Regulation of Swi6/HP1-dependent Heterochromatin Assembly by Cooperation of Components of the Mitogen-activated Protein Kinase Pathway and a Histone Deacetylase Clr6

Abstract

A study of gene silencing within the mating-type region of fission yeast defines two distinct pathways responsible for the establishment of heterochromatin assembly. One is RNA interference-dependent and acts on centromere-homologous repeats (cenH). The other is a stochastic Swi6 (the fission yeast HP1 homolog)-dependent mechanism that is not fully understood. Here we find that activating transcription factor (Atf1) and Pcr1, the fission yeast bZIP transcription factors homologous to human ATF-2, are crucial for proper histone deacetylation of both H3 and H4. This deacetylation is a prerequisite for subsequent H3 lysine 9 methylation and Swi6-dependent heterochromatin assembly across the rest of the silent mating-type (mat) region lacking the RNA interference-dependent cenH repeat. Moreover, Atf1 and Pcr1 can form complexes with both a histone deacetylase, Clr6, and Swi6, and clr6 mutations affected the H3/H4 acetylation patterns, similar to the atf1 and pcr1 deletion mutant phenotypes at the endogenous mat loci and at the ctt1 ؉ promoter region surrounding ATF/CREbinding site. These data suggest that Atf1 and Pcr1 participate in an early step essential for heterochromatin assembly at the mat locus and silencing of transcriptional targets of Atf1. Furthermore, a phosphorylation event catalyzed by the conserved mitogen-activated protein kinase pathway is important for regulation of heterochromatin silencing by Atf1 and Pcr1. These findings suggest a role for the mitogen-activated protein kinase pathway and histone deacetylase in Swi6-based heterochromatin assembly.Methylation of histone H3 lysine 9 (H3 Lys-9) by the conserved H3 Lys-9-specific methyltransferase, Su(var)3-9 in flies, SUV39H1 in human, and Clr4 in the fission yeast Schizosaccharomyces pombe (1-4) correlates with heterochromatin assembly. The methylated Lys-9 residue recruits another conserved heterochromatin protein, which is called Swi6 in S. pombe and HP1 (heterochromatin protein 1) in higher eukaryotes (5, 6), leading to regional silencing of chromatin. In the fission yeast, recent studies (6 -8) addressing the silencing of the mating-type region provide insights for understanding the regulation of heterochromatin assembly in eukaryotes. Of particular interest, previous work (9) has defined sequential requirements for the establishment and maintenance of regional heterochromatic domains.Heterochromatin assembly at the mating-type region containing the mat2 and mat3 silent donor loci and an 11-kb interval (K region) between them requires several cis-acting DNA sequences as well as trans-acting factors (8, 10 -13). Heterochromatin formation at the centromeres and within the silent mat2/3 interval requires many of the same silencing factors, including Clr3 and Clr6 (H3/H4-specific histone deacetylases), the Clr4-Rik1 complexes, and Swi6 (2, 14 -19). The DNA elements involved in silencing within the entire 20 kb of the mat2/3 silent mating-type interval include REII (20), the mat3-M element including putative ATF 1 /CREB-binding sites (21), and the ...

Interaction of APC/C-E3 Ligase with Swi6/HP1 and Clr4/Suv39 in Heterochromatin Assembly in Fission Yeast

“…Subsequently, multimerization of Swi6 together with an interaction with the histone methyltransferase Clr4 helps in spreading the heterochromatin. (Alternative pathways exist where Atf1-mediated recruitment of Clr4 occurs independently of RNAi (29,40).) Swi6, in turn, recruits the Cohesin complex, which is important for sister chromatid cohesion and for imparting greater chromatin integrity to the mating type region.…”

Mutations in deoxyribonucleotide biosynthesis pathway cause spreading of silencing across heterochromatic barriers at the mating‐type region of the fission yeast

“…Thus, the levels of both Atf1 and Pcr1 proteins are expected to be elevated in stress conditions. Atf1/Pcr1 factors have been shown to target Swi6 and Clr4 to the silent domain through their physical interaction with these proteins (Jia et al, 2004;Kim et al, 2004). These factors are critical for deacetylation of H3 and H4 histones and the dosage-critical role of Swi6 in silencing is dependent upon the presence of Atf1 (Kim et al, 2004).…”

“…Earlier studies have shown that two independent pathways operate to nucleate heterochromatin assembly at the mat2,3 region (Grewal and Jia, 2007). One of these pathways is through stress-activated ATF/CREB family proteins, Atf1 and Pcr1 (Jia et al, 2004;Kim et al, 2004); the other involves the RNAi machinery (Hall et al, 2002).…”

“…In addition, another pathway involving the CREB family of proteins, Atf1 (activating transcription factor 1) and Pcr1, and components of a multi-enzyme effector complex (SHREC), plays an overlapping but important role in recruiting heterochromatin assembly factors to a site other than the cenH (Jia et al, 2004;Kim et al, 2004;Sugiyama et al, 2007). Once heterochromatin assembly-promoting factors such as Clr4 are recruited at cenH or at Atf1/Pcr1-binding sites, modification of histone tails in chromatin by methylation of H3K9 occurs.…”

Elimination of shelterin components bypasses RNAi for pericentric heterochromatin assembly

“…For example, ATF/CREB family proteins Atf1 and Pcr1 cooperate with RNAi to establish heterochromatin at the silent mating type region (Jia et al 2004;Kim et al 2004). Similarly, telomere shelterin component Taz1 and the telomere-associated sequence (TAS) function together with RNAi to establish heterochromatin at telomeres (Kanoh et al 2005).…”