The Histone Chaperone FACT Contributes to DNA Replication-Coupled Nucleosome Assembly

ANP32E drives vulnerability to ATR inhibitors by inducing R-loops-dependent transcription replication conflicts in triple negative breast cancer

Oncogene-induced replicative stress (RS) drives tumor progression by disrupting genome stability, primarily through transcription-replication conflicts (TRCs), which promote R-loop accumulation and trigger the DNA damage response (DDR). In this study, we investigate the role of chromatin regulators in exacerbating TRCs and R-loop accumulation in cancer. We find that in breast cancer patients, the simultaneous upregulation of MYC and the H2A.Z-specific chaperone ANP32E correlates with increased genomic instability. Genome-wide analyses reveal that ANP32E-driven H2A.Z turnover alters RNA polymerase II processivity, leading to the accumulation of long R-loops at TRC sites. Furthermore, we show that ANP32E overexpression enhances TRC formation and activates an ATR-dependent DDR, predisposing cancer cells to R-loop-mediated genomic fragility. By exploiting the vulnerability of ANP32E-expressing cancer cells to ATR inhibitors, we find that tumors relied on this DDR pathway, whose inhibition halts their pro-metastatic capacity. These findings identify ANP32E as a key driver of TRC-induced genomic instability, indicating ATR inhibition as a potential therapeutic strategy for ANP32E-overexpressing tumors.

A Rfa1-MN-based system reveals new factors involved in the rescue of broken replication forks

The integrity of the replication forks is essential for an accurate and timely completion of genome duplication. However, little is known about how cells deal with broken replication forks. We have generated in yeast a system based on a chimera of the largest subunit of the ssDNA binding complex RPA fused to the micrococcal nuclease (Rfa1-MN) to induce double-strand breaks (DSBs) at replication forks and searched for mutants affected in their repair. Our results show that the core homologous recombination (HR) proteins involved in the formation of the ssDNA/Rad51 filament are essential for the repair of DSBs at forks, whereas non-homologous end joining plays no role. Apart from the endonucleases Mus81 and Yen1, the repair process employs fork-associated HR factors, break-induced replication (BIR)-associated factors and replisome components involved in sister chromatid cohesion and fork stability, pointing to replication fork restart by BIR followed by fork restoration. Notably, we also found factors controlling the length of G1, suggesting that a minimal number of active origins facilitates the repair by converging forks. Our study has also revealed a requirement for checkpoint functions, including the synthesis of Dun1-mediated dNTPs. Finally, our screening revealed minimal impact from the loss of chromatin factors, suggesting that the partially disassembled nucleosome structure at the replication fork facilitates the accessibility of the repair machinery. In conclusion, this study provides an overview of the factors and mechanisms that cooperate to repair broken forks.

Roles for the 3D genome in the cell cycle, DNA replication, and double strand break repair

Large eukaryotic genomes are packaged into the restricted area of the nucleus to protect the genetic code and provide a dedicated environment to read, copy and repair DNA. The physical organisation of the genome into chromatin loops and self-interacting domains provides the basic structural units of genome architecture. These structural arrangements are complex, multi-layered, and highly dynamic and influence how different regions of the genome interact. The role of chromatin structures during transcription via enhancer-promoter interactions is well established. Less understood is how nuclear architecture influences the plethora of chromatin transactions during DNA replication and repair. In this review, we discuss how genome architecture is regulated during the cell cycle to influence the positioning of replication origins and the coordination of DNA double strand break repair. The role of genome architecture in these cellular processes highlights its critical involvement in preserving genome integrity and cancer prevention.

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.

The MCM2-7 Complex: Roles beyond DNA Unwinding

The MCM2-7 complex is a hexameric protein complex that serves as a DNA helicase. It unwinds the DNA double helix during DNA replication, thereby providing the single-stranded replication template. In recent years, it has become clear that the MCM2-7 complex has additional functions that extend well beyond its role in DNA replication. Through physical and functional interactions with different pathways, it impacts other nuclear events and activities, including folding of the genome, histone inheritance, chromosome segregation, DNA damage sensing and repair, and gene transcription. Collectively, the diverse roles of the MCM2-7 complex suggest it plays a critical role in maintaining genome integrity by integrating the regulation of DNA replication with other pathways in the nucleus.

A fluorescent assay for cryptic transcription in Saccharomyces cerevisiae reveals novel insights into factors that stabilize chromatin structure on newly replicated DNA

The disruption of chromatin structure can result in transcription initiation from cryptic promoters within gene bodies. While the passage of RNA polymerase II is a well-characterized chromatin-disrupting force, numerous factors, including histone chaperones, normally stabilize chromatin on transcribed genes, thereby repressing cryptic transcription. DNA replication, which employs a partially overlapping set of histone chaperones, is also inherently disruptive to chromatin, but a role for DNA replication in cryptic transcription has never been examined. In this study, we tested the hypothesis that, in the absence of chromatin-stabilizing factors, DNA replication can promote cryptic transcription in Saccharomyces cerevisiae. Using a novel fluorescent reporter assay, we show that multiple factors, including Asf1, CAF-1, Rtt106, Spt6, and FACT, block transcription from a cryptic promoter, but are entirely or partially dispensable in G1-arrested cells, suggesting a requirement for DNA replication in chromatin disruption. Collectively, these results demonstrate that transcription fidelity is dependent on numerous factors that function to assemble chromatin on nascent DNA.

Nucleosome Dynamics Derived at the Single-Molecule Level Bridges Its Structures and Functions

Nucleosome, the building block of chromatin, plays pivotal roles in all DNA-related processes. While cryogenic-electron microscopy (cryo-EM) has significantly advanced our understanding of nucleosome structures, the emerging field of single-molecule force spectroscopy is illuminating their dynamic properties. This technique is crucial for revealing how nucleosome behavior is influenced by chaperones, remodelers, histone variants, and post-translational modifications, particularly in their folding and unfolding mechanisms under tension. Such insights are vital for deciphering the complex interplay in nucleosome assembly and structural regulation, highlighting the nucleosome's versatility in response to DNA activities. In this Perspective, we aim to consolidate the latest advancements in nucleosome dynamics, with a special focus on the revelations brought forth by single-molecule manipulation. Our objective is to highlight the insights gained from studying nucleosome dynamics through this innovative approach, emphasizing the transformative impact of single-molecule manipulation techniques in the field of chromatin research.

