In the lac operon, which statement accurately captures the roles of LacI and CAP in transcription regulation?

In the lac operon, which statement accurately captures the roles of LacI and CAP in transcription regulation?

• A. LacI binds the operator to repress transcription; CAP-cAMP has no role in transcription.
• B. LacI is an activator that increases transcription; CAP-cAMP is the sole regulator.
• C. LacI represses transcription by binding the operator; CAP-cAMP enhances transcription when glucose is low.
• D. Lactose binds directly to CAP to promote transcription.

Answer: (C)

The main idea here is how two separate regulatory inputs control the lac operon: a repressor that blocks transcription and an activator that helps kick transcription up when energy is scarce. The LacI repressor binds to the operator and physically blocks RNA polymerase from transcribing the operon. When lactose is present (actually the inducer allolactose), it binds LacI and causes the repressor to release from the operator, lifting that block and allowing transcription to begin. At the same time, CAP acts as a positive regulator. It binds cAMP, and this CAP-cAMP complex helps RNA polymerase bind to the promoter and initiate transcription. This activation happens most when glucose is low, because low glucose raises cAMP levels, making CAP active. So, the statement that LacI represses transcription by binding the operator, while CAP-cAMP enhances transcription when glucose is low, captures both the negative control by the repressor and the positive control by CAP-cAMP. The other options clash with these roles: LacI is not an activator, CAP-cAMP does have a role, lactose does not bind CAP, and CAP is activated by cAMP rather than by lactose.

The main idea here is how two separate regulatory inputs control the lac operon: a repressor that blocks transcription and an activator that helps kick transcription up when energy is scarce. The LacI repressor binds to the operator and physically blocks RNA polymerase from transcribing the operon. When lactose is present (actually the inducer allolactose), it binds LacI and causes the repressor to release from the operator, lifting that block and allowing transcription to begin.

At the same time, CAP acts as a positive regulator. It binds cAMP, and this CAP-cAMP complex helps RNA polymerase bind to the promoter and initiate transcription. This activation happens most when glucose is low, because low glucose raises cAMP levels, making CAP active.

So, the statement that LacI represses transcription by binding the operator, while CAP-cAMP enhances transcription when glucose is low, captures both the negative control by the repressor and the positive control by CAP-cAMP. The other options clash with these roles: LacI is not an activator, CAP-cAMP does have a role, lactose does not bind CAP, and CAP is activated by cAMP rather than by lactose.