Long-range nucleosome ordering is associated with gene silencing in Drosophila melanogaster pericentric heterochromatin

Epigenetic inheritance and gene expression regulation in early Drosophila embryos

Precise spatiotemporal regulation of gene expression is of paramount importance for eukaryotic development. The maternal-to-zygotic transition (MZT) during early embryogenesis in Drosophila involves the gradual replacement of maternally contributed mRNAs and proteins by zygotic gene products. The zygotic genome is transcriptionally activated during the first 3 hours of development, in a process known as "zygotic genome activation" (ZGA), by the orchestrated activities of a few pioneer factors. Their decisive role during ZGA has been characterized in detail, whereas the contribution of chromatin factors to this process has been historically overlooked. In this review, we aim to summarize the current knowledge of how chromatin regulation impacts the first stages of Drosophila embryonic development. In particular, we will address the following questions: how chromatin factors affect ZGA and transcriptional silencing, and how genome architecture promotes the integration of these processes early during development. Remarkably, certain chromatin marks can be intergenerationally inherited, and their presence in the early embryo becomes critical for the regulation of gene expression at later stages. Finally, we speculate on the possible roles of these chromatin marks as carriers of epialleles during transgenerational epigenetic inheritance (TEI).

Epigenetic Silencing of P-Element Reporter Genes Induced by Transcriptionally Active Domains of Constitutive Heterochromatin in Drosophila melanogaster

Reporter genes inserted via P-element integration into different locations of the Drosophila melanogaster genome have been routinely used to monitor the functional state of chromatin domains. It is commonly thought that P-element-derived reporter genes are subjected to position effect variegation (PEV) when transposed into constitutive heterochromatin because they acquire heterochromatin-like epigenetic modifications that promote silencing. However, sequencing and annotation of the D. melanogaster genome have shown that constitutive heterochromatin is a genetically and molecularly heterogeneous compartment. In fact, in addition to repetitive DNAs, it harbors hundreds of functional genes, together accounting for a significant fraction of its entire genomic territory. Notably, most of these genes are actively transcribed in different developmental stages and tissues, irrespective of their location in heterochromatin. An open question in the genetic and molecular studies on PEV in D. melanogaster is whether functional heterochromatin domains, i.e., heterochromatin harboring active genes, are able to silence reporter genes therein transposed or, on the contrary, can drive their expression. In this work, we provide experimental evidence showing that strong silencing of the Pw⁺ reporters is induced even when they are integrated within or near actively transcribed loci in the pericentric regions of chromosome 2. Interestingly, some Pw⁺ reporters were found insensitive to the action of a known PEV suppressor. Two of them are inserted within Yeti, a gene expressed in the deep heterochromatin of chromosome 2 which carries active chromatin marks. The difference sensitivity to suppressors-exhibited Pw⁺ reporters supports the view that different epigenetic regulators or mechanisms control different regions of heterochromatin. Together, our results suggest that there may be more complexity regarding the molecular mechanisms underlying PEV.

ATP hydrolysis coordinates the activities of two motors in a dimeric chromatin remodeling enzyme

ATP-dependent chromatin remodelers are essential enzymes that restructure eukaryotic genomes to enable all DNA-based processes. The diversity and complexity of these processes are matched by the complexity of the enzymes that carry them out, making remodelers a challenging class of molecular motors to study by conventional methods. Here we use a single molecule biophysical assay to overcome some of these challenges, enabling a detailed mechanistic dissection of a paradigmatic remodeler reaction, that of sliding a nucleosome towards the longer DNA linker. We focus on how two motors of a dimeric remodeler coordinate to accomplish such directional sliding. We find that ATP hydrolysis by both motors promotes coordination, suggesting a role for ATP in resolving the competition for directional commitment. Furthermore, we show an artificially constitutive dimer is no more or less coordinated, but is more processive, suggesting a cell could modulate a remodeler's oligomeric state to modulate local chromatin dynamics.

Nucleosome-induced homology recognition in chromatin

One of the least understood properties of chromatin is the ability of its similar regions to recognize each other through weak interactions. Theories based on electrostatic interactions between helical macromolecules suggest that the ability to recognize sequence homology is an innate property of the non-ideal helical structure of DNA. However, this theory does not account for the nucleosomal packing of DNA. Can homologous DNA sequences recognize each other while wrapped up in the nucleosomes? Can structural homology arise at the level of nucleosome arrays? Here, we present a theoretical model for the recognition potential well between chromatin fibres sliding against each other. This well is different from the one predicted for bare DNA; the minima in energy do not correspond to literal juxtaposition, but are shifted by approximately half the nucleosome repeat length. The presence of this potential well suggests that nucleosome positioning may induce mutual sequence recognition between chromatin fibres and facilitate the formation of chromatin nanodomains. This has implications for nucleosome arrays enclosed between CTCF-cohesin boundaries, which may form stiffer stem-like structures instead of flexible entropically favourable loops. We also consider switches between chromatin states, e.g. through acetylation/deacetylation of histones, and discuss nucleosome-induced recognition as a precursory stage of genetic recombination.

Targeting EHMT2/ G9a for cancer therapy: Progress and perspective

Euchromatic histone lysine methyltransferase-2, also known as G9a, is a ubiquitously expressed SET domain-containing histone lysine methyltransferase linked with both facultative and constitutive heterochromatin formation and transcriptional repression. It is an essential developmental gene and reported to play role in embryonic development, establishment of proviral silencing in ES cells, tumor cell growth, metastasis, T-cell immune response, cocaine induced neural plasticity and cognition and adaptive behavior. It is mainly responsible for carrying out mono, di and tri methylation of histone H3K9 in euchromatin. G9a levels are elevated in many cancers and its selective inhibition is known to reduce the cell growth and induce autophagy, apoptosis and senescence. We carried out a thorough search of online literature databases including Pubmed, Scopus, Journal websites, Clinical trials etc to gather the maximum possible information related to the G9a. The main messages from the cited papers are presented in a systematic manner. Chemical structures were drawn by Chemdraw software. In this review, we shed light on current understanding of structure and biological activity of G9a, the molecular events directing its targeting to genomic regions and its post-translational modification. Finally, we discuss the current strategies to target G9a in different cancers and evaluate the available compounds and agents used to inhibit G9a functions. The review provides the present status and future directions of research in targeting G9a and provides the basis to persuade the development of novel strategies to target G9a -related effects in cancer cells.

