Trp Operon vs Lac Operon for the MCAT: Repressible vs Inducible

The lac operon is inducible: it is normally off and is switched on when lactose is present. The trp operon is repressible: it is normally on and is switched off when tryptophan is plentiful. Both are negative control systems in which a repressor protein blocks transcription.

Side by side

How the trp operon works

The trp repressor is made in an inactive form. When tryptophan is abundant, it binds the repressor, the complex binds the operator, and RNA polymerase is blocked. When tryptophan is scarce, the repressor stays inactive, the operator is free, and the enzymes for tryptophan synthesis are made. This makes sense because the cell should not spend energy making an amino acid it already has.

Mutation logic

• A mutation that stops the trp repressor from binding tryptophan leaves the operon on all the time (constitutive).
• A mutation in the operator that prevents repressor binding also leaves the operon on.
• A mutation that makes the repressor bind the operator regardless of tryptophan keeps the operon off.

Attenuation

Bacteria also regulate the trp operon by attenuation, because transcription and translation are coupled. When tryptophan is high, the ribosome moves through the leader sequence quickly, a terminator hairpin forms, and transcription stops early.

Common traps

• Calling tryptophan an inducer. It is a corepressor.
• Saying the trp operon is turned on by its product. High tryptophan turns it off.
• Forgetting that repressible operons are usually anabolic and inducible operons catabolic.

One-minute recap

• Lac: inducible, off by default, lactose turns it on.
• Trp: repressible, on by default, tryptophan turns it off.
• Inducer releases the repressor; corepressor activates it.