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.

lac operon: gene regulation via cAMP

The lac operon in *E. coli* is a well studied inducible operon in *E. coli*. The lac operon genes code for proteins needed to break down lactose. When lactose is not present, the lac repressor binds to the lac operator sequence and prevents transcription. When lactose is present in the cell, it is converted to allolactose which is recognized by the lac repressor. Allolactose binds to the lac repressor and prevents it from binding the DNA. This allows transcription of genes needed to break down lactose in the cell. This processes allows for the cell to only encode genes needed for lactose break down when in the presence of lactose.

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Inducible operons typically encode proteins that use or break down specific molecules in the cell. Their expression is induced when in the presence of that molecule so that the operon is only expressed when the genes are needed to break down that compound.

Inducible Operons

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