After the lac operon was sequenced, it eventually became clear that it has two additional operators. The original operator with its center of symmetry at position + 11 is now designated lacO1. Auxiliary operators lacO2 and lacO3 have their centers of symmetry at positions+412 and-82, respectively. Thus, lacO3 is upstream of the lac promoter while lacO2 is located in lacZ.
The discoveries of lacO2 and lacO3 led investigators to ask whether these auxiliary operators play a role in lac operon regulation. Millier-Hill and coworkers performed a series of genetic experiments that answered this question. Their approach was to alter one or more lac operators and then determine the alteration's effect on repression. Point mutations in taco, caused a 5-to 50-fold decrease in repression, but some repression was still observed. Destruction of either lacO2 or lacO3 caused a twofold decrease in repression and destruction of both auxiliary operators caused a 70-fold decrease in repression. The ability to repress the lac system was completely lost in cells in which lacO1, and either of the auxiliary opera-tors were destroyed. These re X indicate that lacO1 plays a major role in repression but the two auxiliary operators also make important contributions. We will now examine the structure of the Lac repressor to see how it interacts with the lac operators.
