Explain gene expression regulation in prokaryotes using the lac operon as an example.

Explain gene expression regulation in prokaryotes using the lac operon as an example.

• A. Presence of lactose leads to allolactose formation which inactivates the LacI repressor; CAP-cAMP enhances transcription when glucose is low.
• B. Lactose presence prevents transcription entirely by activating the LacI repressor.
• C. Allolactose activates the LacI repressor, increasing transcription.
• D. CAP-cAMP inhibits transcription irrespective of glucose levels.

Answer: (C)

Gene expression in the lac operon is controlled by two regulatory signals: a repressor that blocks transcription and an activator that boosts transcription when energy is scarce. In the absence of lactose, the LacI repressor binds to the operator and blocks RNA polymerase, so the operon is not transcribed. When lactose is present, it is converted to allolactose, which binds to LacI and changes its shape so LacI releases from the operator. This derepression allows RNA polymerase to proceed with transcription. At the same time, glucose availability modulates transcription through CAP and cAMP. When glucose is low, adenylyl cyclase makes more cAMP, and CAP binds it to form CAP-cAMP. This complex binds near the promoter and helps recruit RNA polymerase, increasing transcription. When glucose is plentiful, cAMP levels are low, CAP-cAMP formation is reduced, and transcription is not as strongly activated, even if lactose is present. So the high expression of lac genes requires both lactose to inactivate LacI (derepression) and low glucose to activate CAP-cAMP (enhanced transcription). The idea that allolactose activates the LacI repressor is incorrect; allolactose inactivates LacI, enabling transcription.

Gene expression in the lac operon is controlled by two regulatory signals: a repressor that blocks transcription and an activator that boosts transcription when energy is scarce. In the absence of lactose, the LacI repressor binds to the operator and blocks RNA polymerase, so the operon is not transcribed. When lactose is present, it is converted to allolactose, which binds to LacI and changes its shape so LacI releases from the operator. This derepression allows RNA polymerase to proceed with transcription.

At the same time, glucose availability modulates transcription through CAP and cAMP. When glucose is low, adenylyl cyclase makes more cAMP, and CAP binds it to form CAP-cAMP. This complex binds near the promoter and helps recruit RNA polymerase, increasing transcription. When glucose is plentiful, cAMP levels are low, CAP-cAMP formation is reduced, and transcription is not as strongly activated, even if lactose is present.

So the high expression of lac genes requires both lactose to inactivate LacI (derepression) and low glucose to activate CAP-cAMP (enhanced transcription). The idea that allolactose activates the LacI repressor is incorrect; allolactose inactivates LacI, enabling transcription.