Specialized replication of heterochromatin domains ensures self-templated chromatin assembly and epigenetic inheritance

Heterochromatin, defined by histone H3 lysine 9 methylation (H3K9me), spreads across large domains and can be epigenetically inherited in a self-propagating manner. Heterochromatin propagation depends upon a read-write mechanism, where the Clr4/Suv39h methyltransferase binds to preexisting trimethylated H3K9 (H3K9me3) and further deposits H3K9me. How the parental methylated histone template is preserved during DNA replication is not well understood. Here, we demonstrate using Schizosaccharomyces pombe that heterochromatic regions are specialized replication domains demarcated by their surrounding boundary elements. DNA replication throughout these domains is distinguished by an abundance of replisome components and is coordinated by Swi6/HP1. Although mutations in the replicative helicase subunit Mcm2 that affect histone binding impede the maintenance of a heterochromatin domain at an artificially targeted ectopic site, they have only a modest impact on heterochromatin propagation via the read-write mechanism at an endogenous site. Instead, our findings suggest a crucial role for the replication factor Mcl1 in retaining parental histones and promoting heterochromatin propagation via a mechanism involving the histone chaperone FACT. Engagement of FACT with heterochromatin requires boundary elements, which position the heterochromatic domain at the nuclear peripheral subdomain enriched for heterochromatin factors. Our findings highlight the importance of replisome components and boundary elements in creating a specialized environment for the retention of parental methylated histones, which facilitates epigenetic inheritance of heterochromatin.

Pluripotency state transition of embryonic stem cells requires the turnover of histone chaperone FACT on chromatin

The differentiation of embryonic stem cells (ESCs) begins with the transition from the naive to the primed state. The formative state was recently established as a critical intermediate between the two states. Here, we demonstrate the role of the histone chaperone FACT in regulating the naive-to-formative transition. We found that the Q265K mutation in the FACT subunit SSRP1 increased the binding of FACT to histone H3-H4, impaired nucleosome disassembly in vitro, and reduced the turnover of FACT on chromatin in vivo. Strikingly, mouse ESCs harboring this mutation showed elevated naive-to-formative transition. Mechanistically, the SSRP1-Q265K mutation enriched FACT at the enhancers of formative-specific genes to increase targeted gene expression. Together, these findings suggest that the turnover of FACT on chromatin is crucial for regulating the enhancers of formative-specific genes, thereby mediating the naive-to-formative transition. This study highlights the significance of FACT in fine-tuning cell fate transition during early development.

Transcriptional regulation of FACT involves Coordination of chromatin accessibility and CTCF binding

Histone chaperone FACT (facilitates chromatin transcription) is well known to promote chromatin recovery during transcription. However, the mechanism how FACT regulates genome-wide chromatin accessibility and transcription factor binding has not been fully elucidated. Through loss-of-function studies, we show here that FACT component Ssrp1 is required for DNA replication and DNA damage repair and is also essential for progression of cell phase transition and cell proliferation in mouse embryonic fibroblast cells. On the molecular level, absence of the Ssrp1 leads to increased chromatin accessibility, enhanced CTCF binding, and a remarkable change in dynamic range of gene expression. Our study thus unequivocally uncovers a unique mechanism by which FACT complex regulates transcription by coordinating genome-wide chromatin accessibility and CTCF binding.

Mismatch Repair Protein Msh6Tt Is Necessary for Nuclear Division and Gametogenesis in Tetrahymena thermophila

DNA mismatch repair (MMR) improves replication accuracy by up to three orders of magnitude. The MutS protein in E. coli or its eukaryotic homolog, the MutSα (Msh2-Msh6) complex, recognizes base mismatches and initiates the mismatch repair mechanism. Msh6 is an essential protein for assembling the heterodimeric complex. However, the function of the Msh6 subunit remains elusive. Tetrahymena undergoes multiple DNA replication and nuclear division processes, including mitosis, amitosis, and meiosis. Here, we found that Msh6Tt localized in the macronucleus (MAC) and the micronucleus (MIC) during the vegetative growth stage and starvation. During the conjugation stage, Msh6Tt only localized in MICs and newly developing MACs. MSH6Tt knockout led to aberrant nuclear division during vegetative growth. The MSH6TtKO mutants were resistant to treatment with the DNA alkylating agent methyl methanesulfonate (MMS) compared to wild type cells. MSH6Tt knockout affected micronuclear meiosis and gametogenesis during the conjugation stage. Furthermore, Msh6Tt interacted with Msh2Tt and MMR-independent factors. Downregulation of MSH2Tt expression affected the stability of Msh6Tt. In addition, MSH6Tt knockout led to the upregulated expression of several MSH6Tt homologs at different developmental stages. Msh6Tt is involved in macronuclear amitosis, micronuclear mitosis, micronuclear meiosis, and gametogenesis in Tetrahymena.

The N-terminus of Spt16 anchors FACT to MCM2-7 for parental histone recycling

Parental histone recycling is vital for maintaining chromatin-based epigenetic information during replication, yet its underlying mechanisms remain unclear. Here, we uncover an unexpected role of histone chaperone FACT and its N-terminus of the Spt16 subunit during parental histone recycling and transfer in budding yeast. Depletion of Spt16 and mutations at its middle domain that impair histone binding compromise parental histone recycling on both the leading and lagging strands of DNA replication forks. Intriguingly, deletion of the Spt16-N domain impairs parental histone recycling, with a more pronounced defect observed on the lagging strand. Mechanistically, the Spt16-N domain interacts with the replicative helicase MCM2-7 and facilitates the formation of a ternary complex involving FACT, histone H3/H4 and Mcm2 histone binding domain, critical for the recycling and transfer of parental histones to lagging strands. Lack of the Spt16-N domain weakens the FACT-MCM interaction and reduces parental histone recycling. We propose that the Spt16-N domain acts as a protein-protein interaction module, enabling FACT to function as a shuttle chaperone in collaboration with Mcm2 and potentially other replisome components for efficient local parental histone recycling and inheritance.

Chromatin assembly factor-1 preserves genome stability in ctf4Δ cells by promoting sister chromatid cohesion

Chromatin assembly and the establishment of sister chromatid cohesion are intimately connected to the progression of DNA replication forks. Here we examined the genetic interaction between the heterotrimeric chromatin assembly factor-1 (CAF-1), a central component of chromatin assembly during replication, and the core replisome component Ctf4. We find that CAF-1 deficient cells as well as cells affected in newly-synthesized H3-H4 histones deposition during DNA replication exhibit a severe negative growth with ctf4Δ mutant. We dissected the role of CAF-1 in the maintenance of genome stability in ctf4Δ yeast cells. In the absence of CTF4, CAF-1 is essential for viability in cells experiencing replication problems, in cells lacking functional S-phase checkpoint or functional spindle checkpoint, and in cells lacking DNA repair pathways involving homologous recombination. We present evidence that CAF-1 affects cohesin association to chromatin in a DNA-damage-dependent manner and is essential to maintain cohesion in the absence of CTF4. We also show that Eco1-catalyzed Smc3 acetylation is reduced in absence of CAF-1. Furthermore, we describe genetic interactions between CAF-1 and essential genes involved in cohesin loading, cohesin stabilization, and cohesin component indicating that CAF-1 is crucial for viability when sister chromatid cohesion is affected. Finally, our data indicate that the CAF-1-dependent pathway required for cohesion is functionally distinct from the Rtt101-Mms1-Mms22 pathway which functions in replicated chromatin assembly. Collectively, our results suggest that the deposition by CAF-1 of newly-synthesized H3-H4 histones during DNA replication creates a chromatin environment that favors sister chromatid cohesion and maintains genome integrity.

