DNA polymerase epsilon, acetylases and remodellers cooperate to form a specialized chromatin structure at a tRNA insulator

TFIIIC-based chromatin insulators through eukaryotic evolution

Eukaryotic chromosomes are divided into domains with distinct structural and functional properties, such as differing levels of chromatin compaction and gene transcription. Domains of relatively compact chromatin and minimal transcription are termed heterochromatic, whereas euchromatin is more open and actively transcribed. Insulators separate these domains and maintain their distinct features. Disruption of insulators can cause diseases such as cancer. Many insulators contain tRNA genes (tDNAs), examples of which have been shown to block the spread of activating or silencing activities. This characteristic of specific tDNAs is conserved through evolution, such that human tDNAs can serve as barriers to the spread of silencing in fission yeast. Here we demonstrate that tDNAs from the methylotrophic fungus Pichia pastoris can function effectively as insulators in distantly-related budding yeast. Key to the function of tDNAs as insulators is TFIIIC, a transcription factor that is also required for their expression. TFIIIC binds additional loci besides tDNAs, some of which have insulator activity. Although the mechanistic basis of TFIIIC-based insulation has been studied extensively in yeast, it is largely uncharacterized in metazoa. Utilising publicly-available genome-wide ChIP-seq data, we consider the extent to which mechanisms conserved from yeast to man may suffice to allow efficient insulation by TFIIIC in the more challenging chromatin environments of metazoa and suggest features that may have been acquired during evolution to cope with new challenges. We demonstrate the widespread presence at human tDNAs of USF1, a transcription factor with well-established barrier activity in vertebrates. We predict that tDNA-based insulators in higher organisms have evolved through incorporation of modules, such as binding sites for factors like USF1 and CTCF that are absent from yeasts, thereby strengthening function and providing opportunities for regulation between cell types.

Measuring the buffering capacity of gene silencing in Saccharomyces cerevisiae

Gene silencing, once established, is stably maintained for several generations. Despite the high fidelity of the inheritance of the silent state, individual components of silenced chromatin are in constant flux. Models suggest that silent loci can tolerate fluctuations in Sir proteins and histone acetylation levels, but the level of tolerance is unknown. To understand the quantitative relationships between H4K16 acetylation, Sir proteins, and silencing, we developed assays to quantitatively alter a H4K16 acetylation mimic allele and Sir protein levels and measure the effects of these changes on silencing. Our data suggest that a two- to threefold change in levels of histone marks and specific Sir proteins affects the stability of the silent state of a large chromatin domain.

Gene silencing in budding yeast is mediated by Sir protein binding to unacetylated nucleosomes to form a chromatin structure that inhibits transcription. Transcriptional silencing is characterized by the high-fidelity transmission of the silent state. Despite its relative stability, the constituent parts of the silent state are in constant flux, giving rise to a model that silent loci can tolerate such fluctuations without functional consequences. However, the level of tolerance is unknown, and we developed methods to measure the threshold of histone acetylation that causes the silent chromatin state to switch to the active state as well as to measure the levels of the enzymes and structural proteins necessary for silencing. We show that loss of silencing required 50 to 75% acetyl-mimic histones, though the precise levels were influenced by silencer strength and upstream activating sequence (UAS) enhancer/promoter strength. Measurements of repressor protein levels necessary for silencing showed that reducing SIR4 gene dosage two- to threefold significantly weakened silencing, though reducing the gene copy numbers for Sir2 or Sir3 to the same extent did not significantly affect silencing suggesting that Sir4 was a limiting component in gene silencing. Calculations suggest that a mere twofold reduction in the ability of acetyltransferases to acetylate nucleosomes across a large array of nucleosomes may be sufficient to generate a transcriptionally silent domain.

COMPASS and SWI/SNF complexes in development and disease

The Trithorax group (TrxG) of proteins is a large family of epigenetic regulators that form multiprotein complexes to counteract repressive developmental gene expression programmes established by the Polycomb group of proteins and to promote and maintain an active state of gene expression. Recent studies are providing new insights into how two crucial families of the TrxG - the COMPASS family of histone H3 lysine 4 methyltransferases and the SWI/SNF family of chromatin remodelling complexes - regulate gene expression and developmental programmes, and how misregulation of their activities through genetic abnormalities leads to pathologies such as developmental disorders and malignancies.

Genome-wide analyses of chromatin interactions after the loss of Pol I, Pol II, and Pol III

BACKGROUND

The relationship between transcription and the 3D chromatin structure is debated. Multiple studies have shown that transcription affects global Cohesin binding and 3D genome structures. However, several other studies have indicated that inhibited transcription does not alter chromatin conformations.

RESULTS

We provide the most comprehensive evidence to date to demonstrate that transcription plays a relatively modest role in organizing the local, small-scale chromatin structures in mammalian cells. We show degraded Pol I, Pol II, and Pol III proteins in mESCs cause few or no changes in large-scale 3D chromatin structures, selected RNA polymerases with a high abundance of binding sites or active promoter-associated interactions appear to be relatively more affected after the degradation, transcription inhibition alters local, small loop domains, as indicated by high-resolution chromatin interaction maps, and loops with bound Pol II but without Cohesin or CTCF are identified and found to be largely unchanged after transcription inhibition. Interestingly, Pol II depletion for a longer time significantly affects the chromatin accessibility and Cohesin occupancy, suggesting that RNA polymerases are capable of affecting the 3D genome indirectly. These direct and indirect effects explain the previous inconsistent findings on the influence of transcription inhibition on the 3D genome.

