In Turning Genes On and Off you saw that cells express only some genes at a time. Bacteria show the logic clearly. Their genes for one job often sit together in an operon: one promoter, one operator (an on-off switch where a repressor can bind), and several genes read as one mRNA. François Jacob and Jacques Monod worked out the lac operon of E. coli in the early 1960s.
- lac operon (inducible): genes for using lactose. A repressor protein, made by the lacI gene, normally sits on the operator: off. When lactose is present, a form of it called allolactose binds the repressor and changes its shape, so it lets go: on.
- Glucose and CAP: when glucose is scarce, cAMP rises and binds a protein called CAP, which helps RNA polymerase bind the lac promoter. So the operon is fully on only when lactose is present and glucose is scarce.
- trp operon (repressible): genes for making tryptophan. Its repressor is inactive alone, so the genes are normally on. When tryptophan is plentiful, it binds the repressor (as a corepressor), activating it; the repressor binds the operator: off.
Key idea
The lac operon is inducible: normally off, switched on by lactose. The trp operon is repressible: normally on, switched off by tryptophan. In both, the small molecule binds the repressor, never the operator.
Why it works
Each design saves energy in its own situation. Enzymes to digest lactose are a waste unless lactose is around, so their default is off. Tryptophan is needed all the time, so its genes default to on and shut off only when there is plenty, a form of negative feedback. Glucose control adds a priority: glucose is the easier fuel, so bacteria use it first.
Worked examples
Worked example: Predicting the lac operon
E. coli is grown with lactose and no glucose. Is the lac operon on?
- Lactose present: allolactose binds the repressor, which leaves the operator.
- Glucose absent: cAMP is high, and cAMP-CAP helps RNA polymerase bind.
Answer: Yes, strongly on.
Worked example: A broken repressor
A mutant makes no working lac repressor. What happens with no lactose (and no glucose)?
- With no repressor, nothing blocks the operator.
- RNA polymerase can transcribe the genes whether or not lactose is present.
Answer: The genes are on all the time (constitutive), even without lactose.
Common mistake
Saying lactose binds the operator, or mixing up the defaults. The inducer (allolactose) and the corepressor (tryptophan) both bind the repressor protein and change its shape. Lac is off by default; trp is on by default. A check: ask what the cell needs when the small molecule is present. Lactose present: make the enzymes. Tryptophan present: stop making it.