CHROMATIN-REMODELING FACTOR CHRAC CONTAINS THE ATPASES ISWI AND TOPOISOMERASE-II

Repressive chromatin structures need to be unravelled to allow DNA-bin ding proteins access to their target sequences. This de-repression con stitutes an important point at which transcription and presumably othe r nuclear processes can be regulated(1,2). Energy-consuming enzyme com plexes that facilitate the interaction of transcription factors with c hromatin by modifying nucleosome structure are involved in this regula tion(3-5). One such factor, nucleosome-remodelling factor (NURF), has been isolated from Drosophila embryo extracts(4,6,7), We have now iden tified a chromatin-accessibility complex (CHRAC) which uses energy to increase the general accessibility of DNA in chromatin. However, unlik e other known chromatin remodelling factors, CHRAC can also function d uring chromatin assembly: it uses ATP to convert irregular chromatin i nto a regular array of nucleosomes with even spacing. CHRAC combines e nzymes that modulate nucleosome structure and DNA topology. Using mass spectrometry, we identified two of the five CHRAC subunits as the ATP ase ISWI, which is also part of NURF6,8, and topoisomerase II, The pre sence of ISWI in different contexts suggests that chromatin remodellin g machines have a modular nature and that ISWI has a central role in d ifferent chromatin remodelling reactions.