The lac operon also acts as an activator in response to cyclic AMP (cAMP). cAMP is a key substrate in involved energy metabolism in E. coli. cAMP is higher in the cell once glucose levels are low. When cAMP levels arae high in the cell, the cAMP receptor protein (CRP) binds cAMP. CRP binds the promoter region of the lac operon. Binding cAMP causes a conformation change in CRP that enhances the binding of RNA Polymerase to the lac operon to encode the genes to metabolize lactose, which is a secondary carbon source needed for E. coli to grow. In this way, one operon is regulated by two different transcription factors when certain conditions are met in the cell.