Catalytic activity of the yeast SWI/SNF complex on reconstituted nucleosome arrays.

A novel, quantitative nucleosome array assay has been developed that couples the activity of a nucleosome 'remodeling' activity to restriction endonuclease activity. This assay has been used to determine the kinetic parameters of ATP-dependent nucleosome disruption by the yeast SWI/SNF complex. Our results support a catalytic mode of action for SWI/SNF in the absence of nucleosome targeting. In this quantitative assay SWI/SNF and ATP lead to a 100-fold increase in nucleosomal DNA accessibility, and initial rate measurements indicate that the complex can remodel one nucleosome every 4.5 min on an 11mer nucleosome array. In contrast to SWI/SNF action on mononucleosomes, we find that the SWI/SNF remodeling reaction on a nucleosome array is a highly reversible process. This result suggests that recovery from SWI/SNF action involves interactions among nucleosomes. The biophysical properties of model nucleosome arrays, coupled with the ease with which homogeneous arrays can be reconstituted and the DNA accessibility analyzed, makes the described array system generally applicable for functional analysis of other nucleosome remodeling enzymes, including histone acetyltransferases.