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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 22a
Chapter 16, Problem 22a

The SOS repair genes in E. coli are negatively regulated by the lexA gene product, called the LexA repressor. When a cell's DNA sustains extensive damage, the LexA repressor is inactivated by the recA gene product (RecA), and transcription of the SOS genes is increased dramatically. One of the SOS genes is the uvrA gene. You are a student studying the function of the UvrA gene product in DNA repair. You isolate a mutant strain that shows constitutive expression of the UvrA protein. Naming this mutant strain uvrAᶜ, you construct the diagram shown above in the right-hand column showing the lexA and uvrA operons:
Describe two different mutations that would result in a uvrA constitutive phenotype. Indicate the actual genotypes involved.
Diagram showing lexA and uvrA operons with mutations causing constitutive uvrA expression and their genotypes.

Verified step by step guidance
1
Understand the concept of constitutive expression: Constitutive expression means that a gene is expressed continuously, regardless of environmental conditions or regulatory mechanisms. In this case, the uvrA gene is being expressed constitutively, meaning its expression is no longer regulated by the LexA repressor.
Identify the regulatory mechanism: Normally, the LexA repressor binds to the operator region of the uvrA operon, preventing transcription. When DNA damage occurs, the RecA protein inactivates LexA, allowing transcription of the uvrA gene. A mutation leading to constitutive expression would disrupt this regulatory mechanism.
Mutation 1: A mutation in the operator region of the uvrA operon could prevent the LexA repressor from binding. This would result in the uvrA gene being transcribed continuously, as the repressor cannot inhibit transcription. The genotype for this mutation could be represented as uvrAᶜ (operator mutation).
Mutation 2: A mutation in the lexA gene itself could result in a nonfunctional LexA repressor protein. If LexA cannot bind to the operator region of the uvrA operon, the uvrA gene would be expressed constitutively. The genotype for this mutation could be represented as lexA⁻ (loss-of-function mutation in LexA).
Summarize the two mutations: The two mutations that could lead to a uvrA constitutive phenotype are (1) a mutation in the operator region of the uvrA operon that prevents LexA binding, and (2) a loss-of-function mutation in the lexA gene that produces a nonfunctional LexA repressor. Both mutations disrupt the normal regulation of the uvrA gene, leading to its continuous expression.

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Key Concepts

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

LexA Repressor Function

The LexA repressor is a protein that negatively regulates the SOS response in E. coli by binding to the operator region of SOS genes, including uvrA. Under normal conditions, LexA prevents transcription of these genes. However, when DNA damage occurs, the RecA protein facilitates the cleavage of LexA, leading to derepression and allowing the expression of SOS genes to initiate DNA repair processes.
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Constitutive Expression

Constitutive expression refers to the continuous production of a gene product regardless of environmental conditions. In the context of the uvrA gene, a mutation that leads to constitutive expression would result in the UvrA protein being produced at all times, even when the LexA repressor is functional. This can occur through mutations that either inactivate the LexA binding site or alter the promoter region of the uvrA gene.
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Mutations Leading to Constitutive Phenotype

Two types of mutations can lead to a constitutive phenotype for the uvrA gene. One example is a mutation in the lexA gene that results in a nonfunctional LexA repressor, allowing continuous transcription of uvrA. Another example is a mutation in the uvrA promoter region that enhances its activity, making it less dependent on LexA regulation, thus leading to constant expression of the UvrA protein.
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Related Practice
Textbook Question

In a theoretical operon, genes A, B, C, and D represent the repressor gene, the promoter sequence, the operator gene, and the structural gene, but not necessarily in the order named. This operon is concerned with the metabolism of a theoretical molecule (tm). From the data provided in the accompanying table, first decide whether the operon is inducible or repressible. Then assign A, B, C, and D to the four parts of the operon. Explain your rationale. (AE=active enzyme; IE=inactive enzyme; NE=no enzyme.) Genotype tm Present tm Absent A⁺B⁺C⁺D⁺ AE NE A⁻B⁺C⁺D⁺ AE AE A⁺B⁻C⁺D⁺ NE NE A⁺B⁺C⁻D⁺ IE NE A⁺B⁺C⁺D⁻ AE AE A⁻B⁺C⁺D⁺/F'A⁺B⁺C⁺D⁺ AE AE A⁺B⁻C⁺D⁺/F'A⁺B⁺C⁺D⁺ AE NE A⁺B⁺C⁻D⁺/F'A⁺B⁺C⁺D⁺ AE+IE NE A⁺B⁺C⁺D⁻/F'A⁺B⁺C⁺D⁺ AE NE

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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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Textbook Question

A marine bacterium is isolated and shown to contain an inducible operon whose genetic products metabolize oil when it is encountered in the environment. Investigation demonstrates that the operon is under positive control and that there is a reg gene whose product interacts with an operator region (o) to regulate the structural genes, designated sg. In an attempt to understand how the operon functions, a constitutive mutant strain and several partial diploid strains were isolated and tested with the results shown in the following table. Host Chromosome F' Factor Phenotype Wild type None Inducible Wild type reg gene from mutant strain Inducible Wild type Operon from mutant strain Constitutive Mutant strain reg gene from wild type Constitutive Draw all possible conclusions about the mutation as well as the nature of regulation of the operon. Is the constitutive mutation in the trans-acting reg element or in the cis-acting o operator element?

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Textbook Question

The SOS repair genes in E. coli are negatively regulated by the lexA gene product, called the LexA repressor. When a cell's DNA sustains extensive damage, the LexA repressor is inactivated by the recA gene product (RecA), and transcription of the SOS genes is increased dramatically. One of the SOS genes is the uvrA gene. You are a student studying the function of the uvrA gene product in DNA repair. You isolate a mutant strain that shows constitutive expression of the UvrA protein. Naming this mutant strain uvrAC, you construct the diagram shown above in the right-hand column showing the lexA and uvrA operons:

Outline a series of genetic experiments that would use partial diploid strains to determine which of the two possible mutations you have isolated.

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Textbook Question

A fellow student considers the issues in Problem 22 and argues that there is a more straightforward, nongenetic experiment that could differentiate between the two types of mutations. The experiment requires no fancy genetics and would allow you to easily assay the products of the other SOS genes. Propose such an experiment.

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Textbook Question

The following figure depicts numerous critical regions of the leader sequence of mRNA that play important roles during the process of attenuation in the trp operon. A closer view of the leader sequence, which begins at about position 30 downstream from the 5' end, is shown below, running along both columns. Within this molecule are the sequences that cause the formation of the alternative hairpins. It also contains the successive triplets that encode tryptophan, where stalling during translation occurs.

Take a large piece of paper (such as manila wrapping paper) and, along with several other students from your genetics class, work through the base sequence to identify the trp codons and the parts of the molecule representing the base-pairing regions that form the terminator and antiterminator hairpins shown in the following figure.

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