Textbook Question
What properties demonstrate that the lac repressor is a protein? Describe the evidence that it indeed serves as a repressor within the operon system.
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Ch. 15 - Regulation of Gene Expression in Bacteria
Klug10th EditionEssentials of GeneticsISBN: 9780135588789
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Table of contents
- Ch. 1 - Introduction to Genetics16
- Ch. 2 - Mitosis and Meiosis28
- Ch. 3 - Mendelian Genetics37
- Ch. 4 - Modification of Mendelian Ratios53
- Ch. 5 - Sex Determination and Sex Chromosomes32
- Ch. 6 - Chromosome Mutations: Variation in Number and Arrangement37
- Ch. 7 - Linkage and Chromosome Mapping in Eukaryotes36
- Ch. 8 - Genetic Analysis and Mapping in Bacteria and Bacteriophages24
- Ch. 9 - DNA Structure and Analysis38
- Ch. 10 - DNA Replication29
- Ch. 11 - Chromosome Structure and DNA Sequence Organization22
- Ch. 12 - The Genetic Code and Transcription36
- Ch. 13 - Translation and Proteins27
- Ch. 14 - Gene Mutation, DNA Repair, and Transposition34
- Ch. 15 - Regulation of Gene Expression in Bacteria22
- Ch. 16 - Regulation of Gene Expression in Eukaryotes37
- Ch. 17 - Recombinant DNA Technology27
- Ch. 18 - Genomics, Bioinformatics, and Proteomics30
- Ch. 19 - The Genetics of Cancer21
- Ch. 20 - Quantitative Genetics and Multifactorial Traits37
- Ch. 21 - Population and Evolutionary Genetics45
Chapter 15, Problem 12
Describe the role of attenuation in the regulation of tryptophan biosynthesis.
Verified step by step guidance1
Understand that attenuation is a regulatory mechanism used by bacteria to control gene expression, particularly in operons like the tryptophan (trp) operon, which is involved in tryptophan biosynthesis.
Recognize that attenuation occurs at the level of transcription termination, where the formation of specific secondary structures in the mRNA leader sequence determines whether transcription proceeds or terminates prematurely.
Identify that the trp operon leader sequence contains a leader peptide coding region with consecutive tryptophan codons, which acts as a sensor for tryptophan availability through the ribosome's translation process.
Explain that when tryptophan levels are high, the ribosome quickly translates the leader peptide, allowing the formation of a terminator hairpin structure in the mRNA that causes RNA polymerase to stop transcription (attenuation).
Conversely, when tryptophan is scarce, the ribosome stalls at the tryptophan codons, leading to the formation of an alternative anti-terminator hairpin structure that allows transcription of the downstream genes necessary for tryptophan synthesis.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Attenuation Mechanism
Attenuation is a regulatory mechanism in prokaryotes where transcription is prematurely terminated based on the formation of specific RNA secondary structures. It allows fine-tuning of gene expression in response to metabolite levels, such as amino acids, by controlling whether RNA polymerase continues transcription.
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Trp Attenuation
Tryptophan Operon Structure
The tryptophan operon contains genes needed for tryptophan synthesis and includes a leader sequence with a short peptide coding region. This leader region plays a critical role in attenuation by sensing tryptophan levels through ribosome stalling during translation of the leader peptide.
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09:11Trp Attenuation
Coupling of Transcription and Translation
In bacteria, transcription and translation occur simultaneously, allowing the ribosome translating the leader peptide to influence RNA secondary structure formation. When tryptophan is abundant, the ribosome quickly translates the leader peptide, causing formation of a terminator hairpin and halting transcription; when scarce, ribosome stalling leads to an anti-terminator structure, allowing gene expression.
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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
A bacterial operon is responsible for the production of the biosynthetic enzymes needed to make the hypothetical amino acid tisophane (tis). The operon is regulated by a separate gene, R. The deletion of R causes the loss of enzyme synthesis. In the wild-type condition, when tis is present, no enzymes are made; in the absence of tis, the enzymes are made. Mutations in the operator gene (O⁻) result in repression regardless of the presence of tis. Is the operon under positive or negative control? Propose a model for:
(a) Repression of the genes in the presence of tis in wild-type cells
(b) The mutations.
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