Lac Operon Mechanism: Regulation, Repressor, CAP, and Diauxic Growth

Give E. coli both glucose and lactose, and it will not touch the lactose until the glucose is finished. The bacterium eats its preferred sugar first, pauses, then switches on a whole set of genes to digest the second. That pause, and the switch that ends it, is the lac operon at work.

It is a molecular decision: make the lactose-digesting enzymes only when lactose is present and the better fuel, glucose, is not. This article walks through how that decision is made, one control at a time, and works through the exact combinations of glucose and lactose that exams ask about.

The lac operon is a well-known example of an inducible gene network that regulates the transport and metabolism of lactose in Escherichia coli. It carries the genes for taking up lactose from outside the cell and breaking it down into glucose and galactose.

The lactose operon of E. coli is turned ON only when lactose is available (and glucose, the preferred energy source, is absent). When there is an absence of lactose the transcription of the lac operon genes is blocked by a repressor protein (as there will be no use of operon’s gene products).

Structure of the lac operon

The lac operon consists of a promoter (P) and operator (O) region followed by three structural genes lacZ, lacY, and lacA in the downstream. A regulatory gene lacI (I) preceding the lac operon is responsible for producing a repressor (R) protein.

## The regulatory parts of the lac operon

The lac operon contains regulatory DNA sequences: regions of DNA where control proteins bind to switch transcription on or off.

Promoter (P)
The promoter is the binding site for RNA polymerase, the enzyme that carries out transcription. The lac promoter sits at the 5′ end of lacZ and directs transcription of all three structural genes as a single mRNA (a polycistronic message).

Operator (O)
The operator is a short DNA sequence just after the promoter. It is the docking site for the repressor protein. When the repressor sits on the operator, it physically blocks RNA polymerase from moving forward, so transcription cannot proceed. The operator is the switch the repressor controls.

Regulatory gene (lacI) and the repressor
The lacI gene sits just before the operon and codes for the lac repressor protein. lacI is transcribed on its own, all the time, so the repressor is always available. The repressor is the negative control of the operon: by default it binds the operator and keeps the operon off until lactose signals otherwise.

The lac operon responds to two signals from the growth medium, lactose and glucose, through two control proteins: the repressor (which senses lactose, via allolactose) and CAP (which senses glucose). The next sections take each in turn.

DO YOU KNOW?

François Jacob and Jacques Monod proposed the lac operon model of gene regulation in 1961. Together with their colleague André Lwoff, they received the Nobel Prize in Physiology or Medicine in 1965. Since its discovery, lac operon has been serving as a model system for understanding different aspects of gene regulations.

Allolactose: the true inducer

Students are often taught that "lactose induces the lac operon." The precise answer, and the one exams reward, is that allolactose is the inducer, not lactose itself.

Here is why. When a little lactose enters the cell, the enzyme β-galactosidase converts a small fraction of it into allolactose, an isomer of lactose. Allolactose, not lactose, is the molecule that binds the repressor and switches the operon on. Lactose is the signal in the environment; allolactose is the internal messenger that actually flips the switch.

This is also why the system can start at all: a small basal amount of β-galactosidase is always present, enough to make the first allolactose when lactose appears.

Mechanism of the lac operon

When lactose is absent (operon OFF)

When there is no lactose, there is no allolactose. With no allolactose to bind it, the repressor protein (R) stays bound to the operator (O), where it blocks RNA polymerase from transcribing the structural genes. The operon is off, and the cell makes almost none of the lactose-digesting enzymes. This is the default state.

### When lactose is present (operon ON)

When lactose is present in a medium, some of it will be converted to allolactose. This allolactose binds to the repressor protein. Binding of allolactose with repressor protein changes the shape of the repressor protein so it can no longer bind to the operator region. This allows the RNA polymerase to bind to the promoter site, starting the initiation of transcription of the structural genes lacZ, lacY, and lacA to produce mRNA.

Once the mRNA is produced, it is translated to produce enzymes. The lacZ gene encodes for the production of the enzyme β-galactosidase (B), lacY gene for the production of the enzyme permease (P) and the lacA gene is responsible for the production of the enzyme thiogalactoside transacetylase.

Catabolite repression: the glucose effect

It would be wasteful for the cell to make lactose-digesting enzymes while a better fuel, glucose, is available. Catabolite repression is the control that ensures the cell uses the best carbon source first. It works through a second protein, separate from the repressor.

Figure: Expression of Lac Operon gene in the presence of Glucose

CAP (CRP) and cAMP: the positive control
Efficient transcription of the lac operon needs the catabolite activator protein (CAP), also called CRP (cAMP receptor protein). CAP binds the DNA near the promoter only after it has first bound cyclic AMP (cAMP). Once the CAP–cAMP complex is on the DNA, it helps RNA polymerase bind the promoter firmly, raising transcription. CAP is the positive control of the operon. The gene for CAP sits elsewhere on the chromosome, not in the lac operon.

