Mutations in the fission yeast silencing factors clr4+ and rik1+ disrupt the localisation of the chromo domain protein Swi6p and impair centromere function

Mechanistic insights into the stimulation of the histone H3K9 methyltransferase Clr4 by proximal H3K14 ubiquitination

H3K9 methylation, a conserved heterochromatin marker, is crucial for chromosome segregation and gene regulation. Clr4 is the sole known methyltransferase catalyzing H3K9 methylation in Schizosaccharomyces pombe. Clr4 K455/K472 automethylation and H3K14 ubiquitination (H3K14Ub) are vital activators of Clr4, ensuring appropriate heterochromatin deposition and preventing deleterious silencing. While automethylation's activation mechanism is uncovered, the mechanism of H3K14Ub's significantly stronger stimulation on Clr4 remains unclear. Here, we determined the crystal structures of Clr4 bound to ubiquitinated and unmodified H3 peptides at 2.60 and 2.39 angstrom, which revealed a synergistic mechanism underlying the pronounced stimulatory effect: H3K14Ub increases substrate affinity through multivalent interactions and facilitates the allosteric transition of Clr4 from an inactive apo conformation to a hyperactive "catalyzing state," including conformational changes in the αC-SET-insertion region, autoregulatory loop, and the β9/10 loop. We finally propose a multilevel structural model for the Clr4 catalytic-regulatory cycle. This work provides structural insights into the interplay between histone modifications and their collective impact on epigenetic regulation.

Tracking live-cell single-molecule dynamics enables measurements of heterochromatin-associated protein-protein interactions

Visualizing and measuring molecular-scale interactions in living cells represents a major challenge, but recent advances in single-molecule super-resolution microscopy are bringing us closer to achieving this goal. Single-molecule super-resolution microscopy enables high-resolution and sensitive imaging of the positions and movement of molecules in living cells. HP1 proteins are important regulators of gene expression because they selectively bind and recognize H3K9 methylated (H3K9me) histones to form heterochromatin-associated protein complexes that silence gene expression, but several important mechanistic details of this process remain unexplored. Here, we extended live-cell single-molecule tracking studies in fission yeast to determine how HP1 proteins interact with their binding partners in the nucleus. We measured how genetic perturbations that affect H3K9me alter the diffusive properties of HP1 proteins and their binding partners, and we inferred their most likely interaction sites. Our results demonstrate that H3K9 methylation spatially restricts HP1 proteins and their interactors, thereby promoting ternary complex formation on chromatin while simultaneously suppressing off-chromatin binding. As opposed to being an inert platform to direct HP1 binding, our studies propose a novel function for H3K9me in promoting ternary complex formation by enhancing the specificity and stimulating the assembly of HP1-protein complexes in living cells.

Inheritance of H3K9 methylation regulates genome architecture in Drosophila early embryos

Constitutive heterochromatin is essential for transcriptional silencing and genome integrity. The establishment of constitutive heterochromatin in early embryos and its role in early fruitfly development are unknown. Lysine 9 trimethylation of histone H3 (H3K9me3) and recruitment of its epigenetic reader, heterochromatin protein 1a (HP1a), are hallmarks of constitutive heterochromatin. Here, we show that H3K9me3 is transmitted from the maternal germline to the next generation. Maternally inherited H3K9me3, and the histone methyltransferases (HMT) depositing it, are required for the organization of constitutive heterochromatin: early embryos lacking H3K9 methylation display de-condensation of pericentromeric regions, centromere-centromere de-clustering, mitotic defects, and nuclear shape irregularities, resulting in embryo lethality. Unexpectedly, quantitative CUT&Tag and 4D microscopy measurements of HP1a coupled with biophysical modeling revealed that H3K9me2/3 is largely dispensable for HP1a recruitment. Instead, the main function of H3K9me2/3 at this developmental stage is to drive HP1a clustering and subsequent heterochromatin compaction. Our results show that HP1a binding to constitutive heterochromatin in the absence of H3K9me2/3 is not sufficient to promote proper embryo development and heterochromatin formation. The loss of H3K9 HMTs and H3K9 methylation alters genome organization and hinders embryonic development.

RNA quality control factors nucleate Clr4/SUV39H and trigger constitutive heterochromatin assembly

In eukaryotes, the Suv39 family of proteins tri-methylate lysine 9 of histone H3 (H3K9me) to form constitutive heterochromatin. However, how Suv39 proteins are nucleated at heterochromatin is not fully described. In the fission yeast, current models posit that Argonaute1-associated small RNAs (sRNAs) nucleate the sole H3K9 methyltransferase, Clr4/SUV39H, to centromeres. Here, we show that in the absence of all sRNAs and H3K9me, the Mtl1 and Red1 core (MTREC)/PAXT complex nucleates Clr4/SUV39H at a heterochromatic long noncoding RNA (lncRNA) at which the two H3K9 deacetylases, Sir2 and Clr3, also accumulate by distinct mechanisms. Iterative cycles of H3K9 deacetylation and methylation spread Clr4/SUV39H from the nucleation center in an sRNA-independent manner, generating a basal H3K9me state. This is acted upon by the RNAi machinery to augment and amplify the Clr4/H3K9me signal at centromeres to establish heterochromatin. Overall, our data reveal that lncRNAs and RNA quality control factors can nucleate heterochromatin and function as epigenetic silencers in eukaryotes.

Chromodomain mutation in S. pombe Kat5/Mst1 affects centromere dynamics and DNA repair

KAT5 (S. pombe Mst1, human TIP60) is a MYST family histone acetyltransferase conserved from yeast to humans that is involved in multiple cellular activities. This family is characterized in part by containing a chromodomain, a motif associated with binding methylated histones. We show that a chromodomain mutation in the S. pombe Kat5, mst1-W66R, has defects in pericentromere silencing. mst1-W66R is sensitive to camptothecin (CPT) but only at an increased temperature of 36°C, although it is proficient for growth at this temperature. We also describe a de-silencing effect at the pericentromere by CPT that is independent of RNAi and methylation machinery. We also show that mst1-W66R disrupts recruitment of proteins to repair foci in response to camptothecin-induced DNA damage. Our data suggest a function of Mst1 chromodomain in centromere heterochromatin formation and a separate role in genome-wide damage repair in CPT.

Histone Methyltransferase SsDim5 Regulates Fungal Virulence through H3K9 Trimethylation in Sclerotinia sclerotiorum

Histone post-translational modification is one of the main mechanisms of epigenetic regulation, which plays a crucial role in the control of gene expression and various biological processes. However, whether or not it affects fungal virulence in Sclerotinia sclerotiorum is not clear. In this study, we identified and cloned the histone methyltransferase Defective in methylation 5 (Dim5) in S. sclerotiorum, which encodes a protein containing a typical SET domain. SsDim5 was found to be dynamically expressed during infection. Knockout experiment demonstrated that deletion of SsDim5 reduced the virulence in Ssdim5-1/Ssdim5-2 mutant strains, accompanied by a significant decrease in H3K9 trimethylation levels. Transcriptomic analysis further revealed the downregulation of genes associated with mycotoxins biosynthesis in SsDim5 deletion mutants. Additionally, the absence of SsDim5 affected the fungus's response to oxidative and osmotic, as well as cellular integrity. Together, our results indicate that the H3K9 methyltransferase SsDim5 is essential for H3K9 trimethylation, regulating fungal virulence throug mycotoxins biosynthesis, and the response to environmental stresses in S. sclerotiorum.

