The protooncogenes c-fos and c-jun encode nuclear proteins (fos and jun, respectively) that function cooperatively as a heterodimeric protein complex in the regulation of gene transcription. These proteins dimerize via a structural motif known as the leucine zipper and bind to activator protein-1 sites via a conserved domain that is rich in basic amino acids. Previously, we demonstrated that while fos and jun polypeptides expressed in Escherichia coli dimerize efficiently, they exhibit only a low level of DNA-binding activity. Here we show that the DNA-binding activity of fos-jun heterodimers and jun-jun homodimers is stimulated dramatically by a ubiquitous nuclear protein. This protein does not appear to participate in the DNA-protein complex, and it does not affect the specificity of the interaction with DNA. These results suggest that a nuclear protein regulates the DNA-binding activity of fos and jun indirectly.