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Chapter 14 · Part 3 of 9
  1. Chapter overview
  2. 1Why cells regulate their genes
  3. 2Repressors, activators, and the lac operon
  4. 3Switching the lac operon on and off
  5. 4The trp operon and predicting mutations
  6. 5Eukaryotic promoters and enhancers
  7. 6General transcription factors and mediator
  8. 7Remodeling chromatin and modifying histones
  9. 8Switching genes on and silencing them
  10. 9Regulating splicing and translation

Switching the lac operon on and off

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The lac operon answers two questions before it runs at full speed: is lactose here, and is glucose missing? A repressor handles the first question and an activator handles the second.

Negative control: lac repressor and allolactose

With no lactose around, lac repressor sits on the operator, gripping the DNA in its major groove. RNA polymerase can still bind the promoter, but the repressor is in its path, so it can’t move forward to transcribe lacZ, lacY, and lacA.

The switch that releases it is allolactose, the lactose look-alike that β-galactosidase makes as a side product. Allolactose is the operon’s small effector molecule. It binds lac repressor at an allosteric site, a site separate from the protein’s DNA-binding site (the same kind of site noncompetitive inhibitors use in Chapter 6). That binding changes the shape of the DNA-binding site, so the repressor lets go of the operator.

  1. Lactose appears outside the cell, and a little enters through the few permease molecules already present.
  2. The few β-galactosidase molecules present convert some of it into allolactose.
  3. Allolactose binds lac repressor, which changes shape and releases the operator.
  4. RNA polymerase transcribes lacZ, lacY, and lacA, and the cell makes plenty of all three proteins.

The repressor never shuts the operon off completely, and that small leak matters. Without a few permease and β-galactosidase molecules already in the cell, lactose couldn’t get in or be turned into allolactose, and the operon could never switch on.

A small effector molecule that increases transcription is an inducer, and allolactose is the inducer of the lac operon. An operon that an inducer turns on is an inducible operon.

Lac repressor Allolactose RNApolymerase Promoter Operator lacZ lacY lacA mRNA No lactose Lactose present
Figure 14.2 Without lactose, lac repressor sits on the operator and RNA polymerase can’t get past it. Allolactose binds the repressor and changes its shape. The repressor lets go of the operator. RNA polymerase then transcribes all three genes into one mRNA.
Check yourselfWhy does the lac operon stay off when lactose is absent?

With no lactose, there’s no allolactose to bind lac repressor. The repressor stays on the operator and blocks RNA polymerase from transcribing the genes.

Positive control: CAP and cAMP

The lac operon also has an activator, the catabolite activator protein (CAP). Its small effector molecule is cAMP, the second messenger from Chapter 9 that adenylyl cyclase makes from ATP. When cAMP binds CAP, the pair binds the CAP site, bends the DNA, and helps RNA polymerase bind the promoter. Transcription speeds up.

Glucose controls this activator. Glucose inhibits the production of cAMP, so when glucose is plentiful, cAMP is scarce, CAP stays off the DNA, and the lac operon runs slowly. E. coli uses glucose first and turns to lactose only when glucose runs low.

This kind of control, where a preferred energy source holds back the use of others, is catabolite repression. It saves energy because the cell uses one sugar at a time.

Four combinations of sugars

Putting both controls together, the lac operon runs at full speed only when lactose is present and glucose is absent.

Lactose Glucose Lac repressor CAP Transcription
High High Released Not bound Low (glucose is used first)
High Low Released Bound High (lactose is used)
Low High Bound to operator Not bound Off
Low Low Bound to operator Bound Off
Check yourselfE. coli is growing on a mix of glucose and lactose. What happens to the lac operon when the glucose runs out?

cAMP levels rise, cAMP binds CAP, and CAP binds the CAP site. Lactose is still present, so the repressor is already off the operator, and transcription jumps from low to high.

Practice questions

  1. 1 of 3

    Allolactose binds lac repressor at a site separate from the protein’s DNA-binding site. What is this kind of site called?

  2. 2 of 3

    Lac repressor never shuts the lac operon off completely. Why does this small leak matter?

  3. 3 of 3

    When cAMP-CAP binds the CAP site, how does it speed transcription of the lac operon?