Patterns and mechanisms of ancestral histone protein inheritance in budding yeast

A replisome-associated histone H3-H4 chaperone required for epigenetic inheritance

Faithful transfer of parental histones to newly replicated daughter DNA strands is critical for inheritance of epigenetic states. Although replication proteins that facilitate parental histone transfer have been identified, how intact histone H3-H4 tetramers travel from the front to the back of the replication fork remains unknown. Here, we use AlphaFold-Multimer structural predictions combined with biochemical and genetic approaches to identify the Mrc1/CLASPIN subunit of the replisome as a histone chaperone. Mrc1 contains a conserved histone-binding domain that forms a brace around the H3-H4 tetramer mimicking nucleosomal DNA and H2A-H2B histones, is required for heterochromatin inheritance, and promotes parental histone recycling during replication. We further identify binding sites for the FACT histone chaperone in Swi1/TIMELESS and DNA polymerase α that are required for heterochromatin inheritance. We propose that Mrc1, in concert with FACT acting as a mobile co-chaperone, coordinates the distribution of parental histones to newly replicated DNA.

DNA polymerase delta governs parental histone transfer to DNA replication lagging strand

Chromatin replication is intricately intertwined with the recycling of parental histones to the newly duplicated DNA strands for faithful genetic and epigenetic inheritance. The transfer of parental histones occurs through two distinct pathways: leading strand deposition, mediated by the DNA polymerase ε subunits Dpb3/Dpb4, and lagging strand deposition, facilitated by the MCM helicase subunit Mcm2. However, the mechanism of the facilitation of Mcm2 transferring parental histones to the lagging strand while moving along the leading strand remains unclear. Here, we show that the deletion of Pol32, a nonessential subunit of major lagging-strand DNA polymerase δ, results in a predominant transfer of parental histone H3-H4 to the leading strand during replication. Biochemical analyses further demonstrate that Pol32 can bind histone H3-H4 both in vivo and in vitro. The interaction of Pol32 with parental histone H3-H4 is disrupted through the mutation of the histone H3-H4 binding domain within Mcm2. Our findings identify the DNA polymerase δ subunit Pol32 as a critical histone chaperone downstream of Mcm2, mediating the transfer of parental histones to the lagging strand during DNA replication.

Hexasomal particles: consequence or also consequential?

It is long known that an RNA polymerase transcribing through a nucleosome can generate subnucleosomal particles called hexasomes. These particles lack an H2A-H2B dimer, breaking the symmetry of a nucleosome and revealing new interfaces. Whether hexasomes are simply a consequence of RNA polymerase action or they also have a regulatory impact remains an open question. Recent biochemical and structural studies of RNA polymerases and chromatin remodelers with hexasomes motivated us to revisit this question. Here, we build on previous models to discuss how formation of hexasomes can allow sophisticated regulation of transcription and also significantly impact chromatin folding. We anticipate that further cellular and biochemical analysis of these subnucleosomal particles will uncover additional regulatory roles.

A single fiber view of the nucleosome organization in eukaryotic chromatin

Eukaryotic cells are thought to arrange nucleosomes into extended arrays with evenly spaced nucleosomes phased at genomic landmarks. Here we tested to what extent this stereotypic organization describes the nucleosome organization in Saccharomyces cerevisiae using Fiber-Seq, a long-read sequencing technique that maps entire nucleosome arrays on individual chromatin fibers in a high throughput manner. With each fiber coming from a different cell, Fiber-Seq uncovers cell-to-cell heterogeneity. The long reads reveal the nucleosome architecture even over repetitive DNA such as the ribosomal DNA repeats. The absolute nucleosome occupancy, a parameter that is difficult to obtain with conventional sequencing approaches, is a direct readout of Fiber-Seq. We document substantial deviations from the stereotypical nucleosome organization with unexpectedly long linker DNAs between nucleosomes, gene bodies missing entire nucleosomes, cell-to-cell heterogeneity in nucleosome occupancy, heterogeneous phasing of arrays and irregular nucleosome spacing. Nucleosome array structures are indistinguishable throughout the gene body and with respect to the direction of transcription arguing against transcription promoting array formation. Acute nucleosome depletion destroyed most of the array organization indicating that nucleosome remodelers cannot efficiently pack nucleosomes under those conditions. Given that nucleosomes are cis-regulatory elements, the cell-to-cell heterogeneity uncovered by Fiber-Seq provides much needed information to understand chromatin structure and function.

Diffusion controls local versus dispersed inheritance of histones during replication and shapes epigenomic architecture

The dynamics of inheritance of histones and their associated modifications across cell divisions can have major consequences on maintenance of the cellular epigenomic state. Recent experiments contradict the long-held notion that histone inheritance during replication is always local, suggesting that active and repressed regions of the genome exhibit fundamentally different histone dynamics independent of transcription-coupled turnover. Here we develop a stochastic model of histone dynamics at the replication fork and demonstrate that differential diffusivity of histones in active versus repressed chromatin is sufficient to quantitatively explain these recent experiments. Further, we use the model to predict patterns in histone mark similarity between pairs of genomic loci that should be developed as a result of diffusion, but cannot originate from either PRC2 mediated mark spreading or transcriptional processes. Interestingly, using a combination of CHIP-seq, replication timing and Hi-C datasets we demonstrate that all the computationally predicted patterns are consistently observed for both active and repressive histone marks in two different cell lines. While direct evidence for histone diffusion remains controversial, our results suggest that dislodged histones in euchromatin and facultative heterochromatin may exhibit some level of diffusion within "Diffusion-Accessible-Domains" (DADs), leading to redistribution of epigenetic marks within and across chromosomes. Preservation of the epigenomic state across cell divisions therefore might be achieved not by passing on strict positional information of histone marks, but by maintaining the marks in somewhat larger DADs of the genome.

Nucleosomes unwrapped: Structural perspectives on transcription through chromatin

Transcription of most protein-coding genes requires the passage of RNA polymerase II through chromatin. Chromatin with its fundamental unit, the nucleosome, represents a barrier to transcription. How RNA polymerase II and associated factors traverse through nucleosomes and how chromatin architecture is maintained have remained largely enigmatic. Only recently, cryo-EM structures have visualized the transcription process through chromatin. These structures have elucidated how transcription initiation and transcription elongation influence and are influenced by a chromatinized DNA substrate. This review provides a summary of our current structural understanding of transcription through chromatin, highlighting common mechanisms during nucleosomal traversal and novel regulatory mechanisms that have emerged in the last five years.

