H2A.Z has a function reminiscent of an activator required for preferential binding to intergenic DNA

Characterizing the regulatory effects of H2A.Z and SWR1-C on gene expression during hydroxyurea exposure in Saccharomyces cerevisiae

Chromatin structure and DNA accessibility are partly modulated by the incorporation of histone variants. H2A.Z, encoded by the non-essential HTZ1 gene in S. cerevisiae, is an evolutionarily conserved H2A histone variant that is predominantly incorporated at transcription start sites by the SWR1-complex (SWR1-C). While H2A.Z has often been implicated in transcription regulation, htz1Δ mutants exhibit minimal changes in gene expression compared to wild-type. However, given that growth defects of htz1Δ mutants are alleviated by simultaneous deletion of SWR1-C subunits, previous work examining the role of H2A.Z in gene expression regulation may be confounded by deleterious activity caused by SWR1-C when missing its H2A.Z substrate (apo-SWR1-C). Furthermore, as H2A.Z mutants only display significant growth defects in genotoxic stress conditions, a more substantive role for H2A.Z in gene expression may only be uncovered after exposure to cellular stress. To explore this possibility, we generated mRNA transcript profiles for wild-type, htz1Δ, swr1Δ, and htz1Δswr1Δ mutants before and after exposure to hydroxyurea (HU), which induces DNA replication stress. Our data showed that H2A.Z played a more prominent role in gene activation than repression during HU exposure, and its incorporation was important for proper upregulation of several HU-induced genes. We also observed that apo-SWR1-C contributed to gene expression defects in the htz1Δ mutant, particularly for genes involved in phosphate homeostasis regulation. Furthermore, mapping H2A.Z incorporation before and after treatment with HU revealed that decreases in H2A.Z enrichment at transcription start sites was correlated with, but generally not required for, the upregulation of genes during HU exposure. Together this study characterized the regulatory effects of H2A.Z incorporation during the transcriptional response to HU.

Cloning and characterization of the histone variant gene H2A.Z in Bombyx mori

H2A.Z, the most evolutionarily conserved variant of histone H2A, plays a pivotal role in chromatin remodeling and contributes significantly to gene transcription and genome stability. However, the role of H2A.Z in the silkworm (Bombyx mori) remains unclear. In this study, we cloned the BmH2A.Z from B. mori. The open reading frame of BmH2A.Z is 390 bp, encoding 129 amino acids, with a confirmed molecular weight of 13.4 kDa through prokaryotic expression analysis. Sequence analysis revealed that BmH2A.Z has a conserved H2A.Z domain and is closely related to the systemic evolution of other known H2A.Zs. The expression profile of BmH2A.Z at various developmental stages of the B. mori exhibited the highest expression level in the 1st instar, followed by the grain stage and the 2nd instar, and the lowest expression level in the moth. The highest transcript level of BmH2A.Z was observed in the head, with relatively lower levels detected in the blood than in the other tissues under consideration. In addition, the upregulation of BmH2A.Z resulted in the amplified expression of B. mori nucleopolyhedrovirus (BmNPV) genes, thus facilitating the proliferation of BmNPV. This study establishes a foundation for investigating the role of BmH2A.Z in B. mori and its participation in virus-host interactions.

The Histone Variant H2A.Z C-Terminal Domain Has Locus-Specific Differential Effects on H2A.Z Occupancy and Nucleosome Localization

The incorporation of histone variant H2A.Z into nucleosomes creates specialized chromatin domains that regulate DNA-templated processes, such as gene transcription. In Saccharomyces cerevisiae, the diverging H2A.Z C terminus is thought to provide the H2A.Z exclusive functions. To elucidate the roles of this H2A.Z C terminus genome-wide, we used derivatives in which the C terminus was replaced with the corresponding region of H2A (ZA protein), or the H2A region plus a transcriptional activating peptide (ZA-rII'), with the intent of regenerating the H2A.Z-dependent regulation globally. The distribution of these H2A.Z derivatives indicates that the H2A.Z C-terminal region is crucial for both maintaining the occupation level of H2A.Z and the proper positioning of targeted nucleosomes. Interestingly, the specific contribution on incorporation efficiency versus nucleosome positioning varies enormously depending on the locus analyzed. Specifically, the role of H2A.Z in global transcription regulation relies on its C-terminal region. Remarkably, however, this mostly involves genes without a H2A.Z nucleosome in the promoter. Lastly, we demonstrate that the main chaperone complex which deposits H2A.Z to gene regulatory region (SWR1-C) is necessary to localize all H2A.Z derivatives at their specific loci, indicating that the differential association of these derivatives is not due to impaired interaction with SWR1-C. IMPORTANCE We provide evidence that the Saccharomyces cerevisiae C-terminal region of histone variant H2A.Z can mediate its special function in performing gene regulation by interacting with effector proteins and chaperones. These functional interactions allow H2A.Z not only to incorporate to very specific gene regulatory regions, but also to facilitate the gene expression process. To achieve this, we used a chimeric protein which lacks the native H2A.Z C-terminal region but contains an acidic activating region, a module that is known to interact with components of chromatin-remodeling entities and/or transcription modulators. We reasoned that because this activating region can fulfill the role of the H2A.Z C-terminal region, at least in part, the role of the latter would be to interact with these activating region targets.

What makes a histone variant a variant: Changing H2A to become H2A.Z

Chromatin structure and underlying DNA accessibility is modulated by the incorporation of histone variants. H2A.Z, a variant of the H2A core histone family, plays a distinct and essential role in a diverse set of biological functions including gene regulation and maintenance of heterochromatin-euchromatin boundaries. Although it is currently unclear how the replacement of H2A with H2A.Z can regulate gene expression, the variance in their amino acid sequence likely contributes to their functional differences. To tease apart regions of H2A.Z that confer its unique identity, a set of plasmids expressing H2A-H2A.Z hybrids from the native H2A.Z promoter were examined for their ability to recapitulate H2A.Z function. First, we found that the H2A.Z M6 region was necessary and sufficient for interaction with the SWR1-C chromatin remodeler. Remarkably, the combination of only 9 amino acid changes, the H2A.Z M6 region, K79 and L81 (two amino acids in the α2-helix), were sufficient to fully rescue growth phenotypes of the htz1Δ mutant. Furthermore, combining three unique H2A.Z regions (K79 and L81, M6, C-terminal tail) was sufficient for expression of H2A.Z-dependent heterochromatin-proximal genes and GAL1 derepression. Surprisingly, hybrid constructs that restored the transcription of H2A.Z-dependent genes, did not fully recapitulate patterns of H2A.Z-specific enrichment at the tested loci. This suggested that H2A.Z function in transcription regulation may be at least partially independent of its specific localization in chromatin. Together, this work has identified three regions that can confer specific H2A.Z-identity to replicative H2A, furthering our understanding of what makes a histone variant a variant.

