Editing of plant mitochondrial transfer RNAs

Editing in plant mitochondria consists in C to U changes and mainly affects messenger RNAs, thus providing the correct genetic information for the biosynthesis of mitochondrial (mt) proteins. But editing can also affect some of the plant mt tRNAs encoded by the mt genome. In dicots, a C to U editing event corrects a C:A mismatch into a U:A base-pair in the acceptor stem of mt tRNAPhe (GAA). In larch mitochondria, three C to U editing events restore U:A base-pairs in the acceptor stem, D stem and anticodon stem, respectively, of mt tRNAHis (GUG). For both these mt tRNAs editing of the precursors is a prerequisite for their processing into mature tRNAs. In potato mt tRNACys (GCA), editing converts a C28:U42 mismatch in the anticodon stem into a U28:U42 non-canonical base-pair, and reverse transcriptase minisequencing has shown that the mature mt tRNACys is fully edited. In the bryophyte Marchantia polymorpha this U residue is encoded in the mt genome and evolutionary studies suggest that restoration of the U28 residue is necessary when it is not encoded in the gene. However, in vitro studies have shown that neither processing of the precursor nor aminoacylation of tRNACys requires C to U editing at this position. But sequencing of the purified mt tRNACys has shown that psi is present at position 28, indicating that C to U editing is a prerequisite for the subsequent isomerization of U into psi at position 28.