Swi/Snf dynamics on stress-responsive genes is governed by competitive bromodomain interactions

Swi/Snf chromatin remodeling regulates transcriptional interference and gene repression

Alternative transcription start sites can affect transcript isoform diversity and translation levels. In a recently described form of gene regulation, coordinated transcriptional and translational interference results in transcript isoform-dependent changes in protein expression. Specifically, a long undecoded transcript isoform (LUTI) is transcribed from a gene-distal promoter, interfering with expression of the gene-proximal promoter. Although transcriptional and chromatin features associated with LUTI expression have been described, the mechanism underlying LUTI-based transcriptional interference is not well understood. Using an unbiased genetic approach followed by functional genomics, we uncovered that the Swi/Snf chromatin remodeling complex is required for co-transcriptional nucleosome remodeling that leads to LUTI-based repression. We identified genes with tandem promoters that rely on Swi/Snf function for transcriptional interference during protein folding stress, including LUTI-regulated genes. This study provides clear evidence for Swi/Snf playing a direct role in gene repression via a cis transcriptional interference mechanism.

The SWI/SNF chromatin remodeling complex: a critical regulator of metabolism

The close relationship between chromatin and metabolism has been well-studied in recent years. Many metabolites have been found to be cofactors used to modify chromatin, and these modifications can in turn affect gene transcription. One chromatin-associated factor responsible for regulating transcription is the SWI/SNF complex, an ATP-dependent chromatin remodeler conserved throughout eukaryotes. SWI/SNF was originally described in yeast as regulating genes involved in carbon source metabolism and mating type switching, and its mammalian counterpart has been extensively studied for its role in diseases such as cancer. The yeast SWI/SNF complex is closely associated with activation of stress response genes, many of which have metabolic functions. It is now recognized that this is a conserved function of the complex, and recent work has shown that mammalian SWI/SNF is also a key regulator of metabolic transcription. Emerging evidence suggests that loss of SWI/SNF introduces vulnerabilities to cells due to this metabolic influence, and that this may present opportunities for treatment of SWI/SNF-deficient cancers.

The Swi-Snf chromatin remodeling complex mediates gene repression through metabolic control

In eukaryotes, ATP-dependent chromatin remodelers regulate gene expression in response to nutritional and metabolic stimuli. However, altered transcription of metabolic genes may have significant indirect consequences which are currently poorly understood. In this study, we use genetic and molecular approaches to uncover a role for the remodeler Swi-Snf as a critical regulator of metabolism. We find that snfΔ mutants display a cysteine-deficient phenotype, despite growth in nutrient-rich media. This correlates with widespread perturbations in sulfur metabolic gene transcription, including global redistribution of the sulfur-sensing transcription factor Met4. Our findings show how a chromatin remodeler can have a significant impact on a whole metabolic pathway by directly regulating an important gene subset and demonstrate an emerging role for chromatin remodeling complexes as decisive factors in metabolic control.

Quantitative Acetylomics Reveals Substrates of Lysine Acetyltransferase GCN5 in Adult and Aging Drosophila

Protein lysine acetylation is a dynamic post-translational modification (PTM) that regulates a wide spectrum of cellular events including aging. General control nonderepressible 5 (GCN5) is a highly conserved lysine acetyltransferase (KAT). However, the acetylation substrates of GCN5 in vivo remain poorly studied, and moreover, how lysine acetylation changes with age and the contribution of KATs to aging remain to be addressed. Here, using Drosophila, we perform label-free quantitative acetylomic analysis, identifying new substrates of GCN5 in the adult and aging process. We further characterize the dynamics of protein acetylation with age, which exhibits a trend of increase. Since the expression of endogenous fly Gcn5 progressively increases during aging, we reason that, by combining the substrate analysis, the increase in acetylation with age is triggered, at least in part, by GCN5. Collectively, our study substantially expands the atlas of GCN5 substrates in vivo, provides a resource of protein acetylation that naturally occurs with age, and demonstrates how individual KAT contributes to the aging acetylome.

Swi/Snf Chromatin Remodeling Regulates Transcriptional Interference and Gene Repression

Alternative transcription start sites can affect transcript isoform diversity and translation levels. In a recently described form of gene regulation, coordinated transcriptional and translational interference results in transcript isoform-dependent changes in protein expression. Specifically, a long undecoded transcript isoform (LUTI) is transcribed from a gene-distal promoter, interfering with expression of the gene-proximal promoter. While transcriptional and chromatin features associated with LUTI expression have been described, the mechanism underlying LUTI-based transcriptional interference is not well understood. Using an unbiased genetic approach followed by integrated genomic analysis, we uncovered that the Swi/Snf chromatin remodeling complex is required for co-transcriptional nucleosome remodeling that leads to LUTI-based repression. We identified genes with tandem promoters that rely on Swi/Snf function for transcriptional interference during protein folding stress, including LUTI-regulated genes. To our knowledge, this study is the first to observe Swi/Snf's direct involvement in gene repression via a cis transcriptional interference mechanism.

