Structural basis of nucleosome disassembly and reassembly by RNAPII elongation complex with FACT

Multiple structures of RNA polymerase II isolated from human nuclei by ChIP-CryoEM analysis

RNA polymerase II (RNAPII) is a central transcription enzyme that exists as multiple forms with or without accessory factors, and transcribes the genomic DNA packaged in chromatin. To understand how RNAPII functions in the human genome, we isolate transcribing RNAPII complexes from human nuclei by chromatin immunopurification, and determine the cryo-electron microscopy structures of RNAPII elongation complexes (ECs) associated with genomic DNA in distinct forms, without or with the elongation factors SPT4/5, ELOF1, and SPT6. This ChIP-cryoEM method also reveals the two EC-nucleosome complexes corresponding nucleosome disassembly/reassembly processes. In the structure of EC-downstream nucleosome, EC paused at superhelical location (SHL) -5 in the nucleosome, suggesting that SHL(-5) pausing occurs in a sequence-independent manner during nucleosome disassembly. In the structure of the EC-upstream nucleosome, EC directly contacts the nucleosome through the nucleosomal DNA-RPB4/7 stalk and the H2A-H2B dimer-RPB2 wall interactions, suggesting that EC may be paused during nucleosome reassembly. These representative EC structures transcribing the human genome provide mechanistic insights into understanding RNAPII transcription on chromatin.

Always on the Move: Overview on Chromatin Dynamics within Nuclear Processes

Our genome is organized into chromatin, a dynamic and modular structure made of nucleosomes. Chromatin organization controls access to the DNA sequence, playing a fundamental role in cell identity and function. How nucleosomes enable these processes is an active area of study. In this review, we provide an overview of chromatin dynamics, its properties, mechanisms, and functions. We highlight the diverse ways by which chromatin dynamics is controlled during transcription, DNA replication, and repair. Recent technological developments have promoted discoveries in this area, to which we provide an outlook on future research directions.

High-resolution sequencing reveals that the Paf1 complex may be a conserved transcription elongation factor for eukaryotic RNA polymerase I

In eukaryotes, at least three Pols (I, II, and III) are responsible for synthesizing unique RNA products. Many trans-acting factors affect the efficiency of transcription by the three Pols. Some of these factors influence more than one of the nuclear Pols. One such factor is polymerase-associated factor 1 complex (Paf1C). Paf1C, composed of five subunits in Saccharomyces cerevisiae (yeast), has been shown to promote transcription by Pols I and II and is conserved across eukaryotes. Although several studies have demonstrated that Paf1C associates with Pol I machinery, its roles in ribosomal RNA synthesis are not well-defined. In this study, we used native elongating transcript sequencing (NET-seq), to investigate the effect of the loss of two of the five Paf1C subunits (Paf1 and Cdc73) on Pol I occupancy at single-nucleotide resolution in yeast. We found that in both paf1Δ and cdc73Δ mutants, there was a significant reduction in Pol I occupancy at the 5' end of the DNA template as compared to WT yeast, accompanied by other occupancy pattern changes throughout the gene. To complement these results, we also analyzed a PRO-seq dataset that was generated with DLD1 mammalian cells. Interestingly, we found that when Paf1C was knocked-down, there was also a reduction in the occupancy of Pol I at the 5' end of the gene, consistent with our NET-seq analysis. Overall, our results support the conclusion that Paf1C is an important transcription elongation factor for Pol I and may play a conserved role across species.

NDF/GLYR1 Promotes RNA Polymerase II Processivity via Pol II Binding and Nucleosome Destabilization

The Nucleosome Destabilizing Factor (NDF) facilitates transcription through chromatin, but its precise mechanism remains incompletely understood. Here, we identify a critical region (amino acids 140-160) within NDF that specifically interacts with phosphorylated RPB1, the largest subunit of elongating RNA Polymerase II (Pol II). Mutations in this region disrupt Pol II interaction and impair Pol II elongation both in vitro and in cells, yet do not affect NDF's ability to destabilize nucleosomes, establishing a functional separation between these two activities. Cellular studies reveal that NDF knockout cells display faster Pol II elongation rates but produce fewer nascent transcripts, demonstrating NDF's primary role in maintaining transcriptional processivity throughout gene bodies. Our findings demonstrate that NDF uses distinct mechanisms to ensure productive transcription elongation rather than simply enhancing elongation speed, offering new insights into how transcription efficiency is maintained in chromatin.

FACT weakens the nucleosomal barrier to transcription and preserves its integrity by forming a hexasome-like intermediate

Transcription of yeast RNA polymerase II through nucleosomes requires the assistance of the histone chaperone FACT (facilitates chromatin transcription). Yet, how FACT modulates the nucleosomal mechanical barrier to affect the polymerase's elongation dynamics is poorly understood. Using high-resolution single-molecule optical tweezers, we show that FACT greatly decreases the magnitude of the barrier by favoring the unwrapping of DNA from the distal H2A-H2B dimer, which, in turn, weakens the contacts near the dyad, significantly reducing the enzyme's crossing time. We show that barrier crossing depends on the asymmetric flexibility of the nucleosome arms, an asymmetry we find across the genome. Mechanical unwrapping of Cy3-H2A nucleosomes reveals that FACT reduces their unwrapping force and stabilizes a hexasome-like intermediate that retains both labeled dimers during successive unwrapping cycles. This intermediate is also observed after transcription. In conclusion, FACT facilitates nucleosomal transcription by weakening the barrier and actively assisting the maintenance of nucleosomal integrity after enzyme passage.

PAF1C-mediated activation of CDK12/13 kinase activity is critical for CTD phosphorylation and transcript elongation

The transcription cycle is regulated by dynamic changes in RNA polymerase II (RNAPII) C-terminal domain (CTD) phosphorylation, which are crucial for gene expression. However, the mechanisms regulating the transcription-specific cyclin-dependent kinases (CDKs) during the transcription cycle remain poorly understood. Here, we show that human CDK12 co-phosphorylates CTD Serine2 and Serine5. This di-phosphorylated Serine2-Serine5 CTD mark may then act as a precursor for Serine2 mono-phosphorylated CTD through Serine5 de-phosphorylation. Notably, CDK12 is specifically regulated by association with the elongation-specific factor PAF1 complex (PAF1C), in which the CDC73 subunit contains a metazoan-specific peptide motif, capable of allosteric CDK12/cyclin K activation. This motif is essential for cell proliferation and required for normal levels of CTD phosphorylation in chromatin, and for transcript elongation, particularly across long human genes. Together, these findings provide insight into the mechanisms governing RNAPII phospho-CTD dynamics that ensure progression through the human transcription cycle.

