Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

12. Gene Regulation in Prokaryotes

Lac Operon

Genetics

12. Gene Regulation in Prokaryotes

Lac Operon

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Problem 15

Textbook Question

The creation of milk products such as cheeses and yogurts is dependent on the conversion by various anaerobic bacteria, including several Lactobacillus species, of lactose to glucose and galactose, ultimately producing lactic acid. These conversions are dependent on both permease and β-galactosidase as part of the lac operon. After selection for rapid fermentation for the production of yogurt, one Lactobacillus subspecies lost its ability to regulate lac operon expression [Lapierre, L., et al. (2002). J. Bacteriol. 184:928–935]. Would you consider it likely that in this subspecies the lac operon is on or off? What genetic events would likely contribute to the loss of regulation as described above?

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Understand the lac operon: The lac operon is a set of genes in bacteria that are involved in the metabolism of lactose. It includes structural genes (lacZ, lacY, and lacA), a promoter, an operator, and a regulatory gene (lacI). The operon is regulated by the presence or absence of lactose and glucose, and its expression is controlled by the lac repressor and catabolite activator protein (CAP).

Determine the likely state of the lac operon: In the described Lactobacillus subspecies, the lac operon is likely to be 'on' continuously. This is because the bacteria have lost the ability to regulate the operon, which suggests that the mechanisms for turning it 'off' in the absence of lactose are no longer functional.

Identify genetic events that could lead to loss of regulation: A mutation in the lacI gene (which encodes the lac repressor) could prevent the repressor protein from binding to the operator, resulting in constitutive expression of the operon. Alternatively, a mutation in the operator sequence itself could prevent the repressor from binding, leading to the same outcome.

Consider other possible mutations: A mutation in the promoter region could lead to constitutive expression if it enhances RNA polymerase binding. Additionally, mutations in regulatory pathways, such as those affecting CAP or cAMP levels, could disrupt the normal regulation of the operon.

Relate the loss of regulation to the selection process: The selection for rapid fermentation in yogurt production likely favored mutations that resulted in constitutive expression of the lac operon. This would allow the bacteria to continuously metabolize lactose without the need for regulatory control, enhancing their efficiency in producing lactic acid.

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Key Concepts

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

Lac Operon

The lac operon is a set of genes in bacteria that are involved in the metabolism of lactose. It includes genes for permease, which facilitates lactose entry into the cell, and β-galactosidase, which breaks down lactose into glucose and galactose. The operon is regulated by the presence or absence of lactose, allowing bacteria to efficiently use this sugar when available. Understanding its regulation is crucial for analyzing how bacteria adapt to their environments.

Anaerobic Fermentation

Anaerobic fermentation is a metabolic process that occurs in the absence of oxygen, allowing organisms like Lactobacillus to convert sugars into energy. During this process, lactose is fermented into lactic acid, which is essential for producing dairy products like yogurt and cheese. This pathway is vital for understanding how certain bacteria thrive in specific environments and how they contribute to food production.

Gene Regulation and Mutations

Gene regulation refers to the mechanisms that control the expression of genes, ensuring that proteins are produced at the right time and in the right amounts. In the context of the lac operon, mutations can lead to a loss of regulation, causing the operon to be constitutively expressed (always 'on') or not expressed at all. Such genetic events can include deletions, point mutations, or changes in regulatory sequences, which can significantly impact the organism's metabolic capabilities.

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Related Practice

Textbook Question

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of:

(a) The CRP gene, which encodes the CAP protein

(b) The CAP-binding site within the promoter.

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Textbook Question

Erythritol, a natural sugar abundant in fruits and fermenting foods, is about 65 percent as sweet as table sugar and has about 95 percent fewer calories. It is 'tooth friendly' and generally devoid of negative side effects as a human consumable product. Pathogenic Brucella strains that catabolize erythritol contain four closely spaced genes, all involved in erythritol metabolism. One of the four genes (eryD) encodes a product that represses the expression of the other three genes. Erythritol catabolism is stimulated by erythritol. Present a simple regulatory model to account for the regulation of erythritol catabolism in Brucella. Does this system appear to be under inducible or repressible control?

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Textbook Question

Consider the transcription of genes of the lac operon under two conditions: (1) when both glucose and lactose are present and (2) when glucose is absent and lactose is present. Describe the comparative levels of transcription of lac operon genes under these conditions, and explain the molecular basis for the difference.

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Textbook Question

Neelaredoxin is a 15-kDa protein that is a gene product common in anaerobic bacteria. It has superoxide-scavenging activity, and it is constitutively expressed. In addition, its expression is not further induced during its exposure to O₂ or H₂O₂ [Silva, G. et al. (2001). J. Bacteriol. 183:4413 4420]. What do the terms constitutively expressed and induced mean in terms of neelaredoxin synthesis?

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of the CAP binding site of the lac promoter

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of the lacI gene affecting the DNA-binding site of the protein

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