The Gcn5-Ada2-Ada3 histone acetyltransferase module has divergent roles in pathogenesis of Candida glabrata

Candida glabrata is an opportunistic fungal pathogen and the second most prevalent species isolated from candidiasis patients. C. glabrata has intrinsic tolerance to antifungal drugs and oxidative stresses and the ability to adhere to mucocutaneous surfaces. However, knowledge about the regulation of its virulence traits is limited. The Spt-Ada-Gcn5 acetyltransferase (SAGA) complex modulates gene transcription by histone acetylation through the histone acetyltransferase (HAT) module comprised of Gcn5-Ada2-Ada3. Previously, we showed that the ada2 mutant was hypervirulent but displayed decreased tolerance to antifungal drugs and cell wall perturbing agents. In this study, we further characterized the functions of Ada3 and Gcn5 in C. glabrata. We found that single, double, or triple deletions of the HAT module, as expected, resulted in a decreased level of acetylation on histone H3 lysine 9 (H3K9) and defective growth. These mutants were more susceptible to antifungal drugs, oxidative stresses, and cell wall perturbing agents compared with the wild-type. In addition, HAT module mutants exhibited enhanced agar invasion and upregulation of adhesin and proteases encoding genes, whereas the biofilm formation of those mutants was impaired. Interestingly, HAT module mutants exhibited enhanced induction of catalases (CTA1) expression upon treatment with H2O2 compared with the wild-type. Lastly, although ada3 and gcn5 exhibited marginal hypervirulence, the HAT double and triple mutants were hypervirulent in a murine model of candidiasis. In conclusion, the HAT module of the SAGA complex plays unique roles in H3K9 acetylation, drug tolerance, oxidative stress response, adherence, and virulence in C. glabrata.

Identifying Endogenous Cellular Proteins Destabilizing the Propagation of Swi1 Prion upon Overproduction

(1) Background: Numerous prions exist in the budding yeast, including [SWI+], the prion form of Swi1-a subunit of the chromatin-remodeling complex SWI/SNF. Despite decades of research, the molecular mechanisms underlying prion initiation and propagation are not fully understood. In this study, we aimed to identify endogenous cellular proteins that destabilize [SWI+]. (2) Methods: We screened the MoBY-ORF 2.0 library for proteins that destabilize [SWI+] upon overproduction. We further explored the effects of the identified candidates against other yeast prions and analyzed their potential prion-curing mechanisms. (3) Results: Eighty-two [SWI+] suppressors were identified, and their effects were shown to be [SWI+]-specific. Interestingly, a few documented [SWI+] suppressors were not among the identified hits. Further experiments indicate that, for some of these [SWI+] suppressors, their overproduction, and thus their prion-curing activities, are regulated by environmental conditions. Bioinformatics analyses show that our identified [SWI+] suppressors are involved in diverse biological functions, with gene ontology term enrichments specifically for transcriptional regulation and translation. Competition for Swi1 monomers between [SWI+] and Swi1 interactors, including the SWI/SNF complex, is a potential prion-curing mechanism. (4) Conclusions: We identified a number of [SWI+]-specific suppressors that highlight unique features of [SWI+] in maintaining its self-perpetuating conformations.

SWI/SNF and the histone chaperone Rtt106 drive expression of the Pleiotropic Drug Resistance network genes

The Pleiotropic Drug Resistance (PDR) network is central to the drug response in fungi, and its overactivation is associated with drug resistance. However, gene regulation of the PDR network is not well understood. Here, we show that the histone chaperone Rtt106 and the chromatin remodeller SWI/SNF control expression of the PDR network genes and confer drug resistance. In Saccharomyces cerevisiae, Rtt106 specifically localises to PDR network gene promoters dependent on transcription factor Pdr3, but not Pdr1, and is essential for Pdr3-mediated basal expression of the PDR network genes, while SWI/SNF is essential for both basal and drug-induced expression. Also in the pathogenic fungus Candida glabrata, Rtt106 and SWI/SNF regulate drug-induced PDR gene expression. Consistently, loss of Rtt106 or SWI/SNF sensitises drug-resistant S. cerevisiae mutants and C. glabrata to antifungal drugs. Since they cooperatively drive PDR network gene expression, Rtt106 and SWI/SNF represent potential therapeutic targets to combat antifungal resistance.

Emerging roles of SWI/SNF remodelers in fungal pathogens

Fungal pathogens constantly sense and respond to the environment they inhabit, and this interaction is vital for their survival inside hosts and exhibiting pathogenic traits. Since such responses often entail specific patterns of gene expression, regulators of chromatin structure contribute to the fitness and virulence of the pathogens by modulating DNA accessibility to the transcriptional machinery. Recent studies in several human and plant fungal pathogens have uncovered the SWI/SNF group of chromatin remodelers as an important determinant of pathogenic traits and provided insights into their mechanism of function. Here, we review these studies and highlight the differential functions of these remodeling complexes and their subunits in regulating fungal fitness and pathogenicity. As an extension of our previous study, we also show that loss of specific RSC subunits can predispose the human fungal pathogen Candida albicans cells to filamentous growth in a context-dependent manner. Finally, we consider the potential of targeting the fungal SWI/SNF remodeling complexes for antifungal interventions.

