Regulation of gene expression: Operon concept, Transcription regulation in prokaryotes (lac operon and tryptophan operon)

 

Regulation of Gene Expression: Operon Concept

B.Sc. Zoology · Molecular Biology · Zoologys.co.in

UGC 4-Quadrant

Quadrant I · e-Text Content

Gene Regulation & the Operon Concept

Understand how prokaryotes control gene expression at the transcriptional level through elegant molecular switches.

✓ Define operon ✓ Explain lac operon induction ✓ Explain trp operon repression ✓ Compare inducible vs repressible ✓ Understand catabolite repression

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1.1 Introduction to Gene Regulation

Not all genes are expressed at all times. Organisms must regulate which genes are switched ON or OFF depending on environmental conditions, available nutrients, or developmental stage. This selective expression conserves energy and resources.

In prokaryotes (bacteria), gene regulation occurs primarily at the level of transcription initiation. The bacterium Escherichia coli has about 4,000 genes, but only a fraction are expressed at any given moment.

🔑 Key Concept Regulation of gene expression = controlling WHEN, WHERE, and HOW MUCH a gene product (mRNA → protein) is made.

⚡ Why it matters A bacterial cell making enzymes it doesn't need wastes ATP and amino acids. The operon system allows bacteria to produce metabolic enzymes only when required — an evolutionary advantage.

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1.2 The Operon Concept

The operon model was proposed by François Jacob and Jacques Monod in 1961, for which they received the Nobel Prize in Physiology or Medicine in 1965. An operon is a cluster of functionally related genes under the control of a single regulatory region.

📌 Definition of Operon An operon is a unit of gene expression consisting of one or more structural genes, an operator, and a promoter — all transcribed as a single polycistronic mRNA.

Components of an operon:

5'————————————————————————————————————————3' PROMOTER | OPERATOR | GENE 1 | GENE 2 | GENE 3 | TERMINATOR ↑ RNA Pol ↑ Repressor ←————— Structural Genes —————→

Regulatory Gene (i gene) Codes for the repressor protein. Located upstream; transcribed constitutively (always ON).

Promoter (P) Binding site for RNA polymerase. Transcription starts here.

Operator (O) DNA sequence where repressor binds to block transcription. The "switch."

Structural Genes Code for enzymes or proteins with related metabolic function. Transcribed together as polycistronic mRNA.

Types of operons based on regulation:

Inducible Operon Normally OFF. Switched ON in presence of an inducer molecule. Example: lac operon (induced by lactose/allolactose).

Repressible Operon Normally ON. Switched OFF when end-product (corepressor) accumulates. Example: trp operon (repressed by tryptophan).

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1.3 The Lac Operon — An Inducible System

The lac operon of E. coli encodes enzymes for lactose metabolism. It is expressed only when lactose is present AND glucose is absent. This demonstrates two levels of control: negative regulation (repressor) and positive regulation (CAP/CRP).

🧪 Structural Genes of the Lac Operon • lacZ → β-galactosidase (cleaves lactose → glucose + galactose)
• lacY → β-galactoside permease (transports lactose into cell)
• lacA → β-galactoside transacetylase (detoxification function)

Regulatory elements:

i gene CAP site P O lacZ lacY lacA —[i]———[CAP]——[P]—[O]——[lacZ]——[lacY]——[lacA]—— ↓ ↓ ↓ Repressor cAMP-CAP Blocked by repressor

Negative Control — The Repressor Mechanism:

Active Repressor (No Lactose) The lacI gene produces a repressor protein that binds to the operator, physically blocking RNA polymerase from transcribing the structural genes. The operon is OFF.

Inducer Action (Lactose Present) Lactose is converted to allolactose (the true inducer) by residual β-galactosidase. Allolactose binds to the repressor, causing a conformational change → repressor can no longer bind operator → RNA polymerase transcribes the operon → enzymes produced.

Positive Control — Catabolite Repression (CAP/CRP system):

Even when lactose is present, if glucose is also available, the lac operon is NOT fully expressed because glucose is the preferred carbon source. This is called catabolite repression.