CONCLUSIONS

We conclude that Pol I, Pol II, and Pol III loss alters local, small-scale chromatin interactions in mammalian cells, suggesting that the 3D chromatin structures are pre-established and relatively stable.

SUMOylated SNF2PH promotes variant surface glycoprotein expression in bloodstream trypanosomes

SUMOylation is a post-translational modification that positively regulates monoallelic expression of the trypanosome variant surface glycoprotein (VSG). The presence of a highly SUMOylated focus associated with the nuclear body, where the VSG gene is transcribed, further suggests an important role of SUMOylation in regulating VSG expression. Here, we show that SNF2PH, a SUMOylated plant homeodomain (PH)-transcription factor, is upregulated in the bloodstream form of the parasite and enriched at the active VSG telomere. SUMOylation promotes the recruitment of SNF2PH to the VSG promoter, where it is required to maintain RNA polymerase I and thus to regulate VSG transcript levels. Further, ectopic overexpression of SNF2PH in insect forms, but not of a mutant lacking the PH domain, induces the expression of bloodstream stage-specific surface proteins. These data suggest that SNF2PH SUMOylation positively regulates VSG monoallelic transcription, while the PH domain is required for the expression of bloodstream-specific surface proteins. Thus, SNF2PH functions as a positive activator, linking expression of infective form surface proteins and VSG regulation, thereby acting as a major regulator of pathogenicity.

tRNA Genes Affect Chromosome Structure and Function via Local Effects

The genome is packaged and organized in an ordered, nonrandom manner, and specific chromatin segments contact nuclear substructures to mediate this organization. tRNA genes (tDNAs) are binding sites for transcription factors and architectural proteins and are thought to play an important role in the organization of the genome. In this study, we investigate the roles of tDNAs in genomic organization and chromosome function by editing a chromosome so that it lacked any tDNAs. Surprisingly our analyses of this tDNA-less chromosome show that loss of tDNAs does not grossly affect chromatin architecture or chromosome tethering and mobility. However, loss of tDNAs affects local nucleosome positioning and the binding of SMC proteins at these loci. The absence of tDNAs also leads to changes in centromere clustering and a reduction in the frequency of long-range HML-HMR heterochromatin clustering with concomitant effects on gene silencing. We propose that the tDNAs primarily affect local chromatin structure, which results in effects on long-range chromosome architecture.

Regulation of tRNA gene transcription by the chromatin structure and nucleosome dynamics

The short, non-coding genes transcribed by the RNA polymerase (pol) III, necessary for survival of a cell, need to be repressed under the stress conditions in vivo. The pol III-transcribed genes have adopted several novel chromatin-based regulatory mechanisms to their advantage. In the budding yeast, the sub-nucleosomal size tRNA genes are found in the nucleosome-free regions, flanked by positioned nucleosomes at both the ends. With their chromosomes-wide distribution, all tRNA genes have a different chromatin context. A single nucleosome dynamics controls the accessibility of the genes for transcription. This dynamics operates under the influence of several chromatin modifiers in a gene-specific manner, giving the scope for differential regulation of even the isogenes within a tRNA gene family. The chromatin structure around the pol III-transcribed genes provides a context conducive for steady-state transcription as well as gene-specific transcriptional regulation upon signaling from the environmental cues. This article is part of a Special Issue entitled: SI: Regulation of tRNA synthesis and modification in physiological conditions and disease edited by Dr. Boguta Magdalena.

The Nuts and Bolts of Transcriptionally Silent Chromatin in Saccharomyces cerevisiae

Transcriptional silencing in Saccharomyces cerevisiae occurs at several genomic sites including the silent mating-type loci, telomeres, and the ribosomal DNA (rDNA) tandem array. Epigenetic silencing at each of these domains is characterized by the absence of nearly all histone modifications, including most prominently the lack of histone H4 lysine 16 acetylation. In all cases, silencing requires Sir2, a highly-conserved NAD(+)-dependent histone deacetylase. At locations other than the rDNA, silencing also requires additional Sir proteins, Sir1, Sir3, and Sir4 that together form a repressive heterochromatin-like structure termed silent chromatin. The mechanisms of silent chromatin establishment, maintenance, and inheritance have been investigated extensively over the last 25 years, and these studies have revealed numerous paradigms for transcriptional repression, chromatin organization, and epigenetic gene regulation. Studies of Sir2-dependent silencing at the rDNA have also contributed to understanding the mechanisms for maintaining the stability of repetitive DNA and regulating replicative cell aging. The goal of this comprehensive review is to distill a wide array of biochemical, molecular genetic, cell biological, and genomics studies down to the "nuts and bolts" of silent chromatin and the processes that yield transcriptional silencing.

Methods to Study the Atypical Roles of DNA Repair and SMC Proteins in Gene Silencing

Silenced heterochromatin influences all nuclear processes including chromosome structure, nuclear organization, transcription, replication, and repair. Proteins that mediate silencing affect all of these nuclear processes. Similarly proteins involved in replication, repair, and chromosome structure play a role in the formation and maintenance of silenced heterochromatin. In this chapter we describe a handful of simple tools and methods that can be used to study the atypical role of proteins in gene silencing.

