Which condition leads to inactivation of the lac repressor and transcription of lac operon genes?

Which condition leads to inactivation of the lac repressor and transcription of lac operon genes?

• A. Absence of lactose.
• B. High glucose concentration.
• C. Presence of lactose.
• D. Neither lactose nor glucose affect transcription.

Answer: (C)

Presence of lactose leads to inactivation of the lac repressor. Lactose is converted to allolactose, which binds the LacI repressor and changes its shape so it can no longer bind the operator. With the repressor out of the way, RNA polymerase can access the promoter and transcribe the lac operon genes. Glucose can influence how strongly this transcription occurs by affecting CAP-cAMP levels, but the key trigger for lifting repression is lactose. If lactose isn’t present, the repressor stays bound and transcription stays off.

Presence of lactose leads to inactivation of the lac repressor. Lactose is converted to allolactose, which binds the LacI repressor and changes its shape so it can no longer bind the operator. With the repressor out of the way, RNA polymerase can access the promoter and transcribe the lac operon genes. Glucose can influence how strongly this transcription occurs by affecting CAP-cAMP levels, but the key trigger for lifting repression is lactose. If lactose isn’t present, the repressor stays bound and transcription stays off.