The Core Histone Tail Domains Contribute to Sequence-dependent Nucleosome Positioning

High-Speed Atomic Force Microscopy Reveals the Nucleosome Sliding and DNA Unwrapping/Wrapping Dynamics of Tail-less Nucleosomes

Each nucleosome contains four types of histone proteins, each with a histone tail. These tails are essential for the epigenetic regulation of gene expression through post-translational modifications (PTMs). However, their influence on nucleosome dynamics at the single-molecule level remains undetermined. Here, we employed high-speed atomic force microscopy to visualize nucleosome dynamics in the absence of the N-terminal tail of each histone or all of the N-terminal tails. Loss of all tails stripped 6.7 base pairs of the nucleosome from the histone core, and the DNA entry-exit angle expanded by 18° from that of wild-type nucleosomes. Tail-less nucleosomes, particularly those without H2B and H3 tails, showed a 10-fold increase in dynamics, such as nucleosome sliding and DNA unwrapping/wrapping, within 0.3 s, emphasizing their role in histone-DNA interactions. Our findings illustrate that N-terminal histone tails stabilize the nucleosome structure, suggesting that histone tail PTMs modulate nucleosome dynamics.

Histone N-tails modulate sequence-specific positioning of nucleosomes

The precise mechanisms governing sequence-dependent positioning of nucleosomes on DNA remain unknown in detail. Existing algorithms, taking into account the sequence-dependent deformability of DNA and its interactions with the histone globular domains, predict rotational setting of only 65% of human nucleosomes mapped in vivo. To uncover novel factors responsible for the nucleosome positioning, we analyzed potential involvement of the histone N-tails in this process. To this aim, we reconstituted the H2A/H4 N-tailless nucleosomes on human BRCA1 DNA (~100 kb) and compared their positions and sequences with those of the wild-type nucleosomes. In the case of H2A tailless nucleosomes, the AT content of DNA sequences is changed locally at superhelical location (SHL) ±4, while maintaining the same rotational setting as their wild-type counterparts. Conversely, the H4 tailless nucleosomes display widespread changes of the AT content near SHL ±1 and SHL ±2, where the H4 N-tails interact with DNA. Furthermore, a substantial number of H4 tailless nucleosomes exhibit rotational setting opposite to that of the wild-type nucleosomes. Thus, our findings strongly suggest that the histone N-tails are operative in selection of nucleosome positions, which may have wide-ranging implications for epigenetic modulation of chromatin states.

Retroviral prototype foamy virus intasome binding to a nucleosome target does not determine integration efficiency

Retroviral integrases must navigate host DNA packaged as chromatin during integration of the viral genome. Prototype foamy virus (PFV) integrase (IN) forms a tetramer bound to two viral DNA (vDNA) ends in a complex termed an intasome. PFV IN consists of four domains: the amino terminal extension domain (NED), amino terminal domain (NTD), catalytic core domain (CCD), and carboxyl terminal domain (CTD). The domains of the two inner IN protomers have been visualized, as well as the CCDs of the two outer IN protomers. However, the roles of the amino and carboxyl terminal domains of the PFV intasome outer subunits during integration to a nucleosome target substrate are not clear. We used the well-characterized 601 nucleosome to assay integration activity as well as intasome binding. PFV intasome integration to 601 nucleosomes occurs in clusters at four independent sites. We find that the outer protomer NED and NTD domains have no significant effects on integration efficiency, site selection, or binding. The CTDs of the outer PFV intasome subunits dramatically affect nucleosome binding but have little effect on total integration efficiency. The outer PFV IN CTDs did significantly alter the integration efficiency at one site. Histone tails also significantly affect intasome binding, but have little impact on PFV integration efficiency or site selection. These results indicate that binding to nucleosomes does not correlate with integration efficiency and suggests most intasome-binding events are unproductive.