Macrophages, Metabolites, and Nucleosomes: Chromatin at the Intersection between Aging and Inflammation

Inflammation is the body's means of defense against harmful stimuli, with the ultimate aim being to restore homeostasis. Controlled acute inflammation transiently activates an immune response and can be beneficial as protection against infection or injury. However, dysregulated inflammatory responses, including chronic inflammation, disrupt the immune system's ability to maintain homeostatic balance, leading to increased susceptibility to infection, continuous tissue damage, and dysfunction. Aging is a risk factor for chronic inflammation; their coincidence is termed "inflammaging". Metabolic disorders including obesity, neurodegenerative diseases, and atherosclerosis are often encountered in old age. Therefore, it is important to understand the mechanistic relationship between aging, chronic inflammation, and metabolism. It has been established that the expression of inflammatory mediators is transcriptionally and translationally regulated. In addition, the post-translational modification of the mediators plays a crucial role in the response to inflammatory signaling. Chromatin regulation responds to metabolic status and controls homeostasis. However, chromatin structure is also changed by aging. In this review, we discuss the functional contributions of chromatin regulation to inflammaging.

The ATP-dependent SWI/SNF and RSC chromatin remodelers cooperatively induce unfolded protein response genes during endoplasmic reticulum stress

The SWI/SNF subfamily remodelers (SWI/SNF and RSC) generally promote gene expression by displacing or evicting nucleosomes at the promoter regions. Their action creates a nucleosome-depleted region where transcription machinery accesses the DNA. Their function has been shown critical for inducing stress-responsive transcription programs. Although the role of SWI/SNF and RSC complexes in transcription regulation of heat shock responsive genes is well studied, their involvement in other pathways such as unfolded protein response (UPR) and protein quality control (PQC) is less known. This study shows that SWI/SNF occupies the promoters of UPR, HSP and PQC genes in response to unfolded protein stress, and its recruitment at UPR promoters depends on Hac1 transcription factor and other epigenetic factors like Ada2 and Ume6. Disruption of SWI/SNF's activity does not affect the remodeling of these promoters or gene expression. However, inactivation of RSC and SWI/SNF together diminishes induction of most of the UPR, HSP and PQC genes tested. Furthermore, RSC and SWI/SNF colocalize at these promoters, suggesting that these two remodelers functionally cooperate to induce stress-responsive genes under proteotoxic conditions.

The Bromodomains of the mammalian SWI/SNF (mSWI/SNF) ATPases Brahma (BRM) and Brahma Related Gene 1 (BRG1) promote chromatin interaction and are critical for skeletal muscle differentiation

Skeletal muscle regeneration is mediated by myoblasts that undergo epigenomic changes to establish the gene expression program of differentiated myofibers. mSWI/SNF chromatin remodeling enzymes coordinate with lineage-determining transcription factors to establish the epigenome of differentiated myofibers. Bromodomains bind to acetylated lysines on histone N-terminal tails and other proteins. The mutually exclusive ATPases of mSWI/SNF complexes, BRG1 and BRM, contain bromodomains with undefined functional importance in skeletal muscle differentiation. Pharmacological inhibition of mSWI/SNF bromodomain function using the small molecule PFI-3 reduced differentiation in cell culture and in vivo through decreased myogenic gene expression, while increasing cell cycle-related gene expression and the number of cells remaining in the cell cycle. Comparative gene expression analysis with data from myoblasts depleted of BRG1 or BRM showed that bromodomain function was required for a subset of BRG1- and BRM-dependent gene expression. Reduced binding of BRG1 and BRM after PFI-3 treatment showed that the bromodomain is required for stable chromatin binding at target gene promoters to alter gene expression. Our findings demonstrate that mSWI/SNF ATPase bromodomains permit stable binding of the mSWI/SNF ATPases to promoters required for cell cycle exit and establishment of muscle-specific gene expression.

Variation, Variegation and Heritable Gene Repression in S. cerevisiae

Phenotypic heterogeneity provides growth advantages for a population upon changes of the environment. In S. cerevisiae, such heterogeneity has been observed as "on/off" states in the expression of individual genes in individual cells. These variations can persist for a limited or extended number of mitotic divisions. Such traits are known to be mediated by heritable chromatin structures, by the mitotic transmission of transcription factors involved in gene regulatory circuits or by the cytoplasmic partition of prions or other unstructured proteins. The significance of such epigenetic diversity is obvious, however, we have limited insight into the mechanisms that generate it. In this review, we summarize the current knowledge of epigenetically maintained heterogeneity of gene expression and point out similarities and converging points between different mechanisms. We discuss how the sharing of limiting repression or activation factors can contribute to cell-to-cell variations in gene expression and to the coordination between short- and long- term epigenetic strategies. Finally, we discuss the implications of such variations and strategies in adaptation and aging.

Yeast Nuak1 phosphorylates histone H3 threonine 11 in low glucose stress by the cooperation of AMPK and CK2 signaling

Changes in available nutrients are inevitable events for most living organisms. Upon nutritional stress, several signaling pathways cooperate to change the transcription program through chromatin regulation to rewire cellular metabolism. In budding yeast, histone H3 threonine 11 phosphorylation (H3pT11) acts as a marker of low glucose stress and regulates the transcription of nutritional stress-responsive genes. Understanding how this histone modification 'senses' external glucose changes remains elusive. Here, we show that Tda1, the yeast ortholog of human Nuak1, is a direct kinase for H3pT11 upon low glucose stress. Yeast AMP-activated protein kinase (AMPK) directly phosphorylates Tda1 to govern Tda1 activity, while CK2 regulates Tda1 nuclear localization. Collectively, AMPK and CK2 signaling converge on histone kinase Tda1 to link external low glucose stress to chromatin regulation.

