Effects of histone H2B ubiquitylation on the nucleosome structure and dynamics

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.

Histone ubiquitination: Role in genome integrity and chromatin organization

Maintenance of genome integrity is a precise but tedious and complex job for the cell. Several post-translational modifications (PTMs) play vital roles in maintaining the genome integrity. Although ubiquitination is one of the most crucial PTMs, which regulates the localization and stability of the nonhistone proteins in various cellular and developmental processes, ubiquitination of the histones is a pivotal epigenetic event critically regulating chromatin architecture. In addition to genome integrity, importance of ubiquitination of core histones (H2A, H2A, H3, and H4) and linker histone (H1) have been reported in several cellular processes. However, the complex interplay of histone ubiquitination and other PTMs, as well as the intricate chromatin architecture and dynamics, pose a significant challenge to unravel how histone ubiquitination safeguards genome stability. Therefore, further studies are needed to elucidate the interactions between histone ubiquitination and other PTMs, and their role in preserving genome integrity. Here, we review all types of histone ubiquitinations known till date in maintaining genomic integrity during transcription, replication, cell cycle, and DNA damage response processes. In addition, we have also discussed the role of histone ubiquitination in regulating other histone PTMs emphasizing methylation and acetylation as well as their potential implications in chromatin architecture. Further, we have also discussed the involvement of deubiquitination enzymes (DUBs) in controlling histone ubiquitination in modulating cellular processes.

Epigenetic regulators controlling osteogenic lineage commitment and bone formation

Bone formation and homeostasis are controlled by environmental factors and endocrine regulatory cues that initiate intracellular signaling pathways capable of modulating gene expression in the nucleus. Bone-related gene expression is controlled by nucleosome-based chromatin architecture that limits the accessibility of lineage-specific gene regulatory DNA sequences and sequence-specific transcription factors. From a developmental perspective, bone-specific gene expression must be suppressed during the early stages of embryogenesis to prevent the premature mineralization of skeletal elements during fetal growth in utero. Hence, bone formation is initially inhibited by gene suppressive epigenetic regulators, while other epigenetic regulators actively support osteoblast differentiation. Prominent epigenetic regulators that stimulate or attenuate osteogenesis include lysine methyl transferases (e.g., EZH2, SMYD2, SUV420H2), lysine deacetylases (e.g., HDAC1, HDAC3, HDAC4, HDAC7, SIRT1, SIRT3), arginine methyl transferases (e.g., PRMT1, PRMT4/CARM1, PRMT5), dioxygenases (e.g., TET2), bromodomain proteins (e.g., BRD2, BRD4) and chromodomain proteins (e.g., CBX1, CBX2, CBX5). This narrative review provides a broad overview of the covalent modifications of DNA and histone proteins that involve hundreds of enzymes that add, read, or delete these epigenetic modifications that are relevant for self-renewal and differentiation of mesenchymal stem cells, skeletal stem cells and osteoblasts during osteogenesis.

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.

Transcriptional co-activators: emerging roles in signaling pathways and potential therapeutic targets for diseases

Specific cell states in metazoans are established by the symphony of gene expression programs that necessitate intricate synergic interactions between transcription factors and the co-activators. Deregulation of these regulatory molecules is associated with cell state transitions, which in turn is accountable for diverse maladies, including developmental disorders, metabolic disorders, and most significantly, cancer. A decade back most transcription factors, the key enablers of disease development, were historically viewed as 'undruggable'; however, in the intervening years, a wealth of literature validated that they can be targeted indirectly through transcriptional co-activators, their confederates in various physiological and molecular processes. These co-activators, along with transcription factors, have the ability to initiate and modulate transcription of diverse genes necessary for normal physiological functions, whereby, deregulation of such interactions may foster tissue-specific disease phenotype. Hence, it is essential to analyze how these co-activators modulate specific multilateral processes in coordination with other factors. The proposed review attempts to elaborate an in-depth account of the transcription co-activators, their involvement in transcription regulation, and context-specific contributions to pathophysiological conditions. This review also addresses an issue that has not been dealt with in a comprehensive manner and hopes to direct attention towards future research that will encompass patient-friendly therapeutic strategies, where drugs targeting co-activators will have enhanced benefits and reduced side effects. Additional insights into currently available therapeutic interventions and the associated constraints will eventually reveal multitudes of advanced therapeutic targets aiming for disease amelioration and good patient prognosis.

