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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# 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.

## In detail

- 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.

**Also called**
Lactose operon

## Clinical notes

- 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.

## Pathways

- [Galactose metabolism](https://durgical.com/en/biochemistry/pathways/galactose-metabolism/)

## Related

- [Transcription](https://durgical.com/en/biochemistry/topics/transcription/)
- [Epigenetics: DNA and histone methylation](https://durgical.com/en/biochemistry/topics/epigenetics-dna-and-histone-methylation/)
- [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/)

## Test yourself

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.

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

Sources: [Can genes be turned on and off in cells?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/howgeneswork/geneonoff/) (MedlinePlus, US National Library of Medicine, 2026); [Gene expression (Transcription)](https://reactome.org/PathwayBrowser/#/R-HSA-74160) (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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/the-lac-operon-and-gene-regulation-in-bacteria/