Chromatin remodeling complexes help regulate the structure of chromatin to facilitate transcription. family of remodeling complexes. Chromatin acts as a barrier to eukaryotic transcription by blocking transcription factor access and polymerase movement. To contend with this barrier, cells make Adam23 use of a variety of evolutionarily conserved ATP-dependent chromatin remodeling complexes. These complexes have been shown to influence transcription and chromatin access in vivo and in vitro, but the extent of their roles and TMC-207 kinase activity assay their mechanisms of action are only beginning to be understood (15, 17, 40, 43). Human cells contain a family of SWI-SNF complexes that are closely related to the yeast (y) SWI-SNF and RSC complexes. Human (h) SWI-SNF has been implicated in transcriptional activation of several genes (1, 6, 10, 16, 20, 25), as well as in transcriptional repression and growth control through the p105Rb retinoblastoma protein (9, 26, 34, 38). To determine the function of SWI-SNF in gene regulation, we have examined its effects on chromatin in vitro. TMC-207 kinase activity assay hSWI-SNF contains at least eight subunits and can be isolated in two forms, which contain either BRG1 or hBRM as a central ATPase subunit (18, 41). These complexes possess DNA- and nucleosome-stimulated ATPase activity and have an ATP-dependent ability to remodel mononucleosome core particles (referred to as nucleosomes or cores herein) (as assayed by changes in DNase I digestion patterns) and plasmid chromatin (as assayed by a reduction in nucleosome-constrained negative supercoils) (18). Each ATPase alone, when purified from insect cells, appears capable of these activities, although at reduced levels (28). Addition of three other conserved subunitsnamely BAF155, BAF170, and Ini1to the ATPase subunit generates a minimal hSWI-SNF complex that is almost as active as the intact complex. While ATP is required for remodeling by intact hSWI-SNF, the remodeled state is stable in the absence of ATP (13) or SWI-SNF in several in vitro assays. These observations led to the discovery that SWI-SNF can act enzymatically to create a stable, altered dimer of mononucleosome cores (30). This product, which we refer to here as the remodeled dimer, could be separated from hSWI-SNF and was shown to have the proper ratio of the four core histones and DNA but an apparent molecular weight twice that of a normal nucleosome. It was determined to have distinct DNase and micrococcal nuclease digestion patterns and altered susceptibility to restriction enzyme digestion. Increased affinity of GAL4 for this product suggested that it might be more amenable to transcriptional activation. The hSWI-SNF complex also converts this product back to cores, and both the creation of the remodeled dimer and reconversion to cores requires ATP hydrolysis. In general, these activities and properties are shared by hSWI-SNF and the related yeast complexes. Notably, yeast RSC generates and reconverts a highly similar stable product (22), and the remodeling of mononucleosomes by ySWI-SNF is stable after ySWI-SNF removal (8). Many different chromatin remodeling complexes have been identified to date. Some can be classified as SWI-SNF-like, by virtue of their subunit composition and the similarity of their ATPases to ySWI2-SNF2 (for a review, see reference 17). A second major family contains ISWI or a related protein as its central ATPase (for reviews, see references 11 and 40). The complexes from this ISWI-based family are unlike SWI-SNF-like complexes in that their ATPase activity is stimulated primarily by nucleosomes and not by DNA alone. Nevertheless, ISWI is similar to BRG1 and hBRM in that it can also perform at least some of the in vitro activities of the complex in which it is found (7, 19). TMC-207 kinase activity assay A third family, which includes the nucleosome remodeling and deacetylase (NuRD) complexes, contains a CHD/Mi-2 protein(s) as the central ATPase(s) (for a review, see reference 2). All of these complexes have been shown to remodel chromatin in vitro in some of the many available assays. An exhaustive comparison across all assays has not been done, but current data indicate that each remodeling complex can perform only a subset of known remodeling activities (see, e.g., references 17 and 40 for reviews). These activities.