Photochemically induced dynamic nuclear polarization investigation of complex formation of the NH2-terminal DNA-binding domain of lac repressor with poly[d(AT)].

The interaction of the NH2-terminal DNA-binding domain of lac repressor with synthetic oligo[d(AT)] was studied by a photo-CIDNP technique (CIDNP is chemically induced dynamic nuclear polarization). Three of the four tyrosines of the NH2-terminal region were found to be accessible to the photosensitive dye. The corresponding ring proton resonances were enhanced in the photo-CIDNP 1H NMR spectrum, and the only histidine (histidine 29) was located at the surface of the domain, which is supposed to be linked to the core protein of lac repressor by a flexible hinge region. After complex formation of the NH2-terminal region with oligo[d(AT)], two of the three tyrosine residues were no longer accessible to solvent or to photosensitive dye, which is strong evidence that the two tyrosines are part of the contact region.