Genome-Wide Mapping Targets of the Metazoan Chromatin Remodeling Factor NURF Reveals Nucleosome Remodeling at Enhancers, Core Promoters and Gene Insulators

摘要

NURF is a conserved higher eukaryotic ISWI-containing chromatin remodeling complex that catalyzes ATP-dependent nucleosome sliding. By sliding nucleosomes, NURF is able to alter chromatin dynamics to control transcription and genome organization. Previous biochemical and genetic analysis of the specificity-subunit of Drosophila NURF (Nurf301/Enhancer of Bithorax (E(bx)) has defined NURF as a critical regulator of homeotic, heat-shock and steroid-responsive gene transcription. It has been speculated that NURF controls pathway specific transcription by co-operating with sequence-specific transcription factors to remodel chromatin at dedicated enhancers. However, conclusive in vivo demonstration of this is lacking and precise regulatory elements targeted by NURF are poorly defined. To address this, we have generated a comprehensive map of in vivo NURF activity, using MNase-sequencing to determine at base pair resolution NURF target nucleosomes, and ChIP-sequencing to define sites of NURF recruitment. Our data show that, besides anticipated roles at enhancers, NURF interacts physically and functionally with the TRF2/DREF basal transcription factor to organize nucleosomes downstream of active promoters. Moreover, we detect NURF remodeling and recruitment at distal insulator sites, where NURF functionally interacts with and co-localizes with DREF and insulator proteins including CP190 to establish nucleosome-depleted domains. This insulator function of NURF is most apparent at subclasses of insulators that mark the boundaries of chromatin domains, where multiple insulator proteins co-associate. By visualizing the complete repertoire of in vivo NURF chromatin targets, our data provide new insights into how chromatin remodeling can control genome organization and regulatory interactions. Author Summary In eukaryotes DNA is folded and compacted into manageable units by wrapping around a protein spool of histone proteins to form nucleosomes. By varying the position and dynamics of nucleosomes using energy-dependent chromatin remodeling enzymes, genes can be selectively turned off or on in cells, controlling development and cellular function. Distinct sub-families of ATP-dependent chromatin remodeling enzymes have been characterised. However, their specific nucleosome targets in the genome and how they are recruited to these are not completely defined. Here we have identified nucleosome targets of the conserved higher eukaryotic chromatin remodeling enzyme NURF. Our data indicate three distinct functions for NURF during transcription. NURF organizes nucleosome positions at gene enhancer elements to regulate transcription initiation, but is also required to maintain nucleosome position downstream of the transcription start site of active genes. In addition, we detect NURF remodeling and recruitment at distal insulator sites that are required for functional organisation of the genome. We postulate that NURF function at insulators as well as promoters reflects functional interaction between distant insulators and active promoters, with functional clustering of regulatory elements providing a solution to how chromatin remodeling enzymes engage multiple targets in the genome.