SWI-SNF-mediated nucleosome remodeling: role of histone octamer mobility in the persistence of the remodeled state.

SWI-SNF is an ATP-dependent chromatin remodeling complex that disrupts DNA-histone interactions. Several studies of SWI-SNF activity on mononucleosome substrates have suggested that remodeling leads to novel, accessible nucleosomes which persist in the absence of continuous ATP hydrolysis. In contrast, we have reported that SWI-SNF-dependent remodeling of nucleosomal arrays is rapidly reversed after removal of ATP. One possibility is that these contrasting results are due to the different assays used; alternatively, the lability of the SWI-SNF-remodeled state might be different on mononucleosomes versus nucleosomal arrays. To investigate these possibilities, we use a coupled SWI-SNF remodeling-restriction enzyme assay to directly compare the remodeling of mononucleosome and nucleosomal array substrates. We find that SWI-SNF action causes a mobilization of histone octamers for both the mononucleosome and nucleosomal array substrates, and these changes in nucleosome positioning persist in the absence of continued ATP hydrolysis or SWI-SNF binding. In the case of mononucleosomes, the histone octamers accumulate at the DNA ends even in the presence of continued ATP hydrolysis. On nucleosomal arrays, SWI-SNF and ATP lead to a more dynamic state where nucleosomes appear to be constantly redistributed and restriction enzyme sites throughout the array have increased accessibility. This random positioning of nucleosomes within the array persists after removal of ATP, but inactivation of SWI-SNF is accompanied by an increased occlusion of many restriction enzyme sites. Our results also indicate that remodeling of mononucleosomes or nucleosomal arrays does not lead to an accumulation of novel nucleosomes that maintain an accessible state in the absence of continuous ATP hydrolysis.