Structure of the lac repressor

a. The repressor–DNA complex, with DNA (in green) bound separately to each of two dimeric units—one shown with the monomers in green and purple, and monomers of the other shown in yellow-green and red.

b. The effect of IPTG binding. Shown in yellow-green are the extreme N-termini of two monomeric units of the protein (residues 1–68), bound to DNA, in purple. The dashed structures represent the presumed positions of the DNA-binding helices after binding of inducer IPTG to the protein, which drives the DNA-binding helices apart.