The Paf1 complex factors Leo1 and Paf1 promote local histone turnover to modulate chromatin states in fission yeast

The maintenance of open and repressed chromatin states is crucial for the regulation of gene expression. To study the genes involved in maintaining chromatin states, we generated a random mutant library in Schizosaccharomyces pombe and monitored the silencing of reporter genes inserted into the euchromatic region adjacent to the heterochromatic mating type locus. We show that Leo1–Paf1 [a subcomplex of the RNA polymerase II‐associated factor 1 complex (Paf1C)] is required to prevent the spreading of heterochromatin into euchromatin by mapping the heterochromatin mark H3K9me2 using high‐resolution genomewide ChIP (ChIP–exo). Loss of Leo1–Paf1 increases heterochromatin stability at several facultative heterochromatin loci in an RNAi‐independent manner. Instead, deletion of Leo1 decreases nucleosome turnover, leading to heterochromatin stabilization. Our data reveal that Leo1–Paf1 promotes chromatin state fluctuations by enhancing histone turnover.     

Molecular regulation of histone H3 trimethylation by COMPASS and the regulation of gene expression

The Set1-containing complex COMPASS, which is the yeast homolog of the human MLL complex, is required for mono-, di-, and trimethylation of lysine 4 of histone H3. We have performed a comparative global proteomic screen to better define the role of COMPASS in histone trimethylation. We report that both Cps60 and Cps40 components of COMPASS are required for proper histone H3 trimethylation, but not for proper regulation of telomere-associated gene silencing. Purified COMPASS lacking Cps60 can mono- and dimethylate but is not capable of trimethylating H3(K4). Chromatin immunoprecipitation (ChIP) studies indicate that the loss subunits of COMPASS required for histone trimethylation do not affect the localization of Set1 to chromatin for the genes tested. Collectively, our results suggest a molecular requirement for several components of COMPASS for proper histone H3 trimethylation and regulation of telomere-associated gene expression, indicating multiple roles for different forms of histone methylation by COMPASS.     

组蛋白赖氨酸甲基化在表观遗传调控中的作用

组蛋白赖氨酸的甲基化在表观遗传调控中起着关键作用。组蛋白H3的K4、K9、K27、K36、K79和H4的K20均可被甲基化。组蛋白H3第9位赖氨酸的甲基化与基因的失活相关连; 组蛋白H3第4位赖氨酸和第36位赖氨酸的甲基化与基因的激活相关连; 组蛋白H3第27位赖氨酸的甲基化与同源盒基因沉默、X染色体失活、基因印记等基因沉默现象有关; 组蛋白H3第79位赖氨酸的甲基化与防止基因失活和DNA修复有关。与此同时, 组蛋白的去甲基化也受到更为广泛的关注。 关键词: 组蛋白赖氨酸甲基转移酶; 组蛋白赖氨酸甲基化; 组蛋白去甲基化     

DNA and histone methylation in plants

Heritable patterns of gene activity and gene silencing arise by the formation and the propagation of specific chromatin states that restrict or permit gene expression. In mammals and in plants, restrictive heterochromatin is associated with the hypermethylation of DNA at CG sites and with the specific modification of histones, such as the methylation of histone H3 at lysine 9 (H3K9(Me)). In addition to CG methylation, plant nuclear DNA packaged in restrictive chromatin is also usually methylated in cytosines outside a CG sequence context. The functional relationship between an unexpectedly complex plant DNA-methylation system and histone modifications that lead to chromatin compaction and gene silencing is under intense scrutiny. The results of recent studies indicate intriguing links between chromatin remodeling, histone methylation, DNA methylation and RNA interference.     

Histone crosstalk between H2B monoubiquitination and H3 methylation mediated by COMPASS

COMPASS, the yeast homolog of the mammalian MLL complex, is a histone H3 lysine 4 (H3K4) methylase consisting of Set1 (KMT2) and seven other polypeptides, including Cps35, the only essential subunit. Histone H2B monoubiquitination by Rad6/Bre1 is required for both H3K4 methylation by COMPASS, and H3K79 methylation by Dot1. However, the molecular mechanism for such histone crosstalk is poorly understood. Here, we demonstrate that histone H2B monoubiquitination controls the binding of Cps35 with COMPASS complex. Cps 35 is required for COMPASS' catalytic activity in vivo, and the addition of exogenous purified Cps35 to COMPASS purified from a Deltarad6 background results in the generation of a methylation competent COMPASS. Cps35 associates with the chromatin of COMPASS-regulated genes in a H2BK123 monoubiquitination-dependent but Set1-independent manner. Cps35 is also required for proper H3K79 trimethylation. These findings offer insight into the molecular role of Cps35 in translating the H2B monoubiquitination signal into H3 methylation.     

Dot1p modulates silencing in yeast by methylation of the nucleosome core

DOT1 was originally identified as a gene affecting telomeric silencing in S. cerevisiae. We now find that Dot1p methylates histone H3 on lysine 79, which maps to the top and bottom of the nucleosome core. Methylation occurs only when histone H3 is assembled in chromatin. In vivo, Dot1p is solely responsible for this methylation and methylates approximately 90% of histone H3. In dot1delta cells, silencing is compromised and silencing proteins become redistributed at the expense of normally silenced loci. We suggest that methylation of histone H3 lysine 79 limits silencing to discrete loci by preventing the binding of Sir proteins elsewhere along the genome. Because Dot1p and histone H3 are conserved, similar mechanisms are likely at work in other eukaryotes.