Transcription and FACT facilitate the restoration of replication-coupled chromatin assembly defects

Genome duplication occurs through the coordinated action of DNA replication and nucleosome assembly at replication forks. Defective nucleosome assembly causes DNA lesions by fork breakage that need to be repaired. In addition, it causes a loss of chromatin integrity. These chromatin alterations can be restored, even though the mechanisms are unknown. Here, we show that the process of chromatin restoration can deal with highly severe chromatin defects induced by the absence of the chaperones CAF1 and Rtt106 or a strong reduction in the pool of available histones, and that this process can be followed by analyzing the topoisomer distribution of the 2µ plasmid. Using this assay, we demonstrate that chromatin restoration is slow and independent of checkpoint activation, whereas it requires the action of transcription and the FACT complex. Therefore, cells are able to "repair" not only DNA lesions but also chromatin alterations associated with defective nucleosome assembly.

The conserved histone chaperone Spt6 is strongly required for DNA replication and genome stability

Histone chaperones are an important class of proteins that regulate chromatin accessibility for DNA-templated processes. Spt6 is a conserved histone chaperone and key regulator of transcription and chromatin structure. However, its functions outside of these roles have been little explored. In this work, we demonstrate a requirement for S. cerevisiae Spt6 in DNA replication and, more broadly, as a regulator of genome stability. Depletion or mutation of Spt6 impairs DNA replication in vivo. Additionally, spt6 mutants are sensitive to DNA replication stress-inducing agents. Interestingly, this sensitivity is independent of the association of Spt6 with RNA polymerase II (RNAPII), suggesting that spt6 mutants have a transcription-independent impairment of DNA replication. Specifically, genomic studies reveal that spt6 mutants have decreased loading of the MCM replicative helicase at replication origins, suggesting that Spt6 promotes origin licensing. Our results identify Spt6 as a regulator of genome stability, at least in part through a role in DNA replication.

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.

Adaptive partitioning of a gene locus to the nuclear envelope in Saccharomyces cerevisiae is driven by polymer-polymer phase separation

Partitioning of active gene loci to the nuclear envelope (NE) is a mechanism by which organisms increase the speed of adaptation and metabolic robustness to fluctuating resources in the environment. In the yeast Saccharomyces cerevisiae, adaptation to nutrient depletion or other stresses, manifests as relocalization of active gene loci from nucleoplasm to the NE, resulting in more efficient transport and translation of mRNA. The mechanism by which this partitioning occurs remains a mystery. Here, we demonstrate that the yeast inositol depletion-responsive gene locus INO1 partitions to the nuclear envelope, driven by local histone acetylation-induced polymer-polymer phase separation from the nucleoplasmic phase. This demixing is consistent with recent evidence for chromatin phase separation by acetylation-mediated dissolution of multivalent histone association and fits a physical model where increased bending stiffness of acetylated chromatin polymer causes its phase separation from de-acetylated chromatin. Increased chromatin spring stiffness could explain nucleation of transcriptional machinery at active gene loci.

Parental histone distribution and location of the replication obstacle at nascent strands control homologous recombination

The advance and stability of replication forks rely on a tight co-regulation of DNA synthesis and nucleosome assembly. We show that mutants affected in parental histone recycling are impaired in the recombinational repair of the single-stranded DNA gaps generated in response to DNA adducts that hamper replication, which are then filled in by translesion synthesis. These recombination defects are in part due to an excess of parental nucleosomes at the invaded strand that destabilizes the sister chromatid junction formed after strand invasion through a Srs2-dependent mechanism. In addition, we show that a dCas9^∗/R-loop is more recombinogenic when the dCas9^∗/DNA-RNA hybrid interferes with the lagging than with the leading strand, and this recombination is particularly sensitive to problems in the deposition of parental histones at the strand that contains the hindrance. Therefore, parental histone distribution and location of the replication obstacle at the lagging or leading strand regulate homologous recombination.

The Histone Chaperone Network Is Highly Conserved in Physarum polycephalum

The nucleosome is composed of histones and DNA. Prior to their deposition on chromatin, histones are shielded by specialized and diverse proteins known as histone chaperones. They escort histones during their entire cellular life and ensure their proper incorporation in chromatin. Physarum polycephalum is a Mycetozoan, a clade located at the crown of the eukaryotic tree. We previously found that histones, which are highly conserved between plants and animals, are also highly conserved in Physarum. However, histone chaperones differ significantly between animal and plant kingdoms, and this thus probed us to further study the conservation of histone chaperones in Physarum and their evolution relative to animal and plants. Most of the known histone chaperones and their functional domains are conserved as well as key residues required for histone and chaperone interactions. Physarum is divergent from yeast, plants and animals, but PpHIRA, PpCABIN1 and PpSPT6 are similar in structure to plant orthologues. PpFACT is closely related to the yeast complex, and the Physarum genome encodes the animal-specific APFL chaperone. Furthermore, we performed RNA sequencing to monitor chaperone expression during the cell cycle and uncovered two distinct patterns during S-phase. In summary, our study demonstrates the conserved role of histone chaperones in handling histones in an early-branching eukaryote.

AtMCM10 promotes DNA replication-coupled nucleosome assembly in Arabidopsis

Minichromosome Maintenance protein 10 (MCM10) is essential for DNA replication initiation and DNA elongation in yeasts and animals. Although the functions of MCM10 in DNA replication and repair have been well documented, the detailed mechanisms for MCM10 in these processes are not well known. Here, we identified AtMCM10 gene through a forward genetic screening for releasing a silenced marker gene. Although plant MCM10 possesses a similar crystal structure as animal MCM10, AtMCM10 is not essential for plant growth or development in Arabidopsis. AtMCM10 can directly bind to histone H3-H4 and promotes nucleosome assembly in vitro. The nucleosome density is decreased in Atmcm10, and most of the nucleosome density decreased regions in Atmcm10 are also regulated by newly synthesized histone chaperone Chromatin Assembly Factor-1 (CAF-1). Loss of both AtMCM10 and CAF-1 is embryo lethal, indicating that AtMCM10 and CAF-1 are indispensable for replication-coupled nucleosome assembly. AtMCM10 interacts with both new and parental histones. Atmcm10 mutants have lower H3.1 abundance and reduced H3K27me1/3 levels with releasing some silenced transposons. We propose that AtMCM10 deposits new and parental histones during nucleosome assembly, maintaining proper epigenetic modifications and genome stability during DNA replication.