Structural basis of RNA polymerase II transcription on the H3-H4 octasome

The H3-H4 octasome is a nucleosome-like particle in which two DNA gyres are wrapped around each H3-H4 tetramer disk, forming a clamshell-like configuration. In the present study, we performed in vitro RNAPII transcription assays with the H3-H4 octasome and found that RNAPII transcribed the H3-H4 octasome more efficiently than the nucleosome. RNAPII paused at only one position, superhelical location (SHL) -4 in the H3-H4 octasome, in contrast to pausing at the SHL(-5), SHL(-2), and SHL(-1) positions in the nucleosome. Cryo-electron microscopy analysis revealed that two H3-H4 tetramer disks are retained when the RNAPII paused at the SHL(-4) position of the H3-H4 octasome. However, when RNAPII reached the SHL(-0.5) position, five base pairs before the dyad position of the H3-H4 octasome, the proximal H3-H4 tetramer was disassembled but the distal H3-H4 tetramer still remained on the DNA. Therefore, RNAPII efficiently transcribes the H3-H4 octasome by stepwise H3-H4 tetramer disassembly.

Nanoscale analysis of human G1 and metaphase chromatin in situ

The structure of chromatin at the nucleosome level inside cells is still incompletely understood. Here we present in situ electron cryotomography analyses of chromatin in both G1 and metaphase RPE-1 cells. G1 nucleosomes are concentrated in globular chromatin domains, and metaphase nucleosomes are concentrated in the chromatids. Classification analysis reveals that canonical mononucleosomes, and in some conditions ordered stacked dinucleosomes and mononucleosomes with a disordered gyre-proximal density, are abundant in both cell-cycle states. We do not detect class averages that have more than two stacked nucleosomes or side-by-side dinucleosomes, suggesting that groups of more than two nucleosomes are heterogeneous. Large multi-megadalton structures are abundant in G1 nucleoplasm, but not found in G1 chromatin domains and metaphase chromatin. The macromolecular phenotypes studied here represent a starting point for the comparative analysis of compaction in normal vs. unhealthy human cells, in other cell-cycle states, other organisms, and in vitro chromatin assemblies.

Spt6-Spn1 interaction is required for RNA polymerase II association and precise nucleosome positioning along transcribed genes

Spt6-Spn1 is an essential histone chaperone complex that associates with RNA Polymerase II (RNAPII) and reassembles nucleosomes during gene transcription. While the interaction between Spt6 and Spn1 is important for its histone deposition and transcription functions, a precise mechanistic understanding is still limited. Here, using temperature-sensitive alleles of spt6 and spn1 that disrupt their interaction in yeast, we show that the Spt6-Spn1 association is important for its stable interaction with the elongating RNAPII complex and nucleosomes. Using micrococcal nuclease (MNase)-based chromatin occupancy profiling, we further find that Spt6-Spn1 interaction is required to maintain a preferred nucleosome positioning at actively transcribed genes; in the absence of Spt6-Spn1 interaction, we observe a return to replication-dependent phasing. In addition to positioning defects, Spt6-Spn1 disrupting mutants also resulted in an overall shift of nucleosomes toward the 5' end of genes that were correlated with decreased RNAPII levels. As loss of Spt6-Spn1 association results in cryptic transcription at a subset of genes, we examined these genes for their nucleosome profiles. These findings revealed that the chromatin organization at these loci is similar to other active genes, thus underscoring the critical role of DNA sequence in mediating cryptic transcription when nucleosome positioning is altered. Taken together, these findings reveal that Spt6-Spn1 interaction is key to its association with elongating RNAPII and to its ability to precisely organize nucleosomes across transcription units.

Structural diversity of noncanonical nucleosomes: Functions in chromatin

In eukaryotes, genomic DNA is compacted into chromatin, with nucleosomes acting as its basic structural units. In addition to canonical nucleosomes, noncanonical nucleosomes, such as hexasomes, H3-H4 octasomes, and overlapping dinucleosomes, exhibit alternative histone compositions and play key roles in chromatin remodeling, transcription, and replication. Recent cryo-electron microscopy (cryo-EM) studies have elucidated the structural details of these noncanonical nucleosomes and their interactions with histone chaperones and chromatin remodelers. This review highlights recent advances in the structural and functional understanding of noncanonical nucleosomes and their roles in maintaining chromatin integrity and facilitating transcriptional dynamics.

The histone chaperone Spn1 preserves chromatin protections at promoters and nucleosome positioning in open reading frames

Spn1 is a multifunctional histone chaperone that associates with RNA polymerase II during elongation and is essential for life in eukaryotes. While previous work has elucidated regions of the protein important for its many interactions, it is unknown how these domains contribute to the maintenance of chromatin structure. Here, we employ digestion by micrococcal nuclease followed by single-stranded library preparation and sequencing to characterize chromatin structure in Saccharomyces cerevisiae expressing wild-type or mutants of Spn1 (spn1K192N or spn1141-305). We mapped protections of all sizes genome wide. Surprisingly, we observed a widespread loss of short fragments over nucleosome-depleted regions (NDRs) at promoters in the spn1K192N-containing strain, indicating critical functions of Spn1 in maintaining normal chromatin architecture outside open reading frames. Additionally, there are shifts in DNA protections in both Spn1 mutant-expressing strains over open reading frames, which indicate changes in nucleosome and subnucleosome positioning. This was observed in markedly different Spn1 mutant strains, demonstrating that multiple functions of Spn1 are required to maintain proper chromatin structure in open reading frames. Changes in chromatin structure correlate positively with changes in gene expression, as shown by RNA-seq analysis in the Spn1 mutant strains. Taken together, our results reveal a previously unknown role of Spn1 in the maintenance of NDR architecture and deepen our understanding of Spn1-dependent chromatin maintenance over transcribed regions.

Structural basis of human replisome progression into a nucleosome

Epigenetic inheritance requires the transfer of parental histones to newly synthesized DNA during eukaryotic chromosome replication, yet the structural mechanisms underlying replisome engagement with nucleosomes remain unclear. Here we establish an in vitro chromatin replication system and report four cryo-EM structures of the human replisome in complex with a parental nucleosome. The structures capture distinct states of nucleosomal DNA unwrapping and nucleosome integrity during nucleosome disassembly by the encroaching replisome.