Nucleosome Core Particles Lacking H2B or H3 Tails Are Altered Structurally and Have Differential Base Excision Repair Fingerprints

A recently discovered post-translational modification of histone proteins is the irreversible proteolytic clipping of the histone N-terminal tail domains. This modification is involved in the regulation of various biological processes, including the DNA damage response. In this work, we used chemical footprinting to characterize the structural alterations to nucleosome core particles (NCPs) that result from a lack of a histone H2B or H3 tail. We also examine the influence of these histone tails on excision of the mutagenic lesion 1,N⁶-ethenoadenine (εA) by the repair enzyme alkyladenine DNA glycosylase. We found that the absence of the H2B or H3 tail results in altered DNA periodicity relative to that of native NCPs. We correlated these structural alterations to εA excision by utilizing a global analysis of 21 εA sites in NCPs and unincorporated duplex DNA. In comparison to native NCPs, there is enhanced excision of εA in tailless H2B NCPs in regions that undergo DNA unwrapping. This enhanced excision is not observed for tailless H3 NCPs; rather, excision is inhibited in more static areas of the NCP not prone to unwrapping. Our results support in vivo observations of alkylation damage profiles and the potential role of tail clipping as a mechanism for overcoming physical obstructions caused by packaging in NCPs but also reveal the potential inhibition of repair by tail clipping in some locations. Taken together, these results further our understanding of how base excision repair can be facilitated or diminished by histone tail removal and contribute to our understanding of the underlying mechanism that leads to mutational hot spots.

Chromatin-modifying drugs and metabolites in cell fate control

For multicellular organisms, it is essential to produce a variety of specialized cells to perform a dazzling panoply of functions. Chromatin plays a vital role in determining cellular identities, and it dynamically regulates gene expression in response to changing nutrient metabolism and environmental conditions. Intermediates produced by cellular metabolic pathways are used as cofactors or substrates for chromatin modification. Drug analogues of metabolites that regulate chromatin-modifying enzyme reactions can also regulate cell fate by adjusting chromatin organization. In recent years, there have been many studies about how chromatin-modifying drug molecules or metabolites can interact with chromatin to regulate cell fate. In this review, we systematically discuss how DNA and histone-modifying molecules alter cell fate by regulating chromatin conformation and propose a mechanistic model that explains the process of cell fate transitions in a concise and qualitative manner.

Sequence-dependent nucleosome nanoscale structure characterized by atomic force microscopy

DNA sequence plays an important role in the assembly of nucleosomes, and DNA motifs with high specificity to nucleosomes have been identified. At the same time, important questions such as how the DNA sequence changes DNA wrapping and how the DNA sequence contributes to the interaction between the nucleosomes remain unclear. Here, we addressed these questions by comparing nanoscale properties of nucleosomes assembled on the highest nucleosome positioning sequence, the 601 motif, and essentially random DNA sequences. We used atomic force microscopy to measure the nucleosome positions and the DNA wrapping. The studies showed that nucleosomes assemble on the nonspecific sequence without any preference for position on DNA, but they wrap the same DNA length as DNA of the same length with the 601 sequence. Experiments with longer DNA containing the 601 motif along with nonspecific DNA, capable of forming dinucleosomes, revealed that dinucleosomes assembled on hybrid sequences show a preference for positioning near each other, with one always assembling on the 601 motif. These findings point to the interaction between the nucleosomes and suggest that internucleosomal interactions may play a large role in nucleosome positioning.-Stormberg, T., Stumme-Diers, M., Lyubchenko, Y. L. Sequence-dependent nucleosome nanoscale structure characterized by atomic force microscopy.

A cassette of basic amino acids in histone H2B regulates nucleosome dynamics and access to DNA damage

Nucleosome dynamics, such as spontaneous DNA unwrapping, are postulated to have a critical role in regulating the access of DNA repair machinery to DNA lesions within nucleosomes. However, the specific histone domains that regulate nucleosome dynamics and the impact of such changes in intrinsic nucleosome dynamics on DNA repair are not well understood. Previous studies identified a highly conserved region in the N-terminal tail of histone H2B known as the histone H2Brepression (or HBR) domain, which has a significant influence on gene expression, chromatin assembly, and DNA damage formation and repair. However, the molecular mechanism(s) that may account for these observations are limited. In this study, we characterized the stability and dynamics of ΔHBR mutant nucleosome core particles (NCPs) in vitro by restriction enzyme accessibility (REA), FRET, and temperature-induced sliding of histone octamers. Our results indicate that ΔHBR-NCPs are more dynamic, with a larger steady-state fraction of the NCP population occupying the unwrapped state than for WT-NCPs. Additionally, ΔHBR-histone octamers are more susceptible to temperature-induced sliding on DNA than WT histone octamers. Furthermore, we show that the activity of base excision repair enzymes at uracil lesions and single nucleotide gaps is enhanced in a site-specific manner in ΔHBR-NCPs. This enhanced activity correlates well with regions exhibiting increased DNA unwrapping. Finally, removal of the HBR domain is not sufficient to completely alleviate the structural constraints imposed by histone octamers on the activity of base excision repair enzymes.

