U01.01.008 Lac operon

The lac operon in E. coli is a classic example of a genetic response to environmental change. This operon regulates the metabolism of lactose in the absence of glucose.

Preferred Metabolic Substrate

Glucose: Preferred metabolic substrate for E. coli.

Activation of Lac Operon

When glucose is absent and lactose is available, the lac operon is activated to switch to lactose metabolism.

Mechanism of Shift

1. Low Glucose Levels:

Decrease in Glucose:
- ↓ Glucose levels
- ↓ Adenylate cyclase activity
- ↓ Generation of cAMP from ATP
- Activation of catabolite activator protein (CAP)
- ↑ Transcription of lac operon genes

2. High Lactose Levels:

Increase in Lactose:
- Lactose binds to repressor protein
- Unbinding of repressor from the operator site
- ↑ Transcription of lac operon genes

Summary of Key Points

Low Glucose: Leads to activation of CAP via cAMP, enhancing transcription.
High Lactose: Prevents the repressor from binding to the operator, allowing transcription.

Table: Mechanism of Lac Operon Activation

Condition	Molecular Change	Result
Low Glucose	↓ cAMP from ATP	Activation of CAP
	Activation of CAP	↑ Transcription
High Lactose	Lactose binds repressor protein	Repressor unbinds operator
	Unbinding of repressor from operator	↑ Transcription

Points to Remember

Glucose is the preferred substrate.
The lac operon is activated in the absence of glucose and the presence of lactose.
cAMP and CAP play crucial roles in transcription regulation.
Lactose acts by inactivating the repressor protein.

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