CTCF-dependent chromatin boundaries formed by asymmetric nucleosome arrays with decreased linker length

The CCCTC-binding factor (CTCF) organises the genome in 3D through DNA loops and in 1D by setting boundaries isolating different chromatin states, but these processes are not well understood. Here we investigate chromatin boundaries in mouse embryonic stem cells, defined by the regions with decreased Nucleosome Repeat Length (NRL) for ∼20 nucleosomes near CTCF sites, affecting up to 10% of the genome. We found that the nucleosome-depleted region (NDR) near CTCF is asymmetrically located >40 nucleotides 5'-upstream from the centre of CTCF motif. The strength of CTCF binding to DNA and the presence of cohesin is correlated with the decrease of NRL near CTCF, and anti-correlated with the level of asymmetry of the nucleosome array. Individual chromatin remodellers have different contributions, with Snf2h having the strongest effect on the NRL decrease near CTCF and Chd4 playing a major role in the symmetry breaking. Upon differentiation, a subset of preserved, common CTCF sites maintains asymmetric nucleosome pattern and small NRL. The sites which lost CTCF upon differentiation are characterized by nucleosome rearrangement 3'-downstream, with unchanged NDR 5'-upstream of CTCF motifs. Boundaries of topologically associated chromatin domains frequently contain several inward-oriented CTCF motifs whose effects, described above, add up synergistically.

Nucleosome positioning and spacing: from genome-wide maps to single arrays

The positioning of nucleosomes relative to DNA and their neighboring nucleosomes represents a fundamental layer of chromatin organization. Changes in nucleosome positioning and spacing affect the accessibility of DNA to regulatory factors and the formation of higher order chromatin structures. Sequencing of mononucleosomal fragments allowed mapping nucleosome positions on a genome-wide level in many organisms. This revealed that successions of evenly spaced and well-positioned nucleosomes-so called phased nucleosome arrays-occur at the 5' end of many active genes and in the vicinity of transcription factor and other protein binding sites. Phased arrays arise from the interplay of barrier elements on the DNA, which position adjacent nucleosomes, and the nucleosome spacing activity of ATP-dependent chromatin remodelers. A shortcoming of classic mononucleosomal mapping experiments is that they only reveal nucleosome spacing and array regularity at select sites in the genome with well-positioned nucleosomes. However, new technological approaches elucidate nucleosome array structure throughout the genome and with single-cell resolution. In the future, it will be interesting to see whether changes in nucleosome array regularity and spacing contribute to the formation of higher order chromatin structures and the spatial organization of the genome in vivo.

Nucleosome Positioning by an Evolutionarily Conserved Chromatin Remodeler Prevents Aberrant DNA Methylation in Neurospora

In the filamentous fungus Neurospora crassa, constitutive heterochromatin is marked by tri-methylation of histone H3 lysine 9 (H3K9me3) and DNA methylation. We identified mutations in the Neurospora defective in methylation-1 (dim-1) gene that cause defects in cytosine methylation and implicate a putative AAA-ATPase chromatin remodeler. Although it was well-established that chromatin remodelers can affect transcription by influencing DNA accessibility with nucleosomes, little was known about the role of remodelers on chromatin that is normally not transcribed, including regions of constitutive heterochromatin. We found that dim-1 mutants display both reduced DNA methylation in heterochromatic regions as well as increased DNA methylation and H3K9me3 in some intergenic regions associated with highly expressed genes. Deletion of dim-1 leads to atypically spaced nucleosomes throughout the genome and numerous changes in gene expression. DIM-1 localizes to both heterochromatin and intergenic regions that become hyper-methylated in dim-1 strains. Our findings indicate that DIM-1 normally positions nucleosomes in both heterochromatin and euchromatin and that the standard arrangement and density of nucleosomes is required for the proper function of heterochromatin machinery.

Drosophila melanogaster retrotransposon and inverted repeat-derived endogenous siRNAs are differentially processed in distinct cellular locations

BACKGROUND

Endogenous small interfering (esi)RNAs repress mRNA levels and retrotransposon mobility in Drosophila somatic cells by poorly understood mechanisms. 21 nucleotide esiRNAs are primarily generated from retrotransposons and two inverted repeat (hairpin) loci in Drosophila culture cells in a Dicer2 dependent manner. Additionally, proteins involved in 3' end processing, such as Symplekin, CPSF73 and CPSR100, have been recently implicated in the esiRNA pathway.

RESULTS

Here we present evidence of overlap between two essential RNA metabolic pathways: esiRNA biogenesis and mRNA 3' end processing. We have identified a nucleus-specific interaction between the essential esiRNA cleavage enzyme Dicer2 (Dcr2) and Symplekin, a component of the core cleavage complex (CCC) required for 3' end processing of all eukaryotic mRNAs. This interaction is mediated by the N-terminal 271 amino acids of Symplekin; CCC factors CPSF73 and CPSF100 do not contact Dcr2. While Dcr2 binds the CCC, Dcr2 knockdown does not affect mRNA 3' end formation. RNAi-depletion of CCC components Symplekin and CPSF73 causes perturbations in esiRNA abundance that correlate with fluctuations in retrotransposon and hairpin esiRNA precursor levels. We also discovered that esiRNAs generated from retrotransposons and hairpins have distinct physical characteristics including a higher predominance of 22 nucleotide hairpin-derived esiRNAs and differences in 3' and 5' base preference. Additionally, retrotransposon precursors and derived esiRNAs are highly enriched in the nucleus while hairpins and hairpin derived esiRNAs are predominantly cytoplasmic similar to canonical mRNAs. RNAi-depletion of either CPSF73 or Symplekin results in nuclear retention of both hairpin and retrotransposon precursors suggesting that polyadenylation indirectly affects cellular localization of Dcr2 substrates.

CONCLUSIONS

Together, these observations support a novel mechanism in which differences in localization of esiRNA precursors impacts esiRNA biogenesis. Hairpin-derived esiRNAs are generated in the cytoplasm independent of Dcr2-Symplekin interactions, while retrotransposons are processed in the nucleus.

Silencio/CG9754 connects the Piwi-piRNA complex to the cellular heterochromatin machinery

In this study, Sienski et al. characterize CG9754/Silencio as an essential piRNA pathway factor that is required for Piwi's nuclear function in guiding the transcriptional silencing of transposons. These results provide novel insight into the transcriptional silencing process downstream from Piwi and the regulation of piRNA-guided heterochromatin formation.