Global histone H2B degradation regulates insulin/IGF signaling-mediated nutrient stress

Eukaryotic organisms adapt to environmental fluctuations by altering their epigenomic landscapes and transcriptional programs. Nucleosomal histones carry vital epigenetic information and regulate gene expression, yet the mechanisms underlying chromatin-bound histone exchange remain elusive. Here, we found that histone H2Bs are globally degraded in Caenorhabditis elegans during starvation. Our genetic screens identified mutations in ubiquitin and ubiquitin-related enzymes that block H2B degradation in starved animals, identifying lysine 31 as the crucial residue for chromatin-bound H2B ubiquitination and elimination. Retention of aberrant nucleosomal H2B increased the association of the FOXO transcription factor DAF-16 with chromatin, generating an ectopic gene expression profile detrimental to animal viability when insulin/IGF signaling was reduced in well-fed animals. Furthermore, we show that the ubiquitin-proteasome system regulates chromosomal histone turnover in human cells. During larval development, C. elegans epidermal cells undergo H2B turnover after fusing with the epithelial syncytium. Thus, histone degradation may be a widespread mechanism governing dynamic changes of the epigenome.

The role and prospect of lysine-specific demethylases in cancer chemoresistance

Histone methylation plays a key function in modulating gene expression, and preserving genome integrity and epigenetic inheritance. However, aberrations of histone methylation are commonly observed in human diseases, especially cancer. Lysine methylation mediated by histone methyltransferases can be reversed by lysine demethylases (KDMs), which remove methyl marks from histone lysine residues. Currently, drug resistance is a main impediment for cancer therapy. KDMs have been found to mediate drug tolerance of many cancers via altering the metabolic profile of cancer cells, upregulating the ratio of cancer stem cells and drug-tolerant genes, and promoting the epithelial-mesenchymal transition and metastatic ability. Moreover, different cancers show distinct oncogenic addictions for KDMs. The abnormal activation or overexpression of KDMs can alter gene expression signatures to enhance cell survival and drug resistance in cancer cells. In this review, we describe the structural features and functions of KDMs, the KDMs preferences of different cancers, and the mechanisms of drug resistance resulting from KDMs. We then survey KDM inhibitors that have been used for combating drug resistance in cancer, and discuss the opportunities and challenges of KDMs as therapeutic targets for cancer drug resistance.

Effects of Histone H2B Ubiquitylations and H3K79me3 on Transcription Elongation

Post-translational modifications of histone proteins often mediate gene regulation by altering the global and local stability of the nucleosome, the basic gene-packing unit of eukaryotes. We employed semisynthetic approaches to introduce histone H2B ubiquitylations at K34 (H2BK34ub) and K120 (H2BK120ub) and H3K79 trimethylation (H3K79me3). With these modified histones, we investigated their effects on the kinetics of transcription elongation by RNA polymerase II (Pol II) using single-molecule FRET. Pol II pauses at several locations within the nucleosome for a few seconds to minutes, which governs the overall transcription efficiency. We found that H2B ubiquitylations suppress pauses and shorten the pause durations near the nucleosome entry while H3K79me3 shortens the pause durations and increases the rate of RNA elongation near the center of the nucleosome. We also found that H2BK34ub facilitates partial rewrapping of the nucleosome upon Pol II passage. These observations suggest that H2B ubiquitylations promote transcription elongation and help maintain the chromatin structure by inducing and stabilizing nucleosome intermediates and that H3K79me3 facilitates Pol II progression possibly by destabilizing the local structure of the nucleosome. Our results provide the mechanisms of how these modifications coupled by a network of regulatory proteins facilitate transcription in two different regions of the nucleosome and help maintain the chromatin structure during active transcription.

The Effects of Histone H2B ubiquitylations and H3K79me 3 on Transcription Elongation

Post-translational modifications of histone proteins often mediate gene regulation by altering the global and local stability of the nucleosome, the basic gene-packing unit of eukaryotes. We employed semi-synthetic approaches to introduce histone H2B ubiquitylations at K34 (H2BK34ub) and K120 (H2BK120ub) and H3 K79 trimethylation (H3K79me3). With these modified histones, we investigated their effects on the kinetics of transcription elongation by RNA Polymerase II (Pol II) using single-molecule FRET. Pol II pauses at several locations within the nucleosome for a few seconds to minutes, which governs the overall transcription efficiency. We found that H2B ubiquitylations suppress pauses and shorten the pause durations near the nucleosome entry while H3K79me3 shortens the pause durations and increases the rate of RNA elongation near the center of the nucleosome. We also found that H2BK34ub facilitates partial rewrapping of the nucleosome upon Pol II passage. These observations suggest that H2B ubiquitylations promote transcription elongation and help maintain the chromatin structure by inducing and stabilizing nucleosome intermediates and that H3K79me3 facilitates Pol II progression possibly by destabilizing the local structure of the nucleosome. Our results provide the mechanisms of how these modifications coupled by a network of regulatory proteins facilitate transcription in two different regions of the nucleosome and help maintain the chromatin structure during active transcription.