Improving the Level of the Tyrosine Biosynthesis Pathway in Saccharomyces cerevisiae through HTZ1 Knockout and Atmospheric and Room Temperature Plasma (ARTP) Mutagenesis

In recent years, many studies have been conducted on the expression of multiple aromatic compounds by Saccharomyces cerevisiae. The concentration of l-tyrosine, as a precursor of such valuable compounds, is crucial for the biosynthesis of aromatic metabolites. In this study, a novel function of HTZ1 was found to be related to tyrosine biosynthesis, which has not yet been reported. Knockout of this gene could significantly improve the ability of yeast cells to synthesize tyrosine, and its p-coumaric acid (p-CA) titer was approximately 3.9-fold higher than that of the wild-type strain BY4742. Subsequently, this strain was selected for random mutagenesis through an emerging mutagenesis technique, namely, atmospheric and room temperature plasma (ARTP). After two rounds of mutagenesis, five tyrosine high-producing mutants were obtained. The highest production of p-CA was 7.6-fold higher than that of the wild-type strain. Finally, transcriptome data of the htz1Δ strain and the five mutants were analyzed. The genome of mutagenic strains was also resequenced to reveal the mechanism underlying the high titer of tyrosine. This system of target engineering combined with random mutagenesis to screen excellent mutants provides a new basis for synthetic biology.

Histone variant H2A.Z antagonizes the positive effect of the transcriptional activator CPC1 to regulate catalase-3 expression under normal and oxidative stress conditions

In eukaryotes, deposition of the histone variant H2A.Z into nucleosomes through the chromatin remodeling complex, SWR1, is a crucial step in modulating gene transcription. Recently, H2A.Z has been shown to control the expression of responsive genes, but the underlying mechanism of how H2A.Z responds to physiological stimuli is not well understood. Here, we reveal that, in Neurospora crassa, H2A.Z is a negative regulator of catalase-3 gene, which is responsible for resistance to oxidative stress. H2A.Z represses cat-3 gene expression through direct incorporation at cat-3 locus in a SWR1 complex dependent pathway. Notably, loss of H2A.Z or SWR1 subunits leads to increased binding of a transcription factor, CPC1, at cat-3 locus. Additionally, introduction of plasmids containing gene encoding H2A.Z or SWR1 complex subunits into wild-type strains decreased CAT-3 expression, indicating that H2A.Z counteracts the positive effect of CPC1 to achieve low level cat-3 expression under non-inductive condition. Furthermore, upon oxidative stress, H2A.Z is rapidly evicted from cat-3 locus for the recruitment of CPC1, resulting in robust and full cat-3 gene expression in response to external stimuli. Collectively, this study strongly demonstrates that H2A.Z antagonizes the function of transcription factor to regulate responsive gene transcription under normal conditions and to poise for gene full activation under oxidative stress.

Genome-wide identification of histone H2A and histone variant H2A.Z-interacting proteins by bPPI-seq

H2A is a nucleosome core subunit involved in organizing DNA into a chromatin structure that is often inaccessible to regulatory enzymes. Replacement of H2A by its variant H2A.Z renders chromatin accessible at enhancers and promoters. However, it remains unclear how H2A.Z functions so differently from canonical H2A. Here we report the genome-wide identification of proteins that directly interact with H2A and H2A.Z in vivo using a novel strategy, bPPI-seq. We show that bPPI-seq is a sensitive and robust technique to identify protein-protein interactions in vivo. Our data indicate that H2A.Z-interacting proteins and H2A-interacting proteins participate in distinct biological processes. In contrast to H2A-interacting proteins, the H2A.Z-interacting proteins are involved in transcriptional regulation. We found that the transcription factor Osr1 interacts with H2A.Z both in vitro and in vivo. It also mediates H2A.Z incorporation to a large number of target sites and regulates gene expression. Our data indicate that bPPI-seq can be widely applied to identify genome-wide interacting proteins under physiological conditions.

Progress in epigenetic histone modification analysis by mass spectrometry for clinical investigations

Chromatin biology and epigenetics are scientific fields that are rapid expanding due to their fundamental role in understanding cell development, heritable characters and progression of diseases. Histone post-translational modifications (PTMs) are major regulators of the epigenetic machinery due to their ability to modulate gene expression, DNA repair and chromosome condensation. Large-scale strategies based on mass spectrometry have been impressively improved in the last decade, so that global changes of histone PTM abundances are quantifiable with nearly routine proteomics analyses and it is now possible to determine combinatorial patterns of modifications. Presented here is an overview of the most utilized and newly developed proteomics strategies for histone PTM characterization and a number of case studies where epigenetic mechanisms have been comprehensively characterized. Moreover, a number of current epigenetic therapies are illustrated, with an emphasis on cancer.

Expression and functionality of histone H2A variants in cancer

Regulation of gene expression includes the replacement of canonical histones for non-allelic histone variants, as well as their multiple targeting by postranslational modifications. H2A variants are highly conserved between species suggesting they execute important functions that cannot be accomplished by canonical histones. Altered expression of many H2A variants is associated to cancer. MacroH2A variants are enriched in heterocromatic foci and are necessary for chromatin condensation. MacroH2A1.1 and macroH2A1.2 are two mutually exclusive isoforms. MacroH2A1.1 and macroH2A2 inhibit proliferation and are associated with better cancer prognosis; while macroH2A1.2 is associated to cancer progression. H2AX variant functions as a sensor of DNA damage and defines the cellular response towards DNA repair or apoptosis; therefore, screening approaches and therapeutic options targeting H2AX have been proposed. H2A.Z is enriched in euchromatin, acting as a proto-oncogene with established roles in hormone responsive cancers and overexpressed in endocrine-resistant disease. Other H2A family members have also been found altered in cancer, but their function remains unknown. Substantial progress has been made to understand histone H2A variants, their contribution to normal cellular function and to cancer development and progression. Yet, implementation of high resolution mass spectrometry is needed to further our knowledge on highly homologous H2A variants expression and function.

Expression of Non-acetylatable H2A.Z in Myoblast Cells Blocks Myoblast Differentiation through Disruption of MyoD Expression

H2A.Z is a histone H2A variant that is essential for viability in Tetrahymena and Drosophila and also during embryonic development of mice. Although implicated in diverse cellular processes, including transcriptional regulation, chromosome segregation, and heterochromatin formation, its essential function in cells remains unknown. Cellular differentiation is part of the developmental process of multicellular organisms. To elucidate the roles of H2A.Z and H2A.Z acetylation in cellular differentiation, we examined the effects of expressing wild type (WT) or a non-acetylatable form of H2A.Z in the growth and differentiation of the myoblast C2C12 cell line. Ectopic expression of wild type or mutant H2A.Z resulted in distinct phenotypes in the differentiation of the C2C12 cells and the formation of myotubes. Most strikingly, expression of the H2A.Z non-acetylatable mutant (H2A.Z-Ac-mut) resulted in a complete block of myoblast differentiation. We determined that this phenotype is caused by a loss of MyoD expression in the Ac-mut-expressing cells prior to and after induction of differentiation. Moreover, chromatin accessibility assays showed that the promoter region of MyoD is less accessible in the differentiation-defective cells. Altogether, these new findings show that expression of the Ac-mut form of H2A.Z resulted in a dominant phenotype that blocked differentiation due to chromatin changes at the MyoD promoter.

