Human SWI/SNF directs sequence-specific chromatin changes on promoter polynucleosomes.

Studies in humans and other species have revealed that a surprisingly large fraction of nucleosomes adopt specific positions on promoters, and that these positions appear to be determined by nucleosome positioning DNA sequences (NPSs). Recent studies by our lab, using minicircles containing only one nucleosome, indicated that the human SWI/SNF complex (hSWI/SNF) prefers to relocate nucleosomes away from NPSs. We now make use of novel mapping techniques to examine the hSWI/SNF sequence preference for nucleosome movement in the context of polynucleosomal chromatin, where adjacent nucleosomes can limit movement and where hSWI/SNF forms altered dinucleosomal structures. Using two NPS templates (5S rDNA and 601) and two hSWI/SNF target promoter templates (c-myc and UGT1A1), we observed hSWI/SNF-driven depletion of normal mononucleosomes from almost all positions that were strongly favored by assembly. In some cases, these mononucleosomes were moved to hSWI/SNF-preferred sequences. In the majority of other cases, one repositioned mononucleosome appeared to combine with an unmoved mononucleosome forming a specifically localized altered or normal dinucleosome. These effects result in dramatic, template-specific changes in nucleosomal distribution. Taken together, these studies indicate hSWI/SNF is likely to activate or repress transcription of its target genes by generating promoter sequence-specific changes in chromatin configuration.