Functions of HP1 in preventing chromosomal instability

Chromosomal instability (CIN), caused by errors in the segregation of chromosomes during mitosis, is a hallmark of many types of cancer. The fidelity of chromosome segregation is governed by a sophisticated cellular signaling network, one crucial orchestrator of which is Heterochromatin protein 1 (HP1). HP1 dynamically localizes to distinct sites at various stages of mitosis, where it regulates key mitotic events ranging from chromosome-microtubule attachment to sister chromatid cohesion to cytokinesis. Our evolving comprehension of HP1's multifaceted role has positioned it as a central protein in the orchestration of mitotic processes.

The Cross-Regulation Between Set1, Clr4, and Lsd1/2 in Schizosaccharomyces pombe

Eukaryotic chromatin is organized into either silenced heterochromatin or relaxed euchromatin regions, which controls the accessibility of transcriptional machinery and thus regulates gene expression. In fission yeast, Schizosaccharomyces pombe, Set1 is the sole H3K4 methyltransferase and is mainly enriched at the promoters of actively transcribed genes. In contrast, Clr4 methyltransferase initiates H3K9 methylation, which has long been regarded as a hallmark of heterochromatic silencing. Lsd1 and Lsd2 are two highly conserved H3K4 and H3K9 demethylases. As these histone-modifying enzymes perform critical roles in maintaining histone methylation patterns and, consequently, gene expression profiles, cross-regulations among these enzymes are part of the complex regulatory networks. Thus, elucidating the mechanisms that govern their signaling and mutual regulations remains crucial. Here, we demonstrated that C-terminal truncation mutants, lsd1-ΔHMG and lsd2-ΔC, do not compromise the integrity of the Lsd1/2 complex but impair their chromatin-binding capacity at the promoter region of target genomic loci. We identified protein-protein interactions between Lsd1/2 and Raf2 or Swd2, which are the subunits of the Clr4 complex (CLRC) and Set1-associated complex (COMPASS), respectively. We showed that Clr4 and Set1 modulate the protein levels of Lsd1 and Lsd2 in opposite ways through the ubiquitin-proteasome-dependent pathway. During heat stress, the protein levels of Lsd1 and Lsd2 are upregulated in a Set1-dependent manner. The increase in protein levels is crucial for differential gene expression under stress conditions. Together, our results support a cross-regulatory model by which Set1 and Clr4 methyltransferases control the protein levels of Lsd1/2 demethylases to shape the dynamic chromatin landscape.

Diverse and dynamic forms of gene regulation by the S. cerevisiae histone methyltransferase Set1

Gene transcription is an essential and highly regulated process. In eukaryotic cells, the structural organization of nucleosomes with DNA wrapped around histone proteins impedes transcription. Chromatin remodelers, transcription factors, co-activators, and histone-modifying enzymes work together to make DNA accessible to RNA polymerase. Histone lysine methylation can positively or negatively regulate gene transcription. Methylation of histone 3 lysine 4 by SET-domain-containing proteins is evolutionarily conserved from yeast to humans. In higher eukaryotes, mutations in SET-domain proteins are associated with defects in the development and segmentation of embryos, skeletal and muscle development, and diseases, including several leukemias. Since histone methyltransferases are evolutionarily conserved, the mechanisms of gene regulation mediated by these enzymes are also conserved. Budding yeast Saccharomyces cerevisiae is an excellent model system to study the impact of histone 3 lysine 4 (H3K4) methylation on eukaryotic gene regulation. Unlike larger eukaryotes, yeast cells have only one enzyme that catalyzes H3K4 methylation, Set1. In this review, we summarize current knowledge about the impact of Set1-catalyzed H3K4 methylation on gene transcription in S. cerevisiae. We describe the COMPASS complex, factors that influence H3K4 methylation, and the roles of Set1 in gene silencing at telomeres and heterochromatin, as well as repression and activation at euchromatic loci. We also discuss proteins that "read" H3K4 methyl marks to regulate transcription and summarize alternate functions for Set1 beyond H3K4 methylation.

The Fission Yeast Mating-Type Switching Motto: "One-for-Two" and "Two-for-One"

Schizosaccharomyces pombe is an ascomycete fungus that divides by medial fission; it is thus commonly referred to as fission yeast, as opposed to the distantly related budding yeast Saccharomyces cerevisiae. The reproductive lifestyle of S. pombe relies on an efficient genetic sex determination system generating a 1:1 sex ratio and using alternating haploid/diploid phases in response to environmental conditions. In this review, we address how one haploid cell manages to generate two sister cells with opposite mating types, a prerequisite to conjugation and meiosis. This mating-type switching process depends on two highly efficient consecutive asymmetric cell divisions that rely on DNA replication, repair, and recombination as well as the structure and components of heterochromatin. We pay special attention to the intimate interplay between the genetic and epigenetic partners involved in this process to underscore the importance of basic research and its profound implication for a better understanding of chromatin biology.

HP1 proteins regulate nucleolar structure and function by secluding pericentromeric constitutive heterochromatin

Nucleoli are nuclear compartments regulating ribosome biogenesis and cell growth. In embryonic stem cells (ESCs), nucleoli containing transcriptionally active ribosomal genes are spatially separated from pericentromeric satellite repeat sequences packaged in largely repressed constitutive heterochromatin (PCH). To date, mechanisms underlying such nuclear partitioning and the physiological relevance thereof are unknown. Here we show that repressive chromatin at PCH ensures structural integrity and function of nucleoli during cell cycle progression. Loss of heterochromatin proteins HP1α and HP1β causes deformation of PCH, with reduced H3K9 trimethylation (H3K9me3) and HP1γ levels, absence of H4K20me3 and upregulated major satellites expression. Spatially, derepressed PCH aberrantly associates with nucleoli accumulating severe morphological defects during S/G2 cell cycle progression. Hp1α/β deficiency reduces cell proliferation, ribosomal RNA biosynthesis and mobility of Nucleophosmin, a major nucleolar component. Nucleolar integrity and function require HP1α/β proteins to be recruited to H3K9me3-marked PCH and their ability to dimerize. Correspondingly, ESCs deficient for both Suv39h1/2 H3K9 HMTs display similar nucleolar defects. In contrast, Suv4-20h1/2 mutant ESCs lacking H4K20me3 at PCH do not. Suv39h1/2 and Hp1α/β deficiency-induced nucleolar defects are reminiscent of those defining human ribosomopathy disorders. Our results reveal a novel role for SUV39H/HP1-marked repressive constitutive heterochromatin in regulating integrity, function and physiology of nucleoli.

