Fig. 1.

Illustrations of major steps in microRNA (miRNA) biogenesis. RNA polymerase II (Pol II)-mediated miRNA gene (MIR) transcription is regulated by multiple transcription factors (TFs). Pol II activity itself is also subjected to phospho-regulation at its C-terminal domain (CTD). miRNA precursors are processed at the dicing bodies by the dicing complex, which is mainly composed of DICER-LIKE 1 (DCL1), HYPONASTIC LEAVES 1 (HYL1) and SERRATE (SE). Many other protein factors contribute to miRNA precursor processing through phospho-regulation, RNA splicing and other unknown molecular mechanisms. It remains unclear whether the dicing complex interacts with HUA ENHANCER 1 (HEN1) (question mark) and contributes to miRNA/miRNA* duplex export and RNA-induced silencing complex (RISC) assembly. During RISC loading, one strand of the small RNA duplex is selected as the guide strand (red) and incorporated into ARGONAUTE 1 (AGO1) to form a functional RISC, whereas the other strand (the passenger strand) is removed and degraded. Proteins are color-coded according to their known molecular functions in phospho-regulation of Pol II (red), MIR transcription (pink), phospho-regulation of HYL1 (orange), splicing/RNA-binding(dark blue)and potentially splicing/RNA-binding (light blue), and RISC assembly (brown). The core dicing complex components are colored green and protein with unknown molecular functions is colored purple. m⁷G,7-methylguanylate cap at the 5′ end of primary miRNAs; CDKF;1, CYCLIN-DEPENDENT KINASE F;1; CDKDs, CYCLIN-DEPENDENT KINASE D; NOT2, NEGATIVE ON TATA LESS 2; CDC5, CELL DIVISION CYCLE 5; CPL, C-TERMINAL DOMAIN PHOSPHATASE-LIKE; RCF3, REGULATOR OF CBF GENE EXPRESSION 3; PP4, Protein Phosphatase 4 complex; MPK3, MITOGEN-ACTIVATED PROTEIN KINASE 3; SnRK2s,SNF1-related protein kinase subfamily 2; CBC, Cap Binding Complex; AtGRP7, GLYCINE-RICH RNA-BINDING PROTEIN 7; STA1, STABILIZED 1; PRL1, PROTEIN PLEIOTROPIC REGULATORY LOCUS 1; MAC, MOS4-associated Complex; MOS2, MODIFIER OF SNC1, 2; THO/TREX, suppressor of the Transcription defects of Hpr1 mutants by Overexpression/TRanscription-EXport complex; PINP1, PSR1-INTERACTING PROTEIN 1; DBR1, LARIAT DEBRANCHING ENZYME 1; DDL, DAWDLE; HST, HASTY; HSP90, HEAT SHOCK PROTEIN 90; EMA1, ENHANCED MIRNA ACTIVITY 1; TRN1, TRANSPORTIN 1.