How glucose lowers cAMP
cAMP is made from ATP by the enzyme adenylate cyclase. When glucose is being taken up, cAMP levels fall. Low cAMP means little CAP–cAMP complex forms, so CAP cannot help RNA polymerase, and transcription stays low even if lactose is present. In short: glucose present → low cAMP → CAP inactive → weak transcription.

High glucose (transcription low)
When glucose is high, cAMP is low, the CAP–cAMP complex does not form, CAP cannot bind DNA, and RNA polymerase binds the promoter poorly. Transcription runs only at a low level.

Low or no glucose (transcription high)
When glucose is low or absent, cAMP rises. Abundant cAMP binds CAP to form the CAP–cAMP complex, which binds the DNA and boosts RNA polymerase binding at the lac promoter. If lactose is also present (repressor off), transcription is strong.

A note on how glucose really acts (beyond exams)
The cAMP–CAP story above is the standard model and the one exams test, so learn it as written. For accuracy: research since then shows glucose also lowers lac expression by a second route called inducer exclusion. When glucose is being transported, a transport protein (EIIA^Glc) binds lactose permease and blocks lactose entry, so little allolactose forms and the repressor stays on. Current evidence suggests inducer exclusion contributes at least as much as the cAMP drop. Take-home: glucose keeps the operon off by two cooperating routes, lowering cAMP (less CAP activation) and blocking lactose uptake (less allolactose).

Negative and positive regulation of the lac operon

The lac operon is controlled in two opposite ways at once, which is why it is a favorite exam example.

Negative regulation (the repressor). Regulation by the repressor is called negative because the control protein, the repressor, acts to turn the operon off. Its default action is to block transcription; removing it (by allolactose) allows transcription. Control that works by blocking is negative regulation. This is the answer to "why is the lac operon called negatively regulated" and "regulation by the repressor is referred to as": negative regulation.

Positive regulation (CAP). Regulation by CAP is called positive because CAP acts to turn transcription up. When CAP–cAMP binds, it increases transcription; it activates rather than blocks. Control that works by activating is positive regulation.

So the same operon is under negative control by the repressor (lactose signal) and positive control by CAP (glucose signal) at the same time. Two proteins, two signals, opposite logic.

Putting it together: the four glucose-lactose states

The operon reads two signals, glucose and lactose, and only one combination gives strong transcription. The rule: the operon is strongly ON only when lactose is present (repressor off) AND glucose is absent (CAP on).

For the transcription of structural genes by the lac operon, two requirements must be met:

1. Glucose must be absent: The level of cyclic AMP must be high enough so that the CAP protein binds to the CAP binding site. Bound CAP helps to attach RNA polymerase efficiently to the lac operon promoter.
2. Lactose must be present: There must be an inducer (allolactose, made when lactose is present) so that the repressor leaves the operator and does not block transcription.

Each of the regulatory proteins (CAP and lac repressor) responds to one environmental signal and communicates it to the lac genes. The combined effect of these two regulators ensures that the genes are expressed at significant levels only when lactose is present and glucose is absent. Now, let’s observe the transcription of the operon in various environmental conditions:

1. Glucose present, lactose absent

As glucose is present, cAMP level is low so activator CAP remains inactive. Lac repressor remains bound to the operator and prevents binding of RNA polymerase. In this condition, no transcription of the lac operon occurs.

2.Glucose present, lactose present

Activator CAP remains inactive. The lac repressor is off because allolactose (from the lactose present) has bound it. In this condition, the basal level transcription of the lac operon occurs.

3.Glucose absent, lactose absent

Activator CAP is active as a high level of cAMP is present (as glucose is absent) but lac repressor is functional (active). Lac repressor remains bound to the operator and prevents transcription.

4.Glucose absent, lactose present

cAMP levels are high so CAP is active and bound to the DNA. CAP helps the efficient binding of RNA polymerase to the promoter. The lac repressor is inactive because the inducer allolactose (made when lactose is present) has bound it. In this condition, strong transcription of the lac operon occurs.

| Glucose | Lactose | Repressor | CAP–cAMP | Transcription | | --- | --- | --- | --- | --- | | Present | Absent | On (bound to operator) | Inactive | None | | Present | Present | Off | Inactive | Very low (basal) | | Absent | Absent | On (bound to operator) | Active | None | | Absent | Present | Off | Active | High (maximal) |

Reading the table: transcription needs two green lights at once, the repressor off (needs lactose) and CAP on (needs glucose absent). Any other combination gives little or no transcription. This is why "maximal transcription requires" the answer: lactose present and glucose absent.

