Swi2/Snf2 remodelers: hybrid views on hybrid molecular machines

Immunoprecipitation of RNA-DNA hybrid interacting proteins in Trypanosoma brucei reveals conserved and novel activities, including in the control of surface antigen expression needed for immune evasion by antigenic variation

RNA-DNA hybrids are epigenetic features of genomes that provide a diverse and growing range of activities. Understanding of these functions has been informed by characterising the proteins that interact with the hybrids, but all such analyses have so far focused on mammals, meaning it is unclear if a similar spectrum of RNA-DNA hybrid interactors is found in other eukaryotes. The African trypanosome is a single-cell eukaryotic parasite of the Discoba grouping and displays substantial divergence in several aspects of core biology from its mammalian host. Here, we show that DNA-RNA hybrid immunoprecipitation coupled with mass spectrometry recovers 602 putative interactors in T. brucei mammal- and insect-infective cells, some providing activities also found in mammals and some lineage-specific. We demonstrate that loss of three factors, two putative helicases and a RAD51 paralogue, alters T. brucei nuclear RNA-DNA hybrid and DNA damage levels. Moreover, loss of each factor affects the operation of the parasite immune survival mechanism of antigenic variation. Thus, our work reveals the broad range of activities contributed by RNA-DNA hybrids to T. brucei biology, including new functions in host immune evasion as well as activities likely fundamental to eukaryotic genome function.

Ubiquitylome analysis reveals the involvement of ubiquitination in the cold responses of banana seedling leaves

Low temperature is a crucial environmental factor limiting the productivity and distribution of banana. Ubiquitination (Kub) is one of the main posttranslational modifications (PTMs) involved in plant responses to abiotic stresses. However, little information is available on the effects of Kub on banana under cold stress. In this study, we used label-free quantification (LFQ) to identify changes in the protein expression and Kub levels in banana seedling leaves after chilling treatment. In total, 4156 proteins, 1089 ubiquitinated proteins and 2636 Kub sites were quantified. Western blot assays showed that Kub was abundant in leaves after low-temperature treatment. Our results show that the proteome and ubiquitylome were negatively correlated, indicating that Kub could be involved in the degradation of proteins in banana after chilling treatment. Based on bioinformatics analysis, low-temperature stress-related signals and metabolic pathways such as cold acclimation, glutathione metabolism, calcium signaling, and photosynthesis signaling were identified. In addition, we found that transcription factors and chromatin remodeling factors related to low-temperature stress were ubiquitinated. Overall, our work presents the first systematic analysis of the Kub proteome in banana under cold stress and provides support for future studies on the regulatory mechanisms of Kub during the cold stress response in plants. SIGNIFICANCE: Banana is a typical tropical fruit tree with poor low-temperature tolerance,however, the role of PTMs such as Kub in the cold response of banana remains unclear. This study highlights the fact that the effects of low-temperature on proteome and ubiquitylome in the banana seedling leaves, we discussed the correlation between transcriptome and proteome, ubiquitylome and proteome, and we analyzed the expression and the changes of ubiquitination levels of low-temperature related proteins and pathway after chilling treatment, and we found that transcription factors and chromatin remodeling factors related to low-temperature stress were ubiquitinated. This study provides new insights into the ubiquitination pathway of banana under cold stress.

Chromatin remodeling of histone H3 variants by DDM1 underlies epigenetic inheritance of DNA methylation

Nucleosomes block access to DNA methyltransferase, unless they are remodeled by DECREASE in DNA METHYLATION 1 (DDM1LSH/HELLS), a Snf2-like master regulator of epigenetic inheritance. We show that DDM1 promotes replacement of histone variant H3.3 by H3.1. In ddm1 mutants, DNA methylation is partly restored by loss of the H3.3 chaperone HIRA, while the H3.1 chaperone CAF-1 becomes essential. The single-particle cryo-EM structure at 3.2 Å of DDM1 with a variant nucleosome reveals engagement with histone H3.3 near residues required for assembly and with the unmodified H4 tail. An N-terminal autoinhibitory domain inhibits activity, while a disulfide bond in the helicase domain supports activity. DDM1 co-localizes with H3.1 and H3.3 during the cell cycle, and with the DNA methyltransferase MET1Dnmt1, but is blocked by H4K16 acetylation. The male germline H3.3 variant MGH3/HTR10 is resistant to remodeling by DDM1 and acts as a placeholder nucleosome in sperm cells for epigenetic inheritance.

Chromatin remodeling of histone H3 variants underlies epigenetic inheritance of DNA methylation

Epigenetic inheritance refers to the faithful replication of DNA methylation and histone modification independent of DNA sequence. Nucleosomes block access to DNA methyltransferases, unless they are remodeled by DECREASE IN DNA METHYLATION1 (DDM1 Lsh/HELLS ), a Snf2-like master regulator of epigenetic inheritance. We show that DDM1 activity results in replacement of the transcriptional histone variant H3.3 for the replicative variant H3.1 during the cell cycle. In ddm1 mutants, DNA methylation can be restored by loss of the H3.3 chaperone HIRA, while the H3.1 chaperone CAF-1 becomes essential. The single-particle cryo-EM structure at 3.2 Å of DDM1 with a variant nucleosome reveals direct engagement at SHL2 with histone H3.3 at or near variant residues required for assembly, as well as with the deacetylated H4 tail. An N-terminal autoinhibitory domain binds H2A variants to allow remodeling, while a disulfide bond in the helicase domain is essential for activity in vivo and in vitro . We show that differential remodeling of H3 and H2A variants in vitro reflects preferential deposition in vivo . DDM1 co-localizes with H3.1 and H3.3 during the cell cycle, and with the DNA methyltransferase MET1 Dnmt1 . DDM1 localization to the chromosome is blocked by H4K16 acetylation, which accumulates at DDM1 targets in ddm1 mutants, as does the sperm cell specific H3.3 variant MGH3 in pollen, which acts as a placeholder nucleosome in the germline and contributes to epigenetic inheritance.

Structural basis for TBP displacement from TATA box DNA by the Swi2/Snf2 ATPase Mot1

The Swi2/Snf2 family transcription regulator Modifier of Transcription 1 (Mot1) uses adenosine triphosphate (ATP) to dissociate and reallocate the TATA box-binding protein (TBP) from and between promoters. To reveal how Mot1 removes TBP from TATA box DNA, we determined cryogenic electron microscopy structures that capture different states of the remodeling reaction. The resulting molecular video reveals how Mot1 dissociates TBP in a process that, intriguingly, does not require DNA groove tracking. Instead, the motor grips DNA in the presence of ATP and swings back after ATP hydrolysis, moving TBP to a thermodynamically less stable position on DNA. Dislodged TBP is trapped by a chaperone element that blocks TBP's DNA binding site. Our results show how Swi2/Snf2 proteins can remodel protein-DNA complexes through DNA bending without processive DNA tracking and reveal mechanistic similarities to RNA gripping DEAD box helicases and RIG-I-like immune sensors.

The Chromatin Remodeling Factor BrCHR39 Targets DNA Methylation to Positively Regulate Apical Dominance in Brassica rapa

The SHPRH (SNF2, histone linker, PHD, RING, helicase) subfamily belonging to ATP-dependent chromatin remodeling factor is the effective tumor-suppressor, which can polyubiquitinate PCNA (proliferating cell nuclear antigen) and participate in post-replication repair in human. However, little is known about the functions of SHPRH proteins in plants. In this study, we identified a novel SHPRH member BrCHR39 and obtained BrCHR39-silenced transgenic Brassica rapa. In contrast to wild-type plants, transgenic Brassica plants exhibited a released apical dominance phenotype with semi-dwarfism and multiple lateral branches. Furthermore, a global alteration of DNA methylation in the main stem and bud appeared after silencing of BrCHR39. Based on the GO (gene ontology) functional annotation and KEGG (Kyoto encyclopedia of genes and genomes) pathway analysis, the plant hormone signal transduction pathway was clearly enriched. In particular, we found a significant increase in the methylation level of auxin-related genes in the stem, whereas auxin- and cytokinin-related genes were hypomethylated in the bud of transgenic plants. In addition, further qRT-PCR (quantitative real-time PCR) analysis revealed that DNA methylation level always had an opposite trend with gene expression level. Considered together, our findings indicated that suppression of BrCHR39 expression triggered the methylation divergence of hormone-related genes and subsequently affected transcription levels to regulate the apical dominance in Brassica rapa.