The repression of transposable elements in eukaryotes often involves their transcriptional silencing via targeted chromatin modifications. In animal gonads, nuclear Argonaute proteins of the PIWI clade complexed with small guide RNAs (piRNAs) serve as sequence specificity determinants in this process. How binding of nuclear PIWI–piRNA complexes to nascent transcripts orchestrates heterochromatin formation and transcriptional silencing is unknown. Here, we characterize CG9754/Silencio as an essential piRNA pathway factor that is required for Piwi-mediated transcriptional silencing in Drosophila. Ectopic targeting of Silencio to RNA or DNA is sufficient to elicit silencing independently of Piwi and known piRNA pathway factors. Instead, Silencio requires the H3K9 methyltransferase Eggless/SetDB1 for its silencing ability. In agreement with this, SetDB1, but not Su(var)3-9, is required for Piwi-mediated transcriptional silencing genome-wide. Due to its interaction with the target-engaged Piwi–piRNA complex, we suggest that Silencio acts as linker between the sequence specificity factor Piwi and the cellular heterochromatin machinery.

Chromatin Unfolding by Epigenetic Modifications Explained by Dramatic Impairment of Internucleosome Interactions: A Multiscale Computational Study

Histone tails and their epigenetic modifications play crucial roles in gene expression regulation by altering the architecture of chromatin. However, the structural mechanisms by which histone tails influence the interconversion between active and inactive chromatin remain unknown. Given the technical challenges in obtaining detailed experimental characterizations of the structure of chromatin, multiscale computations offer a promising alternative to model the effect of histone tails on chromatin folding. Here we combine multimicrosecond atomistic molecular dynamics simulations of dinucleosomes and histone tails in explicit solvent and ions, performed with three different state-of-the-art force fields and validated by experimental NMR measurements, with coarse-grained Monte Carlo simulations of 24-nucleosome arrays to describe the conformational landscape of histone tails, their roles in chromatin compaction, and the impact of lysine acetylation, a widespread epigenetic change, on both. We find that while the wild-type tails are highly flexible and disordered, the dramatic increase of secondary-structure order by lysine acetylation unfolds chromatin by decreasing tail availability for crucial fiber-compacting internucleosome interactions. This molecular level description of the effect of histone tails and their charge modifications on chromatin folding explains the sequence sensitivity and underscores the delicate connection between local and global structural and functional effects. Our approach also opens new avenues for multiscale processes of biomolecular complexes.

Nucleosome spacing generated by ISWI and CHD1 remodelers is constant regardless of nucleosome density

Arrays of regularly spaced nucleosomes are a hallmark of chromatin, but it remains unclear how they are generated. Recent genome-wide studies, in vitro and in vivo, showed constant nucleosome spacing even if the histone concentration was experimentally reduced. This counters the long-held assumption that nucleosome density determines spacing and calls for factors keeping spacing constant regardless of nucleosome density. We call this a clamping activity. Here, we show in a purified system that ISWI- and CHD1-type nucleosome remodelers have a clamping activity such that they not only generate regularly spaced nucleosome arrays but also generate constant spacing regardless of nucleosome density. This points to a functionally attractive nucleosome interaction that could be mediated either directly by nucleosome-nucleosome contacts or indirectly through the remodelers. Mutant Drosophila melanogaster ISWI without the Hand-Sant-Slide (HSS) domain had no detectable spacing activity even though it is known to remodel and slide nucleosomes. This suggests that the role of ISWI remodelers in generating constant spacing is not just to mediate nucleosome sliding; they actively contribute to the attractive interaction. Additional factors are necessary to set physiological spacing in absolute terms.

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.

Targeting heterochromatin formation to transposable elements in Drosophila: potential roles of the piRNA system

Successful heterochromatin formation is critical for genome stability in eukaryotes, both to maintain structures needed for mitosis and meiosis and to silence potentially harmful transposable elements. Conversely, inappropriate heterochromatin assembly can lead to inappropriate silencing and other deleterious effects. Hence targeting heterochromatin assembly to appropriate regions of the genome is of utmost importance. Here we focus on heterochromatin assembly in Drosophila melanogaster, the model organism in which variegation, or cell-to-cell variable gene expression resulting from heterochromatin formation, was first described. In particular, we review the potential role of transposable elements as genetic determinants of the chromatin state and examine how small RNA pathways may participate in the process of targeted heterochromatin formation.

HP1a: a structural chromosomal protein regulating transcription

Heterochromatin protein 1 (HP1a in Drosophila) is a conserved eukaryotic chromosomal protein that is prominently associated with pericentric heterochromatin and mediates the concomitant gene silencing. Mechanistic studies implicate HP1 family proteins as 'hub proteins,' able to interact with a variety of chromosomal proteins through the chromo-shadow domain (CSD), as well as to recognize key histone modification sites [primarily histone H3 di/trimethyl Lys9 (H3K9me2/3)] through the chromodomain (CD). Consequently, HP1 has many important roles in chromatin architecture and impacts both gene expression and gene silencing, utilizing a variety of mechanisms. Clearly, HP1 function is altered by context, and potentially by post-translational modifications (PTMs). Here, we report on recent ideas as to how this versatile protein accomplishes its diverse functions.

Human heterochromatin protein 1α promotes nucleosome associations that drive chromatin condensation

HP1(Hsα)-containing heterochromatin is located near centric regions of chromosomes and regulates DNA-mediated processes such as DNA repair and transcription. The higher-order structure of heterochromatin contributes to this regulation, yet the structure of heterochromatin is not well understood. We took a multidisciplinary approach to determine how HP1(Hsα)-nucleosome interactions contribute to the structure of heterochromatin. We show that HP1(Hsα) preferentially binds histone H3K9Me3-containing nucleosomal arrays in favor of non-methylated nucleosomal arrays and that nonspecific DNA interactions and pre-existing chromatin compaction promote binding. The chromo and chromo shadow domains of HP1(Hsα) play an essential role in HP1(Hsα)-nucleosome interactions, whereas the hinge region appears to have a less significant role. Electron microscopy of HP1(Hsα)-associated nucleosomal arrays showed that HP1(Hsα) caused nucleosome associations within an array, facilitating chromatin condensation. Differential sedimentation of HP1(Hsα)-associated nucleosomal arrays showed that HP1(Hsα) promotes interactions between arrays. These strand-to-strand interactions are supported by in vivo studies where tethering the Drosophila homologue HP1a to specific sites promotes interactions with distant chromosomal sites. Our findings demonstrate that HP1(Hsα)-nucleosome interactions cause chromatin condensation, a process that regulates many chromosome events.