WiFi Related Radiofrequency Electromagnetic Fields Promote Transposable Element Dysregulation and Genomic Instability in Drosophila melanogaster

Exposure to artificial radio frequency electromagnetic fields (RF-EMFs) has greatly increased in recent years, thus promoting a growing scientific and social interest in deepening the biological impact of EMFs on living organisms. The current legislation governing the exposure to RF-EMFs is based exclusively on their thermal effects, without considering the possible non-thermal adverse health effects from long term exposure to EMFs. In this study we investigated the biological non-thermal effects of low-level indoor exposure to RF-EMFs produced by WiFi wireless technologies, using Drosophila melanogaster as the model system. Flies were exposed to 2.4 GHz radiofrequency in a Transverse Electromagnetic (TEM) cell device to ensure homogenous controlled fields. Signals were continuously monitored during the experiments and regulated at non thermal levels. The results of this study demonstrate that WiFi electromagnetic radiation causes extensive heterochromatin decondensation and thus a general loss of transposable elements epigenetic silencing in both germinal and neural tissues. Moreover, our findings provide evidence that WiFi related radiofrequency electromagnetic fields can induce reactive oxygen species (ROS) accumulation, genomic instability, and behavioural abnormalities. Finally, we demonstrate that WiFi radiation can synergize with Ras^V12 to drive tumor progression and invasion. All together, these data indicate that radiofrequency radiation emitted from WiFi devices could exert genotoxic effects in Drosophila and set the stage to further explore the biological effects of WiFi electromagnetic radiation on living organisms.

Targets of histone H3 lysine 9 methyltransferases

Histone H3 lysine 9 di- and trimethylation are well-established marks of constitutively silenced heterochromatin domains found at repetitive DNA elements including pericentromeres, telomeres, and transposons. Loss of heterochromatin at these sites causes genomic instability in the form of aberrant DNA repair, chromosome segregation defects, replication stress, and transposition. H3K9 di- and trimethylation also regulate cell type-specific gene expression during development and form a barrier to cellular reprogramming. However, the role of H3K9 methyltransferases extends beyond histone methylation. There is a growing list of non-histone targets of H3K9 methyltransferases including transcription factors, steroid hormone receptors, histone modifying enzymes, and other chromatin regulatory proteins. Additionally, two classes of H3K9 methyltransferases modulate their own function through automethylation. Here we summarize the structure and function of mammalian H3K9 methyltransferases, their roles in genome regulation and constitutive heterochromatin, as well as the current repertoire of non-histone methylation targets including cases of automethylation.

Differential phosphorylation of Clr4SUV39H by Cdk1 accompanies a histone H3 methylation switch that is essential for gametogenesis

Methylation of histone H3 at lysine 9 (H3K9) is a hallmark of heterochromatin that plays crucial roles in gene silencing, genome stability, and chromosome segregation. In Schizosaccharomyces pombe, Clr4 mediates both di- and tri-methylation of H3K9. Although H3K9 methylation has been intensely studied in mitotic cells, its role during sexual differentiation remains unclear. Here, we map H3K9 methylation genome-wide during meiosis and show that constitutive heterochromatin temporarily loses H3K9me2 and becomes H3K9me3 when cells commit to meiosis. Cells lacking the ability to tri-methylate H3K9 exhibit meiotic chromosome segregation defects. Finally, the H3K9 methylation switch is accompanied by differential phosphorylation of Clr4 by the cyclin-dependent kinase Cdk1. Our results suggest that a conserved master regulator of the cell cycle controls the specificity of an H3K9 methyltransferase to prevent ectopic H3K9 methylation and to ensure faithful gametogenesis.

HP1 oligomerization compensates for low-affinity H3K9me recognition and provides a tunable mechanism for heterochromatin-specific localization

HP1 proteins traverse a complex and crowded chromatin landscape to bind with low affinity but high specificity to histone H3K9 methylation (H3K9me) and form transcriptionally inactive genomic compartments called heterochromatin. Here, we visualize single-molecule dynamics of an HP1 homolog, the fission yeast Swi6, in its native chromatin environment. By tracking single Swi6 molecules, we identify mobility states that map to discrete biochemical intermediates. Using Swi6 mutants that perturb H3K9me recognition, oligomerization, or nucleic acid binding, we determine how each biochemical property affects protein dynamics. We estimate that Swi6 recognizes H3K9me3 with ~94-fold specificity relative to unmodified nucleosomes in living cells. While nucleic acid binding competes with Swi6 oligomerization, as few as four tandem chromodomains can overcome these inhibitory effects to facilitate Swi6 localization at heterochromatin formation sites. Our studies indicate that HP1 oligomerization is essential to form dynamic, higher-order complexes that outcompete nucleic acid binding to enable specific H3K9me recognition.

Chl1 coordinates with H3K9 methyltransferase Clr4 to reduce the accumulation of RNA-DNA hybrids and maintain genome stability

A genome-wide analysis in Schizosaccharomyces pombe indicated that double-deletion mutants of Chl1 and histone H3K9 methyltransferase complex factors are synthetically sick. Here, we show that loss of Chl1 increases the accumulation of RNA-DNA hybrids at pericentromeric dg and dh repeats in the absence of the H3K9 methyltransferase Clr4, which leads to genome instability, including more severe defects in chromosome segregation and increased chromatin accessibility. Localization of Chl1 at pericentromeric regions depends on a subunit of replication protein A (RPA), Ssb1. In wild-type (WT) cells, transcriptionally repressed heterochromatin prevents the formation of RNA-DNA hybrids. When Clr4 is deleted, dg and dh repeats are highly transcribed. Then Ssb1 associates with the displaced single-stranded DNA (ssDNA) and recruits Chl1 to resolve the RNA-DNA hybrids. Together, our data suggest that Chl1 coordinates with Clr4 to eliminate RNA-DNA hybrids, which contributes to the maintenance of genome integrity.

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Double mutant of Chl1 and Chl1 leads to the accumulation of RNA-DNA hybrids

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RNA-DNA hybrids at pericentromeric regions affect genome stability and cell viability

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Ssb1 recruits Chl1 to unwind RNA-DNA hybrids in the absence of Clr4

Subtelomeric Chromatin in the Fission Yeast S. pombe

Telomeres play important roles in safeguarding the genome. The specialized repressive chromatin that assembles at telomeres and subtelomeric domains is key to this protective role. However, in many organisms, the repetitive nature of telomeric and subtelomeric sequences has hindered research efforts. The fission yeast S. pombe has provided an important model system for dissection of chromatin biology due to the relative ease of genetic manipulation and strong conservation of important regulatory proteins with higher eukaryotes. Telomeres and the telomere-binding shelterin complex are highly conserved with mammals, as is the assembly of constitutive heterochromatin at subtelomeres. In this review, we seek to summarize recent work detailing the assembly of distinct chromatin structures within subtelomeric domains in fission yeast. These include the heterochromatic SH subtelomeric domains, the telomere-associated sequences (TAS), and ST chromatin domains that assemble highly condensed chromatin clusters called knobs. Specifically, we review new insights into the sequence of subtelomeric domains, the distinct types of chromatin that assemble on these sequences and how histone H3 K36 modifications influence these chromatin structures. We address the interplay between the subdomains of chromatin structure and how subtelomeric chromatin is influenced by both the telomere-bound shelterin complexes and by euchromatic chromatin regulators internal to the subtelomeric domain. Finally, we demonstrate that telomere clustering, which is mediated via the condensed ST chromatin knob domains, does not depend on knob assembly within these domains but on Set2, which mediates H3K36 methylation.