Histone modification and chromatin remodeling in plant response to pathogens

As sessile organisms, plants are constantly exposed to changing environments frequently under diverse stresses. Invasion by pathogens, including virus, bacterial and fungal infections, can severely impede plant growth and development, causing important yield loss and thus challenging food/feed security worldwide. During evolution, plants have adapted complex systems, including coordinated global gene expression networks, to defend against pathogen attacks. In recent years, growing evidences indicate that pathogen infections can trigger local and global epigenetic changes that reprogram the transcription of plant defense genes, which in turn helps plants to fight against pathogens. Here, we summarize up plant defense pathways and epigenetic mechanisms and we review in depth current knowledge's about histone modifications and chromatin-remodeling factors found in the epigenetic regulation of plant response to biotic stresses. It is anticipated that epigenetic mechanisms may be explorable in the design of tools to generate stress-resistant plant varieties.

Probing the structure and function of polymerase θ helicase-like domain

DNA Polymerase θ is the key actuator of the recently identified double-strand break repair pathway, theta-mediated end joining (TMEJ). It is the only known polymerase to have a 3-domain architecture containing an independently functional family A DNA polymerase tethered by a long central region to an N-terminal helicase-like domain (HLD). Full-length polymerase θ and the isolated HLD hydrolyze ATP in the presence of DNA, but no processive DNA duplex unwinding has been observed. Based on sequence and structure conservation, the HLD is classified as a member of helicase superfamily II and, more specifically, the Ski2-like family. The specific subdomain composition and organization most closely resemble that of archaeal DNA repair helicases Hel308 and Hjm. The underlying structural basis as to why the HLD is not able to processively unwind duplex DNA, despite its similarity to bona fide helicases, remains elusive. Activities of the HLD include ATP hydrolysis, protein displacement, and annealing of complementary DNA. These observations have led to speculation about the role of the HLD within the context of double-strand break repair via TMEJ, such as removal of single-stranded DNA binding proteins like RPA and RAD51 and microhomology alignment. This review summarizes the structural classification and organization of the polymerase θ HLD and its homologs and explores emerging data on its biochemical activities. We conclude with a simple, speculative model for the HLD's role in TMEJ.

Characterization of Chromatin Remodeling Genes Involved in Thermal Tolerance of Biologically Invasive Bemisia tabaci

As an invasive species, Bemisia tabaci Mediterranean (MED) has notable potential to adapt to a wide range of environmental temperatures, which enables it to successfully spread after invasion and occupy habitats over a wide latitude range. It has been postulated that chromatin remodeling mechanisms are related to the rapid acquisition of adaptive traits and thermal resistance in invasive species; however, relevant experimental evidence is scarce. To identify the molecular characteristics and assess the role of chromatin remodelers in thermal stress within invasive MED and native Asia II 1 of the B. tabaci species complex, we identified 13 switching defective/sucrose non-fermenting (SWI/SNF) and 10 imitation switch (ISWI) family members in the B. tabaci genome, analyzed their molecular characteristics and structures, and identified key mutation sites between MED and Asia II 1, then cloned the catalytic subunits, and revealed the difference in thermal tolerance function. The results showed that the expression levels of Bt-BRM-1 and Bt-BRM-2 were significantly higher in MED than in Asia II 1 during heat stress, and Bt-BRM-2 expression was significantly higher during cold stress. In addition, RNA interference results indicated that the two target genes had similar temperature tolerance function in the both two cryptic species. This study is the first to identify and analyze the molecular characteristics of SWI/SNF and ISWI family members and reveal their potential key roles in temperature tolerance in poikilothermic ectotherms. The results will assist in understanding the underlying temperature adaptation mechanism of invasive insects and will enrich stress adaptation research systems from an epigenetic perspective.

Chromatin-Remodelling ATPases ISWI and BRM Are Essential for Reproduction in the Destructive Pest Tuta absoluta

The tomato leaf miner (Tuta absoluta) is one of the top 20 plant pests worldwide. We cloned and identified the chromatin-remodelling ATPase genes ISWI and BRM by RACE and bioinformatic analysis, respectively; used RT-qPCR to examine their expression patterns during different life cycle stages; and elucidated their roles in insect reproduction using double-stranded RNA injections. The full-length cDNA of TaISWI was 3428 bp and it encoded a 1025-aa polypeptide. The partial-length cDNA of TaBRM was 3457 bp and it encoded a 1030-aa polypeptide. TaISWI and TaBRM were upregulated at the egg stage. Injection of TaISWI or TaBRM dsRNA at the late pupa stage significantly inhibited adult ovary development and reduced fecundity, hatchability, and longevity in the adult females. To the best of our knowledge, the present study was the first to perform molecular characterisations of two chromatin-remodelling ATPase genes and clarify their roles in T. absoluta fecundity. Chromatin-remodelling ATPases are potential RNAi targets for the control of T. absoluta and other insect pests. The present study was also the first to demonstrate the feasibility of reproductive inhibitory RNAi as a putative approach for the suppression of T. absoluta and other Lepidopteran insect populations.

An insight into understanding the coupling between homologous recombination mediated DNA repair and chromatin remodeling mechanisms in plant genome: an update

