Model Answer
0 min readIntroduction
The *lac* operon in *Escherichia coli* is a classic example of an inducible gene regulatory system. It controls the metabolism of lactose, a disaccharide, by encoding enzymes necessary for its uptake and breakdown. In the presence of glucose, *E. coli* preferentially utilizes glucose as an energy source, effectively repressing the expression of the *lac* operon genes. However, when glucose is scarce and lactose is available, the operon is induced, allowing the bacterium to utilize lactose. Understanding the fate of the *lac* operon when cells are transferred from a glucose-rich to a lactose-rich medium is fundamental to comprehending gene regulation in prokaryotes.
Initial State: Glucose Growth
When *E. coli* cells are grown in a glucose-rich medium, the *lac* operon is largely repressed. This repression is primarily due to two mechanisms:
- Repressor Protein (LacI): The *lacI* gene, located upstream of the operon, constitutively produces the LacI repressor protein. This repressor binds to the operator region of the *lac* operon, physically blocking RNA polymerase from transcribing the structural genes (*lacZ*, *lacY*, and *lacA*).
- Catabolite Repression: Even if the repressor is not bound, transcription is still low due to catabolite repression. When glucose levels are high, the concentration of cyclic AMP (cAMP) is low. cAMP is required to bind to the Catabolite Activator Protein (CAP), forming a cAMP-CAP complex. This complex enhances RNA polymerase binding to the promoter region of the *lac* operon. With low cAMP, CAP remains inactive, and transcription is inefficient.
Transfer to Lactose Medium: Induction
When glucose-grown *E. coli* cells are transferred to a medium containing lactose, the following events occur:
1. Lactose Uptake and Allolactose Formation
Lactose enters the cell via a passive transporter. A small amount of lactose is converted into allolactose, an isomer of lactose, by the enzyme β-galactosidase (encoded by *lacZ*). Allolactose acts as the inducer.
2. Repressor Inactivation
Allolactose binds to the LacI repressor protein, causing a conformational change. This change renders the repressor unable to bind to the operator region of the *lac* operon. The repressor dissociates from the operator, removing the primary block to transcription.
3. Catabolite Activation (Gradual)
As glucose is absent, the intracellular concentration of cAMP begins to rise. cAMP binds to CAP, forming the cAMP-CAP complex. This complex then binds to a specific site upstream of the *lac* operon promoter, enhancing RNA polymerase binding.
4. Increased Transcription and Translation
With the repressor removed and CAP activated, RNA polymerase can efficiently bind to the promoter and transcribe the *lacZ*, *lacY*, and *lacA* genes. This leads to the production of β-galactosidase, permease, and transacetylase, respectively. These enzymes facilitate lactose uptake and metabolism.
5. Negative Feedback Regulation
As lactose is metabolized, allolactose levels decrease. This leads to the dissociation of allolactose from the repressor, restoring its ability to bind to the operator. As glucose levels remain low, CAP remains activated, maintaining a high level of transcription until lactose is depleted.
Stages of *Lac* Operon Activation
| Stage | Repressor | CAP | Transcription |
|---|---|---|---|
| Glucose Present, Lactose Absent | Bound to Operator | Inactive (low cAMP) | Very Low |
| Lactose Present, Glucose Absent | Not Bound to Operator (Allolactose bound) | Active (high cAMP) | High |
| Lactose Present, Glucose Present | Not Bound to Operator (Allolactose bound) | Inactive (low cAMP) | Low |
Conclusion
In summary, when glucose-grown *E. coli* cells are transferred to a lactose medium, the *lac* operon undergoes a carefully orchestrated induction process. The removal of repression by allolactose, coupled with the activation of transcription by the cAMP-CAP complex, leads to increased expression of the *lac* operon genes. This allows the cells to efficiently utilize lactose as an alternative energy source. The *lac* operon serves as a powerful model for understanding gene regulation and its responsiveness to environmental cues.
Answer Length
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