Histone H2Bub dynamics in the 5' region of active genes are tightly linked to the UV-induced transcriptional response

The timing and location of writing and erasing of histone modifications determine gene expression programs and are tightly controlled processes. One such modification is the monoubiquitination of histone H2B (H2Bub), whose precise level during transcription elongation is dynamically regulated by the synergistic action of RNF20/40 ubiquitin-ligase and the de-ubiquitinase (DUB) of the ATXN7L3-containing DUB modules. Here, we characterize the dynamics of H2Bub in transcription and explore its role in perspective with the recently updated model of UV damage-induced transcription reorganization. Employing integrative analysis of genome-wide high-throughput approaches, transcription inhibitors and ATXN7L3-DUB knockdown cells, we find that H2Bub levels and patterns depend on intron-exon architecture both in steady state and upon UV. Importantly, our analysis reveals a widespread redistribution of this histone mark, rather than a uniform loss as previously suggested, which closely mirrors the post-UV dynamics of elongating RNA Polymerase II (RNAPII) at transcribed loci. The observed effects are due to a direct inter-dependence on RNAPII local concentration and speed, and we show that deficient ATXN7L3-mediated DUB activity leads to increased elongation rates in both non-irradiated and irradiated conditions. Our data and the implementation of a high-resolution computational framework reveal that the H2Bub pattern follows that of RNAPII, both in the ATXNL3 knockdown and in response to UV guaranteeing faithful elongation speed, especially in the context of the transcription-driven DNA damage response.

Characterizing and exploiting the many roles of aberrant H2B monoubiquitination in cancer pathogenesis

Monoubiquitination of histone H2B on lysine 120 (H2Bub1) is implicated in the control of multiple essential processes, including transcription, DNA damage repair and mitotic chromosome segregation. Accordingly, aberrant regulation of H2Bub1 can induce transcriptional reprogramming and genome instability that may promote oncogenesis. Remarkably, alterations of the ubiquitin ligases and deubiquitinating enzymes regulating H2Bub1 are emerging as ubiquitous features in cancer, further supporting the possibility that the misregulation of H2Bub1 is an underlying mechanism contributing to cancer pathogenesis. To date, aberrant H2Bub1 dynamics have been reported in multiple cancer types and are associated with transcriptional changes that promote oncogenesis in a cancer type-specific manner. Owing to the multi-functional nature of H2Bub1, misregulation of its writers and erasers may drive disease initiation and progression through additional synergistic processes. Accordingly, understanding the molecular determinants and pathogenic impacts associated with aberrant H2Bub1 regulation may reveal novel drug targets and therapeutic vulnerabilities that can be exploited to develop innovative precision medicine strategies that better combat cancer. In this review, we present the normal functions of H2Bub1 in the control of DNA-associated processes and describe the pathogenic implications associated with its misregulation in cancer. We further discuss the challenges coupled with the development of therapeutic strategies targeting H2Bub1 misregulation and expose the potential benefits of designing treatments that synergistically exploit the multiple functionalities of H2Bub1 to improve treatment selectivity and efficacy.

The Effects of Histone H2B Ubiquitylations on the Nucleosome Structure and Internucleosomal Interactions

Eukaryotic gene compaction takes place at multiple levels to package DNA to chromatin and chromosomes. Two of the most fundamental levels of DNA packaging are at the nucleosome and dinucleosome stacks. The nucleosome is the basic gene-packing unit and is composed of DNA wrapped around a histone core. Nucleosomes stack with one another for further compaction of DNA. The first stacking step leads to dinucleosome formation, which is driven by internucleosomal interactions between various parts of two nucleosomes. Histone proteins are rich targets for post-translational modifications, some of which affect the structure of the nucleosome and the interactions between nucleosomes. These effects are often implicated in the regulation of various genomic transactions. In particular, histone H2B ubiquitylation has been associated with facilitated transcription and hexasome formation. Here, we employed semi-synthetically ubiquitylated histone H2B and single-molecule FRET to investigate the effects of H2B ubiquitylations at lysine 34 (H2BK34) and lysine 120 (H2BK120) on the structure of the nucleosome and the interactions between two nucleosomes. Our results suggest that H2BK34 ubiquitylation widens the DNA gyre gap in the nucleosome and stabilizes long- and short-range internucleosomal interactions while H2BK120 ubiquitylation does not affect the nucleosome structure or internucleosomal interactions. These results suggest potential roles for H2B ubiquitylations in facilitated transcription and hexasome formation while maintaining the structural integrity of chromatin.

