Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast
Aurélia Vavasseur, Leila Touat-Todeschini and André Verdel
from: RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity (Edited by: Kevin V. Morris). Caister Academic Press, U.K. (2008)
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. Here, we review studies conducted in the fission yeast Schizosaccharomyces pombe, which 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. Finally, 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. As RNAi-dependent chromatin modifications have been observed throughout the eukaryotic kingdom the mechanisms reviewed here are susceptible to occur in a large range of eukaryotes read more ...