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Ch. 16 - Regulation of Gene Expression in Bacteria
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 16 - Regulation of Gene Expression in BacteriaProblem 12
Chapter 16, Problem 12

Describe the role of attenuation in the regulation of tryptophan biosynthesis.

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1
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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Trp 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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Translation initiation
Related Practice
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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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.

951
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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
Attenuation of the trp operon was viewed as a relatively inefficient way to achieve genetic regulation when it was first discovered in the 1970s. Since then, however, attenuation has been found to be a relatively common regulatory strategy. Assuming that attenuation is a relatively inefficient way to achieve genetic regulation, what might explain its widespread occurrence?
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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

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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