H2A-H2B Histone Dimer Plasticity and Its Functional Implications

The protein core of the nucleosome is composed of an H3-H4 histone tetramer and two H2A-H2B histone dimers. The tetramer organizes the central 60 DNA bp, while H2A-H2B dimers lock the flanking DNA segments. Being positioned at the sides of the nucleosome, H2A-H2B dimers stabilize the overall structure of the nucleosome and modulate its dynamics, such as DNA unwrapping, sliding, etc. Such modulation at the epigenetic level is achieved through post-translational modifications and the incorporation of histone variants. However, the detailed connection between the sequence of H2A-H2B histones and their structure, dynamics and implications for nucleosome functioning remains elusive. In this work, we present a detailed study of H2A-H2B dimer dynamics in the free form and in the context of nucleosomes via atomistic molecular dynamics simulations (based on X. laevis histones). We supplement simulation results by comparative analysis of information in the structural databases. Particularly, we describe a major dynamical mode corresponding to the bending movement of the longest H2A and H2B α-helices. This overall bending dynamics of the H2A-H2B dimer were found to be modulated by its interactions with DNA, H3-H4 tetramer, the presence of DNA twist-defects with nucleosomal DNA and the amino acid sequence of histones. Taken together, our results shed new light on the dynamical mechanisms of nucleosome functioning, such as nucleosome sliding, DNA-unwrapping and their epigenetic modulation.

H2B Lys34 Ubiquitination Induces Nucleosome Distortion to Stimulate Dot1L Activity

Ubiquitination-dependent histone crosstalk plays critical roles in chromatin-associated processes and is highly associated with human diseases. Mechanism studies of the crosstalk have been of the central focus. Here our study on the crosstalk between H2BK34ub and Dot1L-catalyzed H3K79me suggests a novel mechanism of ubiquitination-induced nucleosome distortion to stimulate the activity of an enzyme. We determined the cryo-electron microscopy structures of Dot1L-H2BK34ub nucleosome complex and the H2BK34ub nucleosome alone. The structures reveal that H2BK34ub induces an almost identical orientation and binding pattern of Dot1L on nucleosome as H2BK120ub, which positions Dot1L for the productive conformation through direct ubiquitin-enzyme contacts. However, H2BK34-anchored ubiquitin does not directly interact with Dot1L as occurs in the case of H2BK120ub, but rather induces DNA and histone distortion around the modified site. Our findings establish the structural framework for understanding the H2BK34ub-H3K79me trans-crosstalk and highlight the diversity of mechanisms for histone ubiquitination to activate chromatin-modifying enzymes.

Decoding histone ubiquitylation

Histone ubiquitylation is a critical part of both active and repressed transcriptional states, and lies at the heart of DNA damage repair signaling. The histone residues targeted for ubiquitylation are often highly conserved through evolution, and extensive functional studies of the enzymes that catalyze the ubiquitylation and de-ubiquitylation of histones have revealed key roles linked to cell growth and division, development, and disease in model systems ranging from yeast to human cells. Nonetheless, the downstream consequences of these modifications have only recently begun to be appreciated on a molecular level. Here we review the structure and function of proteins that act as effectors or “readers” of histone ubiquitylation. We highlight lessons learned about how ubiquitin recognition lends specificity and function to intermolecular interactions in the context of transcription and DNA repair, as well as what this might mean for how we think about histone modifications more broadly.

