The lac operon of E . coli is a cluster of three structural genes, encoding proteins involved in lactose metabolism and the sites on the DNA involved in regulation of the operon.
Normally lac operon is turned off. The operon takes charge only when glucose levels are low. When glucose is unavailable the lac operon helps in the effective digestion of lactose. This is done through the activity of beta galactosidase. The operon uses a control mechanism that ensures the encoding of enzymes only when it is necessary. In the absence of lactose, the lac operon halts production of the enzymes encoded by the lac operon.
This control mechanism causes the sequential utilisation of glucose and lactose metabolism and the sites on the DNA involved in the regulation of the operon.