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Ch. 16 - Regulation of Gene Expression in Bacteria
All textbooksKlug 12th EditionCh. 16 - Regulation of Gene Expression in BacteriaProblem 22
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Chapter 16, Problem 22

The SOS repair genes in E. coli (discussed in Chapter 15) 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. (Leader sequence for Problem 24 above)

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Identify the normal regulation mechanism of the uvrA gene, which involves the LexA repressor binding to the operator region to inhibit transcription.
Consider a mutation in the lexA gene that could lead to a constitutive expression of the uvrA gene. A possible mutation is a loss-of-function mutation in the lexA gene, resulting in a non-functional LexA repressor that cannot bind to the operator.
Explore another mutation possibility within the operator region of the uvrA gene. A mutation in the operator sequence could prevent the LexA repressor from binding, leading to constitutive expression of the uvrA gene.
Define the genotypes for these mutations: for the lexA mutation, it could be lexA⁻, indicating a non-functional LexA repressor; for the operator mutation, it could be uvrAᵒᶜ, indicating a constitutive operator mutation.
Summarize that both mutations disrupt the normal repression mechanism, leading to continuous expression of the uvrA gene, regardless of DNA damage presence.

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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. When DNA damage occurs, the RecA protein facilitates the cleavage of LexA, leading to the derepression of SOS genes and allowing for their expression 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 constitutive phenotype means that the UvrA protein is produced at all times, even when DNA damage is not present. This can occur due to mutations that either inactivate the LexA repressor or alter the uvrA promoter to be permanently active.
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Mutations Leading to Constitutive Phenotype

Two types of mutations can lead to a constitutive expression of the uvrA gene. One mutation could be a point mutation in the lexA gene that results in a nonfunctional LexA repressor, preventing it from binding to the uvrA operator. Another mutation could be a promoter mutation in the uvrA gene itself, enhancing its transcription independently of the LexA regulation, thus leading to continuous UvrA protein production.
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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 and (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
Figure 16.13 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 Figure 16.13.
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