Global position and recruitment of HATs and HDACs in the yeast genome

The histone chaperone anti-silencing function 1 stimulates the acetylation of newly synthesized histone H3 in S-phase

Anti-silencing function 1 (Asf1) is a highly conserved chaperone of histones H3/H4 that assembles or disassembles chromatin during transcription, replication, and repair. We have found that budding yeast lacking Asf1 has greatly reduced levels of histone H3 acetylated at lysine 9. Lysine 9 is acetylated on newly synthesized budding yeast histone H3 prior to its assembly onto newly replicated DNA. Accordingly, we found that the vast majority of H3 Lys-9 acetylation peaked in S-phase, and this S-phase peak of H3 lysine 9 acetylation was absent in yeast lacking Asf1. By contrast, deletion of ASF1 has no effect on the S-phase specific peak of H4 lysine 12 acetylation; another modification carried by newly synthesized histones prior to chromatin assembly. We show that Gcn5 is the histone acetyltransferase responsible for the S-phase-specific peak of H3 lysine 9 acetylation. Strikingly, overexpression of Asf1 leads to greatly increased levels of H3 on acetylation on lysine 56 and Gcn5-dependent acetylation on lysine 9. Analysis of a panel of Asf1 mutations that modulate the ability of Asf1 to bind to histones H3/H4 demonstrates that the histone binding activity of Asf1 is required for the acetylation of Lys-9 and Lys-56 on newly synthesized H3. These results demonstrate that Asf1 does not affect the stability of the newly synthesized histones per se, but instead histone binding by Asf1 promotes the efficient acetylation of specific residues of newly synthesized histone H3.

High-resolution computational models of genome binding events

Direct physical information that describes where transcription factors, nucleosomes, modified histones, RNA polymerase II and other key proteins interact with the genome provides an invaluable mechanistic foundation for understanding complex programs of gene regulation. We present a method, joint binding deconvolution (JBD), which uses additional easily obtainable experimental data about chromatin immunoprecipitation (ChIP) to improve the spatial resolution of the transcription factor binding locations inferred from ChIP followed by DNA microarray hybridization (ChIP-Chip) data. Based on this probabilistic model of binding data, we further pursue improved spatial resolution by using sequence information. We produce positional priors that link ChIP-Chip data to sequence data by guiding motif discovery to inferred protein-DNA binding sites. We present results on the yeast transcription factors Gcn4 and Mig2 to demonstrate JBD's spatial resolution capabilities and show that positional priors allow computational discovery of the Mig2 motif when a standard approach fails.

Gcn5p contributes to the bidirectional character of the UGA3-GLT1 yeast promoter

Analysis of the UGA3-GLT1 bidirectional promoter has indicated that its transcriptional activation is determined by the combined action of Gcn4p and Gln3p, and that its bidirectional character is influenced by chromatin organization, through the action of an Abf1p binding site and a polydAdTtract. Results presented in this paper show that lack of Gcn5p impairs histone acetylation and nucleosomal organization of the UGA3-GLT1 promoter, resulting in an asymmetrical transcriptional activation response of UGA3 and GLT1. The phenotype displayed by a double mutant impaired in GCN5 and in the Abf1p binding site indicates that the combined action of these two elements determines the bidirectional capacity of the UGA3-GLT1 intergenic region.

Tip60 in DNA damage response and growth control: many tricks in one HAT

The Tip60 histone acetyltransferase is part of an evolutionarily conserved multisubunit complex, NuA4, which is recruited by many transcription factors to their target promoters, where it is thought to participate in histone acetylation and transcriptional activation. These transcription factors include tumor promoters and also tumor suppressors, such as p53, which links Tip60 to DNA damage responses. Tip60 also has transcription-independent roles in DNA damage responses. First, independently from NuA4, Tip60 binds the kinases ataxia-telangiectasia mutated (ATM) and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and participates in their activation by DNA double-strand breaks. Second, NuA4 is recruited to the chromatin surrounding the breaks and, through a series of chromatin modifications, contributes to the dynamics of DNA repair. These molecular activities might endow Tip60 with multiple and potentially antagonistic biological functions.

Negative and positive regulation of gene expression by mouse histone deacetylase 1

Histone deacetylases (HDACs) catalyze the removal of acetyl groups from core histones. Because of their capacity to induce local condensation of chromatin, HDACs are generally considered repressors of transcription. In this report, we analyzed the role of the class I histone deacetylase HDAC1 as a transcriptional regulator by comparing the expression profiles of wild-type and HDAC1-deficient embryonic stem cells. A specific subset of mouse genes (7%) was deregulated in the absence of HDAC1. We identified several putative tumor suppressors (JunB, Prss11, and Plagl1) and imprinted genes (Igf2, H19, and p57) as novel HDAC1 targets. The majority of HDAC1 target genes showed reduced expression accompanied by recruitment of HDAC1 and local reduction in histone acetylation at regulatory regions. At some target genes, the related deacetylase HDAC2 partially masks the loss of HDAC1. A second group of genes was found to be downregulated in HDAC1-deficient cells, predominantly by additional recruitment of HDAC2 in the absence of HDAC1. Finally, a small set of genes (Gja1, Irf1, and Gbp2) was found to require HDAC activity and recruitment of HDAC1 for their transcriptional activation. Our study reveals a regulatory cross talk between HDAC1 and HDAC2 and a novel function for HDAC1 as a transcriptional coactivator.

