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Ch. 18 - Control of Gene Expression in Bacteria
All textbooksFreeman 8th EditionCh. 18 - Control of Gene Expression in BacteriaProblem 13
Chapter 17, Problem 13

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.

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The lux operon in V. fischeri includes genes essential for bioluminescence (luxCDABE) and a gene (luxI) that produces an inducer molecule. This inducer can diffuse freely across the cell membrane, allowing it to move between cells in the environment.
LuxI catalyzes the production of the inducer molecule. As the population of V. fischeri increases, the concentration of the inducer in the environment also increases because more cells are producing the inducer.
The inducer molecules bind to the LuxR protein, which is coded by the luxR gene. When the inducer binds to LuxR, it forms a LuxR-inducer complex.
This LuxR-inducer complex can then bind to a regulatory site on the DNA, leading to the activation of the transcription of the lux operon. This results in the production of the enzymes necessary for light production (bioluminescence).
The system is designed such that significant light production only occurs at high cell densities. This is because a high concentration of the inducer (reflecting high cell density) is necessary to form enough LuxR-inducer complexes to activate the lux operon effectively. This mechanism of gene regulation in response to cell density is known as quorum sensing.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
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Lux Operon

The lux operon is a set of genes in V. fischeri responsible for bioluminescence, including luxCDABE, which encodes proteins necessary for light production. The operon is regulated by the LuxR protein and its inducer, allowing for the activation of luminescence genes when conditions are favorable, such as high cell density, ensuring that light production is a collective response rather than an individual one.
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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 transcription of the lux operon, it simultaneously inhibits the transcription of luxR. This ensures that as luminescence increases, the production of the activator LuxR is downregulated, maintaining a balance in gene expression and preventing overproduction.
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Related Practice
Textbook Question

In a mutant that lacks adenylyl cyclase, the enzyme that synthesizes cAMP, predict which of the following conditions of extracellular lactose and glucose would cause regulation of the lac operon to differ from that of wild-type cells. a. no lactose, no glucose b. no lactose, abundant glucose c. abundant lactose, no glucose d. abundant lactose, abundant glucose

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Textbook Question

X-gal is a colorless, lactose-like molecule that can be split into two fragments by ββ-galactosidase. One of these product molecules creates a blue color. The photograph here shows E. coli colonies growing in a medium that contains X-gal. Find three colonies whose cells have functioning copies of ββ-galactosidase. Find three colonies whose cells might have mutations in the lacZ or the lacY genes. Suppose you analyze the protein-coding sequence of the lacZ and lacY genes of cells from the three mutant colonies and find that these sequences are wild type (normal). What other region of the lac operon might be altered to account for the mutant phenotype of these colonies?

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Textbook Question

The light-producing genes of V. fischeri are organized in an operon that is under positive control by an activator protein called LuxR. Would you expect the genes of this operon to be transcribed when LuxR is bound or not bound to a DNA regulatory sequence? Explain.

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Textbook Question

LuxR is allosterically regulated by the inducer molecule secreted by V. fischeri. What does it mean that LuxR is allosterically regulated?

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Textbook Question

What characteristic of the light-producing regulatory circuit is consistent with the idea that it may be a regulon? What characteristic of this circuit stretches the definition for a regulon?

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Textbook Question

Quorum sensing (introduced in Ch. 11, Section 11.4) allows bacteria to detect the number of neighboring cells and to trigger a response only when this number reaches a critical level. Quorum sensing is used by V. fischeri in light production and by many pathogenic bacteria, including Vibrio cholerae, to turn on genes for toxin production only when a critical cell density is reached. Why might quorum sensing be beneficial to pathogenic bacteria?

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