Editing Heterochromatin (section)

==Yeast heterochromatin==
''[[Saccharomyces cerevisiae]]'', or budding yeast, is a model [[eukaryote]] and its heterochromatin has been defined thoroughly. Although most of its genome can be characterized as euchromatin, ''S. cerevisiae'' has regions of DNA that are transcribed very poorly. These loci are the so-called silent mating type loci (HML and HMR), the rDNA (encoding ribosomal RNA), and the sub-telomeric regions.
Fission yeast (''[[Schizosaccharomyces pombe]]'') uses another mechanism for heterochromatin formation at its centromeres. Gene silencing at this location depends on components of the [[RNAi]] pathway. Double-stranded RNA is believed to result in silencing of the region through a series of steps.

In the fission yeast ''Schizosaccharomyces pombe'', two RNAi complexes, the RITS complex and the RNA-directed RNA polymerase complex (RDRC), are part of an RNAi machinery involved in the initiation, propagation and maintenance of heterochromatin assembly. These two complexes localize in a [[siRNA]]-dependent manner on chromosomes, at the site of heterochromatin assembly. [[RNA polymerase II]] synthesizes a transcript that serves as a platform to recruit RITS, RDRC and possibly other complexes required for heterochromatin assembly.<ref>{{cite journal | vauthors = Kato H, Goto DB, Martienssen RA, Urano T, Furukawa K, Murakami Y | title = RNA polymerase II is required for RNAi-dependent heterochromatin assembly | journal = Science | volume = 309 | issue = 5733 | pages = 467–9 | date = July 2005 | pmid = 15947136 | doi = 10.1126/science.1114955 | bibcode = 2005Sci...309..467K | s2cid = 22636283 }}</ref><ref>{{cite journal | vauthors = Djupedal I, Portoso M, Spåhr H, Bonilla C, Gustafsson CM, Allshire RC, Ekwall K | title = RNA Pol II subunit Rpb7 promotes centromeric transcription and RNAi-directed chromatin silencing | journal = Genes & Development | volume = 19 | issue = 19 | pages = 2301–6 | date = October 2005 | pmid = 16204182 | pmc = 1240039 | doi = 10.1101/gad.344205 }}</ref> Both RNAi and an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These mechanisms of ''Schizosaccharomyces pombe'' may occur in other eukaryotes.<ref name= Vavasseur>{{cite book |chapter-url=http://www.horizonpress.com/rnareg|author= Vavasseur|year=2008 |chapter=Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast |title=RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity |publisher=Caister Academic Press |isbn=978-1-904455-25-7|display-authors=etal}}</ref> A large RNA structure called [[RevCen]] has also been implicated in the production of siRNAs to mediate heterochromatin formation in some fission yeast.<ref name="Dju09">{{cite journal | vauthors = Djupedal I, Kos-Braun IC, Mosher RA, Söderholm N, Simmer F, Hardcastle TJ, Fender A, Heidrich N, Kagansky A, Bayne E, Wagner EG, Baulcombe DC, Allshire RC, Ekwall K | display-authors = 6 | title = Analysis of small RNA in fission yeast; centromeric siRNAs are potentially generated through a structured RNA | journal = The EMBO Journal | volume = 28 | issue = 24 | pages = 3832–44 | date = December 2009 | pmid = 19942857 | pmc = 2797062 | doi = 10.1038/emboj.2009.351 }}</ref>