Nucleosomes unfold completely at a transcriptionally active promoter

Optogenetic Control of Phosphate-Responsive Genes Using Single-Component Fusion Proteins in Saccharomyces cerevisiae

Blue light illumination can be detected by light-oxygen-voltage (LOV) photosensing proteins and translated into a range of biochemical responses, facilitating the generation of novel optogenetic tools to control cellular function. Here, we develop new variants of our previously described VP-EL222 light-dependent transcription factor and apply them to study the phosphate-responsive signaling (PHO) pathway in the budding yeast Saccharomyces cerevisiae, exemplifying the utilities of these new tools. Focusing first on the VP-EL222 protein itself, we quantified the tunability of gene expression as a function of light intensity and duration and demonstrated that this system can tolerate the addition of substantially larger effector domains without impacting function. We further demonstrated the utility of several EL222-driven transcriptional controllers in both plasmid and genomic settings, using the PHO5 and PHO84 promoters in their native chromosomal contexts as examples. These studies highlight the utility of light-controlled gene activation using EL222 tethered to either artificial transcription domains or yeast activator proteins (Pho4). Similarly, we demonstrate the ability to optogenetically repress gene expression with EL222 fused to the yeast Ume6 protein. We finally investigated the effects of moving EL222 recruitment sites to different locations within the PHO5 and PHO84 promoters, as well as determining how this artificial light-controlled regulation could be integrated with the native controls dependent on inorganic phosphate (Pi) availability. Taken together, our work expands the applicability of these versatile optogenetic tools in the types of functionalities that they can deliver and the biological questions that can be probed.

Optogenetic control of phosphate-responsive genes using single component fusion proteins in Saccharomyces cerevisiae

Blue light illumination can be detected by Light-Oxygen-Voltage (LOV) photosensing proteins and translated into a range of biochemical responses, facilitating the generation of novel optogenetic tools to control cellular function. Here, we develop new variants of our previously described VP-EL222 light-dependent transcription factor and apply them to study the phosphate-responsive signaling (PHO) pathway in the budding yeast Saccharomyces cerevisiae, exemplifying the utilities of these new tools. Focusing first on the VP-EL222 protein itself, we quantified the tunability of gene expression as a function of light intensity and duration, and demonstrated that this system can tolerate the addition of substantially larger effector domains without impacting function. We further demonstrated the utility of several EL222-driven transcriptional controllers in both plasmid and genomic settings, using the PHO5 and PHO84 promoters in their native chromosomal contexts as examples. These studies highlight the utility of light-controlled gene activation using EL222 tethered to either artificial transcription domains or yeast activator proteins (Pho4). Similarly, we demonstrate the ability to optogenetically repress gene expression with EL222 fused to the yeast Ume6 protein. We finally investigated the effects of moving EL222 recruitment sites to different locations within the PHO5 and PHO84 promoters, as well as determining how this artificial light-controlled regulation could be integrated with the native controls dependent on inorganic phosphate (Pi) availability. Taken together, our work expands the applicability of these versatile optogenetic tools in the types of functionalities they can deliver and biological questions that can be probed.

Regulation of Catalase Expression and Activity by DhHog1 in the Halotolerant Yeast Debaryomyces hansenii Under Saline and Oxidative Conditions

Efficient transcriptional regulation of the stress response is critical for microorganism survival. In yeast, stress-related gene expression, particularly for antioxidant enzymes like catalases, mitigates reactive oxygen species such as hydrogen peroxide (H2O2), preventing cell damage. The halotolerant yeast Debaryomyces hansenii shows oxidative stress tolerance, largely due to high catalase activity from DhCTA and DhCTT genes. This study evaluates D. hansenii's response to oxidative stress caused by H2O2 under saline conditions, focusing on cell viability, gene expression, and catalase activity. Chromatin organization in the promoter of DhCTA and DhCTT was analyzed, revealing low nucleosome occupancy in promoter regions, correlating with active gene expression. Stress-related motifs for transcription factors like Msn2/4 and Sko1 were found, suggesting regulation by the DhHog1 MAP kinase. Analysis of a Dhhog1Δ mutant showed DhHog1's role in DhCTA expression under H2O2 or NaCl conditions. These findings highlight DhHog1's critical role in regulating the stress response in D. hansenii, offering insights for enhancing stress tolerance in halotolerant yeasts, particularly for industrial applications in saline wastewater management.

Minor Spliceosomal 65K/RNPC3 Interacts with ANKRD11 and Mediates HDAC3-Regulated Histone Deacetylation and Transcription

RNA splicing is crucial in the multilayer regulatory networks for gene expression, making functional interactions with DNA- and other RNA-processing machineries in the nucleus. However, these established couplings are all major spliceosome-related; whether the minor spliceosome is involved remains unclear. Here, through affinity purification using Drosophila lysates, an interaction is identified between the minor spliceosomal 65K/RNPC3 and ANKRD11, a cofactor of histone deacetylase 3 (HDAC3). Using a CRISPR/Cas9 system, Deletion strains are constructed and found that both Dm65KΔ/Δ and Dmankrd11Δ/Δ mutants have reduced histone deacetylation at Lys9 of histone H3 (H3K9) and Lys5 of histone H4 (H4K5) in their heads, exhibiting various neural-related defects. The 65K-ANKRD11 interaction is also conserved in human cells, and the HsANKRD11 middle-uncharacterized domain mediates Hs65K association with HDAC3. Cleavage under targets and tagmentation (CUT&Tag) assays revealed that HsANKRD11 is a bridging factor, which facilitates the synergistic common chromatin-binding of HDAC3 and Hs65K. Knockdown (KD) of HsANKRD11 simultaneously decreased their common binding, resulting in reduced deacetylation of nearby H3K9. Ultimately, this study demonstrates that expression changes of many genes caused by HsANKRD11-KD are due to the decreased common chromatin-binding of HDAC3 and Hs65K and subsequently reduced deacetylation of H3K9, illustrating a novel and conserved coupling mechanism that links the histone deacetylation with minor spliceosome for the regulation of gene expression.

Single gene analysis in yeast suggests nonequilibrium regulatory dynamics for transcription

Fluctuations in the initiation rate of transcription, the first step in gene expression, ensue from the stochastic behavior of the molecular process that controls transcription. In steady state, the regulatory process is often assumed to operate reversibly, i.e., in equilibrium. However, reversibility imposes fundamental limits to information processing. For instance, the assumption of equilibrium is difficult to square with the precision with which the regulatory process executes its task in eukaryotes. Here we provide evidence - from microscopic analyses of the transcription dynamics at a single gene copy of yeast - that the regulatory process for transcription is cyclic and irreversible (out of equilibrium). The necessary coupling to reservoirs of free energy occurs via sequence-specific transcriptional activators and the recruitment, in part, of ATP-dependent chromatin remodelers. Our findings may help explain how eukaryotic cells reconcile the dual but opposing requirements for fast regulatory kinetics and high regulatory specificity.

Apparent RNA bridging between PRC2 and chromatin is an artifact of non-specific chromatin precipitation upon RNA degradation

Polycomb repressive complex 2 (PRC2) modifies chromatin to maintain repression of genes specific for other cell lineages. In vitro, RNA inhibits PRC2 activity, but the effect of RNA on PRC2 in cells is less clear, with studies concluding that RNA either antagonizes or promotes PRC2 chromatin association. The addition of RNase A to chromatin immunoprecipitation reactions has been reported to reduce detection of PRC2 target sites, suggesting the existence of RNA bridges connecting PRC2 to chromatin. Here, we show that the apparent loss of PRC2 chromatin association after RNase A treatment is due to non-specific chromatin precipitation. RNA degradation precipitates chromatin out of solution, thereby masking enrichment of specific DNA sequences in chromatin immunoprecipitation reactions. Maintaining chromatin solubility by the addition of poly-L-glutamic acid rescues detection of PRC2 chromatin occupancy upon RNA degradation. These findings undermine support for the model that RNA bridges PRC2 and chromatin in cells.

