Comparison of ABF1 and RAP1 in chromatin opening and transactivator potentiation in the budding yeast Saccharomyces cerevisiae.

Autonomously replicating sequence binding factor 1 (ABF1) and repressor/activator protein 1 (RAP1) from budding yeast are multifunctional, site-specific DNA-binding proteins, with roles in gene activation and repression, replication, and telomere structure and function. Previously we have shown that RAP1 can prevent nucleosome positioning in the vicinity of its binding site and have provided evidence that this ability to create a local region of "open" chromatin contributes to RAP1 function at the HIS4 promoter by facilitating binding and activation by GCN4. Here we examine and directly compare to that of RAP1 the ability of ABF1 to create a region of open chromatin near its binding site and to contribute to activated transcription at the HIS4, ADE5,7, and HIS7 promoters. ABF1 behaves similarly to RAP1 in these assays, but it shows some subtle differences from RAP1 in the character of the open chromatin region near its binding site. Furthermore, although the two factors can similarly enhance activated transcription at the promoters tested, RAP1 binding is continuously required for this enhancement, but ABF1 binding is not. These results indicate that ABF1 and RAP1 achieve functional similarity in part via mechanistically distinct pathways.