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Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
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Tutti i libri di testoSanders 3rd EditionCh. 12 - Regulation of Gene Expression in Bacteria and BacteriophageProblema 25a
Capitolo 12, Problema 25a

What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.
Region 3 is deleted.

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Understand the role of the trpL region in attenuation control: The trpL region is part of the leader sequence of the trp operon and contains sequences that form stem-loop structures (regions 1-4). These structures are critical for regulating transcription termination based on tryptophan levels.
Identify the function of Region 3: Region 3 is involved in forming the antiterminator stem-loop (with Region 2) and the terminator stem-loop (with Region 4). The antiterminator prevents transcription termination, while the terminator promotes it.
Analyze the effect of deleting Region 3: Without Region 3, the antiterminator stem-loop cannot form, and the terminator stem-loop (Region 3-4 pairing) is also disrupted. This likely prevents attenuation control, as the operon cannot respond to tryptophan levels effectively.
Consider the transcriptional outcome: If attenuation control is lost, transcription of the trp operon genes may proceed unchecked, regardless of tryptophan levels. This could lead to overproduction of tryptophan biosynthesis enzymes even when tryptophan is abundant.
Summarize the reasoning: The deletion of Region 3 disrupts the structural elements required for attenuation control, impairing the operon's ability to regulate transcription termination in response to tryptophan availability.

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Attenuation in Gene Regulation

Attenuation is a regulatory mechanism in prokaryotes, particularly in the trp operon, that controls gene expression based on the availability of tryptophan. It involves the formation of specific RNA structures during transcription that can either promote or inhibit the continuation of transcription, depending on the concentration of tryptophan in the cell.
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Trp Attenuation

trp Operon Structure

The trp operon consists of a set of genes responsible for tryptophan biosynthesis in bacteria. It includes regulatory regions such as the leader sequence (trpL), which contains sequences that can form alternative secondary structures, influencing whether transcription is terminated early or continues to produce the enzymes needed for tryptophan synthesis.
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Trp Attenuation

Role of Region 3 in Attenuation

Region 3 of the trpL sequence plays a critical role in the formation of the RNA secondary structures that determine whether transcription will be terminated. If Region 3 is deleted, it disrupts the ability to form the necessary structures for attenuation, likely leading to increased transcription of the trp operon genes, regardless of tryptophan levels.
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Trp Attenuation
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A repressible operon system, like the trp operon, contains three genes, G, Z, and W. Operon genes are synthesized when the end product of the operon synthesis pathway is absent, but there is no synthesis when the end product is present. One of these genes is an operator, one is a regulatory protein, and the other is a structural enzyme involved in synthesis of the end product. In the table below, '+' indicates that the enzyme is synthesized by the operon and '−' means that no enzyme synthesis occurs. Use this information to determine which gene corresponds to each operon function.

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What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

Region 4 is deleted.

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What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

The start (AUG) codon of the trpL polypeptide is deleted.

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What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.

The entire trpL region is deleted.

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