Comparison of nucleosome remodeling by the yeast transcription factor Pho4and the glucocorticoid receptor

Chromatin reorganization of the PHO5 and murine mammary tumor virus (MMTV) promoters is triggered by binding of either Pho4 or the glucocorticoid rece ptor (GR), respectively. In order to compare the ability of Pho4 and GR to remodel chromatin and activate transcription, hybrid promoter constructs we re created by insertion of the MMTV B nucleosome sequence into the PHO5 pro moter and then transformed into a yeast strain expressing GR, Activation of either Pho4 (by phosphate depletion) or GR (by hormone addition) resulted in only slight induction of hybrid promoter activity. However, simultaneous activation of both Pho4 and GR resulted in synergistic activation to level s exceeding that of the wild type PHO5 promoter. Under these conditions, Ph o4 completely disrupted the nucleosome containing its binding site. In cont rast, GR had little effect on the stability of the MMTV B nucleosome. A min imal transactivation domain of the GR fused to the Pho4 DNA-binding domain is capable of efficiently disrupting the nucleosome with a Pho4-binding sit e, whereas the complementary hybrid protein (Pho4 activation domain, GR DNA -binding domain) does not labilize the B nucleosome. Therefore, we conclude that significant activation by Pho4 requires nucleosome disruption, wherea s equivalent transcriptional activation by GR is not accompanied by overt p erturbation of nucleosome structure. Our results show that the DNA-binding domains of the two factors play critical roles in determining how chromatin structure is modified during promoter activation.