THE GCN5 ADA COMPLEX POTENTIATES THE HISTONE ACETYLTRANSFERASE ACTIVITY OF GCN5

The Gcn5 histone acetyltransferase (HAT) is part of a large multimeric complex that is required for transcriptional activation in yeast. Thi s complex can acetylate in vitro and in a Gcn5-dependent manner both n ucleosomal and free core histones. For this reason it is believed that part of the function of the GcnF5.Ada complex is chromatin remodeling effected by histone acetylation. The roles of the other subunits of t his complex are not yet known. We have generated mutated Gcn5 proteins with severely attenuated in vitro HAT activities. Despite their appar ent loss in HAT activity, these GCN5 derivatives complemented all the defects of a gcn5 strain. We have shown that when these mutated protei ns were produced in yeast cells in the absence of another component of the complex, Ada2, their activity was still compromised. By contrast, when produced in the wild type context, they were partially capable o f acetylating free histones and were even more active when nucleosomal arrays were used as substrates. Kinetic enzymatic analyses showed tha t the rate of catalysis by Gcn5 was enhanced when the mutated proteins were produced in yeast in the presence of Ada2. Because Ada2 is requi red for the assembly of Gcn5, we conclude that one role for components of the Gcn5 Ada complex is the potentiation of its HAT activity.