The genetics of RNA silencing.

Although initially recognized as a handy tool to reduce gene expression, RNA silencing, triggered by double-stranded RNA molecules, is now recognized as a mechanism for cellular protection and cleansing: It defends the genome against molecular parasites such as viruses and transposons, while removing abundant but aberrant nonfunctional messenger RNAs. The underlying mechanisms in distinct gene silencing phenomena in different genetic systems, such as cosuppression in plants and RNAi in animals, are very similar. There are common RNA intermediates, and similar genes are required in RNA silencing pathways in protozoa, plants, fungi, and animals, thus indicating an ancient pathway. This chapter gives an overview of both biochemical and genetic approaches leading to the current understanding of the molecular mechanism of RNA silencing and its probable biological function.