The solution structure of Lac repressor headpiece 62 complexed to a symmetrical lac operator

Background: Lactose repressor protein (Lac) controls the expression of the lactose metabolic genes in Escherichia coli by binding to an operator seque nce in the promoter of the lac operon, Binding of inducer molecules to the Lac core domain induces changes in tertiary structure that are propagated t o the DNA-binding domain through the connecting hinge region, thereby reduc ing the affinity for the operator. Protein-protein and protein-DNA interact ions involving the hinge region play a crucial role in the allosteric chang es occurring upon induction, but have not, as yet, been analyzed in atomic detail. Results: We have used nuclear magnetic resonance (NMR) spectroscopy and res trained molecular dynamics (rMD) to determine the structure of the Lac repr essor DNA-binding domain (headpeice 62; HP62) in complex with a symmetrized lac operator. Analysis of the structures reveals specific interactions bet ween Lac repressor and DNA that were not found in previously investigated L ac repressor-DNA complexes, Important differences with the previously repor ted structures of the HP56-DNA complex were found in the loop following the helix-turn-helix (HTH) motif, The protein-protein and protein-DNA interact ions involving the hinge region and the deformations in the DNA structure c ould be delineated in atomic detail. The structures were also used for comp arison with the available crystallographic data on the Lac and Pur represso r-DNA complexes. Conclusions: The structures of the HP62-DNA complex provide the basis for a better understanding of the specific recognition in the Lac repressor-oper ator complex. In addition, the structural features of the hinge region prov ide detailed insight into the protein-protein and protein-DNA interactions responsible for the high affinity of the repressor for operator DNA.