Interaction of the Lac repressor and the operator region. (a) The lac operon contains three operator sequences to which Lac repressor can bind. O1 is adjacent to the lac operon promoter. The inverted repeat of the O1 site is shown (sequence repeats are shaded orange). (b) The Lac repressor tetramer can bind to O1 and O2 or to O1 and O3. The intervening DNA is looped out. The molecular model shows the Lac repressor, a tetramer formed from two homodimers. Each homodimer can bind one operator sequence.