Is cAMP an Inducer in the Lac Operon? (MCAT)

Is cAMP an Inducer? cAMP vs Allolactose in the Lac Operon

By StudyStore · Published 2026-11-09 · 1 min read · 281 words

No. cAMP is not the inducer of the lac operon. The inducer is allolactose (an isomer of lactose made from it). cAMP is the signal that tells the cell glucose is low, and it works through a different protein.

Two molecules, two jobs

	Allolactose	cAMP
Role	Inducer	Activating signal (positive control)
Binds	The lac repressor (LacI)	CAP (catabolite activator protein)
Effect of binding	Repressor leaves the operator, so RNA polymerase can pass	CAP binds the promoter region and helps RNA polymerase bind
Present when	Lactose is present	Glucose is low

Allolactose removes a block. cAMP-CAP adds a boost.

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The four cases

Glucose	Lactose	Repressor	cAMP-CAP	Expression
High	Absent	On operator	Low	None
High	Present	Off operator	Low	Low
Low	Absent	On operator	High	None
Low	Present	Off operator	High	High

Both conditions must be met for high expression: lactose present (repressor off) and glucose low (CAP on).

Why glucose lowers cAMP

High glucose lowers adenylate cyclase activity, so cAMP falls. Without cAMP, CAP cannot bind and transcription stays weak even when the repressor is off. This is catabolite repression.

Common traps

Calling cAMP the inducer. Inducer means it inactivates the repressor. Only allolactose does that.
Thinking low glucose alone turns the operon on. Without lactose the repressor still blocks it.
Mixing up the two proteins. The repressor responds to allolactose; CAP responds to cAMP.

One-minute recap

Allolactose = inducer, binds the repressor.
cAMP binds CAP, a positive regulator.
High expression needs lactose present and glucose low.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. What is the inducer of the lac operon?

Allolactose.

2. Which protein does cAMP bind?

CAP (catabolite activator protein).

3. What does allolactose binding do to the lac repressor?

It releases the repressor from the operator.

4. Does high glucose raise or lower cAMP?

It lowers cAMP.

5. In which condition is lac operon expression highest?

Lactose present and glucose low.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.