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Eukaryotic transcriptional control is complicated. Prokaryotic control is elegantly simple, and the lac operon of Escherichia coli is the textbook example. First described by François Jacob and Jacques Monod in 1961 (who won the Nobel Prize for it), the lac operon allows E. coli to switch on the enzymes needed to digest lactose only when lactose is present and glucose is absent. It is a beautiful illustration of both negative and positive regulation in the same system. OCR A-Level Biology A specification module 6.1.1(b)(ii) requires you to describe the structure and function of the lac operon.
Key Definitions:
- Operon — a cluster of genes under the control of a single promoter, transcribed together as one mRNA (common in prokaryotes).
- Structural gene — a gene encoding a functional protein.
- Regulatory gene — a gene encoding a protein (usually a repressor) that controls the expression of other genes.
- Promoter — the DNA sequence where RNA polymerase binds to start transcription.
- Operator — a DNA sequence adjacent to the promoter that binds a repressor protein and blocks transcription.
- Inducer — a small molecule that binds to a repressor and inactivates it, allowing transcription.
E. coli is a gut bacterium that must respond quickly to changes in the nutrient environment. Its preferred fuel is glucose, but it can also use lactose if that is all that is available. However, the enzymes for lactose digestion cost energy to make, so E. coli only makes them when needed. Grouping the three lactose-metabolism genes into a single operon lets the cell turn them all on or off together — an efficient solution.
The lac operon consists of:
| Component | Description |
|---|---|
| Regulatory gene (lacI) | Has its own promoter; constitutively expressed; encodes the lac repressor protein |
| Promoter (P) | RNA polymerase binding site |
| Operator (O) | Overlaps the promoter; binds the repressor |
| Structural gene lacZ | Encodes β-galactosidase — hydrolyses lactose to glucose + galactose |
| Structural gene lacY | Encodes lactose permease — pumps lactose into the cell |
| Structural gene lacA | Encodes thiogalactoside transacetylase — detoxifies some analogues |
flowchart LR
lacI[lacI] --> P1[P]
P1 --> O[O]
O --> Z[lacZ]
Z --> Y[lacY]
Y --> A[lacA]
Note that the regulatory gene lacI is not part of the operon itself — it lies just upstream and has its own promoter. The three structural genes are transcribed as a single polycistronic mRNA (one transcript, several proteins).
When lactose is exhausted, allolactose disappears, the repressor re-binds the operator, and the operon switches off again.
If both glucose and lactose are available, E. coli preferentially uses glucose. This catabolite repression is achieved by the cAMP-CAP system.
Therefore, maximum lac operon expression requires both conditions:
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