Transcription elongation factor ELOF1 is required for efficient somatic hypermutation and class switch recombination

Somatic hypermutation (SHM) and class switch recombination (CSR) diversify immunoglobulin (Ig) genes and are initiated by the activation-induced deaminase (AID), a single-stranded DNA cytidine deaminase thought to engage its substrate during RNA polymerase II (RNAPII) transcription. Through a genetic screen, we identified numerous potential factors involved in SHM, including elongation factor 1 homolog (ELOF1), a component of the RNAPII elongation complex that functions in transcription-coupled nucleotide excision repair (TC-NER) and transcription elongation. Loss of ELOF1 compromises SHM, CSR, and AID action in mammalian B cells and alters RNAPII transcription by reducing RNAPII pausing downstream of transcription start sites and levels of serine 5 but not serine 2 phosphorylated RNAPII throughout transcribed genes. ELOF1 must bind to RNAPII to be a proximity partner for AID and to function in SHM and CSR, and TC-NER is not required for SHM. We propose that ELOF1 helps create the appropriate stalled RNAPII substrate on which AID acts.

Replication-transcription symbiosis in the mammalian nucleus: The art of living together

Similarly to life in our planet, where thousands of species inhabit the same ecosystem, the cell nucleus hosts different essential processes that share the same territory, making the interaction between them unavoidable. DNA replication and transcription are essential processes that copy and decode the information contained in our genomes, sharing -and competing for- the same chromatin template. Both activities are executed by large macromolecular machines with similar requirements to access the DNA, remodel the nucleosomes ahead of them and reassemble the chromatin make-up behind. Mechanistically, both processes cannot simultaneously act on the same DNA sequence, but emerging evidence shows that they frequently interact. Here we revise recent data on how transcription and replication occur in chromatin highlighting the symbiotic relationship between both processes, which might help explain how their activities contribute to shape the structure and function of the mammalian genome.

APC/C-mediated ubiquitylation of extranucleosomal histone complexes lacking canonical degrons

Non-degradative histone ubiquitylation plays a myriad of well-defined roles in the regulation of gene expression and choreographing DNA damage repair pathways. In contrast, the contributions of degradative histone ubiquitylation on genomic processes has remained elusive. Recently, the APC/C has been shown to ubiquitylate histones to regulate gene expression in pluripotent cells, but the molecular mechanism is unclear. Here we show that despite directly binding to the nucleosome through subunit APC3, the APC/C is unable to ubiquitylate nucleosomal histones. In contrast, extranucleosomal H2A/H2B and H3/H4 complexes are broadly ubiquitylated by the APC/C in an unexpected manner. Using a combination of cryo-electron microscopy (cryo-EM) and biophysical and enzymatic assays, we demonstrate that APC8 and histone tails direct APC/C-mediated polyubiquitylation of core histones in the absence of traditional APC/C substrate degron sequences. Taken together, our work implicates APC/C-nucleosome tethering in the degradation of diverse chromatin-associated proteins and extranucleosomal histones for the regulation of transcription and the cell cycle and for preventing toxicity due to excess histone levels.

Structural insights into promoter-proximal pausing of RNA polymerase II at +1 nucleosome

The metazoan transcription elongation complex (EC) of RNA polymerase II (RNAPII) generally stalls between the transcription start site and the first (+1) nucleosome. This promoter-proximal pausing involves negative elongation factor (NELF), 5,6-dichloro-1-β-d-ribobenzimidazole sensitivity-inducing factor (DSIF), and transcription elongation factor IIS (TFIIS) and is critical for subsequent productive transcription elongation. However, the detailed pausing mechanism and the involvement of the +1 nucleosome remain enigmatic. Here, we report cryo-electron microscopy structures of ECs stalled on nucleosomal DNA. In the absence of TFIIS, the EC is backtracked/arrested due to conflicts between NELF and the nucleosome. We identified two alternative binding modes of NELF, one of which reveals a critical contact with the downstream DNA through the conserved NELF-E basic helix. Upon binding with TFIIS, the EC progressed to the nucleosome to establish a paused EC with a partially unwrapped nucleosome. This paused EC strongly restricts EC progression further downstream. These structures illuminate the mechanism of RNAPII pausing/stalling at the +1 nucleosome.

Overlapping and distinct functions of SPT6, PNUTS, and PCF11 in regulating transcription termination

The histone chaperone and transcription elongation factor SPT6 is integral to RNA polymerase II (RNAPII) activity. SPT6 also plays a crucial role in regulating transcription termination, although the mechanisms involved are largely unknown. In an attempt to identify the pathways employed by SPT6 in this regulation, we found that, while SPT6 and its partner IWS1 interact and co-localize with RNAPII, their functions diverge significantly at gene termination sites. Depletion of SPT6, but not of IWS1, results in extensive readthrough transcription, indicating that SPT6 independently regulates transcription termination. Further analysis identified that the cleavage and polyadenylation factor PCF11 and the phosphatase regulatory protein PNUTS collaborate with SPT6 in this process. These findings suggest that SPT6 may facilitate transcription termination by recruiting PNUTS and PCF11 to RNAPII. Additionally, SPT6 and PNUTS jointly restrict promoter upstream transcripts (PROMPTs), whereas PCF11 presence is essential for their accumulation in the absence of SPT6 at hundreds of genes. Thus, SPT6, PCF11, and PNUTS have both distinct and overlapping functions in transcription termination. Our data highlight the pivotal role of SPT6 in ensuring proper transcription termination at the 5' and 3'-ends of genes.

Abo1 ATPase facilitates the dissociation of FACT from chromatin

The histone chaperone FAcilitates Chromatin Transcription (FACT) is a heterodimeric complex consisting of Spt16 and Pob3, crucial for preserving nucleosome integrity during transcription and DNA replication. Loss of FACT leads to cryptic transcription and heterochromatin defects. FACT was shown to interact with Abo1, an AAA + family histone chaperone involved in nucleosome dynamics. Depletion of Abo1 causes FACT to stall at transcription start sites and mimics FACT mutants, indicating a functional association between Abo1 and FACT. However, the precise role of Abo1 in FACT function remains poorly understood. Here, we reveal that Abo1 directly interacts with FACT and facilitates the dissociation of FACT from nucleosome. Specifically, the N-terminal region of Abo1 utilizes its FACT-interacting helix to bind to the N-terminal domain of Spt16. In addition, using single-molecule fluorescence imaging, we discovered that Abo1 facilitates the ATP-dependent dissociation of FACT from nucleosomes. Furthermore, we demonstrate that the interaction between Abo1 and FACT is essential for maintaining heterochromatin in fission yeast. In summary, our findings suggest that Abo1 regulates FACT turnover in an ATP-dependent manner, proposing a model of histone chaperone recycling driven by inter-chaperone interactions.

