Chromatin remodeling and transcription activation by the Pho4 transcription factor in vitro and in vivo Chromatin plays an essential regulatory role in transcription regulation. However, many of the molecular mechanisms underlying activation domain-mediated chromatin reorganisation remain unknown. Perhaps the best characterised example of an activator-induced chromatin transition is found in the activation of the yeast acid phosphatase gene PHO5 by the transcription factor Pho4. As a consequence of Pho4 binding the PH05 UAS in cooperation with the Pho2 homeodomain protein, four positioned nucleosomes undergo a transition resulting in the entire promoter nd UAS becoming nuclease sensitive. Importantly. the Pho4 activation domain is essential for the Chromatin transition to occur. How the Pho4 activation domain mediates the cromatin transition is not known. By isolating minichromosomes from yeast we have recently been able to demonstrate A ATP and Pho4 dependent chromatin remodelling across the PH05 promoter in vitro The aims of this proposal are: I . To characterise further the ability of Pho4 to remodel chromatin in vitro 2. To define the targets of the Pho4 activation domain vitro and in vivo 3. To determine whether the requirements for transcription activation by Pho4 are the same on different Pho4-regulated promoters.