Strong DNA binding by covalently linked dimeric Lac headpiece: evidence for the crucial role of the hinge helices.

The combined structural and biochemical studies on Lac repressor bound to operator DNA have demonstrated the central role of the hinge helices in operator bending and the induction mechanism. We have constructed a covalently linked dimeric Lac-headpiece that binds DNA with four orders of magnitude higher affinity as compared with the monomeric form. This enabled a detailed biochemical and structural study of Lac binding to its cognate wild-type and selected DNA operators. The results indicate a profound contribution of hinge helices to the stability of the protein-DNA complex and highlight their central role in operator recognition. Furthermore, protein-DNA interactions in the minor groove appear to modulate hinge helix stability, thus accounting for affinity differences and protein-induced DNA bending among the various operator sites. Interestingly, the in vitro DNA-binding affinity of the reported dimeric Lac construct can de readily modulated by simple adjustment of redox conditions, thus rendering it a potential artificial gene regulator.