Role of inverted DNA repeats in transcriptional and post-transcriptional gene silencing

Transgenes and endogenous genes are sensitive to silencing, in particular w hen the genes are tandemly repeated. Their expression can be transcriptiona lly or post-transcriptionally repressed, or both. It is remarkable that ver y often, two or more genes or parts of the genes are arranged as inverted r epeats (IR). Many of such IRs are dominant silencing loci. They can repress the expression of homologous genes elsewhere in the genome in trans which is usually associated with an increase in the level of DNA methylation. Tra ns-silencing has been explained by DNA-DNA pairing between a repetitive sil encing locus and a homologous target locus. However, there is accumulating evidence that the trans effect might be mediated by dsRNA transcribed from the IR (trans)genes. Besides dsRNA-directed DNA methylation, dsRNA in plant s as well as in other systems also induces the degradation of homologous RN As and silence genes post-transcriptionally. These findings indicate that s everal features associated with gene silencing can be attributed to the act ivities of dsRNA, which would explain why inverted transgene repeats are su ch efficient silencing loci.