Insertional editing in mitochondria of Physarum

RNA produced from a number of genes on the mitochondrial (mt) DNA of Physarum polycephalum have nucleotides inserted at specific sites in their sequence. These insertions are spaced at approximately 25 nucleotide intervals and create open reading frames in mRNA and functional structure in tRNAs and rRNAs. Although most of the insertions at a site are single cytidines; single uridines and certain dinucleotides containing adenosine and guanosine as well as cytidine and uridine are also occasionally inserted at certain sites. This mixed nucleotide insertional RNA editing is unique among currently characterized editing systems.     

RNA editing in plant mitochondria

RNA editing in plant mitochondria alters nearly all mRNAs by C to U and U to C transitions. In some species more than 400 edited sites have been identified with significant effects on the encoded proteins. RNA editing occurs in higher and lower plants and presumably has evolved before the differentiation of land plants. Current research focuses on the elucidation of the biochemistry and the specificity determinants of RNA editing in plant mitochrondria.     

The process of RNA editing in plant mitochondria

RNA editing changes more than 400 cytidines to uridines in the mRNAs of mitochondria in flowering plants. In other plants such as ferns and mosses, RNA editing reactions changing C to U and U to C are observed at almost equal frequencies. Development of transfection systems with isolated mitochondria and of in vitro systems with extracts from mitochondria has considerably improved our understanding of the recognition of specific editing sites in the last few years. These assays have also yielded information about the biochemical parameters, but the enzymes involved have not yet been identified. Here we summarize our present understanding of the process of RNA editing in flowering plant mitochondria.