Transcriptional Control of Gene Expression by MicroRNAs

SummaryMicroRNAs (miRNAs) control gene expression in animals and plants. Like another class of small RNAs, siRNAs, they affect gene expression posttranscriptionally. While siRNAs in addition act in transcriptional gene silencing, a role of miRNAs in transcriptional regulation has been less clear. We show here that in moss Physcomitrella patens mutants without a DICER-LIKE1b gene, maturation of miRNAs is normal but cleavage of target RNAs is abolished and levels of these transcripts are drastically reduced. These mutants accumulate miRNA:target-RNA duplexes and show hypermethylation of the genes encoding target RNAs, leading to gene silencing. This pathway occurs also in the wild-type upon hormone treatment. We propose that initiation of epigenetic silencing by DNA methylation depends on the ratio of the miRNA and its target RNA.

Graphical SummaryFigure optionsDownload high-quality image (212 K)Download as PowerPoint slideHighlights
► The moss DICER-LIKE1a (PpDCL1a) protein is required for miRNA biogenesis
► The related PpDCL1b protein is required for target cleavage but not miRNA biogenesis
► In PpDCL1b mutants, genes encoding miRNA targets are silenced by DNA methylation
► This epigenetic gene silencing is initiated by high miRNA to target RNA ratios