Mature RISC consists of a single-stranded small RNA bound to an Argonaute protein. Small RNAs that occupy WAGO clade proteins are usually direct products of RNA synthesis 6, 7, 9. Piwi subfamily proteins load small RNAs derived from single-stranded precursors (piRNAs) and AGO clade proteins usually associate with small RNA duplexes processed by RNase III endonucleases (miRNAs and siRNAs reviewed in REFS 1, 2). ![]() Argonaute proteins can be classified into three subgroups according to their sequence relationships: the AGO subfamily, the Piwi subfamily and the worm-specific WAGO clade 9- 11. Some small RNA classes, including Piwi-interacting RNAs (piRNAs) and secondary siRNAs in worms, however, are not derived from dsRNA precursors and are produced through alternative biogenesis mechanisms independently of RNase III enzymes 4- 8.įollowing their production, small RNAs are sorted to confer association with specific Argonaute family proteins, which function as the core of the rna-induced silencing complex (RISC). The biogenesis of most small RNA classes, including microRNAs (miRNAs) and many small interfering RNAs (siRNAs), requires the action of RNase III family proteins (reviewed in REFS 1– 3). Conceptually, all small RNA-mediated regulatory events can be considered as the culmination of several consecutive steps: small RNA biogenesis, strand selection (in which dsRNA is the precursor), loading into Argonaute, target recognition and effector function. Only then can the target specificity conferred on Argonaute proteins by their small RNA guides enable their myriad important roles, which include the regulation of gene expression, modification of chromosome structure and protection from mobile elements. Therefore, it is crucial that these very similar species are appropriately sorted among closely related partners. The precise combination of a small RNA with a particular Argonaute protein determines its biological function. They are mainly ~20–30 nucleotides (nt) in length, have 5′ phosphate groups and 3′ hydroxyl (−OH) (although sometimes modified) termini, and they associate with specific members of a large protein family - the Argonautes. ![]() Small RNAs that engage RNAi-related pathways share several characteristic features. As yet, we know very little about many newly discovered groups of small RNAs, but our understanding of the biogenesis and biological functions of RNAi-related small RNA classes is growing rapidly. A growing number of small RNA classes has since emerged from studies of eukaryotic organisms, and these RNAs can be approximately divided into two groups: small RNAs that engage RNAi-related machinery and those that do not. The discovery of RNA interference (RNAi) in the late 1990s sparked a renaissance in our understanding of RNAs as regulatory molecules.
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