H3K9me3 maintenance on a human artificial chromosome is required for segregation but not centromere epigenetic memory

Most eukaryotic centromeres are located within heterochromatic regions. Paradoxically, heterochromatin can also antagonize de novo centromere formation, and some centromeres lack it altogether. In order to investigate the importance of heterochromatin at centromeres, we used epigenetic engineering of a synthetic alphoidtetO human artificial chromosome (HAC), to which chimeric proteins can be targeted. By tethering the JMJD2D demethylase (also known as KDM4D), we removed heterochromatin mark H3K9me3 (histone 3 lysine 9 trimethylation) specifically from the HAC centromere. This caused no short-term defects, but long-term tethering reduced HAC centromere protein levels and triggered HAC mis-segregation. However, centromeric CENP-A was maintained at a reduced level. Furthermore, HAC centromere function was compatible with an alternative low-H3K9me3, high-H3K27me3 chromatin signature, as long as residual levels of H3K9me3 remained. When JMJD2D was released from the HAC, H3K9me3 levels recovered over several days back to initial levels along with CENP-A and CENP-C centromere levels, and mitotic segregation fidelity. Our results suggest that a minimal level of heterochromatin is required to stabilize mitotic centromere function but not for maintaining centromere epigenetic memory, and that a homeostatic pathway maintains heterochromatin at centromeres.This article has an associated First Person interview with the first authors of the paper.

Vigilin protein Vgl1 is required for heterochromatin-mediated gene silencing in Schizosaccharomyces pombe

Heterochromatin is a conserved feature of eukaryotic genomes and regulates various cellular processes, including gene silencing, chromosome segregation, and maintenance of genome stability. In the fission yeast Schizosaccharomyces pombe, heterochromatin formation involves methylation of lysine 9 in histone H3 (H3K9), which recruits Swi6/HP1 proteins to heterochromatic loci. The Swi6/HP1-H3K9me3 chromatin complex lies at the center of heterochromatic macromolecular assemblies and mediates many functions of heterochromatin by recruiting a diverse set of regulators. However, additional factors may be required for proper heterochromatin organization, but they are not fully known. Here, using several molecular and biochemical approaches, we report that Vgl1, a member of a large family of multiple KH-domain proteins, collectively known as vigilins, is indispensable for the heterochromatin-mediated gene silencing in S. pombe ChIP analysis revealed that Vgl1 binds to pericentromeric heterochromatin in an RNA-dependent manner and that Vgl1 deletion leads to loss of H3K9 methylation and Swi6 recruitment to centromeric and telomeric heterochromatic loci. Furthermore, we show that Vgl1 interacts with the H3K9 methyltransferase, Clr4, and that loss of Vgl1 impairs Clr4 recruitment to heterochromatic regions of the genome. These findings uncover a novel role for Vgl1 as a key regulator in heterochromatin-mediated gene silencing in S. pombe.

Shelterin and subtelomeric DNA sequences control nucleosome maintenance and genome stability

Telomeres and the shelterin complex cap and protect the ends of chromosomes. Telomeres are flanked by the subtelomeric sequences that have also been implicated in telomere regulation, although their role is not well defined. Here, we show that, in Schizosaccharomyces pombe, the telomere-associated sequences (TAS) present on most subtelomeres are hyper-recombinogenic, have metastable nucleosomes, and unusual low levels of H3K9 methylation. Ccq1, a subunit of shelterin, protects TAS from nucleosome loss by recruiting the heterochromatic repressor complexes CLRC and SHREC, thereby linking nucleosome stability to gene silencing. Nucleosome instability at TAS is independent of telomeric repeats and can be transmitted to an intrachromosomal locus containing an ectopic TAS fragment, indicating that this is an intrinsic property of the underlying DNA sequence. When telomerase recruitment is compromised in cells lacking Ccq1, DNA sequences present in the TAS promote recombination between chromosomal ends, independent of nucleosome abundance, implying an active function of these sequences in telomere maintenance. We propose that Ccq1 and fragile subtelomeres co-evolved to regulate telomere plasticity by controlling nucleosome occupancy and genome stability.

Heterochromatin: Guardian of the Genome

Constitutive heterochromatin is a major component of the eukaryotic nucleus and is essential for the maintenance of genome stability. Highly concentrated at pericentromeric and telomeric domains, heterochromatin is riddled with repetitive sequences and has evolved specific ways to compartmentalize, silence, and repair repeats. The delicate balance between heterochromatin epigenetic maintenance and cellular processes such as mitosis and DNA repair and replication reveals a highly dynamic and plastic chromatin domain that can be perturbed by multiple mechanisms, with far-reaching consequences for genome integrity. Indeed, heterochromatin dysfunction provokes genetic turmoil by inducing aberrant repeat repair, chromosome segregation errors, transposon activation, and replication stress and is strongly implicated in aging and tumorigenesis. Here, we summarize the general principles of heterochromatin structure and function, discuss the importance of its maintenance for genome integrity, and propose that more comprehensive analyses of heterochromatin roles in tumorigenesis will be integral to future innovations in cancer treatment.

Sfh1, an essential component of the RSC chromatin remodeling complex, maintains genome integrity in fission yeast

Abp1 is a fission yeast CENP-B homologue that contributes to centromere function, silencing at pericentromeric heterochromatin and silencing of retrotransposons. We identified the sfh1 gene, encoding a core subunit of the fission yeast chromatin remodeling complex RSC as an Abp1-interacting protein. Because sfh1 is essential for growth, we isolated temperature-sensitive sfh1 mutants. These mutants showed defects in centromere functions, reflected by sensitivity to an inhibitor of spindle formation and minichromosome instability. Sfh1 localized at both kinetochore and pericentromeric heterochromatin regions. Although sfh1 mutations had minor effect on silencing at these regions, they decreased the levels of cohesin on centromeric heterochromatin. Sfh1 also localized at a retrotransposon, Tf2, in a partly Abp1-dependent manner, and assisted in silencing of Tf2 by Abp1 probably in the same pathway as a histone chaperon, HIRA, which is also known to involve in Tf2 repression. Furthermore, sfh1 mutants were sensitive to several DNA-damaging treatments (HU, MMS, UV and X-ray). Increase in spontaneous foci of Rad22, a recombination Mediator protein Rad52 homologue, in sfh1 mutant suggests that RSC functions in homologous recombination repair of double-stranded break downstream of the Rad22 recruitment. These results indicate that RSC plays multiple roles in the maintenance of genome integrity.

Ten principles of heterochromatin formation and function

Heterochromatin is a key architectural feature of eukaryotic chromosomes, which endows particular genomic domains with specific functional properties. The capacity of heterochromatin to restrain the activity of mobile elements, isolate DNA repair in repetitive regions and ensure accurate chromosome segregation is crucial for maintaining genomic stability. Nucleosomes at heterochromatin regions display histone post-translational modifications that contribute to developmental regulation by restricting lineage-specific gene expression. The mechanisms of heterochromatin establishment and of heterochromatin maintenance are separable and involve the ability of sequence-specific factors bound to nascent transcripts to recruit chromatin-modifying enzymes. Heterochromatin can spread along the chromatin from nucleation sites. The propensity of heterochromatin to promote its own spreading and inheritance is counteracted by inhibitory factors. Because of its importance for chromosome function, heterochromatin has key roles in the pathogenesis of various human diseases. In this Review, we discuss conserved principles of heterochromatin formation and function using selected examples from studies of a range of eukaryotes, from yeast to human, with an emphasis on insights obtained from unicellular model organisms.

