Regulation of gene expression: the lac operon
The lac operon of E. coli is switched on only when lactose is present. Lactose, the inducer, inactivates the repressor, so genes z, y and a are transcribed.
Why genes are regulated
A cell does not make every protein all the time. E. coli makes β-galactosidase only when it must use lactose as food. In eukaryotes, gene expression can be regulated at four levels: transcription (making the primary transcript), processing (splicing), transport of mRNA from nucleus to cytoplasm, and translation. In prokaryotes, the main control point is the start of transcription. Here, proteins bind DNA sequences called operators, next to the promoter, and so decide whether RNA polymerase can reach the promoter. The development of an embryo into an adult is also the result of well-coordinated gene regulation.
Parts of the lac operon
| Part | What it codes for or does |
|---|---|
| i gene (regulatory) | Codes for the repressor; made all the time. 'i' comes from inhibitor, not inducer |
| Promoter (p) | Where RNA polymerase binds |
| Operator (o) | Where the repressor binds |
| z gene | β-galactosidase, which breaks lactose into glucose and galactose |
| y gene | Permease, which makes the cell more permeable to β-galactosides such as lactose |
| a gene | Transacetylase |
Lactose absent vs lactose present
When all the lactose has been used up, the repressor binds the operator again and the operon switches off.
Switching the lac operon on
Because the repressor switches the operon off, this control is called negative regulation.
Negative regulation
Control of the lac operon by the repressor is called negative regulation: a protein stops transcription until an inducer removes it. The lac operon is also under positive regulation, which your syllabus does not cover. A very low level of lac expression is always present. Without it, there would be no permease, and lactose could not enter the cell to act as the inducer.
How to draw the lac operon (off and on)
- Draw a long DNA bar and divide it, left to right, into i, p, o, z, y, a. Label each.
- (a) Off: show the i gene's mRNA making a repressor protein, and the repressor sitting on the operator. Put a cross on the arrow for transcription.
- (b) On: draw the inducer (lactose) stuck to the repressor, with the repressor drawn away from the operator.
- Show RNA polymerase on the promoter and a wavy mRNA coming from z, y and a.
- Below the mRNA, draw three ovals labelled β-galactosidase, permease, transacetylase.
People with lactose intolerance make too little lactase, the human enzyme that does the job of β-galactosidase, so milk upsets their stomach. Curd is easier for them because bacteria have already broken down some of its lactose.
3 marks: “What happens to the lac operon in E. coli when lactose is (a) absent and (b) present in the medium?” (a) The repressor from the i gene binds the operator; RNA polymerase is blocked; z, y, a not transcribed. (b) Lactose enters via permease, binds and inactivates the repressor; operator free; RNA polymerase transcribes z, y, a; enzymes digest lactose.
2 marks: “Why is regulation of the lac operon by the repressor called negative regulation?” Because the repressor protein, when bound to the operator, stops transcription. Genes are expressed only when the inducer removes this block.
5 marks: “Explain the lac operon with labelled diagrams.” Operon concept; Jacob and Monod; i, p, o, z, y, a and their products; lactose absent: repressor on operator; lactose present: inducer inactivates repressor, transcription; negative regulation; two labelled diagrams.
Gene order i p o z y a: 'I Pray Only Zen Yoga Asanas'. Lactose present = repressor busy = genes on.