Coupling between Histone Conformations and DNA Geometry in Nucleosomes on a Microsecond Timescale: Atomistic Insights into Nucleosome Functions

An octamer of histone proteins wraps about 200bp of DNA into two superhelical turns to form nucleosomes found in chromatin. Although the static structure of the nucleosomal core particle has been solved, details of the dynamic interactions between histones and DNA remain elusive. We performed extensively long unconstrained, all-atom microsecond molecular dynamics simulations of nucleosomes including linker DNA segments and full-length histones in explicit solvent. For the first time, we were able to identify and characterize the rearrangements in nucleosomes on a microsecond timescale including the coupling between the conformation of the histone tails and the DNA geometry. We found that certain histone tail conformations promoted DNA bulging near its entry/exit sites, resulting in the formation of twist defects within the DNA. This led to a reorganization of histone-DNA interactions, suggestive of the formation of initial nucleosome sliding intermediates. We characterized the dynamics of the histone tails upon their condensation on the core and linker DNA and showed that tails may adopt conformationally constrained positions due to the insertion of "anchoring" lysines and arginines into the DNA minor grooves. Potentially, these phenomena affect the accessibility of post-translationally modified histone residues that serve as important sites for epigenetic marks (e.g., at H3K9, H3K27, H4K16), suggesting that interactions of the histone tails with the core and linker DNA modulate the processes of histone tail modifications and binding of the effector proteins. We discuss the implications of the observed results on the nucleosome function and compare our results to different experimental studies.

Accessing DNA damage in chromatin: Preparing the chromatin landscape for base excision repair

DNA damage in chromatin comes in many forms, including single base lesions that induce base excision repair (BER). We and others have shown that the structural location of DNA lesions within nucleosomes greatly influences their accessibility to repair enzymes. Indeed, a difference in the location of uracil as small as one-half turn of the DNA backbone on the histone surface can result in a 10-fold difference in the time course of its removal in vitro. In addition, the cell has evolved several interdependent processes capable of enhancing the accessibility of excision repair enzymes to DNA lesions in nucleosomes, including post-translational modification of histones, ATP-dependent chromatin remodeling and interchange of histone variants in nucleosomes. In this review, we focus on different factors that affect accessibility of BER enzymes to nucleosomal DNA.

Global nucleosome positioning regulates salicylic acid mediated transcription in Arabidopsis thaliana

BACKGROUND

The nucleosome positioning regulates the gene expression and many other DNA-related processes in eukaryotes. Genome-wide mapping of nucleosome positions and correlation of genome-wide nucleosomal remodeling with the changes in the gene expression can help us understanding gene regulation on genome level.

RESULTS

In the present study, we correlate the gene expression and the genomic nucleosomal remodeling in response to salicylic acid (SA) treatment in A. thaliana. We have mapped genome-wide nucleosomes by performing tiling microarray using 146 bp mononucleosomal template DNA. The average nucleosomal coverage is approximately 346 bp per nucleosome both under the control and the SA-treated conditions. The nucleosomal coverage is more in the coding region than in the 5' regulatory regions. We observe approximately 50% nucleosomal remodeling on SA treatment where significant nucleosomal depletion and nucleosomal enrichment around the transcription start site (TSS) occur in SA induced genes and SA repressed genes respectively in response to SA treatment. Especially in the case of the SA-induced group, the nucleosomal remodeling over the minimal promoter in response to SA is especially significant in the Non-expresser of PR1 (NPR1)-dependent genes. A detailed investigation of npr1-1 mutant confirms a distinct role of NPR1 in the nucleosome remodeling over the core promoter. We have also identified several motifs for various hormonal responses; including ABRE elements in the remodeled nucleosomal regions around the promoter region in the SA regulated genes. We have further identified that the W-box and TGACG/C motif, reported to play an important role in SA-mediated induction, are enriched in nucleosome free regions (NFRs) of the promoter region of the SA induced genes.

