Difficulty: Intermediate | Prerequisites: Basic understanding of DNA, RNA, transcription and translation (Ch. 10–13 or equivalent).
Gene regulation is the reason every cell in your body has the same DNA yet looks and behaves differently. In prokaryotes (bacteria), the main mechanism is the operon: a cluster of genes under one switch that can be flipped on or off depending on what the cell needs right now. This chapter focuses on how bacteria use operons, repressors, inducers and activators to respond to their environment, particularly in the classic lac and trp systems. You need a solid grasp of transcription and translation before this will click, so revisit those if they are hazy.
Bacteria control which proteins they produce by grouping related genes into operons and regulating them with repressors and activators. The trp operon shuts off when tryptophan is abundant (repressible); the lac operon turns on when lactose is present and glucose is absent (inducible, with both negative and positive control). Understanding these two systems covers most of what exams test on prokaryotic gene regulation.
Operon
A stretch of DNA consisting of a promoter, an operator and a set of structural genes that are transcribed together as one mRNA. Think of it as a single switch that controls an entire assembly line of related proteins.
Operator
A short DNA sequence between the promoter and the structural genes that acts as a control switch. When a repressor binds here, transcription is blocked. In simple terms, it is the on/off button for the operon.
Promoter
The specific nucleotide sequence where RNA polymerase binds to begin transcription. Think of it as the landing pad that positions the transcription machinery.
Repressor
A protein that prevents gene expression by binding to the operator (or to DNA sequences near the promoter). In simple terms, it is the molecule that physically blocks the road so RNA polymerase cannot pass.
Inducer
A small molecule that binds to a repressor protein and changes its shape so it can no longer sit on the operator, thereby switching the operon on. Think of it as the key that unlocks the gate.
Corepressor
A small molecule that binds to a repressor protein and activates it, enabling it to bind the operator and shut transcription down. In simple terms, it is the missing piece that makes the repressor functional.
Activator
A protein that facilitates gene expression by binding to DNA sequences or other proteins, helping RNA polymerase bind or work more efficiently. Think of it as a welcome mat that encourages RNA polymerase to sit down and start reading.
Regulatory gene
A gene that codes for a protein (such as a repressor) that controls the transcription of another gene or group of genes. These are the genes whose job is to manage other genes, not to code for structural or metabolic products.
Inducible operon
An operon whose default state is off and whose expression is switched on by an inducer. The lac operon is the textbook example.
Repressible operon
An operon whose default state is on and whose expression is shut down when a corepressor is present. The trp operon is the textbook example.
Constitutive expression
A gene that is always expressed regardless of conditions. In simple terms, it is always on, no switches involved.
β-galactosidase (beta-galactosidase)
The enzyme that hydrolyses lactose into glucose and galactose. It is one of the structural gene products of the lac operon.
Permease
A membrane protein that transports lactose into the bacterial cell. Another product of the lac operon.
Allolactose
An isomer of lactose that acts as the natural inducer of the lac operon. When lactose enters the cell, a small amount is converted to allolactose, which then inactivates the lac repressor.
Allosteric regulation
A process in which a small molecule binds to a large protein and changes the protein's shape and activity. Both inducers and corepressors work through allosteric regulation of repressor proteins.
Negative regulation
Regulation of gene expression by the binding of a repressor protein to DNA, turning transcription off.
Positive regulation
Regulation of gene expression by the binding of an activator protein to DNA, turning transcription on or increasing its rate.
CRP (cAMP receptor protein) / CAP (catabolite activator protein)
A regulatory protein that, when bound to cyclic AMP (cAMP), acts as an activator for the lac operon. It is the positive-control arm of the lac system. Think of it as the boost pedal: it does not start the engine (that requires the repressor to be off), but it makes the engine run faster.
Every cell in an organism carries the same DNA, yet different cells produce different proteins.
Two broad strategies keep protein activity appropriate:
Adjust the activity of proteins already present (e.g. feedback inhibition of enzymes).
Control which genes are transcribed and translated in the first place.
Negative feedback inhibition is the most common method at the enzyme level: the end product of a metabolic pathway inhibits an enzyme earlier in the pathway, slowing its own production.
Regulatory genes code for proteins (repressors, activators) that control the expression of other genes.
Small changes in regulatory genes can have large phenotypic effects. Humans and chimpanzees share nearly identical genomes; the key differences lie in the expression of roughly 19 regulatory genes, not in the structural genes themselves.
An operon is a cluster of functionally related structural genes under the control of a single promoter and operator.
All genes in the operon are transcribed together into one mRNA (polycistronic mRNA), starting from one transcription start site.
The operator sits between the promoter and the structural genes. It is the binding site for the repressor.
When the repressor binds the operator, RNA polymerase cannot proceed, and transcription is blocked.
The trp operon codes for enzymes that synthesise the amino acid tryptophan.
Default state: on. The operon is transcribed unless it is actively shut down.
When tryptophan is abundant, it acts as a corepressor: it binds to the inactive trp repressor, activating it.
The activated repressor then binds the operator, blocking transcription.
When tryptophan is scarce, the repressor remains inactive, the operator is clear, and the biosynthetic enzymes are produced.
The trp operon uses negative control only. There is no positive-control layer.
Repressible operons typically code for anabolic (biosynthetic) enzymes: the cell makes the product until it has enough, then shuts the pathway down.
The lac operon codes for enzymes needed to import and break down lactose, including β-galactosidase (hydrolyses lactose into glucose and galactose) and permease (transports lactose into the cell).
