AP Biology

## Learning Outcomes

- Define an operon and identify the roles of promoters, operators, regulatory genes, and structural genes.
- Explain how inducible operons are switched on by an inducer molecule.
- Explain how repressible operons are switched off by a corepressor molecule.
- Predict lac operon expression under different lactose and glucose conditions.
- Predict trp operon expression under different tryptophan conditions.
- Compare negative regulation by repressors with positive regulation by activator proteins.
- Apply operon logic to unfamiliar AP Biology scenarios, such as arabinose or amino acid synthesis operons.

## AP Biology Syllabus

Operons are part of AP Biology Unit 6, Gene Expression and Regulation, which tests how cells control the use of genetic information. Unit 6 is a significant part of the multiple-choice exam, and operons are a common way to assess prokaryotic regulation. You do not need to memorize every named operon, but you should be able to apply the logic of inducible and repressible systems to new examples.

- Prokaryotes often regulate related genes together in operons.
- Operons include regulatory DNA sequences and coding sequences.
- A promoter is a binding site for RNA polymerase.
- An operator is a binding site for a repressor protein.
- Inducible operons are usually involved in catabolic pathways, such as lactose breakdown.
- Repressible operons are usually involved in anabolic pathways, such as amino acid synthesis.
- The lac operon is affected by lactose availability and glucose availability.
- The trp operon is controlled by tryptophan acting as a corepressor.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What is the difference between a promoter and an operator in a bacterial operon?
2. Why is the lac operon usually off when lactose is absent?
3. How does tryptophan shut down transcription of the trp operon?
4. Under which conditions is lac operon transcription highest: high glucose and high lactose, or low glucose and high lactose?
5. A bacterial operon codes for enzymes that synthesize an amino acid. The amino acid lowers transcription of the operon. Is this more likely an inducible or repressible operon?

## Introduction

Cells do not express every gene at the same time. Gene expression uses genetic information to make a functional product, usually an RNA molecule and often a protein. Regulation of gene expression allows a cell to produce proteins only when they are useful. This saves energy and materials, and it helps cells respond to changing environments.

In prokaryotes such as bacteria, genes with related functions are often grouped together and controlled as a unit called an operon. Operons are especially efficient for bacteria because several genes needed for the same pathway can be turned on or off together. The lac operon and trp operon are the classic AP Biology examples: the lac operon helps bacteria break down lactose, while the trp operon helps bacteria make tryptophan.

> **Key Term: operon**
> A cluster of genes with a related function that is controlled by a common promoter and regulated as a unit, especially in prokaryotes.

### The basic parts of a bacterial operon

An operon contains coding sequences and regulatory sequences. The coding sequences are the genes that will be transcribed and translated into proteins. The regulatory sequences are DNA regions that help determine whether transcription happens.

A simplified operon includes four main parts: a regulatory gene, a promoter, an operator, and structural genes.

The regulatory gene codes for a regulatory protein, often a repressor. The repressor can bind to a specific DNA sequence and reduce transcription. In many textbook diagrams, the regulatory gene is shown near the operon, but it is not always part of the same transcribed unit as the structural genes.

> **Key Term: regulatory gene**
> A gene that codes for a regulatory protein, such as a repressor, that affects transcription of other genes.

The promoter is the DNA sequence where RNA polymerase binds to start transcription. If RNA polymerase can bind and move along the DNA, the structural genes can be transcribed into mRNA.

> **Key Term: promoter**
> A noncoding DNA sequence that serves as a binding site for RNA polymerase to begin transcription.

The operator is another noncoding regulatory sequence. It acts like a control point. If a repressor protein binds to the operator, it can block RNA polymerase from transcribing the structural genes.

> **Key Term: operator**
> A noncoding DNA sequence in an operon that serves as a binding site for a repressor protein.

The structural genes code for proteins that perform the function of the operon. In the lac operon, the structural genes code for proteins involved in lactose digestion, including beta-galactosidase and lactose permease. In the trp operon, the structural genes code for enzymes needed to synthesize the amino acid tryptophan.

> **Key Term: structural genes**
> Coding sequences in an operon that contain the information for proteins needed to carry out the operon’s function.

A useful AP Biology shortcut is to ask what would benefit the bacterium. If the operon breaks down a nutrient, the bacterium should turn it on only when that nutrient is present. If the operon makes a molecule, the bacterium should turn it off when enough of that molecule is already available.

> **Test Tip:**
> When you see an unfamiliar operon on the exam, first identify the function. If it breaks down a substance, think inducible. If it makes a substance, think repressible. Then use the information in the question to confirm your reasoning.