Precise deposition of histone H2A.Z in chromatin for genome expression and maintenance

Histone variant H2A.Z is essential in higher eukaryotes and has different functions in the cell. Several studies indicate that H2A.Z is found at specific loci in the genome such as regulatory-gene regions, where it poises genes for transcription. Itsdeposition creates chromatin regions with particular structural characteristics which could favor rapid transcription activation. This review focuses on the highly regulated mechanism of H2A.Z deposition in chromatin which is essential for genome integrity. Chaperones escort H2A.Z to large ATP-dependent chromatin remodeling enzymes which are responsible for its deposition/eviction. Over the last ten years, biochemical, genetic and genomic studies helped us understand the precise role of these complexes in this process. It hasbeen suggested that a cooperation occurs between histone acetyltransferase and chromatin remodeling activities to incorporate H2A.Z in chromatin. Its regulated deposition near centromeres and telomeres also shows its implication in chromosomal structure integrity and parallels a role in DNA damage response. Thedynamics of H2A.Z deposition/eviction at specific loci was shown to be critical for genome expression andmaintenance, thus cell fate. Altogether, recent findings reassert the importance of the regulated depositionof this histone variant. This article is part of a Special Issue entitled: Histone chaperones and Chromatin assembly.

Maintenance of heterochromatin boundary and nucleosome composition at promoters by the Asf1 histone chaperone and SWR1-C chromatin remodeler in Saccharomyces cerevisiae

Chromatin remodeling complexes cooperate to regulate gene promoters and to define chromatin neighborhoods. Here, we identified genetic and functional connections between two silencing-related chromatin factors in the maintenance of native heterochromatic structures and nucleosome composition at promoters. Building on a previously reported link between the histone chaperone Asf1 and the Yaf9 subunit of the SWR1-C chromatin remodeler, we found that ASF1 broadly interacted with genes encoding for SWR1-C subunits. Asf1 and Yaf9 were required for maintaining expression of heterochromatin-proximal genes and they worked cooperatively to prevent repression of telomere-proximal genes by limiting the spread of SIR complexes into nearby regions. Genome-wide Sir2 profiling, however, revealed that the cooperative heterochromatin regulation of Asf1 and SWR1-C occurred only on a subset of yeast telomeres. Extensive analyses demonstrated that formation of aberrant heterochromatin structures in the absence of ASF1 and YAF9 was not causal for the pronounced growth and transcriptional defects in cells lacking both these factors. Instead, genetic and molecular analysis revealed that H3K56 acetylation was required for efficient deposition of H2A.Z at subtelomeric and euchromatic gene promoters, pointing to a role for Asf1-dependent H3K56 acetylation in SWR1-C biology.

Mutations in non-acid patch residues disrupt H2A.Z's association with chromatin through multiple mechanisms

The incorporation of histone variants into nucleosomes is a critical mechanism for regulating essential DNA-templated processes and for establishing distinct chromatin architectures with specialised functions. H2A.Z is an evolutionarily conserved H2A variant that has diverse roles in transcriptional regulation, heterochromatin boundary definition, chromosome stability and DNA repair. The H2A.Z C-terminus diverges in sequence from canonical H2A and imparts unique functions to H2A.Z in the yeast S. cerevisiae. Although mediated in part through the acid patch-containing M6 region, many molecular determinants of this divergent structure-function relationship remain unclear. Here, by using an unbiased random mutagenesis screen of H2A.Z alleles, we identify point mutations in the C-terminus outside of the M6 region that disrupt the normal function of H2A.Z in response to cytotoxic stress. These functional defects correlate with reduced chromatin association, which we attribute to reduced physical stability within chromatin, but also to altered interactions with the SWR and INO80 chromatin remodeling complexes. Together with experimental data, computational modelling of these residue changes in the context of protein structure suggests the importance of C-terminal domain integrity and configuration for maintaining the level of H2A.Z in nucleosomes.

Single-molecule tools elucidate H2A.Z nucleosome composition

Although distinct epigenetic marks correlate with different chromatin states, how they are integrated within single nucleosomes to generate combinatorial signals remains largely unknown. We report the successful implementation of single molecule tools constituting fluorescence correlation spectroscopy (FCS), pulse interleave excitation-based Förster resonance energy transfer (PIE-FRET) and fluorescence lifetime imaging-based FRET (FLIM-FRET) to elucidate the composition of single nucleosomes containing histone variant H2A.Z (Htz1p in yeast) in vitro and in vivo. We demonstrate that yeast nucleosomes containing Htz1p are primarily composed of H4 K12ac and H3 K4me3 but not H3 K36me3 and that these patterns are conserved in mammalian cells. Quantification of epigenetic modifications in nucleosomes will provide a new dimension to epigenetics research and lead to a better understanding of how these patterns contribute to the targeting of chromatin-binding proteins and chromatin structure during gene regulation.

Analysis of histones and histone variants in plants

Histone proteins are the major protein components of chromatin - the physiologically relevant form of the genome (or epigenome) in all eukaryotic cells. For many years, histones were considered passive structural components of eukaryotic chromatin. In recent years, it has been demonstrated that dynamic association of histones and their variants to the genome plays a very important role in gene regulation. Histones are extensively modified during posttranslation viz. acetylation, methylation, phosphorylation, ubiquitylation, etc., and the identification of these covalent marks on canonical and variant histones is crucial for the understanding of their biological significance. Different biochemical techniques have been developed to purify and separate histone proteins; here, we describe techniques for analysis of histones from plant tissues.

Regulation of DCC localization by HTZ-1/H2A.Z and DPY-30 does not correlate with H3K4 methylation levels

Dosage compensation is a specialized form of gene regulation that balances sex-chromosome linked gene expression between the sexes. In C. elegans, dosage compensation is achieved by the activity of the dosage compensation complex (DCC). The DCC binds along both X chromosomes in hermaphrodites to down-regulate gene expression by half, limiting X-linked gene products to levels produced in XO males. Sequence motifs enriched on the X chromosome play an important role in targeting the DCC to the X. However, these motifs are not strictly X-specific and therefore other factors, such as the chromatin environment of the X chromosome, are likely to aid in DCC targeting. Previously, we found that loss of HTZ-1 results in partial disruption of dosage compensation localization to the X chromosomes. We wanted to know whether other chromatin components coordinated with HTZ-1 to regulate DCC localization. One candidate is DPY-30, a protein known to play a role in DCC localization. DPY-30 homologs in yeast, flies, and mammals are highly conserved members of histone H3 lysine 4 (H3K4) methyltransferase Set1/MLL complexes. Therefore, we investigated the hypothesis that the dosage compensation function of DPY-30 involves H3K4 methylation. We found that in dpy-30 animals the DCC fails to stably bind chromatin. Interestingly, of all the C. elegans homologs of Set1/MLL complex subunits, only DPY-30 is required for stable DCC binding to chromatin. Additionally, loss of H3K4 methylation does not enhance DCC mislocalization in htz-1 animals. We conclude that DPY-30 and HTZ-1 have unique functions in DCC localization, both of which are largely independent of H3K4 methylation.

H2A.Z-Mediated Genome-Wide Chromatin Specialization

The characterization of the involvement of different histone post-translational modifications (PTMs) and histone variants in chromatin structure has represented one of the most recurrent topics in molecular biology during the last decade (since 1996). The interest in this topic underscores the critical roles played by chromatin in such important processes as DNA packaging, DNA repair and recombination, and regulation of gene expression. The genomic information currently available has pushed the boundaries of this research a step further, from the study of local domains to the genome-wide characterization of the mechanisms governing chromatin dynamics. How the heterchromatin and euchromatin compartmentalization is established has been the subject of recent extensive research. Many PTMs, as well as histone variants have been identified to play a role, including the replacement of histone H2A by the histone variant H2A.Z. Several studies have provided support to a role for H2A.Z (known as Htz1 in yeast) in transcriptional regulation, chromosome structure, DNA repair and heterochromatin formation. Although the mechanisms by which H2A.Z defines different structural regions in the chromatin have long remained elusive, various reports published last year have shed new insight into this process. The present mini review focuses its attention on the genome-wide distribution of H2A.Z, with special attention to the mechanisms involved in its distribution and exchange as well as on the role of its N-terminal acetylation.

