Solution AFM studies of human Swi-Snf and its interactions with MMTV DNA and chromatin

ATP-dependent nucleosome remodeling complexes are crucial for relieving nucleosome repression during transcription, DNA replication, recombination, and repair. Remodeling complexes can carry out a variety of reactions on chromatin substrates but precisely how they do so remains a topic of active inquiry. Here, a novel recognition atomic force microscopy (AFM) approach is used to characterize human Swi-Snf (hSwi-Snf) nucleosome remodeling complexes in solution. This information is then used to locate hSwi-Snf complexes bound to mouse mammary tumor virus promoter nucleosomal arrays, a natural target of hSwi-Snf action, in solution topographic AFM images of surface-tethered arrays. By comparing the same individual chromatin arrays before and after hSwi-Snf activation, remodeling events on these arrays can be monitored in relation to the complexes bound to them. Remodeling is observed to be: inherently heterogeneous; nonprocessive; able to occur near and far from bound complexes; often associated with nucleosome height decreases. These height decreases frequently occur near sites of DNA release from chromatin. hSwi-Snf is usually incorporated into nucleosomal arrays, with multiple DNA strands entering into it from various directions, + or - ATP; these DNA paths can change after hSwi-Snf activation. hSwi-Snf appears to interact with naked mouse mammary tumor virus DNA somewhat differently than with chromatin and ATP activation of surface-bound DNA/hSwi-Snf produces no changes detectable by AFM.