The lac operon is the gene which is incorporated in the metabolism of glucose. The lactose metabolism is regulated my the expression and non-expression of the lac operon. The gene which is transcribed contains the CAP ( the region where the activator binds), and then there is the lacI gene, which independently expresses the repressor (which binds to the lacO site; see below).
The lac operon also contains structural genes such as lacZ which expresses β-galactosidase ( breaks down lactose into glucose and galacatose), lacY gene which encodes lactose permase (which imports lactose into the cell), and lacA which encodes galactosidase transacetylase (breaks down toxic byproducts of β-galactosidase.
Regulatory proteins known as an activator (CAP) protein or a Repressor protein can increase or decrease gene expression respectively.
The CAP site allows the activator to bind. The promoter region allows RNA polymerase to bind. The operator site binds the lac repressor, which was made by lacI.
The repressor inactivate the expression of these genes, and this is done when there are high levels of lactose. When there is no lactose there is no substance in the repressor which activates the expression of the genes.
Expression also is dependent on the activator site. The binding of the activator is dependant on glucose levels. When there is low amounts of glucose the activator is bound to the lac operon CAP protein allowing the expression as high levels of cAMP is available. When glucose is present, lower levels of cAMP is present, and the activator is inactive.
Thus, the lac operon needs both the cAMP bound to the CAP protein, and the lactose to not be bound to the repressor protein in order to be functional.
See more information with our Gene Regulation and Operons Explained post.