Genomic organization of H2Av containing nucleosomes in Drosophila heterochromatin

H2Av is a versatile histone variant that plays both positive and negative roles in transcription, DNA repair, and chromatin structure in Drosophila. H2Av, and its broader homolog H2A.Z, tend to be enriched toward 5′ ends of genes, and exist in both euchromatin and heterochromatin. Its organization around euchromatin genes and other features have been described in many eukaryotic model organisms. However, less is known about H2Av nucleosome organization in heterochromatin. Here we report the properties and organization of individual H2Av nucleosomes around genes and transposable elements located in Drosophila heterochromatic regions. We compare the similarity and differences with that found in euchromatic regions. Our analyses suggest that nucleosomes are intrinsically positioned on inverted repeats of DNA transposable elements such as those related to the “1360” element, but are not intrinsically positioned on retrotransposon-related elements.

Loss of H2A.Z Is Not Sufficient to Determine Transcriptional Activity of Snf2-Related CBP Activator Protein or p400 Complexes

The p400 and SRCAP (Snf2-related CBP activator protein) complexes remodel chromatin by catalyzing deposition of histone H2A.Z into nucleosomes. This remodeling activity has been proposed as a basis for regulation of transcription by these complexes. Transcript levels of p21 or Sp1 mRNAs after knockdown of p400 or SRCAP reveals that each regulates transcription of these promoters differently. In this study, we asked whether deposition of H2A.Z within specific nucleosomes by p400 or SRCAP dictates transcriptional activity. Our data indicates that nucleosome density at specific p21 or Sp1 promoter positions is not altered by the loss of either remodeling complex. However, knockdown of SRCAP or p400 reduces deposition of H2A.Z∼50% into all p21 and Sp1 promoter nucleosomes. Thus, H2A.Z deposition is not targeted to specific nucleosomes. These results indicate that the deposition of H2A.Z by the p400 or SRCAP complexes is not sufficient to determine how each regulates transcription. This conclusion is further supported by studies that demonstrate a SRCAP_ΔATP mutant unable to deposit H2A.Z has similar transcriptional activity as wild-type SRCAP.

Global analysis for functional residues of histone variant Htz1 using the comprehensive point mutant library

Histone variants perform unique functions and are deposited onto DNA by mechanisms distinct from those of canonical histones. The H2A variant, H2A.Z, also known as Htz1 in Saccharomyces cerevisiae, is not uniformly distributed across the genome but facilitates transcriptional activation at target gene promoters and anti-silencing at heterochromatin loci. Htz1 is also involved in DNA replication, DNA repair, chromosome segregation and cell cycle control. Its sequence identity to canonical H2A is only ∼60%, and it is likely that the nonconserved residues are responsible for Htz1-specific functions. However, precise roles of these variant-specific residues are not well understood. To gain insights into the molecular basis underlying the functional differences between canonical and variant histones, 117 alanine-scanning point mutants of Htz1 were constructed for this study, and chemical genetic screens were carried out. Consequently, seven Htz1 residues that conferred one or more abnormal phenotypes when mutated were identified. Based on primary sequence and functional conservation between H2A and Htz1, two of these residues (F32 and I109) appear to have an Htz1-specific role, whereas the rest seem to have functions shared between H2A and Htz1. This study provides a useful resource for future investigations into functional convergence and divergence between canonical and variant histones.

Histone variant H2A.Z and RNA polymerase II transcription elongation

Nucleosomes containing histone variant H2A.Z (Htz1) serve to poise quiescent genes for activation and transcriptional initiation. However, little is known about their role in transcription elongation. Here we show that dominant mutations in the elongation genes SPT5 and SPT16 suppress the hypersensitivity of htz1Δ strains to drugs that inhibit elongation, indicating that Htz1 functions at the level of transcription elongation. Direct kinetic measurements of RNA polymerase II (Pol II) movement across the 9.5-kb GAL10p-VPS13 gene revealed that the elongation rate of polymerase is 24% slower in the absence of Htz1. We provide evidence for two nonexclusive mechanisms. First, we observed that both the phospho-Ser2 levels in the elongating isoform of Pol II and the loading of Spt5 and Elongator over the GAL1 open reading frame (ORF) depend on Htz1. Second, in the absence of Htz1, the density of nucleosome occupancy is increased over the GAL10p-VPS13 ORF and the chromatin is refractory to remodeling during active transcription. These results establish a mechanistic role for Htz1 in transcription elongation and suggest that Htz1-containing nucleosomes facilitate Pol II passage by affecting the correct assembly and modification status of Pol II elongation complexes and by favoring efficient nucleosome remodeling over the gene.

Histone H2A.Z acid patch residues required for deposition and function

The incorporation of histone variants is one mechanism used by the eukaryotic cell to alter the generally repressive chromatin template. However, the exact molecular mechanisms that direct this incorporation are not well understood. The SWR1 chromatin remodeling complex that binds to and directs incorporation of histone variant H2A.Z into chromatin has been characterized, but significantly less information is available concerning the requirements on the H2A.Z target molecule. We performed an unbiased mutagenic screen designed to elucidate the function of H2A.Z in Saccharomyces cerevisiae. The screen identified residues within the conserved acidic patch of H2A.Z as being important for the function of the variant. We characterized single point mutations in the patch that are phenotypically sensitive to a variety of growth conditions and are expressed at lower protein levels, but are functionally defective (htz1-D99A, htz1-D99K, and htz1-E101K). The mutants were significantly less detectable by chromatin immunoprecipitation at PHO5, a gene previously described to be enriched for H2A.Z. These results identify acidic patch residues of H2A.Z that are critical for mediating deposition and function in chromatin, and represent potential candidates for the interaction of H2A.Z with its deposition and/or targeting machinery.