Elucidating the regulatory mechanism of Swi1 prion in global transcription and stress responses

Transcriptional regulators are prevalent among identified prions in Saccharomyces cerevisiae, however, it is unclear how prions affect genome-wide transcription. We show here that the prion ([SWI⁺]) and mutant (swi1∆) forms of Swi1, a subunit of the SWI/SNF chromatin-remodeling complex, confer dramatically distinct transcriptomic profiles. In [SWI⁺] cells, genes encoding for 34 transcription factors (TFs) and 24 Swi1-interacting proteins can undergo transcriptional modifications. Several TFs show enhanced aggregation in [SWI⁺] cells. Further analyses suggest that such alterations are key factors in specifying the transcriptomic signatures of [SWI⁺] cells. Interestingly, swi1∆ and [SWI⁺] impose distinct and oftentimes opposite effects on cellular functions. Translation-associated activities, in particular, are significantly reduced in swi1∆ cells. Although both swi1∆ and [SWI⁺] cells are similarly sensitive to thermal, osmotic and drought stresses, harmful, neutral or beneficial effects were observed for a panel of tested chemical stressors. Further analyses suggest that the environmental stress response (ESR) is mechanistically different between swi1∆ and [SWI⁺] cells—stress-inducible ESR (iESR) are repressed by [SWI⁺] but unchanged by swi1∆ while stress-repressible ESR (rESR) are induced by [SWI⁺] but repressed by swi1∆. Our work thus demonstrates primarily gain-of-function outcomes through transcriptomic modifications by [SWI⁺] and highlights a prion-mediated regulation of transcription and phenotypes in yeast.

Non-histone protein acetylation by the evolutionarily conserved GCN5 and PCAF acetyltransferases

GCN5, conserved from yeast to humans, and the vertebrate specific PCAF, are lysine acetyltransferase enzymes found in large protein complexes. Both enzymes have well documented roles in the histone acetylation and the concomitant regulation of transcription. However, these enzymes also acetylate non-histone substrates to impact diverse aspects of cell physiology. Here, I review our current understanding of non-histone acetylation by GCN5 and PCAF across eukaryotes, from target identification to molecular mechanism and regulation. I focus mainly on budding yeast, where Gcn5 was first discovered, and mammalian systems, where the bulk of non-histone substrates have been characterized. I end the review by defining critical caveats and open questions that apply to all models.

RNA Sequencing Reveals Specific TranscriptomicSignatures Distinguishing Effects of the [SWI⁺] Prion and SWI1 Deletion in Yeast Saccharomyces cerevisiae

Prions are infectious, self-perpetuating protein conformers. In mammals, pathological aggregation of the prion protein causes incurable neurodegenerative disorders, while in yeast Saccharomyces cerevisiae, prion formation may be neutral or even beneficial. According to the prevailing contemporary point of view, prion formation is considered to be a functional inactivation of the corresponding protein whose conformational state shifts from the functional monomeric one to the infectious aggregated one. The Swi1 protein forms the [SWI⁺] prion and belongs to the nucleosome remodeler complex SWI/SNF controlling the expression of a significant part of the yeast genome. In this work, we performed RNA sequencing of isogenic S. cerevisiae strains grown on the media containing galactose as the sole carbon source. These strains bore the [SWI⁺] prion or had its structural gene SWI1 deleted. The comparative analysis showed that [SWI⁺] affects genome expression significantly weaker as compared to the SWI1 deletion. Moreover, in contrast to [SWI⁺], the SWI1 deletion causes the general inhibition of translation-related genes expression and chromosome I disomy. At the same time, the [SWI⁺] prion exhibits a specific pattern of modulation of the metabolic pathways and some biological processes and functions, as well as the expression of several genes. Thus, the [SWI⁺] prion only partially corresponds to the loss-of-function of SWI1 and demonstrates several gain-of-function traits.

Acetylation-Dependent Recruitment of the FACT Complex and Its Role in Regulating Pol II Occupancy Genome-Wide in Saccharomyces cerevisiae

Histone chaperones, chromatin remodelers, and histone modifying complexes play a critical role in alleviating the nucleosomal barrier for DNA-dependent processes. Here, we have examined the role of two highly conserved yeast (Saccharomyces cerevisiae) histone chaperones, facilitates chromatin transcription (FACT) and Spt6, in regulating transcription. We show that the H3 tail contributes to the recruitment of FACT to coding sequences in a manner dependent on acetylation. We found that deleting a H3 histone acetyltransferase Gcn5 or mutating lysines on the H3 tail impairs FACT recruitment at ADH1 and ARG1 genes. However, deleting the H4 tail or mutating the H4 lysines failed to dampen FACT occupancy in coding regions. Additionally, we show that FACT depletion reduces RNA polymerase II (Pol II) occupancy genome-wide. Spt6 depletion leads to a reduction in Pol II occupancy toward the 3'-end, in a manner dependent on the gene length. Severe transcription and histone-eviction defects were also observed in a strain that was impaired for Spt6 recruitment (spt6Δ202) and depleted of FACT. Importantly, the severity of the defect strongly correlated with wild-type Pol II occupancies at these genes, indicating critical roles for Spt6 and Spt16 in promoting high-level transcription. Collectively, our results show that both FACT and Spt6 are important for transcription globally and may participate during different stages of transcription.

