Recently it has become clear that general chromatin proteins as well as sequence-specific DNA binding proteins are important in the control of gene expression. SIN1 in Saccharomyces cerevislae is a chromatin component that regulates the transcription of a family of genes. Previously, we identified a 32 bp unique sequence (here termed XBS) in the promoter of one of those genes, HO, which specifically binds a protein that interacts with SIN1. We also found that this sequence can function as a weak UAS in a heterologous promoter that is dependent on the presence of SIN1. Here we report a relationship between the level of HO expression and the presence of the short sequence in situ in the HO gene. By comparing the expression of HO from wild type or XBS deleted HO promoters, we concluded that XBS serves as a weak UAS in situ in the HO gene, that it influences HO transcription via the SWI/SNF complex, and that sequences other than the XBS mediate the affect of SIN1 on HO transcription. In addition, we show that a portion of the SIN1 protein that has sequence similarity to mammalian HMG1 preferentially binds the XBS.