Expression of P. falciparum var genes involves exchange of the histone variant H2A.Z at the promoter

Plasmodium falciparum employs antigenic variation to evade the human immune response by switching the expression of different variant surface antigens encoded by the var gene family. Epigenetic mechanisms including histone modifications and sub-nuclear compartmentalization contribute to transcriptional regulation in the malaria parasite, in particular to control antigenic variation. Another mechanism of epigenetic control is the exchange of canonical histones with alternative variants to generate functionally specialized chromatin domains. Here we demonstrate that the alternative histone PfH2A.Z is associated with the epigenetic regulation of var genes. In many eukaryotic organisms the histone variant H2A.Z mediates an open chromatin structure at promoters and facilitates diverse levels of regulation, including transcriptional activation. Throughout the asexual, intraerythrocytic lifecycle of P. falciparum we found that the P. falciparum ortholog of H2A.Z (PfH2A.Z) colocalizes with histone modifications that are characteristic of transcriptionally-permissive euchromatin, but not with markers of heterochromatin. Consistent with this finding, antibodies to PfH2A.Z co-precipitate the permissive modification H3K4me3. By chromatin-immunoprecipitation we show that PfH2A.Z is enriched in nucleosomes around the transcription start site (TSS) in both transcriptionally active and silent stage-specific genes. In var genes, however, PfH2A.Z is enriched at the TSS only during active transcription in ring stage parasites. Thus, in contrast to other genes, temporal var gene regulation involves histone variant exchange at promoter nucleosomes. Sir2 histone deacetylases are important for var gene silencing and their yeast ortholog antagonises H2A.Z function in subtelomeric yeast genes. In immature P. falciparum parasites lacking Sir2A or Sir2B high var transcription levels correlate with enrichment of PfH2A.Z at the TSS. As Sir2A knock out parasites mature the var genes are silenced, but PfH2A.Z remains enriched at the TSS of var genes; in contrast, PfH2A.Z is lost from the TSS of de-repressed var genes in mature Sir2B knock out parasites. This result indicates that PfH2A.Z occupancy at the active var promoter is antagonized by PfSir2A during the intraerythrocytic life cycle. We conclude that PfH2A.Z contributes to the nucleosome architecture at promoters and is regulated dynamically in active var genes.

Plasmodium falciparum is a protist parasite that causes malaria and kills more than 800,000 people per year. The parasite escapes from the human immune response by antigenic variation through switching between expression of different var genes. These encode different variant antigens that are expressed on the surface of the infected erythrocyte and mediate pathogenic adhesion of the infected erythrocytes to host receptors. Understanding how this process is regulated may lead to the identification of factors that are essential for immune evasion and that could represent novel drug targets. Here, we have identified the parasite's histone variant PfH2A.Z as a novel contributor to the transcriptional regulation of antigenic variation. PfH2A.Z is enriched in the promoter of many genes, but enrichment correlates with gene expression only in var genes. Furthermore we show that PfH2A.Z enrichment in var promoters is antagonised by the var gene silencing factor PfSir2A. These findings further extend our knowledge of the complex mechanisms regulating gene expression in P. falciparum.

USP10 deubiquitylates the histone variant H2A.Z and both are required for androgen receptor-mediated gene activation

H2A.Z, a variant of H2A, is found at the promoters of inducible genes in both yeast and higher eukaryotes. However, its role in transcriptional regulation is complex since it has been reported to function both as a repressor and activator. We have previously found that mono-ubiquitylation of H2A.Z is linked to transcriptional silencing. Here, we provide new evidence linking H2A.Z deubiquitylation to transcription activation. We found that H2A.Z and ubiquitin-specific protease 10 (USP10) are each required for transcriptional activation of the androgen receptor (AR)-regulated PSA and KLK3 genes. USP10 directly deubiquitylates H2A.Z in vitro and in vivo, and reducing USP10 expression in prostate cancer cells results in elevated steady-state levels of mono-ubiquitylated H2A.Z (H2A.Zub1). Moreover, knockdown of USP10 ablates hormone-induced deubiquitylation of chromatin proteins at the AR-regulated genes. Finally, by sequential ChIP assays, we found that H2A.Zub1 is enriched at the PSA and KLK3 regulatory regions, and loss of H2A.Zub1 is associated with transcriptional activation of these genes. Together, these data provide novel insights into how H2A.Z ubiquitylation/deubiquitylation and USP10 function in AR-regulated gene expression.

H2A.Z (Htz1) controls the cell-cycle-dependent establishment of transcriptional silencing at Saccharomyces cerevisiae telomeres

The establishment of transcriptional silencing in Saccharomyces cerevisiae requires progression through the cell cycle. We have previously found that transit through M-phase is necessary and sufficient to establish silencing at telomeres following induction of the Sir3 silencing factor. In this study we find that halting cell-cycle progression in either G(1) or at the beginning of M-phase limits the ability of Sir3 to associate with a telomere-linked reporter gene and prevents the changes in histone modifications associated with gene repression. Deletion of genes coding for the histone variant H2A.Z (Htz1 in yeast) and histone acetyltransferase Sas2 abolish the cell-cycle progression requirement for the establishment of silencing. Cells blocked in telophase (but not at metaphase) are also able to establish silencing. We show that H2A.Z binds to the promoter of our telomere-linked reporter gene and that this binding diminishes in silenced cells. Finally, we observe a specific displacement of H2A.Z from chromatin in telophase-blocked cells, regardless of the silencing status of the reporter gene. These results suggest that the requirement for M-phase in the establishment of silencing may reflect a cell-cycle regulated relaxation of heterochromatin barriers.

Decoding the epigenetic language of plant development

Epigenetics refers to the study of heritable changes in gene expression or cellular phenotype without changes in DNA sequence. Epigenetic regulation of gene expression is accomplished by DNA methylation, histone modifications, histone variants, chromatin remodeling, and may involve small RNAs. DNA methylation at cytosine is carried out by enzymes called DNA Methyltransferases and is involved in many cellular processes, such as silencing of transposable elements and pericentromeric repeats, X-chromosome inactivation and genomic imprinting, etc. Histone modifications refer to posttranslational covalent attachment of chemical groups onto histones such as phosphorylation, acetylation, and methylation, etc. Histone variants, the non-canonical histones with amino acid sequences divergent from canonical histones, can have different epigenetic impacts on the genome from canonical histones. Higher-order chromatin structures maintained or modified by chromatin remodeling proteins also play important roles in regulating gene expression. Small non-coding RNAs play various roles in the regulation of gene expression at pre- as well as posttranscriptional levels. A special issue of Molecular Plant on 'Epigenetics and Plant Development' (Volume 4, Number 2, 2009) published a variety of articles covering many aspects of epigenetic regulation of plant development. We have tried here to present a bird's-eye view of these credible efforts towards understanding the mysterious world of epigenetics. The majority of the articles are about the chromatin modifying proteins, including histone modifiers, histone variants, and chromatin remodeling proteins that regulate various developmental processes, such as flowering time, vernalization, stem cell maintenance, and response to hormonal and environmental stresses, etc. Regulation of expression of seed transcriptome, involvement of direct tandem repeat elements in the PHE1 imprinting in addition to PcG proteins activity, paramutation, and epigenetic barriers in species hybridization are described well. The last two papers are about the Pol V-mediated heterochromatin formation independent of the 24nt-siRNA and the effect of genome position and tissue type on epigenetic regulation of gene expression. These findings not only further our current understanding of epigenetic mechanisms involved in many biological phenomena, but also pave the path for the future work, by raising many new questions that are discussed in the following lines.

The role of histone modifications and variants in regulating gene expression in breast cancer

The role of epigenetic phenomena in cancer biology is increasingly being recognized. Here we focus on the mechanisms and enzymes involved in regulating histone methylation and acetylation, and the modulation of histone variant expression and deposition. Implications of these epigenetic marks for tumor development, progression and invasiveness are discussed with a particular emphasis on breast cancer progression.

