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Fig. 2. A model linking RNAi and GW body (GWB) assembly and function. RNAi activity is triggered by siRNA/miRNA duplexes, which are first processed from either double-stranded RNA (dsRNA) or precursor microRNA (pre-miRNA), respectively, by the RNase-III-type endonuclease Dicer. These duplexes are then incorporated into RISC, where the passenger strand (black) is either cleaved and degraded or removed by the bypass mechanism (Matranga et al., 2005). This activation process and assembly of the guide strand (red) into RISC is thought to initiate early stages of GWB formation. Subsequent targeting by RISC results in further recruitment of one or more heteromeric protein complexes (which could include GW182 and RCK/p54) on the mRNA, which forms a specific RNP structure that causes post-transcriptional inhibition of gene expression (through siRNA-mediated cleavage or miRNA-mediated translational repression, depending on the degree of complementarity between the guide-strand RNA and its target mRNA). The targeted mRNA is eventually degraded by further recruitment of 5'-to-3' mRNA decay factors, which include the deadenylase Ccr4, decapping factors (LSm1-7 ring, Dcp complex) and the 5'-to-3' exonuclease Xrn1. The accumulation of RNA decay factors during degradation could be responsible for the size increases in GWBs, which makes them visible by conventional light microscopy. ORF, open reading frame.
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