Plants, with their obligatory immobility, are vastly exposed to a wide range of environmental agents and also various endogenous processes, which frequently cause damage to DNA and impose genotoxic stress. These factors subsequently increase genome instability, thus affecting plant growth and productivity. Therefore, to survive under frequent and extreme environmental stress conditions, plants have developed highly efficient and powerful defense mechanisms to repair the damages in the genome for maintaining genome stability. Such multi-dimensional signaling response, activated in presence of damage in the DNA, is collectively known as DNA Damage Response (DDR). DDR plays a crucial role in the remarkably efficient detection, signaling, and repair of damages in the genome for maintaining plant genome stability and normal growth responses. Like other highly advanced eukaryotic systems, chromatin dynamics play a key role in regulating cell cycle progression in plants through remarkable orchestration of environmental and developmental signals. The regulation of chromatin architecture and nucleosomal organization in DDR is mainly modulated by the ATP dependent chromatin remodelers (ACRs), chromatin modifiers, and histone chaperones. ACRs are mainly responsible for transcriptional regulation of several homologous recombination (HR) repair genes in plants under genotoxic stress. The HR-based repair of DNA damage has been considered as the most error-free mechanism of repair and represents one of the essential sources of genetic diversity and new allelic combinations in plants. The initiation of DDR signaling and DNA damage repair pathway requires recruitment of epigenetic modifiers for remodeling of the damaged chromatin while accumulating evidence has shown that chromatin remodeling and DDR share part of the similar signaling pathway through the altered epigenetic status of the associated chromatin region. In this review, we have integrated information to provide an overview on the association between chromatin remodeling mediated regulation of chromatin structure stability and DDR signaling in plants, with emphasis on the scope of the utilization of the available knowledge for the improvement of plant health and productivity.Abbreviation: ADH: Alcohol Dehydrogenase; AGO2: Argonaute 2; ARP: Actin-Related Protein; ASF:1- Anti-Silencing Function-1; ATM: Ataxia Telangiectasia Mutated; ATR: ATM and Rad3- Related; AtSWI3c: Arabidopsis thaliana Switch 3c; ATXR5: Arabidopsis Trithorax-Related5; ATXR6: Arabidopsis Trithorax-Related6; BER: Base Excision Repair; BRCA1: Breast Cancer Associated 1; BRM: BRAHMA; BRU1: BRUSHY1; CAF:1- Chromatin Assembly Factor-1; CHD: Chromodomain Helicase DNA; CHR5: Chromatin Remodeling Protein 5; CHR11/17: Chromatin Remodeling Protein 11/17; CIPK11- CBL- Interacting Protein Kinase 11; CLF: Curly Leaf; CMT3: Chromomethylase 3; COR15A: Cold Regulated 15A; COR47: Cold Regulated 47; CRISPR: Clustered Regulatory Interspaced Short Palindromic Repeats; DDM1: Decreased DNA Methylation1; DRR: DNA Repair and Recombination; DSBs: Double-Strand Breaks; DDR: DNA Damage Response; EXO1: Exonuclease 1; FAS1/2: Fasciata1/2; FACT: Facilitates Chromatin Transcription; FT: Flowering Locus T; GMI1: Gamma-Irradiation And Mitomycin C Induced 1; HAC1: Histone Acetyltransferase of the CBP Family 1; HAM1: Histone Acetyltransferase of the MYST Family 1; HAM2: Histone Acetyltransferase of the MYST Family 2; HAF1: Histone Acetyltransferase of the TAF Family 1; HAT: Histone Acetyl Transferase; HDA1: Histone Deacetylase 1; HDA6: Histone Deacetylase 6; HIRA: Histone Regulatory Homolog A; HR- Homologous recombination; HAS: Helicase SANT Associated; HSS: HAND-SLANT-SLIDE; ICE1: Inducer of CBF Expression 1; INO80: Inositol Requiring Mutant 80; ISW1: Imitation Switch 1; KIN1/2: Kinase 1 /2; MET1: Methyltransferase 1; MET2: Methyltransferase 2; MINU: MINUSCULE; MMS: Methyl Methane Sulfonate; MMS21: Methyl Methane Sulfonate Sensitivity 21; MRN: MRE11, RAD50 and NBS1; MSI1: Multicopy Suppressor Of Ira1; NAP1: Nucleosome Assembly Protein 1; NRP1/NRP2: NAP1-Related Protein; NER: Nucleotide Excision Repair; NHEJ: Non-Homologous End Joining; PARP1: Poly-ADP Ribose Polymerase; PIE1: Photoperiod Independent Early Flowering 1; PIKK: Phosphoinositide 3-Kinase-Like Kinase; PKL: PICKLE; PKR1/2: PICKLE Related 1/2; RAD: Radiation Sensitive Mutant; RD22: Responsive To Desiccation 22; RD29A: Responsive To Desiccation 29A; ROS: Reactive Oxygen Species; ROS1: Repressor of Silencing 1; RPA1E: Replication Protein A 1E; SANT: Swi3, Ada2, N-Cor and TFIIIB; SEP3: SEPALLATA3; SCC3: Sister Chromatid Cohesion Protein 3; SMC1: Structural Maintenance of Chromosomes Protein 1; SMC3: Structural Maintenance of Chromosomes Protein 3; SOG1: Suppressor of Gamma Response 1; SWC6: SWR1 Complex Subunit 6; SWR1: SWI2/SNF2-Related 1; SYD: SPLAYED; SMC5: Structural Maintenance of Chromosome 5; SWI/SNF: Switch/Sucrose Non-Fermentable; TALENs: Transcription Activators Like Effector Nucleases; TRRAP: Transformation/Transactivation Domain-Associated Protein; ZFNs: Zinc Finger Nucleases.

Chromatin remodeling factors regulate environmental stress responses in plants

Environmental stress from climate change and agricultural activity threatens global plant biodiversity as well as crop yield and quality. As sessile organisms, plants must maintain the integrity of their genomes and adjust gene expression to adapt to various environmental changes. In eukaryotes, nucleosomes are the basic unit of chromatin around which genomic DNA is packaged by condensation. To enable dynamic access to packaged DNA, eukaryotes have evolved Snf2 (sucrose nonfermenting 2) family proteins as chromatin remodeling factors (CHRs) that modulate the position of nucleosomes on chromatin. During plant stress responses, CHRs are recruited to specific genomic loci, where they regulate the distribution or composition of nucleosomes, which in turn alters the accessibility of these loci to general transcription or DNA damage repair machinery. Moreover, CHRs interplay with other epigenetic mechanisms, including DNA methylation, histone modifications, and deposition of histone variants. CHRs are also involved in RNA processing at the post-transcriptional level. In this review, we discuss major advances in our understanding of the mechanisms by which CHRs function during plants' response to environmental stress.

The chromatin-remodeling protein BAF60/SWP73A regulates the plant immune receptor NLRs

In both plant and animal innate immune responses, surveillance of pathogen infection is mediated by membrane-associated receptors and intracellular nucleotide-binding domain and leucine-rich-repeat receptors (NLRs). Homeostasis of NLRs is under tight multilayered regulation to avoid over-accumulation or over-activation, which often leads to autoimmune responses that have detrimental effects on growth and development. How NLRs are regulated epigenetically at the chromatin level remains unclear. Here, we report that SWP73A, an ortholog of the mammalian switch/sucrose nonfermentable (SWI/SNF) chromatin-remodeling protein BAF60, suppresses the expression of NLRs either directly by binding to the NLR promoters or indirectly by affecting the alternative splicing of some NLRs through the suppression of cell division cycle 5 (CDC5), a key regulator of RNA splicing. Upon infection, bacteria-induced small RNAs silence SWP73A to activate a group of NLRs and trigger robust immune responses. SWP73A may function as a H3K9me2 reader to enhance transcription suppression.

Role of Chromatin Architecture in Plant Stress Responses: An Update

Sessile plants possess an assembly of signaling pathways that perceive and transmit environmental signals, ultimately resulting in transcriptional reprogramming. Histone is a key feature of chromatin structure. Numerous histone-modifying proteins act under different environmental stress conditions to help modulate gene expression. DNA methylation and histone modification are crucial for genome reprogramming for tissue-specific gene expression and global gene silencing. Different classes of chromatin remodelers including SWI/SNF, ISWI, INO80, and CHD are reported to act upon chromatin in different organisms, under diverse stresses, to convert chromatin from a transcriptionally inactive to a transcriptionally active state. The architecture of chromatin at a given promoter is crucial for determining the transcriptional readout. Further, the connection between somatic memory and chromatin modifications may suggest a mechanistic basis for a stress memory. Studies have suggested that there is a functional connection between changes in nuclear organization and stress conditions. In this review, we discuss the role of chromatin architecture in different stress responses and the current evidence on somatic, intergenerational, and transgenerational stress memory.

Megadalton chromatin remodelers: common principles for versatile functions

ATP-dependent chromatin remodelers are enigmatic macromolecular machines that govern the arrangement and composition of nucleosomes across eukaryotic genomes. Here, we review the recent breakthrough provided by cryo-electron microscopy that reveal the first high-resolution insights into all four families of remodelers. We highlight the emerging structural and mechanistic principles with a particular focus on multi-subunit SWI/SNF and INO80/SWR1 complexes. A conserved architecture comprising a motor, rotor, stator and grip suggests a unifying mechanism for how stepwise DNA translocation enables large scale reconfigurations of nucleosomes. A molecular circuitry involving the nuclear actin containing module establishes a framework for understanding allosteric regulation. Remodelers emerge as programable hubs that enable differential processing of genetic and epigenetic information in response to the physiological state of a cell.

Chromatin-remodeling links metabolic signaling to gene expression

BACKGROUND

ATP-dependent chromatin remodelers are evolutionarily conserved complexes that alter nucleosome positioning to influence many DNA-templated processes, such as replication, repair, and transcription. In particular, chromatin remodeling can dynamically regulate gene expression by altering accessibility of chromatin to transcription factors.

SCOPE OF REVIEW

This review provides an overview of the importance of chromatin remodelers in the regulation of metabolic gene expression. Particular emphasis is placed on the INO80 and SWI/SNF (BAF/PBAF) chromatin remodelers in both yeast and mammals. This review details discoveries from the initial identification of chromatin remodelers in Saccharomyces cerevisiae to recent discoveries in the metabolic requirements of developing embryonic tissues in mammals.

