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Fig. 1 | Epigenetics & Chromatin

Fig. 1

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

Fig. 1

Heterochromatin formation in Saccharomyces cerevisiae. A Heterochromatin formation in silent mating-type loci of S. cerevisiae. At budding yeast chromosome III, two silent mating-type loci—HMLα and HMRa—surround the mating-type (MAT) locus. Each homothallic mating (HM) locus is surrounded by two proto-silencers, E and I, which nucleate the heterochromatin assembly. Silencer elements are bound by Orc1, Rap1, and Abf1. Orc1 interacts with Sir1 and Abf1 interacts with Sir3. Through the self-reinforcing mechanism of the SIR complex composed of Sir2, Sir3, and Sir4, silent chromatin is formed at HM loci. B Heterochromatin formation in telomeres of S. cerevisiae. Telomeres consist of TG1-3 repeat regions and chromosomal ends. Chromosomal ends are bound by yKu70/80 heterodimeric complexes. Telomeric repeats contain multiple Rap1 binding sites and the SIR complex is recruited to telomeric repeats through Rap1. Rif1 competes with Sir4 for binding to Rap1. The yKu complex regulates this competition process for Sir4 recruitment and SIR complex assembly. C Heterochromatin formation in rDNA repeats of S. cerevisiae. Approximately 100 to 200 rDNA repeats are positioned at chromosome XII. Each repeat consists of the 35S pre-rRNA gene and 5S rRNA gene which are separated by intergenic spacer 1(IGS1). IGS2 is located upstream of the 5S rRNA gene. A replication fork barrier (RFB) site is positioned within IGS1 and the binding site for Fob1. The binding of Fob1 into RFB sites causes recombination of rDNA repeats, which should be prevented by the binding of additional proteins. Net1 tethers to Fob1 and recruits Cdc14 and Sir2 into rDNA loci, thereby forming the regulator of nucleolar silencing and telophase exit (RENT) complex. Tof2 binds to Fob1, leading to the recruitment of two cohibin complex components, Lrs4 and Csm1, for Sir2-independent rDNA silencing. Lrs4/Csm1 interacts with Heh1/Nur1 (two nuclear membrane proteins)

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