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Examples of operon regulation: lac operon mechanism

  • When lactose is absent, a regulator protein is bound to the operator sequence and inhibits gene expression by preventing RNA polymerase from binding to the promoter region.
  • When lactose is present, lactose binds to the regulator protein and inactivates it, so it can no longer bind to the operator.

    This allows RNA polymerase to bind to the promoter and gene expression to occur.

The molecule cAMP (cyclic AMP, related to ATP) also regulates gene expression in the lac operon. There is an inverse relationship between glucose concentration and cAMP concentration in the cell: high cAMP levels indicate the cell is low on energy.

When cAMP concentration is high, it is especially important to break down lactose into glucose. cAMP binds to the receptor protein - the catabolite activator protein (CAP). CAP aids the binding of RNA polymerase to the promoter region.

The lac operon represents a negative inducible system.

The lac operon.
The lac operon.