The INO80 chromatin remodeler sustains metabolic stability by promoting TOR signaling and regulating histone acetylation

Chromatin remodeling complexes are essential for gene expression programs that coordinate cell function with metabolic status. However, how these remodelers are integrated in metabolic stability pathways is not well known. Here, we report an expansive genetic screen with chromatin remodelers and metabolic regulators in Saccharomyces cerevisiae. We found that, unlike the SWR1 remodeler, the INO80 chromatin remodeling complex is composed of multiple distinct functional subunit modules. We identified a strikingly divergent genetic signature for the Ies6 subunit module that links the INO80 complex to metabolic homeostasis. In particular, mitochondrial maintenance is disrupted in ies6 mutants. INO80 is also needed to communicate TORC1-mediated signaling to chromatin, as ino80 mutants exhibit defective transcriptional profiles and altered histone acetylation of TORC1-responsive genes. Furthermore, comparative analysis reveals subunits of INO80 and mTORC1 have high co-occurrence of alterations in human cancers. Collectively, these results demonstrate that the INO80 complex is a central component of metabolic homeostasis that influences histone acetylation and may contribute to disease when disrupted.

Sas3 and Ada2(Gcn5)-dependent histone H3 acetylation is required for transcription elongation at the de-repressed FLO1 gene

The Saccharomyces cerevisiae FLO1 gene encodes a cell wall protein that imparts cell-cell adhesion. FLO1 transcription is regulated via the antagonistic activities of the Tup1-Cyc8 co-repressor and Swi-Snf co-activator complexes. Tup1-Cyc8 represses transcription through the organization of strongly positioned, hypoacetylated nucleosomes across gene promoters. Swi-Snf catalyzes remodeling of these nucleosomes in a mechanism involving histone acetylation that is poorly understood. Here, we show that FLO1 de-repression is accompanied by Swi-Snf recruitment, promoter histone eviction and Sas3 and Ada2(Gcn5)-dependent histone H3K14 acetylation. In the absence of H3K14 acetylation, Swi-Snf recruitment and histone eviction proceed, but transcription is reduced, suggesting these processes, while essential, are not sufficient for de-repression. Further analysis in the absence of H3K14 acetylation reveals RNAP II recruitment at the FLO1 promoter still occurs, but RNAP II is absent from the gene-coding region, demonstrating Sas3 and Ada2-dependent histone H3 acetylation is required for transcription elongation. Analysis of the transcription kinetics at other genes reveals shared mechanisms coupled to a distinct role for histone H3 acetylation, essential at FLO1, downstream of initiation. We propose histone H3 acetylation in the coding region provides rate-limiting control during the transition from initiation to elongation which dictates whether the gene is permissive for transcription.

Diverse Activities of Histone Acylations Connect Metabolism to Chromatin Function

Modifications of histones play important roles in balancing transcriptional output. The discovery of acyl marks, besides histone acetylation, has added to the functional diversity of histone modifications. Since all modifications use metabolic intermediates as substrates for chromatin-modifying enzymes, the prevalent landscape of histone modifications in any cell type is a snapshot of its metabolic status. Here, we review some of the current findings of how differential use of histone acylations regulates gene expression as response to metabolic changes and differentiation programs.

The chromatin remodeling complex Swi/Snf regulates splicing of meiotic transcripts in Saccharomyces cerevisiae

Despite its relatively streamlined genome, there are important examples of regulated RNA splicing in Saccharomyces cerevisiae, such as splicing of meiotic transcripts. Like other eukaryotes, S. cerevisiae undergoes a dramatic reprogramming of gene expression during meiosis, including regulated splicing of a number of crucial meiosis-specific RNAs. Splicing of a subset of these is dependent upon the splicing activator Mer1. Here we show a crucial role for the chromatin remodeler Swi/Snf in regulation of splicing of meiotic genes and find that the complex affects meiotic splicing in two ways. First, we show that Swi/Snf regulates nutrient-dependent downregulation of ribosomal protein encoding RNAs, leading to the redistribution of spliceosomes from this abundant class of intron-containing RNAs (the ribosomal protein genes) to Mer1-regulated transcripts. We also demonstrate that Mer1 expression is dependent on Snf2, its acetylation state and histone H3 lysine 9 acetylation at the MER1 locus. Hence, Snf2 exerts systems level control of meiotic gene expression through two temporally distinct mechanisms, demonstrating that it is a key regulator of meiotic splicing in S. cerevisiae. We also reveal an evolutionarily conserved mechanism whereby the cell redirects its energy from maintaining its translational capacity to the process of meiosis.

