Heterochromatin is a prevalent chromatin state among eukaryotes that has critical functions in chromosome segregation, control of genomic stability and epigenetic regulation of gene expression. Recent studies (for a review see: Vavasseur et al, 2008) conducted in the fission yeast Schizosaccharomyces pombe, reveal that two RNAi complexes, the RNAi-induced transcriptional gene silencing (RITS) complex and the RNA-directed RNA polymerase complex (RDRC), are part of a RNAi machinery involved in the initiation, propagation and maintenance of heterochromatin assembly. It appears that these two complexes localize in a siRNA-dependent manner on chromosomes, at the site of heterochromatin assembly. Moreover, these studies reveal an unprecedented and central role for RNA polymerase II (RNApII) in RNAi-dependent heterochromatin assembly. RNApII synthesizes a nascent transcript that is believed to serve as a RNA platform to recruit, RITS, RDRC and possibly other complexes required for heterochromatin assembly. Recent findings indicate that RNAi as well as an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These findings challenge the widely accepted view that heterochromatic gene silencing is caused strictly by chromatin compaction. RNAi-dependent chromatin modifications have been observed throughout the eukaryotic kingdom the mechanisms considered here may occur in a large range of eukaryotes.

Further reading:

Vavasseur et al, 2008. Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast. In: RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Ed, Morris K.V., Caister Academic Press, Norfolk, UK

Tost, J. 2008 Epigenetics Caister Academic Press, Norfolk, UK