The product of the jun proto-oncogene has been identified as one form of the transcription factor AP-1. The p55fos protein associates with jun/AP-1 by means of a heterodimer which requires intact'leucine zipper'domains of both proteins. The fos/jun heterodimer binds to and activates transcription from TPA-responsive promoter elements (TGACTCA), which represent one final target of the protein kinase C pathway. The other main signal transduction pathway, initiated by the activation of the adenylate cyclase, involves the transcription factor CREB. The promoter element recognized by CREB, a cyclic AMP responsive element (CRE), consist of a palyndromic sequence similar to a TRE (TGACGTCA). We show that jun efficiently trans-activates CRE sequences and that fos and jun efficiently bind and cooperate in activating CRE promoter elements. The similarity between TRE and CRE sequences may involve an interplay in transcriptional regulation and'cross-talk'between components of the two major signal transduction pathways.