The lac operon in E. coli is a classic model of gene regulation in response to environmental substrates.
- E. coli preferentially metabolizes glucose.
- When glucose is absent and lactose is available, the lac operon is activated to enable lactose metabolism.
Mechanism of Lac Operon Regulation
Positive Control: CAP-cAMP System
| Condition | Mechanism | Effect on Transcription |
|---|---|---|
| Low glucose | ↑ Adenylate cyclase → ↑ cAMP → cAMP binds CAP → CAP-cAMP binds promoter | ↑ Lac operon transcription |
| High glucose | ↓ Adenylate cyclase → ↓ cAMP → CAP not activated | ↓ Lac operon transcription |
Negative Control: Repressor-Operator System
| Condition | Mechanism | Effect on Transcription |
|---|---|---|
| High lactose | Lactose (allolactose) binds repressor → repressor is inactivated → cannot bind operator | ↑ Transcription |
| Low lactose | The repressor binds the operator | Transcription blocked |
Summary Table of Lac Operon Expression
| Glucose | Lactose | CAP-cAMP | Repressor | Lac Operon Expression |
|---|---|---|---|---|
| Low | High | Active | Inactive | Strong expression |
| High | Low | Inactive | Active | No expression |
| High | High | Inactive | Inactive | Very low (basal) |
| Low | Low | Active | Active | No expression |
Key Points
- Glucose availability dominates lac operon regulation.
- Lactose acts as an inducer, binding the repressor and permitting transcription.
- CAP-cAMP is a positive regulator that enhances RNA polymerase binding.
- Lac Z, Lac Y, and Lac A genes encode enzymes for lactose metabolism.