RNA-dependent stabilization of SUV39H1 at constitutive heterochromatin

Heterochromatin formed by the SUV39 histone methyltransferases represses transcription from repetitive DNA sequences and ensures genomic stability. How SUV39 enzymes localize to their target genomic loci remains unclear. Here, we demonstrate that chromatin-associated RNA contributes to the stable association of SUV39H1 with constitutive heterochromatin in human cells. We find that RNA associated with mitotic chromosomes is concentrated at pericentric heterochromatin, and is encoded, in part, by repetitive α-satellite sequences, which are retained in cis at their transcription sites. Purified SUV39H1 directly binds nucleic acids through its chromodomain; and in cells, SUV39H1 associates with α-satellite RNA transcripts. Furthermore, nucleic acid binding mutants destabilize the association of SUV39H1 with chromatin in mitotic and interphase cells – effects that can be recapitulated by RNase treatment or RNA polymerase inhibition – and cause defects in heterochromatin function. Collectively, our findings uncover a previously unrealized function for chromatin-associated RNA in regulating constitutive heterochromatin in human cells.

DOI:http://dx.doi.org/10.7554/eLife.25299.001

Each cell in a human body contains the same DNA sequence, which serves as a set of instructions for how the body should develop and operate. However, only certain sections of DNA are “active” at any particular time and in any given type of cell. When a section of DNA is active, cells make many copies of it using a molecule called RNA. When a section of DNA in inactive, very little RNA is made. Some sections of DNA must always be kept inactive to avoid damaging the cell.

DNA is packaged around proteins called histones, and enzymes that modify histones control which sections of DNA are switched on or off. One such modifying enzyme, called SUV39H1, is important for inactivating sections of DNA that could cause harm to the cell if they are active. Previous studies showed that the loss of SUV39H1 and related proteins cause abnormalities and cancer in mice. However, it is not clear how this enzyme identifies and inactivates the DNA it needs to target.

Johnson, Yewdell et al. studied SUV39H1 in human cells. The experiments show that RNA binds to the SUV39H1 enzyme and controls how it interacts with DNA. Specifically, Johnson, Yewdell et al. found that sections of DNA that are inactive can still make a small amount of RNA, and that this RNA tethers SUV39H1 to the DNA to keep the DNA switched off. Mutant forms of SUV39H1 that are unable to interact with RNA fall off the DNA, which allows DNA sequences that are normally switched off to become active.

The findings of Johnson, Yewdell et al. reveal a new role for RNAs in regulating whether DNA is switched on or off. The next step is to determine whether other enzymes that can also modify histones use the same mechanism to activate or inactivate DNA. Differences in how the activity of DNA is regulated between individuals plays a crucial role in generating the diversity we see in nature. Therefore, this work helps us to understand our basic biology and may provide new opportunities for treating disease.

DOI:http://dx.doi.org/10.7554/eLife.25299.002

Histone H3G34R mutation causes replication stress, homologous recombination defects and genomic instability in S. pombe

Recurrent somatic mutations of H3F3A in aggressive pediatric high-grade gliomas generate K27M or G34R/V mutant histone H3.3. H3.3-G34R/V mutants are common in tumors with mutations in p53 and ATRX, an H3.3-specific chromatin remodeler. To gain insight into the role of H3-G34R, we generated fission yeast that express only the mutant histone H3. H3-G34R specifically reduces H3K36 tri-methylation and H3K36 acetylation, and mutants show partial transcriptional overlap with set2 deletions. H3-G34R mutants exhibit genomic instability and increased replication stress, including slowed replication fork restart, although DNA replication checkpoints are functional. H3-G34R mutants are defective for DNA damage repair by homologous recombination (HR), and have altered HR protein dynamics in both damaged and untreated cells. These data suggest H3-G34R slows resolution of HR-mediated repair and that unresolved replication intermediates impair chromosome segregation. This analysis of H3-G34R mutant fission yeast provides mechanistic insight into how G34R mutation may promote genomic instability in glioma.

Writing, erasing and reading histone lysine methylations

Histone modifications are key epigenetic regulatory features that have important roles in many cellular events. Lysine methylations mark various sites on the tail and globular domains of histones and their levels are precisely balanced by the action of methyltransferases ('writers') and demethylases ('erasers'). In addition, distinct effector proteins ('readers') recognize specific methyl-lysines in a manner that depends on the neighboring amino-acid sequence and methylation state. Misregulation of histone lysine methylation has been implicated in several cancers and developmental defects. Therefore, histone lysine methylation has been considered a potential therapeutic target, and clinical trials of several inhibitors of this process have shown promising results. A more detailed understanding of histone lysine methylation is necessary for elucidating complex biological processes and, ultimately, for developing and improving disease treatments. This review summarizes enzymes responsible for histone lysine methylation and demethylation and how histone lysine methylation contributes to various biological processes.

Centromere Stability: The Replication Connection

The fission yeast centromere, which is similar to metazoan centromeres, contains highly repetitive pericentromere sequences that are assembled into heterochromatin. This is required for the recruitment of cohesin and proper chromosome segregation. Surprisingly, the pericentromere replicates early in the S phase. Loss of heterochromatin causes this domain to become very sensitive to replication fork defects, leading to gross chromosome rearrangements. This review examines the interplay between components of DNA replication, heterochromatin assembly, and cohesin dynamics that ensures maintenance of genome stability and proper chromosome segregation.

Rad51 and Rad54 promote noncrossover recombination between centromere repeats on the same chromatid to prevent isochromosome formation

Centromeres consist of DNA repeats in many eukaryotes. Non-allelic homologous recombination (HR) between them can result in gross chromosomal rearrangements (GCRs). In fission yeast, Rad51 suppresses isochromosome formation that occurs between inverted repeats in the centromere. However, how the HR enzyme prevents homology-mediated GCRs remains unclear. Here, we provide evidence that Rad51 with the aid of the Swi/Snf-type motor protein Rad54 promotes non-crossover recombination between centromere repeats to prevent isochromosome formation. Mutations in Rad51 and Rad54 epistatically increased the rates of isochromosome formation and chromosome loss. In sharp contrast, these mutations decreased gene conversion between inverted repeats in the centromere. Remarkably, analysis of recombinant DNAs revealed that rad51 and rad54 increase the proportion of crossovers. In the absence of Rad51, deletion of the structure-specific endonuclease Mus81 decreased both crossovers and isochromosomes, while the cdc27/pol32-D1 mutation, which impairs break-induced replication, did not. We propose that Rad51 and Rad54 promote non-crossover recombination between centromere repeats on the same chromatid, thereby suppressing crossover between non-allelic repeats on sister chromatids that leads to chromosomal rearrangements. Furthermore, we found that Rad51 and Rad54 are required for gene silencing in centromeres, suggesting that HR also plays a role in the structure and function of centromeres.