Genome-wide patterns of histone modifications in yeast

Post-translational histone modifications and histone variants generate complexity in chromatin to enable the many functions of the chromosome. Recent studies have mapped histone modifications across the Saccharomyces cerevisiae genome. These experiments describe how combinations of modified and unmodified states relate to each other and particularly to chromosomal landmarks that include heterochromatin, subtelomeric chromatin, centromeres, origins of replication, promoters and coding regions. Such patterns might be important for the regulation of heterochromatin-mediated silencing, chromosome segregation, DNA replication and gene expression.

Fine-structure analysis of ribosomal protein gene transcription

The ribosomal protein genes of Saccharomyces cerevisiae, responsible for nearly 40% of the polymerase II transcription initiation events, are characterized by the constitutive tight binding of the transcription factor Rap1. Rap1 binds at many places in the yeast genome, including glycolytic enzyme genes, the silent MAT loci, and telomeres, its specificity arising from specific cofactors recruited at the appropriate genes. At the ribosomal protein genes two such cofactors have recently been identified as Fhl1 and Ifh1. We have now characterized the interaction of these factors at a bidirectional ribosomal protein promoter by replacing the Rap1 sites with LexA operator sites. LexA-Gal4(AD) drives active transcription at this modified promoter, although not always at the correct initiation site. Tethering Rap1 to the promoter neither drives transcription nor recruits Fhl1 or Ifh1, showing that Rap1 function requires direct DNA binding. Tethering Fhl1 also fails to activate transcription, even though it does recruit Ifh1, suggesting that Fhl1 does more than simply provide a platform for Ifh1. Tethering Ifh1 to the promoter leads to low-level transcription, at the correct initiation sites. Remarkably, activation by tethered LexA-Gal4(AD) is strongly reduced when TOR kinase is inhibited by rapamycin. Thus, TOR can act independently of Fhl1/Ifh1 at ribosomal protein promoters. We also show that, in our strain background, the response of ribosomal protein promoters to TOR inhibition is independent of the Ifh1-related protein Crf1, indicating that the role of this corepressor is strain specific. Fine-structure chromatin mapping of several ribosomal protein promoters revealed that histones are essentially absent from the Rap1 sites, while Fhl1 and Ifh1 are coincident with each other but distinct from Rap1.

Statistical assessment of the global regulatory role of histone acetylation in Saccharomyces cerevisiae

An analysis of genome-wide histone acetylation data using a few complementary statistical models gives support to a cumulative effect model for global histone acetylation.

Background

Histone acetylation plays important but incompletely understood roles in gene regulation. A comprehensive understanding of the regulatory role of histone acetylation is difficult because many different histone acetylation patterns exist and their effects are confounded by other factors, such as the transcription factor binding sequence motif information and nucleosome occupancy.

Results

We analyzed recent genomewide histone acetylation data using a few complementary statistical models and tested the validity of a cumulative model in approximating the global regulatory effect of histone acetylation. Confounding effects due to transcription factor binding sequence information were estimated by using two independent motif-based algorithms followed by a variable selection method. We found that the sequence information has a significant role in regulating transcription, and we also found a clear additional histone acetylation effect. Our model fits well with observed genome-wide data. Strikingly, including more complicated combinatorial effects does not improve the model's performance. Through a statistical analysis of conditional independence, we found that H4 acetylation may not have significant direct impact on global gene expression.

Conclusion

Decoding the combinatorial complexity of histone modification requires not only new data but also new methods to analyze the data. Our statistical analysis confirms that histone acetylation has a significant effect on gene transcription rates in addition to that attributable to upstream sequence motifs. Our analysis also suggests that a cumulative effect model for global histone acetylation is justified, although a more complex histone code may be important at specific gene loci. We also found that the regulatory roles among different histone acetylation sites have important differences.

Genome-wide location analysis: insights on transcriptional regulation

Gene expression analysis of microarray data can provide a global view of the transcriptome of a cell or specific tissue type, revealing important information about the kinds of signaling pathways, genes and protein classifications that are active. However, transcript profiles alone do not reveal how expression levels are controlled or which transcription factors (TFs) are responsible. Establishing transcriptional regulatory networks requires knowledge of TFs bound to promoter, enhancer and repressor elements. Accessibility of these sites and an additional level of control are mediated by chromatin and DNA modifications. Genome-wide location analysis is a tool for identifying protein-DNA interaction sites on a genomic scale. Applications of this tool are proving invaluable in determining in vivo target genes of TFs, epigenetic marks and cis-regulatory elements. Here, we will discuss how advances have been made in each of these categories and how this has helped to elucidate regulatory networks and control mechanisms.