A single fiber view of the nucleosome organization in eukaryotic chromatin

Eukaryotic cells are thought to arrange nucleosomes into extended arrays with evenly spaced nucleosomes phased at genomic landmarks. Here we tested to what extent this stereotypic organization describes the nucleosome organization in Saccharomyces cerevisiae using Fiber-Seq, a long-read sequencing technique that maps entire nucleosome arrays on individual chromatin fibers in a high throughput manner. With each fiber coming from a different cell, Fiber-Seq uncovers cell-to-cell heterogeneity. The long reads reveal the nucleosome architecture even over repetitive DNA such as the ribosomal DNA repeats. The absolute nucleosome occupancy, a parameter that is difficult to obtain with conventional sequencing approaches, is a direct readout of Fiber-Seq. We document substantial deviations from the stereotypical nucleosome organization with unexpectedly long linker DNAs between nucleosomes, gene bodies missing entire nucleosomes, cell-to-cell heterogeneity in nucleosome occupancy, heterogeneous phasing of arrays and irregular nucleosome spacing. Nucleosome array structures are indistinguishable throughout the gene body and with respect to the direction of transcription arguing against transcription promoting array formation. Acute nucleosome depletion destroyed most of the array organization indicating that nucleosome remodelers cannot efficiently pack nucleosomes under those conditions. Given that nucleosomes are cis-regulatory elements, the cell-to-cell heterogeneity uncovered by Fiber-Seq provides much needed information to understand chromatin structure and function.

Integrating Omics and CRISPR Technology for Identification and Verification of Genomic Safe Harbor Loci in the Chicken Genome

BACKGROUND

One of the most prominent questions in the field of transgenesis is 'Where in the genome to integrate a transgene?'. Escape from epigenetic silencing and promoter shutdown of the transgene needs reliable genomic safe harbor (GSH) loci. Advances in genome engineering technologies combined with multi-omics bioinformatics data have enabled rational evaluation of GSH loci in the host genome. Currently, no validated GSH loci have been evaluated in the chicken genome.

RESULTS

Here, we analyzed and experimentally examined two GSH loci in the genome of chicken cells. To this end, putative GSH loci including chicken HIPP-like (cHIPP; between DRG1 and EIF4ENIF1 genes) and chicken ROSA-like (cROSA; upstream of the THUMPD3 gene) were predicted using multi-omics bioinformatics data. Then, the durable expression of the transgene was validated by experimental characterization of continuously-cultured isogenous cell clones harboring DsRed2-ΔCMV-EGFP cassette in the predicted loci. The weakened form of the CMV promoter (ΔCMV) allowed the precise evaluation of GSH loci in a locus-dependent manner compared to the full-length CMV promoter.

CONCLUSIONS

cHIPP and cROSA loci introduced in this study can be reliably exploited for consistent bio-manufacturing of recombinant proteins in the genetically-engineered chickens. Also, results showed that the genomic context dictates the expression of transgene controlled by ΔCMV in GSH loci.

Development of Human Adrenocortical Adenoma (HAA1) Cell Line from Zona Reticularis

The human adrenal cortex is composed of distinct zones that are the main source of steroid hormone production. The mechanism of adrenocortical cell differentiation into several functionally organized populations with distinctive identities remains poorly understood. Human adrenal disease has been difficult to study, in part due to the absence of cultured cell lines that faithfully represent adrenal cell precursors in the early stages of transformation. Here, Human Adrenocortical Adenoma (HAA1) cell line derived from a patient's macronodular adrenocortical hyperplasia and was treated with histone deacetylase inhibitors (HDACis) and gene expression was examined. We describe a patient-derived HAA1 cell line derived from the zona reticularis, the innermost zone of the adrenal cortex. The HAA1 cell line is unique in its ability to exit a latent state and respond with steroidogenic gene expression upon treatment with histone deacetylase inhibitors. The gene expression pattern of differentiated HAA1 cells partially recreates the roster of genes in the adrenal layer that they have been derived from. Gene ontology analysis of whole genome RNA-seq corroborated increased expression of steroidogenic genes upon HDAC inhibition. Surprisingly, HDACi treatment induced broad activation of the Tumor Necrosis Factor (TNF) alpha pathway. This novel cell line we developed will hopefully be instrumental in understanding the molecular and biochemical mechanisms controlling adrenocortical differentiation and steroidogenesis.

Antisense non-coding transcription represses the PHO5 model gene at the level of promoter chromatin structure

Pervasive transcription of eukaryotic genomes generates non-coding transcripts with regulatory potential. We examined the effects of non-coding antisense transcription on the regulation of expression of the yeast PHO5 gene, a paradigmatic case for gene regulation through promoter chromatin remodeling. A negative role for antisense transcription at the PHO5 gene locus was demonstrated by leveraging the level of overlapping antisense transcription through specific mutant backgrounds, expression from a strong promoter in cis, and use of the CRISPRi system. Furthermore, we showed that enhanced elongation of PHO5 antisense leads to a more repressive chromatin conformation at the PHO5 gene promoter, which is more slowly remodeled upon gene induction. The negative effect of antisense transcription on PHO5 gene transcription is mitigated upon inactivation of the histone deacetylase Rpd3, showing that PHO5 antisense RNA acts via histone deacetylation. This regulatory pathway leads to Rpd3-dependent decreased recruitment of the RSC chromatin remodeling complex to the PHO5 gene promoter upon induction of antisense transcription. Overall, the data in this work reveal an additional level in the complex regulatory mechanism of PHO5 gene expression by showing antisense transcription-mediated repression at the level of promoter chromatin structure remodeling.

Distinct functions of three chromatin remodelers in activator binding and preinitiation complex assembly

The nucleosome remodeling complexes (CRs) SWI/SNF, RSC, and Ino80C cooperate in evicting or repositioning nucleosomes to produce nucleosome depleted regions (NDRs) at the promoters of many yeast genes induced by amino acid starvation. We analyzed mutants depleted of the catalytic subunits of these CRs for binding of transcriptional activator Gcn4 and recruitment of TATA-binding protein (TBP) during preinitiation complex (PIC) assembly. RSC and Ino80 were found to enhance Gcn4 binding to both UAS elements in NDRs upstream of promoters and to unconventional binding sites within nucleosome-occupied coding sequences; and SWI/SNF contributes to UAS binding when RSC is depleted. All three CRs are actively recruited by Gcn4 to most UAS elements and appear to enhance Gcn4 binding by reducing nucleosome occupancies at the binding motifs, indicating a positive regulatory loop. SWI/SNF acts unexpectedly in WT cells to prevent excessive Gcn4 binding at many UAS elements, indicating a dual mode of action that is modulated by the presence of RSC. RSC and SWI/SNF collaborate to enhance TBP recruitment at Gcn4 target genes, together with Ino80C, in a manner associated with nucleosome eviction at the TBP binding sites. Cooperation among the CRs in TBP recruitment is also evident at the highly transcribed ribosomal protein genes, while RSC and Ino80C act more broadly than SWI/SNF at the majority of other constitutively expressed genes to stimulate this step in PIC assembly. Our findings indicate a complex interplay among the CRs in evicting promoter nucleosomes to regulate activator binding and stimulate PIC assembly.