Insights into chromatin structure and dynamics in plants

The packaging of chromatin into the nucleus of a eukaryotic cell requires an extraordinary degree of compaction and physical organization. In recent years, it has been shown that this organization is dynamically orchestrated to regulate responses to exogenous stimuli as well as to guide complex cell-type-specific developmental programs. Gene expression is regulated by the compartmentalization of functional domains within the nucleus, by distinct nucleosome compositions accomplished via differential modifications on the histone tails and through the replacement of core histones by histone variants. In this review, we focus on these aspects of chromatin organization and discuss novel approaches such as live cell imaging and photobleaching as important tools likely to give significant insights into our understanding of the very dynamic nature of chromatin and chromatin regulatory processes. We highlight the contribution plant studies have made in this area showing the potential advantages of plants as models in understanding this fundamental aspect of biology.

Large-scale periodicity of nucleosome positioning signal in pericentric regions of chromosomes (Drosophila melanogaster)

Nucleosome positioning signal (NPS) in heterochromatin is not uniform. We suggest the analysis of its heterogeneity by correlation with periodic function (analog of Furrier analysis). It was established the periodical repetition of the nucleosome clusters of large size in pericentric regions in a discontinuous manner. In the 3L pericentric region, it was revealed the domination of 78-85 kbp wavelength in the correlation coefficient profile and also strong presentation of 50 kbp signal. In further to centromere position, the 69 kbp value strongly dominates as well as the 50 kbp value in the closest proximity. In addition to the long wavelength signals, there are plenty of short wavelengths signals especially in the closest vicinity to centromere. In some positions throughout pericentric region of 2L chromosome, there are two sizes of repeated intermingled correlation signals (50, and 75 kbp) with dominating value of 75 kbp in proximity and 50 kbp distantly to centromere, the situation for 2R is analogous. Some genes with long introns support these quantitative characteristics of NPSs and to some extent their dominating character in each region. The characteristic repeat periods for 3L pericentric region coincide with the distances between heterochromatin epigenetic mark clusters and their distribution throughout this region for fly embryos, larvae, and some cell lines.

Maternal depletion of Piwi, a component of the RNAi system, impacts heterochromatin formation in Drosophila

A persistent question in epigenetics is how heterochromatin is targeted for assembly at specific domains, and how that chromatin state is faithfully transmitted. Stable heterochromatin is necessary to silence transposable elements (TEs) and maintain genome integrity. Both the RNAi system and heterochromatin components HP1 (Swi6) and H3K9me2/3 are required for initial establishment of heterochromatin structures in S. pombe. Here we utilize both loss of function alleles and the newly developed Drosophila melanogaster transgenic shRNA lines to deplete proteins of interest at specific development stages to dissect their roles in heterochromatin assembly in early zygotes and in maintenance of the silencing chromatin state during development. Using reporters subject to Position Effect Variegation (PEV), we find that depletion of key proteins in the early embryo can lead to loss of silencing assayed at adult stages. The piRNA component Piwi is required in the early embryo for reporter silencing in non-gonadal somatic cells, but knock-down during larval stages has no impact. This implies that Piwi is involved in targeting HP1a when heterochromatin is established at the late blastoderm stage and possibly also during embryogenesis, but that the silent chromatin state created is transmitted through cell division independent of the piRNA system. In contrast, heterochromatin structural protein HP1a is required for both initial heterochromatin assembly and the following mitotic inheritance. HP1a profiles in piwi mutant animals confirm that Piwi depletion leads to decreased HP1a levels in pericentric heterochromatin, particularly in TEs. The results suggest that the major role of the piRNA system in assembly of heterochromatin in non-gonadal somatic cells occurs in the early embryo during heterochromatin formation, and further demonstrate that failure of heterochromatin formation in the early embryo impacts the phenotype of the adult.

Position-effect variegation, heterochromatin formation, and gene silencing in Drosophila

Position-effect variegation (PEV) results when a gene normally in euchromatin is juxtaposed with heterochromatin by rearrangement or transposition. When heterochromatin packaging spreads across the heterochromatin/euchromatin border, it causes transcriptional silencing in a stochastic pattern. PEV is intensely studied in Drosophila using the white gene. Screens for dominant mutations that suppress or enhance white variegation have identified many conserved epigenetic factors, including the histone H3 lysine 9 methyltransferase SU(VAR)3-9. Heterochromatin protein HP1a binds H3K9me2/3 and interacts with SU(VAR)3-9, creating a core memory system. Genetic, molecular, and biochemical analysis of PEV in Drosophila has contributed many key findings concerning establishment and maintenance of heterochromatin with concomitant gene silencing.

Heterochromatinization induced by GAA-repeat hyperexpansion in Friedreich's ataxia can be reduced upon HDAC inhibition by vitamin B3

Large intronic expansions of the triplet-repeat sequence (GAA.TTC) cause transcriptional repression of the Frataxin gene (FXN) leading to Friedreich's ataxia (FRDA). We previously found that GAA-triplet expansions stimulate heterochromatinization in vivo in transgenic mice. We report here using chromosome conformation capture (3C) coupled with high-throughput sequencing that the GAA-repeat expansion in FRDA cells stimulates a higher-order structure as a fragment containing the GAA-repeat expansion showed an increased interaction frequency with genomic regions along the FXN locus. This is consistent with a more compacted chromatin and coincided with an increase in both constitutive H3K9me3 and facultative H3K27me3 heterochromatic marks in FRDA. Consistent with this, DNase I accessibility in regions flanking the GAA repeats in patients was decreased compared with healthy controls. Strikingly, this effect could be antagonized with the class III histone deactylase (HDAC) inhibitor vitamin B3 (nicotinamide) which activated the silenced FXN gene in several FRDA models. Examination of the FXN locus revealed a reduction of H3K9me3 and H3K27me3, an increased accessibility to DNase I and an induction of euchromatic H3 and H4 histone acetylations upon nicotinamide treatment. In addition, transcriptomic analysis of nicotinamide treated and untreated FRDA primary lymphocytes revealed that the expression of 67% of genes known to be dysregulated in FRDA was ameliorated by the treatment. These findings show that nictotinamide can up-regulate the FXN gene and reveal a potential mechanism of action for nicotinamide in reactivating the epigenetically silenced FXN gene and therefore support the further assessment of HDAC inhibitors (HDACi's) in FRDA and diseases caused by a similar mechanism.