CONCLUSIONS

This is the first study reporting genome-wide effects of SA treatment on the chromatin architecture of A. thaliana. It also reports significant role of NPR1 in genome-wide nucleosomal remodeling in response to SA.

Binding of NF-κB to nucleosomes: effect of translational positioning, nucleosome remodeling and linker histone H1

NF-κB is a key transcription factor regulating the expression of inflammatory responsive genes. How NF-κB binds to naked DNA templates is well documented, but how it interacts with chromatin is far from being clear. Here we used a combination of UV laser footprinting, hydroxyl footprinting and electrophoretic mobility shift assay to investigate the binding of NF-κB to nucleosomal templates. We show that NF-κB p50 homodimer is able to bind to its recognition sequence, when it is localized at the edge of the core particle, but not when the recognition sequence is at the interior of the nucleosome. Remodeling of the nucleosome by the chromatin remodeling machine RSC was not sufficient to allow binding of NF-κB to its recognition sequence located in the vicinity of the nucleosome dyad, but RSC-induced histone octamer sliding allowed clearly detectable binding of NF-κB with the slid particle. Importantly, nucleosome dilution-driven removal of H2A–H2B dimer led to complete accessibility of the site located close to the dyad to NF-κB. Finally, we found that NF-κB was able to displace histone H1 and prevent its binding to nucleosome. These data provide important insight on the role of chromatin structure in the regulation of transcription of NF-κB dependent genes.

In eukaryotes DNA is hierarchically packaged into chromatin by histones. The fundamental subunit of chromatin is the nucleosome. The packaging of DNA into nucleosomes not only restricts DNA accessibility for regulatory proteins but also provides opportunities to regulate DNA based processes. Accessibility of transcription factor NF-κB to their recognition sequences embedded in nucleosomes is highly controversial. On one hand in vivo studies have suggested that packaging of DNA into chromatin plays an important role in regulating the expression of NF-κB dependent genes, and on the other hand some in vitro studies reported that NF-κB can bind by itself to its recognition sequences embedded in the nucleosome. In this study, we show that NF-κB can specifically bind to its recognition sequences placed at the end of the nucleosome but not when placed inside the nucleosome core. We then demonstrate that disruption of nucleosome is necessary for the productive binding of NF-κB. Finally, we show that the presence of histone H1 does not affect the specific binding of NF-κB to its cognate sequence, when its binding region overlaps with the binding site of NF-κB. We propose that histone eviction is needed for NF-κB to bind specifically to its recognition sequence embedded in the nucleosome.

Contribution of histone N-terminal tails to the structure and stability of nucleosomes

Histones are the protein components of the nucleosome, which forms the basic architecture of eukaryotic chromatin. Histones H2A, H2B, H3, and H4 are composed of two common regions, the "histone fold" and the "histone tail". Many efforts have been focused on the mechanisms by which the post-translational modifications of histone tails regulate the higher-order chromatin architecture. On the other hand, previous biochemical studies have suggested that histone tails also affect the structure and stability of the nucleosome core particle itself. However, the precise contributions of each histone tail are unclear. In the present study, we determined the crystal structures of four mutant nucleosomes, in which one of the four histones, H2A, H2B, H3, or H4, lacked the N-terminal tail. We found that the deletion of the H2B or H3 N-terminal tail affected histone-DNA interactions and substantially decreased nucleosome stability. These findings provide important information for understanding the complex roles of histone tails in regulating chromatin structure.

Probing the effects of DNA-protein interactions on DNA hole transport: the N-terminal histone tails modulate the distribution of oxidative damage and chemical lesions in the nucleosome core particle