ATP-dependent chromatin remodelers (CRs), including SWI/SNF and RSC in budding yeast, are thought to stimulate transcription by repositioning or evicting promoter nucleosomes, and we recently implicated the CR Ino80C in this process as well. The relative importance of these CRs in stimulating activator binding and recruitment of TATA-binding protein (TBP) to promoters is incompletely understood. Examining mutants depleted of the catalytic subunits of these CRs, we determined that RSC and Ino80C stimulate binding of transcription factor Gcn4 to nucleosome-depleted regions, or linkers between genic nucleosomes, at multiple target genes activated by Gcn4 in amino acid-starved cells, frequently via evicting nucleosomes from the Gcn4 binding motifs. At some genes, SWI/SNF functionally complements RSC, while opposing RSC at others to limit Gcn4 binding. The CRs in turn are recruited by Gcn4, consistent with a positive feedback loop that enhances Gcn4 binding. The three CRs also cooperate to enhance TBP recruitment, again involving nucleosome depletion, at both Gcn4 target and highly expressed ribosomal protein genes, whereas only RSC and Ino80C act broadly throughout the genome to enhance this key step in preinitiation complex assembly. Our findings illuminate functional cooperation among multiple CRs in regulating activator binding and promoter activation.

Kinetic Proofreading

Biochemistry and molecular biology rely on the recognition of structural complementarity between molecules. Molecular interactions must be both quickly reversible, i.e., tenuous, and specific. How the cell reconciles these conflicting demands is the subject of this article. The problem and its theoretical solution are discussed within the wider theoretical context of the thermodynamics of stochastic processes (stochastic thermodynamics). The solution-an irreversible reaction cycle that decreases internal error at the expense of entropy export into the environment-is shown to be widely employed by biological processes that transmit genetic and regulatory information. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A set of accessible enhancers enables the initial response of breast cancer cells to physiological progestin concentrations

Here, we report that in T47D breast cancer cells 50 pM progestin is sufficient to activate cell cycle entry and the progesterone gene expression program. At this concentration, equivalent to the progesterone blood levels found around the menopause, progesterone receptor (PR) binds only to 2800 genomic sites, which are accessible to ATAC cleavage prior to hormone exposure. These highly accessible sites (HAs) are surrounded by well-organized nucleosomes and exhibit breast enhancer features, including estrogen receptor alpha (ERα), higher FOXA1 and BRD4 (bromodomain containing 4) occupancy. Although HAs are enriched in RAD21 and CTCF, PR binding is the driving force for the most robust interactions with hormone-regulated genes. HAs show higher frequency of 3D contacts among themselves than with other PR binding sites, indicating colocalization in similar compartments. Gene regulation via HAs is independent of classical coregulators and ATP-activated remodelers, relying mainly on MAP kinase activation that enables PR nuclear engagement. HAs are also preferentially occupied by PR and ERα in breast cancer xenografts derived from MCF-7 cells as well as from patients, indicating their potential usefulness as targets for therapeutic intervention.

HDAC6 inactivates Runx2 promoter to block osteogenesis of bone marrow stromal cells in age-related bone loss of mice

Background

Senile osteoporosis can cause bone fragility and increased risk for fractures and has been one of the most prevalent and severe diseases affecting the elderly population worldwidely. The underlying mechanisms are currently intensive areas of investigation. In age-related bone loss, decreased bone formation overweighs increased bone resorption. The molecular mechanisms underlying defective bone formation in age-related bone loss are not completely understood. In particular, the specific role of histone acetylation in age-related bone loss has not been examined thoroughly.

Methods

We employed 6- and 18-month-old mice to investigate the mechanisms of defective bone formation in age-related bone loss. Bone marrow stromal cells (BMSCs) were induced to undergo in vitro osteogenic differentiation. Chromatin immunoprecipitation (ChIP) was used to investigate the binding of histone deacetylases (HDACs) on Runx2 promoter in BMSCs. Luciferase reporter and transient transfection assay were employed to study Runx2 gene expression modulation by HDAC and androgen receptor (AR). siRNA and HDAC6 inhibitor, Tubastatin A, were used to inhibit HDAC6 in vitro. And systemic administration of Tubastatin A was used to block HDAC6 in vivo.

Results

Age-related trabecular bone loss was observed in 18-month-old mice compared with 6-month-old mice. In vitro osteogenic differentiation potential of BMSCs from 18-month-old mice was weaker than 6-month-old mice, in which there was Runx2 expression inactivation in BMSCs of 18-month-old mice compared with 6-month-old mice, which was attributable to HDAC6-mediated histone hypoacetylation in Runx2 promoter. There was competitive binding of HDAC6 and AR on Runx2 promoter to modulate Runx2 expression in BMSCs. More importantly, through siRNA- or specific inhibitor-mediated HDAC6 inhibition, we could activate Runx2 expression, rescue in vitro osteogenesis potential of BMSCs, and alleviate in vivo age-related bone loss of mice.

Conclusion

HDAC6 accumulation and histone hypoacetylation on Runx2 promoter contributed to the attenuation of in vitro osteogenic differentiation potential of BMSCs from aged mice. Through HDAC6 inhibition, we could activate Runx2 expression and osteogenic differentiation potential of BMSCs from aged mice and alleviate the age-related bone loss of aged mice. Our study will benefit not only for understanding the age-related bone loss, but also for finding new therapies to treat senile osteoporosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02545-w.

Genome information processing by the INO80 chromatin remodeler positions nucleosomes

The fundamental molecular determinants by which ATP-dependent chromatin remodelers organize nucleosomes across eukaryotic genomes remain largely elusive. Here, chromatin reconstitutions on physiological, whole-genome templates reveal how remodelers read and translate genomic information into nucleosome positions. Using the yeast genome and the multi-subunit INO80 remodeler as a paradigm, we identify DNA shape/mechanics encoded signature motifs as sufficient for nucleosome positioning and distinct from known DNA sequence preferences of histones. INO80 processes such information through an allosteric interplay between its core- and Arp8-modules that probes mechanical properties of nucleosomal and linker DNA. At promoters, INO80 integrates this readout of DNA shape/mechanics with a readout of co-evolved sequence motifs via interaction with general regulatory factors bound to these motifs. Our findings establish a molecular mechanism for robust and yet adjustable +1 nucleosome positioning and, more generally, remodelers as information processing hubs that enable active organization and allosteric regulation of the first level of chromatin.

DNA sequence preferences or statistical positioning of histones has not explained genomic patterns of nucleosome organisation in vivo. Here, the authors establish DNA shape/mechanics as key elements that have evolved together with binding sites of DNA sequence-specific barriers so that such information directs nucleosome positioning by chromatin remodelers.

Emerging roles of centromeric RNAs in centromere formation and function

BACKGROUND

Centromeres are specialized chromosomal domains involved in kinetochore formation and faithful chromosome segregation. Despite a high level of functional conservation, centromeres are not identified by DNA sequences, but by epigenetic means. Universally, centromeres are typically formed on highly repetitive DNA, which were previously considered to be silent. However, recent studies have shown that transcription occurs in this region, known as centromeric-derived RNAs (cenRNAs). CenRNAs that contribute to fundamental aspects of centromere function have been recently investigated in detail. However, the distribution, behavior and contributions of centromeric transcripts are still poorly understood.