Structural biology of the chromodomain: form and function

The chromodomain motif is found among certain chromosomal proteins of all eukaryotes. The chromodomain fold - three beta strands packed against a C-terminal alpha helix - mediates protein-protein and/or protein-nucleic acid interactions. In some cases, the affinity of chromodomain binding is regulated by lysine methylation, which appears to target chromodomain proteins and associated complexes to specific sites in chromatin. In this review, our current knowledge of chromodomain structure and function is summarized.

The chromatin code of fungal secondary metabolite gene clusters

Secondary metabolite biosynthesis genes in fungi are usually physically linked and organized in large gene clusters. The physical linkage of genes involved in the same biosynthetic pathway minimizes the amount of regulatory steps necessary to regulate the biosynthetic machinery and thereby contributes to physiological economization. Regulation by chromatin accessibility is a proficient molecular mechanism to synchronize transcriptional activity of large genomic regions. Chromatin regulation largely depends on DNA and histone modifications and the histone code hypothesis proposes that a certain combination of modifications, such as acetylation, methylation or phosphorylation, is needed to perform a specific task. A number of reports from several laboratories recently demonstrated that fungal secondary metabolite (SM) biosynthesis clusters are controlled by chromatin-based mechanisms and histone acetyltransferases, deacetylases, methyltransferases, and proteins involved in heterochromatin formation were found to be involved. This led to the proposal that establishment of repressive chromatin domains over fungal SM clusters under primary metabolic conditions is a conserved mechanism that prevents SM production during the active growth phase. Consequently, transcriptional activation of SM clusters requires reprogramming of the chromatin landscape and replacement of repressive histone marks by activating marks. This review summarizes recent advances in our understanding of chromatin-based SM cluster regulation and highlights some of the open questions that remain to be answered before we can draw a more comprehensive picture.

Structure and function of active chromatin and DNase I hypersensitive sites

Chromatin is by its very nature a repressive environment which restricts the recruitment of transcription factors and acts as a barrier to polymerases. Therefore the complex process of gene activation must operate at two levels. In the first instance, localized chromatin decondensation and nucleosome displacement is required to make DNA accessible. Second, sequence-specific transcription factors need to recruit chromatin modifiers and remodellers to create a chromatin environment that permits the passage of polymerases. In this review I will discuss the chromatin structural changes that occur at active gene loci and at regulatory elements that exist as DNase I hypersensitive sites.

Differential contributions of histone H3 and H4 residues to heterochromatin structure

Transcriptional silencing in Saccharomyces cerevisiae is mediated by heterochromatin. There is a plethora of information regarding the roles of histone residues in transcriptional silencing, but exactly how histone residues contribute to heterochromatin structure is not resolved. We address this question by testing the effects of a series of histone H3 and H4 mutations involving residues in their aminoterminal tails, on the solvent-accessible and lateral surfaces of the nucleosome, and at the interface of the H3/H4 tetramer and H2A/H2B dimer on heterochromatin structure and transcriptional silencing. The general state, stability, and conformational heterogeneity of chromatin are examined with a DNA topology-based assay, and the primary chromatin structure is probed by micrococcal nuclease. We demonstrate that the histone mutations differentially affect heterochromatin. Mutations of lysine 16 of histone H4 (H4-K16) and residues in the LRS (loss of rDNA silencing) domain of nucleosome surface markedly alter heterochromatin structure, supporting the notion that H4-K16 and LRS play key roles in heterochromatin formation. Deletion of the aminoterminal tail of H3 moderately alters heterochromatin structure. Interestingly, a group of mutations in the globular domains of H3 and H4 that abrogate or greatly reduce transcriptional silencing increase the conformational heterogeneity and/or reduce the stability of heterochromatin without affecting its overall structure. Surprisingly, yet another series of mutations abolish or reduce silencing without significantly affecting the structure, stability, or conformational heterogeneity of heterochromatin. Therefore, histone residues may contribute to the structure, stability, conformational heterogeneity, or other yet-to-be-characterized features of heterochromatin important for transcriptional silencing.

Roles of chromatin remodeling factors in the formation and maintenance of heterochromatin structure

Heterochromatin consists of highly ordered nucleosomes with characteristic histone modifications. There is evidence implicating chromatin remodeling proteins in heterochromatin formation, but their exact roles are not clear. We demonstrate in Saccharomyces cerevisiae that the Fun30p and Isw1p chromatin remodeling factors are similarly required for transcriptional silencing at the HML locus, but they differentially contribute to the structure and stability of HML heterochromatin. In the absence of Fun30p, only a partially silenced structure is established at HML. Such a structure resembles fully silenced heterochromatin in histone modifications but differs markedly from both fully silenced and derepressed chromatin structures regarding nucleosome arrangement. This structure likely represents an intermediate state of heterochromatin that can be converted by Fun30p to the mature state. Moreover, Fun30p removal reduces the rate of de novo establishment of heterochromatin, suggesting that Fun30p assists the silencing machinery in forming heterochromatin. We also find evidence suggesting that Fun30p functions together with, or after, the action of the silencing machinery. On the other hand, Isw1p is dispensable for the formation of heterochromatin structure but is instead critically required for maintaining its stability. Therefore, chromatin remodeling proteins may rearrange nucleosomes during the formation of heterochromatin or serve to stabilize/maintain heterochromatin structure.

Drosophila melanogaster heterochromatin protein HP1b plays important roles in transcriptional activation and development

The condensed heterochromatic domains are known to be associated with transcriptional repression and cell differentiation. Here, we investigate the function of heterochromatin protein HP1b, a member of the HP1 family in Drosophila melanogaster, in transcription and development. Both knockdown and overexpression of HP1b resulted in partial lethality, indicating that HP1b is essential for the normal development. In contrast to the positive role of HP1a in heterochromatin formation, overexpression of HP1b decondensed the pericentromeric heterochromatin and reduced the association of HP1a and H3K9me2 with it, both known markers of pericentric heterochromatin. Interestingly, the structure of the heterochromatic fourth chromosome appeared not to be affected. Further experiments showed that the presence of HP1a partially rescued the lethality caused by HP1b overexpression in males, and it fully rescued the lethality in females. Consistent with this observation, the defective transcription of heterochromatic genes was also partially restored in the presence of HP1a. Overall, this study argues that HP1b counteracts HP1a function both in heterochromatin formation and in the transcriptional regulation of euchromatic genes.