Swi1Timeless Prevents Repeat Instability at Fission Yeast Telomeres

Genomic instability associated with DNA replication stress is linked to cancer and genetic pathologies in humans. If not properly regulated, replication stress, such as fork stalling and collapse, can be induced at natural replication impediments present throughout the genome. The fork protection complex (FPC) is thought to play a critical role in stabilizing stalled replication forks at several known replication barriers including eukaryotic rDNA genes and the fission yeast mating-type locus. However, little is known about the role of the FPC at other natural impediments including telomeres. Telomeres are considered to be difficult to replicate due to the presence of repetitive GT-rich sequences and telomere-binding proteins. However, the regulatory mechanism that ensures telomere replication is not fully understood. Here, we report the role of the fission yeast Swi1^Timeless, a subunit of the FPC, in telomere replication. Loss of Swi1 causes telomere shortening in a telomerase-independent manner. Our epistasis analyses suggest that heterochromatin and telomere-binding proteins are not major impediments for telomere replication in the absence of Swi1. Instead, repetitive DNA sequences impair telomere integrity in swi1Δ mutant cells, leading to the loss of repeat DNA. In the absence of Swi1, telomere shortening is accompanied with an increased recruitment of Rad52 recombinase and more frequent amplification of telomere/subtelomeres, reminiscent of tumor cells that utilize the alternative lengthening of telomeres pathway (ALT) to maintain telomeres. These results suggest that Swi1 ensures telomere replication by suppressing recombination and repeat instability at telomeres. Our studies may also be relevant in understanding the potential role of Swi1^Timeless in regulation of telomere stability in cancer cells.

In every round of the cell cycle, cells must accurately replicate their full genetic information. This process is highly regulated, as defects during DNA replication cause genomic instability, leading to various genetic disorders including cancers. To thwart these problems, cells carry an array of complex mechanisms to deal with various obstacles found across the genome that can hamper DNA replication and cause DNA damage. Understanding how these mechanisms are regulated and orchestrated is of paramount importance in the field. In this report, we describe how Swi1, a Timeless-related protein in fission yeast, regulates efficient replication of telomeres, which are considered to be difficult to replicate due to the presence of repetitive DNA and telomere-binding proteins. We show that Swi1 prevents telomere damage and maintains telomere length by protecting integrity of telomeric repeats. Swi1-mediated telomere maintenance is independent of telomerase activity, and loss of Swi1 causes hyper-activation of recombination-based telomere maintenance, which generates heterogeneous telomeres. Similar telomerase-independent and recombination-dependent mechanism is utilized by approximately 15% of human cancers, linking telomere replication defects with cancer development. Thus, our study may be relevant in understanding the role of telomere replication defects in the development of cancers in humans.

Regulating retrotransposon activity through the use of alternative transcription start sites

Retrotransposons, the ancestors of retroviruses, have the potential for gene disruption and genomic takeover if not kept in check. Paradoxically, although host cells repress these elements by multiple mechanisms, they are transcribed and are even activated under stress conditions. Here, we describe a new mechanism of retrotransposon regulation through transcription start site (TSS) selection by altered nucleosome occupancy. We show that Fun30 chromatin remodelers cooperate to maintain a high level of nucleosome occupancy at retrotransposon-flanking long terminal repeat (LTR) elements. This enforces the use of a downstream TSS and the production of a truncated RNA incapable of reverse transcription and retrotransposition. However, in stressed cells, nucleosome occupancy at LTR elements is reduced, and the TSS shifts to allow for productive transcription. We propose that controlled retrotransposon transcription from a nonproductive TSS allows for rapid stress-induced activation, while preventing uncontrolled transposon activity in the genome.

Control of heterochromatin localization and silencing by the nuclear membrane protein Lem2

Transcriptionally silent chromatin localizes to the nuclear periphery, which provides a special microenvironment for gene repression. A variety of nuclear membrane proteins interact with repressed chromatin, yet the functional role of these interactions remains poorly understood. Here, we show that, in Schizosaccharomyces pombe, the nuclear membrane protein Lem2 associates with chromatin and mediates silencing and heterochromatin localization. Unexpectedly, we found that these functions can be separated and assigned to different structural domains within Lem2, excluding a simple tethering mechanism. Chromatin association and tethering of centromeres to the periphery are mediated by the N-terminal LEM (LAP2-Emerin-MAN1) domain of Lem2, whereas telomere anchoring and heterochromatin silencing require exclusively its conserved C-terminal MSC (MAN1-Src1 C-terminal) domain. Particularly, silencing by Lem2 is epistatic with the Snf2/HDAC (histone deacetylase) repressor complex SHREC at telomeres, while its necessity can be bypassed by deleting Epe1, a JmjC protein with anti-silencing activity. Furthermore, we found that loss of Lem2 reduces heterochromatin association of SHREC, which is accompanied by increased binding of Epe1. This reveals a critical function of Lem2 in coordinating these antagonistic factors at heterochromatin. The distinct silencing and localization functions mediated by Lem2 suggest that these conserved LEM-containing proteins go beyond simple tethering to play active roles in perinuclear silencing.

Escape from Mitotic Arrest: An Unexpected Connection Between Microtubule Dynamics and Epigenetic Regulation of Centromeric Chromatin in Schizosaccharomyces pombe

Accurate chromosome segregation is necessary to ensure genomic integrity. Segregation depends on the proper functioning of the centromere, kinetochore, and mitotic spindle microtubules and is monitored by the spindle assembly checkpoint (SAC). In the fission yeast Schizosaccharomyces pombe, defects in Dis1, a microtubule-associated protein that influences microtubule dynamics, lead to mitotic arrest as a result of an active SAC and consequent failure to grow at low temperature. In a mutant dis1 background (dis1-288), loss of function of Msc1, a fission yeast homolog of the KDM5 family of proteins, suppresses the growth defect and promotes normal mitosis. Genetic analysis implicates a histone deacetylase (HDAC)-linked pathway in suppression because HDAC mutants clr6-1, clr3∆, and sir2∆, though not hos2∆, also promote normal mitosis in the dis1-288 mutant. Suppression of the dis phenotype through loss of msc1 function requires the spindle checkpoint protein Mad2 and is limited by the presence of the heterochromatin-associated HP1 protein homolog Swi6. We speculate that alterations in histone acetylation promote a centromeric chromatin environment that compensates for compromised dis1 function by allowing for successful kinetochore-microtubule interactions that can satisfy the SAC. In cells arrested in mitosis by mutation of dis1, loss of function of epigenetic determinants such as Msc1 or specific HDACs can promote cell survival. Because the KDM5 family of proteins has been implicated in human cancers, an appreciation of the potential role of this family of proteins in chromosome segregation is warranted.

An Ancient Yeast for Young Geneticists: A Primer on the Schizosaccharomyces pombe Model System

The fission yeast Schizosaccharomyces pombe is an important model organism for the study of eukaryotic molecular and cellular biology. Studies of S. pombe, together with studies of its distant cousin, Saccharomyces cerevisiae, have led to the discovery of genes involved in fundamental mechanisms of transcription, translation, DNA replication, cell cycle control, and signal transduction, to name but a few processes. However, since the divergence of the two species approximately 350 million years ago, S. pombe appears to have evolved less rapidly than S. cerevisiae so that it retains more characteristics of the common ancient yeast ancestor, causing it to share more features with metazoan cells. This Primer introduces S. pombe by describing the yeast itself, providing a brief description of the origins of fission yeast research, and illustrating some genetic and bioinformatics tools used to study protein function in fission yeast. In addition, a section on some key differences between S. pombe and S. cerevisiae is included for readers with some familiarity with budding yeast research but who may have an interest in developing research projects using S. pombe.

