Which of the following statements is correct about the role of regulatory proteins in transcription in prokary

NCERT Exemplar · Q17

Q. Which of the following statements is correct about the role of regulatory proteins in transcription in prokaryotes?

(a) They only increase expression

(b) They only decrease expression

(d) They can act both as activators and as repressors

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Concept understanding — Lac Operon Catabolite Repression

Imagine you are a factory manager. You have two raw materials: a high-grade fuel that your machines run on perfectly, and a low-grade backup fuel that works but is harder to use. As long as the good fuel is available, you would never waste time and energy switching to the backup. But if the good fuel runs out, you immediately switch to the backup to keep production going.

That is exactly what catabolite repression does inside a bacterium like E. coli. It is the cell's way of saying: "Use the best fuel first; don't bother with the second-best until you absolutely have to."

The Two Fuels: Glucose and Lactose

E. coli bacteria love glucose. It is their favourite energy source — easy to break down, gives quick energy. Lactose (milk sugar) is harder to digest; the cell needs to build special enzymes (like β-galactosidase) to break it down. These enzymes are coded by the lac operon.

The cell has a simple rule: If glucose is present, do not waste energy making lactose-digesting enzymes. That is catabolite repression. It is a global regulatory mechanism that ensures glucose is used first, even when lactose is also available.

Note

Catabolite repression is sometimes called the glucose effect. It is not unique to the lac operon — it affects many operons that break down alternative sugars. But the lac operon is the classic textbook example.

How It Works: The Molecular Switch

The key player is a molecule called cAMP (cyclic AMP). Its level inside the cell is inversely related to glucose concentration:

When glucose is high: cAMP levels are low.
When glucose is low: cAMP levels rise.

cAMP binds to a protein called CAP (Catabolite Activator Protein). The cAMP–CAP complex then binds to a specific site near the lac operon's promoter. This binding dramatically increases the rate of transcription — it is like pressing the accelerator pedal.

So here is the logic:

Glucose present (high): Low cAMP → CAP cannot bind → lac operon is barely transcribed, even if lactose is around. The cell ignores lactose.
Glucose absent (low): High cAMP → CAP binds → lac operon is fully activated. Now, if lactose is also present, the operon switches on fully and the cell digests lactose.

Important

Catabolite repression is a positive control mechanism. The CAP–cAMP complex activates transcription. This is different from the lac repressor, which blocks transcription when lactose is absent. The lac operon is controlled by two switches: a negative one (repressor) and a positive one (CAP–cAMP). Both must be in the "on" position for maximum expression.

Why It Matters (Exam Perspective)

The NCERT textbook (Class 12 Biology, Chapter 6) presents catabolite repression as a fine-tuning mechanism. It explains that even when the lac repressor is removed (by lactose binding), transcription is still low unless glucose is absent. The CAP–cAMP complex is the "second key" that unlocks full expression.

Key points to remember for exams: …

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Regulatory proteins in prokaryotes are versatile molecular switches that can both activate and repress transcription depending on cellular needs, making them essential for adaptive gene expression.

Prokaryotic cells face a constant challenge: how to respond rapidly to environmental changes while conserving energy. The solution lies in their elegant regulatory machinery, where proteins act as molecular decision-makers that control which genes get transcribed and when.

The classical example that illuminates this dual role is the lac operon system in E. coli. When lactose is absent, the lac repressor protein binds to the operator region and physically blocks RNA polymerase from transcribing the genes needed for lactose metabolism. This is negative regulation—the protein decreases expression. But the story doesn't end there.

Even when lactose is present and the repressor is inactivated, the cell has another layer of control. If glucose is also available, the cell preferentially uses glucose because it's metabolically cheaper. Here's where catabolite activator protein (CAP) enters the picture. When glucose levels are low, CAP binds to cAMP and attaches to a specific site near the promoter, actually helping RNA polymerase bind more efficiently. This is positive regulation—the same operon now has a protein that increases expression.

Important

The key insight is that prokaryotic regulatory proteins are not locked into a single function. The same gene can be controlled by different proteins acting in opposite directions, or even the same protein can have different effects in different operons. …

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Q.Which of the following statements is correct about the role of regulatory proteins in transcription in prokaryotes?

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