Structure and dynamics of the lac repressor-operator complex as determined by NMR

The structures of the lac repressor headpiece and of its complex with an 11 base-pair lac half-operator have been determined by NMR spectroscopy. By 15N relaxation studies the dynamic behavior of the free protein and of the protein in the complex could be established. In the three-helical lac headpiece local backbone mobility was detected in the N-terminal and C-terminal peptide regions and in the loop between helices II and III. Upon DNA binding this loop becomes more rigid and it changes its conformation considerably. The specificity of the protein-DNA interaction follows from a large number of hydrogen-bond and hydrophobic interactions between amino acid side chains and DNA backbone and bases. Restrained molecular dynamics calculations suggest that some of these interactions are dynamic in nature.