### Inducible operons and the lac operon

The lac operon is an inducible operon. It is usually off, but it can be turned on when a specific molecule is present. The lac operon helps bacteria digest lactose, a sugar. If lactose is absent, making lactose-digesting enzymes would waste energy. If lactose is present, making those enzymes is useful.

> **Key Term: inducible operon**
> An operon that is usually off but can be turned on when an inducer molecule is present.

In the absence of lactose, the lac repressor binds to the operator. This blocks RNA polymerase from transcribing the structural genes. As a result, the operon is off, and the cell makes little or none of the proteins needed to digest lactose.

When lactose is present, it acts as an inducer. In many AP Biology questions, lactose is described as the inducer; more precisely, a lactose-derived molecule called allolactose binds the repressor. The inducer changes the shape of the lac repressor so that the repressor can no longer bind effectively to the operator. RNA polymerase can then transcribe the structural genes.

> **Key Term: inducer**
> A molecule that turns on an inducible operon by binding to a repressor and preventing it from blocking transcription.

This is negative regulation because a repressor protein prevents transcription. The inducer does not directly “push” RNA polymerase forward. Instead, it removes the block. Once lactose is used up, the inducer is no longer available, the repressor can bind the operator again, and the operon turns off.

The lac operon can be summarized in two key conditions:

- No lactose: repressor binds operator, RNA polymerase is blocked, lac operon is off.
- Lactose present: inducer binds repressor, repressor leaves operator, RNA polymerase transcribes structural genes, lac operon is on.

This mechanism is a feedback system. The cell produces lactose-digesting enzymes only when lactose is available. After lactose is digested, transcription decreases again.

> **Exam Warning:**
> Do not say that lactose binds to the operator. The inducer binds to the repressor protein. The repressor protein binds to the operator.

The lac operon is also affected by glucose. Bacteria prefer glucose because it is easier to use as an energy source. When glucose is abundant, there is less need to break down lactose, even if lactose is present. When glucose is low, the cell increases expression of genes that help use other sugars.

This involves cyclic AMP, often called cAMP, and catabolite activator protein, often called CAP. When glucose levels are low, cAMP levels rise. cAMP binds CAP, activating it. The CAP-cAMP complex binds near the lac promoter and increases RNA polymerase binding. This increases transcription of the lac operon.

> **Key Term: CAP-cAMP complex**
> An activator complex that forms when glucose is low and increases lac operon transcription by helping RNA polymerase bind near the promoter.

The highest transcription of the lac operon occurs when lactose is present and glucose is low. Lactose removes the repressor, and low glucose causes CAP-cAMP activation. Both conditions favor transcription.

If lactose is absent, the operon remains off or nearly off, even if glucose is low, because the repressor is still bound to the operator. If lactose is present but glucose is high, the repressor is removed, but CAP-cAMP activation is weak, so transcription is lower than the maximum level.

For AP Biology, remember these lac operon outcomes:

- Low lactose, high glucose: operon off.
- Low lactose, low glucose: operon off or very low, because the repressor remains bound.
- High lactose, high glucose: operon on at a moderate or lower level.
- High lactose, low glucose: operon on at the highest level.

> **Test Tip:**
> For lac operon questions, answer in two steps: first check lactose to decide whether the repressor is removed; then check glucose to decide whether CAP-cAMP boosts transcription.

### Repressible operons and the trp operon

The trp operon is a repressible operon. It is usually on, but it can be turned off when the product of the pathway is abundant. The trp operon codes for enzymes that synthesize tryptophan, an amino acid.

> **Key Term: repressible operon**
> An operon that is usually on but can be turned off when a corepressor molecule allows a repressor to block transcription.

When tryptophan is absent or scarce, the bacterium needs to make tryptophan. In that condition, the trp repressor alone cannot bind effectively to the operator. RNA polymerase can bind the promoter and transcribe the structural genes. The resulting enzymes help synthesize tryptophan.

When tryptophan is abundant, the bacterium does not need to spend energy making more. Tryptophan acts as a corepressor. It binds to the trp repressor and changes the repressor’s shape. The repressor-corepressor complex can then bind to the operator and block transcription.

> **Key Term: corepressor**
> A molecule that binds to a repressor protein and enables the repressor to bind to the operator, reducing transcription.

The trp operon can be summarized in two key conditions:

- No tryptophan: repressor cannot bind operator, RNA polymerase transcribes structural genes, trp operon is on.
- Tryptophan present: tryptophan binds repressor as a corepressor, the repressor-corepressor complex binds operator, transcription stops, trp operon is off.

