Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

16. Regulation of Expression

Prokaryotic Gene Regulation via Operons

General Biology

16. Regulation of Expression

Prokaryotic Gene Regulation via Operons

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4:48 minutes

Problem 13

Textbook Question

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

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.

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.

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.

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.

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.

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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. 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.

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Textbook Question

A regulon is a set of genes controlled by

a. One type of regulator of transcription

b. Two or more different alternative sigma proteins

c. Many different types of promoters

d. Glucose

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Textbook Question

What would occur if the repressor of an inducible operon were mutated so it could not bind the operator?

a. Irreversible binding of the repressor to the promoter

b. Reduced transcription of the operon's genes

c. Buildup of a substrate for the pathway controlled by the operon

d. Continuous transcription of the operon's genes

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Textbook Question

Explain why it makes sense for the lexA regulatory gene of the SOS regulon to be expressed constitutively.

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Textbook Question

Explain why it makes sense for the lexA regulatory gene of the SOS regulon to be expressed constitutively.

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Textbook Question

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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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

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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