A proposal for kinetic proof reading by ISWI family chromatin remodeling motors

ATP-dependent chromatin remodeling motors play fundamental roles in nuclear processes by regulating access to DNA. Yet compared to other cellular motors less is known about how these motors couple the energy of ATP to alter their substrates. Here we use recent studies on a key chromatin remodeling motor from the ISWI class, human ACF and its yeast counterpart, ISW2, to propose a model for how these motors use ATP to read structural cues presented by nucleosomal substrates. Substantial earlier work has shown that ACF activity is strongly regulated by the length of the DNA flanking a nucleosome as well as by the histone H4 tail. Recent bulk and single-molecule studies of human ACF suggest that this complex functions as a dimeric motor. These studies, together with studies of yeast ISW2 imply that at least two types of ATP hydrolysis events accompany each cycle of nucleosome movement. We propose that ISWI motors may employ a kinetic proof reading type of mechanism to favor action on nucleosomes that are poised to be in condensed chromatin while inhibiting action on nucleosomes that are in fully active or fully condensed chromatin.

The role of DNA methylation, nucleosome occupancy and histone modifications in paramutation

Paramutation is the transfer of epigenetic information between alleles that leads to a heritable change in expression of one of these alleles. Paramutation at the tissue-specifically expressed maize (Zea mays) b1 locus involves the low-expressing B' and high-expressing B-I allele. Combined in the same nucleus, B' heritably changes B-I into B'. A hepta-repeat located 100-kb upstream of the b1 coding region is required for paramutation and for high b1 expression. The role of epigenetic modifications in paramutation is currently not well understood. In this study, we show that the B' hepta-repeat is DNA-hypermethylated in all tissues analyzed. Importantly, combining B' and B-I in one nucleus results in de novo methylation of the B-I repeats early in plant development. These findings indicate a role for hepta-repeat DNA methylation in the establishment and maintenance of the silenced B' state. In contrast, nucleosome occupancy, H3 acetylation, and H3K9 and H3K27 methylation are mainly involved in tissue-specific regulation of the hepta-repeat. Nucleosome depletion and H3 acetylation are tissue-specifically regulated at the B-I hepta-repeat and associated with enhancement of b1 expression. H3K9 and H3K27 methylation are tissue-specifically localized at the B' hepta-repeat and reinforce the silenced B' chromatin state. The B' coding region is H3K27 dimethylated in all tissues analyzed, indicating a role in the maintenance of the silenced B' state. Taken together, these findings provide insight into the mechanisms underlying paramutation and tissue-specific regulation of b1 at the level of chromatin structure.

Combinatorial, site-specific requirement for heterochromatic silencing factors in the elimination of nucleosome-free regions

High-resolution nucleosome occupancy maps of heterochromatic regions of wild-type and silencing-defective mutants of the fission yeast Schizosaccharomyces pombe revealed that heterochromatin induces the elimination of nucleosome-free regions (NFRs). NFRs associated with transcription initiation sites as well as those not associated with promoters are affected. We dissected the roles of the histone H3K9 methyltransferase Clr4 and the HP1 proteins Swi6 and Chp2, as well as the two catalytic activities of the SHREC histone deacetylase (HDAC)/ATPase effector complex. Strikingly, different DNA sites have distinct combinatorial requirements for these factors: Five classes of NFRs were identified that are eliminated by silencing factors through a mechanistic hierarchy governed by Clr4. The SHREC HDAC activity plays a major role in the elimination of class I-IV NFRs by antagonizing the action of RSC, a remodeling complex implicated in NFR formation. We propose that heterochromatin formation involves the deployment in several sequence-specific mechanisms to eliminate gaps between nucleosomes, thereby blocking access to the DNA.

Relationship between nucleosome positioning and DNA methylation

Nucleosomes compact and regulate access to DNA in the nucleus, and are composed of approximately 147 bases of DNA wrapped around a histone octamer. Here we report a genome-wide nucleosome positioning analysis of Arabidopsis thaliana using massively parallel sequencing of mononucleosomes. By combining this data with profiles of DNA methylation at single base resolution, we identified 10-base periodicities in the DNA methylation status of nucleosome-bound DNA and found that nucleosomal DNA was more highly methylated than flanking DNA. These results indicate that nucleosome positioning influences DNA methylation patterning throughout the genome and that DNA methyltransferases preferentially target nucleosome-bound DNA. We also observed similar trends in human nucleosomal DNA, indicating that the relationships between nucleosomes and DNA methyltransferases are conserved. Finally, as has been observed in animals, nucleosomes were highly enriched on exons, and preferentially positioned at intron-exon and exon-intron boundaries. RNA polymerase II (Pol II) was also enriched on exons relative to introns, consistent with the hypothesis that nucleosome positioning regulates Pol II processivity. DNA methylation is also enriched on exons, consistent with the targeting of DNA methylation to nucleosomes, and suggesting a role for DNA methylation in exon definition.

The chromatin remodeller ACF acts as a dimeric motor to space nucleosomes

Evenly spaced nucleosomes directly correlate with condensed chromatin and gene silencing. The ATP-dependent chromatin assembly factor (ACF) forms such structures in vitro and is required for silencing in vivo. ACF generates and maintains nucleosome spacing by constantly moving a nucleosome towards the longer flanking DNA faster than the shorter flanking DNA. But how the enzyme rapidly moves back and forth between both sides of a nucleosome to accomplish bidirectional movement is unknown. We show that nucleosome movement depends cooperatively on two ACF molecules, suggesting that ACF functions as a dimer of ATPases. Further, the nucleotide state determines whether the dimer closely engages one vs. both sides of the nucleosome. Three-dimensional reconstruction by single particle electron microscopy of the ATPase-nucleosome complex in an activated ATP state reveals a dimer architecture in which the two ATPases face each other. Our results suggest a model in which the two ATPases work in a coordinated manner, taking turns to engage either side of a nucleosome, thereby allowing processive bidirectional movement. This novel dimeric motor mechanism differs from that of dimeric motors such as kinesin and dimeric helicases that processively translocate unidirectionally and reflects the unique challenges faced by motors that move nucleosomes.

The molecular basis for stability of heterochromatin-mediated silencing in mammals

The archetypal epigenetic phenomenon of position effect variegation (PEV) in Drosophila occurs when a gene is brought abnormally close to heterochromatin, resulting in stochastic silencing of the affected gene in a proportion of cells that would normally express it. PEV has been instrumental in unraveling epigenetic mechanisms. Using an in vivo mammalian model for PEV we have extensively investigated the molecular basis for heterochromatin-mediated gene silencing. Here we distinguish 'epigenetic effects' from other cellular differences by studying ex vivo cells that are identical, apart from the expression of the variegating gene which is silenced in a proportion of the cells. By separating cells according to transgene expression we show here that silencing appears to be associated with histone H3 lysine 9 trimethylation (H3K9me3), DNA methylation and the localization of the silenced gene to a specific nuclear compartment enriched in these modifications. In contrast, histone H3 acetylation (H3Ac) and lysine 4 di or tri methylation (H3K4me2/3) are the predominant modifications associated with expression where we see the gene in a euchromatic compartment. Interestingly, DNA methylation and inaccessibility, rather than H3K9me3, correlated most strongly with resistance to de-repression by cellular activation. These results have important implications for understanding the contribution of specific factors involved in the establishment and maintenance of gene silencing and activation in vivo.