A function of TPL/TBL1-type corepressors is to nucleate the assembly of the preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole-genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5, and SPT6 as necessary for repression with SPT4 acting as a bridge connecting TPL to SPT5 and SPT6. We discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved early in transcription initiation. These findings were validated in yeast and plants, including a novel method to analyze the conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate the rapid onset of transcription once repression is relieved.

Structural basis of H3K36 trimethylation by SETD2 during chromatin transcription

During transcription, RNA polymerase II traverses through chromatin, and posttranslational modifications including histone methylations mark regions of active transcription. Histone protein H3 lysine 36 trimethylation (H3K36me3), which is established by the histone methyltransferase SET domain containing 2 (SETD2), suppresses cryptic transcription, regulates splicing, and serves as a binding site for transcription elongation factors. The mechanism by which the transcription machinery coordinates the deposition of H3K36me3 is not well understood. Here we provide cryo-electron microscopy structures of mammalian RNA polymerase II-DSIF-SPT6-PAF1c-TFIIS-IWS1-SETD2-nucleosome elongation complexes, revealing that the transcription machinery regulates H3K36me3 deposition by SETD2 on downstream and upstream nucleosomes. SPT6 binds the exposed H2A-H2B dimer during transcription, and the SPT6 death-like domain mediates an interaction with SETD2 bound to a nucleosome upstream of RNA polymerase II.

THE HISTONE CHAPERONE SPN1 PRESERVES CHROMATIN PROTECTIONS AT PROMOTERS AND NUCLEOSOME POSITIONING IN OPEN READING FRAMES

Spn1 is a multifunctional histone chaperone that associates with RNA polymerase II during elongation and is essential for life in eukaryotes. While previous work has elucidated regions of the protein important for its many interactions, it is unknown how these domains contribute to the maintenance of chromatin structure. Here, we employ digestion by micrococcal nuclease followed by single-stranded library preparation and sequencing (MNase-SSP) to characterize chromatin structure in Saccharomyces cerevisiae expressing wild-type or mutants of Spn1 (spn1 K192N or spn1 141-305 ). We mapped protections of all sizes genome-wide. Surprisingly, we observed a widespread loss of short fragments over nucleosome-depleted regions (NDRs) at promoters in the spn1 K192N -containing strain, indicating critical functions of Spn1 in maintaining normal chromatin architecture outside open reading frames. Additionally, there are shifts in DNA protections in both Spn1 mutant expressing strains over open reading frames, which indicate changes in nucleosome and subnucleosome positioning. This was observed in markedly different Spn1 mutant strains, demonstrating that multiple functions of Spn1 are required to maintain proper chromatin structure in open reading frames. Changes in chromatin structure correlate positively with changes in gene expression as shown by RNA-seq analysis in the Spn1 mutant strains. Taken together, our results reveal a previously unknown role of Spn1 in the maintenance of NDR architecture and deepen our understanding of Spn1-dependent chromatin maintenance over transcribed regions.

Nucleosome wrapping states encode principles of 3D genome organization

Nucleosome is the basic structural unit of the genome. During processes like DNA replication and gene transcription, the conformation of nucleosomes undergoes dynamic changes, including DNA unwrapping and rewrapping, as well as histone disassembly and assembly. However, the wrapping characteristics of nucleosomes across the entire genome, including region-specificity and their correlation with higher-order chromatin organization, remains to be studied. In this study, we investigate the wrapping length of DNA on nucleosomes across the whole genome using wrapping-seq. We discover that the chromatin of mouse ES cells forms Nucleosome Wrapping Domains (NRDs), which can also be observed in yeast and fly genomes. We find that the degree of nucleosome wrapping decreases after DNA replication and is promoted by transcription. Furthermore, we observe that nucleosome wrapping domains delineate Hi-C compartments and replication timing domains. In conclusion, we have unveiled a previously unrecognized domainization principle of the chromatin, encoded by nucleosome wrapping states.

Chromatin Transcription Elongation - A Structural Perspective

In eukaryotic cells, transcription by RNA polymerase II occurs in the context of chromatin, requiring the transcription machinery to navigate through nucleosomes as it traverses gene bodies. Recent advances in structural biology have provided unprecedented insights into the mechanisms underlying transcription elongation. This review presents a structural perspective on transcription through chromatin, focusing on the latest findings from high-resolution structures of transcribing RNA polymerase II-nucleosome complexes. I discuss how RNA polymerase II, in concert with elongation factors such as SPT4/5, SPT6, ELOF1, and the PAF1 complex, engages with and transcribes through nucleosomes. The review examines the stepwise unwrapping of nucleosomal DNA as polymerase advances, the roles of elongation factors in facilitating this process, and the mechanisms of nucleosome retention and transfer during transcription. This structural perspective provides a foundation for understanding the intricate interplay between the transcription machinery and chromatin, offering insights into how cells balance the need for genetic accessibility with the maintenance of genome stability and epigenetic regulation.

RNA Polymerase II Activity Control of Gene Expression and Involvement in Disease

Gene expression is dependent on RNA Polymerase II (Pol II) activity in eukaryotes. In addition to determining the rate of RNA synthesis for all protein coding genes, Pol II serves as a platform for the recruitment of factors and regulation of co-transcriptional events, from RNA processing to chromatin modification and remodeling. The transcriptome can be shaped by changes in Pol II kinetics affecting RNA synthesis itself or because of alterations to co-transcriptional events that are responsive to or coupled with transcription. Genetic, biochemical, and structural approaches to Pol II in model organisms have revealed critical insights into how Pol II works and the types of factors that regulate it. The complexity of Pol II regulation generally increases with organismal complexity. In this review, we describe fundamental aspects of how Pol II activity can shape gene expression, discuss recent advances in how Pol II elongation is regulated on genes, and how altered Pol II function is linked to human disease and aging.

