SWI/SNF and Asf1p Cooperate To Displace Histones during Induction of the Saccharomyces cerevisiae HO Promoter

Regulation of the Saccharomyces cerevisiae HO promoter has been shown to require the recruitment of chromatin-modifying and -remodeling enzymes. Despite this, relatively little is known about what changes to chromatin structure occur during the course of regulation at HO. Here, we used indirect end labeling in synchronized cultures to show that the chromatin structure is disrupted in a region that spans bp 600 to 1800 relative to the transcriptional start site. Across this region, there is a loss of canonical nucleosomes and a reduction in histone DNA cross-linking, as monitored by chromatin immunoprecipitation. The ATPase Snf2 is required for these alterations, but the histone acetyltransferase Gcn5 is not. This suggests that the SWI/SNF complex is directly involved in nucleosome removal at HO. We also present evidence indicating that the histone chaperone Asf1 assists in this. These observations suggest that SWI/SNF-related complexes in concert with histone chaperones act to remove histone octamers from DNA during the course of gene regulation. The Saccharomyces cerevisiae HO gene encodes an endonuclease that generates a double-stranded break at the matingtype locus that allows the yeast to switch between a and mating types (53). HO is transcribed transiently during the late G1 phase only in mother cells, but not in daughter cells (40). As such, it has been considered a paradigm for a developmentally and cell cycle-regulated gene in a relatively simple eukaryote.