Figure 1.

microRNA biogenesis and mode of action. The biogenesis of miRNAs starts in the nucleus where miRNA genes from distinct genomic loci are transcribed by RNA polymerase II into long primary transcripts (pri-miRNAs) followed by cleavage to precursor mRNA (pre-miRNA) by the nuclear RNase III-like enzyme, called DICER-LIKE (DCL1) in association with other proteins such as hyponastic leaves 1 (HYL1) and serrate (SE). The imperfect stem-loop secondary structure of the pre-miRNA hairpins are cut by DCL1 enzymes to generate the miRNA:miRNA^* duplexes. The duplexes are methylated by S-adenosyl-L-methionine-dependent RNA methyltransferase, HUA enhancer 1 (HEN1); and exported to the cytoplasm by HASTY where they undergo RNA-induced silencing complex (RISC) loading. The miR^* is released and the mature miRNA loads onto ARGONAUTE (AGO) ribonucleases in the RISC complex. Extensive base-pairing with mRNA targets is required for plant miRNA function to regulate gene expression. miRNAs guide Ago proteins to their specific targets through sequence complementarity which then leads to degrading the target mRNA transcript or by repressing its translation. On binding with perfect complementarity to a target mRNA, the Ago-miRNA complex induces its cleavage and degradation. An Ago-miRNA complex binding imperfectly to the 3′ UTR of the target mRNA induces translational inhibition, or deadenylation and subsequent decapping (DCP1,2) and degradation of the target mRNA.