Cell cycle-regulated transcriptional pausing of Drosophila replication-dependent histone genes

Coordinated expression of replication-dependent (RD) histones genes occurs within the Histone Locus Body (HLB) during S phase, but the molecular steps in transcription that are cell cycle regulated are unknown. We report that Drosophila RNA Pol II promotes HLB formation and is enriched in the HLB outside of S phase, including G1-arrested cells that do not transcribe RD histone genes. In contrast, the transcription elongation factor Spt6 is enriched in HLBs only during S phase. Proliferating cells in the wing and eye primordium express full-length histone mRNAs during S phase but express only short nascent transcripts in cells in G1 or G2 consistent with these transcripts being paused and then terminated. Full-length transcripts are produced when Cyclin E/Cdk2 is activated as cells enter S phase. Thus, activation of transcription elongation by Cyclin E/Cdk2 and not recruitment of RNA pol II to the HLB is the critical step that links histone gene expression to cell cycle progression in Drosophila.

A direct interaction between the Chd1 CHCT domain and Rtf1 controls Chd1 distribution and nucleosome positioning on active genes

The nucleosome remodeler Chd1 is required for the re-establishment of nucleosome positioning in the wake of transcription elongation by RNA Polymerase II. Previously, we found that Chd1 occupancy on gene bodies depends on the Rtf1 subunit of the Paf1 complex in yeast. Here, we identify an N-terminal region of Rtf1 and the CHCT domain of Chd1 as sufficient for their interaction and demonstrate that this interaction is direct. Mutations that disrupt the Rtf1-Chd1 interaction result in an accumulation of Chd1 at the 5' ends of Chd1-occupied genes, increased cryptic transcription, altered nucleosome positioning, and concordant shifts in histone modification profiles. We show that a homologous region within mouse RTF1 interacts with the CHCT domains of mouse CHD1 and CHD2. This work supports a conserved mechanism for coupling Chd1 family proteins to the transcription elongation complex and identifies a cellular function for a domain within Chd1 about which little is known.

Chromatin endogenous cleavage provides a global view of yeast RNA polymerase II transcription kinetics

Chromatin immunoprecipitation (ChIP-seq) is the most common approach to observe global binding of proteins to DNA in vivo. The occupancy of transcription factors (TFs) from ChIP-seq agrees well with an alternative method, chromatin endogenous cleavage (ChEC-seq2). However, ChIP-seq and ChEC-seq2 reveal strikingly different patterns of enrichment of yeast RNA polymerase II (RNAPII). We hypothesized that this reflects distinct populations of RNAPII, some of which are captured by ChIP-seq and some of which are captured by ChEC-seq2. RNAPII association with enhancers and promoters - predicted from biochemical studies - is detected well by ChEC-seq2 but not by ChIP-seq. Enhancer/promoter-bound RNAPII correlates with transcription levels and matches predicted occupancy based on published rates of enhancer recruitment, preinitiation assembly, initiation, elongation, and termination. The occupancy from ChEC-seq2 allowed us to develop a stochastic model for global kinetics of RNAPII transcription which captured both the ChEC-seq2 data and changes upon chemical-genetic perturbations to transcription. Finally, RNAPII ChEC-seq2 and kinetic modeling suggests that a mutation in the Gcn4 transcription factor that blocks interaction with the NPC destabilizes promoter-associated RNAPII without altering its recruitment to the enhancer.

The histone chaperone Spt6 controls chromatin structure through its conserved N-terminal domain

The disassembly and reassembly of nucleosomes by histone chaperones is an essential activity during eukaryotic transcription elongation. This highly conserved process maintains chromatin integrity by transiently removing nucleosomes as barriers and then restoring them in the wake of transcription. While transcription elongation requires multiple histone chaperones, there is little understanding of how most of them function and why so many are required. Here, we show that the histone chaperone Spt6 acts through its acidic, intrinsically disordered N-terminal domain (NTD) to bind histones and control chromatin structure. The Spt6 NTD is essential for viability and its histone binding activity is conserved between yeast and humans. The essential nature of the Spt6 NTD can be bypassed by changes in another histone chaperone, FACT, revealing a close functional connection between the two. Our results have led to a mechanistic model for dynamic cooperation between multiple histone chaperones during transcription elongation.

In vivo assembly of complete eukaryotic nucleosomes and (H3-H4)-only non-canonical nucleosomal particles in the model bacterium Escherichia coli

As a fundamental unit for packaging genomic DNA into chromatin, the eukaryotic nucleosome core comprises a canonical octamer with two copies for each histone, H2A, H2B, H3, and H4, wrapped around with 147 base pairs of DNA. While H3 and H4 share structure-fold with archaeal histone-like proteins, the eukaryotic nucleosome core and the complete nucleosome (the core plus H1 histone) are unique to eukaryotes. To explore whether the eukaryotic nucleosome can assemble in prokaryotes and to reconstruct the possible route for its emergence in eukaryogenesis, we developed an in vivo system for assembly of nucleosomes in the model bacterium, Escherichia coli, and successfully reconstituted the core nucleosome, the complete nucleosome, and unexpectedly the non-canonical (H3-H4)4 octasome. The core and complete nucleosomes assembled in E. coli exhibited footprints typical of eukaryotic hosts after in situ micrococcal nuclease digestion. Additionally, they caused condensation of E. coli nucleoid. We also demonstrated the stable formation of non-canonical (H3-H4)2 tetrasome and (H3-H4)4 octasomes in vivo, which are suggested to be 'fossil complex' that marks the intermediate in the progressive development of eukaryotic nucleosome. The study presents a unique platform in a bacterium for in vivo assembly and studying the properties of non-canonical variants of nucleosome.

