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Operons

Operons

4 min read Updated Apr 18, 2026

Bacteria often organize several related genes as an operon - a cluster under a single promoter, producing a polycistronic mRNA. Operons let bacteria switch multiple genes on or off at once in response to environmental signals. The lac and trp operons are the textbook examples.

Two operons, opposite defaults

Gene regulation
lac operon Inducible job: digest lactose by default OFF · repressor is bound promoter operator structural genes repressor what flips it
lactose (as allolactose) binds the repressor and pulls it off
Only build the enzymes when the substrate is actually there.
Glucose overrides it: high glucose means low cAMP, so CAP cannot bind and the operon stays quiet even with lactose present.
 trp operon Repressible job: build tryptophan by default ON · repressor cannot bind alone promoter operator structural genes repressor (cannot bind yet) what flips it
tryptophan binds the repressor and switches it on
Keep building until there is enough of the product, then stop.
Also controlled by attenuation: a ribosome stalling on two tryptophan codons decides whether transcription continues at all.
 Every operon, same three parts Promoter
where RNA polymerase binds
 Operator
where the repressor binds, blocking the polymerase
 Structural genes
the enzymes themselves, transcribed as one message
 The rule catabolic pathways are inducible, anabolic pathways are repressible. Both of these operons are that sentence made into DNA.
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Inducible: off by default, substrate switches it on Repressible: on by default, product switches it off Operator: where control happens Positive control (CAP and cAMP)

The rule that generates bothCatabolic pathways are inducible and anabolic pathways are repressible. A pathway that breaks something down should only run when that something is present. A pathway that builds something should run until there is enough. Both operons follow from that one sentence.

Why glucose beats lactoseThe lac operon has a second, positive control. CAP only binds when cAMP is high, and cAMP is only high when glucose is low. So the cell eats glucose first and only turns to lactose when glucose runs out, which is exactly what diauxic growth curves show.

Eukaryotes do it differentlyNo operons, because each gene has its own promoter. Control is spread across chromatin state (histone acetylation opens it, methylation usually closes it), distant enhancers looped in by transcription factors, alternative splicing, and microRNAs acting after the message is made.

Both operons use a repressor and an operator; only the default differs. Ask what the pathway is for. If it breaks something down, the substrate turns it on. If it builds something, the product turns it off.

The Lac Operon

Controls three genes needed to metabolize lactose:

  • lacZ: beta-galactosidase (splits lactose into glucose + galactose).
  • lacY: lactose permease (imports lactose into the cell).
  • lacA: transacetylase (role less clear).

Regulation logic:

  • Negative regulation: The lac repressor (product of lacI gene) binds the operator and blocks transcription when lactose is absent. When lactose is present, allolactose (a lactose derivative) binds the repressor, causing it to release the operator - transcription turns ON. Lactose is the inducer.
  • Positive regulation: When glucose is scarce, cAMP levels rise. cAMP binds the catabolite activator protein (CAP / CRP), which binds upstream of the promoter and enhances transcription. When glucose is abundant, cAMP is low, CAP is inactive, and even with lactose, the lac operon expresses only weakly. This is catabolite repression - the cell prefers glucose, and only turns to lactose when glucose is gone.

The Trp Operon

Controls five genes needed to synthesize tryptophan from chorismate. When tryptophan is plentiful, the cell does not need to make more, so the operon should be OFF.

Regulation logic:

  • Repression: The trp repressor is inactive on its own. When tryptophan is abundant, Trp binds the repressor as a corepressor, activating it. The active repressor binds the operator and shuts off transcription. When Trp is scarce, the repressor is inactive, and the operon expresses.
  • Attenuation: An additional fine-tuning mechanism using the leader region of the trp mRNA. If Trp is abundant, ribosomes translating a leader peptide coast smoothly over trp codons, and the mRNA forms a structure that terminates transcription early. If Trp is scarce, ribosomes stall at trp codons, a different mRNA structure forms, and transcription continues. The MCAT does not typically require deep attenuation knowledge, but recognizing that the trp operon uses attenuation is a useful concept.

Induction vs. Repression

  • Inducible operon: normally OFF, turned ON by a substrate (inducer). Lac is the classic example. The substrate is scarce most of the time, and the cell only expresses the enzymes when the substrate appears.
  • Repressible operon: normally ON, turned OFF by the pathway’s product (corepressor). Trp is the classic example. The cell makes tryptophan constantly unless there is already plenty, in which case it shuts down synthesis.
How does the lac operon behave when both glucose and lactose are present in the medium?
Click to reveal answer
Lactose inactivates the repressor, so the operator is unbound. But glucose keeps cAMP levels low, so CAP does not activate the promoter strongly. The operon expresses only weakly. The cell uses the available glucose first; once glucose is gone, cAMP rises, CAP activates, and lac expression turns fully on. This is catabolite repression - the diauxic growth pattern.
Why is the trp operon called “repressible” rather than “inducible”?
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A repressible operon is ON by default and is turned OFF when a signal is present. The trp operon defaults to ON (because the cell usually needs to make Trp). When Trp accumulates, it binds the trp repressor as a corepressor, activating the repressor and turning the operon OFF. “Repressed by its end product” is the signature of a biosynthetic operon.
What would a lacI- mutation cause phenotypically?
Click to reveal answer
Constitutive expression of lacZ, lacY, lacA. lacI encodes the lac repressor. If repressor cannot be made or cannot bind the operator, the operon expresses regardless of whether lactose is present. The cell wastes energy producing enzymes it may not need. The same phenotype arises from an operator mutation that prevents repressor binding.
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