A recently described class of DNA binding proteins is characterized by the bZIP motif, which consists of a basic region that contacts DNA and an adjacent leucine zipper that mediates protein dimerization. A peptide model for the basic region of the yeast transcriptional activator GCN4 has been developed in which the leucine zipper has been replaced by a disulfide bond. The 34-residue peptide dimer, but not the reduced monomer, binds DNA with nanomolar affinity at 4 degrees C. DNA binding is sequence-specific as judged by deoxyribonuclease I footprinting. Circular dichroism spectroscopy suggests that the peptide adopts a helical structure when bound to DNA. These results demonstrate directly that the GCN4 basic region is sufficient for sequence-specific DNA binding and suggest that a major function of the GCN4 leucine zipper is simply to mediate protein dimerization. Our approach provides a strategy for the design of short sequence-specific DNA binding peptides.