Repeat-induced gene silencing: common mechanisms in plants and fungi.

One of the most surprising observations made in plant science in recent years is the inactivation of transgenes triggered by interactions between DNA repeats. In plants, we can differentiate between transcriptional silencing, most likely reflecting a regulation at the DNA level, and post-transcriptional silencing that affects steady state RNA levels. In the filamentous fungi Ascobolus immersus and Neurospora crassa, we find two premeiotic silencing processes that are also based on the interaction of repeated sequences. A common feature of transcriptional silencing in plants and premeiotic gene inactivation in filamentous fungi is that the repeated sequences undergo cytosine methylation. DNA methylation, which is either the cause or the consequence of gene silencing, can be associated with changes in chromatin structure. These structural changes are reminiscent of homology-based silencing mechanisms in Drosophila, an organism that lacks DNA methylation. Repeat-induced silencing may therefore reflect the activity of an endogenous mechanism, present in some species, which screens for homology and has significant implications for the organization and evolution of the genome.