The ability of DNA to transport positive charges, or holes, over long distances is well-established, but the mechanistic details of how this process is influenced by packaging into DNA-protein complexes have not been fully delineated. In eukaryotes, genomic DNA is packaged into chromatin through its association with the core histone octamer to form the nucleosome core particle (NCP), a complex whose structure can be modulated through changes in the local environment and the histone proteins. Because (i) varying the salt concentration and removing the histone tails influence the structure of the NCP in known ways and (ii) previous studies have shown that DNA hole transport (HT) occurs in the nucleosome, we have used our previously described 601 sequence NCPs to test the hypothesis that DNA HT dynamics can be modulated by structural changes in a DNA-protein complex. We show that at low salt concentrations there is a sharp increase in long-range DNA HT efficiency in the NCP as compared to naked DNA. This enhancement of HT can be negated by either removal of the histone tails at low salt concentrations or disruption of the interaction of the packaged DNA and the histone tails by increasing the buffer's ionic strength. Association of the histone tails with 601 DNA at low salt concentrations shifts the guanine damage spectrum to favor lesions like 8-oxoguanine in the NCP, most likely through modulation of the rate of the reaction of the guanine radical cation with oxygen. These experimental results indicate that for most genomic DNA, the influence of DNA-protein interactions on DNA HT will depend strongly on the level of protection of the DNA nucleobases from oxygen. Further, these results suggest that the oxidative damage arising from DNA HT may vary in different genomic regions depending on the presence of either euchromatin or heterochromatin.

Preparation of nucleosomes containing a specific H2A-H2A cross-link forming a DNA-constraining loop structure

ATP-dependent chromatin-remodeling complexes use the energy of ATP hydrolysis to alter nucleosome structure, increase the accessibility of trans-acting factors, and induce nucleosome movement on the nucleosomal DNA. Recent studies suggest that bulge propagation is a major component of the mechanism for SWI/SNF remodeling. We describe in detail a method to prepare a mononucleosomal substrate in which the two H2A N-terminal tails are cross-linked in an intranucleosomal fashion, forming a closed loop around the two superhelical winds of DNA. This substrate is useful for researchers who wish to test processes in which the DNA is transiently or permanently lifted off the histone surface, such as in the bulge propagation model. Our method allows assessment of the extent of cross-linking within the population of nucleosomes used in small-scale experiments, such as assays to test SWI/SNF-remodeling activities.

The divalent cations Ca2+ and Mg2+ play specific roles in stabilizing histone-DNA interactions within nucleosomes that are partially redundant with the core histone tail domains

We previously reported that reconstituted nucleosomes undergo sequence-dependent translational repositioning upon removal of the core histone tail domains under physiological conditions, indicating that the tails influence the choice of position. We report here that removal of the core histone tail domains increases the exposure of the DNA backbone in nucleosomes to hydroxyl radicals, a nonbiased chemical cleavage reagent, indicative of an increase in the motility of the DNA on the histone surface. Moreover, we demonstrate that the divalent cations Mg(2+) and Ca(2+) can replace the role of the tail domains with regard to stabilization of histone-DNA interactions within the nucleosome core and restrict repositioning of nucleosomes upon tail removal. However, when nucleosomes were incubated with Mg(2+) after tail removal, the original distribution of translational positions was not re-established, indicating that divalent cations increase the energy barrier between translational positions rather than altering the free energy differences between positions. Interestingly, other divalent cations such as Zn(2+), Fe(2+), Co(2+), and Mn(2+) had little or no effect on the stability of histone-DNA interactions within tailless nucleosomes. These results support the idea that specific binding sites for Mg(2+) and Ca(2+) ions exist within the nucleosome and play a critical role in nucleosome stability that is partially redundant with the core histone tail domains.

SWI/SNF- and RSC-catalyzed nucleosome mobilization requires internal DNA loop translocation within nucleosomes

The multisubunit SWI/SNF and RSC complexes utilize energy derived from ATP hydrolysis to mobilize nucleosomes and render the DNA accessible for various nuclear processes. Here we test the idea that remodeling involves intermediates with mobile DNA bulges or loops within the nucleosome by cross-linking the H2A N- or C-terminal tails together to generate protein "loops" that constrict separation of the DNA from the histone surface. Analyses indicate that this intranucleosomal cross-linking causes little or no change in remodeling-dependent exposure of DNA sequences within the nucleosome to restriction enzymes. However, cross-linking inhibits nucleosome mobilization and blocks complete movement of nucleosomes to extreme end positions on the DNA fragments. These results are consistent with evidence that nucleosome remodeling involves intermediates with DNA loops on the nucleosome surface but indicate that such loops do not freely diffuse about the surface of the histone octamer. We propose a threading model for movement of DNA loops around the perimeter of the nucleosome core.