The patterns and participants of parental histone recycling during DNA replication in Saccharomyces cerevisiae

Epigenetic information carried by histone modifications not only reflects the state of gene expression, but also participates in the maintenance of chromatin states and the regulation of gene expression. Recycling of parental histones to daughter chromatin after DNA replication is vital to mitotic inheritance of epigenetic information and the maintenance of cell identity, because the locus-specific modifications of the parental histones need to be maintained. To assess the precision of parental histone recycling, we developed a synthetic local label-chasing system in budding yeast Saccharomyces cerevisiae. Using this system, we observed that parental histone H3 can be recycled to their original position, thereby recovering their position information after DNA replication at all tested loci, including heterochromatin boundary, non-transcribed region, and actively transcribed regions. Moreover, the recycling rate appears to be affected by local chromatin environment. We surveyed a number of potential regulatory factors and observed that histone H3-H4 chaperon Asf1 contributed to parental histone recycling, while the eukaryotic replisome-associated components Mcm2 and Dpb3 displayed compounding effects in this process. In addition, the FACT complex also plays a role in the recycling of parental histones and helps to stabilize the nucleosomes.

Regulation, functions and transmission of bivalent chromatin during mammalian development

Cells differentiate and progress through development guided by a dynamic chromatin landscape that mediates gene expression programmes. During development, mammalian cells display a paradoxical chromatin state: histone modifications associated with gene activation (trimethylated histone H3 Lys4 (H3K4me3)) and with gene repression (trimethylated H3 Lys27 (H3K27me3)) co-occur at promoters of developmental genes. This bivalent chromatin modification state is thought to poise important regulatory genes for expression or repression during cell-lineage specification. In this Review, we discuss recent work that has expanded our understanding of the molecular basis of bivalent chromatin and its contributions to mammalian development. We describe the factors that establish bivalency, especially histone-lysine N-methyltransferase 2B (KMT2B) and Polycomb repressive complex 2 (PRC2), and consider evidence indicating that PRC1 shapes bivalency and may contribute to its transmission between generations. We posit that bivalency is a key feature of germline and embryonic stem cells, as well as other types of stem and progenitor cells. Finally, we discuss the relevance of bivalent chromtin to human development and cancer, and outline avenues of future research.

Structural basis of nucleosome retention during transcription elongation

In eukaryotes, RNA polymerase (Pol) II transcribes chromatin and must move past nucleosomes, often resulting in nucleosome displacement. How Pol II unwraps the DNA from nucleosomes to allow transcription and how DNA rewraps to retain nucleosomes has been unclear. Here, we report the 3.0-angstrom cryo-electron microscopy structure of a mammalian Pol II-DSIF-SPT6-PAF1c-TFIIS-nucleosome complex stalled 54 base pairs within the nucleosome. The structure provides a mechanistic basis for nucleosome retention during transcription elongation where upstream DNA emerging from the Pol II cleft has rewrapped the proximal side of the nucleosome. The structure uncovers a direct role for Pol II and transcription elongation factors in nucleosome retention and explains how nucleosomes are retained to prevent the disruption of chromatin structure across actively transcribed genes.

SETD2: from chromatin modifier to multipronged regulator of the genome and beyond

Histone modifying enzymes play critical roles in many key cellular processes and are appealing proteins for targeting by small molecules in disease. However, while the functions of histone modifying enzymes are often linked to epigenetic regulation of the genome, an emerging theme is that these enzymes often also act by non-catalytic and/or non-epigenetic mechanisms. SETD2 (Set2 in yeast) is best known for associating with the transcription machinery and methylating histone H3 on lysine 36 (H3K36) during transcription. This well-characterized molecular function of SETD2 plays a role in fine-tuning transcription, maintaining chromatin integrity, and mRNA processing. Here we give an overview of the various molecular functions and mechanisms of regulation of H3K36 methylation by Set2/SETD2. These fundamental insights are important to understand SETD2’s role in disease, most notably in cancer in which SETD2 is frequently inactivated. SETD2 also methylates non-histone substrates such as α-tubulin which may promote genome stability and contribute to the tumor-suppressor function of SETD2. Thus, to understand its role in disease, it is important to understand and dissect the multiple roles of SETD2 within the cell. In this review we discuss how histone methylation by Set2/SETD2 has led the way in connecting histone modifications in active regions of the genome to chromatin functions and how SETD2 is leading the way to showing that we also have to look beyond histones to truly understand the physiological role of an ‘epigenetic’ writer enzyme in normal cells and in disease.

Role of Chromatin Replication in Transcriptional Plasticity, Cell Differentiation and Disease

Chromatin organization is essential to maintain a correct regulation of gene expression and establish cell identity. However, during cell division, the replication of the genetic material produces a global disorganization of chromatin structure. In this paper, we describe the new scientific breakthroughs that have revealed the nature of the post-replicative chromatin and the mechanisms that facilitate its restoration. Moreover, we highlight the implications of these chromatin alterations in gene expression control and their impact on key biological processes, such as cell differentiation, cell reprogramming or human diseases linked to cell proliferation, such as cancer.

Rtt109 promotes nucleosome replacement ahead of the replication fork

DNA replication perturbs chromatin by triggering the eviction, replacement, and incorporation of nucleosomes. How this dynamic is orchestrated in time and space is poorly understood. Here, we apply a genetically encoded sensor for histone exchange to follow the time-resolved histone H3 exchange profile in budding yeast cells undergoing slow synchronous replication in nucleotide-limiting conditions. We find that new histones are incorporated not only behind, but also ahead of the replication fork. We provide evidence that Rtt109, the S-phase-induced acetyltransferase, stabilizes nucleosomes behind the fork but promotes H3 replacement ahead of the fork. Increased replacement ahead of the fork is independent of the primary Rtt109 acetylation target H3K56 and rather results from Vps75-dependent Rtt109 activity toward the H3 N terminus. Our results suggest that, at least under nucleotide-limiting conditions, selective incorporation of differentially modified H3s behind and ahead of the replication fork results in opposing effects on histone exchange, likely reflecting the distinct challenges for genome stability at these different regions.

Microbial Adaptation to Enhance Stress Tolerance

Microbial cell factories have been widely used in the production of various chemicals. Although synthetic biology is useful in improving the cell factories, adaptation is still widely applied to enhance its complex properties. Adaptation is an important strategy for enhancing stress tolerance in microbial cell factories. Adaptation involves gradual modifications of microorganisms in a stressful environment to enhance their tolerance. During adaptation, microorganisms use different mechanisms to enhance non-preferred substrate utilization and stress tolerance, thereby improving their ability to adapt for growth and survival. In this paper, the progress on the effects of adaptation on microbial substrate utilization capacity and environmental stress tolerance are reviewed, and the mechanisms involved in enhancing microbial adaptive capacity are discussed.