Chromosome segregation and organization are targets of 5'-Fluorouracil in eukaryotic cells

The antimetabolite 5'-Fluorouracil (5FU) is an analog of uracil commonly employed as a chemotherapeutic agent in the treatment of a range of cancers including colorectal tumors. To assess the cellular effects of 5FU, we performed a genome-wide screening of the haploid deletion library of the eukaryotic model Schizosaccharomyces pombe. Our analysis validated previously characterized drug targets including RNA metabolism, but it also revealed unexpected mechanisms of action associated with chromosome segregation and organization (post-translational histone modification, histone exchange, heterochromatin). Further analysis showed that 5FU affects the heterochromatin structure (decreased levels of histone H3 lysine 9 methylation) and silencing (down-regulation of heterochromatic dg/dh transcripts). To our knowledge, this is the first time that defects in heterochromatin have been correlated with increased cytotoxicity to an anticancer drug. Moreover, the segregation of chromosomes, a process that requires an intact heterochromatin at centromeres, was impaired after drug exposure. These defects could be related to the induction of genes involved in chromatid cohesion and kinetochore assembly. Interestingly, we also observed that thiabendazole, a microtubule-destabilizing agent, synergistically enhanced the cytotoxic effects of 5FU. These findings point to new targets and drug combinations that could potentiate the effectiveness of 5FU-based treatments.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction

The segregation of centromeres and telomeres at mitosis is coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. Mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during fission yeast mitosis. Telomere dispersion initiates in metaphase, whereas disjunction takes place in anaphase. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres, whereas disjunction occurs at anaphase after the phosphorylation of condensin subunit Cnd2. Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescued cell death after Aurora inhibition by promoting the loading of condensin on chromosome arms. Our findings reveal an essential role for telomeres in chromosome arm segregation.

KdmA, a histone H3 demethylase with bipartite function, differentially regulates primary and secondary metabolism in Aspergillus nidulans

Aspergillus nidulans kdmA encodes a member of the KDM4 family of jumonji histone demethylase proteins, highly similar to metazoan orthologues both within functional domains and in domain architecture. This family of proteins exhibits demethylase activity towards lysines 9 and 36 of histone H3 and plays a prominent role in gene expression and chromosome structure in many species. Mass spectrometry mapping of A. nidulans histones revealed that around 3% of bulk histone H3 carried trimethylated H3K9 (H3K9me3) but more than 90% of histones carried either H3K36me2 or H3K36me3. KdmA functions as H3K36me3 demethylase and has roles in transcriptional regulation. Genetic manipulation of KdmA levels is tolerated without obvious effect in most conditions, but strong phenotypes are evident under various conditions of stress. Transcriptome analysis revealed that – in submerged early and late cultures – between 25% and 30% of the genome is under KdmA influence respectively. Transcriptional imbalance in the kdmA deletion mutant may contribute to the lethal phenotype observed upon exposure of mutant cells to low‐density visible light on solid medium. Although KdmA acts as transcriptional co‐repressor of primary metabolism genes, it is required for full expression of several genes involved in biosynthesis of secondary metabolites.

The telomere bouquet regulates meiotic centromere assembly

The role of the conserved meiotic telomere bouquet has been enigmatic for over a century. We showed previously that disruption of the fission yeast bouquet impairs spindle formation in approximately half of meiotic cells. Surprisingly, bouquet-deficient meiocytes with functional spindles harbour chromosomes that fail to achieve spindle attachment. Kinetochore proteins and the centromeric histone H3 variant Cnp1 fail to localize to those centromeres that exhibit spindle attachment defects in the bouquet's absence. The HP1 orthologue Swi6 also fails to bind these centromeres, suggesting that compromised pericentromeric heterochromatin underlies the kinetochore defects. We find that centromeres are prone to disassembly during meiosis, but this is reversed by localization of centromeres to the telomere-proximal microenvironment, which is conducive to heterochromatin formation and centromere reassembly. Accordingly, artificially tethering a centromere to a telomere rescues the tethered centromere but not other centromeres. These results reveal an unanticipated level of control of centromeres by telomeres.

The RFTS domain of Raf2 is required for Cul4 interaction and heterochromatin integrity in fission yeast

Centromeric heterochromatin assembly in fission yeast is critical for faithful chromosome segregation at mitosis. Its assembly requires a concerted pathway of events whereby the RNA interference (RNAi) pathway guides H3K9 methylation to target sequences. H3K9 methylation, a hallmark of heterochromatin structure, is mediated by the single histone methyltransferase Clr4 (equivalent to metazoan Suv3-9), a component of the CLRC complex. Loss of or defects in CLRC components disrupts heterochromatin formation due to loss of H3K9 methylation, thus an intact, fully functional CLRC complex is required for heterochromatin integrity. Despite its importance, little is known about the contribution of the CLRC component Raf2 to H3K9 methylation and heterochromatin assembly. We demonstrate that Raf2 is concentrated at centromeres and contrary to other analyses, we find that loss of Raf2 does not affect CENP-A^Cnp1 localisation or recruitment to centromeres. Our sequence alignments show that Raf2 contains a Replication Foci Targeting Sequence (RFTS) domain homologous to the RFTS domain of the human DNA methyltransferase DNMT1. We show that the Raf2 RFTS domain is required for centromeric heterochromatin formation as its mutation disrupts H3K9 methylation but not the processing of centromeric transcripts into small interfering RNAs (siRNAs) by the RNAi pathway. Analysis of biochemical interactions demonstrates that the RFTS domain mediates an interaction between Raf2 and the CLRC component Cul4. We conclude that the RFTS domain of Raf2 is a protein interaction module that plays an important role in heterochromatin formation at centromeres.

The proper splicing of RNAi factors is critical for pericentric heterochromatin assembly in fission yeast

Heterochromatin preferentially assembles at repetitive DNA elements, playing roles in transcriptional silencing, recombination suppression, and chromosome segregation. The RNAi machinery is required for heterochromatin assembly in a diverse range of organisms. In fission yeast, RNA splicing factors are also required for pericentric heterochromatin assembly, and a prevailing model is that splicing factors provide a platform for siRNA generation independently of their splicing activity. Here, by screening the fission yeast deletion library, we discovered four novel splicing factors that are required for pericentric heterochromatin assembly. Sequencing total cellular RNAs from the strongest of these mutants, cwf14Δ, showed intron retention in mRNAs of several RNAi factors. Moreover, introducing cDNA versions of RNAi factors significantly restored pericentric heterochromatin in splicing mutants. We also found that mutations of splicing factors resulted in defective telomeric heterochromatin assembly and mis-splicing the mRNA of shelterin component Tpz1, and that replacement of tpz1+ with its cDNA partially rescued heterochromatin defects at telomeres in splicing mutants. Thus, proper splicing of RNAi and shelterin factors contributes to heterochromatin assembly at pericentric regions and telomeres.

Targeting heterochromatin formation to transposable elements in Drosophila: potential roles of the piRNA system

Successful heterochromatin formation is critical for genome stability in eukaryotes, both to maintain structures needed for mitosis and meiosis and to silence potentially harmful transposable elements. Conversely, inappropriate heterochromatin assembly can lead to inappropriate silencing and other deleterious effects. Hence targeting heterochromatin assembly to appropriate regions of the genome is of utmost importance. Here we focus on heterochromatin assembly in Drosophila melanogaster, the model organism in which variegation, or cell-to-cell variable gene expression resulting from heterochromatin formation, was first described. In particular, we review the potential role of transposable elements as genetic determinants of the chromatin state and examine how small RNA pathways may participate in the process of targeted heterochromatin formation.

