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 concept of quorum sensing: Quorum sensing is a mechanism by which bacteria communicate and coordinate behavior based on their population density. In this case, V. fischeri uses quorum sensing to regulate light production.
2. Identify the role of the inducer: The inducer is a signaling molecule produced by the enzyme encoded by the luxI gene. This inducer can diffuse across the plasma membrane and accumulate both inside and outside the cell.
3. Explain the accumulation of the inducer: At low cell density, the concentration of the inducer is low because it diffuses away into the environment. However, as the bacterial population increases, the concentration of the inducer in the environment also increases, leading to a higher concentration of the inducer inside the cells.
4. Describe the activation of the lux operon: When the inducer concentration reaches a threshold level, it binds to the LuxR protein, forming the LuxR–inducer complex. This complex binds to the regulatory site of the lux operon, activating its transcription. This leads to the production of luminescence genes (luxCDABE) and more inducer (positive feedback).
5. Explain the high cell density requirement: The system ensures that light production only occurs at high cell density because the inducer concentration must reach a critical threshold to activate the lux operon. This prevents energy waste by producing light only when there are enough bacteria to make the light visible.

Key Concepts

Quorum Sensing

Quorum sensing is a communication mechanism used by bacteria to coordinate behavior based on population density. When the cell density is high, the concentration of signaling molecules, such as the inducer produced by luxI, increases. This allows bacteria to collectively regulate gene expression, such as the lux operon, leading to behaviors like bioluminescence only when a sufficient number of cells are present.

Lux Operon

The lux operon is a set of genes in V. fischeri that are responsible for bioluminescence. It includes genes that encode proteins necessary for light production (luxCDABE) and the luxI gene, which produces an inducer. The operon is regulated by the LuxR protein, which, when activated by the inducer, enhances the transcription of the lux genes, allowing the bacteria to emit light in response to 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 operon, it simultaneously inhibits the transcription of luxR. This ensures that as luminescence increases, the production of the activator LuxR decreases, creating a balance that prevents overexpression of the luminescent genes.