How CAP works:
• Low glucose → Adenylyl cyclase active → High cAMP
• cAMP + CAP (Catabolite Activator Protein) → cAMP-CAP complex forms
• cAMP-CAP binds to CAP site upstream of promoter → stimulates RNA polymerase binding → maximum transcription
• High glucose → low cAMP → CAP cannot bind → low transcription

📊 Summary: Four States of Lac Operon 1. Glucose present, Lactose absent → OFF (repressor bound)
2. Glucose present, Lactose present → Low (inducer removes repressor; but low cAMP)
3. Glucose absent, Lactose absent → OFF (repressor bound; CAP active but no induction)
4. Glucose absent, Lactose present → MAXIMUM ON (repressor removed + CAP activates)

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1.4 The Tryptophan (trp) Operon — A Repressible System

The trp operon encodes enzymes for the biosynthesis of the amino acid tryptophan. Unlike the lac operon, it is normally ON and is turned OFF only when tryptophan is abundant. It employs two regulatory mechanisms: repression and attenuation.

🧪 Structural Genes of the Trp Operon • trpE + trpD → Anthranilate synthase complex
• trpC → Indole glycerol phosphate synthase
• trpB + trpA → Tryptophan synthase (α and β subunits)

trpR Leader Attenuator trpE trpD trpC trpB trpA —[trpR]——[P][O][L][att]——[E]—[D]—[C]—[B]—[A]— ↓ ↑ ↑ Aporepressor Operator Attenuation site

Mechanism 1 — Repression:

Aporepressor + Corepressor:
• The trpR gene makes an aporepressor (inactive form) that cannot bind the operator alone.
• When tryptophan is abundant, Trp acts as a corepressor — it binds the aporepressor, forming an active repressor complex.
• The repressor complex binds the operator → blocks RNA polymerase → operon switches OFF.

Mechanism 2 — Transcriptional Attenuation:

Even if some transcription occurs (repressor doesn't always bind), a second control operates in the leader sequence upstream of the structural genes.

Attenuation — Key Points:
• A leader sequence (162 nt) is transcribed before the structural genes
• The leader contains a short leader peptide with two Trp codons (UGG-UGG)
• The leader mRNA can form alternative secondary structures: terminator hairpin or anti-terminator hairpin
• High Trp: Ribosomes translate leader peptide rapidly → terminator hairpin forms → premature termination (attenuation occurs, operon OFF)
• Low Trp: Ribosomes stall at Trp codons → anti-terminator hairpin forms → transcription continues → operon ON

🔑 Significance of Attenuation Attenuation is a fine-tuning mechanism that responds directly to the availability of charged tRNA^Trp — making it a highly sensitive and rapid response system. It couples transcription and translation.

📊 Comparison: Lac vs Trp Operon

Feature	Lac Operon	Trp Operon
Type	Inducible	Repressible
Default state	OFF	ON
Function	Catabolic (breakdown)	Anabolic (biosynthesis)
Inducer/Corepressor	Allolactose (inducer)	Tryptophan (corepressor)
Repressor	Active repressor by itself; allolactose inactivates it	Aporepressor activated by Trp
Second control	Catabolite repression (CAP/cAMP)	Attenuation (leader sequence)
Positive regulation	Yes (cAMP-CAP)	No
Proposed by	Jacob & Monod (1961)	Yanofsky et al. (1970s)

Quadrant II · e-Tutorial / Interactive Simulations

Visualise & Simulate the Operons

Interact with animated diagrams. Toggle conditions and observe how the molecular switches respond in real time.

🎛️ Interactive controls 🔬 Step-by-step animations 📊 Visual comparisons

🥛 Lac Operon — Interactive Diagram

Click on each segment for details ↓

i gene

Regulatory gene — produces the lac repressor protein (LacI). Transcribed constitutively.

CAP

Catabolite Activator Protein binding site. cAMP-CAP complex binds here to stimulate transcription (positive regulation).

P

Promoter — RNA polymerase binding site. Transcription of lacZYA mRNA begins here.

O

Operator — repressor binding site. When repressor is bound here, RNA Pol CANNOT proceed.

lacZ

β-galactosidase — cleaves lactose into glucose + galactose. Also converts lactose → allolactose (the inducer).

lacY

Permease — membrane protein that transports lactose into the bacterial cell.

lacA

Transacetylase — transfers acetyl group; involved in detoxification of non-metabolizable sugars.

T

Terminator — signals the end of transcription. RNA polymerase detaches here.