A genomic code for nucleosome positioning

Eukaryotic genomes are packaged into nucleosome particles that occlude the DNA from interacting with most DNA binding proteins. Nucleosomes have higher affinity for particular DNA sequences, reflecting the ability of the sequence to bend sharply, as required by the nucleosome structure. However, it is not known whether these sequence preferences have a significant influence on nucleosome position in vivo, and thus regulate the access of other proteins to DNA. Here we isolated nucleosome-bound sequences at high resolution from yeast and used these sequences in a new computational approach to construct and validate experimentally a nucleosome-DNA interaction model, and to predict the genome-wide organization of nucleosomes. Our results demonstrate that genomes encode an intrinsic nucleosome organization and that this intrinsic organization can explain approximately 50% of the in vivo nucleosome positions. This nucleosome positioning code may facilitate specific chromosome functions including transcription factor binding, transcription initiation, and even remodelling of the nucleosomes themselves.

Chromosome-wide, allele-specific analysis of the histone code on the human X chromosome

Variation in the composition of chromatin has been proposed to generate a 'histone code' that epigenetically regulates gene expression in a variety of eukaryotic systems. As a result of the process of X chromosome inactivation, chromatinon the mammalian inactive X chromosome (Xi) is marked by several modifications, including histone hypoacetylation, trimethylation of lysine 9 on histone H3 (H3TrimK9) and substitution of core histone H2A with the histone variant MacroH2A. H3TrimK9 is a well-studied marker for heterochromatin in many organisms, but the distribution and function of MacroH2A are less clear. Cytologically, the Xi in human cells comprises alternating and largely non-overlapping approximately 10-15 Mb domains marked by MacroH2A and H3TrimK9. To examine the genomic deposition of MacroH2A, H3TrimK9 and acetylated histone H4 modifications on the Xi at higher resolution, we used chromatin immunoprecipitation in combination with a SNP-based assay to distinguish the Xi and active X (Xa) in a diploid female cell line and to determine quantitatively the relative enrichment of these histone code elements on the Xi relative to the Xa. Although we found a majority of sites were enriched for either MacroH2A or H3TrimK9 in a manner consistent with the cytological appearance of the Xi, a range of different histone code types were detected at different sites along the X. These findings suggest that the nature of the heterochromatin histone code associated with X inactivation may be more heterogeneous than previously thought and imply that gene silencing can be achieved by a variety of different epigenetic mechanisms whose genomic, evolutionary or developmental basis is now amenable to investigation.

Genome-wide location of the coactivator mediator: Binding without activation and transient Cdk8 interaction on DNA

Mediator is a general coactivator of RNA polymerase II (Pol II) transcription. Genomic location analyses of different Mediator subunits indicate a uniformly composed core complex upstream of active genes but unexpectedly also upstream of inactive genes and on the coding regions of some highly active genes. The repressive Cdk8 submodule is associated with core Mediator at all sites but with a lower degree of occupancy, indicating transient interaction, regardless of promoter activity. This suggests gene-specific regulation of Cdk8 activity, rather than regulated Cdk8 recruitment. Mediator presence is not necessarily linked to transcription. This goes beyond Cdk8-repressed genes, indicating that Mediator can mark some regulatory regions ahead of additional signals. Overlap with intergenic Pol II location in stationary phase points to a role as a binding platform for inactive Pol II during quiescence. These results shed light on Cdk8 repression, suggest additional roles for Mediator, and query models of recruitment-coupled regulation.

Transcription Impairment and Cell Migration Defects in Elongator-Depleted Cells: Implication for Familial Dysautonomia

Mutations in IKBKAP, encoding a subunit of Elongator, cause familial dysautonomia (FD), a severe neurodevelopmental disease with complex clinical characteristics. Elongator was previously linked not only with transcriptional elongation and histone acetylation but also with other cellular processes. Here, we used RNA interference (RNAi) and fibroblasts from FD patients to identify Elongator target genes and study the role of Elongator in transcription. Strikingly, whereas Elongator is recruited to both target and nontarget genes, only target genes display histone H3 hypoacetylation and progressively lower RNAPII density through the coding region in FD cells. Interestingly, several target genes encode proteins implicated in cell motility. Indeed, characterization of IKAP/hELP1 RNAi cells, FD fibroblasts, and neuronal cell-derived cells uncovered defects in this cellular function upon Elongator depletion. These results indicate that defects in Elongator function affect transcriptional elongation of several genes and that the ensuing cell motility deficiencies may underlie the neuropathology of FD patients.

