Question

In Catabolite repression model of gene regulation, which of the following compound signals the absence of metabolite of choice for the energy to regulate the gene expression?

• A. cAMP
• B. cGMP
• C. Inositol Phosphate
• D. Tyrosine Kinase

Hint:

Glucose concentration and cAMP concentration are inversely related:
High Glucose $\rightarrow$ Low cAMP $\rightarrow$ Operon Off.
Low Glucose $\rightarrow$ High cAMP $\rightarrow$ cAMP-CAP binds $\rightarrow$ Transcription Activated.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Catabolite repression is a global regulatory mechanism in bacteria (such as E. coli) that ensures the cell preferentially metabolizes glucose (the preferred energy source) before activating pathways for alternative sugars (such as lactose or arabinose).
Detailed Explanation:
- In the presence of glucose, the rate of intracellular cyclic AMP (cAMP) synthesis by the enzyme adenylate cyclase is highly suppressed.
- When glucose is depleted (signaling the absence of the preferred energy metabolite), the transport of glucose stops, and adenylate cyclase is activated.
- This activation causes a rapid accumulation of cyclic AMP (cAMP) in the cytoplasm.
- cAMP acts as an intracellular hunger signal; it binds specifically to the Catabolite Activator Protein (CAP), also known as cAMP Receptor Protein (CRP).
- The cAMP-CAP complex undergoes a conformational change, enabling it to bind to specific promoter regions upstream of operons for alternative sugars (such as the *lac* operon), recruiting RNA polymerase to activate transcription.
- Therefore, cAMP is the signaling compound that indicates glucose starvation.

Step 2: Final Answer:
The compound that signals the absence of glucose is cAMP, corresponding to option (A).