Gene regulation in bacteria: The catabolite activator protein (CAP, also called CRP) exemplifies which control mode of transcription?

Introduction / Context:Bacterial transcription is regulated by repressors and activators. CAP (CRP) integrates carbon source availability (via cAMP) to upregulate genes like the lac operon when glucose is scarce.

Given Data / Assumptions:

• CAP binds DNA with cAMP and enhances RNA polymerase recruitment.
• We distinguish positive control (activation) from negative control (repression).
• Focus is classic E. coli paradigms.

Concept / Approach:In positive control, regulatory proteins increase transcription when bound. CAP–cAMP binds upstream of target promoters, stabilizing RNA polymerase–promoter interactions and boosting transcription initiation.

Step-by-Step Solution:Identify CAP function: transcriptional activator.Relate to mechanism: CAP–cAMP enhances RNAP binding.Therefore, CAP is positive control.

Verification / Alternative check:In high glucose, cAMP levels fall, CAP activation decreases; lac operon transcription drops—consistent with activator logic.

Why Other Options Are Wrong:

• Negative control: involves repressors blocking transcription (e.g., LacI), not CAP.
• “Second type” is vague and not standard nomenclature.
• “None” is incorrect because CAP clearly activates.

Common Pitfalls:Confusing CAP with LacI; remembering that operon output requires both derepression (allolactose) and activation (CAP–cAMP) for full expression.

Final Answer:positive control of gene expression