The asymmetric distribution of RNA polymerase II and nucleosomes on replicated daughter genomes is caused by differences in replication timing between the lagging and the leading strand

Chromatin features are thought to have a role in the epigenetic transmission of transcription states from one cell generation to the next. It is unclear how chromatin structure survives disruptions caused by genomic replication or whether chromatin features are instructive of the transcription state of the underlying gene. We developed a method to monitor budding yeast replication, transcription, and chromatin maturation dynamics on each daughter genome in parallel, with which we identified clusters of secondary origins surrounding known origins. We found a difference in the timing of lagging and leading strand replication on the order of minutes at most yeast genes. We propose a model in which the majority of old histones and RNA polymerase II (RNAPII) bind to the gene copy that replicated first, while newly synthesized nucleosomes are assembled on the copy that replicated second. RNAPII enrichment then shifts to the sister copy that replicated second. The order of replication is largely determined by genic orientation: If transcription and replication are codirectional, the leading strand replicates first; if they are counterdirectional, the lagging strand replicates first. A mutation in the Mcm2 subunit of the replicative helicase Mcm2-7 that impairs Mcm2 interactions with histone H3 slows down replication forks but does not qualitatively change the asymmetry in nucleosome distribution observed in the WT. We propose that active transcription states are inherited simultaneously and independently of their underlying chromatin states through the recycling of the transcription machinery and old histones, respectively. Transcription thus actively contributes to the reestablishment of the active chromatin state.

Measuring the buffering capacity of gene silencing in Saccharomyces cerevisiae

Gene silencing, once established, is stably maintained for several generations. Despite the high fidelity of the inheritance of the silent state, individual components of silenced chromatin are in constant flux. Models suggest that silent loci can tolerate fluctuations in Sir proteins and histone acetylation levels, but the level of tolerance is unknown. To understand the quantitative relationships between H4K16 acetylation, Sir proteins, and silencing, we developed assays to quantitatively alter a H4K16 acetylation mimic allele and Sir protein levels and measure the effects of these changes on silencing. Our data suggest that a two- to threefold change in levels of histone marks and specific Sir proteins affects the stability of the silent state of a large chromatin domain.

Gene silencing in budding yeast is mediated by Sir protein binding to unacetylated nucleosomes to form a chromatin structure that inhibits transcription. Transcriptional silencing is characterized by the high-fidelity transmission of the silent state. Despite its relative stability, the constituent parts of the silent state are in constant flux, giving rise to a model that silent loci can tolerate such fluctuations without functional consequences. However, the level of tolerance is unknown, and we developed methods to measure the threshold of histone acetylation that causes the silent chromatin state to switch to the active state as well as to measure the levels of the enzymes and structural proteins necessary for silencing. We show that loss of silencing required 50 to 75% acetyl-mimic histones, though the precise levels were influenced by silencer strength and upstream activating sequence (UAS) enhancer/promoter strength. Measurements of repressor protein levels necessary for silencing showed that reducing SIR4 gene dosage two- to threefold significantly weakened silencing, though reducing the gene copy numbers for Sir2 or Sir3 to the same extent did not significantly affect silencing suggesting that Sir4 was a limiting component in gene silencing. Calculations suggest that a mere twofold reduction in the ability of acetyltransferases to acetylate nucleosomes across a large array of nucleosomes may be sufficient to generate a transcriptionally silent domain.

A composite DNA element that functions as a maintainer required for epigenetic inheritance of heterochromatin

Epigenetic inheritance of heterochromatin requires DNA-sequence-independent propagation mechanisms, coupling to RNAi, or input from DNA sequence, but how DNA contributes to inheritance is not understood. Here, we identify a DNA element (termed "maintainer") that is sufficient for epigenetic inheritance of pre-existing histone H3 lysine 9 methylation (H3K9me) and heterochromatin in Schizosaccharomyces pombe but cannot establish de novo gene silencing in wild-type cells. This maintainer is a composite DNA element with binding sites for the Atf1/Pcr1 and Deb1 transcription factors and the origin recognition complex (ORC), located within a 130-bp region, and can be converted to a silencer in cells with lower rates of H3K9me turnover, suggesting that it participates in recruiting the H3K9 methyltransferase Clr4/Suv39h. These results suggest that, in the absence of RNAi, histone H3K9me is only heritable when it can collaborate with maintainer-associated DNA-binding proteins that help recruit the enzyme responsible for its epigenetic deposition.

Reduce, Retain, Recycle: Mechanisms for Promoting Histone Protein Degradation versus Stability and Retention

The eukaryotic genome is packaged into chromatin. The nucleosome, the basic unit of chromatin, is composed of DNA coiled around a histone octamer. Histones are among the longest-lived protein species in mammalian cells due to their thermodynamic stability and their associations with DNA and histone chaperones. Histone metabolism plays an integral role in homeostasis. While histones are largely stable, the degradation of histone proteins is necessary under specific conditions. Here, we review the physiological and cellular contexts that promote histone degradation. We describe specific known mechanisms that drive histone proteolysis. Finally, we discuss the importance of histone degradation and regulation of histone supply for organismal and cellular fitness.

Efficient and strand-specific profiling of replicating chromatin with enrichment and sequencing of protein-associated nascent DNA in mammalian cells

Faithful duplication of both genetic and epigenetic information is essential for all eukaryotic cells. DNA replication initiates from replication origins and proceeds bidirectionally but asymmetrically, with the leading strand being synthesized continuously and the lagging strand discontinuously as Okazaki fragments by distinct DNA polymerases. Unraveling the underlying mechanisms of chromatin replication at both strands is crucial to better understand DNA replication and its coupled processes, including nucleosome assembly, sister chromatid cohesion and DNA mismatch repair. Here we describe the enrichment and sequencing of protein-associated nascent DNA (eSPAN) method to analyze the enrichment of proteins of interest, including histones and their modifications at replicating chromatin in a strand-specific manner in mammalian cells. Briefly, cells are pulsed with the thymidine analog bromodeoxyuridine to label newly synthesized DNA. After cell permeabilization, the target proteins are sequentially bound by antibodies and protein A-fused transposase, which digests and tags genomic DNA of interest once activated by magnesium. The strand specificity is preserved through oligo-replacement. Finally, the resulting double-strand DNA is denatured and immunoprecipitated with antibodies against bromodeoxyuridine to enrich nascent DNA associated with proteins of interest. After PCR amplification and next-generation sequencing, the mapped reads are used to calculate the relative enrichment of the target proteins around replication origins. Compared with alternative methods, the eSPAN protocol is simple, cost-effective and sensitive, even in a relatively small number of mammalian cells. The whole procedures from cell collection to generation of sequencing-ready libraries can be completed in 2 days.