Acetylation of H3K56 orchestrates UV-responsive chromatin events that generate DNA accessibility during Nucleotide Excision Repair

Histone modifications have long been related to DNA damage response. Nucleotide excision repair pathway that removes helix-distorting lesions necessitates DNA accessibility through chromatin modifications. Previous studies have linked H3 tail residue acetylation to UV-induced NER. Here we present evidences that acetylation of H3K56 is crucial for early phases of NER. Using H3K56 mutants K56Q and K56R, which mimic acetylated and unacetylated lysines respectively, we show that recruitment of the repair factor Rad16, a Swi/Snf family member is dependent on H3K56 acetylation. With constitutive H3K56 acetylation, Rad16 recruitment became UV-independent. Furthermore, H3K56 acetylation promoted UV-induced hyperacetylation of H3K9 and H3K14. Importantly, constitutive H3K56 acetylation prominently increased chromatin accessibility. During NER, lack of H3K56 acetylation that effectively aborted H3 tail residue acetylation and Rad16 recruitment, thus failed to impart essential chromatin modulations. The NER-responsive oscillation of chromatin structure observed in wild type, was distinctly eliminated in absence of H3K56 acetylation. In vitro assay with wild type and H3K56 mutant cell extracts further indicated that absence of H3K56 acetylation negatively affected DNA relaxation during NER. Overall, H3K56 acetylation regulates Rad16 redistribution and UV-induced H3 tail residue hyperacetylation, and the resultant modification code promotes chromatin accessibility and recruitment of subsequent repair factors during NER.

FACT maintains nucleosomes during transcription and stem cell viability in adult mice

Preservation of nucleosomes during replication has been extensively studied, while the maintenance of nucleosomes during transcription has gotten less attention. The histone chaperone FACT has a role in transcription elongation, although whether it disassembles or assembles nucleosomes during this process is unclear. To elucidate the function of FACT in mammals, we deleted the Ssrp1 subunit of FACT in adult mice. FACT loss is lethal, possibly due to the loss of the earliest progenitors in bone marrow and intestine, while more differentiated cells are not affected. Using cells isolated from several tissues, we show that FACT loss reduces the viability of stem cells but not of cells differentiated in vitro. FACT depletion increases chromatin accessibility in a transcription-dependent manner in adipose mesenchymal stem cells, indicating that nucleosomes are lost in these cells during transcription in the absence of FACT. We also observe activation of interferon (IFN) signaling and the accumulation of immunocytes in organs sensitive to FACT loss. Our data indicate that FACT maintains chromatin integrity during transcription in mammalian adult stem cells, suggesting that chromatin transcription in stem cells and differentiated cells is different.

PCNA Loaders and Unloaders-One Ring That Rules Them All

During each cell duplication, the entirety of the genomic DNA in every cell must be accurately and quickly copied. Given the short time available for the chore, the requirement of many proteins, and the daunting amount of DNA present, DNA replication poses a serious challenge to the cell. A high level of coordination between polymerases and other DNA and chromatin-interacting proteins is vital to complete this task. One of the most important proteins for maintaining such coordination is PCNA. PCNA is a multitasking protein that forms a homotrimeric ring that encircles the DNA. It serves as a processivity factor for DNA polymerases and acts as a landing platform for different proteins interacting with DNA and chromatin. Therefore, PCNA is a signaling hub that influences the rate and accuracy of DNA replication, regulates DNA damage repair, controls chromatin formation during the replication, and the proper segregation of the sister chromatids. With so many essential roles, PCNA recruitment and turnover on the chromatin is of utmost importance. Three different, conserved protein complexes are in charge of loading/unloading PCNA onto DNA. Replication factor C (RFC) is the canonical complex in charge of loading PCNA during the S-phase. The Ctf18 and Elg1 (ATAD5 in mammalian) proteins form complexes similar to RFC, with particular functions in the cell’s nucleus. Here we summarize our current knowledge about the roles of these important factors in yeast and mammals.

The H3.3K27M oncohistone affects replication stress outcome and provokes genomic instability in pediatric glioma

While comprehensive molecular profiling of histone H3.3 mutant pediatric high-grade glioma has revealed extensive dysregulation of the chromatin landscape, the exact mechanisms driving tumor formation remain poorly understood. Since H3.3 mutant gliomas also exhibit high levels of copy number alterations, we set out to address if the H3.3K27M oncohistone leads to destabilization of the genome. Hereto, we established a cell culture model allowing inducible H3.3K27M expression and observed an increase in mitotic abnormalities. We also found enhanced interaction of DNA replication factors with H3.3K27M during mitosis, indicating replication defects. Further functional analyses revealed increased genomic instability upon replication stress, as represented by mitotic bulky and ultrafine DNA bridges. This co-occurred with suboptimal 53BP1 nuclear body formation after mitosis in vitro, and in human glioma. Finally, we observed a decrease in ultrafine DNA bridges following deletion of the K27M mutant H3F3A allele in primary high-grade glioma cells. Together, our data uncover a role for H3.3 in DNA replication under stress conditions that is altered by the K27M mutation, promoting genomic instability and potentially glioma development.

HSV-1 ICP22 Is a Selective Viral Repressor of Cellular RNA Polymerase II-Mediated Transcription Elongation

The Herpes Simplex Virus (HSV-1) immediate-early protein ICP22 interacts with cellular proteins to inhibit host cell gene expression and promote viral gene expression. ICP22 inhibits phosphorylation of Ser2 of the RNA polymerase II (pol II) carboxyl-terminal domain (CTD) and productive elongation of pol II. Here we show that ICP22 affects elongation of pol II through both the early-elongation checkpoint and the poly(A)-associated elongation checkpoint of a protein-coding gene model. Coimmunoprecipitation assays using tagged ICP22 expressed in human cells and pulldown assays with recombinant ICP22 in vitro coupled with mass spectrometry identify transcription elongation factors, including P-TEFb, additional CTD kinases and the FACT complex as interacting cellular factors. Using a photoreactive amino acid incorporated into ICP22, we found that L191, Y230 and C225 crosslink to both subunits of the FACT complex in cells. Our findings indicate that ICP22 interacts with critical elongation regulators to inhibit transcription elongation of cellular genes, which may be vital for HSV-1 pathogenesis. We also show that the HSV viral activator, VP16, has a region of structural similarity to the ICP22 region that interacts with elongation factors, suggesting a model where VP16 competes with ICP22 to deliver elongation factors to viral genes.

