Activation domains are functional modules that enable sequence-specific DNA binding proteins to stimulate transcription. The structural basis for the function of activation domains is poorly understood. A combination of nuclear magnetic resonance (NMR) and biochemical experiments revealed that the minimal acidic activation domain of the herpes simplex virus VP16 protein undergoes an induced transition from random coil to or helix upon binding to its target protein, hTAF(parallel to)31 (a human TFIID TATA box-binding protein-associated factor). Identification of the two hydrophobic residues that make nonpolar contacts suggests a general recognition motif of acidic activation domains for hTAF(parallel to)31.