Nucleosome positioning in Saccharomyces cerevisiae

The DNA of eukaryotic cells is spooled around large histone protein complexes, forming nucleosomes that make up the basis for a high-order packaging structure called chromatin. Compared to naked DNA, nucleosomal DNA is less accessible to regulatory proteins and regulatory processes. The exact positions of nucleosomes therefore influence several cellular processes, including gene expression, chromosome segregation, recombination, replication, and DNA repair. Here, we review recent technological advances enabling the genome-wide mapping of nucleosome positions in the model eukaryote Saccharomyces cerevisiae. We discuss the various parameters that determine nucleosome positioning in vivo, including cis factors like AT content, variable tandem repeats, and poly(dA:dT) tracts that function as chromatin barriers and trans factors such as chromatin remodeling complexes, transcription factors, histone-modifying enzymes, and RNA polymerases. In the last section, we review the biological role of chromatin in gene transcription, the evolution of gene regulation, and epigenetic phenomena.

Activity of FEN1 endonuclease on nucleosome substrates is dependent upon DNA sequence but not flap orientation

We demonstrated previously that human FEN1 endonuclease, an enzyme involved in excising single-stranded DNA flaps that arise during Okazaki fragment processing and base excision repair, cleaves model flap substrates assembled into nucleosomes. Here we explore the effect of flap orientation with respect to the surface of the histone octamer on nucleosome structure and FEN1 activity in vitro. We find that orienting the flap substrate toward the histone octamer does not significantly alter the rotational orientation of two different nucleosome positioning sequences on the surface of the histone octamer but does cause minor perturbation of nucleosome structure. Surprisingly, flaps oriented toward the nucleosome surface are accessible to FEN1 cleavage in nucleosomes containing the Xenopus 5S positioning sequence. In contrast, neither flaps oriented toward nor away from the nucleosome surface are cleaved by the enzyme in nucleosomes containing the high-affinity 601 nucleosome positioning sequence. The data are consistent with a model in which sequence-dependent motility of DNA on the nucleosome is a major determinant of FEN1 activity. The implications of these findings for the activity of FEN1 in vivo are discussed.

Activator-independent transcription of Snf1-dependent genes in mutants lacking histone tails

Transcriptional regulation of Snf1-dependent genes occurs in part by histone-acetylation-dependent binding of the transcription factor Adr1. Analysis of previously published microarray data indicated unscheduled transcription of a large number of Snf1- and Adr1-dependent genes when either the histone H3 or H4 tail was deleted. Quantitative real-time PCR confirmed that the tails were important to preserve stringent transcriptional repression of Snf1-dependent genes when glucose was present. The absence of the tails allowed Adr1 and RNA Polymerase II to bind promoters in normally inhibitory conditions. The promoters escaped glucose repression to a limited extent and the weak constitutive ADH2 transcription induced by deletion of the histone tails was transcription factor- and Snf1-independent. These effects were apparently due to a permissive chromatin structure that allowed transcription in the absence of repression mediated by the histone tails. Deleting REG1, and thus activating Snf1 in the H3 tail mutant enhanced transcription in repressing conditions, indicating that Snf1 and the H3 tail influence transcription independently. Deleting REG1 in the histone H4 tail mutant appeared to be lethal, even in the absence of Snf1, suggesting that Reg1 and the H4 tail have redundant functions that are important for cell viability.

Spreading chromatin into chemical biology

Epigenetics, broadly defined as the inheritance of non-Mendelian phenotypic traits, can be more narrowly defined as heritable alterations in states of gene expression ("on" versus "off") that are not linked to changes in DNA sequence. Moreover, these alterations can persist in the absence of the signals that initiate them, thus suggesting some kind of "memory" to epigenetic forms of regulation. How, for example, during early female mammalian development, is one X chromosome selected to be kept in an active state, while the genetically identical sister X chromosome is "marked" to be inactive, even though they reside in the same nucleus, exposed to the same collection of shared trans-factors? Once X inactivation occurs, how are these contrasting chromatin states maintained and inherited faithfully through subsequent cell divisions? Chromatin states, whether active (euchromatic) or silent (heterochromatic) are established, maintained, and propagated with remarkable precision during normal development and differentiation. However, mistakes made in establishing and maintaining these chromatin states, often executed by a variety of chromatin-remodeling activities, can lead to mis-expression or mis-silencing of critical downstream gene targets with far-reaching implications for human biology and disease, notably cancer. Though chromatin biologists have identified many of the "inputs" that are important for controlling chromatin states, the detailed mechanisms by which these processes work remain largely opaque, in part due to the staggering complexity of the chromatin polymer, the physiologically relevant form of our genome. The primary objective of this article is to serve as a "call to arms" for chemists to contribute to the development of the precision tools needed to answer pressing molecular problems in this rapidly moving field.