MAJOR CONCLUSIONS

INO80 and SWI/SNF (BAF/PBAF) chromatin remodelers regulate the expression of energy metabolism pathways in S. cerevisiae and mammals in response to diverse nutrient environments. In particular, the INO80 complex organizes the temporal expression of gene expression in the metabolically synchronized S. cerevisiae system. INO80-mediated chromatin remodeling is also needed to constrain cell division during metabolically favorable conditions. Conversely, the BAF/PBAF remodeler regulates tissue-specific glycolytic metabolism and is disrupted in cancers that are dependent on glycolysis for proliferation. The role of chromatin remodeling in metabolic gene expression is downstream of the metabolic signaling pathways, such as the TOR pathway, a critical regulator of metabolic homeostasis. Furthermore, the INO80 and BAF/PBAF chromatin remodelers have both been shown to regulate heart development, the tissues of which have unique requirements for energy metabolism during development. Collectively, these results demonstrate that chromatin remodelers communicate metabolic status to chromatin and are a central component of homeostasis pathways that optimize cell fitness, organismal development, and prevent disease.

Using RNA Interference to Reveal the Function of Chromatin Remodeling Factor ISWI in Temperature Tolerance in Bemisia tabaci Middle East-Asia Minor 1 Cryptic Species

Invasive species often encounter rapid environmental changes during invasions and only the individuals that successfully overcome environmental stresses can colonize and spread. Chromatin remodeling may be essential in environmental adaptation. To assess the functions of imitation switch (ISWI) in invasive Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) cryptic species, we cloned and characterized the MEAM1 BtISWI gene and determined its functions in response to thermal stress. The full-length cDNA of BtISWI was 3712 bp, with a 3068 bp open reading frame (ORF) encoding a 118.86 kDa protein. BtISWI mRNA expression was significantly up-regulated after exposure to heat shock or cold shock conditions, indicating that BtISWI expression can be induced by thermal stress. After feeding double-stranded RNA (dsRNA), specifically for BtISWI, resistance to both heat and cold decreased significantly, suggesting that BtISWI may function directly in the thermal tolerance of MEAM1. Moreover, the preferred temperature of MEAM1 adults fed dsRNA was 1.9-3.5 °C higher than the control groups. Taken together, our findings highlight the importance of epigenetic gene regulation in the thermal response or thermal adaptation of invasive Bemisia tabaci (B. tabaci), and provide a new potential target for establishing sustainable control strategies for B. tabaci.

Structure of the primed state of the ATPase domain of chromatin remodeling factor ISWI bound to the nucleosome

ATP-dependent chromatin remodeling factors of SWI/SNF2 family including ISWI, SNF2, CHD1 and INO80 subfamilies share a conserved but functionally non-interchangeable ATPase domain. Here we report cryo-electron microscopy (cryo-EM) structures of the nucleosome bound to an ISWI fragment with deletion of the AutoN and HSS regions in nucleotide-free conditions and the free nucleosome at ∼ 4 Å resolution. In the bound conformation, the ATPase domain interacts with the super helical location 2 (SHL 2) of the nucleosomal DNA, with the N-terminal tail of H4 and with the α1 helix of H3. Density for other regions of ISWI is not observed, presumably due to disorder. Comparison with the structure of the free nucleosome reveals that although the histone core remains largely unchanged, remodeler binding causes perturbations in the nucleosomal DNA resulting in a bulge near the SHL2 site. Overall, the structure of the nucleotide-free ISWI-nucleosome complex is similar to the corresponding regions of the recently reported ADP bound ISWI-nucleosome structures, which are significantly different from that observed for the ADP-BeFx bound structure. Our findings are relevant to the initial step of ISWI binding to the nucleosome and provide additional insights into the nucleosome remodeling process driven by ISWI.

Conformational changes and catalytic inefficiency associated with Mot1-mediated TBP-DNA dissociation

The TATA-box Binding Protein (TBP) plays a central role in regulating gene expression and is the first step in the process of pre-initiation complex (PIC) formation on promoter DNA. The lifetime of TBP at the promoter site is controlled by several cofactors including the Modifier of transcription 1 (Mot1), an essential TBP-associated ATPase. Based on ensemble measurements, Mot1 can use adenosine triphosphate (ATP) hydrolysis to displace TBP from DNA and various models for how this activity is coupled to transcriptional regulation have been proposed. However, the underlying molecular mechanism of Mot1 action is not well understood. In this work, the interaction of Mot1 with the DNA/TBP complex was investigated by single-pair Förster resonance energy transfer (spFRET). Upon Mot1 binding to the DNA/TBP complex, a transition in the DNA/TBP conformation was observed. Hydrolysis of ATP by Mot1 led to a conformational change but was not sufficient to efficiently disrupt the complex. SpFRET measurements of dual-labeled DNA suggest that Mot1's ATPase activity primes incorrectly oriented TBP for dissociation from DNA and additional Mot1 in solution is necessary for TBP unbinding. These findings provide a framework for understanding how the efficiency of Mot1's catalytic activity is tuned to establish a dynamic pool of TBP without interfering with stable and functional TBP-containing complexes.

Ubiquitin-proteasomal regulation of chromatin remodeler INO80 in the nucleus accumbens mediates persistent cocaine craving

Neuroadaptations in the nucleus accumbens (NAc) underlie cue-induced cocaine craving that intensifies ("incubates") during abstinence and is believed to contribute to persistent relapse vulnerability. Changes in gene expression often govern perpetual behavioral abnormalities, but epigenetic plasticity during prolonged abstinence from drug exposure is poorly understood. We examined how E3 ubiquitin ligase TRIM3 dysregulates chromatin remodeler INO80 to mediate cocaine craving during prolonged abstinence. We found that INO80 expression increased in the NAc on abstinence day 30 (AD30) but not on AD1 following cocaine self-administration. Furthermore, TRIM3, which mediates degradation of INO80, was reduced on AD30, along with TRIM3-INO80 interaction. Viral-mediated gene transfer of INO80 or TRIM3 governed cocaine craving during prolonged abstinence. Lastly, chromatin immunoprecipitation followed by massively parallel DNA sequencing identified INO80-mediated transcriptional regulation of predicted pathways associated with cocaine plasticity. Together, these results demonstrate a novel ubiquitin-proteasomal-epigenetic mechanism by which TRIM3-INO80 mediates cocaine craving during prolonged abstinence.

Crystal structure of the full Swi2/Snf2 remodeler Mot1 in the resting state

Swi2/Snf2 ATPases remodel protein:DNA complexes in all of the fundamental chromosome-associated processes. The single-subunit remodeler Mot1 dissociates TATA box-binding protein (TBP):DNA complexes and provides a simple model for obtaining structural insights into the action of Swi2/Snf2 ATPases. Previously we reported how the N-terminal domain of Mot1 binds TBP, NC2 and DNA, but the location of the C-terminal ATPase domain remained unclear (Butryn et al., 2015). Here, we report the crystal structure of the near full-length Mot1 from Chaetomium thermophilum. Our data show that Mot1 adopts a ring like structure with a catalytically inactive resting state of the ATPase. Biochemical analysis suggests that TBP binding switches Mot1 into an ATP hydrolysis-competent conformation. Combined with our previous results, these data significantly improve the structural model for the complete Mot1:TBP:DNA complex and suggest a general mechanism for Mot1 action.

