The lac operon and trp operon are classic examples of how bacteria control gene expression to save energy and respond to the environment. This cheat sheet compares how each operon turns genes on or off using repressors, inducers, corepressors, and regulatory DNA sequences. Students need this comparison because the two systems are often tested together and can be confusing when the signals and outcomes are mixed up.
Key Facts
- The lac operon is inducible, meaning it is usually off but can be turned on when lactose is present.
- The trp operon is repressible, meaning it is usually on but can be turned off when tryptophan is abundant.
- In the lac operon, lactose is converted to allolactose, and allolactose binds the lac repressor so it releases the operator.
- In the trp operon, tryptophan acts as a corepressor by binding the trp repressor, allowing the repressor to bind the operator.
- RNA polymerase begins transcription at the promoter, while the operator is the DNA region where a repressor can block transcription.
- High glucose lowers cAMP, which reduces CAP binding and decreases lac operon transcription even if lactose is present.
- Low glucose raises cAMP, and the CAP-cAMP complex helps RNA polymerase strongly transcribe the lac operon when lactose is present.
- Attenuation in the trp operon uses the speed of ribosome movement on the leader sequence to fine-tune transcription based on tryptophan levels.
Vocabulary
- Operon
- An operon is a cluster of bacterial genes controlled together by one promoter and regulatory DNA sequences.
- Promoter
- A promoter is the DNA sequence where RNA polymerase binds to start transcription.
- Operator
- An operator is a regulatory DNA sequence where a repressor protein can bind to block transcription.
- Inducer
- An inducer is a molecule that turns on gene expression by inactivating a repressor or helping an activator work.
- Corepressor
- A corepressor is a molecule that helps a repressor bind DNA and turn off transcription.
- Attenuation
- Attenuation is a bacterial control mechanism that stops transcription early based on conditions during translation.
Common Mistakes to Avoid
- Calling the lac operon repressible is wrong because it is normally off and is turned on by lactose through allolactose.
- Calling the trp operon inducible is wrong because it is normally on and is turned off when tryptophan activates the repressor.
- Forgetting glucose control in the lac operon is wrong because lactose alone does not guarantee maximum transcription when glucose is high.
- Saying repressors bind promoters is wrong because repressors usually bind operators, while RNA polymerase binds promoters.
- Confusing inducer and corepressor is wrong because an inducer usually increases transcription, while a corepressor helps shut transcription down.
Practice Questions
- 1 A bacterium has lactose present and glucose absent. Predict the lac operon transcription level and explain the roles of allolactose and CAP-cAMP.
- 2 A bacterium has high tryptophan levels. Predict whether the trp operon is on or off and describe what happens to the trp repressor.
- 3 In a lac operon mutant, the operator cannot bind the lac repressor. Predict lac gene expression when lactose is absent.
- 4 Explain why the lac operon is useful for digesting a nutrient from the environment, while the trp operon is useful for conserving resources when an amino acid is already available.
Understanding Lac Operon and Trp Operon Comparison
An operon works like a shared control panel for several genes. One promoter can direct production of a single RNA molecule that carries instructions for multiple proteins. This arrangement is useful when those proteins take part in the same pathway.
The operator sits close enough to the promoter that a bound repressor physically interferes with RNA polymerase. This is negative control because the regulator prevents transcription. The lac system has a second layer called positive control.
CAP bound to cyclic AMP helps polymerase grip the promoter more effectively. Therefore, removing the repressor does not always mean the genes are expressed at a high rate. Bacteria give their strongest response when the needed sugar is available and the preferred energy source is scarce.
The two operons manage different kinds of cell economy. Lactose is a food source that must be broken down before it can be used. Producing lactose-processing enzymes without lactose would waste materials.
Tryptophan is a building block for proteins, so cells need a steady supply of it. Making more tryptophan when plenty is already present would waste energy. The trp repressor is inactive by itself.
Its shape changes only after tryptophan binds to it. This is an example of feedback control.
The final product of a pathway helps stop production of more of that same product. Similar feedback systems occur in human metabolism, although they often use enzymes rather than bacterial operons.
Attenuation gives the trp operon a faster adjustment than repressor binding alone. It happens while RNA polymerase is still making the beginning of the RNA. In bacteria, a ribosome can begin translating that RNA before transcription has finished.
The leader region contains a short peptide sequence with tryptophan codons. When tryptophan is scarce, the ribosome pauses because charged transfer RNA carrying tryptophan is hard to obtain. This pause allows the RNA to fold in a way that permits transcription to continue.
When tryptophan is plentiful, the ribosome moves quickly. A different RNA fold forms and causes polymerase to stop early. This mechanism depends on transcription and translation occurring in the same cell space, which is possible in bacteria but not in eukaryotic cells with a nucleus.
When studying these systems, track the condition, the regulatory protein, and the result separately. A common mistake is to see lactose present and assume maximum lac transcription. Glucose level still matters because it changes cyclic AMP and CAP activity.
Another mistake is to treat tryptophan as an inducer. Its role is the opposite because it activates the trp repressor. Practice predicting what happens after mutations.
A broken operator can prevent repressor binding, causing transcription even when it should be blocked. A defective repressor may have a similar effect, but the mutation affects a protein rather than a DNA site. These distinctions help with genetics problems that describe bacterial growth, enzyme production, or altered DNA sequences.