Histone methyltransferase DOT1L controls state-specific identity during B cell differentiation

Differentiation of naïve peripheral B cells into terminally differentiated plasma cells is characterized by epigenetic alterations, yet the epigenetic mechanisms that control B‐cell fate remain unclear. Here, we identified a role for the histone H3K79 methyltransferase DOT1L in controlling B‐cell differentiation. Mouse B cells lacking Dot1L failed to establish germinal centers (GC) and normal humoral immune responses in vivo. In vitro, activated B cells in which Dot1L was deleted showed aberrant differentiation and prematurely acquired plasma cell characteristics. Similar results were obtained when DOT1L was chemically inhibited in mature B cells in vitro. Mechanistically, combined epigenomics and transcriptomics analysis revealed that DOT1L promotes expression of a pro‐proliferative, pro‐GC program. In addition, DOT1L indirectly supports the repression of an anti‐proliferative plasma cell differentiation program by maintaining the repression of Polycomb Repressor Complex 2 (PRC2) targets. Our findings show that DOT1L is a key modulator of the core transcriptional and epigenetic landscape in B cells, establishing an epigenetic barrier that warrants B‐cell naivety and GC B‐cell differentiation.

Measuring acetyl-CoA and acetylated histone turnover in vivo: Effect of a high fat diet

Cellular availability of acetyl-CoA, a central intermediate of metabolism, regulates histone acetylation. The impact of a high-fat diet (HFD) on the turnover rates of acetyl-CoA and acetylated histones is unknown. We developed a method for simultaneous measurement of acetyl-CoA and acetylated histones kinetics using a single ²H₂O tracer, and used it to examine effect of HFD-induced perturbations on hepatic histone acetylation in LDLR^-/- mice, a mouse model of non-alcoholic fatty liver disease (NAFLD). Mice were given ²H₂O in the drinking water and the kinetics of hepatic acetyl-CoA, histones, and acetylated histones were quantified based on their ²H-labeling. Consumption of a high fat Western-diet (WD) for twelve weeks led to decreased acetylation of hepatic histones (p< 0.05), as compared to a control diet. These changes were associated with 1.5-3-fold increased turnover rates of histones without any change in acetyl-CoA flux. Acetylation significantly reduced the stability of histones and the turnover rates of acetylated peptides were correlated with the number of acetyl groups in neighboring lysine sites. We conclude that ²H₂O-method can be used to study metabolically controlled histone acetylation and acetylated histone turnover in vivo.

Inheritance of Histone (H3/H4): A Binary Choice?

Histones carry information in the form of post-translational modifications (PTMs). For this information to be propagated through cell cycles, parental histones and their PTMs need to be maintained at the same genomic locations. Yet, during DNA replication, every nucleosome in the genome is disrupted to allow passage of the replisome. Recent data have identified histone chaperone activities that are intrinsic components of the replisome and implicate them in maintaining parental histones during DNA replication. We propose that structural and kinetic coordination between DNA replication and replisome-associated histone chaperone activities ensures positional inheritance of histones and their PTMs. When this coordination is perturbed, histones may instead be recycled to random genomic locations by alternative histone chaperones.

Mass Spectrometry to Study Chromatin Compaction

Chromatin accessibility is a major regulator of gene expression. Histone writers/erasers have a critical role in chromatin compaction, as they “flag” chromatin regions by catalyzing/removing covalent post-translational modifications on histone proteins. Anomalous chromatin decondensation is a common phenomenon in cells experiencing aging and viral infection. Moreover, about 50% of cancers have mutations in enzymes regulating chromatin state. Numerous genomics methods have evolved to characterize chromatin state, but the analysis of (in)accessible chromatin from the protein perspective is not yet in the spotlight. We present an overview of the most used approaches to generate data on chromatin accessibility and then focus on emerging methods that utilize mass spectrometry to quantify the accessibility of histones and the rest of the chromatin bound proteome. Mass spectrometry is currently the method of choice to quantify entire proteomes in an unbiased large-scale manner; accessibility on chromatin of proteins and protein modifications adds an extra quantitative layer to proteomics dataset that assist more informed data-driven hypotheses in chromatin biology. We speculate that this emerging new set of methods will enhance predictive strength on which proteins and histone modifications are critical in gene regulation, and which proteins occupy different chromatin states in health and disease.

Application of Recombination -Induced Tag Exchange (RITE) to study histone dynamics in human cells

In eukaryotes, nucleosomes form a barrier to DNA templated reactions and must be dynamically disrupted to provide access to the genome. During nucleosome (re)assembly, histones can be replaced by new histones, erasing post-translational modifications. Measuring histone turnover in mammalian cells has mostly relied on inducible overexpression of histones, which may influence and distort natural histone deposition rates. We have previously used recombination-induced tag exchange (RITE) to study histone dynamics in budding yeast. RITE is a method to follow protein turnover by genetic switching of epitope tags using Cre recombinase and does not rely on inducible overexpression. Here, we applied RITE to study the dynamics of the replication-independent histone variant H3.3 in human cells. Epitope tag-switching could be readily detected upon induction of Cre-recombinase, enabling the monitoring old and new H3.3 in the same pool of cells. However, the rate of tag-switching was lower than in yeast cells. Analysis of histone H3.3 incorporation by chromatin immunoprecipitation did not recapitulate previously reported aspects of H3.3 dynamics such as high turnover rates in active promoters and enhancers. We hypothesize that asynchronous Cre-mediated DNA recombination in the cell population leads to a low time resolution of the H3.3-RITE system in human cells. We conclude that RITE enables the detection of old and new proteins in human cells and that the time-scale of tag-switching prevents the capture of high turnover events in a population of cells. Instead, RITE might be more suited for tracking long-lived histone proteins in human cells.

A mass spectrometry-based assay using metabolic labeling to rapidly monitor chromatin accessibility of modified histone proteins

Histone post-translational modifications (PTMs) contribute to chromatin accessibility due to their chemical properties and their ability to recruit enzymes responsible for DNA readout and chromatin remodeling. To date, more than 400 different histone PTMs and thousands of combinations of PTMs have been identified, the vast majority with still unknown biological function. Identification and quantification of histone PTMs has become routine in mass spectrometry (MS) but, since raising antibodies for each PTM in a study can be prohibitive, lots of potential is lost from MS datasets when uncharacterized PTMs are found to be significantly regulated. We developed an assay that uses metabolic labeling and MS to associate chromatin accessibility with histone PTMs and their combinations. The labeling is achieved by spiking in the cell media a 5x concentration of stable isotope labeled arginine and allow cells to grow for at least one cell cycle. We quantified the labeling incorporation of about 200 histone peptides with a proteomics workflow, and we confirmed that peptides carrying PTMs with extensively characterized roles in active transcription or gene silencing were in highly or poorly labeled forms, respectively. Data were further validated using next-generation sequencing to assess the transcription rate of chromatin regions modified with five selected PTMs. Furthermore, we quantified the labeling rate of peptides carrying co-existing PTMs, proving that this method is suitable for combinatorial PTMs. We focus on the abundant bivalent mark H3K27me3K36me2, showing that H3K27me3 dominantly represses histone swapping rate even in the presence of the more permissive PTM H3K36me2. Together, we envision this method will help to generate hypotheses regarding histone PTM functions and, potentially, elucidate the role of combinatorial histone codes.