Chromatin remodeling factor CHR18 interacts with replication protein RPA1A to regulate the DNA replication stress response in Arabidopsis

DNA replication is a fundamental process for the faithful transmission of genetic information in all living organisms. Many endogenous and environmental signals impede fork progression during DNA synthesis, which induces replication errors and DNA replication stress. Chromatin remodeling factors regulate nucleosome occupancy and the histone composition of the nucleosome in chromatin; however, whether chromatin remodeling factors are involved in the DNA replication stress response in plants is unknown. We reveal that chromatin remodeling factor CHR18 plays important roles in DNA replication stress in Arabidopsis thaliana by interacting with the DNA replication protein RPA1A. According to the genetic analysis, the loss of function of either CHR18 or RPA1A confers a high sensitivity to DNA replication stress in Arabidopsis. CHR18 interacts with RPA1A in both yeast cells and tobacco epidermal cells. The coexpression of RPA1A and CHR18 enhances the accumulation of CHR18 in nuclear foci in plants. CHR18 is a typical nuclear-localized chromatin remodeling factor with ATPase activity. Our results demonstrate that during DNA synthesis in plants, RPA1A interacts with CHR18 and recruits CHR18 to nuclear foci to resolve DNA replication stress, which is important for cell propagation and root growth in Arabidopsis plants.

The nuclear actin-containing Arp8 module is a linker DNA sensor driving INO80 chromatin remodeling

Nuclear actin (N-actin) and actin-related proteins (Arps) are critical components of several chromatin modulating complexes, including the chromatin remodeler INO80, but their function is largely elusive. Here, we report the crystal structure of the 180-kDa Arp8 module of Saccharomyces cerevisiae INO80 and establish its role in recognition of extranucleosomal linker DNA. Arp8 engages N-actin in a manner distinct from that of other actin-fold proteins and thereby specifies recruitment of the Arp4-N-actin heterodimer to a segmented scaffold of the helicase-SANT-associated (HSA) domain of Ino80. The helical HSA domain spans over 120 Å and provides an extended binding platform for extranucleosomal entry DNA that is required for nucleosome sliding and genome-wide nucleosome positioning. Together with the recent cryo-electron microscopy structure of INO80^Core-nucleosome complex, our findings suggest an allosteric mechanism by which INO80 senses 40-bp linker DNA to conduct highly processive chromatin remodeling.

Mechanistic Insights into Autoinhibition of the Oncogenic Chromatin Remodeler ALC1

Human ALC1 is an oncogene-encoded chromatin-remodeling enzyme required for DNA repair that possesses a poly(ADP-ribose) (PAR)-binding macro domain. Its engagement with PARylated PARP1 activates ALC1 at sites of DNA damage, but the underlying mechanism remains unclear. Here, we establish a dual role for the macro domain in autoinhibition of ALC1 ATPase activity and coupling to nucleosome mobilization. In the absence of DNA damage, an inactive conformation of the ATPase is maintained by juxtaposition of the macro domain against predominantly the C-terminal ATPase lobe through conserved electrostatic interactions. Mutations within this interface displace the macro domain, constitutively activate the ALC1 ATPase independent of PARylated PARP1, and alter the dynamics of ALC1 recruitment at DNA damage sites. Upon DNA damage, binding of PARylated PARP1 by the macro domain induces a conformational change that relieves autoinhibitory interactions with the ATPase motor, which selectively activates ALC1 remodeling upon recruitment to sites of DNA damage.

NAP1L1 accelerates activation and decreases pausing to enhance nucleosome remodeling by CSB

Cockayne syndrome protein B (CSB) belongs to the SWI2/SNF2 ATP-dependent chromatin remodeler family, and CSB is the only ATP-dependent chromatin remodeler essential for transcription-coupled nucleotide excision DNA repair. CSB alone remodels nucleosomes ∼10-fold slower than the ACF remodeling complex. Strikingly, NAP1-like histone chaperones interact with CSB and greatly enhance CSB-mediated chromatin remodeling. While chromatin remodeling by CSB and NAP1-like proteins is crucial for efficient transcription-coupled DNA repair, the mechanism by which NAP1-like proteins enhance chromatin remodeling by CSB remains unknown. Here we studied CSB's DNA-binding and nucleosome-remodeling activities at the single molecule level in real time. We also determined how the NAP1L1 chaperone modulates these activities. We found that CSB interacts with DNA in two principle ways: by simple binding and a more complex association that involves gross DNA distortion. Remarkably, NAP1L1 suppresses both these interactions. Additionally, we demonstrate that nucleosome remodeling by CSB consists of three distinct phases: activation, translocation and pausing, similar to ACF. Importantly, we found that NAP1L1 promotes CSB-mediated remodeling by accelerating both activation and translocation. Additionally, NAP1L1 increases CSB processivity by decreasing the pausing probability during translocation. Our study, therefore, uncovers the different steps of CSB-mediated chromatin remodeling that can be regulated by NAP1L1.

Polyvalent Proteins, a Pervasive Theme in the Intergenomic Biological Conflicts of Bacteriophages and Conjugative Elements

Intense biological conflicts between prokaryotic genomes and their genomic parasites have resulted in an arms race in terms of the molecular “weaponry” deployed on both sides. Using a recursive computational approach, we uncovered a remarkable class of multidomain proteins with 2 to 15 domains in the same polypeptide deployed by viruses and plasmids in such conflicts. Domain architectures and genomic contexts indicate that they are part of a widespread conflict strategy involving proteins injected into the host cell along with parasite DNA during the earliest phase of infection. Their unique feature is the combination of domains with highly disparate biochemical activities in the same polypeptide; accordingly, we term them polyvalent proteins. Of the 131 domains in polyvalent proteins, a large fraction are enzymatic domains predicted to modify proteins, target nucleic acids, alter nucleotide signaling/metabolism, and attack peptidoglycan or cytoskeletal components. They further contain nucleic acid-binding domains, virion structural domains, and 40 novel uncharacterized domains. Analysis of their architectural network reveals both pervasive common themes and specialized strategies for conjugative elements and plasmids or (pro)phages. The themes include likely processing of multidomain polypeptides by zincin-like metallopeptidases and mechanisms to counter restriction or CRISPR/Cas systems and jump-start transcription or replication. DNA-binding domains acquired by eukaryotes from such systems have been reused in XPC/RAD4-dependent DNA repair and mitochondrial genome replication in kinetoplastids. Characterization of the novel domains discovered here, such as RNases and peptidases, are likely to aid in the development of new reagents and elucidation of the spread of antibiotic resistance.

IMPORTANCE This is the first report of the widespread presence of large proteins, termed polyvalent proteins, predicted to be transmitted by genomic parasites such as conjugative elements, plasmids, and phages during the initial phase of infection along with their DNA. They are typified by the presence of multiple domains with disparate activities combined in the same protein. While some of these domains are predicted to assist the invasive element in replication, transcription, or protection of their DNA, several are likely to target various host defense systems or modify the host to favor the parasite's life cycle. Notably, DNA-binding domains from these systems have been transferred to eukaryotes, where they have been incorporated into DNA repair and mitochondrial genome replication systems.

SUMO-Targeted DNA Translocase Rrp2 Protects the Genome from Top2-Induced DNA Damage

The action of DNA topoisomerase II (Top2) creates transient DNA breaks that are normally concealed inside Top2-DNA covalent complexes. Top2 poisons, including ubiquitously present natural compounds and clinically used anti-cancer drugs, trap Top2-DNA complexes. Here, we show that cells actively prevent Top2 degradation to avoid the exposure of concealed DNA breaks. A genome-wide screen revealed that fission yeast cells lacking Rrp2, an Snf2-family DNA translocase, are strongly sensitive to Top2 poisons. Loss of Rrp2 enhances SUMOylation-dependent ubiquitination and degradation of Top2, which in turn increases DNA damage at sites where Top2-DNA complexes are trapped. Rrp2 possesses SUMO-binding ability and prevents excessive Top2 degradation by competing against the SUMO-targeted ubiquitin ligase (STUbL) for SUMO chain binding and by displacing SUMOylated Top2 from DNA. The budding yeast homolog of Rrp2, Uls1, plays a similar role, indicating that this genome protection mechanism is widely employed, a finding with implications for cancer treatment.

