Role of editing in 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 rut tRNAs encoded by the mt genome. In dicots, a C to U editing event corrects a CA mismatch into a U:A base pair in the acceptor stem of mt tRNA(Phe) (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 tRNA(His) (GUG). For both these mt RNA(Phe) and tRNA(His), editing of the precursors is a prerequisite for their processing into mature tRNAs. In potato mt tRNA(Cys) (GCA), editing converts a C-28:U-42 mismatch in the anticodon stem into a U-28:U-42 non-canonical base pair, and reverse transcriptase minisequencing has shown that the mature mt tRNA(Cys) is fully edited. In the bryophyte Marchantia polymorpha this U residue is encoded in the rut genome and evolutionary studies suggest that restoration of a U-28 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 tRNA(Cys), requires C to U editing at this position. But sequencing of the purified rut tRNA(Cys) 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. (C) 2002 Published by Elsevier Science B.V.