MicroRNA-directed transcriptional gene silencing in mammalian cells

MicroRNAs (miRNAs) regulate gene expression at the posttranscriptional level in the cytoplasm, but recent findings suggest additional roles for miRNAs in the nucleus. To address whether miRNAs might transcriptionally silence gene expression, we searched for miRNA target sites proximal to known gene transcription start sites in the human genome. One conserved miRNA, miR-320, is encoded within the promoter region of the cell cycle gene POLR3D in the antisense orientation. We provide evidence of a cis-regulatory role for miR-320 in transcriptional silencing of POLR3D expression. miR-320 directs the association of RNA interference (RNAi) protein Argonaute-1 (AGO1), Polycomb group (PcG) component EZH2, and tri-methyl histone H3 lysine 27 (H3K27me3) with the POLR3D promoter. Our results suggest the existence of an epigenetic mechanism of miRNA-directed transcriptional gene silencing (TGS) in mammalian cells.