PathwayStep334 PathwayStep333 PathwayStep332 PathwayStep331 PathwayStep337 PathwayStep336 PathwayStep335 Pol II mediated transcription of microRNA genes Exportin complex translocates pre-miRNA to cytosol Exportin-5 recognizes 3' overhang of pre-miRNA Microprocessor complex cleaves pri-miRNA to pre-miRNA Dicer cleaves pre-miRNA to yield duplex miRNA Duplex miRNA is loaded into Argonaute Removal of miRNA passenger strand MicroRNA (miRNA) Biogenesis Authored: Gopinathrao, G, May, B, 2007-11-18 23:55:40 Biogenesis of microRNAs (miRNAs) can be summarized in five steps (reviewed in Ketting 2011, Nowotny and Yang 2009, Kim et al. 2009, Chua et al. 2009, Hannon and He 2004):<br>1. Transcription. miRNA transcripts may come from autonomously transcribed genes, they may be contained in cotranscripts with other genes, or they may be located in introns of host genes. Most miRNAs are transcribed by RNA polymerase II, however a few miRNAs originate as RNA polymerase III cotranscripts with neighboring repetitive elements. The initial transcript, termed a primary microRNA (pri-miRNA), contains an imperfectly double-stranded region within a hairpin loop. Longer sequences extend from the 5' and 3' ends of the hairpin and may also contain double-stranded regions. <br>2. Cleavage by Drosha. The 5' and 3' ends of the pri-miRNA are removed during endoribonucleolytic cleavage by the Drosha nuclease in a complex with the RNA-binding protein DGCR8 (the Microprocessor complex). The cleavage product is a short hairpin of about 60 to 70 nt called the pre-microRNA (pre-miRNA). <br>3. Nuclear export by Exportin-5. The resulting pre-miRNA is bound by Exportin-5 in a complex with Ran and GTP. The complex translocates the pre-miRNA through the nuclear pore into the cytoplasm. <br>4. Cleavage by Dicer. Once in the cytoplasm the pre-miRNA is bound and cleaved by the Dicer ribonuclease in complex with the RNA-binding protein TRBP (TAR binding protein). The product is an imperfectly double-stranded miRNA of about 21 to 23 nucleotides. At this stage the double-stranded miRNA has protruding single-stranded 3' ends of 2-3 nt. <br>5. Strand selection and incorporation into RNA-Induced Silencing Complex (RISC). At this stage the miRNA has two strands: the passenger strand, which will be removed and degraded, and the guide strand, which will be retained and guides the RISC to target mRNAs. An Argonaute protein is already bound to the Dicer:miRNA complex or subsequently binds the complex (the order is unknown). The human genome encodes 4 Argonaute proteins, however only Argonaute2 can cleave target mRNAs with perfect or nearly perfect complementarity to the guide miRNA. In mammalian cells, such endogenous interactions are very rare; most miRNAs only cause translational repression. For complexes that contain Argonaute2, cleavage of the passenger strand of the double-stranded miRNA accompanies removal of the passenger strand. Complexes containing other Argonautes may use a helicase to remove the passenger strand but this is not fully known. The resulting single-stranded miRNA (the guide strand) remains in a complex containing Argonaute, Dicer, TRBP, and other proteins termed the RNA-induced Silencing Complex (RISC). The full set of proteins composing RISC is not yet known. Edited: May, B, 2009-03-17 16:05:37 Pubmed14744438 Pubmed15145345 Pubmed15211354 Pubmed15364901 Pubmed15372072 Pubmed15525708 Pubmed15944707 Pubmed17057362 Pubmed17099701 Pubmed19165215 Pubmed19330724 Pubmed19477631 Pubmed21755468 Pubmed8252621 Reactome Database ID Release 43203927 Reactome, http://www.reactome.org ReactomeREACT_12417 Reviewed: Karginov, F, Hannon, GJ, 2008-02-08 15:41:16