Chromatin-remodelling factor CHRAC contains the ATPases ISWI and topoisomerase II

Abstract

Repressive chromatin structures need to be unravelled to allow DNA-binding proteins access to their target sequences. This de-repression constitutes an important point at which transcription and presumably other nuclear processes can be regulated. Energy-consuming enzyme complexes that facilitate the interaction of transcription factors with chromatin by modifying nucleosome structure are involved in this regulation. One such factor, nucleosome-remodelling factor (NURF), has been isolated from Drosophila embryo extracts. We have now identified a chromatin-accessibility complex (CHRAC) which uses energy to increase the general accessibility of DNA in chromatin. However, unlike other known chromatin remodelling factors, CHRAC can also function during chromatin assembly: it uses ATP to convert irregular chromatin into a regular array of nucleosomes with even spacing. CHRAC combines enzymes that modulate nucleosome structure and DNA topology. Using mass spectrometry, we identified two of the five CHRAC subunits as the ATPase ISWI, which is also part of NURF, and topoisomerase II. The presence of ISWI in different contexts suggests that chromatin remodelling machines have a modular nature and that ISWI has a central role in different chromatin remodelling reactions.

Chromatin remodelling and DNA replication: from nucleosomes to loop domains

“…These functions are accomplished by DNA topoisomerases, enzymes that participate in the metabolism of chromatin by altering the DNA topology and reducing the torsional stress in DNA caused by transcription, replication and chromatin assembly (for review see Wang, 1996). DNA topoisomerase II was found to copurify with the chromatin-remodelling complex CHRAC (Varga-Weisz et al, 1997). Although topoisomerase II is not required for CHRAC activity in vitro, its association with chromatin-remodelling complexes may be required for replication termination.…”

“…The ®rst subset has a nucleosomespacing activity, mediating the formation of regularly spaced nucleosome arrays from randomly positioned nucleosomes. It includes the Drosophila ACF and CHRAC complexes (Ito et al, 1997;Varga-Weisz et al, 1997), human RSF (LeRoy et al, 1998, and yeast ISWI 1 and 2 complexes (Tsukiyama et al, 1999). These complexes share an essential, highly conserved, ATPase subunit with helicase signature motifs which de®ne the ISWI family of ATPase/helicases.…”

Spatial Contacts and Nucleosome Step Movements Induced by the NURF Chromatin Remodeling Complex

“…Three distinct Drosophila ISWI chromatin remodeling complexes have been characterized: nucleosome remodeling factor (NURF) 1 (9,16), chromatin accessibility complex (CHRAC) (10), and ATP-dependent chromatin and remodeling gactor (ACF) (11). Recently, components of NURF have also been found in the TATA box binding protein-related factor 2 (TRF2) complex that controls promoter-selective expression (17).…”

“…In addition, ACF also displays nucleosome assembly activity in vitro (11). The nucleosome sliding activities of these complexes are critically dependent on the ISWI ATPase (10,11,16,(21)(22)(23) and modulated by other subunits that facilitate interactions with the nucleosome substrate and DNA-binding regulators (21, 22, 24 -26). Moreover, the ATPase and nucleosome sliding activities of ISWI complexes are strikingly dependent on specific residues of the N-terminal histone H4 tail, indicating that interaction with histone H4 is a key regulatory step (27)(28)(29)(30).…”

Functional Delineation of Three Groups of the ATP-dependent Family of Chromatin Remodeling Enzymes

“…Seventeen members of the SWI2/SNF2 family have been identified in the yeast genome (6), and to date, four of these ATPases have been purified as subunits of distinct chromatin remodeling complexes ySWI/ SNF (8,9), yRSC (10), ISW1 and ISW2 (11). Additional ATPdependent remodeling complexes that harbor SWI2/SNF2 family members have been identified in Drosophila (dACF (12), dNURF (13), dCHRAC (14), Brahma (15,16)), human (hSWI/ SNF (17), hNURD (18 -20), hRSF (21)), and frog (xMi-2 (22)). Although these complexes have a variable number of subunits (i.e.…”