REQUIREMENTS FOR CHROMATIN MODULATION AND TRANSCRIPTION ACTIVATION BYTHE PHO4 ACIDIC ACTIVATION DOMAIN

Perhaps the best characterized example of an activator-induced chromat in transition is found in the activation of the Saccharomyces cerevisi ae acid phosphatase gene PHO5 by the basic helix-loop-helix (bHLH) tra nscription factor Pho4. Transcription activation of the PNO5 promoter by Pho4 is accompanied by the remodeling of four positioned nucleosome s which is dependent on the Pho4 activation domain but independent of transcription initiation. Whether the requirements for transcription a ctivation through the TATA sequence are different from those necessary for the chromatin transition remains a major outstanding question. In an attempt to understand better the ability of Pho4 to activate trans cription and to remodel chromatin, we have initiated a detailed charac terization of the Pho4 activation domain. Using both deletion and poin t mutational analysis, we have defined residues between positions 75 a nd 99 as being both essential and sufficient to mediate transcription activation, Significantly, there is a marked concordance between the a bility of mutations in the Pho4 activation domain to induct chromatin opening and transcription activation, Interestingly, the requirements for transcription activation within the Pho4 activation domain differ significantly if fused to re heterologous bHLH-leucine zipper DNA-bind ing domain. The implications for transcription activation by Pho4 are discussed.