Final answer:
The lac operon in E. coli is turned on in the presence of lactose. Glucose levels affect the activation of the lac operon, with low levels of glucose leading to operon activation. A repressor protein prevents transcription when glucose is present.
Step-by-step explanation:
In E. coli, the lactose operon (lac operon) is turned on in the presence of lactose. When glucose levels are low, the lac operon is activated and the three genes of the operon produce the proteins needed to break down lactose. The lac operon is controlled by a repressor protein that is bound to the operator, blocking transcription when glucose is present.