Heterochromatin structure and function.

Although heterochromatin has long been used as a model for studying chromatin condensation and heritable gene silencing, it is only relatively recently that detailed information has become available on the mechanisms that underlie its structure. Current evidence suggests that these operate on at least three different levels. A regular nucleosome array may facilitate packaging of the chromatin into a highly condensed configuration. Methylation of histone H3 lysine 9 and lysine 27 generates heterochromatin marks that are recognised through binding of heterochromatin proteins such as HP1. Finally, very recent studies using genetic and biochemical approaches have indicated that the RNAi machinery plays an important role in the formation of heterochromatin.