Making noise about silence: repression of repeated genes in animals

Repeated copies of genes, whether in tandem or dispersed, are often recogni zed by the cell and silenced. Tandem repeat silencing is associated with a heterochromatin-like complex. Dispersed gene silencing can be mediated by t he repressive Polycomb Group complex or involve post-transcriptional silenc ing presumably involving double-stranded RNA. The I retrotransposable eleme nt in Drosophila appears to be susceptible to dispersed gene silencing, pot entially by both posttranscriptional and transcriptional processes. Some mu tations 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 determin e the nature of any relationship between post-transcriptionally and transcr iptionally based mechanisms. The silencing mechanisms potentially act as a protection against high expression of transposons and viruses.