SWI/SNF chromatin remodeling requires changes in DNA topology

ySWI/SNF complex belongs to a family of enzymes that use the energy of ATP hydrolysis to remodel chromatin structure. Here we examine the role of DNA topology in the mechanism of ySWI/SNF remodeling. We find that the ability of ySWI/SNF to enhance accessibility of nucleosomal DNA is nearly eliminate d when DNA topology is constrained in small circular nucleosomal arrays and that this inhibition can be alleviated by topoisomerases. Furthermore, we demonstrate that remodeling of these substrates does not require dramatic h istone octamer movements or displacement. Our results suggest a model in wh ich ySWI/SNF remodels nucleosomes by using the energy of ATP hydrolysis to drive local changes in DNA twist.