FACT interacts with Set3 HDAC and fine-tunes GAL1 transcription in response to environmental stimulation

The histone chaperone facilitates chromatin transactions (FACT) functions in various DNA transactions. How FACT performs these multiple functions remains largely unknown. Here, we found, for the first time, that the N-terminal domain of its Spt16 subunit interacts with the Set3 histone deacetylase complex (Set3C) and that FACT and Set3C function in the same pathway to regulate gene expression in some settings. We observed that Spt16-G132D mutant proteins show defects in binding to Set3C but not other reported FACT interactors. At the permissive temperature, induction of the GAL1 and GAL10 genes is reduced in both spt16-G132D and set3Δ cells, whereas transient upregulation of GAL10 noncoding RNA (ncRNA), which is transcribed from the 3′ end of the GAL10 gene, is elevated. Mutations that inhibit GAL10 ncRNA transcription reverse the GAL1 and GAL10 induction defects in spt16-G132D and set3Δ mutant cells. Mechanistically, set3Δ and FACT (spt16-G132D) mutants show reduced histone acetylation and increased nucleosome occupancy at the GAL1 promoter under inducing conditions and inhibition of GAL10 ncRNA transcription also partially reverses these chromatin changes. These results indicate that FACT interacts with Set3C, which in turn prevents uncontrolled GAL10 ncRNA expression and fine-tunes the expression of GAL genes upon a change in carbon source.

Chromatin dynamics and DNA replication roadblocks

A broad spectrum of spontaneous and genotoxin-induced DNA lesions impede replication fork progression. The DNA damage response that acts to promote completion of DNA replication is associated with dynamic changes in chromatin structure that include two distinct processes which operate genome-wide during S-phase. The first, often referred to as histone recycling or parental histone segregation, is characterized by the transfer of parental histones located ahead of replication forks onto nascent DNA. The second, known as de novo chromatin assembly, consists of the deposition of new histone molecules onto nascent DNA. Because these two processes occur at all replication forks, their potential to influence a multitude of DNA repair and DNA damage tolerance mechanisms is considerable. The purpose of this review is to provide a description of parental histone segregation and de novo chromatin assembly, and to illustrate how these processes influence cellular responses to DNA replication roadblocks.

The box C/D snoRNP assembly factor Bcd1 interacts with the histone chaperone Rtt106 and controls its transcription dependent activity

Biogenesis of eukaryotic box C/D small nucleolar ribonucleoproteins initiates co-transcriptionally and requires the action of the assembly machinery including the Hsp90/R2TP complex, the Rsa1p:Hit1p heterodimer and the Bcd1 protein. We present genetic interactions between the Rsa1p-encoding gene and genes involved in chromatin organization including RTT106 that codes for the H3-H4 histone chaperone Rtt106p controlling H3K56ac deposition. We show that Bcd1p binds Rtt106p and controls its transcription-dependent recruitment by reducing its association with RNA polymerase II, modulating H3K56ac levels at gene body. We reveal the 3D structures of the free and Rtt106p-bound forms of Bcd1p using nuclear magnetic resonance and X-ray crystallography. The interaction is also studied by a combination of biophysical and proteomic techniques. Bcd1p interacts with a region that is distinct from the interaction interface between the histone chaperone and histone H3. Our results are evidence for a protein interaction interface for Rtt106p that controls its transcription-associated activity.

Biogenesis of small nucleolar RNAs ribonucleoproteins (snoRNPs) requires dedicated assembly machinery. Here, the authors show that a subset of snoRNP assembly factors interacts, genetically or directly, with factors modulating chromatin architecture, suggesting a link between ribosome formation and chromatin functions.

The Amazing Acrobat: Yeast's Histone H3K56 Juggles Several Important Roles While Maintaining Perfect Balance

Acetylation on lysine 56 of histone H3 of the yeast Saccharomyces cerevisiae has been implicated in many cellular processes that affect genome stability. Despite being the object of much research, the complete scope of the roles played by K56 acetylation is not fully understood even today. The acetylation is put in place at the S-phase of the cell cycle, in order to flag newly synthesized histones that are incorporated during DNA replication. The signal is removed by two redundant deacetylases, Hst3 and Hst4, at the entry to G2/M phase. Its crucial location, at the entry and exit points of the DNA into and out of the nucleosome, makes this a central modification, and dictates that if acetylation and deacetylation are not well concerted and executed in a timely fashion, severe genomic instability arises. In this review, we explore the wealth of information available on the many roles played by H3K56 acetylation and the deacetylases Hst3 and Hst4 in DNA replication and repair.

Rtt109 slows replication speed by histone N-terminal acetylation

The wrapping of DNA around histone octamers challenges processes that use DNA as their template. In vitro, DNA replication through chromatin depends on histone modifiers, raising the possibility that cells modify histones to optimize fork progression. Rtt109 is an acetyl transferase that acetylates histone H3 before its DNA incorporation on the K56 and N-terminal residues. We previously reported that, in budding yeast, a wave of histone H3 K9 acetylation progresses ∼3–5 kb ahead of the replication fork. Whether this wave contributes to replication dynamics remained unknown. Here, we show that the replication fork velocity increases following deletion of RTT109, the gene encoding the enzyme required for the prereplication H3 acetylation wave. By using histone H3 mutants, we find that Rtt109-dependent N-terminal acetylation regulates fork velocity, whereas K56 acetylation contributes to replication dynamics only when N-terminal acetylation is compromised. We propose that acetylation of newly synthesized histones slows replication by promoting replacement of nucleosomes evicted by the incoming fork, thereby protecting genome integrity.

Parental nucleosome segregation and the inheritance of cellular identity

Gene expression programmes conferring cellular identity are achieved through the organization of chromatin structures that either facilitate or impede transcription. Among the key determinants of chromatin organization are the histone modifications that correlate with a given transcriptional status and chromatin state. Until recently, the details for the segregation of nucleosomes on DNA replication and their implications in re-establishing heritable chromatin domains remained unclear. Here, we review recent findings detailing the local segregation of parental nucleosomes and highlight important advances as to how histone methyltransferases associated with the establishment of repressive chromatin domains facilitate epigenetic inheritance.

Transcription-dependent enrichment of the yeast FACT complex influences nucleosome dynamics on the RNA polymerase III-transcribed genes

The FACT (FAcilitates Chromatin Transactions) complex influences transcription initiation and enables passage of RNA polymerase (pol) II through gene body nucleosomes during elongation. In the budding yeast, ~280 non-coding RNA genes highly transcribed in vivo by pol III are found in the nucleosome-free regions bordered by positioned nucleosomes. The downstream nucleosome dynamics was found to regulate transcription via controlling the gene terminator accessibility and hence, terminator-dependent pol III recycling. As opposed to the enrichment at the 5'-ends of pol II-transcribed genes, our genome-wide mapping found transcription-dependent enrichment of the FACT subunit Spt16 near the 3'-end of all pol III-transcribed genes. Spt16 physically associates with the pol III transcription complex and shows gene-specific occupancy levels on the individual genes. On the non-tRNA pol III-transcribed genes, Spt16 facilitates transcription by reducing the nucleosome occupany on the gene body. On the tRNA genes, it maintains the position of the nucleosome at the 3' gene-end and affects transcription in gene-specific manner. Under nutritional stress, Spt16 enrichment is abolished in the gene downstream region of all pol III-transcribed genes and reciprocally changed on the induced or repressed pol II-transcribed ESR genes. Under the heat and replicative stress, its occupancy on the pol III-transcribed genes increases significantly. Our results show that Spt16 elicits a differential, gene-specific and stress-responsive dynamics, which provides a novel stress-sensor mechanism of regulating transcription against external stress. By primarily influencing the nucleosomal organization, FACT links the downstream nucleosome dynamics to transcription and environmental stress on the pol III-transcribed genes.

