Origin and Evolution of RNA Editing
The RNA-editing system seen in the animal may have evolved from mononucleotide deaminases, which have led to larger gene families that include the apobec-1 and adar genes. These genes share close identity with the bacterial deaminases involved in nucleotide metabolism. The adenosine deaminase of E. coli cannot deaminate a nucleoside in the RNA; the enzyme’s reaction pocket is too small to for the RNA strand to bind to. However, this active site is widened by amino acid changes in the corresponding human analog genes, APOBEC-1 and ADAR, allowing deamination . The gRNA-mediated pan-editing in trypanosome mitochondria, involving templated insertion of U residues, is an entirely different biochemical reaction. The enzymes involved have been shown in other studies to be recruited and adapted from different sources. But, the specificity of nucleotide insertion via the interaction between the gRNA and mRNA are similar to the tRNA editing processes in the animal and Acanthamoeba mithochondria. Eukaryotic ribose methylation of rRNAs by guide RNA molecules is a similar form of modification.
Thus, RNA editing evolved more than once. Several adaptive rationales for editing have been suggested . Editing is often described as a mechanism of correction or repair to compensate for defects in gene sequences. However, in the case of gRNA-mediated editing, this explanation does not seem possible because if a defect happens first, there is no way to generate an error-free gRNA-encoding region, which presumably arises by duplication of the original gene region. This thinking leads to an evolutionary proposal called "constructive neutral evolution" in which the order of steps is reversed, with the gratuitous capacity for editing preceding the "defect". 31
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