The nucleosome core particle remembers its position through DNA replication and RNA transcription

Nucleosomes are the fundamental structural unit of chromatin. In addition to stabilizing the DNA polymer, nucleosomes are modified in ways that reflect and affect gene expression in their vicinity. It has long been assumed that nucleosomes can transmit memory of gene expression through their covalent posttranslational modifications. An unproven assumption of this model, which is essential to most models of epigenetic inheritance, is that a nucleosome present at a locus reoccupies the same locus after DNA replication. We tested this assumption by nucleating a synthetic chromatin domain in vivo, in which ∼4 nucleosomes at an arbitrary locus were covalently labeled with biotin. We tracked the fate of labeled nucleosomes through DNA replication, and established that nucleosomes present at a locus remembered their position during DNA replication. The replication-associated histone chaperones Dpb3 and Mcm2 were essential for nucleosome position memory, and in the absence of both Dpb3 and Mcm2 histone chaperone activity, nucleosomes did not remember their position. Using the same approach, we tested the model that transcription results in retrograde transposition of nucleosomes along a transcription unit. We found no evidence of retrograde transposition. Our results suggest that nucleosomes have the capacity to transmit epigenetic memory across mitotic generations with exquisite spatial fidelity.

Conserved crosstalk between histone deacetylation and H3K79 methylation generates DOT1L-dose dependency in HDAC1-deficient thymic lymphoma

DOT1L methylates histone H3K79 and is aberrantly regulated in MLL-rearranged leukemia. Inhibitors have been developed to target DOT1L activity in leukemia, but cellular mechanisms that regulate DOT1L are still poorly understood. We have identified the histone deacetylase Rpd3 as a negative regulator of budding yeast Dot1. At its target genes, the transcriptional repressor Rpd3 restricts H3K79 methylation, explaining the absence of H3K79me3 at a subset of genes in the yeast genome. Similar to the crosstalk in yeast, inactivation of the murine Rpd3 homolog HDAC1 in thymocytes led to an increase in H3K79 methylation. Thymic lymphomas that arise upon genetic deletion of Hdac1 retained the increased H3K79 methylation and were sensitive to reduced DOT1L dosage. Furthermore, cell lines derived from Hdac1Δ/Δ thymic lymphomas were sensitive to a DOT1L inhibitor, which induced apoptosis. In summary, we identified an evolutionarily conserved crosstalk between HDAC1 and DOT1L with impact in murine thymic lymphoma development.

Monoallelic expression and epigenetic inheritance sustained by a Trypanosoma brucei variant surface glycoprotein exclusion complex

The largest gene families in eukaryotes are subject to allelic exclusion, but mechanisms underpinning single allele selection and inheritance remain unclear. Here, we describe a protein complex sustaining variant surface glycoprotein (VSG) allelic exclusion and antigenic variation in Trypanosoma brucei parasites. The VSG-exclusion-1 (VEX1) protein binds both telomeric VSG-associated chromatin and VEX2, an ortholog of nonsense-mediated-decay helicase, UPF1. VEX1 and VEX2 assemble in an RNA polymerase-I transcription-dependent manner and sustain the active, subtelomeric VSG-associated transcription compartment. VSG transcripts and VSG coats become highly heterogeneous when VEX proteins are depleted. Further, the DNA replication-associated chromatin assembly factor, CAF-1, binds to and specifically maintains VEX1 compartmentalisation following DNA replication. Thus, the VEX-complex controls VSG-exclusion, while CAF-1 sustains VEX-complex inheritance in association with the active-VSG. Notably, the VEX2-orthologue and CAF-1 in mammals are also implicated in exclusion and inheritance functions. In trypanosomes, these factors sustain a highly effective and paradigmatic immune evasion strategy.

Yeast epigenetics: the inheritance of histone modification states

Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) are two of the most recognised and well-studied model systems for epigenetic regulation and the inheritance of chromatin states. Their silent loci serve as a proxy for heterochromatic chromatin in higher eukaryotes, and as such both species have provided a wealth of information on the mechanisms behind the establishment and maintenance of epigenetic states, not only in yeast, but in higher eukaryotes. This review focuses specifically on the role of histone modifications in governing telomeric silencing in S. cerevisiae and centromeric silencing in S. pombe as examples of genetic loci that exemplify epigenetic inheritance. We discuss the recent advancements that for the first time provide a mechanistic understanding of how heterochromatin, dictated by histone modifications specifically, is preserved during S-phase. We also discuss the current state of our understanding of yeast nucleosome dynamics during DNA replication, an essential component in delineating the contribution of histone modifications to epigenetic inheritance.

The histone chaperoning pathway: from ribosome to nucleosome

Nucleosomes represent the fundamental repeating unit of eukaryotic DNA, and comprise eight core histones around which DNA is wrapped in nearly two superhelical turns. Histones do not have the intrinsic ability to form nucleosomes; rather, they require an extensive repertoire of interacting proteins collectively known as ‘histone chaperones’. At a fundamental level, it is believed that histone chaperones guide the assembly of nucleosomes through preventing non-productive charge-based aggregates between the basic histones and acidic cellular components. At a broader level, histone chaperones influence almost all aspects of chromatin biology, regulating histone supply and demand, governing histone variant deposition, maintaining functional chromatin domains and being co-factors for histone post-translational modifications, to name a few. In this essay we review recent structural insights into histone-chaperone interactions, explore evidence for the existence of a histone chaperoning ‘pathway’ and reconcile how such histone-chaperone interactions may function thermodynamically to assemble nucleosomes and maintain chromatin homeostasis.

Transcription-driven chromatin repression of Intragenic transcription start sites

Progression of RNA polymerase II (RNAPII) transcription relies on the appropriately positioned activities of elongation factors. The resulting profile of factors and chromatin signatures along transcription units provides a “positional information system” for transcribing RNAPII. Here, we investigate a chromatin-based mechanism that suppresses intragenic initiation of RNAPII transcription. We demonstrate that RNAPII transcription across gene promoters represses their function in plants. This repression is characterized by reduced promoter-specific molecular signatures and increased molecular signatures associated with RNAPII elongation. The conserved FACT histone chaperone complex is required for this repression mechanism. Genome-wide Transcription Start Site (TSS) mapping reveals thousands of discrete intragenic TSS positions in fact mutants, including downstream promoters that initiate alternative transcript isoforms. We find that histone H3 lysine 4 mono-methylation (H3K4me1), an Arabidopsis RNAPII elongation signature, is enriched at FACT-repressed intragenic TSSs. Our analyses suggest that FACT is required to repress intragenic TSSs at positions that are in part characterized by elevated H3K4me1 levels. In sum, conserved and plant-specific chromatin features correlate with the co-transcriptional repression of intragenic TSSs. Our insights into TSS repression by RNAPII transcription promise to inform the regulation of alternative transcript isoforms and the characterization of gene regulation through the act of pervasive transcription across eukaryotic genomes.

