Effects of nucleosome remodeling factor ACF1 on in vivo chromatin organization

Eukaryotic genomes make use of nucleosomes to considerably reduce their packaging volumes. As a consequence, the underlying DNA is rendered inaccessible. Cells make use of ATP-dependent remodeling factors to disrupt histone-DNA contacts and bring about access to the DNA. ACF1 is the largest regulatory subunit of two nucleosome remodeling factors, namely ACF and CHRAC. These complexes assemble, slide or evenly space nucleosomes on DNA with an ability to sense the linker lengths. However, roles of ACF1 in organizing nucleosomes in vivo and their physiological consequences are largely unclear. To understand the roles of ACF1 on chromatin organization, I compared nucleosome occupancy and transcription profiles in wild-type and ACF1-deficient Drosophila embryos. To further investigate and corroborate these chromatin changes, I performed genomewide mapping of ACF1 using chromatin immunoprecipitation. Nucleosome occupancy was mapped by subjecting DNA obtained from MNase-digested chromatin to deep sequencing and the occupancies were analyzed using advanced analog signal processing methods. We found discontinuous and discrete patches of regularly positioned nucleosomes in wild-type tissue, referred to as ‘regularity regions’. These regions span actively transcribing and silent chromatin domains and show associated variation in the linker lengths across them. A subset of these regions located at sides remote from the transcriptional start sites loses regularity upon ACF1 deletion and show presence of a novel DNA sequence motif. Analyzing nucleosome periodicity by autocorrelation function revealed that nucleosome linker length is longer in ACF1-deficient embryos. Despite profound quantifiable changes in the chromatin organization the RNA expression analyses did not show any major changes. Genomewide localization of ACF1 was studied using by chromatin immunoprecipitation. We observed a strong enrichment of ACF1 along active promoter regions, coinciding strikingly well with another remodeling factor, RSF-1. However, careful analyses using mutant tissues for both proteins demonstrated that the observed enrichments were in fact false positive. We define 3100 genomic sites as false positive ‘Phantom Peaks’ that tend to enrich in the ChIP-seq experiments. By comparing publicly accessible profiles and the Phantom regions, we showed that several ChIP-seq profiles of the epigenetic regulators show strong enrichment along the Phantom Peaks. In conclusion, we identify regions of regularly organized nucleosomes across the genome and show that a subset localized in silent chromatin regions is affected by ACF1 deletion. Moreover, we identified a class of false positive ChIP-seq peaks at active promoters. This list of Phantom Peaks can be used to assess potential false positive signal in a ChIP-seq profile, especially when mutant tissue is not available as a control.

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Effects of nucleosome remodeling factor ACF1 on in vivo chromatin organization is an episode from Fakultät für Biologie - Digitale Hochschulschriften der LMU - Teil 06/06 by Ludwig-Maximilians-Universität München.

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This episode was published on Nov 27, 2015.

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