An example of an inducible operon, its function is to produce the proteins required to digest the sugar lactose. If no lactose is present, the lac repressor protein will bind to the operator, blocking transcription of the operon by RNA polymerase. Lactose serves as the inducer molecule for this. When lactose is present, it binds to the lac repressor protein, changing its shape so that it no longer can bind to the operator sequence, allowing RNA polymerase to transcribe the genes for the proteins that digest lactose. After all the lactose has been digested, the repressor can again bind to the operator sequence, shutting down the operon. This type of feedback mechanism allows the cell to manufacture the proteins needed to digest lactose only when they are needed, saving valuable resources in the cell. There are also ways to positively upregulate gene expression, shown through the interaction between cyclic AMP (cAMP) and the catabolite activator protein (CAP) in this. When glucose levels are low, cAMP levels in the cell increase. cAMP binds to the CAP, stimulating CAP to bind at a CAP binding site near the promoter. This increases the affinity of RNA polymerase for the promoter, stimulating transcription. So transcription of this is increased when glucose, another food source for the bacteria, is absent, allowing the cell to utilize the energy in lactose more efficiently.