6.10
Operon Model
The operon model represents a fundamental mechanism of gene regulation in prokaryotes, enabling coordinated expression of genes involved in related me…
An operon is a cluster of metabolically or functionally related structural genes transcribed under a common promoter.
The operator follows the promoter and binds regulatory proteins, controlling the expression of downstream structural genes.
Inducible operons are expressed only when a specific inducer molecule detaches the repressor bound to the operator.
The inducible lac operon in Escherichia coli has three structural genes involved in lactose metabolism. It is regulated by the LacI repressor.
In the absence of lactose, the operator remains LacI-bound, blocking transcription.
When lactose is available, a small amount of lactose is converted to allolactose, which inactivates LacI and induces transcription.
In contrast, the transcription of repressible operons is typically active unless the repressor binds to their operator.
The arginine operon is repressible. Its operator is open to bind RNA polymerase for active transcription till sufficient arginine is generated.
After arginine levels rise, arginine acts as a corepressor molecule and binds to the repressor.
The corepressor-repressor complex binds to the operator, blocking transcription.
View the full transcript and gain access to JoVE Core videos
Q1: What is an operon and how is it structured in bacteria?
An operon is a cluster of metabolically related structural genes transcribed together under a single promoter. The promoter facilitates RNA polymerase binding to initiate transcription. Downstream lies the operator, a DNA sequence binding regulatory proteins that control whether the structural genes are expressed or silenced.
Q2: How does the lac operon respond to lactose availability?
The lac operon in E. coli remains inactive when lactose is absent because the LacI repressor binds the operator, blocking transcription. When lactose is present, a small amount converts to allolactose, which inactivates LacI and releases it from the operator, allowing RNA polymerase to transcribe the three structural genes for lactose metabolism.
Q3: What distinguishes inducible operons from repressible operons?
Inducible operons like lac are normally off and activate only when an inducer molecule inactivates the repressor. Repressible operons like arginine are normally active but turn off when their end product accumulates. This allows bacteria to produce enzymes only when substrates are available or prevent overproduction of unnecessary metabolic products.
Q4: How does arginine act as a corepressor in the arginine operon?
When arginine levels rise sufficiently, arginine binds to the repressor protein, causing a conformational change. The arginine-repressor complex then binds the operator, blocking transcription and preventing excessive arginine synthesis. This mechanism conserves cellular resources by halting enzyme production when the end product is already abundant.
Q5: Why is the operon model an efficient regulatory system for bacteria?
The operon model enables bacteria to coordinate expression of functionally related genes and respond dynamically to environmental changes. Inducible operons allow adaptation to substrate availability, while repressible operons prevent wasteful synthesis of metabolic products. This efficient resource allocation is critical for prokaryotic survival in fluctuating environments.
Q6: What role do regulatory proteins play in operon function?
Regulatory proteins, such as repressors, bind to the operator region and control whether RNA polymerase can access the structural genes. These proteins respond to small effector molecules like allolactose or arginine, undergoing conformational changes that either block or permit transcription. This mechanism allows operons to integrate metabolic signals and adjust gene expression accordingly.
Q7: How does allolactose differ from lactose in lac operon regulation?
Lactose itself does not directly regulate the lac operon; instead, a small amount of lactose is converted to allolactose, the true inducer molecule. Allolactose binds to the LacI repressor, causing it to release from the operator and enabling transcription. This conversion ensures the operon responds specifically to lactose availability.