Histone Modifications, Internucleosome Dynamics, and DNA Stresses: How They Cooperate to “Functionalize” Nucleosomes

Tight packaging of DNA in chromatin severely constrains DNA accessibility and dynamics. In contrast, nucleosomes in active chromatin state are highly flexible, can exchange their histones, and are virtually “transparent” to RNA polymerases, which transcribe through gene bodies at rates comparable to that of naked DNA. Defining mechanisms that revert nucleosome repression, in addition to their value for basic science, is of key importance for the diagnosis and treatment of genetic diseases. Chromatin activity is largely regulated by histone posttranslational modifications, ranging from small chemical groups up to the yet understudied “bulky” ubiquitylation and sumoylation. However, it is to be revealed how histone marks are “translated” to permissive or repressive changes in nucleosomes: it is a general opinion that histone modifications act primarily as “signals” for recruiting the regulatory proteins or as a “neutralizer” of electrostatic shielding of histone tails. Here, we would like to discuss recent evidence suggesting that histone ubiquitylation, in a DNA stress–dependent manner, can directly regulate the dynamics of the nucleosome and their primary structure and can promote nucleosome decomposition to hexasome particles or additionally stabilize nucleosomes against unwrapping. In addition, nucleosome repression/ derepression studies are usually performed with single mononucleosomes as a model. We would like to review and discuss recent findings showing that internucleosomal interactions could strongly modulate the dynamics and rearrangements of nucleosomes. Our hypothesis is that bulky histone modifications, nucleosome inherent dynamics, internucleosome interactions, and DNA torsions could act in cooperation to orchestrate the formation of different dynamic states of arrayed nucleosomes and thus promote chromatin functionality and diversify epigenetic programming methods.

Probing the Effect of Ubiquitinated Histone on Mononucleosomes by Translocation Dynamics Study through Solid-State Nanopores

Post-translational modifications (PTMs), such as ubiquitination, are critically important in regulating genetic expressions by adjusting the nucleosome stability. A fast and label-free technology inspecting dynamic nucleosome structures can facilitate the interrogation of PTMs effects. Here we leverage the advantages of mechanically stable solid-state nanopores and detect the effect of a ubiquitinated histone on mononucleosomes at the single-molecule level. By comparing the translocation dynamics of natural and cross-linked mononucleosomes, we verified that the nucleosomal DNA unravelled from histones in natural mononucleosomes. Furthermore, we found that a turning point of voltage corresponds to the onset of nucleosome rupture. More importantly, we reveal that ubH2A stabilizes the nucleosome by shifting the turning point to a larger value and investigated the effect of ubiquitination on different histones (ubH2A and ubH2B). These findings open promising possibilities for developing a miniaturized and portable device for the fast screening of PTMs on nucleosomes.

The Expanding Constellation of Histone Post-Translational Modifications in the Epigenetic Landscape

The emergence of a nucleosome-based chromatin structure accompanied the evolutionary transition from prokaryotes to eukaryotes. In this scenario, histones became the heart of the complex and precisely timed coordination between chromatin architecture and functions during adaptive responses to environmental influence by means of epigenetic mechanisms. Notably, such an epigenetic machinery involves an overwhelming number of post-translational modifications at multiple residues of core and linker histones. This review aims to comprehensively describe old and recent evidence in this exciting field of research. In particular, histone post-translational modification establishing/removal mechanisms, their genomic locations and implication in nucleosome dynamics and chromatin-based processes, as well as their harmonious combination and interdependence will be discussed.

Hi-CO: 3D genome structure analysis with nucleosome resolution

The nucleosome is the basic organizational unit of the genome. The folding structure of nucleosomes is closely related to genome functions, and has been reported to be in dynamic interplay with binding of various nuclear proteins to genomic loci. Here, we describe our high-throughput chromosome conformation capture with nucleosome orientation (Hi-CO) technology to derive 3D nucleosome positions with their orientations at every genomic locus in the nucleus. This technology consists of an experimental procedure for nucleosome proximity analysis and a computational procedure for 3D modeling. The experimental procedure is based on an improved method of high-throughput chromosome conformation capture (Hi-C) analysis. Whereas conventional Hi-C allows spatial proximity analysis among genomic loci with 1-10 kbp resolution, our Hi-CO allows proximity analysis among DNA entry or exit points at every nucleosome locus. This analysis is realized by carrying out ligations among the entry/exit points in every nucleosome in a micrococcal-nuclease-fragmented genome, and by quantifying frequencies of ligation products with next-generation sequencing. Our protocol has enabled this analysis by cleanly excluding unwanted non-ligation products that are abundant owing to the frequent genome fragmentation by micrococcal nuclease. The computational procedure is based on simulated annealing-molecular dynamics, which allows determination of optimized 3D positions and orientations of every nucleosome that satisfies the proximity ligation data sufficiently well. Typically, examination of the Saccharomyces cerevisiae genome with 130 million sequencing reads facilitates analysis of a total of 66,360 nucleosome loci with 6.8 nm resolution. The technique requires 2-3 weeks for sequencing library preparation and 2 weeks for simulation.

