The trp operon and predicting mutations

The lac operon codes enzymes that break a food down. Operons for building molecules use the same parts, a repressor, an operator, and a small effector molecule, wired to the opposite logic.

Making tryptophan only when it’s scarce

E. coli’s trp operon holds five genes for the enzymes that make the amino acid tryptophan. A separate gene, trpR, codes trp repressor, which can bind the trp operator and block transcription. Whether it binds depends on tryptophan itself, the end product of the pathway the operon’s enzymes carry out.

• When tryptophan is low, trp repressor can’t bind the operator on its own. RNA polymerase transcribes the operon, and the cell makes the enzymes it needs to build tryptophan.
• When tryptophan is high, tryptophan binds trp repressor and changes the repressor’s shape so that it can bind the operator. Transcription stops, and the cell doesn’t waste energy making something it already has plenty of.

A small effector molecule that binds a repressor and lets it shut down transcription is a corepressor. Tryptophan is the corepressor of the trp operon. An operon that its small effector turns off is a repressible operon.

Check yourselfA bacterium is moved into a medium rich in tryptophan. What happens to transcription of the trp operon, and why?

It stops. Tryptophan enters the cell and binds trp repressor as a corepressor, which lets the repressor bind the operator and block RNA polymerase.

Inducible vs. repressible operons

The two operons use their small effector molecules in opposite ways. Lac repressor binds its operator when its effector is absent; trp repressor binds its operator only when its effector is present.

Predicting what a mutation does

Exam questions often break one part of an operon and ask what happens. Work it out from what the broken part normally does.

An operon that’s expressed whether or not its inducer is present is said to be constitutively expressed, like a housekeeping gene. The first two mutations above both do this, and they can be told apart because the parts work differently. Lac repressor is a protein that diffuses through the cell, so a working copy of lacI anywhere in the cell can supply repressor to every lac operator. The operator is a stretch of DNA, so it controls only the genes right next to it.

Check yourselfA working lacI gene is added to a cell whose own lacI gene is broken. Would it restore normal regulation? Would it help a cell whose operator is broken instead?

It restores the first cell, because lac repressor is a protein that diffuses through the cell and can bind any normal operator. It can’t help the second, because the repressor has nowhere to bind on the broken operator.