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Fig. 2 | Epigenetics & Chromatin

Fig. 2

From: The regional sequestration of heterochromatin structural proteins is critical to form and maintain silent chromatin

Fig. 2

Heterochromatin formation in Schizosaccharomyces pombe. A Heterochromatin formation in centromeres. In centromeres, centromeric repeats (cnt) are surrounded by two innermost repeats (imr) regions with inverted orientation. Outer repeat (otr) elements are positioned further outside, and they consist of dg/dh repeat elements. dg/dh repeats nucleate heterochromatin in an RNAi-dependent manner. Once nucleated, heterochromatin is spread through the H3K9me-Swi6-dependent self-reinforcing mechanism up to its encounter with boundary elements including tRNA gene clusters. B Heterochromatin formation in silent mating-type loci. The mating-type is determined by the MAT1 gene; two mating-type determining regions, mat2P and mat3M, which contain information on both mating types, are maintained in silent chromatin. CenH regions show high homology to centromeric dg/dh repeats and function as RNAi-dependent nucleation centers. The REIII element is the binding site for Atf1/Pcr1, constituting an RNAi-independent heterochromatin nucleation site. Once nucleated, heterochromatin is spread up to its encounter with boundary elements, IR (inverted repeats)-L and IR-R. C Heterochromatin formation in telomeres. Telomeres are composed of double-stranded telomeric DNA repeats and single-stranded overhangs at chromosomal ends. Telomeric DNA repeats and immediately adjacent telomere-associated sequences (TAS) are RNAi-independent heterochromatin nucleation sites. Regions more distal to chromosomal ends contain multiple CenH-like sequences, functioning as RNAi-dependent heterochromatin nucleation sites (e.g., centromeres and silent mating-type loci). The shelterin complex consists of Tpz1/Pot1 subcomplexes bound to single-stranded overhangs and the double-stranded telomeric repeat-binding protein Taz1 connected by Rap1 and Poz1. The shelterin component Ccq1 recruits Clr4 for H3K9 methylation and subsequent Swi6-dependent heterochromatin formation. Additionally, Ccq1 leads to SHREC recruitment for transcriptional gene silencing. There is no known boundary element at telomeres; therefore, telomeres contain a long transition zone showing a gradual decrease in the heterochromatin domain

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