The Many Roles of BAF (mSWI/SNF) and PBAF Complexes in Cancer

During the last decade, a host of epigenetic mechanisms were found to contribute to cancer and other human diseases. Several genomic studies have revealed that ∼20% of malignancies have alterations of the subunits of polymorphic BRG-/BRM-associated factor (BAF) and Polybromo-associated BAF (PBAF) complexes, making them among the most frequently mutated complexes in cancer. Recurrent mutations arise in genes encoding several BAF/PBAF subunits, including ARID1A, ARID2, PBRM1, SMARCA4, and SMARCB1 These subunits share some degree of conservation with subunits from related adenosine triphosphate (ATP)-dependent chromatin remodeling complexes in model organisms, in which a large body of work provides insight into their roles in cancer. Here, we review the roles of BAF- and PBAF-like complexes in these organisms, and relate these findings to recent discoveries in cancer epigenomics. We review several roles of BAF and PBAF complexes in cancer, including transcriptional regulation, DNA repair, and regulation of chromatin architecture and topology. More recent results highlight the need for new techniques to study these complexes.

Mating-type Gene Switching in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has two alternative mating types designated MATa and MATα. These are distinguished by about 700 bp of unique sequences, Ya or Yα, including divergent promoter sequences and part of the open reading frames of genes that regulate mating phenotype. Homothallic budding yeast, carrying an active HO endonuclease gene, HO, can switch mating type through a recombination process known as gene conversion, in which a site-specific double-strand break (DSB) created immediately adjacent to the Y region results in replacement of the Y sequences with a copy of the opposite mating type information, which is harbored in one of two heterochromatic donor loci, HMLα or HMRa. HO gene expression is tightly regulated to ensure that only half of the cells in a lineage switch to the opposite MAT allele, thus promoting conjugation and diploid formation. Study of the silencing of these loci has provided a great deal of information about the role of the Sir2 histone deacetylase and its associated Sir3 and Sir4 proteins in creating heterochromatic regions. MAT switching has been examined in great detail to learn about the steps in homologous recombination. MAT switching is remarkably directional, with MATa recombining preferentially with HMLα and MATα using HMRa. Donor preference is controlled by a cis-acting recombination enhancer located near HML. RE is turned off in MATα cells but in MATa binds multiple copies of the Fkh1 transcription factor whose forkhead-associated phosphothreonine binding domain localizes at the DSB, bringing HML into conjunction with MATa.

Global regulation of heterochromatin spreading by Leo1

Heterochromatin plays important roles in eukaryotic genome regulation. However, the repressive nature of heterochromatin combined with its propensity to self-propagate necessitates robust mechanisms to contain heterochromatin within defined boundaries and thus prevent silencing of expressed genes. Here we show that loss of the PAF complex (PAFc) component Leo1 compromises chromatin boundaries, resulting in invasion of heterochromatin into flanking euchromatin domains. Similar effects are seen upon deletion of other PAFc components, but not other factors with related functions in transcription-associated chromatin modification, indicating a specific role for PAFc in heterochromatin regulation. Loss of Leo1 results in reduced levels of H4K16 acetylation at boundary regions, while tethering of the H4K16 acetyltransferase Mst1 to boundary chromatin suppresses heterochromatin spreading in leo1Δ cells, suggesting that Leo1 antagonises heterochromatin spreading by promoting H4K16 acetylation. Our findings reveal a previously undescribed role for PAFc in regulating global heterochromatin distribution.

Heterochromatin formation via recruitment of DNA repair proteins

Double-strand-break repair proteins interact with and recruit Sir proteins to ectopic sites in the genome. Recruitment results in gene silencing, which depends on Sir proteins, as well as on histone H2A modification. Silencing also results in the localization of the locus to the nuclear periphery.

Heterochromatin formation and nuclear organization are important in gene regulation and genome fidelity. Proteins involved in gene silencing localize to sites of damage and some DNA repair proteins localize to heterochromatin, but the biological importance of these correlations remains unclear. In this study, we examined the role of double-strand-break repair proteins in gene silencing and nuclear organization. We find that the ATM kinase Tel1 and the proteins Mre11 and Esc2 can silence a reporter gene dependent on the Sir, as well as on other repair proteins. Furthermore, these proteins aid in the localization of silenced domains to specific compartments in the nucleus. We identify two distinct mechanisms for repair protein–mediated silencing—via direct and indirect interactions with Sir proteins, as well as by tethering loci to the nuclear periphery. This study reveals previously unknown interactions between repair proteins and silencing proteins and suggests insights into the mechanism underlying genome integrity.

Heterochromatin structure: lessons from the budding yeast

The eukaryotic genome can be roughly divided into euchromatin and heterochromatin domains that are structurally and functionally distinct. Heterochromatin is characterized by its high compactness and its inhibitory effect on DNA transactions such as gene expression. Formation of heterochromatin involves special histone modifications and the recruitment and spread of silencing complexes and causes changes in the primary and higher order structures of chromatin. The past two decades have seen dramatic advances in dissecting the molecular aspects of heterochromatin because of the identification of the histone code for heterochromatin as well as its writers and erasers (histone-modifying enzymes) and readers (silencing factors recognizing histone modifications). How heterochromatic histone modifications and silencing factors contribute to the special primary and higher order structures of heterochromatin has begun to be understood. The budding yeast Saccharomyces cerevisiae has long been used as a model organism for heterochromatin studies. Results from these studies have contributed significantly to the elucidation of the general principles governing the formation, maintenance, and function of heterochromatin. This review is focused on investigations into the structural aspects of heterochromatin in S. cerevisiae. Current understanding of other aspects of heterochromatin including how it promotes gene silencing and its epigenetic inheritance is briefly summarized.