A cassette of N-terminal amino acids of histone H2B are required for efficient cell survival, DNA repair and Swi/Snf binding in UV irradiated yeast

The highly charged histone N-terminal domains are engaged in inter- and intra-nucleosomal interactions, and contain a host of sites used for posttranslational modification. We have studied the effect of deleting residues 30–37 from the N-terminal domain of histone H2B in yeast cells, on nucleotide excision repair (NER) following UV irradiation, as these cells are quite sensitive to UV. We find that H2B Δ30–37 cells exhibit reduced NER efficiency at three specific chromatin loci: the transcriptionally active, RPB2 locus; the transcriptionally silenced, nucleosome-loaded HML locus; and the transcriptionally repressed, non-silenced, GAL10 locus. Nuclease digestion studies indicate that H2B Δ30–37 chromatin has increased nucleosome accessibility and/or nucleosome mobility. In addition, H2B Δ30–37 mutants acquire more DNA damage, compared to wt cells, following the same dose of UV radiation. Reducing the level of damage in H2B Δ30–37 cells to match that of wt cells restores the NER rate to wt levels in the RPB2 and GAL10 loci, but NER efficiency remains low in the silenced HML locus. Interestingly, recruitment of Snf5 to the HML locus is reduced in H2B Δ30–37 cells and more transient following UV irradiation. This may reflect a lower binding affinity of the SWI/SNF complex to H2B Δ30–37 nucleosomes.

Characterizing nucleosome dynamics from genomic and epigenetic information using rule induction learning

BACKGROUND

Eukaryotic genomes are packaged into chromatin, a compact structure containing fundamental repeating units, the nucleosomes. The mobility of nucleosomes plays important roles in many DNA-related processes by regulating the accessibility of regulatory elements to biological machineries. Although it has been known that various factors, such as DNA sequences, histone modifications, and chromatin remodelling complexes, could affect nucleosome stability, the mechanisms of how they regulate this stability are still unclear.

RESULTS

In this paper, we propose a novel computational method based on rule induction learning to characterize nucleosome dynamics using both genomic and histone modification information. When applied on S. cerevisiae data, our method produced totally 98 rules characterizing nucleosome dynamics on chromosome III and promoter regions. Analyzing these rules we discovered that, some DNA motifs and post-translational modifications of histone proteins play significant roles in regulating nucleosome stability. Notably, these DNA motifs are strong determinants for nucleosome forming and inhibiting potential; and these histone modifications have strong relation with transcriptional activities, i.e. activation and repression. We also found some new patterns which may reflect the cooperation between these two factors in regulating the stability of nucleosomes.

CONCLUSION

DNA motifs and histone modifications can individually and, in some cases, cooperatively regulate nucleosome stability. This suggests additional insights into mechanisms by which cells control important biological processes, such as transcription, replication, and DNA repair.

Uracil DNA glycosylase activity on nucleosomal DNA depends on rotational orientation of targets

The activity of uracil DNA glycosylases (UDGs), which recognize and excise uracil bases from DNA, has been well characterized on naked DNA substrates but less is known about activity in chromatin. We therefore prepared a set of model nucleosome substrates in which single thymidine residues were replaced with uracil at specific locations and a second set of nucleosomes in which uracils were randomly substituted for all thymidines. We found that UDG efficiently removes uracil from internal locations in the nucleosome where the DNA backbone is oriented away from the surface of the histone octamer, without significant disruption of histone-DNA interactions. However, uracils at sites oriented toward the histone octamer surface were excised at much slower rates, consistent with a mechanism requiring spontaneous DNA unwrapping from the nucleosome. In contrast to the nucleosome core, UDG activity on DNA outside the core DNA region was similar to that of naked DNA. Association of linker histone reduced activity of UDG at selected sites near where the globular domain of H1 is proposed to bind to the nucleosome as well as within the extra-core DNA. Our results indicate that some sites within the nucleosome core and the extra-core (linker) DNA regions represent hot spots for repair that could influence critical biological processes.