Genes represent DNA elements that are transcribed into mRNA. However, the position where transcription actually starts can be dynamically regulated to expand the diversity of RNA isoforms produced from a single gene. Functionally, alternative Transcription Start Sites (TSSs) may generate protein isoforms with differing N-terminal regions and distinct cellular functions. In plants, light signaling regulates protein isoforms largely through regulated TSS selection, emphasizing the biological significance of this mechanism. Despite the importance of alternative TSS selection, little is known about the underlying molecular mechanisms. Here, we characterize for the first time how transcription initiation from an upstream promoter represses alternative downstream promoter activity in plants. This repression mechanism is associated with chromatin changes that are required to maintain precise gene expression control. Specific chromatin signatures are established during transcription via dynamic interactions between the transcription machinery and associated factors. The conserved histone chaperone complex FACT is one such factor involved in regulating the chromatin environment along genes during transcription. We find that mutant plants with reduced FACT activity specifically initiate transcription from thousands of intragenic positions, thus expanding RNA isoform diversity. Overall, our study reveals conserved and plant-specific chromatin features associated with the co-transcriptional repression of downstream intragenic TSSs. These findings promise to help inform the molecular mechanism underlying environmentally-triggered TSS regulation in plants.

Nucleosome Turnover Regulates Histone Methylation Patterns over the Genome

Recent studies have indicated that nucleosome turnover is rapid, occurring several times per cell cycle. To access the effect of nucleosome turnover on the epigenetic landscape, we investigated H3K79 methylation, which is produced by a single methyltransferase (Dot1l) with no known demethylase. Using chemical-induced proximity (CIP), we find that the valency of H3K79 methylation (mono-, di-, and tri-) is determined by nucleosome turnover rates. Furthermore, propagation of this mark is predicted by nucleosome turnover simulations over the genome and accounts for the asymmetric distribution of H3K79me toward the transcriptional unit. More broadly, a meta-analysis of other conserved histone modifications demonstrates that nucleosome turnover models predict both valency and chromosomal propagation of methylation marks. Based on data from worms, flies, and mice, we propose that the turnover of modified nucleosomes is a general means of propagation of epigenetic marks and a determinant of methylation valence.

Mechanistic insights into histone deposition and nucleosome assembly by the chromatin assembly factor-1

Eukaryotic chromatin is a highly dynamic structure with essential roles in virtually all DNA-dependent cellular processes. Nucleosomes are a barrier to DNA access, and during DNA replication, they are disassembled ahead of the replication machinery (the replisome) and reassembled following its passage. The Histone chaperone Chromatin Assembly Factor-1 (CAF-1) interacts with the replisome and deposits H3-H4 directly onto newly synthesized DNA. Therefore, CAF-1 is important for the establishment and propagation of chromatin structure. The molecular mechanism by which CAF-1 mediates H3-H4 deposition has remained unclear. However, recent studies have revealed new insights into the architecture and stoichiometry of the trimeric CAF-1 complex and how it interacts with and deposits H3-H4 onto substrate DNA. The CAF-1 trimer binds to a single H3-H4 dimer, which induces a conformational rearrangement in CAF-1 promoting its interaction with substrate DNA. Two CAF-1•H3-H4 complexes co-associate on nucleosome-free DNA depositing (H3-H4)2 tetramers in the first step of nucleosome assembly. Here, we review the progress made in our understanding of CAF-1 structure, mechanism of action, and how CAF-1 contributes to chromatin dynamics during DNA replication.

Accurate Recycling of Parental Histones Reproduces the Histone Modification Landscape during DNA Replication

Chromatin organization is disrupted genome-wide during DNA replication. On newly synthesized DNA, nucleosomes are assembled from new naive histones and old modified histones. It remains unknown whether the landscape of histone post-translational modifications (PTMs) is faithfully copied during DNA replication or the epigenome is perturbed. Here we develop chromatin occupancy after replication (ChOR-seq) to determine histone PTM occupancy immediately after DNA replication and across the cell cycle. We show that H3K4me3, H3K36me3, H3K79me3, and H3K27me3 positional information is reproduced with high accuracy on newly synthesized DNA through histone recycling. Quantitative ChOR-seq reveals that de novo methylation to restore H3K4me3 and H3K27me3 levels occurs across the cell cycle with mark- and locus-specific kinetics. Collectively, this demonstrates that accurate parental histone recycling preserves positional information and allows PTM transmission to daughter cells while modification of new histones gives rise to complex epigenome fluctuations across the cell cycle that could underlie cell-to-cell heterogeneity.

A mechanism for preventing asymmetric histone segregation onto replicating DNA strands

How parental histone (H3-H4)2 tetramers, the primary carriers of epigenetic modifications, are transferred onto leading and lagging strands of DNA replication forks for epigenetic inheritance remains elusive. Here we show that parental (H3-H4)2 tetramers are assembled into nucleosomes onto both leading and lagging strands, with a slight preference for lagging strands. The lagging-strand preference increases markedly in budding yeast cells lacking Dpb3 and Dpb4, two subunits of the leading strand DNA polymerase, Pol ε, owing to the impairment of parental (H3-H4)2 transfer to leading strands. Dpb3-Dpb4 binds H3-H4 in vitro and participates in the inheritance of heterochromatin. These results indicate that different proteins facilitate the transfer of parental (H3-H4)2 onto leading versus lagging strands and that Dbp3-Dpb4 plays an important role in this poorly understood process.

Evidence for the nuclear import of histones H3.1 and H4 as monomers

Newly synthesised histones are thought to dimerise in the cytosol and undergo nuclear import in complex with histone chaperones. Here, we provide evidence that human H3.1 and H4 are imported into the nucleus as monomers. Using a tether-and-release system to study the import dynamics of newly synthesised histones, we find that cytosolic H3.1 and H4 can be maintained as stable monomeric units. Cytosolically tethered histones are bound to importin-alpha proteins (predominantly IPO4), but not to histone-specific chaperones NASP, ASF1a, RbAp46 (RBBP7) or HAT1, which reside in the nucleus in interphase cells. Release of monomeric histones from their cytosolic tether results in rapid nuclear translocation, IPO4 dissociation and incorporation into chromatin at sites of replication. Quantitative analysis of histones bound to individual chaperones reveals an excess of H3 specifically associated with sNASP, suggesting that NASP maintains a soluble, monomeric pool of H3 within the nucleus and may act as a nuclear receptor for newly imported histone. In summary, we propose that histones H3 and H4 are rapidly imported as monomeric units, forming heterodimers in the nucleus rather than the cytosol.