A novel tRNA variable number tandem repeat at human chromosome 1q23.3 is implicated as a boundary element based on conservation of a CTCF motif in mouse

The human genome contains numerous large tandem repeats, many of which remain poorly characterized. Here we report a novel transfer RNA (tRNA) tandem repeat on human chromosome 1q23.3 that shows extensive copy number variation with 9-43 repeat units per allele and displays evidence of meiotic and mitotic instability. Each repeat unit consists of a 7.3 kb GC-rich sequence that binds the insulator protein CTCF and bears the chromatin hallmarks of a bivalent domain in human embryonic stem cells. A tRNA containing tandem repeat composed of at least three 7.6-kb GC-rich repeat units reside within a syntenic region of mouse chromosome 1. However, DNA sequence analysis reveals that, with the exception of the tRNA genes that account for less than 6% of a repeat unit, the remaining 7.2 kb is not conserved with the notable exception of a 24 base pair sequence corresponding to the CTCF binding site, suggesting an important role for this protein at the locus.

Silencing near tRNA genes is nucleosome-mediated and distinct from boundary element function

Transfer RNA (tRNA) genes and other RNA polymerase III transcription units are dispersed in high copy throughout nuclear genomes, and can antagonize RNA polymerase II transcription in their immediate chromosomal locus. Previous work in Saccharomyces cerevisiae found that this local silencing required subnuclear clustering of the tRNA genes near the nucleolus. Here we show that the silencing also requires nucleosome participation, though the nature of the nucleosome interaction appears distinct from other forms of transcriptional silencing. Analysis of an extensive library of histone amino acid substitutions finds a large number of residues that affect the silencing, both in the histone N-terminal tails and on the nucleosome disk surface. The residues on the disk surfaces involved are largely distinct from those affecting other regulatory phenomena. Consistent with the large number of histone residues affecting tgm silencing, survey of chromatin modification mutations shows that several enzymes known to affect nucleosome modification and positioning are also required. The enzymes include an Rpd3 deacetylase complex, Hos1 deacetylase, Glc7 phosphatase, and the RSC nucleosome remodeling activity, but not multiple other activities required for other silencing forms or boundary element function at tRNA gene loci. Models for communication between the tRNA gene transcription complexes and local chromatin are discussed.

Single cell visualization of yeast gene expression shows correlation of epigenetic switching between multiple heterochromatic regions through multiple generations

A single-cell method allows the assessment of relationships between the dynamic epigenetic behavior of yeast heterochromatin boundaries over multiple generations.

Differences in gene expression between individual cells can be mediated by epigenetic regulation; thus, methods that enable detailed analyses of single cells are crucial to understanding this phenomenon. In this study, genomic silencing regions of Saccharomyces cerevisiae that are subject to epigenetic regulation, including the HMR, HML, and telomere regions, were investigated using a newly developed single cell analysis method. This method uses fluorescently labeled proteins to track changes in gene expression over multiple generations of a single cell. Epigenetic control of gene expression differed depending on the specific silencing region at which the reporter gene was inserted. Correlations between gene expression at the HMR-left and HMR-right regions, as well as the HMR-right and HML-right regions, were observed in the single-cell level; however, no such correlations involving the telomere region were observed. Deletion of the histone acetyltransferase GCN5 gene from a yeast strain carrying a fluorescent reporter gene at the HMR-left region reduced the frequency of changes in gene expression over a generation. The results presented here suggest that epigenetic control within an individual cell is reversible and can be achieved via regulation of histone acetyltransferase activity.

Although eukaryotic gene repression usually acts on individual genes, cells can also repress larger chromosomal regions via the establishment of a high order chromatin structure called heterochromatin. Once initiated, heterochromatin spreads until halted by a boundary, and in this study we focus on how this boundary is formed. Because the mechanism is epigenetic and can differ from cell to cell, we wanted to assess the dynamics of the process by tracking individual cells over multiple generations. Here we develop a novel method employing protein fluorescence to monitor gene expression at the boundaries of several yeast heterochromatic regions simultaneously. This allows us to assess whether different boundaries within a single cell fluctuate in concert or independently of each other. In addition, we use histone modification mutants to probe the specific types of epigenetic regulation responsible for fluctuations in heterochromatin boundary positioning. Using this method, we show that epigenetic gene expression within individual cells is reversible and that this process is regulated by histone acetylation state. Future work will identify connections between variation in boundary positioning and novel transcription control systems.

Long-range heterochromatin association is mediated by silencing and double-strand DNA break repair proteins

In yeast, the localization of homologous recombination–associated proteins to heterochromatic regions of the genome is necessary for proper nuclear organization.

