RAP1 Is Required for BAS1/BAS2- and GCN4-Dependent Transcription of the Yeast HIS4 Gene

The major in vitro binding activity to the Saccharomyces cerevisiae HIS4 promoter is due to the RAP1 protein. In the absence of GCN4, BAS1, and BAS2, the RAP1 protein binds to the HIS4 promoter in vivo but cannot efficiently stimulate HIS4 transcription. RAP1, which binds adjacently to BAS2 on the HIS4 promoter, is required for BAS1/BAS2-dependent activation of HIS4 basal-level transcription. In addition, the RAP1-binding site overlaps with the single high-affinity HIS4 GCN4-binding site. Even though RAP1 and GCN4 bind competitively in vitro, RAP1 is required in vivo for (i) the normal steady-state levels of GCN4-dependent HIS4 transcription under nonstarvation conditions and (ii) the rapid increase in GCN4-dependent steady-state HIS4 mRNA levels following amino acid starvation. The presence of the RAP1-binding site in the HIS4 promoter causes a dramatic increase in the micrococcal nuclease sensitivity of two adjacent regions within HIS4 chromatin: one region contains the high-affinity GCN4-binding site, and the other region contains the BAS1- and BAS2-binding sites. These results suggest that RAP1 functions at HIS4 by increasing the accessibility of GCN4, BAS1, and BAS2 to their respective binding sites when these sites are present within chromatin.