Genome-wide relationships between TAF1 and histone acetyltransferases in Saccharomyces cerevisiae

Histone acetylation regulates gene expression, yet the functional contributions of the numerous histone acetyltransferases (HATs) to gene expression and their relationships with each other remain largely unexplored. The central role of the putative HAT-containing TAF1 subunit of TFIID in gene expression raises the fundamental question as to what extent, if any, TAF1 contributes to acetylation in vivo and to what extent it is redundant with other HATs. Our findings herein do not support the basic tenet that TAF1 is a major HAT in Saccharomyces cerevisiae, nor do we find that TAF1 is functionally redundant with other HATs, including Gcn5, Elp3, Hat1, Hpa2, Sas3, and Esa1, which is in contrast to previous conclusions regarding Gcn5. Our findings do reveal that of these HATs, only Gcn5 and Esa1 contribute substantially to gene expression genome wide. Interestingly, histone acetylation at promoter regions throughout the genome does not require TAF1 or RNA polymerase II, indicating that most acetylation is likely to precede transcription and not depend upon it. TAF1 function has been linked to Bdf1, which binds TFIID and acetylated histone H4 tails, but no linkage between TAF1 and the H4 HAT Esa1 has been established. Here, we present evidence for such a linkage through Bdf1.

A role for gcn5-mediated global histone acetylation in transcriptional regulation

Transcriptional activators often require histone acetyltransferases (HATs) for full activity. The common explanation is that activators directly recruit HATs to gene promoters to locally hyperacetylate histones and thereby facilitate transcription complex formation. However, in addition to being targeted to specific loci, HATs such as Gcn5 also modify histones genome-wide. Here we provide evidence for a role of this global HAT activity in regulated transcription. We show that activation by direct recruitment of the transcriptional machinery neither recruits Gcn5 nor induces changes in histone acetylation yet can strongly depend on Gcn5 at promoters showing a high basal state of Gcn5-mediated histone acetylation. We also show that Gcn5 dependency varies among core promoters and is influenced by the strength of interaction used to recruit the machinery and by the affinity of the latter for the core promoter. These data support a role for global Gcn5 HAT activity in modulating transcription independently of its known coactivator function.

Evidence that Spt2/Sin1, an HMG-like factor, plays roles in transcription elongation, chromatin structure, and genome stability in Saccharomyces cerevisiae

Spt2/Sin1 is a DNA binding protein with HMG-like domains that has been suggested to play a role in chromatin-mediated transcription in Saccharomyces cerevisiae. Previous studies have suggested models in which Spt2 plays an inhibitory role in the initiation of transcription of certain genes. In this work, we have taken several approaches to study Spt2 in greater detail. Our results have identified previously unknown genetic interactions between spt2Delta and mutations in genes encoding transcription elongation factors, including members of the PAF and HIR/HPC complexes. In addition, genome-wide and gene-specific chromatin immunoprecipitation analyses suggest that Spt2 is primarily associated with coding regions in a transcription-dependent fashion. Furthermore, our results show that Spt2, like other elongation factors, is required for the repression of transcription from a cryptic promoter within a coding region and that Spt2 is also required for repression of recombination within transcribed regions. Finally, we provide evidence that Spt2 plays a role in regulating the levels of histone H3 over transcribed regions. Taken together, our results suggest a direct link for Spt2 with transcription elongation, chromatin dynamics, and genome stability.

Chromatin profiling, DamID and the emerging landscape of gene expression

Determining how genes are normally expressed throughout development and how they are mis-regulated in cancer is challenging. The availability of complete genome sequences, the advances in microarray technologies, and the development of novel functional genomic techniques such as 'chromatin profiling' facilitate dissection of the interplay among transcriptional networks and reveals chromosome organization in vivo. Recently, a novel methylation-based tagging technique, termed DamID (DNA adenine methyltransferase identification), has emerged as a powerful tool to decipher transcriptional networks, to study chromatin-associated proteins, and to monitor higher-order chromatin organization on a genome-wide scale. The molecular picture that emerges from DamID and similar studies is that genomes integrate inputs from both genetic and epigenetic machineries to dynamically regulate gene expression.

Contribution of the histone H3 and H4 amino termini to Gcn4p- and Gcn5p-mediated transcription in yeast

Histone amino termini are post-translationally modified by both transcriptional coactivators and corepressors, but the extent to which the relevant histone modifications contribute to gene expression, and the mechanisms by which they do so, are incompletely understood. To address this issue, we have examined the contributions of the histone H3 and H4 amino termini, and of the coactivator and histone acetyltransferase Gcn5p, to activation of a small group of Gcn4p-activated genes. The histone H3 tail exerts a modest (about 2-fold) but significant effect on activation that correlates with a requirement for Gcn5p and is distributed over multiple lysine residues. The H4 tail also plays a positive role in activation of some of those genes tested, but this does not correlate as closely with Gcn5p coactivation. Microarray experiments did not reveal a close correspondence between those genes activated by Gcn4p and genes requiring the H3 or H4 tail, and analysis of published microarray data indicates that Gcn4p-regulated genes are not in general strongly dependent on Gcn5p. However, a large fraction of genes activated by Gcn4p were found to be repressed by the H3 and H4 amino termini under non-inducing conditions, indicating that one role for Gcn4p is to overcome repression mediated by the histone tails.

