Required for the function of some acidic activation domains, which activate transcription from a distant site. The exact mechanism of action is not yet known (By similarity). ADA2 stimulates the acetyltransferase activity of GCN5 on free histones or nucleosomes, probably by opening up the promoter region. Interacts in vitro with the HAT domain of GCN5 and with the DNA-binding domain of the transcriptional activator DREB1B/CBF1. Expressed in roots and leaves. The middle domain of ADA2a is sufficient for interaction with the HAT catalytic domain of GCN5. Acetylated in vitro by GCN5, but acetylation is not essential for biological activity.