Understanding nucleosome dynamics and their links to gene expression and DNA replication

Advances in genomics technology have provided the means to probe myriad chromatin interactions at unprecedented spatial and temporal resolution. This has led to a profound understanding of nucleosome organization within the genome, revealing that nucleosomes are highly dynamic. Nucleosome dynamics are governed by a complex interplay of histone composition, histone post-translational modifications, nucleosome occupancy and positioning within chromatin, which are influenced by numerous regulatory factors, including general regulatory factors, chromatin remodellers, chaperones and polymerases. It is now known that these dynamics regulate diverse cellular processes ranging from gene transcription to DNA replication and repair.

Downregulation of Aedes aegypti chromodomain helicase DNA binding protein 7/Kismet by Wolbachia and its effect on dengue virus replication

Dengue virus (DENV) is a mosquito-transmitted virus imposing a significant burden on human health around the world. Since current control strategies are not sufficient, there is an urgent need to find alternative methods to control DENV transmission. It has been demonstrated that introduction of Wolbachia pipientis in Aedes aegypti mosquitoes can impede DENV transmission with the mechanism(s) not fully understood. Recently, a number of studies have found the involvement of chromodomain DNA binding helicases in case of Human Immunodeficiency virus (HIV) and Influenza A virus infection. In this study, we have identified three chromodomain helicase DNA binding protein (CHD) genes in Ae. aegypti and looked at their response in the case of Wolbachia and DENV infections. Foremost amongst them we have found that AeCHD7/Kismet is significantly downregulated in the presence of Wolbachia infection only in female mosquitoes. Furthermore, AeCHD7 levels showed significant increase during DENV infection, and AeCHD7 depletion led to severe reduction in the replication of DENV. Our data have identified AeCHD7 as a novel Ae. aegypti host factor that is important for DENV replication, and Wolbachia downregulates it, which may contribute towards the mechanism(s) of limiting DENV replication.

Mechanisms of ATP-Dependent Chromatin Remodeling Motors

Chromatin remodeling motors play essential roles in all DNA-based processes. These motors catalyze diverse outcomes ranging from sliding the smallest units of chromatin, known as nucleosomes, to completely disassembling chromatin. The broad range of actions carried out by these motors on the complex template presented by chromatin raises many stimulating mechanistic questions. Other well-studied nucleic acid motors provide examples of the depth of mechanistic understanding that is achievable from detailed biophysical studies. We use these studies as a guiding framework to discuss the current state of knowledge of chromatin remodeling mechanisms and highlight exciting open questions that would continue to benefit from biophysical analyses.

Molecular Mechanism of Mot1, a TATA-binding Protein (TBP)-DNA Dissociating Enzyme

The essential Saccharomyces cerevisiae ATPase Mot1 globally regulates transcription by impacting the genomic distribution and activity of the TATA-binding protein (TBP). In vitro, Mot1 forms a ternary complex with TBP and DNA and can use ATP hydrolysis to dissociate the TBP-DNA complex. Prior work suggested an interaction between the ATPase domain and a functionally important segment of DNA flanking the TATA sequence. However, how ATP hydrolysis facilitates removal of TBP from DNA is not well understood, and several models have been proposed. To gain insight into the Mot1 mechanism, we dissected the role of the flanking DNA segment by biochemical analysis of complexes formed using DNAs with short single-stranded gaps. In parallel, we used a DNA tethered cleavage approach to map regions of Mot1 in proximity to the DNA under different conditions. Our results define non-equivalent roles for bases within a broad segment of flanking DNA required for Mot1 action. Moreover, we present biochemical evidence for two distinct conformations of the Mot1 ATPase, the detection of which can be modulated by ATP analogs as well as DNA sequence flanking the TATA sequence. We also show using purified complexes that Mot1 dissociation of a stable, high affinity TBP-DNA interaction is surprisingly inefficient, suggesting how other transcription factors that bind to TBP may compete with Mot1. Taken together, these results suggest that TBP-DNA affinity as well as other aspects of promoter sequence influence Mot1 function in vivo.

The mRNA cap-binding protein Cbc1 is required for high and timely expression of genes by promoting the accumulation of gene-specific activators at promoters

The highly conserved Saccharomyces cerevisiae cap-binding protein Cbc1/Sto1 binds mRNA co-transcriptionally and acts as a key coordinator of mRNA fate. Recently, Cbc1 has also been implicated in transcription elongation and pre-initiation complex (PIC) formation. Previously, we described Cbc1 to be required for cell growth under osmotic stress and to mediate osmostress-induced translation reprogramming. Here, we observe delayed global transcription kinetics in cbc1Δ during osmotic stress that correlates with delayed recruitment of TBP and RNA polymerase II to osmo-induced promoters. Interestingly, we detect an interaction between Cbc1 and the MAPK Hog1, which controls most gene expression changes during osmostress, and observe that deletion of CBC1 delays the accumulation of the activator complex Hot1-Hog1 at osmostress promoters. Additionally, CBC1 deletion specifically reduces transcription rates of highly transcribed genes under non-stress conditions, such as ribosomal protein (RP) genes, while having low impact on transcription of weakly expressed genes. For RP genes, we show that recruitment of the specific activator Rap1, and subsequently TBP, to promoters is Cbc1-dependent. Altogether, our results indicate that binding of Cbc1 to the capped mRNAs is necessary for the accumulation of specific activators as well as PIC components at the promoters of genes whose expression requires high and rapid transcription.

The N-terminal Region of Chromodomain Helicase DNA-binding Protein 4 (CHD4) Is Essential for Activity and Contains a High Mobility Group (HMG) Box-like-domain That Can Bind Poly(ADP-ribose)

Chromodomain Helicase DNA-binding protein 4 (CHD4) is a chromatin-remodeling enzyme that has been reported to regulate DNA-damage responses through its N-terminal region in a poly(ADP-ribose) polymerase-dependent manner. We have identified and determined the structure of a stable domain (CHD4-N) in this N-terminal region. The-fold consists of a four-α-helix bundle with structural similarity to the high mobility group box, a domain that is well known as a DNA binding module. We show that the CHD4-N domain binds with higher affinity to poly(ADP-ribose) than to DNA. We also show that the N-terminal region of CHD4, although not CHD4-N alone, is essential for full nucleosome remodeling activity and is important for localizing CHD4 to sites of DNA damage. Overall, these data build on our understanding of how CHD4-NuRD acts to regulate gene expression and participates in the DNA-damage response.

Translocation-coupled DNA cleavage by the Type ISP restriction-modification enzymes

Production of endonucleolytic double-strand DNA breaks requires separate strand cleavage events. Although catalytic mechanisms for simple, dimeric endonucleases are known, there are many complex nuclease machines that are poorly understood. Here we studied the single polypeptide Type ISP restriction-modification (RM) enzymes, which cleave random DNA between distant target sites when two enzymes collide after convergent ATP-driven translocation. We report the 2.7-Å resolution X-ray crystal structure of a Type ISP enzyme-DNA complex, revealing that both the helicase-like ATPase and nuclease are located upstream of the direction of translocation, an observation inconsistent with simple nuclease-domain dimerization. Using single-molecule and biochemical techniques, we demonstrate that each ATPase remodels its DNA-protein complex and translocates along DNA without looping it, leading to a collision complex in which the nuclease domains are distal. Sequencing of the products of single cleavage events suggests a previously undescribed endonuclease model, where multiple, stochastic strand-nicking events combine to produce DNA scission.

