The functional lac repressor is a homoteramer.

The functional lac repressor is a homoteramer. However, the X-ray structure of lac repressor bound to the lac operator sequence centered at +11 reveals that the two half sites that constitute the complete operator sequence with two-fold rotational symmetry are each bound by a monomeric subunit of the repressor. If the repressor tetramer uses only two of its subunits to bind to an operator sequence, why is the tetramer formation so important to the control of lac operon gene expression?