RNA silencing as a plant immune system against viruses

'RNA silencing' refers to related processes of post-trancriptional control of gene expression found in plants, animals and fungi. A unifying feature o f RNA silencing is that it mediates sequence-specific degradation of target transcripts, recruiting RNA molecules of 21-23 nucleotides as specificity determinants. In higher plants, RNA silencing serves as an adaptive, antivi ral defence system, which is transmitted systemically in response to locali zed virus challenge. Plant viruses have elaborated a variety of counter-def ensive measures to overcome the host silencing response. One of these strat egies is to produce proteins that target the cell autonomous or signalling steps of RNA silencing. It is not known whether a similar antiviral mechani sm also operates in animal cells.