Allosteric Activation of Bacterial Swi2/Snf2 (Switch/Sucrose Non-fermentable) Protein RapA by RNA Polymerase: BIOCHEMICAL AND STRUCTURAL STUDIES

Members of the Swi2/Snf2 (switch/sucrose non-fermentable) family depend on their ATPase activity to mobilize nucleic acid-protein complexes for gene expression. In bacteria, RapA is an RNA polymerase (RNAP)-associated Swi2/Snf2 protein that mediates RNAP recycling during transcription. It is known that the ATPase activity of RapA is stimulated by its interaction with RNAP. It is not known, however, how the RapA-RNAP interaction activates the enzyme. Previously, we determined the crystal structure of RapA. The structure revealed the dynamic nature of its N-terminal domain (Ntd), which prompted us to elucidate the solution structure and activity of both the full-length protein and its Ntd-truncated mutant (RapAΔN). Here, we report the ATPase activity of RapA and RapAΔN in the absence or presence of RNAP and the solution structures of RapA and RapAΔN either ligand-free or in complex with RNAP. Determined by small-angle x-ray scattering, the solution structures reveal a new conformation of RapA, define the binding mode and binding site of RapA on RNAP, and show that the binding sites of RapA and σ(70) on the surface of RNAP largely overlap. We conclude that the ATPase activity of RapA is inhibited by its Ntd but stimulated by RNAP in an allosteric fashion and that the conformational changes of RapA and its interaction with RNAP are essential for RNAP recycling. These and previous findings outline the functional cycle of RapA, which increases our understanding of the mechanism and regulation of Swi2/Snf2 proteins in general and of RapA in particular. The new structural information also leads to a hypothetical model of RapA in complex with RNAP immobilized during transcription.

DNA Damage Repair in the Context of Plant Chromatin

The integrity of DNA molecules is constantly challenged. All organisms have developed mechanisms to detect and repair multiple types of DNA lesions. The basic principles of DNA damage repair (DDR) in prokaryotes and unicellular and multicellular eukaryotes are similar, but the association of DNA with nucleosomes in eukaryotic chromatin requires mechanisms that allow access of repair enzymes to the lesions. This is achieved by chromatin-remodeling factors, and their necessity for efficient DDR has recently been demonstrated for several organisms and repair pathways. Plants share many features of chromatin organization and DNA repair with fungi and animals, but they differ in other, important details, which are both interesting and relevant for our understanding of genome stability and genetic diversity. In this Update, we compare the knowledge of the role of chromatin and chromatin-modifying factors during DDR in plants with equivalent systems in yeast and humans. We emphasize plant-specific elements and discuss possible implications.

An ATPase-deficient variant of the SNF2 family member HELLS shows altered dynamics at pericentromeric heterochromatin

The HELLS (helicase, lymphoid specific, also known as lymphoid-specific helicase) protein is related to the SNF2 (sucrose non-fermentable 2) family of chromatin remodeling ATPases. It is required for efficient DNA methylation in mammals, particularly at heterochromatin-located repetitive sequences. In this study, we investigated the interaction of HELLS with chromatin and used an ATPase-deficient HELLS variant to address the role of ATP hydrolysis in this process. Chromatin fractionation experiments demonstrated that, in the absence of the ATPase activity, HELLS is retained at the nuclear matrix compartment, defined in part by lamin B1. Microscopy studies revealed a stronger association of the ATPase-deficient mutant with heterochromatin. These results were further supported by fluorescence recovery after photobleaching measurements, which showed that, at heterochromatic sites, wild-type HELLS is very dynamic, with a recovery half-time of 0.8s and a mobile protein fraction of 61%. In contrast, the ATPase-deficient mutant displayed 4.5-s recovery half-time and a reduction in the mobile fraction to 30%. We also present evidence suggesting that, in addition to the ATPase activity, a functional H3K9me3 signaling pathway contributes to an efficient release of HELLS from pericentromeric chromatin. Overall, our results show that a functional ATPase activity is not required for the recruitment of HELLS to heterochromatin, but it is important for the release of the enzyme from these sites.

The ATP binding site of the chromatin remodeling homolog Lsh is required for nucleosome density and de novo DNA methylation at repeat sequences

Lsh, a chromatin remodeling protein of the SNF2 family, is critical for normal heterochromatin structure. In particular, DNA methylation at repeat elements, a hallmark of heterochromatin, is greatly reduced in Lsh(-/-) (KO) cells. Here, we examined the presumed nucleosome remodeling activity of Lsh on chromatin in the context of DNA methylation. We found that dynamic CG methylation was dependent on Lsh in embryonic stem cells. Moreover, we demonstrate that ATP function is critical for de novo methylation at repeat sequences. The ATP binding site of Lsh is in part required to promote stable association of the DNA methyltransferase 3b with the repeat locus. By performing nucleosome occupancy assays, we found distinct nucleosome occupancy in KO ES cells compared to WT ES cells after differentiation. Nucleosome density was restored to wild-type level by re-expressing wild-type Lsh but not the ATP mutant in KO ES cells. Our results suggest that ATP-dependent nucleosome remodeling is the primary molecular function of Lsh, which may promote de novo methylation in differentiating ES cells.

ALT1, a Snf2 family chromatin remodeling ATPase, negatively regulates alkaline tolerance through enhanced defense against oxidative stress in rice

Alkaline salt stress adversely affects rice growth, productivity and grain quality. However, the mechanism underlying this process remains elusive. We characterized here an alkaline tolerant mutant, alt1 in rice. Map-based cloning revealed that alt1 harbors a mutation in a chromatin remodeling ATPase gene. ALT1-RNAi transgenic plants under different genetic background mimicked the alt1 phenotype, exhibiting tolerance to alkaline stress in a transcript dosage-dependent manner. The predicted ALT1 protein belonged to the Ris1 subgroup of the Snf2 family and was localized in the nucleus, and transcription of ALT1 was transiently suppressed after alkaline treatment. Although the absorption of several metal ions maintained well in the mutant under alkaline stress, expression level of the genes involved in metal ions homeostasis was not altered in the alt1 mutant. Classification of differentially expressed abiotic stress related genes, as revealed by microarray analysis, found that the majority (50/78) were involved in ROS production, ROS scavenging, and DNA repair. This finding was further confirmed by that alt1 exhibited lower levels of H₂O₂ under alkaline stress and tolerance to methyl viologen treatment. Taken together, these results suggest that ALT1 negatively functions in alkaline tolerance mainly through the defense against oxidative damage, and provide a potential two-step strategy for improving the tolerance of rice plants to alkaline stress.

Efficient detection of unpaired DNA requires a member of the rad54-like family of homologous recombination proteins

Meiotic silencing by unpaired DNA (MSUD) is a process that detects unpaired regions between homologous chromosomes and silences them for the duration of sexual development. While the phenomenon of MSUD is well recognized, the process that detects unpaired DNA is poorly understood. In this report, we provide two lines of evidence linking unpaired DNA detection to a physical search for DNA homology. First, we have found that a putative SNF2-family protein (SAD-6) is required for efficient MSUD in Neurospora crassa. SAD-6 is closely related to Rad54, a protein known to facilitate key steps in the repair of double-strand breaks by homologous recombination. Second, we have successfully masked unpaired DNA by placing identical transgenes at slightly different locations on homologous chromosomes. This masking falls apart when the distance between the transgenes is increased. We propose a model where unpaired DNA detection during MSUD is achieved through a spatially constrained search for DNA homology. The identity of SAD-6 as a Rad54 paralog suggests that this process may be similar to the searching mechanism used during homologous recombination.

INO80 and SWR complexes: relating structure to function in chromatin remodeling

Virtually all DNA-dependent processes require selective and controlled access to the DNA sequence. Governing this access are sophisticated molecular machines, nucleosome remodelers, which regulate the composition and structure of chromatin, allowing conversion from open to closed states. In most cases these multisubunit remodelers operate in concert to organize chromatin structure by depositing, moving, evicting, or selectively altering nucleosomes in an ATP-dependent manner. Despite sharing a conserved domain architecture, chromatin remodelers differ significantly in how they bind to their nucleosomal substrates. Recent structural studies link specific interactions between nucleosomes and remodelers to the diverse tasks they carry out. We review here insights into the modular organization of the INO80 family of nucleosome remodelers. Understanding their structural diversity will help to shed light on how these related ATPases modify their nucleosomal substrates.

Two-way communications between ubiquitin-like modifiers and DNA

Many aspects of nucleic acid metabolism, such as DNA replication, repair and transcription, are regulated by the post-translational modifiers ubiquitin and SUMO. Not surprisingly, DNA itself plays an integral part in determining the modification of most chromatin-associated targets. Conversely, ubiquitination or SUMOylation of a protein can impinge on its DNA-binding properties. This review describes mechanistic principles governing the mutual interactions between DNA and ubiquitin or SUMO.

