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PubMed1997

SWI/SNF stimulates the formation of disparate activator-nucleosome complexes but is partially redundant with cooperative binding.

To investigate the potential mechanisms by which the SWI/SNF complex differentially regulates different genes we have tested whether transcription factors with diverse DNA binding domains were able to exploit nucleosome disruption by SWI/SNF. In addition to GAL4-VP16, the SWI/SNF complex stimulated nucleosome binding by the Zn2+ fingers of Sp1, the basic helix-loop-helix domain of USF, and the rel domain of NF-kappaB. In each case SWI/SNF action resulted in the formation of a stable factor-nucleosome complex that persisted after detachment of SWI/SNF from the nucleosome. Thus, stimulation of factor binding by SWI/SNF appears to be universal. The degree of SWI/SNF stimulation of nucleosome binding by a factor appears to be inversely related to the extent that binding is inhibited by the histone octamer. Cooperative binding of 5 GAL4-VP16 dimers to a 5-site nucleosome enhanced GAL4 binding relative to a single-site nucleosome, but this also reduced the degree of stimulation by SWI/SNF. The SWI/SNF complex increased the affinity of 5 GAL4-VP16 dimers for nucleosomes equal to that of DNA but no further. Similarly, multimerized NF-kappaB sites enhanced nucleosome binding by NF-kappaB and reduced the stimulatory effect of SWI/SNF. Thus, cooperative binding of factors to nucleosomes is partially redundant with the function of the SWI/SNF complex.

Adenosine Triphosphate↗
PubMed2003

Chromatin remodeling activities act on UV-damaged nucleosomes and modulate DNA damage accessibility to photolyase.

Nucleosomes inhibit DNA repair in vitro, suggesting that chromatin remodeling activities might be required for efficient repair in vivo. To investigate how structural and dynamic properties of nucleosomes affect damage recognition and processing, we investigated repair of UV lesions by photolyase on a nucleosome positioned at one end of a 226-bp-long DNA fragment. Repair was slow in the nucleosome but efficient outside. No disruption or movement of the nucleosome was observed after UV irradiation and during repair. However, incubation with the nucleosome remodeling complex SWI/SNF and ATP altered the conformation of nucleosomal DNA as judged by UV photo-footprinting and promoted more homogeneous repair. Incubation with yISW2 and ATP moved the nucleosome to a more central position, thereby altering the repair pattern. This is the first demonstration that two different chromatin remodeling complexes can act on UV-damaged nucleosomes and modulate repair. Similar activities might relieve the inhibitory effect of nucleosomes on DNA repair processes in living cells.

Adenosine Triphosphatases↗
PubMed2003

Multiple Nhp6 molecules are required to recruit Spt16-Pob3 to form yFACT complexes and to reorganize nucleosomes.

The Saccharomyces cerevisiae Nhp6 protein contains a DNA-binding motif that is similar to those found in the high mobility group B family of chromatin proteins. Nhp6 bound to nucleosomes and made at least two changes in them: the nucleosomal DNA became more sensitive to DNase I at specific sites, and the nucleosomes became competent to bind Spt16-Pob3 to form yFACT.nucleosome complexes. Both changes occurred at similar concentrations of Nhp6, suggesting that they reflect the same structural reorganization of the nucleosome. Nucleosomes have multiple binding sites for Nhp6, and structural reorganization was associated with a concentration of Nhp6 about 10-fold higher than that needed for simple binding. We propose that the coordinated action of multiple Nhp6 molecules is required to convert nucleosomes to an alternative form as the first step in a two-step reorganization of nucleosomes with the second step being dependent on Spt16-Pob3. The presence of linker DNA had only subtle effects on these processes, indicating that both Nhp6 and yFACT act on core nucleosome structure rather than on the interaction between nucleosomes and adjacent DNA. These results suggest that Nhp6 and the related high mobility group B proteins may have a general role in promoting rearrangements of chromatin by initiating the destabilization of core nucleosomal structure.

Amino Acid Motifs↗
PubMed2006

The histone variant macro-H2A preferentially forms "hybrid nucleosomes".