The eukaryotic genome is highly organized in the nucleus, and this organization affects various nuclear processes. However, the molecular details of higher-order organization of chromatin remain obscure. In the present study, we show that the Saccharomyces cerevisiae silenced loci HML and HMR cluster in three-dimensional space throughout the cell cycle and independently of the telomeres. Long-range HML–HMR interactions require the homologous recombination (HR) repair pathway and phosphorylated H2A (γ-H2A). γ-H2A is constitutively present at silenced loci in unperturbed cells, its localization requires heterochromatin, and it is restricted to the silenced domain by the transfer DNA boundary element. SMC proteins and Scc2 localize to the silenced domain, and Scc2 binding requires the presence of γ-H2A. These findings illustrate a novel pathway for heterochromatin organization and suggest a role for HR repair proteins in genomic organization.

RNA polymerase III transcription - regulated by chromatin structure and regulator of nuclear chromatin organization

RNA polymerase III (Pol III) transcription is regulated by modifications of the chromatin. DNA methylation and post-translational modifications of histones, such as acetylation, phosphorylation and methylation have been linked to Pol III transcriptional activity. In addition to being regulated by modifications of DNA and histones, Pol III genes and its transcription factors have been implicated in the organization of nuclear chromatin in several organisms. In yeast, the ability of the Pol III transcription system to contribute to nuclear organization seems to be dependent on direct interactions of Pol III genes and/or its transcription factors TFIIIC and TFIIIB with the structural maintenance of chromatin (SMC) protein-containing complexes cohesin and condensin. In human cells, Pol III genes and transcription factors have also been shown to colocalize with cohesin and the transcription regulator and genome organizer CCCTC-binding factor (CTCF). Furthermore, chromosomal sites have been identified in yeast and humans that are bound by partial Pol III machineries (extra TFIIIC sites - ETC; chromosome organizing clamps - COC). These ETCs/COC as well as Pol III genes possess the ability to act as boundary elements that restrict spreading of heterochromatin.

Yeast RNA polymerase III transcription factors and effectors

Recent data indicate that the well-defined transcription machinery of RNA polymerase III (Pol III) is probably more complex than commonly thought. In this review, we describe the yeast basal transcription factors of Pol III and their involvements in the transcription cycle. We also present a list of proteins detected on genes transcribed by Pol III (class III genes) that might participate in the transcription process. Surprisingly, several of these proteins are involved in RNA polymerase II transcription. Defining the role of these potential new effectors in Pol III transcription in vivo will be the challenge of the next few years. This article is part of a Special Issue entitled: Transcription by Odd Pols.

TFIIIC bound DNA elements in nuclear organization and insulation

tRNA genes (tDNAs) have been known to have barrier insulator function in budding yeast, Saccharomyces cerevisiae, for over a decade. tDNAs also play a role in genome organization by clustering at sites in the nucleus and both of these functions are dependent on the transcription factor TFIIIC. More recently TFIIIC bound sites devoid of pol III, termed Extra-TFIIIC sites (ETC) have been identified in budding yeast and these sites also function as insulators and affect genome organization. Subsequent studies in Schizosaccharomyces pombe showed that TFIIIC bound sites were insulators and also functioned as Chromosome Organization Clamps (COC); tethering the sites to the nuclear periphery. Very recently studies have moved to mammalian systems where pol III genes and their associated factors have been investigated in both mouse and human cells. Short interspersed nuclear elements (SINEs) that bind TFIIIC, function as insulator elements and tDNAs can also function as both enhancer - blocking and barrier insulators in these organisms. It was also recently shown that tDNAs cluster with other tDNAs and with ETCs but not with pol II transcribed genes. Intriguingly, TFIIIC is often found near pol II transcription start sites and it remains unclear what the consequences of TFIIIC based genomic organization are and what influence pol III factors have on pol II transcribed genes and vice versa. In this review we provide a comprehensive overview of the known data on pol III factors in insulation and genome organization and identify the many open questions that require further investigation. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Mating-type genes and MAT switching in Saccharomyces cerevisiae

Mating type in Saccharomyces cerevisiae is determined by two nonhomologous alleles, MATa and MATα. These sequences encode regulators of the two different haploid mating types and of the diploids formed by their conjugation. Analysis of the MATa1, MATα1, and MATα2 alleles provided one of the earliest models of cell-type specification by transcriptional activators and repressors. Remarkably, homothallic yeast cells can switch their mating type as often as every generation by a highly choreographed, site-specific homologous recombination event that replaces one MAT allele with different DNA sequences encoding the opposite MAT allele. This replacement process involves the participation of two intact but unexpressed copies of mating-type information at the heterochromatic loci, HMLα and HMRa, which are located at opposite ends of the same chromosome-encoding MAT. The study of MAT switching has yielded important insights into the control of cell lineage, the silencing of gene expression, the formation of heterochromatin, and the regulation of accessibility of the donor sequences. Real-time analysis of MAT switching has provided the most detailed description of the molecular events that occur during the homologous recombinational repair of a programmed double-strand chromosome break.