Yeti, an essential Drosophila melanogaster gene, encodes a protein required for chromatin organization

The evolutionarily conserved family of Bucentaur (BCNT) proteins exhibits a widespread distribution in animal and plants, yet its biological role remains largely unknown. Using Drosophila melanogaster as a model organism, we investigated the in vivo role of the Drosophila BCNT member called YETI. We report that loss of YETI causes lethality before pupation and defects in higher-order chromatin organization, as evidenced by severe impairment in the association of histone H2A.V, nucleosomal histones and epigenetic marks with polytene chromosomes. We also find that YETI binds to polytene chromosomes through its conserved BCNT domain and interacts with the histone variant H2A.V, HP1a and Domino-A (DOM-A), the ATPase subunit of the DOM/Tip60 chromatin remodeling complex. Furthermore, we identify YETI as a downstream target of the Drosophila DOM-A. On the basis of these results, we propose that YETI interacts with H2A.V-exchanging machinery, as a chaperone or as a new subunit of the DOM/Tip60 remodeling complex, and acts to regulate the accumulation of H2A.V at chromatin sites. Overall, our findings suggest an unanticipated role of YETI protein in chromatin organization and provide, for the first time, mechanistic clues on how BCNT proteins control development in multicellular organisms.

Different CHD chromatin remodelers are required for expression of distinct gene sets and specific stages during development of Dictyostelium discoideum

Control of chromatin structure is crucial for multicellular development and regulation of cell differentiation. The CHD (chromodomain-helicase-DNA binding) protein family is one of the major ATP-dependent, chromatin remodeling factors that regulate nucleosome positioning and access of transcription factors and RNA polymerase to the eukaryotic genome. There are three mammalian CHD subfamilies and their impaired functions are associated with several human diseases. Here, we identify three CHD orthologs (ChdA, ChdB and ChdC) in Dictyostelium discoideum. These CHDs are expressed throughout development, but with unique patterns. Null mutants lacking each CHD have distinct phenotypes that reflect their expression patterns and suggest functional specificity. Accordingly, using genome-wide (RNA-seq) transcriptome profiling for each null strain, we show that the different CHDs regulate distinct gene sets during both growth and development. ChdC is an apparent ortholog of the mammalian Class III CHD group that is associated with the human CHARGE syndrome, and GO analyses of aberrant gene expression in chdC nulls suggest defects in both cell-autonomous and non-autonomous signaling, which have been confirmed through analyses of chdC nulls developed in pure populations or with low levels of wild-type cells. This study provides novel insight into the broad function of CHDs in the regulation development and disease, through chromatin-mediated changes in directed gene expression.

Rad54 functions as a heteroduplex DNA pump modulated by its DNA substrates and Rad51 during D loop formation

The displacement loop (D loop) is the product of homology search and DNA strand invasion, constituting a central intermediate in homologous recombination (HR). In eukaryotes, the Rad51 DNA strand exchange protein is assisted in D loop formation by the Rad54 motor protein. Curiously, Rad54 also disrupts D loops. How these opposing activities are coordinated toward productive recombination is unknown. Moreover, a seemingly disparate function of Rad54 is removal of Rad51 from heteroduplex DNA (hDNA) to allow HR-associated DNA synthesis. Here, we uncover features of D loop formation/dissociation dynamics, employing Rad51 filaments formed on ssDNAs that mimic the physiological length and structure of in vivo substrates. The Rad54 motor is activated by Rad51 bound to synapsed DNAs and guided by a ssDNA-binding domain. We present a unified model wherein Rad54 acts as an hDNA pump that drives D loop formation while simultaneously removing Rad51 from hDNA, consolidating both ATP-dependent activities of Rad54 into a single mechanistic step.

Structure and subunit topology of the INO80 chromatin remodeler and its nucleosome complex

INO80/SWR1 family chromatin remodelers are complexes composed of >15 subunits and molecular masses exceeding 1 MDa. Their important role in transcription and genome maintenance is exchanging the histone variants H2A and H2A.Z. We report the architecture of S. cerevisiae INO80 using an integrative approach of electron microscopy, crosslinking and mass spectrometry. INO80 has an embryo-shaped head-neck-body-foot architecture and shows dynamic open and closed conformations. We can assign an Rvb1/Rvb2 heterododecamer to the head in close contact with the Ino80 Snf2 domain, Ies2, and the Arp5 module at the neck. The high-affinity nucleosome-binding Nhp10 module localizes to the body, whereas the module that contains actin, Arp4, and Arp8 maps to the foot. Structural and biochemical analyses indicate that the nucleosome is bound at the concave surface near the neck, flanked by the Rvb1/2 and Arp8 modules. Our analysis establishes a structural and functional framework for this family of large remodelers.

Mouse BAZ1A (ACF1) is dispensable for double-strand break repair but is essential for averting improper gene expression during spermatogenesis

ATP-dependent chromatin remodelers control DNA access for transcription, recombination, and other processes. Acf1 (also known as BAZ1A in mammals) is a defining subunit of the conserved ISWI-family chromatin remodelers ACF and CHRAC, first purified over 15 years ago from Drosophila melanogaster embryos. Much is known about biochemical properties of ACF and CHRAC, which move nucleosomes in vitro and in vivo to establish ordered chromatin arrays. Genetic studies in yeast, flies and cultured human cells clearly implicate these complexes in transcriptional repression via control of chromatin structures. RNAi experiments in transformed mammalian cells in culture also implicate ACF and CHRAC in DNA damage checkpoints and double-strand break repair. However, their essential in vivo roles in mammals are unknown. Here, we show that Baz1a-knockout mice are viable and able to repair developmentally programmed DNA double-strand breaks in the immune system and germ line, I-SceI endonuclease-induced breaks in primary fibroblasts via homologous recombination, and DNA damage from mitomycin C exposure in vivo. However, Baz1a deficiency causes male-specific sterility in accord with its high expression in male germ cells, where it displays dynamic, stage-specific patterns of chromosomal localization. Sterility is caused by pronounced defects in sperm development, most likely a consequence of massively perturbed gene expression in spermatocytes and round spermatids in the absence of BAZ1A: the normal spermiogenic transcription program is largely intact but more than 900 other genes are mis-regulated, primarily reflecting inappropriate up-regulation. We propose that large-scale changes in chromatin composition that occur during spermatogenesis create a window of vulnerability to promiscuous transcription changes, with an essential function of ACF and/or CHRAC chromatin remodeling activities being to safeguard against these alterations.

The eukaryotic genome is packaged into a periodic nucleoprotein complex known as chromatin. Wrapping of DNA around nucleosomes, the basic repeat unit of chromatin, enables packing of long stretches of DNA into a compact nucleus but also impedes access by protein factors involved in essential cellular processes such as transcription, replication, recombination and repair. Chromatin remodeling factors are multi-protein complexes that utilize the energy released during ATP-hydrolysis to assemble, reposition, restructure and disassemble nucleosomes. These complexes disrupt histone-DNA contacts to ‘remodel’ the chromatin and grant access to the genome. Alternatively, access can also be denied to repress transcription, for example. Spermatogenesis, the developmental program that produces sperm, comprises a dramatic chromatin makeover and the induction of a transcriptional program that engages nearly one-third of the genome. Here we provide evidence suggesting that these large-scale alterations leave the genomic material vulnerable to spurious transcriptional changes which are normally repressed by ACF1 (BAZ1A in mammals), the defining member of the well-studied ACF/CHRAC chromatin remodeling complex. These findings indicate that Baz1a plays a previously unrealized role in male fertility and may represent a novel target for male contraceptive development.

Assembly, remodelled

Biochemical assays reveal that nucleosome maturation and chromatin remodelling by the motor protein Chd1 are distinct, separable enzymatic activities.