from Jonatha M. Gott writing in RNA Editing: Current Research and Future Trends:
Mitochondrial RNAs in the acellular slime mold Physarum polycephalum are subject to the widest range of editing events observed thus far. Mitochondrial RNAs differ from the mitochondrial genome at over 1300 sites, and both coding (mRNAs) and non-coding RNAs (rRNAs and tRNAs) are affected. At least three distinct editing mechanisms are needed to account for the different forms of editing observed in the mitochondrial transcriptome: nucleotide insertions and deletions, C to U changes, and specific alterations at the 5' end of tRNAs. Nucleotide insertions are co-transcriptional and require flanking regions of the template, but the exact signals that specify the site of nucleotide insertion and the identity of the nucleotide to be added remain an enigma. The rare instances of base changes and replacement of the first nucleotide of mitochondrial tRNAs are not directly linked to transcription and are likely to occur via processes related to those previously described in other mitochondrial editing systems.
Further reading: RNA Editing: Current Research and Future Trends