paralogs" (sensu Bowe and dePamphilis, 1996) has occurred at least a dozen times during flowering plant evolution and reversal of shorter contiguous regions of sequence have occurred at least as frequently. These observations shed light on recombination and reverse transcription mediated events in the plant mitochondrion.

Key words: coxI, mitochondrial DNA, phylogeny, processed paralog, RNA editing