OBJECTIVE

The aim of this article is to provide an overview of the roles of cenRNAs in centromere formation and function.

METHODS

We describe the structure and DNA sequence of centromere from yeast to human. In addition, we briefly introduce the roles of cenRNAs in centromere formation and function, kinetochore structure, accurate chromosome segregation, and pericentromeric heterochromatin assembly. Centromeric circular RNAs (circRNAs) and R-loops are rising stars in centromere function. CircRNAs have been successfully identified in various species with the assistance of high-throughput sequencing and novel computational approaches for non-polyadenylated RNA transcripts. Centromeric R-loops can be identified by the single-strand DNA ligation-based library preparation technique. But the molecular features and function of these centromeric R-loops and circRNAs are still being investigated.

CONCLUSION

In this review, we summarize recent findings on the epigenetic regulation of cenRNAs across species, which would provide useful information about cenRNAs and interesting hints for further studies.

Chromatin accessibility profiling methods

Chromatin accessibility, or the physical access to chromatinized DNA, is a widely studied characteristic of the eukaryotic genome. As active regulatory DNA elements are generally 'accessible', the genome-wide profiling of chromatin accessibility can be used to identify candidate regulatory genomic regions in a tissue or cell type. Multiple biochemical methods have been developed to profile chromatin accessibility, both in bulk and at the single-cell level. Depending on the method, enzymatic cleavage, transposition or DNA methyltransferases are used, followed by high-throughput sequencing, providing a view of genome-wide chromatin accessibility. In this Primer, we discuss these biochemical methods, as well as bioinformatics tools for analysing and interpreting the generated data, and insights into the key regulators underlying developmental, evolutionary and disease processes. We outline standards for data quality, reproducibility and deposition used by the genomics community. Although chromatin accessibility profiling is invaluable to study gene regulation, alone it provides only a partial view of this complex process. Orthogonal assays facilitate the interpretation of accessible regions with respect to enhancer-promoter proximity, functional transcription factor binding and regulatory function. We envision that technological improvements including single-molecule, multi-omics and spatial methods will bring further insight into the secrets of genome regulation.

Chromatin remodeler Ino80C acts independently of H2A.Z to evict promoter nucleosomes and stimulate transcription of highly expressed genes in yeast

The chromatin remodelers SWI/SNF and RSC function in evicting promoter nucleosomes at highly expressed yeast genes, particularly those activated by transcription factor Gcn4. Ino80 remodeling complex (Ino80C) can establish nucleosome-depleted regions (NDRs) in reconstituted chromatin, and was implicated in removing histone variant H2A.Z from the -1 and +1 nucleosomes flanking NDRs; however, Ino80C's function in transcriptional activation in vivo is not well understood. Analyzing the cohort of Gcn4-induced genes in ino80Δ mutants has uncovered a role for Ino80C on par with SWI/SNF in evicting promoter nucleosomes and transcriptional activation. Compared to SWI/SNF, Ino80C generally functions over a wider region, spanning the -1 and +1 nucleosomes, NDR and proximal genic nucleosomes, at genes highly dependent on its function. Defects in nucleosome eviction in ino80Δ cells are frequently accompanied by reduced promoter occupancies of TBP, and diminished transcription; and Ino80 is enriched at genes requiring its remodeler activity. Importantly, nuclear depletion of Ino80 impairs promoter nucleosome eviction even in a mutant lacking H2A.Z. Thus, Ino80C acts widely in the yeast genome together with RSC and SWI/SNF in evicting promoter nucleosomes and enhancing transcription, all in a manner at least partly independent of H2A.Z editing.

Exploring Epigenetic Marks by Analysis of Noncovalent Interactions

Epigenetic marks are modest chemical modifications on DNA and histone proteins that regulate the activation or silencing of genes through modulation of the intermolecular interactions between the DNA strands and the protein machinery. The process is complex and not always well understood. One of the systems studied in greater detail is the epigenetic mark on H3K9: lysine 9 of histone 3. The degree of methylation or acetylation of this histone is linked to silencing or activation of the corresponding gene, but it is not clear which effect each mark has in gene expression. We shed light on this particular methylation process by using density functional theory (DFT) calculations. First, we built a model consisting of a DNA double strand containing three base pairs and a sequence of three amino acids of the histone's tail. Then, we computed the modulation introduced into the intermolecular interactions by each epigenetic modification: from mono- to trimethylation and acetylation. The calculations show that whereas acetylation and trimethylation result in a reduction of the DNA-peptide interaction; non-, mono-, and dimethylation increase the intermolecular interactions. Such observations compare well with the findings reported in the literature, and highlight the correlation between the balance of intermolecular forces and biological properties, simultaneously advancing quantum-mechanical studies of large biochemical systems at molecular level through the use of DFT methods.

Formaldehyde-assisted isolation of regulatory DNA elements from Arabidopsis leaves

Eukaryotic gene transcription is associated with the eviction of nucleosomes and the formation of open chromatin, which enables the recruitment of transcriptional coactivators and other regulatory factors. Open chromatin is thus a hallmark of functional regulatory DNA elements in genomes. In recent years, formaldehyde-assisted isolation of regulatory elements (FAIRE) has proven powerful in identifying open chromatin in the genome of various eukaryotes, particularly yeast, human, and mouse. However, it has proven challenging to adapt the FAIRE protocol for use on plant material, and the few available protocols all have their drawbacks (e.g., applicability only to specific developmental stages). In this Protocol Extension, we describe a reliable FAIRE protocol for mature Arabidopsis (Arabidopsis thaliana) leaves that adapts the original protocol for use on plants. The main differences between this protocol extension and the earlier FAIRE protocol are an increased formaldehyde concentration in the chromatin crosslinking buffer, application of a repeated vacuum to increase crosslinking efficiency, and altered composition of the DNA extraction buffer. The protocol is applicable to leaf chromatin of unstressed and stressed plants and can be completed within 1 week. Here, we also describe downstream analysis using qPCR and next-generation sequencing. However, this Protocol Extension should also be compatible with downstream hybridization to a DNA microarray. In addition, it is likely that only minor adaptations will be necessary to apply this protocol to other Arabidopsis organs or plant species.

Nucleosomal proofreading of activator-promoter interactions

Specificity in transcriptional regulation is imparted by transcriptional activators that bind to specific DNA sequences from which they stimulate transcription. Specificity may be increased by slowing down the kinetics of regulation: by increasing the energy for dissociation of the activator-DNA complex or decreasing activator concentration. In general, higher dissociation energies imply longer DNA dwell times of the activator; the activator-bound gene may not readily turn off again. Lower activator concentrations entail longer pauses between binding events; the activator-unbound gene is not easily turned on again and activated transcription occurs in stochastic bursts. We show that kinetic proofreading of activator-DNA recognition-insertion of an energy-dissipating delay step into the activation pathway for transcription-reconciles high specificity of transcriptional regulation with fast regulatory kinetics. We show that kinetic proofreading results from the stochastic removal and reformation of promoter nucleosomes, at a distance from equilibrium.

