Inducers pull repressors off. Corepressors put them on.
- The idea
- Predict
- See it
- Why
- Check
The idea
“Lactose helps the lac repressor bind, and tryptophan pulls the trp repressor off the DNA.”
Both operons pair a repressor with a small molecule, so the two stories blur. It's easy to give each molecule the other one's job.
The short version
The mistake: Lactose helps the lac repressor bind, and tryptophan pulls the trp repressor off the DNA.
What's actually true: An inducer like allolactose makes a repressor let go of the operator. A corepressor like tryptophan makes it bind.
- A repressor sitting on the operator blocks RNA polymerase, so the genes aren't transcribed.
- In an inducible operon like lac, the repressor binds alone. The inducer, allolactose, makes it let go.
- In a repressible operon like trp, the repressor can't bind alone. The corepressor, tryptophan, lets it grip.
- Breakdown genes are usually inducible. Building genes are usually repressible and shut off when the product piles up.
On the exam: Inducers pull repressors off. Corepressors put them on. Ask which one the molecule is.