<P> RNA editing has been observed in some tRNA, rRNA, mRNA, or miRNA molecules of eukaryotes and their viruses, archaea, and prokaryotes . RNA editing occurs in the cell nucleus and cytosol, as well as within mitochondria and plastids . In vertebrates, editing is rare and usually consists of a small number of changes to the sequence of affected molecules . In other organisms, extensive editing (pan-editing) can occur; in some cases the majority of nucleotides in a mRNA sequence may result from editing . </P> <P> RNA - editing processes show great molecular diversity, and some appear to be evolutionarily recent acquisitions that arose independently . The diversity of RNA editing phenomena includes nucleobase modifications such as cytidine (C) to uridine (U) and adenosine (A) to inosine (I) deaminations, as well as non-templated nucleotide additions and insertions . RNA editing in mRNAs effectively alters the amino acid sequence of the encoded protein so that it differs from that predicted by the genomic DNA sequence . </P> <P> RNA editing through the addition and deletion of uracil has been found in kinetoplasts from the mitochondria of Trypanosoma brucei Because this may involve a large fraction of the sites in a gene, it is sometimes called "pan-editing" to distinguish it from topical editing of one or a few sites . </P> <P> Pan-editing starts with the base - pairing of the unedited primary transcript with a guide RNA (gRNA), which contains complementary sequences to the regions around the insertion / deletion points . The newly formed double - stranded region is then enveloped by an editosome, a large multi-protein complex that catalyzes the editing . The editosome opens the transcript at the first mismatched nucleotide and starts inserting uridines . The inserted uridines will base - pair with the guide RNA, and insertion will continue as long as A or G is present in the guide RNA and will stop when a C or U is encountered . The inserted nucleotides cause a frameshift, and result in a translated protein that differs from its gene . </P>

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