The Mi-2 homolog Mit1 actively positions nucleosomes within heterochromatin to suppress transcription

Mit1 is the putative chromatin remodeling subunit of the fission yeast Snf2/histone deacetylase (HDAC) repressor complex (SHREC) and is known to repress transcription at regions of heterochromatin. However, how Mit1 modifies chromatin to silence transcription is largely unknown. Here we report that Mit1 mobilizes histone octamers in vitro and requires ATP hydrolysis and conserved chromatin tethering domains, including a previously unrecognized chromodomain, to remodel nucleosomes and silence transcription. Loss of Mit1 remodeling activity results in nucleosome depletion at specific DNA sequences that display low intrinsic affinity for the histone octamer, but its contribution to antagonizing RNA polymerase II (Pol II) access and transcription is not restricted to these sites. Genetic epistasis analyses demonstrate that SHREC subunits and the transcription-coupled Set2 histone methyltransferase, which is involved in suppression of cryptic transcription at actively transcribed regions, cooperate to silence heterochromatic transcripts. In addition, we have demonstrated that Mit1's remodeling activity contributes to SHREC function independently of Clr3's histone deacetylase activity on histone H3 K14. We propose that Mit1 is a chromatin remodeling factor that cooperates with the Clr3 histone deacetylase of SHREC and other chromatin modifiers to stabilize heterochromatin structure and to prevent access to the transcriptional machinery.

Epigenetically induced paucity of histone H2A.Z stabilizes fission-yeast ectopic centromeres

In most eukaryotes, centromeres are epigenetically defined by nucleosomes that contain the histone H3 variant centromere protein A (CENP-A). Specific targeting of the CENP-A-loading chaperone to the centromere is vital for stable centromere propagation; however, the existence of ectopic centromeres (neocentromeres) indicates that this chaperone can function in different chromatin environments. The mechanism responsible for accommodating the CENP-A chaperone at noncentromeric regions is poorly understood. Here, we report the identification of transient, immature neocentromeres in Schizosaccharomyces pombe that show reduced association with the CENP-A chaperone Scm3, owing to persistence of the histone H2A variant H2A.Z. After the acquisition of adjacent heterochromatin or relocation of the immature neocentromeres to subtelomeric regions, H2A.Z was depleted and Scm3 was replenished, thus leading to subsequent stabilization of the neocentromeres. These findings provide new insights into histone variant-mediated epigenetic control of neocentromere establishment.

Pericentric heterochromatin generated by HP1 protein interaction-defective histone methyltransferase Suv39h1

Pericentric regions form epigenetically organized silent heterochromatin structures that accumulate histone H3 lysine 9 trimethylation (H3K9me3) and HP1. At pericentric regions, Suv39h is the major enzyme that generates H3K9me3. Suv39h also interacts directly with HP1, a methylated H3K9-binding protein. However, it is not well characterized how HP1 interaction is important for Suv39h accumulation and Suv39h-mediated H3K9me3 formation at the pericentromere. To address this, we introduced the HP1 binding-defective N-terminally truncated mouse Suv39h1 (ΔN) into Suv39h-deficient embryonic stem cells. Interestingly, pericentric accumulation of ΔN and ΔN-mediated H3K9me3 was observed to recover, but HP1 accumulation was only marginally restored. ΔN also rescued DNA methyltransferase Dnmt3a and -3b accumulation and DNA methylation of the pericentromere. In contrast, other pericentric heterochromatin features, such as ATRX protein association and H4K20me3, were not recovered. Finally, derepressed major satellite repeats were partially silenced by ΔN expression. These findings clearly showed that the Suv39h-HP1 binding is dispensable for pericentric H3K9me3 and DNA methylation, but this interaction and HP1 recruitment/accumulation seem to be crucial for complete formation of heterochromatin.

Mutations disrupting histone methylation have different effects on replication timing in S. pombe centromere

The fission yeast pericentromere comprises repetitive sequence elements packaged into heterchromatin marked by histone H3K9 methylation and Swi6 binding. Transient disruption of Swi6 during S phase allows a period of RNA synthesis which programs the RNAi machinery to maintain histone methylation. However, Swi6 is also required for early replication timing. We show that not only Swi6 but also the chromodomain protein Chp1 are delocalized during S phase. Different from loss of swi6, mutations that disrupt histone methylation in the centromere, chp1Δ and clr4Δ, undergo early DNA replication. However, timing is modestly delayed in RNAi mutants dcr1Δ or rdp1Δ, while hrr1Δ mutants resemble swi6Δ in their replication delay. Finally, we show that recruitment of RNA polymerase II in the centromere occurs independently of replication. These different effects indicate that replication timing is not simply linked to histone methylation.

Myb-domain protein Teb1 controls histone levels and centromere assembly in fission yeast

The TTAGGG motif is common to two seemingly unrelated dimensions of chromatin function-the vertebrate telomere repeat and the promoter regions of many Schizosaccharomyces pombe genes, including all of those encoding canonical histones. The essential S. pombe protein Teb1 contains two Myb-like DNA binding domains related to those found in telomere proteins and binds the human telomere repeat sequence TTAGGG. Here, we analyse Teb1 binding throughout the genome and the consequences of reduced Teb1 function. Chromatin immunoprecipitation (ChIP)-on-chip analysis reveals robust Teb1 binding at many promoters, notably including all of those controlling canonical histone gene expression. A hypomorphic allele, teb1-1, confers reduced binding and reduced levels of histone transcripts. Prompted by previously suggested connections between histone expression and centromere identity, we examined localization of the centromeric histone H3 variant Cnp1 and found reduced centromeric binding along with reduced centromeric silencing. These data identify Teb1 as a master regulator of histone levels and centromere identity.

A novel RNAi protein, Dsh1, assembles RNAi machinery on chromatin to amplify heterochromatic siRNA

In fission yeast, siRNA is generated from pericentromeric noncoding RNA by the RNAi machinery. siRNA synthesis and heterochromatin formation are interdependent, forming a self-reinforcing loop on chromatin. In this system, siRNA is amplified by the RNA-dependent RNA polymerase complex (RDRC) and the endoribonuclease Dcr1, which synthesizes dsRNA and processes the dsRNA, respectively. The amplification is essential for stable heterochromatin formation. Here, a novel gene, dsh1(+) (defect of the gene silencing at centromeric heterochromatin), is identified as an essential component of RNAi-directed heterochromatin assembly. Loss of dsh1(+) abolishes normal RNAi function and heterochromatic gene silencing at pericentromeres. Dsh1 interacts with Dcr1 and RDRC and couples the reactions of both proteins to the effective production of siRNA in vivo. Dsh1 binds to heterochromatin in the absence of RDRC, while RDRC requires Dsh1 for its chromatin-binding activity, suggesting that Dsh1 recruits RDRC to chromatin. Immunofluorescence analysis shows that Dsh1 forms foci at the nuclear periphery, and some Dsh1 foci colocalize with Dcr1 and RDRC. Dsh1 is required for the colocalization of Dcr1 and RDRC. Moreover, loss of the nuclear periphery localization of Dsh1 abolishes Dsh1 function. Taken together, these results suggest that Dsh1 assembles the RNAi machinery on heterochromatin and forms a perinuclear compartment for amplification of heterochromatic siRNA.