Which protein binds to the operator in the lac operon to prevent transcription?

Which protein binds to the operator in the lac operon to prevent transcription?

• A. LacI repressor
• B. Lac 1 protein
• C. Operator protein
• D. Repressor

Answer: (B)

In this system, transcription is controlled by a repressor that binds to the operator region to block RNA polymerase from initiating transcription of the lac genes. The protein responsible is the LacI repressor, the product of the lacI gene. When lactose is absent, LacI binds the operator and transcription is off. When lactose is present, allolactose binds LacI, changing its shape and reducing its affinity for the operator, so RNA polymerase can proceed with transcription. So the protein that binds the operator to prevent transcription is the LacI repressor.

In this system, transcription is controlled by a repressor that binds to the operator region to block RNA polymerase from initiating transcription of the lac genes. The protein responsible is the LacI repressor, the product of the lacI gene. When lactose is absent, LacI binds the operator and transcription is off. When lactose is present, allolactose binds LacI, changing its shape and reducing its affinity for the operator, so RNA polymerase can proceed with transcription. So the protein that binds the operator to prevent transcription is the LacI repressor.