RNA EDITING OF LARCH MITOCHONDRIAL TRNA(HIS) PRECURSORS IS A PREREQUISITE FOR PROCESSING

Larch mitochondria contain a 'native' tRNA(His) which is absent from a ngiosperms, Sequence comparisons of genomic DNA and cDNA obtained from unprocessed primary transcripts of the larch mitochondrial gene trnH encoding this tRNA revealed three nucleotide discrepancies. These thre e nucleotide alterations, in the acceptor stem, D stem and anticodon s tem respectively, are conversions of genomic cytidines to thymidines i n the cDNA (uridines in the tRNA) and thus resemble the RNA editing ev ents observed in nearly all plant mitochondrial mRNAs. Two cases of ed iting affecting mitochondrial tRNAs from angiosperms have already been described, but we present here the first example of such events in a gymnosperm mitochondrial tRNA. All three editing events correct mismat ched C . A base pairs which appear when folding the gene sequence into the standard cloverleaf structure, thereby improving the secondary st ructure of the tRNA. When incubated with a heterologous potato mitocho ndrial processing extract, only the edited form of the larch mitochond rial tRNA(His) precursor was efficiently processed in vitro. These dat a strongly suggest that editing of larch mitochondrial tRNA(His) is a prerequisite for its processing.