The protein products of the c-fos and c-jun proto-oncogenes (Fos and Jun, respectively) form a heterodimeric protein complex that interacts with the activator protein-1 (AP-1) binding site and regulates gene transcription in response to extracellular stimuli. Protein dimerization is mediated primarily by a coiled-coil-like structure termed the leucine-zipper and DNA binding occurs primarily through regions of each protein rich in basic amino acids that contact both strands of the AP-1 site. The precise nature of the protein-DNA interaction is unknown as studies concerned with dimerization and DNA binding by Fos and Jun have relied on indirect methods to investigate protein-protein-DNA interactions. Here we have developed assay systems using fluorescence spectroscopy and circular dichroism to monitor dimerization and DNA binding directly. The results indicate that the interaction of Fos and Jun with DNA results in an altered conformation of the protein dimers and an increased alpha-helical content. These techniques may have general application in studies concerning the interaction of transcriptional regulatory proteins with specific DNA target sequences.