Regulatory gene

Promoter/CAP

Operator

lacZ

lacY

lacA

🎛️

Lac Operon State Simulator

Select environmental conditions to see what happens inside the cell:

🔴 Operon: OFF

1

No lactose → no allolactose formed

2

Active repressor (LacI) binds to operator

3

RNA polymerase BLOCKED at operator

4

No mRNA → no enzymes synthesised

No energy wasted — bacteria do not produce enzymes for a substrate that isn't present.

🔴 Operon: OFF

1

Glucose is present — preferred carbon source

2

No lactose → repressor remains active, binds O

3

High glucose → low cAMP → CAP cannot bind

4

Doubly OFF: repressor blocking + no CAP activation

The bacterium uses glucose directly — no need to induce lactose enzymes.

🟢 Operon: MAXIMUM ON ⭐

1

Lactose enters cell → converted to allolactose

2

Allolactose binds LacI repressor → conformational change

3

Repressor released from operator

4

No glucose → adenylyl cyclase active → HIGH cAMP

5

cAMP + CAP → binds CAP site → activates RNA Pol

6

Full transcription of lacZYA → enzymes produced!

Both negative control (repressor removed) AND positive control (CAP active) work together — maximum expression!

🟡 Operon: LOW expression

1

Lactose present → allolactose inactivates repressor

2

Operator free → RNA Pol can bind promoter

3

BUT glucose present → adenylyl cyclase inhibited → LOW cAMP

4

CAP cannot bind CAP site → weak promoter activity

5

LOW level transcription only

Catabolite repression: bacteria prefer glucose, so even with lactose, expression is minimal while glucose lasts.

🔵 Trp Operon — Interactive Diagram

Hover on each segment for details ↓

trpR

Regulatory gene — produces the aporepressor (inactive). Cannot bind operator alone; needs tryptophan as corepressor.

P

Promoter — RNA polymerase binding site. Trp operon is active by default (unlike lac operon).

O

Operator — when Trp-aporepressor complex binds here, transcription is blocked.

L

Leader sequence (162 nt) — encodes leader peptide with 2 Trp codons. Site of attenuation control.

att

Attenuator — intrinsic terminator structure in leader mRNA. Premature termination site when Trp is abundant.

trpE

Anthranilate synthase component I — first enzyme in Trp biosynthesis pathway.

trpD

Anthranilate synthase component II — transfers amino group in the pathway.

trpC

InGPS — converts CDRP to indole glycerol phosphate.

trpB

Tryptophan synthase β-subunit — catalyses last step.

trpA

Tryptophan synthase α-subunit — works with trpB to form Trp from indole + serine.

T

Terminator

trpR (regulatory)

Promoter

Operator

Leader/Attenuator

Structural genes

🎛️

Trp Operon State Simulator

Toggle tryptophan levels to observe repression and attenuation:

🟢 Operon: ON — Trp synthesis proceeds

1

Low Trp → aporepressor remains inactive (no corepressor)

2

Operator free → RNA polymerase binds promoter

3

Ribosome translating leader peptide STALLS at Trp codons (UGG)

4

Anti-terminator hairpin (2:3) forms in leader mRNA

5

Terminator hairpin CANNOT form → read-through transcription

6

trpEDCBA transcribed → Trp biosynthesis enzymes made

Cell needs Trp → both repression and attenuation are INACTIVE → maximum gene expression.

🔴 Operon: OFF — Trp biosynthesis halted

1

High Trp → Trp binds aporepressor → active repressor formed

2

Active repressor binds operator → blocks RNA polymerase

3

If any transcription occurs, ribosomes translate leader rapidly

4

Ribosome reaches region 2 before region 3 is transcribed

5

Terminator hairpin (3:4) forms → premature termination

6

Structural genes NOT transcribed → no Trp enzymes made

Cell has enough Trp → two mechanisms (repression + attenuation) ensure NO wasteful enzyme synthesis.

🔵 Attenuation — The mRNA Hairpin Switch

The leader mRNA has 4 regions (1,2,3,4) that form alternative hairpin structures:

LOW Trp → Operon ON

Ribosome stalls at 1
↓
Region 2 pairs with 3
(anti-terminator)
↓
Region 4 = single strand
↓
Read-through ✓

HIGH Trp → Operon OFF

Ribosome rushes through 1&2
↓
Region 3 pairs with 4
(terminator hairpin)
↓
Rho-independent termination
↓
Premature stop ✗

Attenuation demonstrates transcription-translation coupling — unique to prokaryotes where both occur simultaneously in the cytoplasm.