The euchromatic and heterochromatic landscapes are shaped by antagonizing effects of transcription on H2A.Z deposition

A role for variant histone H2A.Z in gene expression is now well established but little is known about the mechanisms by which it operates. Using a combination of ChIP–chip, knockdown and expression profiling experiments, we show that upon gene induction, human H2A.Z associates with gene promoters and helps in recruiting the transcriptional machinery. Surprisingly, we also found that H2A.Z is randomly incorporated in the genome at low levels and that active transcription antagonizes this incorporation in transcribed regions. After cessation of transcription, random H2A.Z quickly reappears on genes, demonstrating that this incorporation utilizes an active mechanism. Within facultative heterochromatin, we observe a hyper accumulation of the variant histone, which might be due to the lack of transcription in these regions. These results show how chromatin structure and transcription can antagonize each other, therefore shaping chromatin and controlling gene expression.

DNA in living cells is packaged into chromatin by histones and non-histone proteins. This packaging is very dynamic, allowing the controlled access of regulatory proteins such as transcription factors to DNA. Most chromatin is packaged with so-called canonical histones; namely H2A, H2B, H3, and H4. In some regions, however, variant histones replace canonical histones, creating special chromatin regions. Here we show that the variant histone H2A.Z is dynamically recruited to promoter regions where it helps in the recruitment of RNA polymerase II, the enzyme responsible for the first step of gene expression. In addition, we show that H2A.Z also associates randomly in the genome, but these molecules are removed during the passage of RNA polymerase II. In non-transcribed regions, H2A.Z accumulates in large domains called heterochromatin. We propose that a battle between random H2A.Z deposition and RNAPII-dependent H2A.Z eviction shapes the chromatin landscape.

NuA4 and SWR1-C: two chromatin-modifying complexes with overlapping functions and componentsThis paper is one of a selection of papers published in this Special Issue, entitled 30th Annual International Asilomar Chromatin and Chromosomes Conference, and has undergone the Journal's usual peer review process

Chromatin structure is important for the compaction of eukaryotic genomes, thus chromatin modifications play a fundamental role in regulating many cellular processes. The coordinated activities of various chromatin-remodelling and -modifying complexes are crucial in maintaining distinct chromatin neighbourhoods, which in turn ensure appropriate gene expression, as well as DNA replication, repair, and recombination. SWR1-C is an ATP-dependent histone deposition complex for the histone variant H2A.Z, whereas NuA4 is a histone acetyltransferase for histones H4, H2A, and H2A.Z. Together the NuA4 and SWR1-C chromatin-modifying complexes alter the chromatin structure through 3 distinct modifications in yeast: post-translational addition of chemical groups, ATP-dependent chromatin remodelling, and histone variant incorporation. These 2 multi-protein complexes share 4 subunits and function together to regulate the circuitry of H2A.Z biology. The components and functions of both multi-protein complexes are evolutionarily conserved and play important roles in multi-cellular development and cellular differentiation in higher eukaryotes. This review will summarize recent findings about NuA4 and SWR1-C and will focus on the connection between these complexes by investigating their physical and functional interactions through eukaryotic evolution.

The beauty of being a variant: H2A.Z and the SWR1 complex in plants

Numerous studies have shown that the nucleosome is a dynamic structure that strongly influences gene expression. Dynamism concerns different nucleosomal characteristics, including position, posttranslational modifications, and histone composition. Thus, within the nucleosome, canonical histones can be exchanged by histone variant proteins with specific functions-a process known as 'histone replacement'. The histone variant H2A.Z has an important function in transcription and, during the last few years, its role in plant development and immune response has become evident. Compiling genetic and biochemical studies from several laboratories has revealed that plants contain a multiprotein complex, similar to the SWR1/SRCAP complex from yeast and animals, involved in H2A.Z deposition. Despite intense research in different organisms, the mechanism by which H2A.Z influences transcription is still unknown. However, recent results from Arabidopsis have shown a strong inverse correlation between H2A.Z and DNA methylation, suggesting that H2A.Z might protect genes from silencing.

Histone H2A.Z is essential for estrogen receptor signaling

Incorporation of H2A.Z into the chromatin of inactive promoters has been shown to poise genes for their expression. Here we provide strong evidence that H2A.Z is incorporated into the promoter regions of estrogen receptor (ERalpha) target genes only upon gene induction, and that, in a cyclic pattern. Moreover, members of the human H2A.Z-depositing complex, p400, also follow the same gene recruitment kinetics as H2A.Z. Importantly, cellular depletion of H2A.Z or p400 leads to a severe defect in estrogen signaling, including loss of estrogen-specific cell proliferation. We find that incorporation of H2A.Z within TFF1 promoter chromatin allows nucleosomes to adopt preferential positions along the DNA translational axis. Finally, we provide evidence that H2A.Z is essential to allow estrogen-responsive enhancer function. Taken together, our results provide strong mechanistic insight into how H2A.Z regulates ERalpha-mediated gene expression and provide a novel link between H2A.Z-p400 and ERalpha-dependent gene regulation and enhancer function.

Regulation of gene expression and cellular proliferation by histone H2A.Z

The mammalian genome is organized into a structure of DNA and proteins known as chromatin. In general, chromatin presents a barrier to gene expression that is regulated by several pathways, namely by the incorporation of histone variants into the nucleosome. In yeast, H2A.Z is an H2A histone variant that is incorporated into nucleosomes as an H2A.Z/H2B dimer by the Swr1 complex and by the SRCAP and p400/Tip60 complexes in mammalian cells. H2A.Z has been associated with the poising of genes for transcriptional activation in the yeast model system, and is essential for development in higher eukaryotes. Recent studies in our laboratory have demonstrated a p400-dependent deposition of H2A.Z at the promoter of p21WAF1/CIP1, a consequence that prevents the activation of the gene by p53, thereby inhibiting p53-dependent replicative senescence, a form of cell-cycle arrest crucial in the prevention of carcinogenic transformation of cells. Moreover, H2A.Z is overexpressed in several different types of cancers, and its overexpression has been associated functionally with the proliferation state of cells. Therefore, we suggest that H2A.Z is an important regulator of gene expression, and its deregulation may lead to the increased proliferation of mammalian cells.

Connection between histone H2A variants and chromatin remodeling complexes

The organization of the eukaryotic genome into chromatin makes it inaccessible to the factors required for gene transcription and DNA replication, recombination, and repair. In addition to histone-modifying enzymes and ATP-dependent chromatin remodeling complexes, which play key roles in regulating many nuclear processes by altering the chromatin structure, cells have developed a mechanism of modulating chromatin structure by incorporating histone variants. These variants are incorporated into specific regions of the genome throughout the cell cycle. H2A.Z, which is an evolutionarily conserved H2A variant, performs several seemingly unrelated and even contrary functions. Another H2A variant, H2A.X, plays a very important role in the cellular response to DNA damage. This review summarizes the recent developments in our understanding of the role of H2A.Z and H2A.X in the regulation of chromatin structure and function, focusing on their functional links with chromatin modifying and remodeling complexes.

Chapter 5. Nuclear actin-related proteins in epigenetic control

The nuclear actin-related proteins (ARPs) share overall structure and low-level sequence homology with conventional actin. They are indispensable subunits of macromolecular machines that control chromatin remodeling and modification leading to dynamic changes in DNA structure, transcription, and DNA repair. Cellular, genetic, and biochemical studies suggest that the nuclear ARPs are essential to the epigenetic control of the cell cycle and cell proliferation in all eukaryotes, while in plants and animals they also exert epigenetic controls over most stages of multicellular development including organ initiation, the switch to reproductive development, and senescence and programmed cell death. A theme emerging from plants and animals is that in addition to their role in controlling the general compaction of DNA and gene silencing, isoforms of nuclear ARP-containing chromatin complexes have evolved to exert dynamic epigenetic control over gene expression and different phases of multicellular development. Herein, we explore this theme by examining nuclear ARP phylogeny, activities of ARP-containing chromatin remodeling complexes that lead to epigenetic control, expanding developmental roles assigned to several animal and plant ARP-containing complexes, the evidence that thousands of ARP complex isoforms may have evolved in concert with multicellular development, and ARPs in human disease.