Histone H2A-H2B binding by Pol α in the eukaryotic replisome contributes to the maintenance of repressive chromatin

The eukaryotic replisome disassembles parental chromatin at DNA replication forks, but then plays a poorly understood role in the re‐deposition of the displaced histone complexes onto nascent DNA. Here, we show that yeast DNA polymerase α contains a histone‐binding motif that is conserved in human Pol α and is specific for histones H2A and H2B. Mutation of this motif in budding yeast cells does not affect DNA synthesis, but instead abrogates gene silencing at telomeres and mating‐type loci. Similar phenotypes are produced not only by mutations that displace Pol α from the replisome, but also by mutation of the previously identified histone‐binding motif in the CMG helicase subunit Mcm2, the human orthologue of which was shown to bind to histones H3 and H4. We show that chromatin‐derived histone complexes can be bound simultaneously by Mcm2, Pol α and the histone chaperone FACT that is also a replisome component. These findings indicate that replisome assembly unites multiple histone‐binding activities, which jointly process parental histones to help preserve silent chromatin during the process of chromosome duplication.

Ccp1 modulates epigenetic stability at centromeres and affects heterochromatin distribution in Schizosaccharomyces pombe

Distinct chromatin organization features, such as centromeres and heterochromatin domains, are inherited epigenetically. However, the mechanisms that modulate the accuracy of epigenetic inheritance, especially at the individual nucleosome level, are not well-understood. Here, using ChIP and next-generation sequencing (ChIP-Seq), we characterized Ccp1, a homolog of the histone chaperone Vps75 in budding yeast that functions in centromere chromatin duplication and heterochromatin maintenance in fission yeast (Schizosaccharomyces pombe). We show that Ccp1 is enriched at the central core regions of the centromeres. Of note, among all histone chaperones characterized, deletion of the ccp1 gene uniquely reduced the rate of epigenetic switching, manifested as position effect variegation within the centromeric core region (CEN-PEV). In contrast, gene deletion of other histone chaperones either elevated the PEV switching rates or did not affect centromeric PEV. Ccp1 and the kinetochore components Mis6 and Sim4 were mutually dependent for centromere or kinetochore association at the proper levels. Moreover, Ccp1 influenced heterochromatin distribution at multiple loci in the genome, including the subtelomeric and the pericentromeric regions. We also found that Gar2, a protein predominantly enriched in the nucleolus, functions similarly to Ccp1 in modulating the epigenetic stability of centromeric regions, although its mechanism remained unclear. Together, our results identify Ccp1 as an important player in modulating epigenetic stability and maintaining proper organization of multiple chromatin domains throughout the fission yeast genome.

Structure and function of the histone chaperone FACT - Resolving FACTual issues

FAcilitates Chromatin Transcription (FACT) has been considered essential for transcription through chromatin mostly based on cell-free experiments. However, FACT inactivation in cells does not cause a significant reduction in transcription. Moreover, not all mammalian cells require FACT for viability. Here we synthesize information from different organisms to reveal the core function(s) of FACT and propose a model that reconciles the cell-free and cell-based observations. We describe FACT structure and nucleosomal interactions, and their roles in FACT-dependent transcription, replication and repair. The variable requirements for FACT among different tumor and non-tumor cells suggest that various FACT-dependent processes have significantly different levels of relative importance in different eukaryotic cells. We propose that the stability of chromatin, which might vary among different cell types, dictates these diverse requirements for FACT to support cell viability. Since tumor cells are among the most sensitive to FACT inhibition, this vulnerability could be exploited for cancer treatment.

Inheritance of Histones H3 and H4 during DNA Replication In Vitro

Nucleosomes are believed to carry epigenetic information through the cell cycle, including through DNA replication. It has been known for decades that parental histones are reassembled on newly replicated chromatin, but the mechanisms underlying histone inheritance and dispersal during DNA replication are not fully understood. We monitored the fate of histones H3 or H4 from a single nucleosome through DNA replication in two in vitro systems. In the SV40 system, histones assembled on a single nucleosome positioning sequence can be inherited by their own daughter DNA but are dispersed from their original location. In Xenopus laevis extracts, histones are dynamic, and nucleosomes are repositioned independent of and prior to DNA replication. Nevertheless, a high fraction of histones H3 and H4 that are inherited through DNA replication remains near its starting location. Thus, inheritance of histone proteins and their dispersal can be mechanistically uncoupled.

Coupling of replisome movement with nucleosome dynamics can contribute to the parent-daughter information transfer

Positioning of nucleosomes along the genomic DNA is crucial for many cellular processes that include gene regulation and higher order packaging of chromatin. The question of how nucleosome-positioning information from a parent chromatin gets transferred to the daughter chromatin is highly intriguing. Accounting for experimentally known coupling between replisome movement and nucleosome dynamics, we propose a model that can obtain de novo nucleosome assembly similar to what is observed in recent experiments. Simulating nucleosome dynamics during replication, we argue that short pausing of the replication fork, associated with nucleosome disassembly, can be a event crucial for communicating nucleosome positioning information from parent to daughter. We show that the interplay of timescales between nucleosome disassembly (τ_p) at the replication fork and nucleosome sliding behind the fork (τ_s) can give rise to a rich ‘phase diagram’ having different inherited patterns of nucleosome organization. Our model predicts that only when τ_p ≥ τ_s the daughter chromatin can inherit nucleosome positioning of the parent.

Constitutive centromere-associated network contacts confer differential stability on CENP-A nucleosomes in vitro and in the cell

Eukaryotic centromeres are defined by the presence of nucleosomes containing the histone H3 variant, centromere protein A (CENP-A). Once incorporated at centromeres, CENP-A nucleosomes are remarkably stable, exhibiting no detectable loss or exchange over many cell cycles. It is currently unclear whether this stability is an intrinsic property of CENP-A containing chromatin or whether it arises from proteins that specifically associate with CENP-A chromatin. Two proteins, CENP-C and CENP-N, are known to bind CENP-A human nucleosomes directly. Here we test the hypothesis that CENP-C or CENP-N stabilize CENP-A nucleosomes in vitro and in living cells. We show that CENP-N stabilizes CENP-A nucleosomes alone and additively with CENP-C in vitro. However, removal of CENP-C and CENP-N from cells, or mutating CENP-A so that it no longer interacts with CENP-C or CENP-N, had no effect on centromeric CENP-A stability in vivo. Thus, the stability of CENP-A nucleosomes in chromatin does not arise solely from its interactions with CENP-C or CENP-N.

time-ChIP: A Method to Determine Long-Term Locus-Specific Nucleosome Inheritance

Understanding chromatin dynamics is essential to define the contribution of chromatin to heritable gene silencing and the long-term maintenance of gene expression. Here we present a detailed protocol for time-ChIP, a novel method to measure histone turnover at high resolution across long timescales. This method is based on the SNAP-tag, a self-labeling enzyme that can be pulse labeled with small molecules in cells. Upon pulse biotinylation of a cohort of SNAP-tagged histones we can determine their abundance and fate across a chase period using a biotin-specific chromatin pulldown followed by DNA sequencing or quantitative PCR. This method is unique in its ability to trace the long-term fate of a chromatin bound histone pool, genome wide. In addition to a step by step protocol, we outline advantages and limitations of the method in relation to other existing techniques. time-ChIP can define regions of high and low histone turnover and identify the location of pools of long lived histones.

