We have purified and characterized the 50 kd activator protein 2 (AP2), another enhancer-binding protein interacting with the human metailothionein IIA (hMT-iIA) gene control region. Purified AP2 activates transcription in vitro from a hybrid promoter containing hMT-11~ upstream sequences. AP2 also recognizes control elements of the human growth hormone, c-myc, and H-2Kb genes, and the SV40 and bovine papilloma virus enhancers Multiple synthetic copies of the hMT-11~ high-affinity AP2 binding site can act as efficient, cell-type-specific enhancer elements; their activity increases after treatment of cells with phorbol ester or cAMP-elevating agents. In contrast, a synthetic enhancer recognized by factor AP-1 is activated only by phorbol ester. AP-2 appears to mediate transcriptional activation in response to two different signal-transduction pathways, one involving the phorbol-esterand diaclyglycerol-activated protein kinase C, the other involving cAMP-dependent protein kinase A.