The Histone Chaperone HIRA Is a Positive Regulator of Seed Germination

Histone chaperones regulate the flow and dynamics of histone variants and ensure their assembly into nucleosomal structures, thereby contributing to the repertoire of histone variants in specialized cells or tissues. To date, not much is known on the distribution of histone variants and their modifications in the dry seed embryo. Here, we bring evidence that genes encoding the replacement histone variant H3.3 are expressed in Arabidopsis dry seeds and that embryo chromatin is characterized by a low H3.1/H3.3 ratio. Loss of HISTONE REGULATOR A (HIRA), a histone chaperone responsible for H3.3 deposition, reduces cellular H3 levels and increases chromatin accessibility in dry seeds. These molecular differences are accompanied by increased seed dormancy in hira-1 mutant seeds. The loss of HIRA negatively affects seed germination even in the absence of HISTONE MONOUBIQUITINATION 1 or TRANSCRIPTION ELONGATION FACTOR II S, known to be required for seed dormancy. Finally, hira-1 mutant seeds show lower germination efficiency when aged under controlled deterioration conditions or when facing unfavorable environmental conditions such as high salinity. Altogether, our results reveal a dependency of dry seed chromatin organization on the replication-independent histone deposition pathway and show that HIRA contributes to modulating seed dormancy and vigor.

Ubiquitin-dependent regulation of transcription in development and disease

Transcription is an elaborate process that is required to establish and maintain the identity of the more than two hundred cell types of a metazoan organism. Strict regulation of gene expression is therefore vital for tissue formation and homeostasis. An accumulating body of work found that ubiquitylation of histones, transcription factors, or RNA polymerase II is crucial for ensuring that transcription occurs at the right time and place during development. Here, we will review principles of ubiquitin-dependent control of gene expression and discuss how breakdown of these regulatory circuits leads to a wide array of human diseases.

Dynamic modules of the coactivator SAGA in eukaryotic transcription

SAGA (Spt-Ada-Gcn5 acetyltransferase) is a highly conserved transcriptional coactivator that consists of four functionally independent modules. Its two distinct enzymatic activities, histone acetylation and deubiquitylation, establish specific epigenetic patterns on chromatin and thereby regulate gene expression. Whereas earlier studies emphasized the importance of SAGA in regulating global transcription, more recent reports have indicated that SAGA is involved in other aspects of gene expression and thus plays a more comprehensive role in regulating the overall process. Here, we discuss recent structural and functional studies of each SAGA module and compare the subunit compositions of SAGA with related complexes in yeast and metazoans. We discuss the regulatory role of the SAGA deubiquitylating module (DUBm) in mRNA surveillance and export, and in transcription initiation and elongation. The findings suggest that SAGA plays numerous roles in multiple stages of transcription. Further, we describe how SAGA is related to human disease. Overall, in this report, we illustrate the newly revealed understanding of SAGA in transcription regulation and disease implications for fine-tuning gene expression.

A protein that helps add epigenetic information to genome, SAGA, controls many aspects of gene activation, potentially making it a target for cancer therapies. To fit inside the tiny cell nucleus, the genome is tightly packaged, and genes must be unpacked before they can be activated. Known to be important in genome opening, SAGA has now been shown to also play many roles in gene activation. Daeyoup Lee at the KAIST, Daejeon, South Korea, and co-workers have reviewed recent discoveries about SAGA’s structure, function, and roles in disease. They report that SAGA’s complex (19 subunits organized into four modules) allows it to play so many roles, genome opening, initiating transcription, and efficiently exporting mRNAs. Its master role means that malfunction of SAGA may be linked to many diseases.

Pioneer factors and their in vitro identification methods

Pioneer transcription factors are a special group of transcription factors that can interact with nucleosomal DNA and initiate regulatory events. Their binding to regulatory regions is the first event in gene activation and can occur in silent or heterochromatin regions. Several research groups have endeavored to define pioneer factors and study their binding characteristics using various techniques. In this review, we describe the in vitro methods used to define and characterize pioneer factors, paying particular attention to differences in methodologies and how these differences can affect results.

