U01.01.009 Lac operon

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Learning Objectives

  • Explain the dual-control mechanism (Glucose and Lactose) of the lac operon.
  • Identify the roles of the Repressor protein, CAP, and cAMP.
  • Predict the expression status of the operon under different environmental conditions.
  • Define the structural genes (LacZ, LacY, LacA) and their purpose.

1. The Preferred Substrate Logic

E. coli prefers Glucose. The lac operon is a classic example of an inducible system that switches to Lactose metabolism only when two specific conditions are met: Glucose is ABSENT, and Lactose is PRESENT.

2. Mechanism of Control

Negative Control (Lactose Sensor)

  • Condition: Lactose is present.
  • Action: Lactose is converted to allolactose (inducer), which binds to the Repressor protein.
  • Result: The repressor unbinds from the Operator site, allowing RNA polymerase to proceed.

Positive Control (Glucose Sensor)

  • Condition: Glucose is low.
  • Action: \uparrow Adenylate cyclase activity $Latex \rightarrow$ \uparrow cAMP.
  • Result: cAMP binds the Catabolite Activator Protein (CAP). The CAP-cAMP complex binds to the CAP site, inducing strong transcription.

3. Operon State Summary

The level of expression depends on the push-and-pull of both sensors:

Glucose Lactose Repressor Status CAP Status Expression Level
Low Present Unbound (Inducer present) Bound (High cAMP) Strongly Expressed
High Absent Bound Unbound Not Expressed
High Present Unbound Unbound Very low (Basal)
Low Absent Bound Bound Not Expressed

Activity

4. Structural Genes

  • LacZ: Codes for β-galactosidase (cleaves lactose into glucose and galactose).
  • LacY: Codes for Permease (helps lactose enter the cell).
  • LacA: Codes for Transacetylase.

 

Activity