The histone domain of macro-H2A, which constitutes the N-terminal one third of this histone variant, is only 64% identical to major H2A. We have shown previously that the main structural differences in a nucleosome in which both H2A moieties have been replaced by macro-H2A reside in the only point of contact between the two histone dimers, the L1-L1 interface of macro-H2A. Here we show that the L1 loop of macro-H2A is responsible for the increased salt-dependent stability of the histone octamer, with implications for the nucleosome assembly pathway. It is unknown whether only one or both of the H2A-H2B dimers within a nucleosome are replaced with H2A variant containing nucleosomes in vivo. We demonstrate that macro-H2A preferentially forms hybrid nucleosomes containing one chain each of major H2A and macro-HA in vitro. The 2.9-A crystal structure of such a hybrid nucleosome shows significant structural differences in the L1-L1 interface when comparing with homotypic major H2A- and macro-H2A-containing nucleosomes. Both homotypic and hybrid macro-nucleosome core particles (NCPs) are resistant to chaperone-assisted H2A-H2B dimer exchange. Together, our findings suggest that the histone domain of macro-H2A modifies the dynamic properties of the nucleosome. We propose that the possibility of forming hybrid macro-NCP adds yet another level of complexity to variant nucleosome structure and function.

Animals↗
PubMed1983

The release of 3-methyladenine from nucleosomal DNA by a 3-methyladenine DNA glycosylase.

Nucleosomes from chicken erythrocytes, with DNA containing an average of 144 base pairs, were alkylated with [3H]methylnitrosourea. The level of alkylation of the nucleosomal DNA was 48% of that of free DNA. The histones had approximately one tenth the radioactivity of the DNA. There was no statistically significant difference between alkylation of nucleosome bases in the major vs. minor groove. When the first 50 residues of the alkylated nucleosomal DNA were examined on sequencing gels, the 7-methylguanine and 3-methyladenine (3-MeA) residues were distributed randomly. The 3-MeA DNA glycosylase I of E. coli was used to measure the release of 3-MeA from nucleosomal DNA. Incubation at 37 degrees C resulted in a release which reached a plateau of approximately 33% of the 3-MeA groups of the nucleosomal DNA. A partially purified 3-MeA DNA glycosylase from rat liver gave similar results. The limited enzymatic release is most likely due to steric hindrance of the enzyme by the DNA-histone interactions on the surface of the core particle. An alteration of nucleosomal conformation has been suggested as an explanation for repair of nucleosomal DNA. Two model systems have been examined. The addition of ethidium bromide to alkylated nucleosomes increased the enzymatic release of 3-MeA to approximately 75% and altered the electron microscopic appearance. The chemical alkylation of nucleosomes also increased the enzymatic release of 3-MeA as well as decreased the sedimentation coefficient. All of these experiments indicate a limited availability of 3-MeA residues to the glycosylase and suggest that some conformational change must occur in vivo for complete repair.

Adenine↗
PubMed2001

Epstein-Barr nuclear antigen 1 binds and destabilizes nucleosomes at the viral origin of latent DNA replication.

The EBNA1 protein of Epstein-Barr virus (EBV) activates latent-phase DNA replication by an unknown mechanism that involves binding to four recognition sites in the dyad symmetry (DS) element of the viral latent origin of DNA replication. Since EBV episomes are assembled into nucleosomes, we have examined the ability of Epstein-Barr virus nuclear antigen 1 (EBNA1) to interact with the DS element when it is assembled into a nucleosome core particle. EBNA1 bound to its recognition sites within this nucleosome, forming a ternary complex, and displaced the histone octamer upon competitor DNA challenge. The DNA binding and dimerization region of EBNA1 was sufficient for nucleosome binding and destabilization. Although EBNA1 was able to bind to nucleosomes containing two recognition sites from the DS element positioned at the edge of the nucleosome, nucleosome destabilization was only observed when all four sites of the DS element were present. Our results indicate that the presence of a nucleosome at the viral origin will not prevent EBNA1 binding to its recognition sites. In addition, since four EBNA1 recognition sites are required for both nucleosome destabilization and efficient origin activation, our findings also suggest that nucleosome destabilization by EBNA1 is important for origin activation.

Animals↗
PubMed1993

Translational positioning of a nucleosomal glucocorticoid response element modulates glucocorticoid receptor affinity.