The role of FACT in managing chromatin: disruption, assembly, or repair?

FACT (FAcilitates Chromatin Transcription) has long been considered to be a transcription elongation factor whose ability to destabilize nucleosomes promotes RNAPII progression on chromatin templates. However, this is just one function of this histone chaperone, as FACT also functions in DNA replication. While broadly conserved among eukaryotes and essential for viability in many organisms, dependence on FACT varies widely, with some differentiated cells proliferating normally in its absence. It is therefore unclear what the core functions of FACT are, whether they differ in different circumstances, and what makes FACT essential in some situations but not others. Here, we review recent advances and propose a unifying model for FACT activity. By analogy to DNA repair, we propose that the ability of FACT to both destabilize and assemble nucleosomes allows it to monitor and restore nucleosome integrity as part of a system of chromatin repair, in which disruptions in the packaging of DNA are sensed and returned to their normal state. The requirement for FACT then depends on the level of chromatin disruption occurring in the cell, and the cell's ability to tolerate packaging defects. The role of FACT in transcription would then be just one facet of a broader system for maintaining chromatin integrity.

An Insight into Reprogramming Barriers to iPSC Generation

Derivation of induced Pluripotent Stem Cells (iPSCs) by reprogramming somatic cells to a pluripotent state has revolutionized stem cell research. Ensuing this, various groups have used genetic and non-genetic approaches to generate iPSCs from numerous cell types. However, achieving a pluripotent state in most of the reprogramming studies is marred by serious limitations such as low reprogramming efficiency and slow kinetics. These limitations are mainly due to the presence of potent barriers that exist during reprogramming when a mature cell is coaxed to achieve a pluripotent state. Several studies have revealed that intrinsic factors such as non-optimal stoichiometry of reprogramming factors, specific signaling pathways, cellular senescence, pluripotency-inhibiting transcription factors and microRNAs act as a roadblock. In addition, the epigenetic state of somatic cells and specific epigenetic modifications that occur during reprogramming also remarkably impede the generation of iPSCs. In this review, we present a comprehensive overview of the barriers that inhibit reprogramming and the understanding of which will pave the way to develop safe strategies for efficient reprogramming.

Deacetylation of H4 lysine16 affects acetylation of lysine residues in histone H3 and H4 and promotes transcription of constitutive genes

Histone modification map of H4 N-terminal tail residues in Saccharomyces cerevisiae reveals the prominence of lysine acetylation. Previous reports have indicated the importance of lysine acetylation in maintaining chromatin structure and function. H4K16, a residue with highly regulated acetylation dynamics has unique functions not overlapping with the other H4 N- terminal acetylable residues. The present work unravels the role of H4K16 acetylation in regulating expression of constitutive genes. H4K16 gets distinctly deacetylated over the coding region of constitutively expressed genes. Deacetylation of H4K16 reduces H3K9 acetylation at the cellular and gene level. Reduced H3K9 acetylation however did not negatively correlate with active gene transcription. Significantly, H4K16 deacetylation was found to be associated with hypoacetylated H4K12 throughout the locus of constitutive genes. H4K16 and K12 deacetylation is known to favour active transcription. Sas2, the HAT mutant showed similar patterns of hypoacetylated H3K9 and H4K12 at the active loci, clearly implying that the modifications were associated with deacetylation state of H4K16. Deacetylation of H4K16 was also concurrent with increased H3K56 acetylation in the promoter region and ORF of the constitutive genes. Combination of all these histone modifications significantly reduced H3 occupancy, increased promoter accessibility and enhanced RNAPII recruitment at the constitutively active loci. Consequently, we found that expression of active genes was higher in H4K16R mutant which mimic deacetylated state, but not in H4K16Q mimicking constitutive acetylation. To summarize, H4K16 deacetylation linked with H4K12 and H3K9 hypoacetylation along with H3K56 hyperacetylation generate a chromatin landscape that is conducive for transcription of constitutive genes.

The Paramecium histone chaperone Spt16-1 is required for Pgm endonuclease function in programmed genome rearrangements

In Paramecium tetraurelia, a large proportion of the germline genome is reproducibly removed from the somatic genome after sexual events via a process involving small (s)RNA-directed heterochromatin formation and DNA excision and repair. How germline limited DNA sequences are specifically recognized in the context of chromatin remains elusive. Here, we use a reverse genetics approach to identify factors involved in programmed genome rearrangements. We have identified a P. tetraurelia homolog of the highly conserved histone chaperone Spt16 subunit of the FACT complex, Spt16-1, and show its expression is developmentally regulated. A functional GFP-Spt16-1 fusion protein localized exclusively in the nuclei where genome rearrangements take place. Gene silencing of Spt16-1 showed it is required for the elimination of all germline-limited sequences, for the survival of sexual progeny, and for the accumulation of internal eliminated sequence (ies)RNAs, an sRNA population produced when elimination occurs. Normal accumulation of 25 nt scanRNAs and deposition of silent histone marks H3K9me3 and H3K27me3 indicated that Spt16-1 does not regulate the scanRNA-directed heterochromatin pathway involved in the early steps of DNA elimination. We further show that Spt16-1 is required for the correct nuclear localization of the PiggyMac (Pgm) endonuclease, which generates the DNA double-strand breaks required for DNA elimination. Thus, Spt16-1 is essential for Pgm function during programmed genome rearrangements. We propose a model in which Spt16-1 mediates interactions between the excision machinery and chromatin, facilitating endonuclease access to DNA cleavage sites during genome rearrangements.

The genome is generally similar in all the cells of an organism. However, in the ciliate Paramecium tetraurelia, massive and reproducible programmed DNA elimination leads to a highly streamlined somatic genome. In eukaryotes, DNA is packaged into nucleosomes, which ensure genome integrity but act as a barrier to enzymes acting on DNA. How the endonuclease PiggyMac gains access to the genome to initiate DNA elimination remains elusive. Here, we identified four P. tetraurelia genes encoding homologs of the conserved histone chaperone Spt16, which can modulate access to DNA by promoting nucleosome assembly and disassembly. We demonstrated that the most divergent gene, SPT16-1, has a highly specialized expression pattern, similar to that of PiggyMac, and a specific role in programmed DNA elimination. We show that the Spt16-1 protein, like PiggyMac, is exclusively localized in the differentiating somatic nucleus, and is also required for the dramatic elimination of germline-limited sequences. We further show that Spt16-1 directs the correct nuclear localization of the PiggyMac endonuclease. Thus, Spt16-1 is essential for PiggyMac function during programmed DNA elimination. We propose that Spt16-1 mediates the interaction between PiggyMac and chromatin or DNA, facilitating endonuclease access to DNA cleavage sites.