Mutations in the extra sex combs and Enhancer of Polycomb genes increase homologous recombination in somatic cells of Drosophila melanogaster

We found that heterozygous mutant alleles of E(Pc) and esc increased homologous recombination from an allelic template in somatic cells in a P-element-induced double-strand break repair assay. Flies heterozygous for mutant alleles of these genes showed increased genome stability and decreased levels of apoptosis in imaginal discs and a concomitant increase in survival following ionizing radiation. We propose that this was caused by a genomewide increase in homologous recombination in somatic cells. A double mutant of E(Pc) and esc had no additive effect, showing that these genes act in the same pathway. Finally, we found that a heterozygous deficiency for the histone deacetylase, Rpd3, masked the radiation-resistant phenotype of both esc and E(Pc) mutants. These findings provide evidence for a gene dosage-dependent interaction between the esc/E(z) complex and the Tip60 histone acetyltransferase complex. We propose that esc and E(Pc) mutants enhance homologous recombination by modulating the histone acetylation status of histone H4 at the double-strand break.

Systems biology in the cell nucleus

The mammalian nucleus is arguably the most complex cellular organelle. It houses the vast majority of an organism's genetic material and is the site of all major genome regulatory processes. Reductionist approaches have been spectacularly successful at dissecting at the molecular level many of the key processes that occur within the nucleus, particularly gene expression. At the same time, the limitations of analyzing single nuclear processes in spatial and temporal isolation and the validity of generalizing observations of single gene loci are becoming evident. The next level of understanding of genome function is to integrate our knowledge of their sequences and the molecular mechanisms involved in nuclear processes with our insights into the spatial and temporal organization of the nucleus and to elucidate the interplay between protein and gene networks in regulatory circuits. To do so, catalogues of genomes and proteomes as well as a precise understanding of the behavior of molecules in living cells are required. Converging technological developments in genomics, proteomics, dynamics and computation are now leading towards such an integrated biological understanding of genome biology and nuclear function.

Genomic views of chromatin

With the availability of complete genome sequences for a number of organisms, a major challenge has become to understand how chromatin and its epigenetic modifications regulate genome function. High-throughput microarray and sequencing technologies are being combined with biochemical and immunological enrichment methods to obtain genome-scale views of chromatin in a variety of organisms. The data pinpoint novel, genomic elements and expansive chromatin domains, and offer insight into the functions of histone modifications. In parallel, state-of-the-art imaging techniques are being used to investigate higher-order chromatin organization, and are beginning to bridge our understanding of chromatin biology with that of chromosome structure.

Preferential occupancy of histone variant H2AZ at inactive promoters influences local histone modifications and chromatin remodeling

The yeast histone variant H2AZ (Htz1) is implicated in transcription activation, prevention of the ectopic spread of heterochromatin, and genome integrity. Our genome-wide localization analysis revealed that Htz1 is widely, but nonrandomly, distributed throughout the genome in an SWR1-dependent manner. We found that Htz1 is enriched in intergenic regions compared with coding regions. Its occupancy is inversely proportional to transcription rates and the enrichment of the RNA polymerase II under different growth conditions. However, Htz1 does not seem to directly regulate transcription repression genome-wide; instead, the presence of Htz1 under the inactivated condition is essential for optimal activation of a subset of genes. In addition, Htz1 is not generally responsible for nucleosome positioning, even at those promoters where Htz1 is highly enriched. Finally, using a biochemical approach, we demonstrate that incorporation of Htz1 into nucleosomes inhibits activities of histone modifiers associated with transcription, Dot1, Set2, and NuA4 and reduces the nucleosome mobilization driven by chromatin remodeling complexes. These lines of evidence collectively suggest that Htz1 may serve to mark quiescent promoters for proper activation.

