SWI SNF STIMULATES THE FORMATION OF DISPARATE ACTIVATOR-NUCLEOSOME COMPLEXES BUT IS PARTIALLY REDUNDANT WITH COOPERATIVE BINDING/

To investigate the potential mechanisms by which the SWI/SNF complex d ifferentially regulates different genes we have tested whether transcr iption factors with diverse DNA binding domains were able to exploit n ucleosome disruption by SWI/SNF. In addition to GAL4-VP16, the SWI/SNF complex stimulated nucleosome binding by the Zn2+ fingers of Sp1, the basic helix-loop-helix domain of USF, and the rel domain of NF-kappa B. In each case SWI/SNF action resulted in the formation of a stable f actor-nucleosome complex that persisted after detachment of SWI/SNF fr om the nucleosome. Thus, stimulation of factor binding by SWI/SNF appe ars to be universal. The degree of SWI/SNF stimulation of nucleosome b inding by a factor appears to be inversely related to the extent that binding is inhibited by the histone octamer. Cooperative binding of 5 GAL4-VP16 dimers to a 5-site nucleosome enhanced GAL4 binding relative to a single-site nucleosome, but this also reduced the degree of stim ulation by SWI/SNF. The SWI/SNF complex increased the affinity of 5 GA L4-VP16 dimers for nucleosomes equal to that of DNA but no further. Si milarly, multimerized NF-kappa B sites enhanced nucleosome binding by NF-kappa B and reduced the stimulatory effect of SWI/SNF. Thus, cooper ative binding of factors to nucleosomes is partially redundant with th e function of the SWI/SNF complex.