Nucleosome Position-Dependent and -Independent Activation of HIS7 Expression in Saccharomyces cerevisiae by Different Transcriptional Activators

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Abstract
ARO4 and HIS7 are two tandemly orientated genes of Saccharomyces cerevisiae that are transcribed into the same direction. The ARO4 terminator and the HIS7 promoter regions are sensitive to Micrococcus nuclease (Mnase) and separated by a positioned nucleosome. The HIS7 promoter is target for the transcription factors Gcn4p and Bas1p/Bas2p that activate its transcription upon amino acid starvation and purine limitation, respectively. Activation of the HIS7 gene by Gcn4p overexpression but not by Bas1p/Bas2p releases an ordered nucleosome distribution to yield increased Mnase sensitivity throughout the intergenic region. This remodeling is SNF2 dependent but mostly GCN5 independent. Accordingly, SNF2 is necessary for the Gcn4p-mediated transcriptional activation of the HIS7 gene. GCN5 is required for activation upon adenine limitation by Bas1p/Bas2p. Our data suggest that activation of HIS7 transcription by Gcn4p and Bas1p/Bas2p is supported by a nucleosome position-dependent and -independent mechanism, respectively. Whereas Gcn4p activation causes Swi/Snf-mediated remodeling of the nucleosomal architecture at the HIS7 promoter, the Bas1p/Bas2p complex presumably activates in combination with Gcn5p-dependent histone acetylation.