INI-CET 2011

Question 1: The CAP protein in the lac operon serves as an example of which of the following?

• A. Positive regulator (Correct Answer)
• B. Negative regulator
• C. Attenuation
• D. Constitutive expression

Explanation: ### Explanation **1. Why Option A is Correct:** The **Catabolite Activator Protein (CAP)**, also known as cAMP Receptor Protein (CRP), is a classic example of a **positive regulator**. In the absence of glucose, levels of cyclic AMP (cAMP) rise. cAMP binds to CAP, forming a cAMP-CAP complex. This complex binds to a specific site near the *lac* promoter, enhancing the affinity of RNA polymerase for the promoter. This "recruitment" significantly increases the rate of transcription. Therefore, CAP acts as an activator that "turns on" or upregulates gene expression. **2. Why Other Options are Incorrect:** * **B. Negative Regulator:** This refers to the **Lac Repressor protein** (encoded by the *lacI* gene). The repressor binds to the operator to block transcription; CAP does the opposite. * **C. Attenuation:** This is a regulatory mechanism seen in the **Tryptophan (*trp*) operon**, involving premature termination of transcription based on the speed of translation. It is not a feature of the *lac* operon. * **D. Constitutive Expression:** This refers to genes that are expressed at a constant rate regardless of environmental conditions (e.g., "housekeeping genes"). The *lac* operon is **inducible**, not constitutive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Glucose Effect:** The *lac* operon is subject to "Catabolite Repression." Even if lactose is present, the operon is not fully active if glucose is available, because glucose inhibits Adenylate Cyclase, lowering cAMP levels and preventing CAP binding. * **Dual Control:** Remember that for maximal *lac* operon expression, two conditions must be met: **High Lactose** (to remove the repressor) and **Low Glucose** (to allow CAP binding). * **DNA Binding Motif:** CAP utilizes a **Helix-turn-helix** motif to bind to DNA, a common feature in prokaryotic regulatory proteins.