| Popis: |
The term "RNA editing" was first coined more than a decade ago to describe the phenomenon of uridine insertion into trypanosomatid mitochondrial transcripts. Unlike mRNAs, transcripts of tRNA and rRNA genes are themselves converted into the functional entities they encode. RNA editing events represent additional steps in posttranscriptional processing and, like nucleoside modifications, they may occur at different stages in the pathway. To quantify RNA editing one usually compares the intensity of the bands on a sequencing (or primer extension) ladder that correspond to the edited and unedited versions of the RNA. The first example of tRNA editing in a mitochondrial (mt) system is that reported to occur in the ameboid protozoon Acantbamoeba castellanii. The observed nucleotide substitutions consisted of both purine-to-purine and pyrimidine- to-purine changes, suggesting a mechanism involving base or nucleotide replacement. More recently, it has been shown that nascent mRNAs present in stalled RNA polymerase complexes are substrates for editing by both cytidine and dinucleotide insertion. In a particular study the isolated RNAs were found to be edited to within 14 to 22 nucleotides of the stalled polymerase, suggesting that the insertional editing activity in Physarum is able to act quite close to the site of RNA synthesis. The most recently described mode of tRNA editing is that found in the mitochondria of metazoa. |
