Lac operon

• Core Logic
  • Polycistronic mRNA encoding enzymes for lactose metabolism.
  • E. coli prefers Glucose. Operon ON only if: Glucose Absent AND Lactose Present.
• Key Components
  • lacZ: β-galactosidase (breaks down lactose).
  • lacY: Permease (lactose transport).
  • lacI: Repressor (constitutive; binds Operator to block transcription).
• Regulation Mechanism
  1. Glucose Signaling (CAP):
     • ↓ Glucose → ↑ cAMP → CAP binds DNA → Helps RNA Pol bind.
     • ↑ Glucose → ↓ cAMP → No CAP → Low/No Transcription.
  2. Lactose Signaling (Repressor):
     • No Lactose: Repressor binds Operator → No Transcription.
     • High Lactose: Allolactose binds Repressor → Repressor falls off → Transcription Possible.

• Transcription Scenarios
  • Low Glu / High Lac: High Transcription (CAP bound, Repressor off).
  • High Glu / High Lac: Very Low Transcription (No CAP, Repressor off).
  • Any “No Lac”: No Transcription (Repressor bound).
• Genetics (Mutations)
  • lac I⁻ (No repressor): Constitutive (Always ON).
  • lac Oᶜ (Mutated Operator): Constitutive (Repressor can’t bind).
  • lac Iˢ (Super-repressor): Always OFF (Inducer can’t bind repressor).