Histone H1 binding is inhibited by histone variant H3.3

Linker histones are involved in the formation of higher-order chromatin structure and the regulation of specific genes, yet it remains unclear what their principal binding determinants are. We generated a genome-wide high-resolution binding map for linker histone H1 in Drosophila cells, using DamID. H1 binds at similar levels across much of the genome, both in classic euchromatin and heterochromatin. Strikingly, there are pronounced dips of low H1 occupancy around transcription start sites for active genes and at many distant cis-regulatory sites. H1 dips are not due to lack of nucleosomes; rather, all regions with low binding of H1 show enrichment of the histone variant H3.3. Knockdown of H3.3 causes H1 levels to increase at these sites, with a concomitant increase in nucleosome repeat length. These changes are independent of transcriptional changes. Our results show that the H3.3 protein counteracts association of H1, providing a mechanism to keep diverse genomic sites in an open chromatin conformation.

A lot about a little dot - lessons learned from Drosophila melanogaster chromosome 4

The fourth chromosome of Drosophila melanogaster has a number of unique properties that make it a convenient model for the study of chromatin structure. Only 4.2 Mb overall, the 1.2 Mb distal arm of chromosome 4 seen in polytene chromosomes combines characteristics of heterochromatin and euchromatin. This domain has a repeat density of ~35%, comparable to some pericentric chromosome regions, while maintaining a gene density similar to that of the other euchromatic chromosome arms. Studies of position-effect variegation have revealed that heterochromatic and euchromatic domains are interspersed on chromosome 4, and both cytological and biochemical studies have demonstrated that chromosome 4 is associated with heterochromatic marks, such as heterochromatin protein 1 and histone 3 lysine 9 methylation. Chromosome 4 is also marked by POF (painting-of-fourth), a chromosome 4-specific chromosomal protein, and utilizes a dedicated histone methyltransferase, EGG. Studies of chromosome 4 have helped to shape our understanding of heterochromatin domains and their establishment and maintenance. In this review, we provide a synthesis of the work to date and an outlook to the future.

Domains of heterochromatin protein 1 required for Drosophila melanogaster heterochromatin spreading

Centric regions of eukaryotic genomes are packaged into heterochromatin, which possesses the ability to spread along the chromosome and silence gene expression. The process of spreading has been challenging to study at the molecular level due to repetitious sequences within centric regions. A heterochromatin protein 1 (HP1) tethering system was developed that generates "ectopic heterochromatin" at sites within euchromatic regions of the Drosophila melanogaster genome. Using this system, we show that HP1 dimerization and the PxVxL interaction platform formed by dimerization of the HP1 chromo shadow domain are necessary for spreading to a downstream reporter gene located 3.7 kb away. Surprisingly, either the HP1 chromo domain or the chromo shadow domain alone is sufficient for spreading and silencing at a downstream reporter gene located 1.9 kb away. Spreading is dependent on at least two H3K9 methyltransferases, with SU(VAR)3-9 playing a greater role at the 3.7-kb reporter and dSETDB1 predominately acting at the 1.9 kb reporter. These data support a model whereby HP1 takes part in multiple mechanisms of silencing and spreading.

High-resolution mapping of heterochromatin redistribution in a Drosophila position-effect variegation model

BACKGROUND

Position-effect variegation (PEV) is the stochastic transcriptional silencing of a gene positioned adjacent to heterochromatin. white-mottled X-chromosomal inversions in Drosophila are classic PEV models that show variegation of the eye color gene white due to its relocation next to pericentric heterochromatin. It has been suggested that in these models the spreading of heterochromatin across the rearrangement breakpoint causes the silencing of white. However, the extent of this spreading and the precise pattern of heterochromatin redistribution have remained unclear. To obtain insight into the mechanism of PEV, we constructed high-resolution binding maps of Heterochromatin Protein 1 (HP1) on white-mottled chromosomes.

RESULTS

We find that HP1 invades euchromatin across the inversion breakpoints over approximately 175 kb and approximately 30 kb, causing de novo association of HP1 with 20 genes. However, HP1 binding levels in these regions show substantial local variation, and white is the most strongly bound gene. Remarkably, white is also the only gene that is detectably repressed by heterochromatin. Furthermore, we find that HP1 binding to the invaded region is particularly sensitive to the dosage of the histone methyltransferase Su(var)3-9, indicating that the de novo formed heterochromatin is less stable than naturally occurring constitutive heterochromatin.

CONCLUSION

Our molecular maps demonstrate that heterochromatin can invade a normally euchromatic region, yet the strength of HP1 binding and effects on gene expression are highly dependent on local context. Our data suggest that the white gene has an unusual intrinsic affinity for heterochromatin, which may cause this gene to be more sensitive to PEV than most other genes.

Cellular mechanism for targeting heterochromatin formation in Drosophila

Near the end of their 1990 historical perspective article "60 Years of Mystery," Spradling and Karpen (1990) observe: "Recent progress in understanding variegation at the molecular level has encouraged some workers to conclude that the heterochromatization model is essentially correct and that position-effect variegation can now join the mainstream of molecular biology." In the 18 years since those words were written, heterochromatin and its associated position effects have indeed joined the mainstream of molecular biology. Here, we review the findings that led to our current understanding of heterochromatin formation in Drosophila and the mechanistic insights into heterochromatin structural and functional properties gained through molecular genetics and cytology.

A role for RNAi in heterochromatin formation in Drosophila

Heterochromatin is a specialized form of DNA packaging that results in a transcriptionally inactive conformation. While much progress has been made in characterizing the heterochromatin structure biochemically and via its effects on genes and transgenes, very little is known about how heterochromatin formation is initiated. Recent evidence from the yeast Saccharomyces pombe suggests the involvement of the RNA interference (RNAi) machinery in heterochromatin formation, and in particular in the targeting of the heterochromatin machinery to specific sites in the genome. In this article, we review the evidence for an involvement of RNAi in heterochromatin formation in the model system Drosophila melanogaster. It appears that while there are numerous threads that connect heterochromatin formation and gene silencing with the RNAi pathways in Drosophila, a direct role for RNAi in particular in the targeting of heterochromatin formation is still lacking.