Chromatin dynamics and the transcriptional competence of HSV-1 genomes during lytic infections

During latent infections with herpes simplex virus 1 (HSV-1), viral transcription is restricted and the genomes are mostly maintained in silenced chromatin, whereas in lytically infected cells all viral genes are transcribed and the genomes are dynamically chromatinized. Histones in the viral chromatin bear markers of silenced chromatin at early times in lytic infection or of active transcription at later times. The virion protein VP16 activates transcription of the immediate-early (IE) genes by recruiting transcription activators and chromatin remodelers to their promoters. Two IE proteins, ICP0 and ICP4 which modulate chromatin epigenetics, then activate transcription of early and late genes. Although chromatin is involved in the mechanism of activation of HSV- transcription, its precise role is not entirely understood. In the cellular genome, chromatin dynamics often modulate transcription competence whereas promoter-specific transcription factors determine transcription activity. Here, biophysical fractionation of serially digested HSV-1 chromatin followed by short-read deep sequencing indicates that nuclear HSV-1 DNA has different biophysical properties than protein-free or encapsidated HSV-1 DNA. The entire HSV-1 genomes in infected cells were equally accessible. The accessibility of transcribed or non-transcribed genes under any given condition did not differ, and each gene was entirely sampled in both the most and least accessible chromatin. However, HSV-1 genomes fractionated differently under conditions of generalized or restricted transcription. Approximately 1/3 of the HSV-1 DNA including fully sampled genes resolved to the most accessible chromatin when HSV-1 transcription was active, but such enrichment was reduced to only 3% under conditions of restricted HSV-1 transcription. Short sequences of restricted accessibility separated genes with different transcription levels. Chromatin dynamics thus provide a first level of regulation on HSV-1 transcription, dictating the transcriptional competency of the genomes during lytic infections, whereas the transcription of individual genes is then most likely activated by specific transcription factors. Moreover, genes transcribed to different levels are separated by short sequences with limited accessibility.

Although chromatin epigenetics modulate transcription of the nuclear replicating DNA viruses, and play major roles in the process of establishment of, and reactivation from, latency, the specific mechanisms of this modulation are not totally clear. Chromatin often regulates the transcriptional competency of cellular genes, rather than the actual level of transcription of individual genes. Here, we show that chromatin dynamics regulate the transcription competency of entire herpes simplex virus 1 (HSV-1) genomes, rather than the actual transcription level of individual genes. Moreover, CTCF/ insulator containing sequences flanking the immediate-early gene loci are more inaccessible when these genes are highly transcribed in a context of little transcription from the rest of the genome than when no gene was highly transcribed or all genes were. We postulate that chromatin dynamics modulate the transcriptional competency of the HSV-1 genome. Genes in genomes rendered transcriptionally inactive by chromatin dynamics cannot be transcribed, whereas transcription of individual genes, or of group of genes, is regulated separately in the transcriptionally competent genomes.

Opposing chromatin remodelers control transcription initiation frequency and start site selection

Precise nucleosome organization at eukaryotic promoters is thought to be generated by multiple chromatin remodeler (CR) enzymes and to affect transcription initiation. Using an integrated analysis of chromatin remodeler binding and nucleosome occupancy following rapid remodeler depletion, we investigated the interplay between these enzymes and their impact on transcription in yeast. We show that many promoters are affected by multiple CRs that operate in concert or in opposition to position the key transcription start site (TSS)-associated +1 nucleosome. We also show that nucleosome movement after CR inactivation usually results from the activity of another CR and that in the absence of any remodeling activity, +1 nucleosomes largely maintain their positions. Finally, we present functional assays suggesting that +1 nucleosome positioning often reflects a trade-off between maximizing RNA polymerase recruitment and minimizing transcription initiation at incorrect sites. Our results provide a detailed picture of fundamental mechanisms linking promoter nucleosome architecture to transcription initiation.

The coupled effect of nucleosome organization on gene transcription level and transcriptional plasticity

Nucleosomes are the fundamental units of eukaryotic chromatin and can modulate the DNA accessibility for transcriptional regulatory elements. Many studies have demonstrated the effect of nucleosome organization on gene transcription level and transcriptional plasticity upon different conditions. Our recent study showed that nucleosome organization also plays an important role in modulating the plasticity of gene transcriptional status in maize. Here, we integrated our findings with previous studies on the role of nucleosome organization in regulation of gene transcription. We highlighted our recent finding that nucleosome organization plays an important role in determining the plasticity of gene transcription, beyond its role in regulating gene transcription level, particularly for intrinsically DNA-encoded nucleosome organization. We also discussed the features of sequence and structure of genes involved in affecting nucleosome organization around genes, as well as the potential mechanisms for overcoming the coupled effect of nucleosome organization on gene transcription level and transcriptional plasticity.

The TAFs of TFIID Bind and Rearrange the Topology of the TATA-Less RPS5 Promoter

The general transcription factor TFIID is a core promoter selectivity factor that recognizes DNA sequence elements and nucleates the assembly of a pre-initiation complex (PIC). The mechanism by which TFIID recognizes the promoter is poorly understood. The TATA-box binding protein (TBP) is a subunit of the multi-protein TFIID complex believed to be key in this process. We reconstituted transcription from highly purified components on a ribosomal protein gene (RPS5) and discovered that TFIIDΔTBP binds and rearranges the promoter DNA topology independent of TBP. TFIIDΔTBP binds ~200 bp of the promoter and changes the DNA topology to a larger extent than the nucleosome core particle. We show that TBP inhibits the DNA binding activities of TFIIDΔTBP and conclude that the complete TFIID complex may represent an auto-inhibited state. Furthermore, we show that the DNA binding activities of TFIIDΔTBP are required for assembly of a PIC poised to select the correct transcription start site (TSS).

The synergy between RSC, Nap1 and adjacent nucleosome in nucleosome remodeling

Eukaryotes have evolved a specific strategy to package DNA. The nucleosome is a 147-base-pair DNA segment wrapped around histone core proteins that plays important roles regulating DNA-dependent biosynthesis and gene expression. Chromatin remodeling complexes (RSC, Remodel the Structure of Chromatin) hydrolyze ATP to perturb DNA-histone contacts, leading to nucleosome sliding and ejection. Here, we utilized tethered particle motion (TPM) experiments to investigate the mechanism of RSC-mediated nucleosome remodeling in detail. We observed ATP-dependent RSC-mediated DNA looping and nucleosome ejection along individual mononucleosomes and dinucleosomes. We found that nucleosome assembly protein 1 (Nap1) enhanced RSC-mediated nucleosome ejection in a two-step disassembly manner from dinucleosomes but not from mononucleosomes. Based on this work, we provide an entire reaction scheme for the RSC-mediated nucleosome remodeling process that includes DNA looping, nucleosome ejection, the influence of adjacent nucleosomes, and the coordinated action between Nap1 and RSC.

RSC-Associated Subnucleosomes Define MNase-Sensitive Promoters in Yeast

The classic view of nucleosome organization at active promoters is that two well-positioned nucleosomes flank a nucleosome-depleted region (NDR). However, this view has been recently disputed by contradictory reports as to whether wider (≳150 bp) NDRs instead contain unstable, micrococcal nuclease-sensitive ("fragile") nucleosomal particles. To determine the composition of fragile particles, we introduce CUT&RUN.ChIP, in which targeted nuclease cleavage and release is followed by chromatin immunoprecipitation. We find that fragile particles represent the occupancy of the RSC (remodeling the structure of chromatin) nucleosome remodeling complex and RSC-bound, partially unwrapped nucleosomal intermediates. We also find that general regulatory factors (GRFs) bind to partially unwrapped nucleosomes at these promoters. We propose that RSC binding and its action cause nucleosomes to unravel, facilitate subsequent binding of GRFs, and constitute a dynamic cycle of nucleosome deposition and clearance at the subset of wide Pol II promoter NDRs.

