RNAi extends its reach

摘要

The discovery of RNA interference (RNAi) and a vast new world of tiny regulatory RNAs has profoundly changed the way we think about gene regulation in animals, plants, and many fungi. The tiny RNAs are termed "short interfering RNAs" (siRNAs) or "microRNAs" (miRNAs) depending on their origin. They down-regulate gene expression by binding to complementary messenger RNAs (mRNAs) and either triggering mRNA elimination (RNAi) or arresting mRNA translation into protein (see the figure). Recently we havelearned that the RNAi pathway extends beyond silencing mRNA to act on the genome. For example, in the fission yeast Schizosaccharomyces pombe, the RNAi machinery is required for assembly of silent condensed heterochromatin at centromeres and at the mating-type region. Results reported by Schramke and Allshire on page 1069 of this issue broaden the role of RNAi. They show that normally euchromatic (decondensed) genes can be direct targets of RNAi-dependent heterochromatin formation. In addition, they find that the same silencing pathway acts on dispersed endogenous DNA repeats to repress genes involved in the sexual cycle.