The lac operon

The lac operon is a group of genes in the bacterium E. coli that lets it use lactose as an energy source, but only when lactose is around. When there is no lactose, a repressor protein binds to the operator and blocks transcription. When lactose is present, it binds to the repressor and pulls it off, so the genes for lactose-digesting enzymes are transcribed. It is a classic example of how genes are switched on and off at the level of transcription.

What is an operon?

In bacteria, genes that work together are often grouped side by side and transcribed as one unit, controlled by a single promoter. OpenStax Biology calls these blocks operons. Because the genes share a promoter, the cell can switch them all on or off together.

Some A-level Biology courses use the lac operon as their example of gene regulation in prokaryotes, so check your own specification.

The parts of the lac operon

How the lac operon works

When lactose is absent

1. The regulatory gene is transcribed and translated, making the lac repressor.
2. The repressor binds to the operator.
3. With the operator occupied, RNA polymerase can't get past to the structural genes, so hardly any β-galactosidase or permease is produced.

OpenStax notes that the operon is still transcribed at a very low rate even without lactose, so the cell always has a few copies of the enzymes.

When lactose is present

1. Lactose enters the cell, and some is converted to a related sugar, allolactose.
2. Allolactose attaches to the repressor, altering the repressor's shape.
3. In its new shape the repressor doesn't fit the operator, so it comes away.
4. RNA polymerase can now transcribe lacZ, lacY and lacA.
5. The enzymes are made, and the cell can take in and break down lactose.

Because lactose switches the genes on, the lac operon is called an inducible operon, and allolactose is the inducer. When the lactose has all been used, the repressor is free again and the genes switch off.

The role of glucose

Bacteria use glucose in preference to lactose, so the operon is only strongly switched on when glucose is scarce. OpenStax explains that when glucose levels drop, a molecule called cAMP builds up. cAMP binds to a protein called CAP (catabolite activator protein), and the CAP-cAMP complex binds near the promoter, helping RNA polymerase bind and increasing transcription.

The table, from OpenStax, shows two switches working together: lactose removes the repressor (the brake), and low glucose brings in CAP (the accelerator). Transcription is high only when lactose is present and glucose is low.

Why bacteria regulate genes like this

Making enzymes takes energy and raw materials. A bacterium that made lactose-digesting enzymes all the time would waste resources when there was no lactose to digest. With the lac operon, it only makes them when lactose is present and its preferred food, glucose, has run low. Regulating genes at the level of transcription means no mRNA or protein is wasted.

Inducible vs repressible operons

The lac operon is off unless lactose turns it on. OpenStax contrasts it with the trp operon, which makes the enzymes for the amino acid tryptophan. That operon is on by default; when tryptophan is plentiful, it binds to the trp repressor, which then attaches to the operator and switches the genes off. So the trp operon is repressible.

Common mistakes

• Saying lactose binds to the operator. Lactose (as allolactose) binds to the repressor, which then leaves the operator.
• Mixing up promoter and operator. RNA polymerase binds to the promoter; the repressor binds to the operator.
• Saying the regulatory gene is part of the operon. It is separate and codes for the repressor.
• Saying lactose is digested by the repressor. β-galactosidase, coded by lacZ, breaks lactose down.
• Forgetting glucose. High glucose keeps transcription low even when lactose is present.

Transcription and translation are covered in the Genetics and DNA course. For practice in a short timed paper, try today's A-level Biology: Year 1 or Year 2 paper.

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