Comparison of the Isw1a, Isw1b, and Isw2 Nucleosome Disrupting Activities

The three Saccharomyces cerevisiae ISWI chromatin remodeling complexes, Isw1a, Isw1b, and Isw2, are implicated in the regularization of arrayed nucleosomes and regulation of gene activity. Although Isw1a and Isw1b are based on the same catalytic unit, in general, their functions in vivo do not overlap. To better understand the structural consequences of these complexes, we compared the putative nucleosome disrupting activities of the purified Isw1a, Isw1b, and Isw2. To account for the putative effects of nucleosomal environment, we employed reconstituted dinucleosomes in which the histone octamers were specifically positioned by the 146 base pair high-affinity nucleosome sequence 601. We have compared the MNase and deoxyribonuclease I protection patterns of remodeled nucleosome templates and evaluated the nucleosome destabilizing abilities of the Isw1a/b and Isw2 using restriction endonucleases. Although the Isw2 showed little evidence of nucleosome disassembly, the Isw1b remodeled dinucleosomes exhibited some common features with ySwi-Snf remodelling products. The nuclease digestion data suggest that Isw1a can also promote ATP-dependent distortion of nucleosome structure, although less efficiently than the Isw1b complex.