Transcriptional silencing and promoter methylation triggered by double-stranded RNA

Double-stranded RNA induces a post-transcriptional gene silencing process, termed RNAi, in diverse organisms. It is shown here that transcriptional ge ne silencing accompanied by de novo methylation of a target promoter in pla nts can be triggered by a double-stranded RNA containing promoter sequences . Similar to the double-stranded RNA involved in RNAi, this promoter double -stranded RNA, which is synthesized in the nucleus, is partially cleaved in to small RNAs similar to 23 nucleotides in length. Both transcriptional and post-transcriptional gene silencing can thus be initiated by double-strand ed RNAs that enter the same degradation pathway. The results also implicate double-stranded RNA in directing DNA methylation. Different constructs des igned to produce double-stranded promoter RNA in various ways were evaluate d for their ability to induce gene silencing in tobacco and Arabidopsis, RN A hairpins transcribed from inverted DNA repeats were the most effective tr aits-acting silencing signals. This strategy could be useful for transcript ionally downregulating genes in a variety of plants.