Making noise about silence: repression of repeated genes in animals.

Repeated copies of genes, whether in tandem or dispersed, are often recognized by the cell and silenced. Tandem repeat silencing is associated with a heterochromatin-like complex. Dispersed gene silencing can be mediated by the repressive Polycomb Group complex or involve post-transcriptional silencing presumably involving double-stranded RNA. The I retrotransposable element in Drosophila appears to be susceptible to dispersed gene silencing, potentially by both post-transcriptional and transcriptional processes. Some mutations that eliminate RNA interference in Caenorhabditis elegans result in the mobilization of many transposons and two of these mutations desilence tandem repeats in the germline. One challenge for the future is to determine the nature of any relationship between post-transcriptionally and transcriptionally based mechanisms. The silencing mechanisms potentially act as a protection against high expression of transposons and viruses.