Default state: off. The lac repressor is produced in an active form and sits on the operator, blocking transcription.
When lactose is present, some of it is converted to allolactose, the natural inducer.
Allolactose binds the repressor and changes its shape (allosteric regulation), causing it to fall off the operator.
With the operator clear, RNA polymerase can transcribe the lac genes.
The lac operon is regulated through both negative and positive control.
Negative control (the repressor arm):
Lactose absent: repressor active, bound to operator, transcription off.
Lactose present: allolactose inactivates repressor, transcription on.
Positive control (the CRP/CAP arm):
Glucose absent: cAMP levels rise, cAMP binds CRP, and the CRP–cAMP complex binds near the lac promoter, boosting RNA polymerase binding and increasing transcription rate.
Glucose present: cAMP levels drop, CRP is inactive, RNA polymerase binds poorly, transcription is low even if lactose is present.
The practical result: the lac operon is transcribed at its highest rate only when lactose is present AND glucose is absent. Glucose is the preferred fuel, so the cell does not waste resources on lactose metabolism while glucose is available.
Condition | Trp operon | Lac operon |
|---|---|---|
Trp present | Off (repressed) | n/a |
Trp absent | On | n/a |
Lactose absent | n/a | Off (repressed) |
Lactose present, glucose present | n/a | On, but at a low rate (no positive boost) |
Lactose present, glucose absent | n/a | On at full rate (positive + negative control both favour transcription) |
The lac operon is the reason gut bacteria can switch to digesting milk sugar when you drink a glass of milk. The lactose in milk changes the chemical environment for bacteria in the digestive tract, and the operon model explains how those bacteria ramp up lactose-digesting enzymes within minutes. More broadly, understanding operon-level gene regulation is foundational to genetic engineering: biotechnology routinely uses inducible promoter systems (descendants of the lac system) to switch target genes on and off in bacterial cultures, which is how many pharmaceutical proteins are manufactured.
Students often think "inducible" means the operon is always on. It is the opposite: an inducible operon is normally off and is induced (turned on) by the presence of a substrate.
Students confuse the inducer with the activator. The inducer (allolactose) inactivates the repressor; the activator (CRP–cAMP) is a separate protein that boosts transcription. They are two different molecules doing two different jobs on the same operon.
Students sometimes assume that the trp operon has a positive-control element like the lac operon. It does not. The trp operon is regulated only by negative control.
A common error is thinking that when glucose is present and lactose is also present, the lac operon is completely off. It is not completely off; it is transcribed at a low basal rate because the repressor has been removed. The positive boost from CRP is simply absent, so transcription is sluggish.
⚠️ Know the difference between inducible and repressible operons cold. Exams love to give you a novel operon and ask you to classify it.
⚠️ Be able to predict the state of the lac operon given any combination of lactose and glucose (four possible combinations, four different outcomes).
⚠️ Understand allosteric regulation as the mechanism behind both induction and corepression. A small molecule changes the shape of a protein, altering its ability to bind DNA.
⚠️ The promoter always binds RNA polymerase. The operator is what the repressor binds. Do not confuse these two sequences.
True or False: The trp operon is an example of an inducible operon. (False. It is repressible.)
Fill in the blank: The natural inducer of the lac operon is ________. (Allolactose.)
True or False: When glucose is present, cAMP levels are high. (False. Glucose lowers cAMP levels.)
Fill in the blank: A repressor binds to the ________ to block transcription. (Operator.)
True or False: Repressible operons usually code for catabolic enzymes. (False. They usually code for anabolic/biosynthetic enzymes.)
Q: What is the difference between an inducible and a repressible operon?
A: An inducible operon is normally off and is turned on by an inducer (e.g. allolactose turns on the lac operon). A repressible operon is normally on and is shut down when a corepressor binds the repressor protein (e.g. tryptophan shuts down the trp operon).
Q: Explain why the lac operon is transcribed at a low rate when both lactose and glucose are present.
A: Lactose (via allolactose) removes the repressor from the operator, so transcription can occur. However, glucose keeps cAMP levels low, so CRP is inactive and cannot provide the positive boost. Without CRP–cAMP at the promoter, RNA polymerase binds poorly and transcription proceeds at only a basal rate.
Q: A mutation destroys the operator of the lac operon so that the repressor can never bind. What happens to transcription of the lac genes?
A: The lac genes would be transcribed constitutively (all the time), regardless of whether lactose is present, because the repressor can never block the promoter.
Q: In the trp operon, what role does tryptophan itself play in regulation?
A: Tryptophan acts as a corepressor. It binds to the inactive trp repressor protein, activating it so that the repressor can bind the operator and block transcription of the tryptophan-biosynthesis genes.
Q: What is allosteric regulation, and how does it apply to the lac repressor?
A: Allosteric regulation is a change in a protein's shape and activity caused by the binding of a small molecule at a site other than the active site. Allolactose binds to the lac repressor at an allosteric site, changing its conformation so it can no longer bind the operator, which permits transcription.
This material connects directly to eukaryotic gene regulation (the companion notes), where the principles of activators, repressors and allosteric control reappear in a far more complex setting with chromatin remodelling, enhancers and post-transcriptional layers. It also ties back to transcription and translation (Ch. 10–13): if those mechanisms are unclear, the logic of operons will not land properly. The concept of negative feedback inhibition here is the same principle covered in metabolism and enzyme regulation.
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