The lac Operon - an inducible system

The trp Operon - a repressible system

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The lac Operon - an inducible system

The first control system for enzyme production worked out at the molecular level described the control of enzymes that are produced in response to the presence of the sugar lactose in E. coli cell. The work was performed by Jacob and Monod for which they were awarded the Nobel Prize. The following is the pathway that leads to the production of glucose and galactose.

Lactose  ----------------------------------->  Glucose + Galactose

Several proteins involved in lactose metabolism in the E. coli cell. They are:

  • ß-galactosidase - converts lactose into glucose and galactose
  • ß-galactoside permease - transports lactose into the cell
  • ß-galactoside transacetylase - function unknown

Research with this system was greatly added by the availability of constitutive mutants. A constitutive mutant is one in which the gene product is produced continually, that is there is no control over its expression. In these mutants, the above proteins were produced all the time in comparison to the wild type where the proteins only appeared in the presence of lactose. So in these mutants, the mutation must be a gene other than those responsible for the structural genes.

All of the genes involved in controlling this pathway are located next to each other on the E. coli chromosome. Together they form an operon. The following is the genetic structure of the operon.

Control Circuit for the lac Operon

     I    P   O    ||     Z     |     Y     |     A     |
Controlling || Structural genes

lac Operon Gene Gene Function


Gene for repressor protein





lac Z

Gene for ß-galactosidase

lac Y

Gene for ß-galactoside permease

lac A

Gene for ß-galactoside transacetylase

Operon - a cluster of structural genes that are expressed as a group and their associated promoter and operator

How does the system work? Without lactose in the cell, the repressor protein binds to the operator and prevents the read through of RNA polymerase into the three structural genes. With lactose in the cell, lactose binds to the repressor. This causes a structural change in the repressor and it loses its affinity for the operator. Thus RNA polymerase can then bind to the promoter and transcribe the structural genes. In this system lactose acts as an effector molecule.

Effector molecule - a molecule that interacts with the repressor and affects the affinity of the repressor for the operator

With the above information, we can now predict the effect that various mutants will have on lac operon gene expression.

Mutant lac gene Mutant Phenotype


constitutive expression because the operator is never closed


constitutive expression because the repressor protein can not bind


no expression of the operon because RNA polymerase cannot bind

lac Z-

no glucose or galactose production from lactose

lac Y-

no induction because lactose will not be taken into the cell

Catabolite Repression of the lac Operon

Lactose is not the preferred carbohydrate source for E. coli. If lactose and glucose are present, the cell will use all of the glucose before the lac operon is turned on. This type of control is termed catabolite repression. To prevent lactose metabolism, a second level of control of gene expression exists. The promoter of the lac operon has two binding sites. One site is the location where RNA polymerase binds. The second location is the binding site for a complex between the catabolite activator protein (CAP) and cyclic AMP (cAMP). The binding of the CAP-cAMP complex to the promoter site is required for transcription of the lac operon. The presence of this complex is closely associated with the presence of glucose in the cell. As the concentration of glucose increases the amount of cAMP decreases. As the cAMP decreases, the amount of complex decreases. This decrease in the complex inactivates the promoter, and the lac operon is turned off. Because the CAP-cAMP complex is needed for transcription, the complex exerts a positive control over the expression of the lac operon.

Copyright © 1997. Phillip McClean