Mitotic progression, arrest, exit or death relies on centromere structural integrity, rather than de novo transcription

Recent studies have challenged the prevailing dogma that transcription is repressed during mitosis. Transcription was also proposed to sustain a robust spindle assembly checkpoint (SAC) response. Here, we used live-cell imaging of human cells, RNA-seq and qPCR to investigate the requirement for de novo transcription during mitosis. Under conditions of persistently unattached kinetochores, transcription inhibition with actinomycin D, or treatment with other DNA-intercalating drugs, delocalized the chromosomal passenger complex (CPC) protein Aurora B from centromeres, compromising SAC signaling and cell fate. However, we were unable to detect significant changes in mitotic transcript levels. Moreover, inhibition of transcription independently of DNA intercalation had no effect on Aurora B centromeric localization, SAC response, mitotic progression, exit or death. Mechanistically, we show that DNA intercalating agents reduce the interaction of the CPC with nucleosomes. Thus, mitotic progression, arrest, exit or death is determined by centromere structural integrity, rather than de novo transcription.

Systematic Study of Nucleosome-Displacing Factors in Budding Yeast

Nucleosomes present a barrier for the binding of most transcription factors (TFs). However, special TFs known as nucleosome-displacing factors (NDFs) can access embedded sites and cause the depletion of the local nucleosomes as well as repositioning of the neighboring nucleosomes. Here, we developed a novel high-throughput method in yeast to identify NDFs among 104 TFs and systematically characterized the impact of orientation, affinity, location, and copy number of their binding motifs on the nucleosome occupancy. Using this assay, we identified 29 NDF motifs and divided the nuclear TFs into three groups with strong, weak, and no nucleosome-displacing activities. Further studies revealed that tight DNA binding is the key property that underlies NDF activity, and the NDFs may partially rely on the DNA replication to compete with nucleosome. Overall, our study presents a framework to functionally characterize NDFs and elucidate the mechanism of nucleosome invasion.

SWI/SNF and RSC cooperate to reposition and evict promoter nucleosomes at highly expressed genes in yeast

The nucleosome remodeling complex RSC functions throughout the yeast genome to set the positions of -1 and +1 nucleosomes and thereby determines the widths of nucleosome-depleted regions (NDRs). The related complex SWI/SNF participates in nucleosome remodeling/eviction and promoter activation at certain yeast genes, including those activated by transcription factor Gcn4, but did not appear to function broadly in establishing NDRs. By analyzing the large cohort of Gcn4-induced genes in mutants lacking the catalytic subunits of SWI/SNF or RSC, we uncovered cooperation between these remodelers in evicting nucleosomes from different locations in the promoter and repositioning the +1 nucleosome downstream to produce wider NDRs-highly depleted of nucleosomes-during transcriptional activation. SWI/SNF also functions on a par with RSC at the most highly transcribed constitutively expressed genes, suggesting general cooperation by these remodelers for maximal transcription. SWI/SNF and RSC occupancies are greatest at the most highly expressed genes, consistent with their cooperative functions in nucleosome remodeling and transcriptional activation. Thus, SWI/SNF acts comparably with RSC in forming wide nucleosome-free NDRs to achieve high-level transcription but only at the most highly expressed genes exhibiting the greatest SWI/SNF occupancies.

Chromatin-remodeling for transcription

The nucleosome serves as a general gene repressor, preventing all initiation of transcription except that which is brought about by specific positive regulatory mechanisms. The positive mechanisms begin with chromatin-remodeling by complexes that slide, disrupt, or otherwise alter the structure and organization of nucleosomes. RSC in yeast and its counterpart PBAF in human cells are the major remodeling complexes for transcription. RSC creates a nucleosome-free region in front of a gene, flanked by strongly positioned +1 and -1 nucleosomes, with the transcription start site typically 10-15 bp inside the border of the +1 nucleosome. RSC also binds stably to nucleosomes harboring regulatory elements and to +1 nucleosomes, perturbing their structures in a manner that partially exposes their DNA sequences. The cryo-electron microscope structure of a RSC-nucleosome complex reveals such a structural perturbation, with the DNA largely unwrapped from the nucleosome and likely interacting with a positively charged surface of RSC. Such unwrapping both exposes the DNA and enables its translocation across the histone octamer of the nucleosome by an ATP-dependent activity of RSC. Genetic studies have revealed additional roles of RSC in DNA repair, chromosome segregation, and other chromosomal DNA transactions. These functions of RSC likely involve the same fundamental activities, DNA unwrapping and DNA translocation.

Toehold-enhanced LNA probes for selective pull down and single-molecule analysis of native chromatin

The organization of DNA into chromatin is thought to regulate gene expression in eukaryotes. To study its structure in vitro, there is a need for techniques that can isolate specific chromosomal loci of natively assembled chromatin. Current purification methods often involve chemical cross-linking to preserve the chromatin composition. However, such cross-linking may affect the native structure. It also impedes single molecule force spectroscopy experiments, which have been instrumental to probe chromatin folding. Here we present a method for the incorporation of affinity tags, such as biotin, into native nucleoprotein fragments based on their DNA sequence, and subsequent single molecule analysis by magnetic tweezers. DNA oligos with several Locked Nucleic Acid (LNA) nucleotides are shown to selectively bind to target DNA at room temperature, mediated by a toehold end in the target, allowing for selective purification of DNA fragments. The stability of the probe-target hybrid is sufficient to withstand over 65 pN of force. We employ these probes to obtain force-extension curves of native chromatin fragments of the 18S ribosomal DNA from the yeast Saccharomyces cerevisiae. These experiments yield valuable insights in the heterogeneity in structure and composition of natively assembled chromatin at the single-molecule level.

Epigenetic Regulation of Cellular Senescence and Aging

Aging is characterized by functional decline of diverse organs and an increased risk for several diseases. Therefore, a high interest exists in understanding the molecular mechanisms that stimulate aging at all levels, from cells and tissues to organs and organisms, in order to develop ways to promote healthy aging. While many molecular and biochemical mechanisms are already understood in some detail, the role of changes in epigenetic regulation has only begun to be considered in recent years. The age-dependent global reduction in heterochromatin, along with site-specific changes in the patterns of DNA methylation and modification of histones, have been observed in several aging model systems. However, understanding of the precise role of such changes requires further research. In this review, we will discuss the role of epigenetic regulation in aging and indicate future research directions that will help elucidate the mechanistic details of it.

The Ino80 complex mediates epigenetic centromere propagation via active removal of histone H3

The centromere is the chromosomal locus at which the kinetochore is assembled to direct chromosome segregation. The histone H3 variant, centromere protein A (CENP-A), is known to epigenetically mark active centromeres, but the mechanism by which CENP-A propagates at the centromere, replacing histone H3, remains poorly understood. Using fission yeast, here we show that the Ino80 adenosine triphosphate (ATP)-dependent chromatin-remodeling complex, which removes histone H3-containing nucleosomes from associated chromatin, promotes CENP-A^Cnp1 chromatin assembly at the centromere in a redundant manner with another chromatin-remodeling factor Chd1^Hrp1. CENP-A^Cnp1 chromatin actively recruits the Ino80 complex to centromeres to elicit eviction of histone H3-containing nucleosomes. Artificial targeting of Ino80 subunits to a non-centromeric DNA sequence placed in a native centromere enhances the spreading of CENP-A^Cnp1 chromatin into the non-centromeric DNA. Based on these results, we propose that CENP-A^Cnp1 chromatin employs the Ino80 complex to mediate the replacement of histone H3 with CENP-A^Cnp1, and thereby reinforces itself.