Chromatin-remodeling SWI/SNF complex regulates coenzyme Q6 synthesis and a metabolic shift to respiration in yeast

Despite its relatively streamlined genome, there are many important examples of regulated RNA splicing in Saccharomyces cerevisiae Here, we report a role for the chromatin remodeler SWI/SNF in respiration, partially via the regulation of splicing. We find that a nutrient-dependent decrease in Snf2 leads to an increase in splicing of the PTC7 transcript. The spliced PTC7 transcript encodes a mitochondrial phosphatase regulator of biosynthesis of coenzyme Q₆ (ubiquinone or CoQ₆) and a mitochondrial redox-active lipid essential for electron and proton transport in respiration. Increased splicing of PTC7 increases CoQ₆ levels. The increase in PTC7 splicing occurs at least in part due to down-regulation of ribosomal protein gene expression, leading to the redistribution of spliceosomes from this abundant class of intron-containing RNAs to otherwise poorly spliced transcripts. In contrast, a protein encoded by the nonspliced isoform of PTC7 represses CoQ₆ biosynthesis. Taken together, these findings uncover a link between Snf2 expression and the splicing of PTC7 and establish a previously unknown role for the SWI/SNF complex in the transition of yeast cells from fermentative to respiratory modes of metabolism.

DNA binding drives the association of BRG1/hBRM bromodomains with nucleosomes

BRG1 and BRM, central components of the BAF (mSWI/SNF) chromatin remodelling complex, are critical in chromatin structure regulation. Here, we show that the human BRM (hBRM) bromodomain (BRD) has moderate specificity for H3K14ac. Surprisingly, we also find that both BRG1 and hBRM BRDs have DNA-binding activity. We demonstrate that the BRDs associate with DNA through a surface basic patch and that the BRD and an adjacent AT-hook make multivalent contacts with DNA, leading to robust affinity and moderate specificity for AT-rich elements. Although we show that the BRDs can bind to both DNA and H3K14ac simultaneously, the histone-binding activity does not contribute substantially to nucleosome targeting in vitro. In addition, we find that neither BRD histone nor DNA binding contribute to the global chromatin affinity of BRG1 in mouse embryonic stem cells. Together, our results suggest that association of the BRG1/hBRM BRD with nucleosomes plays a regulatory rather than targeting role in BAF activity.

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.

Mechanisms of action and regulation of ATP-dependent chromatin-remodelling complexes

Cells utilize diverse ATP-dependent nucleosome-remodelling complexes to carry out histone sliding, ejection or the incorporation of histone variants, suggesting that different mechanisms of action are used by the various chromatin-remodelling complex subfamilies. However, all chromatin-remodelling complex subfamilies contain an ATPase-translocase 'motor' that translocates DNA from a common location within the nucleosome. In this Review, we discuss (and illustrate with animations) an alternative, unifying mechanism of chromatin remodelling, which is based on the regulation of DNA translocation. We propose the 'hourglass' model of remodeller function, in which each remodeller subfamily utilizes diverse specialized proteins and protein domains to assist in nucleosome targeting or to differentially detect nucleosome epitopes. These modules converge to regulate a common DNA translocation mechanism, to inform the conserved ATPase 'motor' on whether and how to apply DNA translocation, which together achieve the various outcomes of chromatin remodelling: nucleosome assembly, chromatin access and nucleosome editing.

Composition and Function of Mutant Swi/Snf Complexes

The 12-subunit Swi/Snf chromatin remodeling complex is conserved from yeast to humans. It functions to alter nucleosome positions by either sliding nucleosomes on DNA or evicting histones. Interestingly, 20% of all human cancers carry mutations in subunits of the Swi/Snf complex. Many of these mutations cause protein instability and loss, resulting in partial Swi/Snf complexes. Although several studies have shown that histone acetylation and activator-dependent recruitment of Swi/Snf regulate its function, it is less well understood how subunits regulate stability and function of the complex. Using functional proteomic and genomic approaches, we have assembled the network architecture of yeast Swi/Snf. In addition, we find that subunits of the Swi/Snf complex regulate occupancy of the catalytic subunit Snf2, thereby modulating gene transcription. Our findings have direct bearing on how cancer-causing mutations in orthologous subunits of human Swi/Snf may lead to aberrant regulation of gene expression by this complex.

In Vitro Assembly of Nucleosomes for Binding/Remodeling Assays

In order to study the functions and activities of chromatin remodeling enzymes in vitro, it is necessary to be able to reconstitute nucleosomes on DNA templates. In this chapter, we describe procedures for purification of histones from E. coli, formation of octamers, and reconstitution of nucleosomes that can be further modified by chromatin modifiers. In addition, we describe methods to purify nucleosomes from human cells. Finally, we also describe assays to test binding and remodeling activities of chromatin remodelers.