Positioned nucleosomes are often found in enhancer/promoter regions where they confer defined positioning of trans-active-factor response element(s) relative to the histone octamer. Here, we address how this affects factor/response element recognition. We used 165-bp DNA segments containing one glucocorticoid response element (GRE) and rat liver core histones to reconstitute nucleosomes in vitro. The GREs in these nucleosomes were held in identical helical settings but different translational positions. This was achieved by placing the GRE within or at the flank of a 95-bp DNA-bending sequence. Glucocorticoid receptor (GR)-binding experiments demonstrated that a GRE in free DNA has a 2.5-fold higher affinity for GR than the nucleosomal GRE when positioned 40 bp from the nucleosome dyad. A nucleosomal GRE positioned 40 bp from the nucleosome dyad, on the other hand, binds GR 4.3-fold better as compared to an identical GRE positioned 20 bp from the dyad and 1.4-fold better than a GRE positioned at the dyad. Interruption of the DNA-bending sequence, by a 5-bp AT segment next to a nucleosomal GRE positioned 20 bp from the dyad, restores GR affinity to the same level as when the GRE is placed 40 bp from the dyad. The effect on GR/GRE affinity either by different positioning within the 95-bp bending sequence or by introducing the 5-bp AT-segment is seen only in a nucleosomal context. We conclude that a translationally positioned nucleosome can modulate the affinity of a trans-active factor for its target response element.

Animals↗
PubMed2006

Flexibility and constraint in the nucleosome core landscape of Caenorhabditis elegans chromatin.

Nucleosome positions within the chromatin landscape are known to serve as a major determinant of DNA accessibility to transcription factors and other interacting components. To delineate nucleosomal patterns in a model genetic organism, Caenorhabditis elegans, we have carried out a genome-wide analysis in which DNA fragments corresponding to nucleosome cores were liberated using an enzyme (micrococcal nuclease) with a strong preference for cleavage in non-nucleosomal regions. Sequence analysis of 284,091 putative nucleosome cores obtained in this manner from a mixed-stage population of C. elegans reveals a combined picture of flexibility and constraint in nucleosome positioning. As has previously been observed in studies of individual loci in diverse biological systems, we observe areas in the genome where nucleosomes can adopt a wide variety of positions in a given region, areas with little or no nucleosome coverage, and areas where nucleosomes reproducibly adopt a specific positional pattern. In addition to illuminating numerous aspects of chromatin structure for C. elegans, this analysis provides a reference from which to begin an investigation of relationships between the nucleosomal pattern, chromosomal architecture, and lineage-based gene activity on a genome-wide scale.

Amino Acid Sequence↗
PubMed2001

Nucleosomes are translationally positioned on the active allele and rotationally positioned on the inactive allele of the HPRT promoter.

Differential chromatin structure is one of the hallmarks distinguishing active and inactive genes. For the X-linked human hypoxanthine phosphoribosyltransferase gene (HPRT), this difference in chromatin structure is evident in the differential general DNase I sensitivity and hypersensitivity of the promoter regions on active versus inactive X chromosomes. Here we characterize the nucleosomal organization responsible for the differential chromatin structure of the active and inactive HPRT promoters. The micrococcal nuclease digestion pattern of chromatin from the active allele in permeabilized cells reveals an ordered array of translationally positioned nucleosomes in the promoter region except over a 350-bp region that is either nucleosome free or contains structurally altered nucleosomes. This 350-bp region includes the entire minimal promoter and all of the multiple transcription initiation sites of the HPRT gene. It also encompasses all of the transcription factor binding sites identified by either dimethyl sulfate or DNase I in vivo footprinting of the active allele. In contrast, analysis of the inactive HPRT promoter reveals no hypersensitivity to either DNase I or a micrococcal nuclease and no translational positioning of nucleosomes. Although nucleosomes on the inactive promoter are not translationally positioned, high-resolution DNase I cleavage analysis of permeabilized cells indicates that nucleosomes are rotationally positioned over a region of at least 210 bp on the inactive promoter, which coincides with the 350-bp nuclease-hypersensitive region on the active allele, including the entire minimal promoter. This rotational positioning of nucleosomes is not observed on the active promoter. These results suggest a model in which the silencing of the HPRT promoter during X chromosome inactivation involves remodeling a transcriptionally competent, translationally positioned nucleosomal array into a transcriptionally repressed architecture consisting of rotationally but not translationally positioned nucleosomal arrays.

Alleles↗
PubMed1998

Alteration of nucleosome structure as a mechanism of transcriptional regulation.

