What is Gene Regulation in Prokaryotes?
Bacteria constantly adjust which genes they transcribe so they only make proteins they actually need. Prokaryotic gene regulation happens mainly at the transcription level, using clusters of genes called operons that switch on or off together in response to the environment.
Gene regulation in prokaryotes is the control of transcription through operons — DNA segments with a shared promoter and operator that let repressor or activator proteins turn a whole set of genes on or off together, such as the lac operon in E. coli.
- 1↓No lactose presentThe lac repressor binds the operator and blocks RNA polymerase from transcribing lacZ, lacY, lacA.
- 2↓Lactose enters the cellLactose is converted to allolactose, which binds the repressor and changes its shape.
- 3↓Repressor releases the operatorThe repressor can no longer bind DNA, so the operator site is now free.
- 4↓RNA polymerase transcribes the operonA single mRNA is made for all three structural genes (lacZ, lacY, lacA).
- 5Enzymes are translatedβ-galactosidase, permease and transacetylase are produced to metabolize lactose.
Step-by-step worked examples
In E. coli, glucose is absent and lactose is present. Explain what happens to the lac operon.
Low glucose raises cAMP levels, so CAP-cAMP binds near the promoter and enhances RNA polymerase binding Lactose is converted to allolactose, which binds and inactivates the lac repressor With the repressor off the operator and CAP boosting the promoter, the lac operon is transcribed at a high rate
The trp operon: tryptophan levels are high in the cell. What happens to transcription?
Excess tryptophan acts as a corepressor and binds the inactive trp repressor protein The tryptophan-repressor complex changes shape so it can now bind the operator RNA polymerase is blocked, so the trp operon (tryptophan biosynthesis genes) is switched off
Both glucose and lactose are present in the medium. What happens to lac operon expression?
Glucose lowers cAMP levels, so CAP cannot bind the promoter efficiently (catabolite repression) Lactose still inactivates the repressor, so the operator is free Without CAP-cAMP help, RNA polymerase binds weakly, so lac operon transcription stays low until glucose runs out
Flashcards
Quick quiz
Q1.What directly blocks RNA polymerase in the lac operon when lactose is absent?
Q2.What molecule inactivates the lac repressor?
Q3.In the trp operon, tryptophan acts as a…
Q4.Why is lac operon expression low even with lactose present if glucose is also present?
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Common mistakes
Operons exist in eukaryotes just like in prokaryotes. — Correct: Operons (shared promoter for multiple genes) are a prokaryotic feature; eukaryotic genes are regulated individually.
The lac repressor always blocks transcription. — Correct: It only blocks transcription when lactose (allolactose) is absent; lactose inactivates it.
Inducible and repressible operons work the same way. — Correct: Inducible operons are off by default and switched on; repressible operons are on by default and switched off.
CAP and the repressor do the same job. — Correct: The repressor controls on/off at the operator; CAP fine-tunes transcription strength at the promoter.
FAQ
What is gene regulation in prokaryotes?
It's the control of which genes are transcribed, mainly through operons — DNA regions with a shared promoter/operator that switch several genes on or off together.
What is the lac operon a good example of?
It shows how bacteria use a repressor, an inducer (allolactose) and CAP-cAMP together to only make lactose-digesting enzymes when lactose is present and glucose is scarce.
How do prokaryotes regulate genes so quickly?
Because transcription and translation are not separated by a nuclear membrane and operons let one signal control many genes at once, bacteria can respond to environmental change within minutes.
What's the difference between an inducible and a repressible operon?
An inducible operon (like lac) is normally off and turned on by an inducer; a repressible operon (like trp) is normally on and turned off by a corepressor.