[RNAi and the formation of heterochromatin in Schizosaccharomyces pombe.]

In the fission yeast Schizosaccharomyces pombe, formation of pericentromeric heterochromatin involves RNA interference (RNAi). Recent data indicate that two RNAi complexes, RITS (RNA-induced transcriptional silencing complex) and RDRC (RNA-directed RNA polymerase complex), their respective enzymatic activity, and RNA polymerase II are essential for RNAi-mediated heterochromatin formation. At the site where heterochromatin formation takes place, RNA polymerase II synthesizes an RNA that would serve as an RNA platform to recruit in a siRNA-dependent manner RITS and RDRC, and thereby initiate heterochromatin assembly. Once recruited, RITS and RDRC seem to also contribute to the processing of the RNA platform. Therefore, RNAi-driven heterochromatin assembly appears to take place through a dynamic process of RNA synthesis, RNA-dependant recruitment of RNAi complexes and RNA degradation that all occur in cis.