SWI/SNF Displaces SAGA-Acetylated Nucleosomes

SWI/SNF is a well-characterized chromatin remodeling complex that remodels chromatin by sliding nu-cleosomes in cis and/or displacing nucleosomes in trans. The latter mechanism has the potential to remove promoter nucleosomes, allowing access to transcription factors and RNA polymerase. In vivo, histone acety-lation often precedes apparent nucleosome loss; therefore, we sought to determine whether nucleosomes containing acetylated histones could be displaced by the SWI/SNF chromatin remodeling complex. We found that SAGA-acetylated histones were lost from an immobilized nucleosome array when treated with the SWI/SNF complex. When the nucleosome array was acetylated by SAGA in the presence of bound transcription activators, it generated a peak of acetylation surrounding the activator binding sites. Subsequent SWI/SNF treatment suppressed this acetylation peak. Immunoblots indicated that SWI/SNF preferentially displaced acetylated histones from the array relative to total histones. Moreover, the Swi2/Snf2 bromodomain, an acetyl-lysine binding domain, played a role in the displacement of acetylated histones. These data indicate that targeted histone acetylation by the SAGA complex predisposes promoter nucleosomes for displacement by the SWI/SNF complex. SAGA (Spt-Ada-Gcn5-acetyltransferase) is a 1.8-MDa multi-subunit complex that is an important coactivator fo