How it works
Inducer exclusion is one way glucose keeps the lac operon of E. coli switched off: lactose, the source of the inducer, cannot get in. Label the proteins that do it, then see what changes when the glucose is gone.
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With glucose: lactose is kept out
Glucose enters through a transporter of the phosphotransferase system and gets a phosphate on the way in. That phosphate is drawn off a protein called EIIA. Without its phosphate, EIIA binds lactose permease and blocks it, so lactose stays outside and the lac operon stays off.
No glucose: the operon can be read
With no glucose to take it, EIIA keeps its phosphate and switches on adenylate cyclase, which makes cyclic AMP from ATP. Cyclic AMP binds CAP, and CAP on the DNA helps RNA polymerase start. Lactose now enters through the permease.
Questions and answers
What is inducer exclusion?
It is how glucose keeps lactose out of an E. coli cell. While glucose is being taken up, the protein EIIA binds lactose permease and blocks it, so the inducer of the lac operon cannot enter.
How do I label the diagram, which illustrates the mechanism of inducer exclusion?
Find five things: glucose, its transporter, EIIA without its phosphate, lactose permease and the lactose left outside. The drawing here is the site's own, so its layout may differ from the one in a course.
What does EIIA do when glucose is absent?
It keeps its phosphate. Phosphorylated EIIA switches on adenylate cyclase, which makes cyclic AMP. Cyclic AMP binds CAP, and CAP helps RNA polymerase transcribe the lac operon when lactose is present.
Is inducer exclusion the same as catabolite repression?
They work together, and some texts count inducer exclusion as part of catabolite repression. The name often means the control through cyclic AMP and CAP, which lowers transcription while glucose is present. Inducer exclusion blocks the transport of lactose itself. Both depend on EIIA.