Histone acetylation/deacetylation in Candida albicans and their potential as antifungal targets

Recently, the incidence of invasive fungal infections has significantly increased. Candida albicans (C. albicans) is the most common opportunistic fungal pathogen that infects humans. The limited number of available antifungal agents and the emergence of drug resistance pose difficulties to treatment, thus new antifungals are urgently needed. Through their functions in DNA replication, DNA repair and transcription, histone acetyltransferases (HATs) and histone deacetylases (HDACs) perform essential functions relating to growth, virulence, drug resistance and stress responses of C. albicans. Here, we summarize the physiological and pathological functions of HATs/HDACs, potential antifungal targets and underlying antifungal compounds that impact histone acetylation and deacetylation. We anticipate this review will stimulate the identification of new HAT/HDAC-related antifungal targets and antifungal agents.

Histone chaperone FACT is essential to overcome replication stress in mammalian cells

The histone chaperone FACT is upregulated during mammary tumorigenesis and necessary for the viability and growth of breast tumor cells. We established that only proliferating tumor cells are sensitive to FACT knockdown, suggesting that FACT functions during DNA replication in tumor cells but not in normal cells. We hypothesized that the basal level of replication stress defines the FACT dependence of cells. Using genetic and chemical tools, we demonstrated that FACT is needed to overcome replication stress. In the absence of FACT during replication stress, the MCM2-7 helicase dissociates from chromatin, resulting in the absence of ssDNA accumulation, RPA binding, and activation of the ATR/CHK1 checkpoint response. Without this response, stalled replication forks are not stabilized, and new origin firing cannot be prevented, leading to the accumulation of DNA damage and cell death. Thus, we propose a novel role for FACT as a factor preventing helicase dissociation from chromatin during replication stress.

Chromatin Assembly Factor 1 (CAF-1) facilitates the establishment of facultative heterochromatin during pluripotency exit

Establishment and subsequent maintenance of distinct chromatin domains during embryonic stem cell (ESC) differentiation are crucial for lineage specification and cell fate determination. Here we show that the histone chaperone Chromatin Assembly Factor 1 (CAF-1), which is recruited to DNA replication forks through its interaction with proliferating cell nuclear antigen (PCNA) for nucleosome assembly, participates in the establishment of H3K27me3-mediated silencing during differentiation. Deletion of CAF-1 p150 subunit impairs the silencing of many genes including Oct4, Sox2 and Nanog as well as the establishment of H3K27me3 at these gene promoters during ESC differentiation. Mutations of PCNA residues involved in recruiting CAF-1 to the chromatin also result in defects in differentiation in vitro and impair early embryonic development as p150 deletion. Together, these results reveal that the CAF-1-PCNA nucleosome assembly pathway plays an important role in the establishment of H3K27me3-mediated silencing during cell fate determination.

The replisome guides nucleosome assembly during DNA replication

Nucleosome assembly during DNA replication is tightly coupled to ongoing DNA synthesis. This process, termed DNA replication-coupled (RC) nucleosome assembly, is essential for chromatin replication and has a great impact on both genome stability maintenance and epigenetic inheritance. This review discusses a set of recent findings regarding the role of replisome components contributing to RC nucleosome assembly. Starting with a brief introduction to the factors involved in nucleosome assembly and some aspects of the architecture of the eukaryotic replisome, we discuss studies from yeast to mammalian cells and the interactions of replisome components with histones and histone chaperones. We describe the proposed functions of replisome components during RC nucleosome assembly and discuss their impacts on histone segregation and implications for epigenetic inheritance.

Asymmetric Histone Inheritance in Asymmetrically Dividing Stem Cells

Epigenetic mechanisms play essential roles in determining distinct cell fates during the development of multicellular organisms. Histone proteins represent crucial epigenetic components that help specify cell identities. Previous work has demonstrated that during the asymmetric cell division of Drosophila male germline stem cells (GSCs), histones H3 and H4 are asymmetrically inherited, such that pre-existing (old) histones are segregated towards the self-renewing GSC whereas newly synthesized (new) histones are enriched towards the differentiating daughter cell. In order to further understand the molecular mechanisms underlying this striking phenomenon, two key questions must be answered: when and how old and new histones are differentially incorporated by sister chromatids, and how epigenetically distinct sister chromatids are specifically recognized and segregated. Here, we discuss recent advances in our understanding of the molecular mechanisms and cellular bases underlying these fundamental and important biological processes responsible for generating two distinct cells through one cell division.

Structure-function relationship of H2A-H2B specific plant histone chaperones

Studies on chromatin structure and function have gained a revived popularity. Histone chaperones are significant players in chromatin organization. They play a significant role in vital nuclear functions like transcription, DNA replication, DNA repair, DNA recombination, and epigenetic regulation, primarily by aiding processes such as histone shuttling and nucleosome assembly/disassembly. Like the other eukaryotes, plants also have a highly orchestrated and dynamic chromatin organization. Plants seem to have more isoforms within the same family of histone chaperones, as compared with other organisms. As some of these are specific to plants, they must have evolved to perform functions unique to plants. However, it appears that only little effort has gone into understanding the structural features of plant histone chaperones and their structure-function relationships. Studies on plant histone chaperones are essential for understanding their role in plant chromatin organization and how plants respond during stress conditions. This review is on the structural and functional aspects of plant histone chaperone families, specifically those which bind to H2A-H2B, viz nucleosome assembly protein (NAP), nucleoplasmin (NPM), and facilitates chromatin transcription (FACT). Here, we also present comparative analyses of these plant histone chaperones with available histone chaperone structures. The review hopes to incite interest among researchers to pursue further research in the area of plant chromatin and the associated histone chaperones.

Histone Chaperone FACT and Curaxins: Effects on Genome Structure and Function

The histone chaperone FACT plays important roles in essentially every chromatin-associated process and is an important indirect target of the curaxin class of anti-cancer drugs. Curaxins are aromatiс compounds that intercalate into DNA and can trap FACT in bulk chromatin, thus interfering with its distribution and its functions in cancer cells. Recent studies have provided mechanistic insight into how FACT and curaxins cooperate to promote unfolding of nucleosomes and chromatin fibers, resulting in genome-wide disruption of contact chromatin domain boundaries, perturbation of higher order chromatin organization, and global disregulation of gene expression. Here, we discuss the implications of these insights for cancer biology.