Asymmetric fluctuation of overlapping dinucleosome studied by cryoelectron microscopy and small-angle X-ray scattering

Nucleosome remodelers modify the local structure of chromatin to release the region from nucleosome-mediated transcriptional suppression. Overlapping dinucleosomes (OLDNs) are nucleoprotein complexes formed around transcription start sites as a result of remodeling, and they consist of two nucleosome moieties: a histone octamer wrapped by DNA (octasome) and a histone hexamer wrapped by DNA (hexasome). While OLDN formation alters chromatin accessibility to proteins, the structural mechanism behind this process is poorly understood. Thus, this study investigated the characteristics of structural fluctuations in OLDNs. First, multiple structures of the OLDN were visualized through cryoelectron microscopy (cryoEM), providing an overview of the tilting motion of the hexasome relative to the octasome at the near-atomistic resolution. Second, small-angle X-ray scattering (SAXS) revealed the presence of OLDN conformations with a larger radius of gyration than cryoEM structures. A more complete description of OLDN fluctuation was proposed by SAXS-based ensemble modeling, which included possible transient structures. The ensemble model supported the tilting motion of the OLDN outlined by the cryoEM models, further suggesting the presence of more diverse conformations. The amplitude of the relative tilting motion of the hexasome was larger, and the nanoscale fluctuation in distance between the octasome and hexasome was also proposed. The cryoEM models were found to be mapped in the energetically stable region of the conformational distribution of the ensemble. Exhaustive complex modeling using all conformations that appeared in the structural ensemble suggested that conformational and motional asymmetries of the OLDN result in asymmetries in the accessibility of OLDN-binding proteins.

Four decades of Eukaryotic DNA replication: From yeast genetics to high-resolution cryo-EM structures of the replisome

I had my eyes set on DNA replication research when I took my first molecular biology course in graduate school. My election to the National Academy of Sciences came just when I was retiring from active research. It gives me an opportunity to reflect on my personal journey in eukaryotic DNA replication research, which started as a thought experiment and culminated in witnessing the determination of the cryoelectron microscopic structure of the yeast replisome in the act of transferring histone-encoded epigenetic information at the replication fork. I would like to dedicate this inaugural article to my talented trainees and valuable collaborators in gratitude for the joy they gave me in this journey. I also want to thank my mentors who instilled in me the purpose of science. I hope junior scientists will not be disheartened by the marathon nature of research, but mindful enough to integrate and pause for other equally fun and meaningful activities of life into the marathon.

Chromatin endogenous cleavage provides a global view of yeast RNA polymerase II transcription kinetics

Chromatin immunoprecipitation (ChIP-seq) is the most common approach to observe global binding of proteins to DNA in vivo. The occupancy of transcription factors (TFs) from ChIP-seq agrees well with an alternative method, chromatin endogenous cleavage (ChEC-seq2). However, ChIP-seq and ChEC-seq2 reveal strikingly different patterns of enrichment of yeast RNA polymerase II. We hypothesized that this reflects distinct populations of RNAPII, some of which are captured by ChIP-seq and some of which are captured by ChEC-seq2. RNAPII association with enhancers and promoters - predicted from biochemical studies - is detected well by ChEC-seq2 but not by ChIP-seq. Enhancer/promoter bound RNAPII correlates with transcription levels and matches predicted occupancy based on published rates of enhancer recruitment, preinitiation assembly, initiation, elongation and termination. The occupancy from ChEC-seq2 allowed us to develop a stochastic model for global kinetics of RNAPII transcription which captured both the ChEC-seq2 data and changes upon chemical-genetic perturbations to transcription. Finally, RNAPII ChEC-seq2 and kinetic modeling suggests that a mutation in the Gcn4 transcription factor that blocks interaction with the NPC destabilizes promoter-associated RNAPII without altering its recruitment to the enhancer.

Transcription elongation factor ELOF1 is required for efficient somatic hypermutation and class switch recombination

Somatic hypermutation (SHM) and class switch recombination (CSR) diversify immunoglobulin (Ig) genes and are initiated by the activation induced deaminase (AID), a single-stranded DNA cytidine deaminase that is thought to engage its substrate in the context of RNA polymerase II (RNAPII) transcription. Through a loss of function genetic screen, we identified numerous potential factors involved in SHM including ELOF1, a component of the RNAPII elongation complex that has been shown to function in DNA repair and transcription elongation. Loss of ELOF1 strongly compromises SHM, CSR, and AID targeting and alters RNAPII transcription by reducing RNAPII pausing downstream of transcription start sites and levels of serine 5 but not serine 2 phosphorylated RNAPII throughout transcribed genes. ELOF1 must bind to RNAPII to be a proximity partner for AID and to function in SHM and CSR. We propose that ELOF1 helps create the appropriate stalled RNAPII substrate on which AID acts.

A Compendium of G-Flipon Biological Functions That Have Experimental Validation

As with all new fields of discovery, work on the biological role of G-quadruplexes (GQs) has produced a number of results that at first glance are quite baffling, sometimes because they do not fit well together, but mostly because they are different from commonly held expectations. Like other classes of flipons, those that form G-quadruplexes have a repeat sequence motif that enables the fold. The canonical DNA motif (G3N1-7)3G3, where N is any nucleotide and G is guanine, is a feature that is under active selection in avian and mammalian genomes. The involvement of G-flipons in genome maintenance traces back to the invertebrate Caenorhabditis elegans and to ancient DNA repair pathways. The role of GQs in transcription is supported by the observation that yeast Rap1 protein binds both B-DNA, in a sequence-specific manner, and GQs, in a structure-specific manner, through the same helix. Other sequence-specific transcription factors (TFs) also engage both conformations to actuate cellular transactions. Noncoding RNAs can also modulate GQ formation in a sequence-specific manner and engage the same cellular machinery as localized by TFs, linking the ancient RNA world with the modern protein world. The coevolution of noncoding RNAs and sequence-specific proteins is supported by studies of early embryonic development, where the transient formation of G-quadruplexes coordinates the epigenetic specification of cell fate.

Finish the unfinished: Chd1 resolving hexasome-nucleosome complex with FACT

In this issue of Molecular Cell, Engeholm et al.1 present cryo-EM structures of the chromatin remodeler Chd1 bound to a hexasome-nucleosome complex, an intermediate state during transcription either with or without FACT to restore the missing H2A-H2B dimer. These two binding modes reveal how Chd1 and FACT cooperate in nucleosome re-establishment during transcription.

Resolution of transcription-induced hexasome-nucleosome complexes by Chd1 and FACT

To maintain the nucleosome organization of transcribed genes, ATP-dependent chromatin remodelers collaborate with histone chaperones. Here, we show that at the 5' ends of yeast genes, RNA polymerase II (RNAPII) generates hexasomes that occur directly adjacent to nucleosomes. The resulting hexasome-nucleosome complexes are then resolved by Chd1. We present two cryoelectron microscopy (cryo-EM) structures of Chd1 bound to a hexasome-nucleosome complex before and after restoration of the missing inner H2A/H2B dimer by FACT. Chd1 uniquely interacts with the complex, positioning its ATPase domain to shift the hexasome away from the nucleosome. In the absence of the inner H2A/H2B dimer, its DNA-binding domain (DBD) packs against the ATPase domain, suggesting an inhibited state. Restoration of the dimer by FACT triggers a rearrangement that displaces the DBD and stimulates Chd1 remodeling. Our results demonstrate how chromatin remodelers interact with a complex nucleosome assembly and suggest how Chd1 and FACT jointly support transcription by RNAPII.