The nucleosome, which is the primary building block of chromatin, is not a static structure: It can adopt alternative conformations. Changes in solution conditions or changes in histone acetylation state cause nucleosomes and nucleosomal arrays to behave with altered biophysical properties. Distinct subpopulations of nucleosomes isolated from cells have chromatographic properties and nuclease sensitivity different from those of bulk nucleosomes. Recently, proteins that were initially identified as necessary for transcriptional regulation have been shown to alter nucleosomal structure. These proteins are found in three types of multiprotein complexes that can acetylate nucleosomes, deacetylate nucleosomes, or alter nucleosome structure in an ATP-dependent manner. The direct modification of nucleosome structure by these complexes is likely to play a central role in appropriate regulation of eukaryotic genes.

Acetylation↗
PubMed2004

Global nucleosome occupancy in yeast.

BACKGROUND: Although eukaryotic genomes are generally thought to be entirely chromatin-associated, the activated PHO5 promoter in yeast is largely devoid of nucleosomes. We systematically evaluated nucleosome occupancy in yeast promoters by immunoprecipitating nucleosomal DNA and quantifying enrichment by microarrays. RESULTS: Nucleosome depletion is observed in promoters that regulate active genes and/or contain multiple evolutionarily conserved motifs that recruit transcription factors. The Rap1 consensus was the only binding motif identified in a completely unbiased search of nucleosome-depleted promoters. Nucleosome depletion in the vicinity of Rap1 consensus sites in ribosomal protein gene promoters was also observed by real-time PCR and micrococcal nuclease digestion. Nucleosome occupancy in these regions was increased by the small molecule rapamycin or, in the case of the RPS11B promoter, by removing the Rap1 consensus sites. CONCLUSIONS: The presence of transcription factor-binding motifs is an important determinant of nucleosome depletion. Most motifs are associated with marked depletion only when they appear in combination, consistent with a model in which transcription factors act collaboratively to exclude nucleosomes and gain access to target sites in the DNA. In contrast, Rap1-binding sites cause marked depletion under steady-state conditions. We speculate that nucleosome depletion enables Rap1 to define chromatin domains and alter them in response to environmental cues.

Base Composition↗
PubMedDate not supplied

Binding of polylysine and ethidium bromide to nucleosomal DNA: comparison of biochemical and electron microscopical results.

Ethidium bromide and polylysine interact with nucleosomal DNA and lead to change of biochemical properties and to morphological changes as to the distance between the two core particles of a nucleosome dimer. With increasing polylysine concentration, the buoyant density of nucleosomes decrease and the accessibility of the nucleosomal DNA to micrococcal nuclease is lowered. Electron microscopy of polylysine treated nucleosome dimers reveals a shortening of the internucleosomal distance as compared with controls. Treatment of nucleosomes with ethidium bromide leads to an enhanced accessibility of the nucleosomal DNA to micrococcal nuclease. Electron microscopy reveals an increase in length of the DNA connecting the two nucleosome cores in the presence of the dye. Both the binding of polylysine and the treatment with ethidium bromide apparently do not affect the histone arrangement within the nucleosome core as suggested by chemical cross-linking of histones and DNA with formaldehyde, and no obvious morphological change of the nucleosome cores can be observed.

Animals↗
PubMed2006

Multiscale modeling of nucleosome dynamics.

Nucleosomes form the fundamental building blocks of chromatin. Subtle modifications of the constituent histone tails mediate chromatin stability and regulate gene expression. For this reason, it is important to understand structural dynamics of nucleosomes at atomic levels. We report a novel multiscale model of the fundamental chromatin unit, a nucleosome, using a simplified model for rapid discrete molecular dynamics simulations and an all-atom model for detailed structural investigation. Using a simplified structural model, we perform equilibrium simulations of a single nucleosome at various temperatures. We further reconstruct all-atom nucleosome structures from simulation trajectories. We find that histone tails bind to nucleosomal DNA via strong salt-bridge interactions over a wide range of temperatures, suggesting a mechanism of chromatin structural organization whereby histone tails regulate inter- and intranucleosomal assemblies via binding with nucleosomal DNA. We identify specific regions of the histone core H2A/H2B-H4/H3-H3/H4-H2B/H2A, termed "cold sites", which retain a significant fraction of contacts with adjoining residues throughout the simulation, indicating their functional role in nucleosome organization. Cold sites are clustered around H3-H3, H2A-H4 and H4-H2A interhistone interfaces, indicating the necessity of these contacts for nucleosome stability. Essential dynamics analysis of simulation trajectories shows that bending across the H3-H3 is a prominent mode of intranucleosomal dynamics. We postulate that effects of salts on mononucleosomes can be modeled in discrete molecular dynamics by modulating histone-DNA interaction potentials. Local fluctuations in nucleosomal DNA vary significantly along the DNA sequence, suggesting that only a fraction of histone-DNA contacts make strong interactions dominating mononucleosomal dynamics. Our findings suggest that histone tails have a direct functional role in stabilizing higher-order chromatin structure, mediated by salt-bridge interactions with adjacent DNA.