Weak but uniform enrichment of the histone variant macroH2A1 along the inactive X chromosome

We studied the enrichment and distribution of the histone variant mH2A1 in the condensed inactive X (Xi) chromosome. By using highly specific antibodies against mH2A1 and stable HEK 293 cell lines expressing either green fluorescent protein (GFP)-mH2A1 or GFP-H2A, we found that the Xi chromosome contains approximately 1.5-fold more mH2A1 than the autosomes. To determine the in vivo distribution of mH2A1 along the X chromosome, we used a native chromatin immunoprecipitation-on-chip technique. DNA isolated from mH2A1-immunoprecipitated nucleosomes from either male or female mouse liver were hybridized to tiling microarrays covering 5 kb around most promoters or the entire X chromosome. The data show that mH2A1 is uniformly distributed across the entire Xi chromosome. Interestingly, a stronger mH2A1 enrichment along the pseudoautosomal X chromosome region was observed in both sexes. Our results indicate a potential role for macroH2A in large-scale chromosome structure and genome stability.

H2A.Z: view from the top

For a couple of decades the chromatin field has endured undeserved neglect. Indeed, what could be so exciting about a monotonous repeating structure whose purpose in life was to package DNA? Chromatin glamour is triumphantly back, due to the realization that chromatin is a major player in the regulation of gene expression and other nuclear processes that occur on the DNA template. The dynamics of the structure that regulates transcription is itself regulated by a variety of complex processes, including histone postsynthetic modifications, chromatin remodeling, and the use of nonallelic histone variants. This review is an attempt to understand the mechanisms of action of the evolutionarily conserved variant H2A.Z, a player with a variety of seemingly unrelated, even contrary, functions. This attempt was prompted by the recent avalanche of genome-wide studies that provide insights that were unthinkable until very recently.

Variant histone H2A.Z, but not the HMG proteins Nhp6a/b, is essential for the recruitment of Swi/Snf, Mediator, and SAGA to the yeast GAL1 UAS(G)

Chromatin architecture is very important for the regulation of transcriptional activation. Here, we investigated the role of two different chromatin components, the histone variant H2A.Z and HMG proteins Nhp6a/b, in regulating GAL1 gene expression. We have shown that recruitment of the Mediator complex is significantly affected in the absence of H2A.Z. Furthermore, H2A.Z is also required to fully recruit the SAGA and Swi/Snf complexes to the yeast GAL1-10 UAS(G). On the other hand, the HMG protein Nhp6a/b is not required to recruit the aforementioned components to the GAL1 promoter. The Nhp6 protein has been shown to interact with nucleosomes, and we show that its distribution is unaffected in the absence of H2A.Z. Our results suggest that the incorporation of the histone variant H2A.Z, but not the HMG proteins Nhp6a/b, in promoter regions creates a specialized chromatin domain that is required for pre-initiation complex assembly at the GAL1 locus.

Global dynamics of newly constructed oligonucleosomes of conventional and variant H2A.Z histone

Background

Complexes of nucleosomes, which often occur in the gene promoter areas, are one of the fundamental levels of chromatin organization and thus are important for transcription regulation. Investigating the dynamic structure of a single nucleosome as well as nucleosome complexes is important for understanding transcription within chromatin. In a previous work, we highlighted the influence of histone variants on the functional dynamics of a single nucleosome using normal mode analysis developed by Bahar et al. The present work further analyzes the dynamics of nucleosome complexes (nucleosome oligomers or oligonucleosomes) such as dimer, trimer and tetramer (beads on a string model) with conventional core histones as well as with the H2A.Z histone variant using normal mode analysis.

Results

The global dynamics of oligonucleosomes reveal larger amplitude of motion within the nucleosomes that contain the H2A.Z variant with in-planar and out-of-planar fluctuations as the common mode of relaxation. The docking region of H2A.Z and the L1:L1 interactions between H2A.Z monomers of nucleosome (that are responsible for the highly stable nucleosome containing variant H2A.Z-histone) are highly dynamic throughout the first two dynamic modes.

Conclusion

Dissection of the dynamics of oligonucleosomes discloses in-plane as well as out-of-plane fluctuations as the common mode of relaxation throughout the global motions. The dynamics of individual nucleosomes and the combination of the relaxation mechanisms expressed by the individual nucleosome are quite interesting and highly dependent on the number of nucleosome fragments present in the complexes. Distortions generated by the non-planar dynamics influence the DNA conformation, and hence the histone-DNA interactions significantly alter the dynamics of the DNA. The variant H2A.Z histone is a major source of weaker intra- and inter-molecular correlations resulting in more disordered motions.

p21 transcription is regulated by differential localization of histone H2A.Z

In yeast cells, H2A.Z regulates transcription and is globally associated within a few nucleosomes of the initiator regions of numerous promoters. H2A.Z is deposited at these loci by an ATP-dependent complex, Swr1.com. Here we show that H2A.Z suppresses the p53 --> p21 transcription and senescence responses. Upon DNA damage, H2A.Z is first evicted from the p21 promoter, followed by the recruitment of the Tip60 histone acetyltransferase to activate p21 transcription. p400, a human Swr1 homolog, is required for the localization of H2A.Z, and largely colocalizes with H2A.Z at multiple promoters investigated. Notably, the presence of sequence-specific transcription factors, such as p53 and Myc, provides positioning cues that direct the location of H2A.Z-containing nucleosomes within these promoters. Collectively, this study strongly suggests that certain sequence-specific transcription factors regulate transcription, in part, by preferentially positioning histone variant H2A.Z within chromatin. This H2A.Z-centered process is part of an epigenetic process for modulating gene expression.

Actin-related proteins in chromatin-level control of the cell cycle and developmental transitions

Regulating developmental transitions, cell proliferation and cell death through differential gene expression is essential to the ontogeny of all multicellular organisms. Chromatin remodeling is an active process that is necessary for managing the genome-wide suppression of gene activities resulting from DNA compaction. Recent data in plants suggest a general theme, whereby chromatin remodeling complexes containing nuclear actin-related proteins (ARPs) potentiate the activities of crucial regulatory genes involved in plant growth and development, in addition to their basal activities on a much larger set of genes.

[H2A.Z: a histone variant that decorates gene promoters]

The nucleosome, fundamental unit of chromatin, is composed of four basic histones, H2A, H2B, H3 and H4, around which DNA is wrapped. In order to have access to DNA, cells must modify the structure of chromatin by different known mechanisms. One such mechanism is by replacing canonical histones in the nucleosome with variants, which can confer special functions to chromatin. H2A.Z is an evolutionary conserved variant of H2A that has both a positive and a negative role on gene transcription. The mechanisms by which H2A.Z acts are still poorly understood. However, recent reports have shed some light on this subject. H2A.Z is found associated with almost 2/3 of the promoters of genes in yeast, suggesting that this histone could have a global role on gene expression by poising chromatin for activation. We review here recent literature and discuss different aspects of the biology of this histone variant.