Subnuclear compartmentalization of sequence-specific transcription factors and regulation of eukaryotic gene expression

To date, the majority of the research regarding eukaryotic transcription factors has focused on characterizing their function primarily through in vitro methods. These studies have revealed that transcription factors are essentially modular structures, containing separate regions that participate in such activities as DNA binding, protein-protein interaction, and transcriptional activation or repression. To fully comprehend the behavior of a given transcription factor, however, these domains must be analyzed in the context of the entire protein, and in certain cases the context of a multiprotein complex. Furthermore, it must be appreciated that transcription factors function in the nucleus, where they must contend with a variety of factors, including the nuclear architecture, chromatin domains, chromosome territories, and cell-cycle-associated processes. Recent examinations of transcription factors in the nucleus have clarified the behavior of these proteins in vivo and have increased our understanding of how gene expression is regulated in eukaryotes. Here, we review the current knowledge regarding sequence-specific transcription factor compartmentalization within the nucleus and discuss its impact on the regulation of such processes as activation or repression of gene expression and interaction with coregulatory factors.

Genome-wide analysis of HDAC function

This article focuses on new developments in the genome-wide analysis of histone deacetylase (HDAC) function in yeast. HDACs are highly conserved in many organisms; therefore, their basic functions can be investigated using experimentally tractable model organisms, such as the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. New microarray techniques have enabled the systematic study of HDACs by identifying their direct and indirect gene targets in addition to their physiological functions and enzymatic specificity. These new approaches have already provided new surprising insights into the basic function of HDACs.

Variant histone H2A.Z is globally localized to the promoters of inactive yeast genes and regulates nucleosome positioning

H2A.Z is an evolutionary conserved histone variant involved in transcriptional regulation, antisilencing, silencing, and genome stability. The mechanism(s) by which H2A.Z regulates these various biological functions remains poorly defined, in part due to the lack of knowledge regarding its physical location along chromosomes and the bearing it has in regulating chromatin structure. Here we mapped H2A.Z across the yeast genome at an approximately 300-bp resolution, using chromatin immunoprecipitation combined with tiling microarrays. We have identified 4,862 small regions--typically one or two nucleosomes wide--decorated with H2A.Z. Those "Z loci" are predominantly found within specific nucleosomes in the promoter of inactive genes all across the genome. Furthermore, we have shown that H2A.Z can regulate nucleosome positioning at the GAL1 promoter. Within HZAD domains, the regions where H2A.Z shows an antisilencing function, H2A.Z is localized in a wider pattern, suggesting that the variant histone regulates a silencing and transcriptional activation via different mechanisms. Our data suggest that the incorporation of H2A.Z into specific promoter-bound nucleosomes configures chromatin structure to poise genes for transcriptional activation. The relevance of these findings to higher eukaryotes is discussed.

The Proteasome Regulatory Particle Alters the SAGA Coactivator to Enhance Its Interactions with Transcriptional Activators

Promoter recruitment of the Saccharomyces cerevisiae SAGA histone acetyltransferase complex is required for RNA polymerase II-dependent transcription of several genes. SAGA is targeted to promoters through interactions with sequence-specific DNA binding transcriptional activators and facilitates preinitiation-complex assembly and transcription. Here, we show that the 19S proteasome regulatory particle (19S RP) alters SAGA to stimulate its interaction with transcriptional activators. The ATPase components of the 19S RP are required for stimulation of SAGA/activator interactions and enhance SAGA recruitment to promoters. Proteasomal ATPases genetically interact with SAGA, and their inhibition reduces global histone H3 acetylation levels and SAGA recruitment to target promoters in vivo. These results indicate that the 19S RP modulates SAGA complex using its ATPase components, thereby facilitating subsequent transcription events at promoters.

Individual histone deacetylases in Drosophila modulate transcription of distinct genes

Lysine residues on the N-terminal tails of histones in chromatin are the primary targets of histone acetyltransferases (HATs) and histone deacetylases (HDACs) in eukaryotes. Regulation of histone acetylation by these two classes of enzymes plays significant roles in controlling transcriptional activity in cells. Eukaryotic organisms have several different HDACs, but the biological roles of each HDAC are still not clear. To understand the physiological functions of HDACs, we characterized six different Drosophila HDACs, including Rpd3, HDAC3, HDAC4, HDAC6-S, HDAC6-L, and Sir2, by developmental expression pattern, transcriptional profiles of target genes, and sensitivity to HDAC inhibitors. We found that each HDAC has a distinct temporal expression pattern and regulates transcription of a unique set of genes. Furthermore, we demonstrated differential sensitivity of HDACs to inhibitors. These results show that each individual HDAC plays different roles in regulating genes involved in various biological processes.

Genome-wide map of nucleosome acetylation and methylation in yeast

Eukaryotic genomes are packaged into nucleosomes whose position and chemical modification state can profoundly influence regulation of gene expression. We profiled nucleosome modifications across the yeast genome using chromatin immunoprecipitation coupled with DNA microarrays to produce high-resolution genome-wide maps of histone acetylation and methylation. These maps take into account changes in nucleosome occupancy at actively transcribed genes and, in doing so, revise previous assessments of the modifications associated with gene expression. Both acetylation and methylation of histones are associated with transcriptional activity, but the former occurs predominantly at the beginning of genes, whereas the latter can occur throughout transcribed regions. Most notably, specific methylation events are associated with the beginning, middle, and end of actively transcribed genes. These maps provide the foundation for further understanding the roles of chromatin in gene expression and genome maintenance.