Fub1p, a novel protein isolated by boundary screening, binds the proteasome complex

Silenced chromatin domains are restricted to specific regions. Eukaryotic chromosomes are organized into discrete domains delimited by domain boundaries. From approximately 6,000 genes in Saccharomyces cerevisiae, we previously isolated 55 boundary genes. In this study, we focus on the molecular function of one of boundary genes, YCR076C/FUB1 (function of boundary), whose function has not been clearly defined in vivo. Biochemical analysis of Fub1p revealed that it interacted with multiple subunits of the 20S proteasome core particle (20S CP). To further clarify the functional link between Fub1p and proteasome, several proteasome mutants were analyzed. Although only 20S CP subunits were isolated as Fub1p interactors, a genetic interaction was also observed for component of 19S regulatory particle (19S RP) suggesting involvement of Fub1p with the whole proteasome. We also analyzed the mechanism of boundary establishment by using proteasome composition factor-deficient strains. Deletion of pre9 and ump1, whose products have effects on the 20S CP, resulted in a decrease in boundary function. Domain analyses of Fub1p identified a minimum functional domain in the C terminus that was essential for boundary establishment and showed a limited sequence homology to the human PSMF1, which is known to inhibit proteasome activity. Finally, boundary assay showed that human PSMF1 also exhibited boundary establishment activity in yeast. Our results defined the functional correlation between Fub1p and PSMF1.

Human tRNA genes function as chromatin insulators

Insulators help separate active chromatin domains from silenced ones. In yeast, gene promoters act as insulators to block the spread of Sir and HP1 mediated silencing while in metazoans most insulators are multipartite autonomous entities. tDNAs are repetitive sequences dispersed throughout the human genome and we now show that some of these tDNAs can function as insulators in human cells. Using computational methods, we identified putative human tDNA insulators. Using silencer blocking, transgene protection and repressor blocking assays we show that some of these tDNA-containing fragments can function as barrier insulators in human cells. We find that these elements also have the ability to block enhancers from activating RNA pol II transcribed promoters. Characterization of a putative tDNA insulator in human cells reveals that the site possesses chromatin signatures similar to those observed at other better-characterized eukaryotic insulators. Enhanced 4C analysis demonstrates that the tDNA insulator makes long-range chromatin contacts with other tDNAs and ETC sites but not with intervening or flanking RNA pol II transcribed genes.

Boundaries of transcriptionally silent chromatin in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae, heterochromatic gene silencing has been found within HMR and HML silent mating type loci, the telomeres, and the rRNA-encoding DNA. There may be boundary elements that regulate the spread of silencing to protect genes adjacent to silenced domains from this epigenetic repressive effect. Many assays show that specific DNA regulatory elements separate a euchromatic locus from a neighboring heterochromatic domain and thereby function as a boundary. Alternatively, DNA-independent mechanisms such as competition between acetylated and deacetylated histones are also reported to contribute to gene insulation. However, the mechanism by which boundaries are formed is not clear. Here, the characteristics and functions of boundaries at silenced domains in S. cerevisiae are discussed.

Chromatin insulators: a role in nuclear organization and gene expression

Chromatin insulators are DNA-protein complexes with broad functions in nuclear biology. Based on the ability of insulator proteins to interact with each other, it was originally found that insulators form loops that bring together distant regions of the genome. Data from genome-wide localization studies indicate that insulator proteins can be present in intergenic regions as well as at the 5', introns or 3' of genes, suggesting a variety of roles for insulator loops in chromosome biology. Recent results suggest that insulators mediate intra- and interchromosomal interactions to affect transcription, imprinting, and recombination. Cells have developed mechanisms to control insulator activity by recruiting specialized proteins or by covalent modification of core components. It is then possible that insulator-mediated interactions set up cell-specific blueprints of nuclear organization that may contribute to the establishment of different patterns of gene expression during cell differentiation and development. As a consequence, disruption of insulator activity could result in the development of cancer or other disease states.

Genomic relationship between SINE retrotransposons, Pol III-Pol II transcription, and chromatin organization: the journey from junk to jewel

A typical eukaryotic genome harbors a rich variety of repetitive elements. The most abundant are retrotransposons, mobile retroelements that utilize reverse transcriptase and an RNA intermediate to relocate to a new location within the cellular genomes. A vast majority of the repetitive mammalian genome content has originated from the retrotransposition of SINE (100-300 bp short interspersed nuclear elements that are derived from the structural 7SL RNA or tRNA), LINE (7kb long interspersed nuclear element), and LTR (2-3 kb long terminal repeats) transposable element superfamilies. Broadly labeled as "evolutionary junkyard" or "fossils", this enigmatic "dark matter" of the genome possesses many yet to be discovered properties.

Roles of chromatin insulator proteins in higher-order chromatin organization and transcription regulation

Eukaryotic chromosomes are condensed into several hierarchical levels of complexity: DNA is wrapped around core histones to form nucleosomes, nucleosomes form a higher-order structure called chromatin, and chromatin is subsequently compartmentalized in part by the combination of multiple specific or unspecific long-range contacts. The conformation of chromatin at these three levels greatly influences DNA metabolism and transcription. One class of chromatin regulatory proteins called insulator factors may organize chromatin both locally, by setting up barriers between heterochromatin and euchromatin, and globally by establishing platforms for long-range interactions. Here, we review recent data revealing a global role of insulator proteins in the regulation of transcription through the formation of clusters of long-range interactions that impact different levels of chromatin organization.