Replication-Coupled Nucleosome Assembly in the Passage of Epigenetic Information and Cell Identity

During S phase, replicated DNA must be assembled into nucleosomes using both newly synthesized and parental histones in a process that is tightly coupled to DNA replication. This DNA replication-coupled process is regulated by multitude of histone chaperones as well as by histone-modifying enzymes. In recent years novel insights into nucleosome assembly of new H3-H4 tetramers have been gained through studies on the classical histone chaperone CAF-1 and the identification of novel factors involved in this process. Moreover, in vitro reconstitution of chromatin replication has shed light on nucleosome assembly of parental H3-H4, a process that remains elusive. Finally, recent studies have revealed that the replication-coupled nucleosome assembly is important for the determination and maintenance of cell fate in multicellular organisms.

Determinants of Histone H3K4 Methylation Patterns

Various factors differentially recognize trimethylated histone H3 lysine 4 (H3K4me3) near promoters, H3K4me2 just downstream, and promoter-distal H3K4me1 to modulate gene expression. This methylation "gradient" is thought to result from preferential binding of the H3K4 methyltransferase Set1/complex associated with Set1 (COMPASS) to promoter-proximal RNA polymerase II. However, other studies have suggested that location-specific cues allosterically activate Set1. Chromatin immunoprecipitation sequencing (ChIP-seq) experiments show that H3K4 methylation patterns on active genes are not universal or fixed and change in response to both transcription elongation rate and frequency as well as reduced COMPASS activity. Fusing Set1 to RNA polymerase II results in H3K4me2 throughout transcribed regions and similarly extended H3K4me3 on highly transcribed genes. Tethered Set1 still requires histone H2B ubiquitylation for activity. These results show that higher-level methylations reflect not only Set1/COMPASS recruitment but also multiple rounds of transcription. This model provides a simple explanation for non-canonical methylation patterns at some loci or in certain COMPASS mutants.

Replication stress affects the fidelity of nucleosome-mediated epigenetic inheritance

The fidelity of epigenetic inheritance or, the precision by which epigenetic information is passed along, is an essential parameter for measuring the effectiveness of the process. How the precision of the process is achieved or modulated, however, remains largely elusive. We have performed quantitative measurement of epigenetic fidelity, using position effect variegation (PEV) in Schizosaccharomyces pombe as readout, to explore whether replication perturbation affects nucleosome-mediated epigenetic inheritance. We show that replication stresses, due to either hydroxyurea treatment or various forms of genetic lesions of the replication machinery, reduce the inheritance accuracy of CENP-A/Cnp1 nucleosome positioning within centromere. Mechanistically, we demonstrate that excessive formation of single-stranded DNA, a common molecular abnormality under these conditions, might have correlation with the reduction in fidelity of centromeric chromatin duplication. Furthermore, we show that replication stress broadly changes chromatin structure at various loci in the genome, such as telomere heterochromatin expanding and mating type locus heterochromatin spreading out of the boundaries. Interestingly, the levels of inheritable expanding at sub-telomeric heterochromatin regions are highly variable among independent cell populations. Finally, we show that HU treatment of the multi-cellular organisms C. elegans and D. melanogaster affects epigenetically programmed development and PEV, illustrating the evolutionary conservation of the phenomenon. Replication stress, in addition to its demonstrated role in genetic instability, promotes variable epigenetic instability throughout the epigenome.

In this study, we found replication stresses reduce the fidelity of nucleosome-mediated epigenetic inheritance. Using Position Effect Variegation (PEV) in centromere as an indicator of chromatin epigenetic stability, we quantified the precision of nucleosomal inheritance and found replication stresses reduce the fidelity of nucleosome-mediated epigenetic inheritance. Further analysis of genome-wide heterochromatin distribution showed that replication stresses affect chromatin structure by expanding of heterochromatin with locus specificity. Mechanistically, we provide evidence suggesting that excessive formation of single-stranded DNA might have correlation with the reduction in fidelity of centromeric chromatin duplication. Finally, we demonstrated replication stress perturb the development process by reducing the fidelity of chromatin organization duplication in fruit fly and worm, illustrating the broadness and the evolutionary conservation of the phenomenon. Together, our results shed light on the importance of replication stresses cause epigenetic instability in addition to genetic stability.

Chromosome Duplication in Saccharomyces cerevisiae

The accurate and complete replication of genomic DNA is essential for all life. In eukaryotic cells, the assembly of the multi-enzyme replisomes that perform replication is divided into stages that occur at distinct phases of the cell cycle. Replicative DNA helicases are loaded around origins of DNA replication exclusively during G₁ phase. The loaded helicases are then activated during S phase and associate with the replicative DNA polymerases and other accessory proteins. The function of the resulting replisomes is monitored by checkpoint proteins that protect arrested replisomes and inhibit new initiation when replication is inhibited. The replisome also coordinates nucleosome disassembly, assembly, and the establishment of sister chromatid cohesion. Finally, when two replisomes converge they are disassembled. Studies in Saccharomyces cerevisiae have led the way in our understanding of these processes. Here, we review our increasingly molecular understanding of these events and their regulation.

DNA sequence-dependent epigenetic inheritance of gene silencing and histone H3K9 methylation

Epigenetic inheritance mechanisms play fundamental roles in maintaining cellular memory of gene expression states. In fission yeast, histone H3 lysine 9 (H3K9) is methylated (H3K9me) at heterochromatic domains. These domains can be epigenetically inherited when epe1⁺ , encoding an enzyme that promotes H3K9 demethylation, is deleted. How native epigenetic states are stably maintained in epe1⁺ cells remains unknown. Here, we developed a system to examine the role of DNA sequence and genomic context in propagation of a cis-heritable H3K9me-dependent silenced state. We show that in epe1⁺ cells, in addition to sequence-independent mechanisms that propagate H3K9me, epigenetic inheritance of silencing requires binding sites for sequence-dependent activating transcription factor (ATF)-adenosine 3',5'-monophosphate (cAMP) response element-binding protein (CREB) family transcription factors within their native chromosomal context. Thus, specific DNA sequences contribute to cis inheritance of H3K9me and silent epigenetic states.

Chromatin replication: TRANSmitting the histone code

Efficient overcoming of the nucleosomal barrier and accurate maintenance of associated histone marks during chromatin replication are essential for normal functioning of the cell. Recent studies revealed new protein factors and histone modifications contributing to overcoming the nucleosomal barrier, and suggested an important role for DNA looping in survival of the original histones during replication. These studies suggest new possible mechanisms for transmitting the histone code to next generations of cells.