ABSENCE OF GCN5 HAT ACTIVITY DEFINES A NOVEL STATE IN THE OPENING OF CHROMATIN AT THE PHO5 PROMOTER IN YEAST

Histone acetyltransferase (HAT) activity has been demonstrated for sev eral transcriptional activators, formally connecting chromatin modific ation with gene regulation. However, no effect on chromatin has been d emonstrated. We have investigated the role of the HAT Gcn5 at the nucl eosomally regulated PHO5 promoter. Under conditions of constitutive su bmaximal activation (i.e., in the absence of the negative regulator Ph o80), deletion of Gcn5 determines a novel randomized nucleosomal organ ization across the promoter and leads to a dramatic reduction in activ ity. Furthermore, mutation of amino acids critical for Gcn5 HAT activi ty is sufficient to generate this structure. This intermediate state i n chromatin opening gives way to the fully open structure upon maximal induction (phosphate starvation), even in the absence of Gcn5. Thus, Gcn5 is shown to affect directly the remodeling of chromatin in vivo.