Quadrant III · Self-Assessment

Test Your Understanding

MCQs, drag-and-drop labelling, and fill-in-the-blank exercises aligned to B.Sc. Zoology examination pattern.

📝 15 MCQs 🧩 Drag & Drop ✏️ Fill in Blanks

0/15

Quiz Result

Attempt all questions to see your score.

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Label the Lac Operon Components

Drag & Drop

Drag each label to its correct position in the operon:

Operator

Promoter

lacZ

lacY

i gene

CAP site

Regulatory gene:

cAMP-CAP binding:

RNA Pol binding:

Repressor binding:

β-galactosidase gene:

Permease gene:

✏️

Fill in the Blanks

Complete the sentence

Quadrant IV · Supplementary Resources

Further Reading & References

Curated textbooks, online resources, and revision materials to deepen your understanding.

📚 Recommended Textbooks

📘

Molecular Biology of the Gene

Watson JD et al. — The foundational text. Chapter on gene regulation in prokaryotes is essential reading.

Primary Reference

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Molecular Cell Biology

Lodish H et al. — Excellent diagrams of lac and trp operons; detailed mechanistic explanations.

Recommended

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Genetics — Lewin's Genes

Krebs JE et al. — Detailed attenuation mechanism; covers regulatory RNA and modern gene control.

Advanced

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Biochemistry — Stryer

Berg JM et al. — Excellent biochemical perspective on operon regulation and enzyme induction.

Supplementary

🌐 Online Resources

▶️

Khan Academy — Lac Operon

Free video lecture series explaining operon concept with animations. Highly recommended for visual learners.

Free Video

🔬

NCBI — Original Jacob & Monod Paper

Journal of Molecular Biology (1961). The historic paper proposing the operon model — available free on PubMed.

Free Access

🎓

NPTEL Molecular Biology

IIT/IISc lectures on gene regulation under e-Yantra programme — aligned with Indian University curriculum.

NPTEL

📰

Nature Education / Scitable

Short, peer-reviewed articles on operons and prokaryotic gene regulation. Good for quick revision.

Free Article

📝 Glossary of Key Terms

Operon

A cluster of genes with a common promoter and operator, transcribed as polycistronic mRNA.

Inducer

A small molecule (e.g., allolactose) that inactivates a repressor, allowing transcription of an inducible operon.

Corepressor

A small molecule (e.g., tryptophan) that activates a repressor by binding to it, turning off a repressible operon.

Operator

DNA sequence near the promoter where the repressor binds to block transcription.

Aporepressor

Inactive repressor protein that requires a corepressor to bind and block transcription.

Allolactose

The true inducer of the lac operon; an isomer of lactose formed by β-galactosidase activity.

CAP / CRP

Catabolite Activator Protein — binds cAMP and activates transcription when glucose is absent.

Attenuation

Premature transcription termination in the leader region of the trp operon when Trp is abundant.

Catabolite Repression

Suppression of non-glucose catabolic operons when glucose is available; mediated through cAMP-CAP.

Polycistronic mRNA

A single mRNA molecule encoding more than one protein; characteristic of prokaryotic operons.

Constitutive expression

Genes expressed continuously at a constant level regardless of environmental conditions.

β-galactosidase

Enzyme encoded by lacZ; cleaves β-galactosidic bonds; converts lactose to glucose + galactose.

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Important University Exam Questions

Short Answer (2–4 marks) 1. Define operon. Name its components.
2. What is the role of the operator in gene regulation?
3. Differentiate between inducer and corepressor.
4. What is allolactose? How is it formed?
5. Explain catabolite repression with reference to cAMP.

Long Answer (8–10 marks) 1. Describe the structure and regulation of the lac operon in E. coli with a labelled diagram.
2. Explain the trp operon as an example of a repressible operon. What is attenuation?
3. Compare and contrast the lac operon and trp operon with respect to their regulatory mechanisms.
4. Write an essay on positive and negative control of gene expression in prokaryotes.

Diagram-based Questions 1. Draw a neat labelled diagram of the lac operon showing all regulatory elements.
2. Draw the trp operon and illustrate the mechanism of attenuation.

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