Chromatin↗
PubMedDate not supplied

Anti-nucleosome antibody: significance in lupus patients lacking anti-double-stranded DNA antibody.

OBJECTIVE: To investigate the clinical significance of anti-nucleosome antibodies in SLE patients lacking anti-double stranded DNA (dsDNA) antibodies. METHODS: IgG anti-nucleosome antibodies were detected by enzyme-linked immunosorbent assays (ELISA) in the sera of SLE patients. Anti-dsDNA antibodies were measured by Farr assays and ELISA, not only in the samples taken for anti-nucleosome testing, but also in sera obtained regularly during the follow-up. RESULTS: Ninety-eight (76.0%) out of 129 patients with SLE had anti-nucleosome antibodies. Twenty-five patients (19.4%) consistently showed little or no anti-dsDNA reactivity during the course of their disease, and among these anti-nucleosome antibodies were present in the sera of 15 (60.0%). Of the patients with anti-dsDNA-negative SLE, renal disorders were present in 8 patients (32.0%), all of whom had anti-nucleosome antibodies. Renal disorders were not found in patients (n = 10) who had neither anti-dsDNA nor anti-nucleosome antibodies. Other autoantibodies such as anti-Ro, anti-Sm and anti-cardiolipin were not associated with renal disorders in this group. The levels of anti-nucleosome antibody strongly correlated with the SLEDAI scores, but inversely correlated with serum complement levels in anti-dsDNA negative SLE patients. CONCLUSION: Our data suggest that the anti-nucleosome antibody may be a useful marker for diagnosis and activity assessment of anti-dsDNA negative SLE. Anti-nucleosome antibody may be an important factor for renal involvement in this subgroup of patients.

Adolescent↗
PubMedDate not supplied

Lupus nephritis: a nucleosome waste disposal defect?

Formation of anti-nuclear autoantibodies is a cardinal characteristic of systemic lupus erythematosus (SLE). In recent years the nucleosome has been identified as the major autoantigen, since nucleosome specific T cells have been identified, which also drive the formation of anti-dsDNA and anti-histone antibodies. Nucleosome specific autoantibodies are present in a large majority of SLE patients and lupus mice. Nucleosomes are formed during apoptosis by organized cleavage of chromatin. These nucleosomes together with other lupus autoantigens cluster in apoptotic bodies at the surface of apoptotic cells. Systemic release of these autoantigens is normally prevented by swift removal of apoptotic cels. However, if the rate of apoptosis overflows the removal capacity and/or the cleaning machinery is reduced, nucleosomes are released. Furthermore, during apoptosis autoantigens can be modified, which makes them more immunogenic. Nucleosomes also play a pivotal role in the evolution of tissue lesions, especially glomerulonephritis. In lupus nephritis nucleosomes, anti-nucleosome autoantibodies and nucleosome/Ig complexes have been identified in the glomerular immune deposits. Via their cationic histone part nucleosomes can bind to heparan sulfate, a strong anionic constituent of the glomerular basement membrane.

Animals↗
PubMed1989

Enhanced DNA repair synthesis in hyperacetylated nucleosomes.

