Control of Gene Expression in Prokaryotes
High-Yield Summary
- The Jacob-Monod model: prokaryotes group functionally related genes into an operon, transcribed together under one shared promoter — efficient for fast environmental response.
- Operon parts: regulator gene (codes repressor, not part of operon), promoter (RNA polymerase binding site), operator (repressor binding site, between promoter and structural genes), structural genes (the actual transcribed genes).
- Inducible system (default OFF, e.g. lac operon): repressor normally bound to operator, blocking transcription (negative control). An inducer (allolactose, from lactose) binds repressor → falls off operator → transcription proceeds.
- Lac operon also has positive control: low glucose → cAMP rises → cAMP binds CAP → CAP-cAMP complex binds near promoter → boosts RNA polymerase binding.
- Repressible system (default ON, e.g. trp operon): repressor normally inactive, transcription proceeds. A corepressor (tryptophan) binds and activates the repressor → binds operator → blocks transcription — feedback inhibition.
Key Terms
- Operon
- Group of functionally related genes transcribed together under one shared promoter.
- Operator
- DNA region between promoter and structural genes where a repressor binds to block RNA polymerase.
- Inducer
- Molecule (e.g. allolactose) that inactivates a repressor, turning ON an inducible system.
- Corepressor
- Molecule (e.g. tryptophan) that activates a repressor, turning OFF a repressible system.
- CAP (catabolite activator protein)
- Positive regulator activated by cAMP (when glucose is low) that boosts RNA polymerase binding at the lac promoter.
- Feedback inhibition
- A pathway's own end product shuts down further production of itself (e.g. tryptophan halting the trp operon).
Operon Components
| Component | Function |
|---|---|
| Regulator gene | Near, not part of operon — codes a repressor protein |
| Promoter | Start of operon — RNA polymerase binding site |
| Operator | Between promoter and structural genes — repressor binding site |
| Structural genes | Downstream — the genes actually transcribed |
Inducible (Lac) vs. Repressible (Trp) Operon
| Feature | Lac (Inducible) vs. Trp (Repressible) |
|---|---|
| Default state | Off / On |
| Repressor's default activity | Active (bound to operator) / Inactive (not bound) |
| Switch molecule | Inducer — allolactose / Corepressor — tryptophan |
| Effect of that molecule | Inactivates repressor, turns ON / Activates repressor, turns OFF |
| Biological logic | Make enzymes only when substrate present / Stop once product is abundant (feedback inhibition) |
Common MCAT Trap
- Inducible ≠ repressible defaults are opposite: inducible starts OFF (needs a molecule to turn ON), repressible starts ON (needs a molecule to turn OFF). Mixing these up flips every downstream answer.
- The lac operon has TWO regulatory layers — negative control (repressor/operator) AND positive control (CAP-cAMP) — a question may test either independently.
- CAP activation depends on LOW glucose (via rising cAMP), not on the presence of lactose — the lac repressor system and the CAP system respond to two different signals (lactose vs. glucose).
Quick Recall
What molecule acts as the inducer in the lac operon, and where does it come from?
What happens to cAMP levels when glucose is low, and what does that trigger?
What acts as the corepressor in the trp operon?
Is the trp operon regulation an example of negative or positive control?