Epigenome manipulation as a pathway to new natural product scaffolds and their congeners

The covalent modification of chromatin is an important control mechanism used by fungi to modulate the transcription of genes involved in secondary metabolite production. To date, both molecular-based and chemical approaches targeting histone and DNA posttranslational processes have shown great potential for rationally directing the activation and/or suppression of natural-product-encoding gene clusters. In this Highlight, the organization of the fungal epigenome is summarized and strategies for manipulating chromatin-related targets are presented. Applications of these techniques are illustrated using several recently published accounts in which chemical-epigenetic methods and mutant studies were successfully employed for the de novo or enhanced production of structurally diverse fungal natural products (e.g., anthraquinones, cladochromes, lunalides, mycotoxins, and nygerones).

What controls nucleosome positions?

The DNA of eukaryotic genomes is wrapped in nucleosomes, which strongly distort and occlude the DNA from access to most DNA-binding proteins. An understanding of the mechanisms that control nucleosome positioning along the DNA is thus essential to understanding the binding and action of proteins that carry out essential genetic functions. New genome-wide data on in vivo and in vitro nucleosome positioning greatly advance our understanding of several factors that can influence nucleosome positioning, including DNA sequence preferences, DNA methylation, histone variants and post-translational modifications, higher order chromatin structure, and the actions of transcription factors, chromatin remodelers and other DNA-binding proteins. We discuss how these factors function and ways in which they might be integrated into a unified framework that accounts for both the preservation of nucleosome positioning and the dynamic nucleosome repositioning that occur across biological conditions, cell types, developmental processes and disease.

H2A and H2B tails are essential to properly reconstitute nucleosome core particles

The conformation of recombinant Nucleosome Core Particles (NCPs) lacking H2A and H2B histone tails (gH2AgH2B) are studied. The migration of these particles in acrylamide native gels is slowed down compared to intact reconstituted NCPs. gH2AgH2B NCPs are also much more sensitive to nuclease digestion than intact NCPs. Small angle X-ray scattering (SAXS) experiments point out that the absence of H2A and H2B tails produces small but significant conformational changes of the octamers conformation (without wrapped DNA), whereas gH2AgH2B NCP conformations are significantly altered. A separation of about 25-30 bp from the core could account for the experimental curves, but other types of DNA superhelix deformation cannot be excluded. The distorted gH2AgH2B octamer may not allow the correct winding of DNA around the core. The absence of the H2A and H2B tails would further prevent the secondary sliding of the DNA around the core and therefore impedes the stabilisation of the particle. Cryo-electron microscopy on the same particles also shows a detachment of DNA portions from the particle core. The effect is even stronger because the vitrification of the samples worsens the instability of gH2AgH2B NCPs.

Regulation of gene transcription by the histone H2A N-terminal domain

Histone N-terminal domains play critical roles in regulating chromatin structure and gene transcription. Relatively little is known, however, about the role of the histone H2A N-terminal domain in transcription regulation. We have used DNA microarrays to characterize the changes in genome-wide expression caused by mutations in the N-terminal domain of histone H2A. Our results indicate that the N-terminal domain of histone H2A functions primarily to repress the transcription of a large subset of the Saccharomyces cerevisiae genome and that most of the H2A-repressed genes are also repressed by the histone H2B N-terminal domain. Using the histone H2A microarray data, we selected three reporter genes (BNA1, BNA2, and GCY1), which we subsequently used to map regions in the H2A N-terminal domain responsible for this transcriptional repression. These studies revealed that a small subdomain in the H2A N-terminal tail, comprised of residues 16 to 20, is required for the transcriptional repression of these reporter genes. Deletion of either the entire histone H2A N-terminal domain or just this small subdomain imparts sensitivity to UV irradiation. Finally, we show that two residues in this H2A subdomain, serine-17 and arginine-18, are specifically required for the transcriptional repression of the BNA2 reporter gene.