Multi-gene epigenetic silencing of tumor suppressor genes in T-cell lymphoma cells; delayed expression of the p16 protein upon reversal of the silencing

To understand better T-cell lymphomagenesis, we examined promoter CpG methylation and mRNA expression of closely related genes encoding p16, p15, and p14 tumor suppressor genes in cultured malignant T-cells that were derived from cutaneous, adult type, and anaplastic lymphoma kinase (ALK)-expressing T-cell lymphomas. p16 gene was epigenetically silenced in all but one of the 10 malignant T-cell lines examined, p15 gene silenced in roughly half of the lines, and p14 was the least frequently affected. Extensive methylation of the p16 promoter was seen in six out of 10 cutaneous T-cell lymphoma patient samples and corresponded with lack of p16 protein expression in the cases examined. Treatment of cultured T-cells with the DNA methyltransferase inhibitor, 5-aza-2-deoxy-cytidine, resulted in reversal of the p16 gene silencing. However, expression of p16 protein was delayed in relationship to p16 promoter demethylation and required up to 3 weeks to occur, seemingly reflecting late activation of the p16 gene. These findings indicate that epigenetic silencing affects in T-cell malignancies, often simultaneously, several tumor suppressor genes that impact on key cell functions. The observed differential silencing of p16 and p14, and to a lesser degree p15 gene, indicates that the silencing is governed by precise, promoter region-specific mechanisms. The study provides also further rationale for treatment of at least some types of T-cell lymphomas with DNA methyltransferase inhibitors to target the epigenetically silenced tumor suppressor genes.

Functional interaction between the Drosophila knirps short range transcriptional repressor and RPD3 histone deacetylase

Knirps and other short range transcriptional repressors play critical roles in patterning the early Drosophila embryo. These repressors are known to bind the C-terminal binding protein corepressor, but their mechanism of action is poorly understood. We purified functional recombinant Knirps protein from transgenic embryos to identify possible cofactors that contribute to the activity of this protein. The protein migrates in a complex of approximately 450 kDa and was found to copurify with the Rpd3 histone deacetylase protein during a double affinity purification procedure. Association of Rpd3 with Knirps was dependent on the presence of the C-terminal binding protein-dependent repression domain of Knirps. Previous studies of an rpd3 mutant had not shown defects in the pattern of expression of even-skipped, a target of the Knirps repressor. However, in embryos doubly heterozygous for knirps and rpd3, a marked increase in the frequency of defects in the Knirps-regulated posterior domain of even-skipped expression was found, indicating that Rpd3 contributes to Knirps repression activity in vivo. This finding implicates deacetylation in the mechanism of short range repression in Drosophila.

Single-nucleosome mapping of histone modifications in S. cerevisiae

Covalent modification of histone proteins plays a role in virtually every process on eukaryotic DNA, from transcription to DNA repair. Many different residues can be covalently modified, and it has been suggested that these modifications occur in a great number of independent, meaningful combinations. Published low-resolution microarray studies on the combinatorial complexity of histone modification patterns suffer from confounding effects caused by the averaging of modification levels over multiple nucleosomes. To overcome this problem, we used a high-resolution tiled microarray with single-nucleosome resolution to investigate the occurrence of combinations of 12 histone modifications on thousands of nucleosomes in actively growing S. cerevisiae. We found that histone modifications do not occur independently; there are roughly two groups of co-occurring modifications. One group of lysine acetylations shows a sharply defined domain of two hypo-acetylated nucleosomes, adjacent to the transcriptional start site, whose occurrence does not correlate with transcription levels. The other group consists of modifications occurring in gradients through the coding regions of genes in a pattern associated with transcription. We found no evidence for a deterministic code of many discrete states, but instead we saw blended, continuous patterns that distinguish nucleosomes at one location (e.g., promoter nucleosomes) from those at another location (e.g., over the 3′ ends of coding regions). These results are consistent with the idea of a simple, redundant histone code, in which multiple modifications share the same role.

High-resolution microarrays were used to investigate 12 histone modifications across thousands of yeast nucelosomes in vivo. Two main groups co-occurred, consistent with the redundant histone code hypothesis.

