Final answer:
The lac operon has three operator sites, identified as O1, O2, and O3. O2 and O3 were not initially detected because individual mutations did not significantly affect lactose regulation; only simultaneous mutations did.
Step-by-step explanation:
DNA foot printing experiments have revealed that the lac operon has a total of three operator sites. These are known as O1, O2, and O3. While O1 is the primary operator, the additional O2 and O3 were found within the lacZ and lacI genes, respectively. Initially, these operator sites were not detected because mutations in O2 or O3 alone did not significantly impact the operon's induction by lactose. It was the simultaneous mutation of both O2 and O3 regions that resulted in a substantial reduction in the binding of the repressor to the operon, thereby highlighting their involvement in the regulatory mechanism.