This is also a feedback system. The final product of the pathway, tryptophan, shuts down further production of the enzymes that make tryptophan. This prevents waste.

The trp operon is a strong example of an anabolic pathway being regulated. Anabolic pathways build molecules. If the product is already available, the cell can conserve resources by stopping enzyme production.

> **Exam Warning:**
> Do not confuse an inducer with a corepressor. An inducer removes or inactivates a repressor, turning transcription on. A corepressor helps a repressor bind, turning transcription off.

### Comparing lac and trp logic on exam questions

The lac and trp operons are often tested together because they show opposite regulatory patterns. The lac operon is inducible and catabolic. It breaks down lactose, so it should be on only when lactose is available. The trp operon is repressible and anabolic. It makes tryptophan, so it should be on when tryptophan is unavailable and off when tryptophan is abundant.

In an inducible operon like lac, the default state is off because the repressor can bind the operator by itself. The inducer changes the repressor so it cannot bind the operator. This allows transcription.

In a repressible operon like trp, the default state is on because the repressor cannot bind the operator by itself. The corepressor binds the repressor and allows it to bind the operator. This stops transcription.

Think of the repressor as the immediate switch at the operator:

- Repressor on operator means transcription is blocked.
- Repressor not on operator means RNA polymerase can transcribe, assuming other required conditions are met.

For unfamiliar AP examples, the exam may give you an operon such as the arabinose operon or an operon involved in making serine or lysine. You are expected to apply the same principles.

For example, suppose a bacterial operon codes for enzymes needed to digest the sugar arabinose, and arabinose is described as an inducer. If arabinose is present, it binds the repressor and prevents the repressor from blocking transcription. The operon turns on, and arabinose digestion increases.

Now suppose a bacterial operon codes for enzymes needed to synthesize the amino acid serine, and serine is described as a corepressor. If serine is present, it binds the repressor and helps the repressor bind to the operator. The operon turns off, reducing further serine synthesis.

> **Test Tip:**
> In free-response answers, connect molecules to mechanisms. Instead of writing “the operon turns off,” write “the corepressor binds the repressor, allowing the repressor to bind the operator and block RNA polymerase.”

AP questions also often ask which sequences are coding or noncoding. Promoters and operators are noncoding regulatory DNA sequences. They affect transcription, but they do not code for proteins. Structural genes are coding sequences. Regulatory genes are also coding sequences because they code for regulatory proteins, such as repressors.

A frequent multiple-choice trap is to treat all parts of an operon as genes. The promoter and operator are not genes in the sense of coding for proteins. They are DNA control regions.

Another common trap is to assume all regulation is negative. The lac operon includes both negative regulation and positive regulation. The lac repressor is negative because it blocks transcription. CAP-cAMP is positive because it increases RNA polymerase binding and raises transcription levels.

When writing AP Biology explanations, use cause-and-effect language:

- “If lactose is absent, the repressor binds the operator, so RNA polymerase cannot transcribe the lac structural genes.”
- “If lactose is present, the inducer binds the repressor, so the repressor cannot bind the operator, allowing transcription.”
- “If tryptophan is present, it acts as a corepressor, allowing the repressor to bind the operator and stop transcription.”
- “If glucose is low, cAMP increases and activates CAP, increasing transcription of the lac operon when lactose is also present.”

This style earns points because it links the environmental signal to the regulatory molecule, the DNA binding event, and the transcription outcome.

## Key Point Checklist

This article has covered the following key knowledge points:

- Operons are common in prokaryotes and allow related genes to be regulated together.
- A promoter is a noncoding DNA sequence where RNA polymerase binds.
- An operator is a noncoding DNA sequence where a repressor can bind.
- Structural genes code for proteins that carry out the operon’s function.
- Regulatory genes code for regulatory proteins such as repressors.
- The lac operon is an inducible operon involved in lactose breakdown.
- In the lac operon, lactose or a lactose-derived inducer prevents the repressor from binding the operator.
- The lac operon has highest transcription when lactose is present and glucose is low.
- Low glucose increases cAMP, allowing CAP-cAMP to increase RNA polymerase binding.
- The trp operon is a repressible operon involved in tryptophan synthesis.
- In the trp operon, tryptophan acts as a corepressor that helps the repressor bind the operator.
- Inducible operons are often catabolic, while repressible operons are often anabolic.

## Key Terms and Concepts

- operon
- regulatory gene
- promoter
- operator
- structural genes
- inducible operon
- inducer
- CAP-cAMP complex
- repressible operon
- corepressor


Regulation of gene expression - Operons (lac, trp)