DNA Methylation Mediated by a MicroRNA Pathway

In plants, the known microRNAs (miRNAs) are produced as ~21 nucleotide (nt) duplexes from their precursors by Dicer like 1 (DCL1). They are incorporated into Argonaute 1 (AGO1) protein to regulate target gene expression primarily through mRNA cleavage. We report here the discovery of a new class of miRNAs in the model monocot rice (Oryza sativa). These are 24 nt in length and require another member of the Dicer family, DCL3, for their biogenesis. The 24 nt long miRNAs (lmiRNAs) are loaded into AGO4 clade proteins according to hierarchical rules, depending on the upstream biogenesis machinery and the 5' terminal nucleotide. We demonstrated that lmiRNAs direct DNA methylation at loci from which they are produced as well as in trans at their target genes and play roles in gene regulation. Considered together, our findings define a novel miRNA pathway that mediates DNA methylation. Overall design: Small RNAs were prepared from Rice total extract in wide type, dcl1, dcl3, rdr2 dbsRNA mutant and AGO4a, AGO4b, and AGO16 complexes, ligated to a 3' adaptor and a 5' acceptor sequentially, and then RT-PCR amplified. PCR products were reamplified using a pair of solexa cloning primers and then provided for sequencing. For technical details, see Wu, L., Zhang, Q., Zhou, H., Ni, F., Wu, X., and Qi, Y. (2009). Rice microRNA effector complexes and targets. The Plant Cell, 21: 3421-3435.