Histone crosstalk directed by H2B ubiquitination is required for chromatin boundary integrity

Genomic maps of chromatin modifications have provided evidence for the partitioning of genomes into domains of distinct chromatin states, which assist coordinated gene regulation. The maintenance of chromatin domain integrity can require the setting of boundaries. The HS4 insulator element marks the 3′ boundary of a heterochromatin region located upstream of the chicken β-globin gene cluster. Here we show that HS4 recruits the E3 ligase RNF20/BRE1A to mediate H2B mono-ubiquitination (H2Bub1) at this insulator. Knockdown experiments show that RNF20 is required for H2Bub1 and processive H3K4 methylation. Depletion of RNF20 results in a collapse of the active histone modification signature at the HS4 chromatin boundary, where H2Bub1, H3K4 methylation, and hyperacetylation of H3, H4, and H2A.Z are rapidly lost. A remarkably similar set of events occurs at the HSA/HSB regulatory elements of the FOLR1 gene, which mark the 5′ boundary of the same heterochromatin region. We find that persistent H2Bub1 at the HSA/HSB and HS4 elements is required for chromatin boundary integrity. The loss of boundary function leads to the sequential spreading of H3K9me2, H3K9me3, and H4K20me3 over the entire 50 kb FOLR1 and β-globin region and silencing of FOLR1 expression. These findings show that the HSA/HSB and HS4 boundary elements direct a cascade of active histone modifications that defend the FOLR1 and β-globin gene loci from the pervasive encroachment of an adjacent heterochromatin domain. We propose that many gene loci employ H2Bub1-dependent boundaries to prevent heterochromatin spreading.

The transcription of genes in eukaryotes occurs within the context of chromatin, a complex of DNA, histone proteins, and regulatory factors. Whole-genome profiling of chromatin proteins and histones that are post-translationally modified has revealed that genomes are organized into domains of distinct chromatin states that coordinate gene regulation. The integrity of chromatin domains can require the setting of their boundaries. DNA sequences known as chromatin insulator or boundary elements can establish boundaries between transcriptionally permissive and repressive chromatin domains. We have studied two chromatin boundary elements that flank a condensed chromatin region located between the chicken FOLR1 and β-globin genes, respectively. These elements recruit enzymes that mediate the ubiquitination of histone H2B. Histone H2B ubiquitination directs a cascade of so-called “active” histone modification events that favor chromatin accessibility. We observe a striking collapse of the active histone modification signature at both chromatin boundaries following the depletion of ubiquitinated H2B. This loss of boundary function leads to the comprehensive spreading of repressive chromatin over the entire FOLR1 and β-globin gene region, resulting in gene silencing. We propose that chromatin boundaries at many gene loci employ H2B ubiquitination to restrict the encroachment of repressive chromatin.

Nucleoporin mediated nuclear positioning and silencing of HMR

The organization of chromatin domains in the nucleus is an important factor in gene regulation. In eukaryotic nuclei, transcriptionally silenced chromatin clusters at the nuclear periphery while transcriptionally poised chromatin resides in the nuclear interior. Recent studies suggest that nuclear pore proteins (NUPs) recruit loci to nuclear pores to aid in insulation of genes from silencing and during gene activation. We investigated the role of NUPs at a native yeast insulator and show that while NUPs localize to the native tDNA insulator adjacent to the silenced HMR domain, loss of pore proteins does not compromise insulation. Surprisingly we find that NUPs contribute to silencing at HMR and are able to restore silencing to a silencing-defective HMR allele when tethered to the locus. We show that the perinuclear positioning of heterochromatin is important for the NUP-mediated silencing effect and find that loss of NUPs result in decreased localization of HMR to the nuclear periphery. We also show that loss of telomeric tethering pathways does not eliminate NUP localization to HMR, suggesting that NUPs may mediate an independent pathway for HMR association with the nuclear periphery. We propose that localization of NUPs to the tDNA insulator at HMR helps maintain the intranuclear position of the silent locus, which in turn contributes to the fidelity of silencing at HMR.

Regulation of white and opaque cell-type formation in Candida albicans by Rtt109 and Hst3

How different cell types with the same genotype are formed and heritability maintained is a fundamental question in biology. We utilized white-opaque switching in Candida albicans as a system to study mechanisms of cell-type formation and maintenance. Each cell type has tractable characters, which are maintained over many cell divisions. Cell-type specification is under the control of interlocking transcriptional feedback loops, with Wor1 being the master regulator of the opaque cell type. Here we show that deletion of RTT109, encoding the acetyltransferase for histone H3K56, impairs stochastic and environmentally stimulated white-opaque switching. Ectopic expression of WOR1 mostly bypasses the requirement for RTT109, but opaque cells lacking RTT109 cannot be maintained. We have also discovered that nicotinamide induces opaque cell formation, and this activity of nicotinamide requires RTT109. Reducing the copy number of HST3, which encodes the H3K56 deacetylase, also leads to increased opaque formation. We further show that the Hst3 level is downregulated in the presence of genotoxins and ectopic expression of HST3 blocks genotoxin induced switching. This finding links genotoxin induced switching to Hst3 regulation. Together, these findings suggest RTT109 and HST3 genes play an important role in the regulation of white-opaque switching in C. albicans.

The FUN30 chromatin remodeler, Fft3, protects centromeric and subtelomeric domains from euchromatin formation

The chromosomes of eukaryotes are organized into structurally and functionally discrete domains. This implies the presence of insulator elements that separate adjacent domains, allowing them to maintain different chromatin structures. We show that the Fun30 chromatin remodeler, Fft3, is essential for maintaining a proper chromatin structure at centromeres and subtelomeres. Fft3 is localized to insulator elements and inhibits euchromatin assembly in silent chromatin domains. In its absence, euchromatic histone modifications and histone variants invade centromeres and subtelomeres, causing a mis-regulation of gene expression and severe chromosome segregation defects. Our data strongly suggest that Fft3 controls the identity of chromatin domains by protecting these regions from euchromatin assembly.