Nucleosome destabilization by nuclear non-coding RNAs

In the nucleus, genomic DNA is wrapped around histone octamers to form nucleosomes. In principle, nucleosomes are substantial barriers to transcriptional activities. Nuclear non-coding RNAs (ncRNAs) are proposed to function in chromatin conformation modulation and transcriptional regulation. However, it remains unclear how ncRNAs affect the nucleosome structure. Eleanors are clusters of ncRNAs that accumulate around the estrogen receptor-α (ESR1) gene locus in long-term estrogen deprivation (LTED) breast cancer cells, and markedly enhance the transcription of the ESR1 gene. Here we detected nucleosome depletion around the transcription site of Eleanor2, the most highly expressed Eleanor in the LTED cells. We found that the purified Eleanor2 RNA fragment drastically destabilized the nucleosome in vitro. This activity was also exerted by other ncRNAs, but not by poly(U) RNA or DNA. The RNA-mediated nucleosome destabilization may be a common feature among natural nuclear RNAs, and may function in transcription regulation in chromatin.

The Eleanor cluster of non-coding RNAs is localised upstream of estrogen receptor-α (ESR1) gene locus in estrogen-deprived breast cancer cells. Fujita et al find that RNA fragments of Eleanor2 and of other non-coding RNAs are able to destabilise nucleosomes in vitro, suggesting a role in transcriptional regulation.

"Direct" and "Indirect" Effects of Histone Modifications: Modulation of Sterical Bulk as a Novel Source of Functionality

The chromatin-regulatory principles of histone post-translational modifications (PTMs) are discussed with a focus on the potential alterations in chromatin functional state due to steric and mechanical constraints imposed by bulky histone modifications such as ubiquitin and SUMO. In the classical view, PTMs operate as recruitment platforms for histone "readers," and as determinants of chromatin array compaction. Alterations of histone charges by "small" chemical modifications (e.g., acetylation, phosphorylation) could regulate nucleosome spontaneous dynamics without globally affecting nucleosome structure. These fluctuations in nucleosome wrapping can be exploited by chromatin-processing machinery. In contrast, ubiquitin and SUMO are comparable in size to histones, and it seems logical that these PTMs could conflict with canonical nucleosome organization. An experimentally testable hypothesis that by adding sterical bulk these PTMs can robustly alter nucleosome primary structure is proposed. The model presented here stresses the diversity of mechanisms by which histone PTMs regulate chromatin dynamics, primary structure and, hence, functionality.

The intrinsic stability of H2B-ubiquitylated nucleosomes and their in vitro assembly/disassembly by histone chaperone NAP1

BACKGROUND

Apart the gene-regulatory functions as docking sites for histone 'readers', some histone modifications could directly affect nucleosome structure. The H2BK34-ubiquitylation deposited by MOF-MSL complex, increases nucleosome dynamics in vitro and promotes donation of one H2A/H2B dimer to histone acceptors.

METHODS

We evaluated temperature-depended stability of H2BK34-ubiquitylated nucleosomes under 'physiological' ionic conditions in the presence or absence of histone acceptor, and examined assembly and disassembly of ubiquitylated nucleosomes in vitro by recombinant mouse NAP1.

RESULTS

H2BK34ub modification is sufficient to promote selective eviction of only one H2A/H2B dimer independently of histone-binding agents. Despite the robust H2A/H2B dimer-displacement effect of mNAP1 with the H2BK34ub (but not unmodified) nucleosomes, NAP1 could assemble symmetrically- or asymmetrically ubiquitylated nucleosomes under 'physiological' conditions in vitro.

CONCLUSIONS AND GENERAL SIGNIFICANCE

The increased mobility of one nucleosomal H2A/H2B dimer is an intrinsic nucleosome destabilizing property of H2BK34 ubiquitylation that has the intranucleosome bases. The ability of NAP to reasonably efficiently assemble H2BK34-ubiquitylated nucleosomes supposes a potential mechanism for deposition/distribution of H2BK34ub mark in the MOF-MSL independent manner (for example, during histone dimer exchange upon transcription elongation).

Analysis of histone ubiquitylation by MSL1/MSL2 proteins in vitro

Histone posttranslational modifications (PTM) control gene activity by targeting chromatin-regulatory proteins. By altering histone charges PTMs could also modulate inter- and intra-nucleosomal interactions, and thus affect chromatin high-order compaction and nucleosome stochastic folding, respectively. However, recently it has been shown that histone H2BK34- ubiquitylation (which is deposited in vivo by MOF-MSL) can destabilize one of the nucleosomal H2A-H2B dimers in symmetrically and (albeit to a lesser extend) asymmetrically modified nucleosomes, and thus promote formation of a hexasome particle. Here we have studied ubiquitylation patterns by purified MSL1/MSL2 using nucleosomes and different histone substrates. We have shown that H2B-ubiquitylation by MSL1/2 depends on substrate configuration. In addition, MSL1/2 efficiently ubiquitylate histone substrates but very poorly modify nucleosomes, which implies a requirement for nucleosome structural alteration for efficient ubiquitylation of H2BK34. Nucleosome modification by MSL1/MSL2 in vitro was analyzed directly using nucleosome gel-mobility shift assay, which suggested that MSL1/2 can deposit two ubiquitin moieties in one nucleosome.