Recruitment of Saccharomyces cerevisiae Cmr1/Ydl156w to Coding Regions Promotes Transcription Genome Wide

Cmr1 (changed mutation rate 1) is a largely uncharacterized nuclear protein that has recently emerged in several global genetic interaction and protein localization studies. It clusters with proteins involved in DNA damage and replication stress response, suggesting a role in maintaining genome integrity. Under conditions of proteasome inhibition or replication stress, this protein localizes to distinct sub-nuclear foci termed as intranuclear quality control (INQ) compartments, which sequester proteins for their subsequent degradation. Interestingly, it also interacts with histones, chromatin remodelers and modifiers, as well as with proteins involved in transcription including subunits of RNA Pol I and Pol III, but not with those of Pol II. It is not known whether Cmr1 plays a role in regulating transcription of Pol II target genes. Here, we show that Cmr1 is recruited to the coding regions of transcribed genes of S. cerevisiae. Cmr1 occupancy correlates with the Pol II occupancy genome-wide, indicating that it is recruited to coding sequences in a transcription-dependent manner. Cmr1-enriched genes include Gcn4 targets and ribosomal protein genes. Furthermore, our results show that Cmr1 recruitment to coding sequences is stimulated by Pol II CTD kinase, Kin28, and the histone deacetylases, Rpd3 and Hos2. Finally, our genome-wide analyses implicate Cmr1 in regulating Pol II occupancy at transcribed coding sequences. However, it is dispensable for maintaining co-transcriptional histone occupancy and histone modification (acetylation and methylation). Collectively, our results show that Cmr1 facilitates transcription by directly engaging with transcribed coding regions.

Barcelona Conference on Epigenetics and Cancer: 50 years of histone acetylation

The Barcelona Conference on Epigenetics and Cancer (BCEC) was held in Barcelona, Spain, on October 1(st) and 2(nd), 2014. The meeting was co-organized by the Cancer Epigenetics and Biology Program (PEBC-IDIBELL) and B·Debate, an initiative of Biocat, with the support of "la Caixa" Foundation. The scientific committee was comprised of leading scientists in the field of epigenetics: Dr. Manel Esteller, director of PEBC-IDIBELL, Dr. Alejandro Vaquero and Dr. Esteban Ballestar, from PEBC-IDIBELL, Juan Ausió from the University of Victoria (Canada), and Marcus Buschbeck, from the Institute of Predictive and Personalized Medicine of Cancer (IMPPC), as BCEC series coordinator. This meeting was the second edition of the BCEC series, which was launched by 5 leading Barcelonan institutes to bring together leading investigators in the fields of epigenetics and chromatin research. The topics discussed during the meeting included the current challenges, opportunities, and perspectives surrounding the study of histone modifications (focusing in acetylation), chromatin structure and gene expression, and the involvement of histone acetylation in physiology and diseases, such as cancer or neurological diseases.

Epigenetic Regulation by ATP-Dependent Chromatin-Remodeling Enzymes: SNF-ing Out Crosstalk

Cells utilize precise mechanisms to access genomic DNA with spatiotemporal accuracy. ATP-dependent chromatin-remodeling enzymes (also known simply as "remodelers") comprise a specialized class of enzymes that is intimately involved in genomic organization and accessibility. Remodelers selectively position nucleosomes to either alleviate chromatin compaction or achieve genomic condensation locally, based on a multitude of cellular signals. By dictating nucleosome position, remodelers control local euchromatic and heterochromatic states. These activities govern the accessibility of regulatory regions like promoters and enhancers to transcription factors, RNA polymerases, and coactivators or -repressors. As studies unravel the complexities of epigenetic topography, evidence points to a chromatin-based interactome where regulators interact competitively, cooperatively, and/or codependently through physical and functional means. These types of interactions, or crosstalk, between remodelers raise important questions for tissue development. Here, we briefly review the evidence for remodeler interactions and argue for additional studies examining crosstalk.

Nucleosome positioning: resources and tools online

Nucleosome positioning is an important process required for proper genome packing and its accessibility to execute the genetic program in a cell-specific, timely manner. In the recent years hundreds of papers have been devoted to the bioinformatics, physics and biology of nucleosome positioning. The purpose of this review is to cover a practical aspect of this field, namely, to provide a guide to the multitude of nucleosome positioning resources available online. These include almost 300 experimental datasets of genome-wide nucleosome occupancy profiles determined in different cell types and more than 40 computational tools for the analysis of experimental nucleosome positioning data and prediction of intrinsic nucleosome formation probabilities from the DNA sequence. A manually curated, up to date list of these resources will be maintained at http://generegulation.info.

Structural basis for recognition and remodeling of the TBP:DNA:NC2 complex by Mot1

Swi2/Snf2 ATPases remodel substrates such as nucleosomes and transcription complexes to control a wide range of DNA-associated processes, but detailed structural information on the ATP-dependent remodeling reactions is largely absent. The single subunit remodeler Mot1 (modifier of transcription 1) dissociates TATA box-binding protein (TBP):DNA complexes, offering a useful system to address the structural mechanisms of Swi2/Snf2 ATPases. Here, we report the crystal structure of the N-terminal domain of Mot1 in complex with TBP, DNA, and the transcription regulator negative cofactor 2 (NC2). Our data show that Mot1 reduces DNA:NC2 interactions and unbends DNA as compared to the TBP:DNA:NC2 state, suggesting that Mot1 primes TBP:NC2 displacement in an ATP-independent manner. Electron microscopy and cross-linking data suggest that the Swi2/Snf2 domain of Mot1 associates with the upstream DNA and the histone fold of NC2, thereby revealing parallels to some nucleosome remodelers. This study provides a structural framework for how a Swi2/Snf2 ATPase interacts with its substrate DNA:protein complex.