The histone variant CENP-A marks active centromeres and replaces H3 at centromeres through a poorly understood mechanism. Here, the authors provide evidence that the chromatin remodeller Ino80 promotes CENP-A chromatin assembly at the centromere in fission yeast.

Regulation of DNA Translocation Efficiency within the Chromatin Remodeler RSC/Sth1 Potentiates Nucleosome Sliding and Ejection

The RSC chromatin remodeler slides and ejects nucleosomes, utilizing a catalytic subunit (Sth1) with DNA translocation activity, which can pump DNA around the nucleosome. A central question is whether and how DNA translocation is regulated to achieve sliding versus ejection. Here, we report the regulation of DNA translocation efficiency by two domains residing on Sth1 (Post-HSA and Protrusion 1) and by actin-related proteins (ARPs) that bind Sth1. ARPs facilitated sliding and ejection by improving "coupling"-the amount of DNA translocation by Sth1 relative to ATP hydrolysis. We also identified and characterized Protrusion 1 mutations that promote "coupling," and Post-HSA mutations that improve ATP hydrolysis; notably, the strongest mutations conferred efficient nucleosome ejection without ARPs. Taken together, sliding-to-ejection involves a continuum of DNA translocation efficiency, consistent with higher magnitudes of ATPase and coupling activities (involving ARPs and Sth1 domains), enabling the simultaneous rupture of multiple histone-DNA contacts facilitating ejection.

NicE-seq: high resolution open chromatin profiling

Open chromatin profiling integrates information across diverse regulatory elements to reveal the transcriptionally active genome. Tn5 transposase and DNase I sequencing-based methods prefer native or high cell numbers. Here, we describe NicE-seq (nicking enzyme assisted sequencing) for high-resolution open chromatin profiling on both native and formaldehyde-fixed cells. NicE-seq captures and reveals open chromatin sites (OCSs) and transcription factor occupancy at single nucleotide resolution, coincident with DNase hypersensitive and ATAC-seq sites at a low sequencing burden. OCSs correlate with RNA polymerase II occupancy and active chromatin marks, while displaying a contrasting pattern to CpG methylation. Decitabine-mediated hypomethylation of HCT116 displays higher numbers of OCSs.

The Arabidopsis SWI/SNF protein BAF60 mediates seedling growth control by modulating DNA accessibility

BACKGROUND

Plant adaptive responses to changing environments involve complex molecular interplays between intrinsic and external signals. Whilst much is known on the signaling components mediating diurnal, light, and temperature controls on plant development, their influence on chromatin-based transcriptional controls remains poorly explored.

RESULTS

In this study we show that a SWI/SNF chromatin remodeler subunit, BAF60, represses seedling growth by modulating DNA accessibility of hypocotyl cell size regulatory genes. BAF60 binds nucleosome-free regions of multiple G box-containing genes, opposing in cis the promoting effect of the photomorphogenic and thermomorphogenic regulator Phytochrome Interacting Factor 4 (PIF4) on hypocotyl elongation. Furthermore, BAF60 expression level is regulated in response to light and daily rhythms.

CONCLUSIONS

These results unveil a short path between a chromatin remodeler and a signaling component to fine-tune plant morphogenesis in response to environmental conditions.

Epigenetics regulates transcription and pathogenesis in the parasite Trichomonas vaginalis

Trichomonas vaginalis is a common sexually transmitted parasite that colonizes the human urogenital tract. Infections range from asymptomatic to highly inflammatory, depending on the host and the parasite strain. Different T. vaginalis strains vary greatly in their adherence and cytolytic capacities. These phenotypic differences might be attributed to differentially expressed genes as a consequence of extra-genetic variation, such as epigenetic modifications. In this study, we explored the role of histone acetylation in regulating gene transcription and pathogenesis in T. vaginalis. Here, we show that histone 3 lysine acetylation (H3KAc) is enriched in nucleosomes positioned around the transcription start site of active genes (BAP1 and BAP2) in a highly adherent parasite strain; compared with the low acetylation abundance in contrast to that observed in a less-adherent strain that expresses these genes at low levels. Additionally, exposition of less-adherent strain with a specific histone deacetylases inhibitor, trichostatin A, upregulated the transcription of BAP1 and BAP2 genes in concomitance with an increase in H3KAc abundance and chromatin accessibility around their transcription start sites. Moreover, we demonstrated that the binding of initiator binding protein, the transcription factor responsible for the initiation of transcription of ~75% of known T. vaginalis genes, depends on the histone acetylation state around the metazoan-like initiator to which initiator binding protein binds. Finally, we found that trichostatin A treatment increased parasite aggregation and adherence to host cells. Our data demonstrated for the first time that H3KAc is a permissive histone modification that functions to mediate both transcription and pathogenesis of the parasite T. vaginalis.

Landscape of histone modifications in a sponge reveals the origin of animal cis-regulatory complexity

Combinatorial patterns of histone modifications regulate developmental and cell type-specific gene expression and underpin animal complexity, but it is unclear when this regulatory system evolved. By analysing histone modifications in a morphologically-simple, early branching animal, the sponge Amphimedonqueenslandica, we show that the regulatory landscape used by complex bilaterians was already in place at the dawn of animal multicellularity. This includes distal enhancers, repressive chromatin and transcriptional units marked by H3K4me3 that vary with levels of developmental regulation. Strikingly, Amphimedon enhancers are enriched in metazoan-specific microsyntenic units, suggesting that their genomic location is extremely ancient and likely to place constraints on the evolution of surrounding genes. These results suggest that the regulatory foundation for spatiotemporal gene expression evolved prior to the divergence of sponges and eumetazoans, and was necessary for the evolution of animal multicellularity.

Establishing nucleosome architecture and stability at promoters: Roles of pioneer transcription factors and the RSC chromatin remodeler

Improvements in deep sequencing, together with methods to rapidly deplete essential transcription factors (TFs) and chromatin remodelers, have recently led to a more detailed picture of promoter nucleosome architecture in yeast and its relationship to transcriptional regulation. These studies revealed that ∼40% of all budding yeast protein-coding genes possess a unique promoter structure, where we propose that an unusually unstable nucleosome forms immediately upstream of the transcription start site (TSS). This "fragile" nucleosome (FN) promoter architecture relies on the combined action of the essential RSC (Remodels Structure of Chromatin) nucleosome remodeler and pioneer transcription factors (PTFs). FNs are associated with genes whose expression is high, coupled to cell growth, and characterized by low cell-to-cell variability (noise), suggesting that they may promote these features. Recent studies in metazoans suggest that the presence of dynamic nucleosomes upstream of the TSS at highly expressed genes may be conserved throughout evolution.

Bleomycin analogues preferentially cleave at the transcription start sites of actively transcribed genes in human cells

Bleomycin (BLM) is a cancer chemotherapeutic agent that is used in the treatment of several types of tumours. The cytotoxicity of three BLM analogues, BLM Z, 6'-deoxy-BLM Z and zorbamycin (ZBM), was determined in human HeLa cells in comparison with BLM. It was found that the IC₅₀ values were 2.9μM for 6'-deoxy-BLM Z, 3.2μM for BLM Z, 4.4μM for BLM and 7.9μM for ZBM in HeLa cells. Using next-generation Illumina DNA sequencing techniques, the genome-wide cleavage of DNA by the BLM analogues was determined in human HeLa cells and compared with BLM. It was ascertained that BLM, 6'-deoxy-BLM Z and ZBM preferentially cleaved at the transcription start sites of actively transcribed genes in human cells. The degree of preferential cleavage at the transcription start sites was quantified and an inverse correlation with the IC₅₀ values was observed. This indicated that the degree of preferential cleavage at transcription start sites is an important component in determining the cytotoxicity of BLM analogues.