Structural basis of eukaryotic transcription termination by the Rat1 exonuclease complex

The 5´-3´ exoribonuclease Rat1/Xrn2 is responsible for the termination of eukaryotic mRNA transcription by RNAPII. Rat1 forms a complex with its partner proteins, Rai1 and Rtt103, and acts as a "torpedo" to bind transcribing RNAPII and dissociate DNA/RNA from it. Here we report the cryo-electron microscopy structures of the Rat1-Rai1-Rtt103 complex and three Rat1-Rai1-associated RNAPII complexes (type-1, type-1b, and type-2) from the yeast, Komagataella phaffii. The Rat1-Rai1-Rtt103 structure revealed that Rat1 and Rai1 form a heterotetramer with a single Rtt103 bound between two Rai1 molecules. In the type-1 complex, Rat1-Rai1 forms a heterodimer and binds to the RNA exit site of RNAPII to extract RNA into the Rat1 exonuclease active site. This interaction changes the RNA path in favor of termination (the "pre-termination" state). The type-1b and type-2 complexes have no bound DNA/RNA, likely representing the "post-termination" states. These structures illustrate the termination mechanism of eukaryotic mRNA transcription.

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.

Impact of the interaction between herpes simplex virus 1 ICP22 and FACT on viral gene expression and pathogenesis

Facilitates chromatin transcription (FACT) interacts with nucleosomes to promote gene transcription by regulating the dissociation and reassembly of nucleosomes downstream and upstream of RNA polymerase II (Pol II). A previous study reported that herpes simplex virus 1 (HSV-1) regulatory protein ICP22 interacted with FACT and was required for its recruitment to the viral DNA genome in HSV-1-infected cells. However, the biological importance of interactions between ICP22 and FACT in relation to HSV-1 infection is unclear. Here, we mapped the minimal domain of ICP22 required for its efficient interaction with FACT to a cluster of five basic amino acids in ICP22. A recombinant virus harboring alanine substitutions in this identified cluster led to the decreased accumulation of viral mRNAs from UL54, UL38, and UL44 genes, reduced Pol II occupancy of these genes in MRC-5 cells, and impaired HSV-1 virulence in mice following ocular or intracranial infection. Furthermore, the treatment of mice infected with wild-type HSV-1 with CBL0137, a FACT inhibitor currently being investigated in clinical trials, significantly improved the survival rate of mice. These results suggested that the interaction between ICP22 and FACT was required for efficient HSV-1 gene expression and pathogenicity. Therefore, FACT might be a potential therapeutic target for HSV-1 infection.IMPORTANCEICP22 is a well-known regulatory factor of HSV-1 gene expression, but its mechanism(s) are poorly understood. Although the interaction of FACT with ICP22 was reported previously, its significance in HSV-1 infection is unknown. Given that FACT is involved in gene transcription, it is of interest to investigate this interaction as it relates to HSV-1 gene expression. To determine a direct link between the interaction and HSV-1 infection, we mapped a minimal domain of ICP22 required for its efficient interaction with FACT and generated a recombinant virus carrying mutations in the identified domain. Using the recombinant virus, we obtained evidence suggesting that the interaction between ICP22 and FACT promoted Pol II transcription from HSV-1 genes and viral virulence in mice. In addition, CBL0137, an inhibitor of FACT, effectively protected mice from lethal HSV-1 infection, suggesting FACT might be a potential target for the development of novel anti-HSV drugs.

FACT maintains chromatin architecture and thereby stimulates RNA polymerase II pausing during transcription in vivo

Facilitates chromatin transcription (FACT) is a histone chaperone that supports transcription through chromatin in vitro, but its functional roles in vivo remain unclear. Here, we analyze the in vivo functions of FACT with the use of multi-omics analysis after rapid FACT depletion from human cells. We show that FACT depletion destabilizes chromatin and leads to transcriptional defects, including defective promoter-proximal pausing and elongation, and increased premature termination of RNA polymerase II. Unexpectedly, our analysis revealed that promoter-proximal pausing depends not only on the negative elongation factor (NELF) but also on the +1 nucleosome, which is maintained by FACT.

Histone H2B ubiquitylation: Connections to transcription and effects on chromatin structure

Nucleosomes are major determinants of eukaryotic genome organization and regulation. Many studies, incorporating a diversity of experimental approaches, have been focused on identifying and discerning the contributions of histone post-translational modifications to DNA-centered processes. Among these, monoubiquitylation of H2B (H2Bub) on K120 in humans or K123 in budding yeast is a critical histone modification that has been implicated in a wide array of DNA transactions. H2B is co-transcriptionally ubiquitylated and deubiquitylated via the concerted action of an extensive network of proteins. In addition to altering the chemical and physical properties of the nucleosome, H2Bub is important for the proper control of gene expression and for the deposition of other histone modifications. In this review, we discuss the molecular mechanisms underlying the ubiquitylation cycle of H2B and how it connects to the regulation of transcription and chromatin structure.

Chromatin regulates alternative polyadenylation via the RNA polymerase II elongation rate

The RNA polymerase II (Pol II) elongation rate influences poly(A) site selection, with slow and fast Pol II derivatives causing upstream and downstream shifts, respectively, in poly(A) site utilization. In yeast, depletion of either of the histone chaperones FACT or Spt6 causes an upstream shift of poly(A) site use that strongly resembles the poly(A) profiles of slow Pol II mutant strains. Like slow Pol II mutant strains, FACT- and Spt6-depleted cells exhibit Pol II processivity defects, indicating that both Spt6 and FACT stimulate the Pol II elongation rate. Poly(A) profiles of some genes show atypical downstream shifts; this subset of genes overlaps well for FACT- or Spt6-depleted strains but is different from the atypical genes in Pol II speed mutant strains. In contrast, depletion of histone H3 or H4 causes a downstream shift of poly(A) sites for most genes, indicating that nucleosomes inhibit the Pol II elongation rate in vivo. Thus, chromatin-based control of the Pol II elongation rate is a potential mechanism, distinct from direct effects on the cleavage/polyadenylation machinery, to regulate alternative polyadenylation in response to genetic or environmental changes.