Repression of flowering in Arabidopsis requires activation of FLOWERING LOCUS C expression by the histone variant H2A.Z

The histone variant H2A.Z has been implicated in numerous chromatin-mediated processes, including transcriptional activation, euchromatin maintenance, and heterochromatin formation. In yeast and humans, H2A.Z is deposited into chromatin by a conserved protein complex known as SWR1 and SRCAP, respectively. Here, we show that mutations in the Arabidopsis thaliana homologs of two components of this complex, ACTIN-RELATED PROTEIN6 (ARP6) and PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1), produce similar developmental phenotypes and result in the misregulation of a common set of genes. Using H2A.Z-specific antibodies, we demonstrate that ARP6 and PIE1 are required for the deposition of H2A.Z at multiple loci, including the FLOWERING LOCUS C (FLC) gene, a central repressor of the transition to flowering. Loss of H2A.Z from chromatin in arp6 and pie1 mutants results in reduced FLC expression and premature flowering, indicating that this histone variant is required for high-level expression of FLC. In addition to defining a novel mechanism for the regulation of FLC expression, these results support the existence of a SWR1-like complex in Arabidopsis and show that H2A.Z can potentiate transcriptional activation in plants. The finding that H2A.Z remains associated with chromatin throughout mitosis suggests that it may serve an epigenetic memory function by marking active genes and poising silenced genes for reactivation.

Reuniting the contrasting functions of H2A.ZThis paper is one of a selection of papers published in this Special Issue, entitled 27th International West Coast Chromatin and Chromosome Conference, and has undergone the Journal's usual peer review process

It is now well established that cells modify chromatin to set transcriptionally active or inactive regions. Such control of chromatin structure is essential for proper development of organisms. In addition to the growing number of histone post-translational modifications, cells can exchange canonical histones with different variants that can directly or indirectly change chromatin structure. Moreover, enzymatic complexes that can exchange specific histone variants within the nucleosome have now been identified. One such variant, H2A.Z, has recently been the focus of many studies. H2A.Z is highly conserved in evolution and has many different functions, while defining both active and inactive chromatin in different contexts. Advanced molecular techniques, such as genome-wide binding assays (chromatin immunoprecipitation on chip) have recently given researchers many clues as to how H2A.Z is targeted to chromatin and how it affects nuclear functions. We wish to review the recent literature and summarize our understanding of the mechanisms and functions of H2A.Z.

H2A.Z Stabilizes Chromatin in a Way That Is Dependent on Core Histone Acetylation

The functional and structural chromatin roles of H2A.Z are still controversial. This work represents a further attempt to resolve the current functional and structural dichotomy by characterizing chromatin structures containing native H2A.Z. We have analyzed the role of this variant in mediating the stability of the histone octamer in solution using gel-filtration chromatography at different pH. It was found that decreasing the pH from neutral to acidic conditions destabilized the histone complex. Furthermore, it was shown that the H2A.Z-H2B dimer had a reduced stability. Sedimentation velocity analysis of nucleosome core particles (NCPs) reconstituted from native H2A.Z-containing octamers indicated that these particles exhibit a very similar behavior to that of native NCPs consisting of canonical H2A. Sucrose gradient fractionation of native NCPs under different ionic strengths indicated that H2A.Z had a subtle tendency to fractionate with more stabilized populations. An extensive analysis of the salt-dependent dissociation of histones from hydroxyapatite-adsorbed chromatin revealed that, whereas H2A.Z co-elutes with H3-H4, hyperacetylation of histones (by treatment of chicken MSB cells with sodium butyrate) resulted in a significant fraction of this variant eluting with the canonical H2A. These studies also showed that the late elution of this variant (correlated to enhanced binding stability) was independent of the chromatin size and of the presence or absence of linker histones.

Acetylation of H2AZ Lys 14 is associated with genome-wide gene activity in yeast

Histone variants and their post-translational modifications help regulate chromosomal functions. Htz1 is an evolutionarily conserved H2A variant found at several promoters in the yeast Saccharomyces cerevisiae. In this study, we undertook a genome-wide analysis of Htz1 and its modifications in yeast. Using mass spectrometric analysis, we determined that Htz1 is acetylated at Lys 3, Lys 8, Lys 10, and Lys 14 within its N-terminal tail, with K14 being the most abundant acetylated site. ChIP and microarray analysis were then used to compare the location of Htz1-K14 acetylation to that of Htz1 genome-wide. The data presented here demonstrate that while Htz1 is associated preferentially with the promoters of repressed genes, K14 acetylation is enriched at the promoters of active genes, and requires two known histone acetyltransferases, Gcn5 and Esa1. In support of our genome-wide analysis, we found that the acetylatable lysines of Htz1 are required for its full deposition during nucleosome reassembly upon repression of PHO5. Since the majority of Htz1 acetylation is seen at active promoters, where nucleosomes are known to be disassembled, our data argue for a dynamic process in which reassembly of Htz1 is regulated by its acetylation at promoters during transcription.

Nucleolin is a histone chaperone with FACT-like activity and assists remodeling of nucleosomes

Remodeling machines play an essential role in the control of gene expression, but how their activity is regulated is not known. Here we report that the nuclear protein nucleolin possesses a histone chaperone activity and that this factor greatly enhances the activity of the chromatin remodeling machineries SWI/SNF and ACF. Interestingly, nucleolin is able to induce the remodeling by SWI/SNF of macroH2A, but not of H2ABbd nucleosomes, which are otherwise resistant to remodeling. This new histone chaperone promotes the destabilization of the histone octamer, helping the dissociation of a H2A-H2B dimer, and stimulates the SWI/SNF-mediated transfer of H2A-H2B dimers. Furthermore, nucleolin facilitates transcription through the nucleosome, which is reminiscent of the activity of the FACT complex. This work defines new functions for histone chaperones in chromatin remodeling and regulation of transcription and explains how nucleolin could act on transcription.

H2A.Z functions to regulate progression through the cell cycle

Histone H2A variants are highly conserved proteins found ubiquitously in nature and thought to perform specialized functions in the cell. Studies in yeast on the histone H2A variant H2A.Z have shown a role for this protein in transcription as well as chromosome segregation. Our studies have focused on understanding the role of H2A.Z during cell cycle progression. We found that htz1delta cells were delayed in DNA replication and progression through the cell cycle. Furthermore, cells lacking H2A.Z required the S-phase checkpoint pathway for survival. We also found that H2A.Z localized to the promoters of cyclin genes, and cells lacking H2A.Z were delayed in the induction of these cyclin genes. Several different models are proposed to explain these observations.

Histones in functional diversification. Core histone variants

Recent research suggests that minor changes in the primary sequence of the conserved histones may become major determinants for the chromatin structure regulating gene expression and other DNA-related processes. An analysis of the involvement of different core histone variants in different nuclear processes and the structure of different variant nucleosome cores shows that this may indeed be so. Histone variants may also be involved in demarcating functional regions of the chromatin. We discuss in this review why two of the four core histones show higher variation. A comparison of the status of variants in yeast with those from higher eukaryotes suggests that histone variants have evolved in synchrony with functional requirement of the cell.