VCE Biology (Year 12) · Nucleic Acids and Proteins · 12m read

Regulation of Gene Expression (trp Operon)

Ever wondered how bacteria know when to stop making a chemical they don't need? The trp operon is a cluster of genes that controls tryptophan production in bacteria—when tryptophan is already available, the operon shuts down to save energy. This repressible system shows you how cells are smart managers of their resources, and understanding it is essential for VCE Biology since examiners expect you to explain this elegant mechanism in detail.

Learning objective: Describe how the trp operon regulates tryptophan synthesis in prokaryotes via a repressible operon mechanism.

By the end of this lesson you can:

• I distinguish between promoter, operator and structural genes.
• I explain the role of the trp repressor and tryptophan as the corepressor.
• I compare repressible operons with inducible operons.

The trp operon is a dimmer switch. When tryptophan is plentiful, the repressor turns production down; when scarce, it turns it up.

The trp operon: repressible control

An operon is a group of functionally related genes under the control of a single promoter and operator — found in prokaryotes like E. coli. The trp operon controls five structural genes that code for enzymes making the amino acid tryptophan.

When tryptophan is LOW: the trp repressor protein (made separately from the operon itself) is inactive. RNA polymerase binds the promoter, transcribes the structural genes, and the cell produces more tryptophan.

When tryptophan is HIGH: excess tryptophan binds the trp repressor, activating it. The activated repressor binds the operator, blocking RNA polymerase. The operon is switched OFF — saving resources.

This is a "repressible" operon — normally ON, switched OFF by the end-product. Contrast the lac operon (for lactose metabolism), which is normally OFF and switched ON by the substrate — that's "inducible".

Operon components

• Promoter: DNA sequence where RNA polymerase binds.
• Operator: DNA switch; repressor binds here to block transcription.
• Structural genes: Code for the functional enzymes.
• trp repressor: Protein that blocks the operator when activated.
• Corepressor: Tryptophan — activates the repressor when abundant.

Quick reference

| Condition | Result |
|---|---|
| High tryptophan | Repressor active → genes OFF |
| Low tryptophan | Repressor inactive → genes ON |
| Attenuation | Fine control via leader sequence |
| Structural genes | Enzymes for tryptophan synthesis |

Worked example (Medium)

In the trp operon, describe what happens when tryptophan levels are high.

1. Tryptophan binds repressor. Acts as corepressor.
2. Repressor becomes active. Changes shape.
3. Active repressor binds operator. Physically blocks RNA polymerase.
4. Transcription is blocked. Structural genes not transcribed.
5. No new tryptophan enzymes made. Cell stops producing tryptophan.

Answer: High tryptophan activates the repressor, which binds the operator and switches OFF the operon. This conserves cellular resources by halting tryptophan synthesis.

Worked example (Hard)

In the trp operon, predict what happens if a mutation prevents the repressor protein from binding tryptophan.

1. Normal function. When tryptophan is abundant, it binds the repressor (co-repressor), activating it so it binds the operator and blocks transcription.
2. Mutant repressor. If the repressor cannot bind tryptophan, it remains inactive regardless of tryptophan levels.
3. Consequence. The trp operon genes are transcribed continuously — tryptophan biosynthesis enzymes are always produced, even when tryptophan is plentiful.
4. Biological cost. The cell wastes energy producing unnecessary enzymes. This is metabolically costly and would be selected against.

Answer: Constitutive expression of the trp operon — tryptophan synthesis enzymes are always produced regardless of tryptophan levels, wasting cellular resources.

Common mistakes

• Saying the repressor "turns on" the operon The repressor BLOCKS transcription — it turns the operon OFF, not on. Fix: Repressor + co-repressor (tryptophan) → binds operator → blocks RNA polymerase → operon OFF. No repressor binding → operon ON.
• Confusing inducible and repressible operons The lac operon is inducible (turned on by lactose). The trp operon is repressible (turned off by tryptophan). Fix: Inducible: default OFF, inducer turns it ON (lac). Repressible: default ON, co-repressor turns it OFF (trp).
• Forgetting that gene regulation saves energy The purpose of regulation is efficiency — producing enzymes only when needed. Fix: Always explain WHY regulation matters: producing unnecessary proteins wastes ATP and amino acids. Regulated cells outcompete constitutive ones.

Epigenetics — gene regulation beyond the DNA sequence

While the trp operon demonstrates gene regulation through transcription factors, eukaryotic gene regulation is far more complex. Epigenetics — changes in gene expression that do not alter the DNA sequence — adds another layer of control.

DNA methylation: Adding methyl groups (-CH₃) to cytosine bases in DNA typically silences gene expression. Heavily methylated genes are not transcribed. This is how different cell types (neurons, muscle cells, skin cells) express different genes despite having identical DNA sequences.

Histone modification: DNA wraps around histone proteins to form chromatin. When histones are acetylated (acetyl groups added), chromatin loosens and genes become accessible for transcription. When histones are deacetylated, chromatin condenses and genes are silenced.

Why epigenetics matters for VCE: (1) It explains cell differentiation — how one genome produces hundreds of cell types. (2) It connects to disease — cancer often involves aberrant DNA methylation or histone modification. (3) Some epigenetic changes can be inherited across generations without changing DNA sequence, challenging the traditional view that only DNA sequence is heritable.

Environmental influence: Diet, stress, toxins, and exercise can alter epigenetic marks. Studies on the Dutch Hunger Winter (1944-45) showed that prenatal famine exposure altered DNA methylation patterns that were still detectable 60 years later — and some effects appeared in the next generation.

Practice questions

1. In the trp operon, the corepressor is:

   1. Tryptophan
   2. The repressor protein
   3. RNA polymerase
   4. Lactose

   (2 marks)

2. The trp operon is classified as repressible because:

   1. It is switched OFF in response to end-product accumulation
   2. It is switched ON by a substrate
   3. It is always on
   4. It codes for a repressor

   (3 marks)

3. Explain the advantage to a bacterium of regulating the trp operon.

   (3 marks)

Summary

• Distinguish between promoter, operator and structural genes.
• Explain the role of the trp repressor and tryptophan as the corepressor.
• Compare repressible operons with inducible operons.