Role of the Ada2 and Ada3 transcriptional coactivators in histone acetylation.

Previous studies have shown that the transcriptional coactivator protein Gcn5 functions as a catalytic histone acetyltransferase (HAT). In this work, we examine the roles of the Ada2 and Ada3 coactivator proteins that are functionally linked to Gcn5. We show that yeast Ada2, Ada3, and Gcn5 form a catalytic core of the ADA and Spt-Ada-Gcn5-acetyltransferase HAT complexes, which is necessary and sufficient in vitro for nucleosomal HAT activity and lysine specificity of the intact HAT complexes. We also demonstrate that Ada3 is necessary for Gcn5-dependent nucleosomal HAT activity in yeast extracts. Our results suggest that Ada2 potentiates the Gcn5 catalytic activity and that Ada3 facilitates nucleosomal acetylation and an expanded lysine specificity.