DOI:http://dx.doi.org/10.7554/eLife.07432.001

An organism’s DNA contains thousands of genes, not all of which are active at the same time. Cells use a number of methods to carefully control when particular genes are switched on or off. For example, proteins called transcription factors can activate a gene by binding to particular regions of DNA called promoters. One such transcription factor is called the TATA-binding protein (TBP for short). Mot1 is a remodeling enzyme that can form a “complex” with TBP by binding to it, and in doing so remove TBP from DNA. This silences the genes at those sites. The freed TBP can then bind to other promoters that lack Mot1 and activate the genes found there.

In 2011, researchers revealed the structure of the complex formed between TBP and Mot1 after TBP has been detached from DNA. However, the structure of the complex that forms while TBP is still bound to the DNA molecule was not known. Butryn et al. – including several of the researchers involved in the 2011 work – have now described the structure of this complex using X-ray crystallography and electron microscopy. Another protein called negative cofactor 2 is also part of the complex, and helps to stabilize it.

Butryn et al. found that Mot1 reduces the strength of the interactions between DNA and both TBP and negative cofactor 2. Binding to TBP and negative cofactor 2 causes the DNA molecule to bend; however, if Mot1 is also in the complex, the DNA becomes less bent. By making these changes, Mot1 is likely to prime TBP to detach from the DNA. Since the current structures do not yet reveal the atomic structure of Mot1’s ATP dependent DNA motor domain, the next challenge is to visualize the entire complex at atomic resolution.

DOI:http://dx.doi.org/10.7554/eLife.07432.002

Functional role of histone variant Htz1 in the stress response to oleate in Saccharomyces cerevisiae

In response to oleate stress in Saccharomyces cerevisiaes, Histone Two A Z1 (Htz1) undergoes a global redistribution during the glucose-oleate shift. The number of Htz1-bound genes increases, but the number of Htz1-bound ribosome genes decreases with stress. Citrate cycle-associated genes are enhanced and ribosome genes are repressed. Nucleosome dynamics are coupled with Htz1-binding changes upon stress. Multicopy suppressor of SNF1 protein 2 (Msn2) acts an important role in response to the oleate stress. We highlight the dynamics of Htz1 in the oleate stress.

Chromatin structure is implicated in regulating gene transcription in stress response. Transcription factors, transferases and deacetylases, such as multicopy suppressor of SNF1 protein 2 (Msn2), SET domain-containing protein 1 (Set1) and sucrose NonFermenting protein 1 (Snf1), have been identified as key regulators in stress response. In the present study, we reported the dynamics of nucleosome occupancy, Histone Two A Z1 (Htz1) deposition and histone H3 lysine 4 dimethylation (H3K4me2) and histone H3 lysine 79 trimethylation (H3K79me3) in Saccharomyces cerevisiae under oleate stress. Our results indicated that citrate cycle-associated genes are enhanced and ribosome genes are repressed during the glucose-oleate shift. Importantly, Htz1 acts as a sensor for oleate stress. High-throughput ChIP-chip analysis showed that Htz1 has redistributed across the genome during oleate stress. The number of Htz1-bound genes increases with stress and the number of Htz1-bound ribosome genes decreases with stress. The dynamics of Htz1 and H3K79me3 around transcription factor-binding sites correlate with transcriptional changes. Moreover, we found that nucleosome dynamics are coupled with Htz1 binding changes upon stress. In unstressed conditions (2% glucose), nucleosome occupancy is comparable between Htz1-bound genes and Htz1-depleted genes; in stressed conditions (0.2% oleate for 8 h), the nucleosome occupancy of Htz1-depleted genes is significantly lower than that of Htz1-bound genes. We also found that Msn2 acts an important role in response to the oleate stress and Htz1 is dynamic in Msn2-target genes. Htz1 senses the oleate stress and undergoes a global redistribution and this change couples dynamics of nucleosome occupancy. Our analysis suggests that Htz1 and nucleosome dynamics change in response to oleate stress.

Histone acetyltransferase Enok regulates oocyte polarization by promoting expression of the actin nucleation factor spire

KAT6 histone acetyltransferases (HATs) are highly conserved in eukaryotes and have been shown to play important roles in transcriptional regulation. Here, we demonstrate that the Drosophila KAT6 Enok acetylates histone H3 Lys 23 (H3K23) in vitro and in vivo. Mutants lacking functional Enok exhibited defects in the localization of Oskar (Osk) to the posterior end of the oocyte, resulting in loss of germline formation and abdominal segments in the embryo. RNA sequencing (RNA-seq) analysis revealed that spire (spir) and maelstrom (mael), both required for the posterior localization of Osk in the oocyte, were down-regulated in enok mutants. Chromatin immunoprecipitation showed that Enok is localized to and acetylates H3K23 at the spir and mael genes. Furthermore, Gal4-driven expression of spir in the germline can largely rescue the defective Osk localization in enok mutant ovaries. Our results suggest that the Enok-mediated H3K23 acetylation (H3K23Ac) promotes the expression of spir, providing a specific mechanism linking oocyte polarization to histone modification.