Evolution of RNA editing in trypanosome mitochondria

Two different RNA editing systems have been described in the kinetoplast-mi tochondrion of trypanosomatid protists, The first involves the precise inse rtion and deletion of U residues mostly within the coding regions of maxici rcle-encoded mRNAs to produce open reading frames. This editing is mediated by short overlapping complementary guide RNAs encoded in both the maxicirc le and the minicircle molecules and involves a series of enzymatic cleavage -ligation steps. The second editing system is a C-34 to U-34 modification i n the anticodon of the imported tRNA(Trp), thereby permitting the decoding of the UGA stop codon as tryptophan, U-insertion editing probably originate d in an ancestor of the kinetoplastid lineage and appears to have evolved i n some cases by the replacement of the original pan-edited cryptogene with a partially edited cDNA, The driving force for the evolutionary fixation of these retroposition events was postulated to be the stochastic loss of ent ire minicircle sequence classes and their encoded guide RNAs upon segregati on of the single kinetoplast DNA network into daughter cells at cell divisi on. A large plasticity in the relative abundance of minicircle sequence cla sses has been observed during cell culture in the laboratory. Computer simu lations provide theoretical evidence for this plasticity if a random distri bution and segregation model of minicircles is assumed. The possible evolut ionary relationship of the C to U and U-insertion editing systems is discus sed.