Identification And Characterization Of A Novel Component In The Arabidopsis MicroRNA Pathway

RNA interference (RNAi) is a conserved mechanism of gene regulation in eukaryotes. The key components of RNAi pathways are small RNAs and Argonaute (AGO) family proteins. Small RNAs are associated with AGO proteins to form the RNA-induced silencing complexes (RISCs) to regulate target genes through mRNA cleavage and/or translational repression.There are four types of small RNAs in Arabidopsis, including microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), natural antisense siRNAs (nat-siRNAs) and hetero-chromatic siRNAs (hc-siRNAs). miRNAs are a class of 21 nt sRNAs that play important roles in many biological processes in plants. miRNA genes are first transcribed by DNA-dependent RNA Polymerase Ⅱ (Pol Ⅱ) to generate long transcripts primary miRNAs (pri-miRNAs), which contain hairpin structures. Pri-miRNAs are bound and stabilized by DAWDLE (DDL), which recruits Dicer-like 1 (DCL1) to pri-miRNAs and facilitates its processing. DCL1 processes pri-miRNAs to mature miRNAs with the aid of a double-strands RNA binding protein HYPONASTC LEAVES 1 (HYL1) and a zinc-finger protein SERRATE (SE), which form Dicing bodies (D-bodies) in nucleus. Mature miRNAs are sorted into AGO1 to form RISC, regulating target genes through slicing target mRNAs or translational repression.From a genetic screen that is based on an artificial miRNA causing trichome clustering when overexpressed, we have obtained dozens of mutants with enhanced miRNA activity (ema) or compromised miRNA activity (cma). We have previously characterized EMA1, which encodes an importin β protein that negatively regulates the loading of miRNAs into AGO1. Here, we further performed a suppressor screening of emal (soe) to more sensitively identify new factors in the miRNA pathway. In this study, we characterized one of the soe mutants, soe4. SOE4/AT2G19380 encodes an unknown protein with four zinc fingers domains and a RNA recognition motif (RRM). The soe4 mutation led to reduced accumulation of miRNAs as well as pri-miRNAs. SOE4 localizes in D-bodies and interacts with HYL1, SE, DCL1. In addition, SOE4 is capable of binding to pri-miRNA and pre-miRNA in vitro. These data suggest that SOE4, as a novel component in biogenesis of miRNA, could possibly binds and stabilizes pri- or pre-miRNAs.