We use chromatin immunoprecipitation assays to show that the Gcn5 histone acetyltransferase in SAGA is required for SWI/SNF association with the HO promoter and that binding of SWI/SNF and SAGA are interdependent. Previous results showed that SWI/SNF binding to HO was Gcn5 independent, but that work used a strain with a mutation in the Ash1 daughter-specific repressor of HO expression. Here, we show that Ash1 functions as a repressor that inhibits SWI/SNF binding and that Gcn5 is required to overcome Ash1 repression in mother cells to allow HO transcription. Thus, Gcn5 facilitates SWI/SNF binding by antagonizing Ash1. Similarly, a mutation in SIN3, like an ash1 mutation, allows both HO expression and SWI/SNF binding in the absence of Gcn5. Although Ash1 has recently been identified in a Sin3-Rpd3 complex, our genetic analysis shows that Ash1 and Sin3 have distinct functions in regulating HO. Analysis of mutant strains shows that SWI/SNF binding and HO expression are correlated and regulated by histone acetylation. The defect in HO expression caused by a mutant SWI/SNF with a Swi2(E834K) substitution can be partially suppressed by ash1 or spt3 mutation or by a gain-of-function V71E substitution in the TATA-binding protein (TBP). Spt3 inhibits TBP binding at HO, and genetic analysis suggests that Spt3 and TBP(V71E) act in the same pathway, distinct from that of Ash1. We have detected SWI/SNF binding at the HO TATA region, and our results suggest that SWI/SNF, either directly or indirectly, facilitates TBP binding at HO.The Saccharomyces cerevisiae HO gene encodes an endonuclease that initiates mating-type switching in haploid yeast cells, and the gene is governed by complex transcriptional regulation (for reviews, see references 22, 41, and 62). The gene is expressed only during the late G 1 phase of the cell cycle, and only in mother cells, one of the two progeny after mitotic division. The Ash1 repressor protein is required for this asymmetric expression, as HO is expressed in both mother and daughter cells in an ash1 mutant (14, 76).Chromatin structure plays an important role in transcriptional regulation, including at HO. There are two major classes of chromatin-modifying factors that alleviate the repressive effects of chromatin, the ATP-dependent chromatin-remodeling factors, such as SWI/SNF, and histone acetyltransferases (HATs) that covalently modify the N-terminal tails of histones by acetylation (84). Recent work has shown that transcription factors recruit chromatin-modifying factors to promoters and that at some promoters, the concerted action of chromatinremodeling and HAT complexes is required for gene activation (61). It has been shown for a number of promoters that sequence-specific DNA-binding proteins recruit chromatin remodelers and HATs in a temporal order.Sequential recruitment of transcription factors was first shown at the HO gene (25). HO contains two defined upstream promoter regions, URS1 and URS2, which contain recognition sites for the Swi5 and SBF sequence-specific DNA-binding factors,...