The lac operon and gene regulation in bacteria !

The lac operon is the classic model of how bacteria switch genes on only when needed: they use them to eat lactose when glucose runs out.

Updated 2026-11-01

Key facts

Structural genes lacZ (β-galactosidase), lacY (permease) and lacA (transacetylase) under one promoter: a polycistronic mRNA.
Negative control: the LacI repressor (lacI gene) binds the operator and blocks transcription when lactose is absent.
Inducer: allolactose (made from lactose) binds the repressor and releases it from the operator.
Positive control: when glucose is low, cAMP rises and binds CAP (CRP); CAP-cAMP strongly activates the promoter.
Maximum expression: lactose present and glucose absent.

High glucose + no lactose: repressor bound and low cAMP → operon off.
High glucose + lactose: repressor released but no CAP-cAMP → low expression (catabolite repression).
Low glucose + no lactose: CAP-cAMP present but repressor bound → off.
Low glucose + lactose: repressor released and CAP-cAMP bound → high expression.
Constitutive mutations: lacI⁻ (inactive repressor) or operator Oᶜ (cannot bind repressor) → always on. lacIˢ (cannot bind inducer) → always off.
Eukaryotes lack typical operons: each gene has its own promoter and regulation relies on transcription factors, enhancers and chromatin.

Bacterial β-galactosidase (lacZ) is a classic reporter gene in the lab (blue or white colonies in cloning).
The trp operon is the example of a repressible operon: tryptophan activates its repressor and shuts down its own synthesis; it also uses attenuation.

Think of the answer, then tap the question to see it.

Under which conditions is the lac operon expressed most?

When lactose is present (the repressor lets go) and glucose is absent (cAMP rises and CAP activates the promoter).

Which molecule is the inducer of the lac operon?

Allolactose, which binds the LacI repressor and stops it binding the operator.

What happens with an Oᶜ operator mutation?

The repressor can no longer bind, so the operon is always on (constitutive), even without lactose.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Can genes be turned on and off in cells?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Gene expression (Transcription) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.