The Lactose Operon

Operons can be either inducible or repressible

Inducible operons	Repressible operons
Repressor alone binds Operator Operon usually OFF	Repressor alone cannot bind to Operator Operon usually ON
Inducer binds Repressor Repressor does not bind to Operator, allowing operon to turn ON	Co-Repressor binds to Repressor to form Active Repressor complex Active Repressor complex binds Operator to turn operon OFF

The lac operon is inducible. The following is an outline of how the lac operon operates.

Normally:

• Enzyme synthesis is repressed because transcription is inhibited when the sugar lactose is absent from the environment.
• The regulator gene "i" constantly produces a repressor protein that binds with the operator, blocking RNA polymerase from binding to the promoter

Induction:

• When lactose enters the bacterium it binds to the repressor protein.

• The lactose-repressor complex no longer sticks to the operator. This allows the RNA polymerase to bind to the promoter region. Transcription begins but is slow.

Additional control:

• The promoter region becomes much more attractive to RNA polymerase if:
  • ATP is mostly used up and converted into cyclic AMP
  • The catabolite activator protein (CAP) is available.
• cAMP and CAP form a complex which binds to the lac promoter which stimulates transcription.

• For more information about the lac operon visit this BioTech site.
• How ATP is converted to cAMP; regulation of Adenyl Cyclase in Eukaryotes
• Follow this link to the Access Excellence diagram of the lac operon in action

Modified on Feb. 12, 2000

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