Lac Operon

  • Classic example of a genetic response to an environmental change. Glucose is the preferred metabolic substrate in E coli, but when glucose is absent and lactose is available, the lac operon is activated to switch to lactose metabolism.
  • Lac operon consists
    • Regulatory gene (lac I): encodes Lac I repressor protein, constitutively expressed.  Repressor proteins, when bound to the operator region, prevent binding of RNA polymerase to the promoter region, thus decreasing transcription of the lac Z, lac Y, and lac A genes.
    • Promoter region (lac p): binding site for RNA polymerase during the initiation of transcription.
    • Operator region (lac o)
    • Structural genes (lac Z, lac Y, and lac A)
      • Lac Z gene codes for β-galactosidase, which is responsible for the hydrolysis of lactose to glucose and galactose.
      • Lac Y gene codes for permease, which allows lactose to enter the bacterium.
  • Mechanism of shift:
    • Low glucose → ↑adenylate cyclase activity → ↑generation of cAMP from ATP → activation of catabolite activator protein (CAP) → ↑transcription.
    • High lactose → unbinds repressor protein from repressor/operator site → ↑transcription.

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