Yeast Isw1a and Isw1b exhibit similar nucleosome mobilization capacities for mononucleosomes, but differently mobilize dinucleosome templates.

Nucleosome remodeling studies in vitro have primarily focused on the use of mononucleosome templates, which, however, can provide only limited information on how nucleosome mobilization occurs in the context of chromatin, in which internucleosome interactions might influence remodeling. We tried to evaluate whether nucleosome mobilization by yeast Isw1a, Isw1b and Isw2 could be affected by neighboring nucleosomes. We compared mono- and dinucleosomes positioned by the '601' sequence, the studied constructs contain variation in linker length between nucleosomes and variation in the length of flanking sequences. The data characterizing the remodeling were based on gel retardation of the mono and dinucleosomes, keeping in mind the observation that the relative position of the nucleosome will change the mobility of the complex in well defined ways. We found that Isw1a, Isw1b and Isw2 process nucleosomes differently whether they exist as mononucleosomes or dinucleosomes, such as, the Isw1a and Isw1b nucleosome repositioning patterns, which were very similar for mononucleosomes, appeared to be profoundly different in case of dinucleosome templates. We also examined the DNase I protection patterns of remodeled mono- and dinucleosomes. The data suggest that nucleosome mobilizing activity of Isw1a, Isw1b and Isw2 complexes could be significantly influenced by neighboring nucleosomes.

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