Structural basis of recognition and destabilization of the histone H2B ubiquitinated nucleosome by the DOT1L histone H3 Lys79 methyltransferase

In this study, Jang et al. present cryo-EM structures of DOT1L complexes with unmodified or H2B ubiquitinated nucleosomes, showing that DOT1L recognizes H2B ubiquitin and the H2A/H2B acidic patch through a C-terminal hydrophobic helix and an arginine anchor in DOT1L, respectively. Their results establish the molecular basis of the cross-talk between H2B ubiquitination and H3 Lys79 methylation as well as nucleosome destabilization by DOT1L.

DOT1L is a histone H3 Lys79 methyltransferase whose activity is stimulated by histone H2B Lys120 ubiquitination, suggesting cross-talk between histone H3 methylation and H2B ubiquitination. Here, we present cryo-EM structures of DOT1L complexes with unmodified or H2B ubiquitinated nucleosomes, showing that DOT1L recognizes H2B ubiquitin and the H2A/H2B acidic patch through a C-terminal hydrophobic helix and an arginine anchor in DOT1L, respectively. Furthermore, the structures combined with single-molecule FRET experiments show that H2B ubiquitination enhances a noncatalytic function of the DOT1L-destabilizing nucleosome. These results establish the molecular basis of the cross-talk between H2B ubiquitination and H3 Lys79 methylation as well as nucleosome destabilization by DOT1L.

Developing Targeted Therapies That Exploit Aberrant Histone Ubiquitination in Cancer

Histone ubiquitination is a critical epigenetic mechanism regulating DNA-driven processes such as gene transcription and DNA damage repair. Importantly, the cellular machinery regulating histone ubiquitination is frequently altered in cancers. Moreover, aberrant histone ubiquitination can drive oncogenesis by altering the expression of tumor suppressors and oncogenes, misregulating cellular differentiation and promoting cancer cell proliferation. Thus, targeting aberrant histone ubiquitination may be a viable strategy to reprogram transcription in cancer cells, in order to halt cellular proliferation and induce cell death, which is the basis for the ongoing development of therapies targeting histone ubiquitination. In this review, we present the normal functions of histone H2A and H2B ubiquitination and describe the role aberrant histone ubiquitination has in oncogenesis. We also describe the key benefits and challenges associated with current histone ubiquitination targeting strategies. As these strategies are predicted to have off-target effects, we discuss additional efforts aimed at developing synthetic lethal strategies and epigenome editing tools, which may prove pivotal in achieving effective and selective therapies targeting histone ubiquitination, and ultimately improving the lives and outcomes of those living with cancer.

Effects of DNA Superhelical Stress on the Stability of H2B-Ubiquitylated Nucleosomes

On the nucleosome level, histone posttranslational modifications function mainly as the regulatory signals; in addition, some posttranslational modifications can enhance nucleosome stochastic folding, which is restricted in "canonic" nucleosomes. Recently, it has been shown in vitro that symmetric or asymmetric nucleosome ubiquitylation at H2BK34 (and H2BK120, to a lesser extent) can destabilize one of the nucleosomal H2A-H2B dimers and promote nucleosome conversion to a hexasome particle [Krajewski et al. (2018). Nucleic Acids Res., 46, 7631-7642]. Such lability of H2Bub nucleosomes raises a question of whether they could accommodate transient changes in DNA torsional tensions, which are generated by virtually any process that manipulates DNA strands. Using positively or negatively supercoiled DNA minicircles and homogeneously-modified H2Bub histones, we have found that DNA topology could strongly and selectively affect nucleosome stability depending on its ubiquitylation state (here the term "nucleosome stability" means the nucleosome property to maintain its structural integrity and dynamics characteristic to "canonic" nucleosomes). The results point to a role for H2B ubiquitylation in amplifying or mitigating the effects of a DNA torque on the nucleosome stability and dynamics.