We have investigated the level of "early" DNA repair synthesis in nucleosome subpopulations, varying in histone acetylation, from normal human fibroblasts treated with sodium butyrate. We find that repair synthesis occurring during the first 30 min after UV irradiation is significantly enhanced in hyperacetylated mononucleosomes. Nucleosomes with an average of 2.3 acetyl residues/H4 molecule contained approximately 1.8-fold more repair synthesis than nucleosomes with an average of 1.5 or 1.0 acetyl residues/H4 molecule. Fractionation of highly acetylated nucleosomes by two-dimensional gel electrophoresis yielded an additional 2.0-fold enrichment of repair synthesis in nucleosomes containing 2.7 acetyl residues/H4 molecule as compared to nucleosomes containing 1.9 acetyl residues/H4 molecule. This enhanced repair synthesis is associated primarily with nucleosome core regions and does not appear to result from increased UV damage in hyperacetylated chromatin. In addition, the distribution of repair synthesis within nucleosome core DNA from hyperacetylated chromatin is nonrandom, showing a bias toward the 5' end which is similar to that obtained for bulk (unfractionated) chromatin. These results provide strong evidence that enhanced repair occurs within nucleosome cores of hyperacetylated chromatin in butyrate-treated human cells. Finally, pulse-chase experiments demonstrate that the association of enhanced repair synthesis with hyperacetylated nucleosomes is transient, lasting only about 12 h after UV damage.

Acetylation↗
PubMed1986

Stability of nucleosome placement in newly repaired regions of DNA.

Rearrangements of chromatin structure during excision repair of UV-damaged DNA appear to involve unfolding of nucleosomal DNA while repair is taking place, followed by refolding of this DNA into a native nucleosome structure. Recently, we found that repair patches are not distributed uniformly along the DNA in nucleosome core particles immediately following their refolding into nucleosomes (Lan, S. Y., and Smerdon, M. J. (1985) Biochemistry, 24,7771). Therefore, the distribution of repair patches in nucleosome core DNA was used to monitor the stability of nucleosome placement in these regions. Our results indicate that in nondividing human cells undergoing excision repair there is a slow change in the positioning of nucleosomes in newly repaired regions of chromatin, resulting in the eventual randomization of repair patches in nucleosome core DNA. Furthermore, the nonrandom placement of nucleosomes observed just after the refolding event is not re-established during DNA replication. Possible mechanisms for this change in nucleosome placement along the DNA are discussed.

Carbon Radioisotopes↗
PubMed1980

Comparative studies of histone acetylation in nucleosomes, nuclei, and intact cells. Evidence for special factors which modify acetylase action.

We have studied the pattern of histone acetylation in intact rat hepatoma tissue culture (HTC) cells, in isolated HTC nuclei, and in chromatin prepared from these cells. The results have been compared with the histone acetylation observed in a reconstituted in vitro system consisting of a variety of purified soluble nucleosomal substrates, [3H]acetyl-CoA, and one of two different purified histone N-acetyltransferases. Acetylase A, a highly purified nuclear enzyme, catalyzed the acetylation of 1) nucleosomally bound histones in the order H4 > H2a = H2b > H3, and 2) free histones in the order H4 > H3 > H2b > H2a. Acetylase B, a cytoplasmic enzyme, modified only free histone H4, and it failed to acetylate histones in nucleosomes. The pattern of histone acetylation obtained by in vitro reaction of purified nucleosomes with the purified nuclear acetylase A differed considerably from the corresponding patterns obtained either by acetate labeling of intact cells, or by the acetyl-CoA labeling of nuclei and crude preparations of nucleosomes, as catalyzed by endogenous chromatin-bound acetylase(s). The most striking difference was in the relative preference for acetylation of histone H4 versus acetylation of histone H3: with the purified acetylase, histone H4 in nucleosomes was acetylated to a much greater extent than was histone H3, whereas the reverse preference was found with the endogenous acetylase(s). This result suggests that either a second nuclear acetylase enzyme, or a separate cofactor for acetylase A, is required for histone H3 acetylation in vivo. In support of this view, we find that the acetylation of histones H4, H2a, and H2b in nuclei is inhibited by urea, salt, or N-ethylmaleimide treatments to a very different extent than is the acetylation of histone H3. By comparing n-butyrate-treated HTC cells with untreated cells, classes of nucleosomes specially accessible and inaccessible to acetylation can be distinguished (Cousens, L. S., Gallwitz, D., and Alberts, B. M. (1979) J. Biol. Chem. 254, 1716-1723). Both types of special nucleosomal reactivities were present in isolated nuclei, but were lost as nucleosomes were purified from these cells. OUr data thus suggest the existence of labile specificity factors or structures, which guide the acetylase(s) to restricted groups of otherwise similar nucleosomes in vivo.

Acetylation↗