The diagram shown here is a model of the gene regulatory circuit for light production by V. fischeri cells. The lux operon contains genes for luminescence (luxCDABE) and a gene, luxI, that encodes an enzyme that catalyzes the production of an inducer. This inducer easily moves back and forth across the plasma membrane and acts as a signaling molecule. The lux operon is never completely turned off. The luxR gene codes for the activator LuxR. The inducer can bind to LuxR, and when it does, the LuxR–inducer complex can bind to a regulatory site to activate transcription of the lux operon and inhibit transcription of luxR. Explain how this gene regulatory circuit accounts for bacteria emitting light only when they reach a high cell density.

Verified step by step guidance

1. Understand the components of the lux operon: The lux operon includes genes luxCDABE responsible for luminescence and luxI, which encodes an enzyme for producing an inducer molecule. LuxR is a protein coded by the luxR gene that acts as an activator in this system.
2. Recognize the role of the inducer: The inducer synthesized by the enzyme encoded by luxI can freely diffuse across the plasma membrane. This inducer is crucial as it serves as a signaling molecule within the bacterial community.
3. Examine the interaction between LuxR and the inducer: When the inducer binds to LuxR, it forms a LuxR-inducer complex. This complex is essential for the regulation of the lux operon.
4. Link the LuxR-inducer complex to gene expression: The LuxR-inducer complex binds to a regulatory site on the DNA, activating the transcription of the lux operon, which leads to light production. Simultaneously, it inhibits the transcription of the luxR gene, controlling the levels of LuxR.
5. Correlate cell density with light production: At low cell densities, the concentration of the inducer in the environment is too low to form sufficient LuxR-inducer complexes to trigger significant light production. As the cell density increases, the concentration of the inducer rises, leading to more LuxR-inducer complex formation, thereby activating the lux operon and resulting in luminescence. This system ensures that V. fischeri cells emit light predominantly at high cell densities.

Key Concepts

Quorum Sensing

Quorum sensing is a communication mechanism used by bacteria to coordinate behavior based on population density. Bacteria release signaling molecules, and as the cell density increases, the concentration of these molecules rises, allowing the bacteria to sense when they are in a crowded environment. This process enables them to activate specific genes, such as those involved in bioluminescence, only when a sufficient number of cells are present.

Lux Operon

The lux operon is a set of genes in Vibrio fischeri that are responsible for bioluminescence. It includes genes that encode proteins necessary for light production (luxCDABE) and luxI, which produces an inducer molecule. The operon is regulated by the LuxR protein, which interacts with the inducer to activate transcription of the lux genes, allowing the bacteria to emit light when conditions are favorable, such as at high cell density.

Feedback Inhibition

Feedback inhibition is a regulatory mechanism where the product of a process inhibits its own production. In the context of the lux operon, when the LuxR–inducer complex activates the transcription of the lux genes, it simultaneously inhibits the transcription of luxR. This ensures that as the population grows and light production increases, the levels of LuxR decrease, preventing overproduction and maintaining a balance in gene expression.