Chromatin potentiates transcription

Chromatin isolated from the chromosomal locus of the PHO5 gene of yeast in a transcriptionally repressed state was transcribed with 12 pure proteins (80 polypeptides): RNA polymerase II, six general transcription factors, TFIIS, the Pho4 gene activator protein, and the SAGA, SWI/SNF, and Mediator complexes. Contrary to expectation, a nucleosome occluding the TATA box and transcription start sites did not impede transcription but rather, enhanced it: the level of chromatin transcription was at least sevenfold greater than that of naked DNA, and chromatin gave patterns of transcription start sites closely similar to those occurring in vivo, whereas naked DNA gave many aberrant transcripts. Both histone acetylation and trimethylation of H3K4 (H3K4me3) were important for chromatin transcription. The nucleosome, long known to serve as a general gene repressor, thus also performs an important positive role in transcription.

The Histone Modification Code in the Pathogenesis of Autoimmune Diseases

Autoimmune diseases are chronic inflammatory disorders caused by a loss of self-tolerance, which is characterized by the appearance of autoantibodies and/or autoreactive lymphocytes and the impaired suppressive function of regulatory T cells. The pathogenesis of autoimmune diseases is extremely complex and remains largely unknown. Recent advances indicate that environmental factors trigger autoimmune diseases in genetically predisposed individuals. In addition, accumulating results have indicated a potential role of epigenetic mechanisms, such as histone modifications, in the development of autoimmune diseases. Histone modifications regulate the chromatin states and gene transcription without any change in the DNA sequence, possibly resulting in phenotype alteration in several different cell types. In this paper, we discuss the significant roles of histone modifications involved in the pathogenesis of autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis, primary biliary cirrhosis, and type 1 diabetes.

Chromatin-remodeling and the initiation of transcription

The nucleosome serves as a general gene repressor by the occlusion of regulatory and promoter DNA sequences. Repression is relieved by the SWI/SNF-RSC family of chromatin-remodeling complexes. Research reviewed here has revealed the essential features of the remodeling process.

Distinct transcription factor complexes act on a permissive chromatin landscape to establish regionalized gene expression in CNS stem cells

Spatially distinct gene expression profiles in neural stem cells (NSCs) are a prerequisite to the formation of neuronal diversity, but how these arise from the regulatory interactions between chromatin accessibility and transcription factor activity has remained unclear. Here, we demonstrate that, despite their distinct gene expression profiles, NSCs of the mouse cortex and spinal cord share the majority of their DNase I hypersensitive sites (DHSs). Regardless of this similarity, domain-specific gene expression is highly correlated with the relative accessibility of associated DHSs, as determined by sequence read density. Notably, the binding pattern of the general NSC transcription factor SOX2 is also largely cell type specific and coincides with an enrichment of LHX2 motifs in the cortex and HOXA9 motifs in the spinal cord. Interestingly, in a zebrafish reporter gene system, these motifs were critical determinants of patterned gene expression along the rostral-caudal axis. Our findings establish a predictive model for patterned NSC gene expression, whereby domain-specific expression of LHX2 and HOX proteins act on their target motifs within commonly accessible cis-regulatory regions to specify SOX2 binding. In turn, this binding correlates strongly with these DHSs relative accessibility—a robust predictor of neighboring gene expression.

Role of transcription factor-mediated nucleosome disassembly in PHO5 gene expression

Studying nucleosome dynamics in promoter regions is crucial for understanding gene regulation. Nucleosomes regulate gene expression by sterically occluding transcription factors (TFs) and other non–histone proteins accessing genomic DNA. How the binding competition between nucleosomes and TFs leads to transcriptionally compatible promoter states is an open question. Here, we present a computational study of the nucleosome dynamics and organization in the promoter region of PHO5 gene in Saccharomyces cerevisiae. Introducing a model for nucleosome kinetics that takes into account ATP-dependent remodeling activity, DNA sequence effects, and kinetics of TFs (Pho4p), we compute the probability of obtaining different “promoter states” having different nucleosome configurations. Comparing our results with experimental data, we argue that the presence of local remodeling activity (LRA) as opposed to basal remodeling activity (BRA) is crucial in determining transcriptionally active promoter states. By modulating the LRA and Pho4p binding rate, we obtain different mRNA distributions—Poisson, bimodal, and long-tail. Through this work we explain many features of the PHO5 promoter such as sequence-dependent TF accessibility and the role of correlated dynamics between nucleosomes and TFs in opening/coverage of the TATA box. We also obtain possible ranges for TF binding rates and the magnitude of LRA.

Genome-wide cooperation by HAT Gcn5, remodeler SWI/SNF, and chaperone Ydj1 in promoter nucleosome eviction and transcriptional activation

Chaperones, nucleosome remodeling complexes, and histone acetyltransferases have been implicated in nucleosome disassembly at promoters of particular yeast genes, but whether these cofactors function ubiquitously, as well as the impact of nucleosome eviction on transcription genome-wide, is poorly understood. We used chromatin immunoprecipitation of histone H3 and RNA polymerase II (Pol II) in mutants lacking single or multiple cofactors to address these issues for about 200 genes belonging to the Gcn4 transcriptome, of which about 70 exhibit marked reductions in H3 promoter occupancy on induction by amino acid starvation. Examining four target genes in a panel of mutants indicated that SWI/SNF, Gcn5, the Hsp70 cochaperone Ydj1, and chromatin-associated factor Yta7 are required downstream from Gcn4 binding, whereas Asf1/Rtt109, Nap1, RSC, and H2AZ are dispensable for robust H3 eviction in otherwise wild-type cells. Using ChIP-seq to interrogate all 70 exemplar genes in single, double, and triple mutants implicated Gcn5, Snf2, and Ydj1 in H3 eviction at most, but not all, Gcn4 target promoters, with Gcn5 generally playing the greatest role and Ydj1 the least. Remarkably, these three cofactors cooperate similarly in H3 eviction at virtually all yeast promoters. Defective H3 eviction in cofactor mutants was coupled with reduced Pol II occupancies for the Gcn4 transcriptome and the most highly expressed uninduced genes, but the relative Pol II levels at most genes were unaffected or even elevated. These findings indicate that nucleosome eviction is crucial for robust transcription of highly expressed genes but that other steps in gene activation are more rate-limiting for most other yeast genes.

Regulation of Nucleosome Architecture and Factor Binding Revealed by Nuclease Footprinting of the ESC Genome

Functional interactions between gene regulatory factors and chromatin architecture have been difficult to directly assess. Here, we use micrococcal nuclease (MNase) footprinting to probe the functions of two chromatin-remodeling complexes. By simultaneously quantifying alterations in small MNase footprints over the binding sites of 30 regulatory factors in mouse embryonic stem cells (ESCs), we provide evidence that esBAF and Mbd3/NuRD modulate the binding of several regulatory proteins. In addition, we find that nucleosome occupancy is reduced at specific loci in favor of subnucleosomes upon depletion of esBAF, including sites of histone H2A.Z localization. Consistent with these data, we demonstrate that esBAF is required for normal H2A.Z localization in ESCs, suggesting esBAF either stabilizes H2A.Z-containing nucleosomes or promotes subnucleosome to nucleosome conversion by facilitating H2A.Z deposition. Therefore, integrative examination of MNase footprints reveals insights into nucleosome dynamics and functional interactions between chromatin structure and key gene-regulatory factors.