Can the monomer of the leucine zipper proteins recognize the dimer binding site without dimerization?

It is generally believed that leucine zipper regulatory proteins for DNA transcription recognize their DNA binding sites as dimers preformed in solution (and that the monomers do not bind specifically to these sites). To test this idea, we synthesized the 31-residue peptide v-Jun-br, which contains only the DNA binding region of the v-jun monomer. Footprinting assays show that v-Jun-br monomers specifically protect the DNA binding site of v-Jun in almost identically the same way as dimers. Thus, (i) the monomer recognizes the half-site of the dimer binding site and (ii) dimerization does not appreciably affect the bound conformation of each monomer. These results may have implications in the regulation of transcription by such proteins. Thus, two monomers of v-Jun might bind sequentially to the dimer binding site followed by dimerization of v-Jun while bound. This may allow binding at concentrations too low for dimerization in solution.