Global Regulation Systems

Catabolite Repression

E. coli can use a number of different sugars as energy sources, including glucose and lactose. Glucose is the preferred substrate, and E. coli has elaborate regulation systems to repress other sugar-utilization genes when glucose is present (this repression is called "catabolite repression").

Let's look at an example.

In addition to lactose induction, the lactose operon is controlled by catabolite repression. In the absence of glucose, E. coli uses the enzyme, adenylate cyclase, to convert ATP to cyclic-AMP.

When abundant, cyclic-AMP binds to the catabolite activator protein, (called CAP, the product of the crp gene). This changes the conformation of CAP so that it becomes active-- that is, CAP can then bind near the promoter regions of a number of sugar-utilizing genes, including the lactose operon.

Without CAP, RNA polymerase does not fit well to the promoter sites of these genes, and the genes are only transcribed at a very, very low level.

When glucose is present, it not only inhibits the production of cyclic-AMP but glucose also enhances the transport of cyclic-AMP out of the cell. This causes the concentration of cyclic-AMP to fall dramatically. Without cyclic-AMP, CAP cannot bind near the promoter sites of the various sugar utilizing genes, RNA polymerase will not bind to the promoter sites, and the genes will not be transcribed (and translated).

So, the lactose operon can only be "turned on" when there is lactose in the environment, but NO glucose.

Now that you've read about it, watch the catabolite repression animation!