Tbr1 and Fezf2 regulate alternate corticofugal neuronal identities during neocortical development

The molecular mechanisms regulating fate divergence of closely related, but distinct, layer 6 corticothalamic and layer 5 subcerebral projection neurons are largely unknown. We present evidence for central transcriptional mechanisms that regulate fate specification of corticothalamic (layer 6) and subcerebral (layer 5) projection neurons. We found that TBR1 promotes the identity of corticothalamic neurons and represses subcerebral fates through reducing expression of Fezf2 and CTIP2. These conclusions are based on the following: (1) In Tbr1(-/-) mice, the number of cells expressing layer 6 markers was reduced, and the number of cells expressing layer 5 markers was increased. Early-born (birthdated on E11.5) neurons ectopically expressed subcerebral neuronal markers, and extended their axons into subcerebral targets. (2) Ectopic Tbr1 expression in layer 5 neurons prevented them from extending axons into the brainstem and the spinal cord. (3) Chromatin immunoprecipitation analysis using TBR1 antibodies showed that TBR1 bound to a conserved region in the Fezf2 gene. (4) Analysis of Fezf2 mutants and Tbr1(-/-); Fezf2(-/-) compound mutants provided evidence that Fezf2 blocks corticothalamic fate in layer 5 by reducing Tbr1 expression in subcerebral neurons. All neocortical regions appear to use this core transcriptional program to specify corticothalamic (layer 6) and subcerebral (layer 5) projection neurons.

Dynamics of Sir3 spreading in budding yeast: secondary recruitment sites and euchromatic localization

Chromatin domains are believed to spread via a polymerization-like mechanism in which modification of a given nucleosome recruits a modifying complex, which can then modify the next nucleosome in the polymer. In this study, we carry out genome-wide mapping of the Sir3 component of the Sir silencing complex in budding yeast during a time course of establishment of heterochromatin. Sir3 localization patterns do not support a straightforward model for nucleation and polymerization, instead showing strong but spatially delimited binding to silencers, and weaker and more variable Ume6-dependent binding to novel secondary recruitment sites at the seripauperin (PAU) genes. Genome-wide nucleosome mapping revealed that Sir binding to subtelomeric regions was associated with overpackaging of subtelomeric promoters. Sir3 also bound to a surprising number of euchromatic sites, largely at genes expressed at high levels, and was dynamically recruited to GAL genes upon galactose induction. Together, our results indicate that heterochromatin complex localization cannot simply be explained by nucleation and linear polymerization, and show that heterochromatin complexes associate with highly expressed euchromatic genes in many different organisms.

Insulators and promoters: closer than we think

Insulators prevent promiscuous gene regulation by restricting the action of enhancers and silencers. Recent studies have revealed a number of similarities between insulators and promoters, including binding of specific transcription factors, chromatin-modification signatures and localization to specific subnuclear positions. We propose that enhancer-blockers and silencing barrier-insulators might have evolved as specialized derivatives of promoters and that the two types of element use related mechanisms to mediate their distinct functions. These insights can help to reconcile different models of insulator action.

tRNA insulator function: insight into inheritance of transcription states?

DNA in eukaryotes is invariably present as a complex with histone and non-histone proteins called chromatin. These proteins play an important role in the proper regulation of genes during development and differentiation. Transcription factors and the covalent modifications of DNA, histone and non-histone proteins establish an epigenetic state that is heritable and which does not involve a change in genotype. The heritability of transcription states through cell division brings up specific questions: How are epigenetic marks established and re-established in the daughter cells following DNA replication and mitosis? In this article we explore what is known of the cell cycle dependence of epigenetic inheritance with particular emphasis on yeast loci and discuss the role of specific proteins responsible for the establishment and maintenance of these states.

An auxiliary silencer and a boundary element maintain high levels of silencing proteins at HMR in Saccharomyces cerevisiae

Heterochromatin is notable for its capacity to propagate along a chromosome. The prevailing model for this spreading process postulates that silencing proteins are first recruited to silencer sequences and then spread from these sites independently of the silencers. However, we found that in Saccharomyces cerevisiae silencers also influence the extent of silenced chromatin domains. We compared the abilities of two different silencers, HMR-E and a telomeric repeat, to promote silencing and found that the HMR-E silencer contributed to an increased steady-state association of Sir proteins over a region of several kilobase pairs compared to the telomeric repeat, even though both silencers recruited similar levels of Sir proteins. We also discovered that, although the HMR-E silencer alone was sufficient to block transcription of the HMR locus, a secondary silencer, HMR-I, boosted the level of Sir proteins at HMR, apparently beyond the level necessary to repress transcription. Finally, we discovered that a tRNA(Thr) gene near HMR-I helped maintain silenced chromatin and transcriptional repression under conditions of reduced deacetylase activity. This study highlights the importance of auxiliary elements, such as HMR-I and the tRNA(Thr) gene, in enhancing the association of Sir silencing proteins with appropriate genomic locations, thereby buffering the capacity of silenced chromatin to assemble under suboptimal conditions.