The Lac Operon

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.

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Why an Operon?

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.

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Structure of the Lac Operon

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).

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Negative Regulation: The Lac Repressor

Lactose absent

1. The lacI gene is constitutively expressed — the cell always has some lac repressor protein in its cytoplasm.
2. The repressor is a tetramer that binds tightly to the operator sequence.
3. With the repressor in place, RNA polymerase cannot transcribe lacZ, lacY, lacA.
4. No β-galactosidase, no permease, no wasted energy.

Lactose present

1. A small amount of lactose enters the cell (via the low basal level of permease). Some is converted to the actual inducer, allolactose, by the tiny amount of β-galactosidase always present.
2. Allolactose binds to the repressor, causing a conformational change.
3. The modified repressor can no longer bind the operator.
4. RNA polymerase is now free to transcribe lacZ, lacY, lacA.
5. β-galactosidase and permease levels rise rapidly, and the cell starts digesting lactose.

When lactose is exhausted, allolactose disappears, the repressor re-binds the operator, and the operon switches off again.

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Positive Regulation: cAMP and CAP

If both glucose and lactose are available, E. coli preferentially uses glucose. This catabolite repression is achieved by the cAMP-CAP system.

• CAP (catabolite activator protein, also called CRP) is a transcription factor that must bind cyclic AMP (cAMP) to be active.
• When glucose is abundant, cellular cAMP is low. CAP is inactive, does not bind the promoter, and transcription of the lac operon is inefficient even if lactose is present.
• When glucose is scarce, cellular cAMP is high. CAP-cAMP binds a site upstream of the lac promoter and recruits RNA polymerase, dramatically increasing transcription.

Therefore, maximum lac operon expression requires both conditions: