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.

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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.

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.

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.

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).

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.

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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.

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.