The lac repressor–DNA complex. The DNA-binding domain from a gene-regulatory protein, the lac repressor, binds to a DNA fragment containing its preferred binding site (referred to as operator DNA) by inserting an α helix into the major groove of operator DNA. Notice that a specific contact forms between an arginine residue of the repressor and a G–C base pair in the binding site.
[Drawn from 1EFA.pdb.]