Comparison of the Isw1a, Isw1b, and Isw2 Nucleosome
Disrupting Activities
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Abstract
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 the ySwi–Snf
remodeling products. The nuclease digestion data suggest that Isw1a
can also promote ATP-dependent distortion of nucleosome structure,
although less efficiently than the Isw1b complex