Nucleosome Remodeling by Fun30SMARCAD1 in the DNA Damage Response

Many cellular pathways are dedicated to maintain the integrity of the genome. In eukaryotes, the underlying DNA transactions occur in the context of chromatin. Cells utilize chromatin and its dynamic nature to regulate those genome integrity pathways. Accordingly, chromatin becomes restructured and modified around DNA damage sites. Here, we review the current knowledge of a chromatin remodeler Fun30^SMARCAD1, which plays a key role in genome maintenance. Fun30^SMARCAD1 promotes DNA end resection and the repair of DNA double-stranded breaks (DSBs). Notably, however, Fun30^SMARCAD1 plays additional roles in maintaining heterochromatin and promoting transcription. Overall, Fun30^SMARCAD1 is involved in distinct processes and the specific roles of Fun30^SMARCAD1 at DSBs, replication forks and sites of transcription appear discordant at first view. Nonetheless, a picture emerges in which commonalities within these context-dependent roles of Fun30^SMARCAD1 exist, which may help to gain a more global understanding of chromatin alterations induced by Fun30^SMARCAD1.

Chromatin assembly factor-1 (CAF-1) chaperone regulates Cse4 deposition into chromatin in budding yeast

Correct localization of the centromeric histone variant CenH3/CENP-A/Cse4 is an important part of faithful chromosome segregation. Mislocalization of CenH3 could affect chromosome segregation, DNA replication and transcription. CENP-A is often overexpressed and mislocalized in cancer genomes, but the underlying mechanisms are not understood. One major regulator of Cse4 deposition is Psh1, an E3 ubiquitin ligase that controls levels of Cse4 to prevent deposition into non-centromeric regions. We present evidence that Chromatin assembly factor-1 (CAF-1), an evolutionarily conserved histone H3/H4 chaperone with subunits shown previously to interact with CenH3 in flies and human cells, regulates Cse4 deposition in budding yeast. yCAF-1 interacts with Cse4 and can assemble Cse4 nucleosomes in vitro. Loss of yCAF-1 dramatically reduces the amount of Cse4 deposited into chromatin genome-wide when Cse4 is overexpressed. The incorporation of Cse4 genome-wide may have multifactorial effects on growth and gene expression. Loss of yCAF-1 can rescue growth defects and some changes in gene expression associated with Cse4 deposition that occur in the absence of Psh1-mediated proteolysis. Incorporation of Cse4 into promoter nucleosomes at transcriptionally active genes depends on yCAF-1. Overall our findings suggest CAF-1 can act as a CenH3 chaperone, regulating levels and incorporation of CenH3 in chromatin.

Structural visualization of key steps in nucleosome reorganization by human FACT

Facilitates chromatin transcription (FACT) is a histone chaperone, which accomplishes both nucleosome assembly and disassembly. Our combined cryo-electron microscopy (EM) and native mass spectrometry (MS) studies revealed novel key steps of nucleosome reorganization conducted by a Mid domain and its adjacent acidic AID segment of human FACT. We determined three cryo-EM structures of respective octasomes complexed with the Mid-AID and AID regions, and a hexasome alone. We discovered extensive contacts between a FACT region and histones H2A, H2B, and H3, suggesting that FACT is competent to direct functional replacement of a nucleosomal DNA end by its phosphorylated AID segment (pAID). Mutational assays revealed that the aromatic and phosphorylated residues within pAID are essential for octasome binding. The EM structure of the hexasome, generated by the addition of Mid-pAID or pAID, indicated that the dissociation of H2A-H2B dimer causes significant alteration from the canonical path of the nucleosomal DNA.

Proteomic Analysis of Histones H2A/H2B and Variant Hv1 in Tetrahymena thermophila Reveals an Ancient Network of Chaperones

Epigenetic information, which can be passed on independently of the DNA sequence, is stored in part in the form of histone posttranslational modifications and specific histone variants. Although complexes necessary for deposition have been identified for canonical and variant histones, information regarding the chromatin assembly pathways outside of the Opisthokonts remains limited. Tetrahymena thermophila, a ciliated protozoan, is particularly suitable to study and unravel the chromatin regulatory layers due to its unique physical separation of chromatin states in the form of two distinct nuclei present within the same cell. Using a functional proteomics pipeline, we carried out affinity purification followed by mass spectrometry of endogenously tagged T. thermophila histones H2A, H2B and variant Hv1.We identified a set of interacting proteins shared among the three analyzed histones that includes the FACT-complex, as well as H2A- or Hv1-specific chaperones. We find that putative subunits of T. thermophila versions of SWR- and INO80-complexes, as well as transcription-related histone chaperone Spt6^Tt specifically copurify with Hv1. We also identified importin β6 and the T. thermophila ortholog of nucleoplasmin 1 (cNpl1^Tt) as H2A–H2B interacting partners. Our results further implicate Poly [ADP-ribose] polymerases in histone metabolism. Molecular evolutionary analysis, reciprocal affinity purification coupled to mass spectrometry experiments, and indirect immunofluorescence studies using endogenously tagged Spt16^Tt (FACT-complex subunit), cNpl1^Tt, and PARP6^Tt underscore the validity of our approach and offer mechanistic insights. Our results reveal a highly conserved regulatory network for H2A (Hv1)–H2B concerning their nuclear import and assembly into chromatin.

Dynamics of the Eukaryotic Replicative Helicase at Lagging-Strand Protein Barriers Support the Steric Exclusion Model

Progression of DNA replication depends on the ability of the replisome complex to overcome nucleoprotein barriers. During eukaryotic replication, the CMG helicase translocates along the leading-strand template and unwinds the DNA double helix. While proteins bound to the leading-strand template efficiently block the helicase, the impact of lagging-strand protein obstacles on helicase translocation and replisome progression remains controversial. Here, we show that CMG and replisome progressions are impaired when proteins crosslinked to the lagging-strand template enhance the stability of duplex DNA. In contrast, proteins that exclusively interact with the lagging-strand template influence neither the translocation of isolated CMG nor replisome progression in Xenopus egg extracts. Our data imply that CMG completely excludes the lagging-strand template from the helicase central channel while unwinding DNA at the replication fork, which clarifies how two CMG helicases could freely cross one another during replication initiation and termination.

Chromatin as a Platform for Modulating the Replication Stress Response

Eukaryotic DNA replication occurs in the context of chromatin. Recent years have seen major advances in our understanding of histone supply, histone recycling and nascent histone incorporation during replication. Furthermore, much is now known about the roles of histone remodellers and post-translational modifications in replication. It has also become clear that nucleosome dynamics during replication play critical roles in genome maintenance and that chromatin modifiers are important for preventing DNA replication stress. An understanding of how cells deploy specific nucleosome modifiers, chaperones and remodellers directly at sites of replication fork stalling has been building more slowly. Here we will specifically discuss recent advances in understanding how chromatin composition contribute to replication fork stability and restart.