Figure 1

Micro-RNA biogenesis. MiR's are synthesized initially as large RNA precursors (pri-miRs), processed in the nucleus by RNAse III Drosha, and DGCR8 into approximately 70 nt pre-miR, which are transported to the cytoplasm by exportin-5, with subsequent cleavage by another RNAse III enzyme Dicer, with its co-factor TRBP, releasing the approximately 22-nt mature dsmiR. MiR's can negatively regulate their targets in one of two major ways, depending on the degree of complementarity to its target. First, and probably most commonly, one strand of this duplex is incorporated into the RNA-induced silencing complex (RISC), then binds with imperfect complementarity to the 3'-UTR (untranslated region) of mRNA targets, preventing protein translation. Alternatively, miRs can bind with perfect complementarity to the ORF (open reading frame) of target mRNA's with subsequent degradation. Recent evidence also indicates miRs can also bind to either promoters, or coding regions of mRNAs as additional mechanisms of regulation.