An investigation of heterochromatin domains on the fourth chromosome of Drosophila melanogaster

The banded portion of Drosophila melanogaster chromosome 4 exhibits euchromatic and heterochromatic characteristics. Reminiscent of heterochromatin, it contains a high percentage of repetitive elements, does not undergo recombination, and exhibits high levels of HP1 and histone-3 lysine-9 dimethylation. However, in the distal 1.2 Mb, the gene density is typical of euchromatin, and this region is polytene in salivary gland nuclei. Using P-element reporters carrying a copy of hsp70-white, alternative chromatin packaging domains can be distinguished by the eye color phenotype. Mapping studies identified the repetitive element 1360 as a candidate for heterochromatin targeting in the fourth chromosome Hcf region. We report here two new screens using this reporter to look for additional heterochromatin target sites. We confirm that reporter elements within 10 kb of 1360 are usually packaged as heterochromatin; however, heterochromatin packaging occurs in the sv region in the absence of 1360. Analyses of the sequences adjacent to P-element reporters show no simple association between specific repeated elements and transgene expression phenotype on a whole chromosome level. The data require that heterochromatin formation as a whole depends on a more complex pattern of sequence organization rather than the presence of a single sequence element.

Transcription and RNA interference in the formation of heterochromatin

Transcription in heterochromatin seems to be an oxymoron--surely the 'silenced' form of chromatin should not be transcribed. But there have been frequent reports of low-level transcription in heterochromatic regions, and several hundred genes are found in these regions in Drosophila. Most strikingly, recent investigations implicate RNA interference mechanisms in targeting and maintaining heterochromatin, and these mechanisms are inherently dependent on transcription. Silencing of chromatin might involve trans-acting sources of the crucial small RNAs that carry out RNA interference, but in some cases, transcription of the region to be silenced seems to be required--an apparent contradiction.

SHREC, an effector complex for heterochromatic transcriptional silencing

Transcriptional gene silencing (TGS) is the mechanism generally thought by which heterochromatin effects silencing. However, recent discovery in fission yeast of a cis-acting posttranscriptional gene-silencing (cis-PTGS) pathway operated by the RNAi machinery at heterochromatin challenges the role of TGS in heterochromatic silencing. Here, we describe a multienzyme effector complex (termed SHREC) that mediates heterochromatic TGS in fission yeast. SHREC consists of a core quartet of proteins - Clr1, Clr2, Clr3, and Mit1 - which distribute throughout all major heterochromatin domains to effect TGS via distinct activities associated with the histone deacetylase Clr3 and the SNF2 chromatin-remodeling factor homolog Mit1. SHREC is also recruited to the telomeres by multiple independent mechanisms involving telomere binding protein Ccq1 cooperating with Taz1 and the RNAi machinery, and to euchromatic sites, via mechanism(s) distinct from its heterochromatin localization aided by Swi6/HP1. Our analyses suggest that SHREC regulates nucleosome positioning to assemble higher-order chromatin structures critical for heterochromatin functions.

Heterochromatin protein 2 interacts with Nap-1 and NURF: a link between heterochromatin-induced gene silencing and the chromatin remodeling machinery in Drosophila

Heterochromatin protein 2 (HP2) is a nonhistone chromosomal protein from Drosophila melanogaster that binds to heterochromatin protein 1 (HP1) and has been implicated in heterochromatin-induced gene silencing. Heretofore, HP1 has been the only known binding partner of HP2, a large protein devoid of sequence motifs other than a pair of AT hooks. In an effort to identify proteins that interact with HP2 and assign functions to its various domains, nuclear proteins were fractionated under nondenaturing conditions. On separation of nuclear proteins, nucleosome assembly protein 1 (Nap-1) has an overlapping elution profile with HP2 (assayed by Western blot) and has been identified by mass spectrometry in fractions with HP2. Upon probing fractions in which HP2 and Nap-1 are both present, we find that the nucleosome remodeling factor (NURF), an ISWI-dependent chromatin remodeling complex, is also present. Results from coimmunoprecipitation experiments suggest that HP2 interacts with Nap-1 as well as with NURF; NURF appears to interact directly with both HP2 and Nap-1. Three distinct domains within HP2 mediate the interaction with NURF, allowing us to assign NURF binding domains in addition to the AT hooks and HP1 binding domains already mapped in HP2. Mutations in Nap-1 are shown to suppress position effect variegation, suggesting that Nap-1 functions to help to assemble chromatin into a closed form, as does HP2. On the basis of these interactions, we speculate that HP2 may cooperate with these factors in the remodeling of chromatin for silencing.

Gene regulation by chromatin structure: paradigms established in Drosophila melanogaster

Studies in Drosophila melanogaster have revealed paradigms for regulating gene expression through chromatin structure, including mechanisms of gene activation and silencing. Regulation occurs at the level of individual genes, chromosomal domains, and entire chromosomes. The chromatin state is dynamic, allowing for changes in gene expression in response to cellular signals and/or environmental cues. Changes in chromatin result from the action of ATP-dependent chromatin-remodeling complexes, reversible epigenetic histone modifications, and the incorporation of histone variants. Many of the chromatin-based transcriptional regulatory mechanisms discovered in D. melanogaster are evolutionarily conserved and therefore serve as a foundation for studies in other organisms.

Supranucleosomal Organization of Chromatin Fibers in Nuclei ofDrosophilaS2 Cells

Earlier, the interphase chromatin structures could not be visualized due to the stickiness of the nuclear material. We have reduced stickiness by the reversal of permeabilization allowing the isolation and microscopic imaging of interphase chromatin structures. By using a high resolution of synchronization, collecting 36 elutriation fractions, we show that major intermediates of chromatin condensation include: (a) decondensed veillike chromatin at the unset of the S phase (2.0-2.2 C-value), (b) polarization of veiled chromatin (2.2-2.6 C), (c) fibrous chromatin (2.6-3.0 C), chromatin bodies (3.0-3.3 C), early precondensed chromosomes (3.3-3.6). The compaction of Drosophila chromosomes did not reach that of the mammalian cells in the final stage of condensation (3.6-4.0 C). Drosophila chromosomes consist of smaller units called rodlets. Results demonstrate that nucleosomal chromatin ("beads on string") does not form a solenoid structure; rather, the topological arrangement consists of meandering and plectonemic loops.