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. Author manuscript; available in PMC: 2009 Sep 15.
Published in final edited form as: Cell Cycle. 2008 Sep 5;7(18):2840–2845. doi: 10.4161/cc.7.18.6734

Figure 1.

Figure 1

Models for the biogenesis of canonical miRNAs, mirtrons, and hpRNAs in Drosophila. All three originate in the nucleus as RNA polymerase II transcripts containing inverted repeats, and are presumably capped and polyadenylated species. Canonical miRNAs are first cleaved by the Pasha/Drosha complex to release the pre-miRNA hairpin. Mirtrons are short intronic hairpins that are spliced and debranched to yield pre-miRNA mimics. Both types of pre-miRNAs are exported to the cytoplasm via Exportin-5, where they are cleaved by the Dicer-1/Loqs complex. The usual fate of the resulting small RNA duplex is for one strand to be preferentially loaded into AGO1 as the mature miRNA, while its partner miRNA* strand is preferentially degraded. However, a fraction of miRNA species are transferred into AGO2 and methylated at their 3' ends; a fraction of miRNA* strands are also transferred into AGO proteins. Hairpin RNA processing is not dependent on Exportin-5, and so the relevant factor is not known (?); possibly it transits the conventional mRNA export pathway. In the cytoplasm, Dicer-2 and Loqs are required for hpRNA maturation, but it is not known if these function as a discrete complex or not (?). The typical fate of the resulting small RNA duplexes is their preferential loading into AGO2 and 3' methylation as mature siRNAs. However, some hpRNA products associate with AGO1. A Dicer-2/R2D2 complex loads exogenously-derived siRNAs into AGO2, but it is not known if this is true for hpRNA-derived siRNAs (?).