Transcript elongation by RNA polymerase II in plants: factors, regulation and impact on gene expression

Transcriptional elongation by RNA polymerase II (RNAPII) through chromatin is a dynamic and highly regulated step of eukaryotic gene expression. A combination of transcript elongation factors (TEFs) including modulators of RNAPII activity and histone chaperones facilitate efficient transcription on nucleosomal templates. Biochemical and genetic analyses, primarily performed in Arabidopsis, provided insight into the contribution of TEFs to establish gene expression patterns during plant growth and development. In addition to summarising the role of TEFs in plant gene expression, we emphasise in our review recent advances in the field. Thus, mechanisms are presented how aberrant intragenic transcript initiation is suppressed by repressing transcriptional start sites within coding sequences. We also discuss how transcriptional interference of ongoing transcription with neighbouring genes is prevented. Moreover, it appears that plants make no use of promoter-proximal RNAPII pausing in the way mammals do, but there are nucleosome-defined mechanism(s) that determine the efficiency of mRNA synthesis by RNAPII. Accordingly, a still growing number of processes related to plant growth, development and responses to changing environmental conditions prove to be regulated at the level of transcriptional elongation.

A conserved function of corepressors is to nucleate assembly of the transcriptional preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we have leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5 and SPT6 as necessary for repression with the SPT4 subunit acting as a bridge connecting TPL to SPT5 and SPT6. We also discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved in early transcription initiation events. These findings were validated in yeast and plants through multiple assays, including a novel method to analyze conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate rapid onset of transcription once repression is relieved.

Structural basis of exoribonuclease-mediated mRNA transcription termination

Efficient termination is required for robust gene transcription. Eukaryotic organisms use a conserved exoribonuclease-mediated mechanism to terminate the mRNA transcription by RNA polymerase II (Pol II)1-5. Here we report two cryogenic electron microscopy structures of Saccharomyces cerevisiae Pol II pre-termination transcription complexes bound to the 5'-to-3' exoribonuclease Rat1 and its partner Rai1. Our structures show that Rat1 displaces the elongation factor Spt5 to dock at the Pol II stalk domain. Rat1 shields the RNA exit channel of Pol II, guides the nascent RNA towards its active centre and stacks three nucleotides at the 5' terminus of the nascent RNA. The structures further show that Rat1 rotates towards Pol II as it shortens RNA. Our results provide the structural mechanism for the Rat1-mediated termination of mRNA transcription by Pol II in yeast and the exoribonuclease-mediated termination of mRNA transcription in other eukaryotes.

Cryo-EM structures of RAD51 assembled on nucleosomes containing a DSB site

RAD51 is the central eukaryotic recombinase required for meiotic recombination and mitotic repair of double-strand DNA breaks (DSBs)1,2. However, the mechanism by which RAD51 functions at DSB sites in chromatin has remained elusive. Here we report the cryo-electron microscopy structures of human RAD51-nucleosome complexes, in which RAD51 forms ring and filament conformations. In the ring forms, the N-terminal lobe domains (NLDs) of RAD51 protomers are aligned on the outside of the RAD51 ring, and directly bind to the nucleosomal DNA. The nucleosomal linker DNA that contains the DSB site is recognized by the L1 and L2 loops-active centres that face the central hole of the RAD51 ring. In the filament form, the nucleosomal DNA is peeled by the RAD51 filament extension, and the NLDs of RAD51 protomers proximal to the nucleosome bind to the remaining nucleosomal DNA and histones. Mutations that affect nucleosome-binding residues of the RAD51 NLD decrease nucleosome binding, but barely affect DNA binding in vitro. Consistently, yeast Rad51 mutants with the corresponding mutations are substantially defective in DNA repair in vivo. These results reveal an unexpected function of the RAD51 NLD, and explain the mechanism by which RAD51 associates with nucleosomes, recognizes DSBs and forms the active filament in chromatin.

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.

Safeguarding the epigenome through the cell cycle: a multitasking game

Sustaining cell identity and function across cell division is germane to human development, healthspan, and cancer avoidance. This relies significantly on propagation of chromatin organization between cell generations, as chromatin presents a barrier to cell fate and cell state conversions. Inheritance of chromatin states across the many cell divisions required for development and tissue homeostasis represents a major challenge, especially because chromatin is disrupted to allow passage of the DNA replication fork to synthesize the two daughter strands. This process also leads to a twofold dilution of epigenetic information in histones, which needs to be accurately restored for faithful propagation of chromatin states across cell divisions. Recent research has identified distinct multilayered mechanisms acting to propagate epigenetic information to daughter strands. Here, we summarize key principles of how epigenetic information in parental histones is transferred across DNA replication and how new histones robustly acquire the same information postreplication, representing a core component of epigenetic cell memory.

Cryo-electron microscopy reveals the impact of the nucleosome dynamics on transcription activity

The structural biology of nucleosomes and their complexes with chromatin-associated factors contributes to our understanding of fundamental biological processes in the genome. With the advent of cryo-electron microscopy (cryo-EM), several structures are emerging with histone variants, various species and chromatin-associated proteins that bind to nucleosomes. Cryo-EM enables visualization of the dynamic states of nucleosomes, leading to the accumulation of knowledge on chromatin-templated biology. The cryo-EM structure of nucleosome in Komagataella pastoris, as studied by Fukushima et al., provided the insights into transcription ability of RNAPII with nucleosome dynamics. In this commentary, we review the recent advances in the structural biology of nucleosomes and their related biomolecules.

Structural perspectives on transcription in chromatin

In eukaryotes, all genetic processes take place in the cell nucleus, where DNA is packaged as chromatin in 'beads-on-a-string' nucleosome arrays. RNA polymerase II (RNAPII) transcribes protein-coding and many non-coding genes in this chromatin environment. RNAPII elongates RNA while passing through multiple nucleosomes and maintaining the integrity of the chromatin structure. Recent structural studies have shed light on the detailed mechanisms of this process, including how transcribing RNAPII progresses through a nucleosome and reassembles it afterwards, and how transcription elongation factors, chromatin remodelers, and histone chaperones participate in these processes. Other studies have also illuminated the crucial role of nucleosomes in preinitiation complex assembly and transcription initiation. In this review we outline these advances and discuss future perspectives.