Genomewide analysis of nucleosome density histone acetylation and HDAC function in fission yeast

We have conducted a genomewide investigation into the enzymatic specificity, expression profiles, and binding locations of four histone deacetylases (HDACs), representing the three different phylogenetic classes in fission yeast (Schizosaccharomyces pombe). By directly comparing nucleosome density, histone acetylation patterns and HDAC binding in both intergenic and coding regions with gene expression profiles, we found that Sir2 (class III) and Hos2 (class I) have a role in preventing histone loss; Clr6 (class I) is the principal enzyme in promoter-localized repression. Hos2 has an unexpected role in promoting high expression of growth-related genes by deacetylating H4K16Ac in their open reading frames. Clr3 (class II) acts cooperatively with Sir2 throughout the genome, including the silent regions: rDNA, centromeres, mat2/3 and telomeres. The most significant acetylation sites are H3K14Ac for Clr3 and H3K9Ac for Sir2 at their genomic targets. Clr3 also affects subtelomeric regions which contain clustered stress- and meiosis-induced genes. Thus, this combined genomic approach has uncovered different roles for fission yeast HDACs at the silent regions in repression and activation of gene expression.

Localized histone acetylation and deacetylation triggered by the homologous recombination pathway of double-strand DNA repair

Many recent studies have demonstrated recruitment of chromatin-modifying enzymes to double-strand breaks. Instead, we wanted to examine chromatin modifications during the repair of these double-strand breaks. We show that homologous recombination triggers the acetylation of N-terminal lysines on histones H3 and H4 flanking a double-strand break, followed by deacetylation of H3 and H4. Consistent with a requirement for acetylation and deacetylation during homologous recombination, Saccharomyces cerevisiae with substitutions of the acetylatable lysines of histone H4, deleted for the N-terminal tail of histone H3 or H4, deleted for the histone acetyltransferase GCN5 gene or the histone deacetylase RPD3 gene, shows inviability following induction of an HO lesion that is repaired primarily by homologous recombination. Furthermore, the histone acetyltransferases Gcn5 and Esa1 and the histone deacetylases Rpd3, Sir2, and Hst1 are recruited to the HO lesion during homologous recombinational repair. We have also observed a distinct pattern of histone deacetylation at the donor locus during homologous recombination. Our results demonstrate that dynamic changes in histone acetylation accompany homologous recombination and that the ability to modulate histone acetylation is essential for viability following homologous recombination.

Birth of a metabolic gene cluster in yeast by adaptive gene relocation

Although most eukaryotic genomes lack operons, they contain some physical clusters of genes that are related in function despite being unrelated in sequence. How these clusters are formed during evolution is unknown. The DAL cluster is the largest metabolic gene cluster in yeast and consists of six adjacent genes encoding proteins that enable Saccharomyces cerevisiae to use allantoin as a nitrogen source. We show here that the DAL cluster was assembled, quite recently in evolutionary terms, through a set of genomic rearrangements that happened almost simultaneously. Six of the eight genes involved in allantoin degradation, which were previously scattered around the genome, became relocated to a single subtelomeric site in an ancestor of S. cerevisiae and Saccharomyces castellii. These genomic rearrangements coincided with a biochemical reorganization of the purine degradation pathway, which switched to importing allantoin instead of urate. This change eliminated urate oxidase, one of several oxygen-consuming enzymes that were lost by yeasts that can grow vigorously in anaerobic conditions. The DAL cluster is located in a domain of modified chromatin involving both H2A.Z histone exchange and Hst1-Sum1-mediated histone deacetylation, and it may be a coadapted gene complex formed by epistatic selection.

Mapping of genetic and epigenetic regulatory networks using microarrays

The highly coordinated expression of thousands of genes in an organism is regulated by the concerted action of hundreds of transcription factors and chromatin proteins, as well as by epigenetic mechanisms. Understanding the architecture of these vastly complex regulatory networks is one of the main challenges in the postgenomic era. New microarray-based techniques have become available for the genome-wide mapping of in vivo protein-DNA interactions and epigenetic marks. Data sets obtained with these techniques begin to offer the first comprehensive views of genetic and epigenetic regulatory networks.

A non-random walk through the genome

Genes showing particular expression patterns are not randomly distributed in the genome but are clustered into neighborhoods. This organization may be related to chromatin and the structure of the nucleus.

Recent publications on a wide range of eukaryotes indicate that genes showing particular expression patterns are not randomly distributed in the genome but are clustered into contiguous regions that we call neighborhoods. It seems probable that this organization is related to chromatin and the structure of the nucleus.

Genome-wide analysis of the context-dependence of regulatory networks

Two recent studies in yeast have helped reveal the relatively small number of transcription-factor control strategies that cells employ to maximize their regulatory options using only a small number of components.

Genome-wide analytical tools are now allowing the discovery of the design rules that govern regulatory networks. Two recent studies in yeast have helped reveal the relatively small number of transcription-factor control strategies that cells employ to maximize their regulatory options using only a small number of components.

Histone modifications: from genome-wide maps to functional insights

A large number of histone modifications has been implicated in the regulation of gene expression. Together, these modifications have the potential to form a complex combinatorial regulatory code. Genome-wide mapping approaches provide new opportunities to decipher this code, but they may suffer from systematic biases. Integration of datasets and improved technologies will provide the way forward.