The exam trap, worked through: if both glucose and lactose are high, is allolactose high or low? With lactose present, some allolactose does form, so the repressor comes off, that part is not the limiter. But glucose keeps cAMP low, so CAP is inactive, and glucose also blocks further lactose uptake by inducer exclusion, which keeps allolactose low. The result is only basal (very low) transcription. The lesson: when both sugars are high, it is the missing CAP activation (and reduced allolactose from inducer exclusion), not a bound repressor, that keeps the operon nearly off.

Diauxic growth: why the operon works this way

Grow E. coli in a medium with both glucose and lactose and plot the growth over time. The curve has two phases with a short pause between them. This two-phase pattern is called diauxic growth, first described by Jacques Monod in 1941.

In the first phase, the cells use glucose only. The lac operon is kept off (glucose keeps cAMP low and blocks lactose uptake), so the lactose sits unused. When the glucose runs out, cAMP rises, CAP becomes active, the operon switches on, and the cells spend a short lag building the lactose-digesting enzymes. In the second phase, they grow on lactose.

Diauxic growth is the visible, whole-cell consequence of everything above: the lac operon exists so the cell spends its energy on the better fuel first and only switches to lactose when it must. The growth curve is the operon's logic made visible.

Natural and gratuitous inducers (IPTG)

The natural inducer of the lac operon is allolactose, made from lactose inside the cell.

Laboratories often use a synthetic inducer instead: IPTG (isopropyl β-D-thiogalactoside). IPTG binds and inactivates the repressor just like allolactose, so it switches the operon on. But β-galactosidase cannot break it down, so its level stays constant and it is not consumed. An inducer that switches the operon on without being metabolized is called a gratuitous inducer. This steady, controllable induction is why IPTG is a standard tool in molecular biology and recombinant protein work. (TMG is another gratuitous inducer used in research.)

How to remember

Two signals, two proteins, opposite jobs. Lactose works through the repressor (negative control, turns the operon off by default). Glucose works through CAP (positive control, turns transcription up). Repressor blocks; CAP boosts. Keep those two straight and the whole operon follows.

The operon needs both green lights. Strong transcription needs lactose present (repressor off) and glucose absent (CAP on). One green light is not enough. "Lactose in, glucose out" is the only combination that runs the operon hard.

Allolactose, not lactose, is the inducer. Lactose is the signal outside; allolactose is the messenger inside that actually pulls the repressor off the operator. Exams reward "allolactose."

Glucose is the boss sugar. The cell always eats glucose first. Everything about the operon, low cAMP, inducer exclusion, the diauxic pause, exists to keep lactose enzymes off until the glucose is gone. If you remember "glucose first," you can reconstruct the rest.

Key exam facts

Where students get confused

"Is lactose or allolactose the inducer?" Allolactose. Lactose is the sugar in the medium, but the molecule that actually binds and inactivates the repressor is allolactose, an isomer made from lactose by β-galactosidase inside the cell. Exams want "allolactose." Lactose is the signal; allolactose is the switch.

"When lactose is present, what happens to the repressor?" Allolactose binds the repressor and changes its shape so it can no longer sit on the operator. The repressor falls off, RNA polymerase can transcribe, and the operon turns on. Lactose present → repressor off.

"In the absence of lactose, the repressor is...?" Active and bound to the operator, blocking transcription. No lactose means no allolactose, so nothing pulls the repressor off. This is the default off state.

"Why is it called negative regulation?" Because the controlling protein, the repressor, acts by turning the operon off. Control that works by blocking is negative, whether or not the operon is currently on. CAP, which works by turning transcription up, is the positive control on the same operon.

"If both glucose and lactose are high, why is the operon nearly off?" Not because of the repressor: lactose makes allolactose, so the repressor comes off. It is because glucose keeps cAMP low, so CAP is inactive and cannot boost transcription, and glucose also limits lactose uptake (inducer exclusion). The operon runs only at a basal level. The limiter here is missing CAP activation, not a bound repressor.

"Does CAP turn the operon on by itself?" No. CAP only boosts transcription; it cannot start it while the repressor is blocking the operator. Both conditions must be met: repressor off (lactose) and CAP on (no glucose). CAP is the volume knob, not the on switch.

References

• Jacob F, Monod J. Genetic regulatory mechanisms in the synthesis of proteins. Journal of Molecular Biology. 1961;3(3):318-356. doi:10.1016/S0022-2836(61)80072-7
• Madigan MT, Bender KS, Buckley DH, Sattley WM, Stahl DA. Brock Biology of Microorganisms. 16th ed. Pearson; 2021.
• Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. Molecular Biology of the Gene. 7th ed. Cold Spring Harbor Laboratory Press; 2013.
• Clark DP, Pazdernik NJ, McGehee MR. Molecular Biology. 3rd ed. Academic Press; 2019.
• OpenStax College. Prokaryotic Gene Regulation. In: Biology 2e. OpenStax; 2018. (Source for the glucose/lactose regulation figures.)