The Lactose Operon

Jacob and Monod were the first researchers to identify gene regulation in prokaryotes.

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.


  • 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


  • 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.

Related Information

Modified on Feb. 12, 2000