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Sequence-specific targeting of chromatin remodelers organizes precisely positioned nucleosomes throughout the genome.

ABSTRACT: Eukaryotic genomes are functionally organized into chromatin, a compact packaging of nucleoproteins with the basic repeating unit known as the nucleosome. A major focus for the chromatin field has been understanding what rules govern nucleosome positioning throughout the genome, and here we review recent findings using a novel, sequence-targeted remodeling enzyme. Nucleosomes are often packed into evenly spaced arrays that are reproducibly positioned, but how such organization is established and maintained through dramatic events such as DNA replication is poorly understood. We hypothesize that a major fraction of positioned nucleosomes arises from sequence-specific targeting of chromatin remodelers to generate "founding" nucleosomes, providing reproducible, predictable, and condition-specific nucleation sites against which neighboring nucleosomes are packed into evenly spaced arrays.

SUBMITTER: Bowman GD

PROVIDER: S-EPMC5194215 | biostudies-literature | 2017 Jan

REPOSITORIES: biostudies-literature

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Sequence-specific targeting of chromatin remodelers organizes precisely positioned nucleosomes throughout the genome.

Bowman Gregory D GD McKnight Jeffrey N JN

BioEssays : news and reviews in molecular, cellular and developmental biology 20161116 1

Eukaryotic genomes are functionally organized into chromatin, a compact packaging of nucleoproteins with the basic repeating unit known as the nucleosome. A major focus for the chromatin field has been understanding what rules govern nucleosome positioning throughout the genome, and here we review recent findings using a novel, sequence-targeted remodeling enzyme. Nucleosomes are often packed into evenly spaced arrays that are reproducibly positioned, but how such organization is established and ...[more]

PMID: 27862071

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Project description:The precise placement of nucleosomes has large regulatory effects on gene expression. Recent work suggests that nucleosome placement is regulated in part by the affinity of the underlying DNA sequence for the histone octamer. Nucleosome locations are also regulated by several different ATP-dependent chromatin remodeling enzymes. This raises the question of whether DNA sequence influences the activity of chromatin remodeling enzymes. DNA sequence could most simply regulate nucleosome remodeling through its effect on nucleosome stability. In such a model, unstable nucleosomes would be remodeled faster than stable nucleosomes. It is also possible that certain DNA elements could regulate remodeling by inhibiting the interaction of nucleosomes with the remodeling enzyme. A third possibility is that DNA sequence could regulate the outcome of remodeling by influencing how reaction intermediates collapse into a particular set of stable nucleosomal positions. Here we dissect the contribution from these potential mechanisms to the activities of yeast RSC and human ACF, which are representative members of two major classes of remodeling complexes. We find that varying the histone-DNA affinity over 3 orders of magnitude has negligible effects on the rates of nucleosome remodeling and ATP hydrolysis by these two enzymes. This suggests that the rate-limiting step for nucleosome remodeling may not involve the disruption of histone-DNA contacts. We further find that a specific curved DNA element previously hypothesized to inhibit ACF activity does not inhibit substrate binding or remodeling by ACF. The element, however, does influence the distribution of nucleosome positions generated by ACF. Our data support a model in which remodeling enzymes move nucleosomes to new locations by a general sequence-independent mechanism. However, consequent to the rate-limiting remodeling step, the local DNA sequence promotes a collapse of remodeling intermediates into highly resolved positions that are dictated by thermodynamic differences between adjacent positions.

Project description:BackgroundTranscription is affected by nucleosomal resistance against polymerase passage. In turn, nucleosomal resistance is determined by DNA sequence, histone chaperones and remodeling enzymes. The contributions of these factors are widely debated: one recent title claims "… DNA-encoded nucleosome organization…" while another title states that "histone-DNA interactions are not the major determinant of nucleosome positions." These opposing conclusions were drawn from similar experiments analyzed by idealized methods. We attempt to resolve this controversy to reveal nucleosomal competency for transcription.Methodology/principal findingsTo this end, we analyzed 26 in vivo, nonlinked, and in vitro genome-wide nucleosome maps/replicates by new, rigorous methods. Individual H2A nucleosomes are reconstituted inaccurately by transcription, chaperones and remodeling enzymes. At gene centers, weakly positioned nucleosome arrays facilitate rapid histone eviction and remodeling, easing polymerase passage. Fuzzy positioning is not due to artefacts. At the regional level, transcriptional competency is strongly influenced by intrinsic histone-DNA affinities. This is confirmed by reproducing the high in vivo occupancy of translated regions and the low occupancy of intergenic regions in reconstitutions from purified DNA and histones. Regional level occupancy patterns are protected from invading histones by nucleosome excluding sequences and barrier nucleosomes at gene boundaries and within genes.Conclusions/significanceDense arrays of weakly positioned nucleosomes appear to be necessary for transcription. Weak positioning at exons facilitates temporary remodeling, polymerase passage and hence the competency for transcription. At regional levels, the DNA sequence plays a major role in determining these features but positions of individual nucleosomes are typically modified by transcription, chaperones and enzymes. This competency is reduced at intergenic regions by sequence features, barrier nucleosomes, and proteins, preventing accessibility regulation of untargeted genes. This combination of DNA- and protein-influenced positioning regulates DNA accessibility and competence for regulatory protein binding and transcription. Interactive nucleosome displays are offered at http://chromatin.unl.edu/cgi-bin/skyline.cgi.

Dataset Information

Sequence-specific targeting of chromatin remodelers organizes precisely positioned nucleosomes throughout the genome.

Publications

Sequence-specific targeting of chromatin remodelers organizes precisely positioned nucleosomes throughout the genome.

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