Ch. 16 - Regulation of Gene Expression in Bacteria

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 16 - Regulation of Gene Expression in Bacteria

Problem 20

Chapter 16, Problem 20

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.

Verified step by step guidance

Step 1: Understand the system and terminology. The operon produces enzymes for making the amino acid tisophane (tis). The regulatory gene R controls the operon. In wild-type cells, enzymes are made only when tis is absent, and not made when tis is present. This suggests that the presence of tis turns off enzyme production.

Step 2: Determine the type of control (positive or negative). Since the presence of tis prevents enzyme synthesis, and deletion of R causes loss of enzyme synthesis, R likely acts as an activator that is active only when tis is absent. This indicates positive control, where the regulator R activates transcription in the absence of the effector (tis).

Step 3: Model repression in wild-type cells. In the absence of tis, R is active and binds to the operator to promote transcription of the enzymes. When tis is present, it binds to R, causing R to lose its ability to activate transcription, leading to repression of enzyme synthesis.

Step 4: Analyze the effect of operator mutations (O⁻). Mutations in the operator cause repression regardless of tis presence, meaning the operon is always off. This suggests that the mutated operator prevents R from binding or functioning properly, blocking activation and thus transcription.

Step 5: Summarize the model. (a) In wild-type, R activates transcription when tis is absent; tis binding to R inhibits this activation, repressing the operon. (b) In O⁻ mutants, the operator mutation prevents R-mediated activation, causing constant repression regardless of tis.

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Key Concepts

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

Operon Model and Gene Regulation

An operon is a cluster of genes under the control of a single promoter and operator, allowing coordinated regulation of gene expression. Regulatory proteins bind to the operator to either block or promote transcription. Understanding how operons function is essential to explain how enzyme synthesis is controlled in response to environmental signals.

Negative vs. Positive Control of Gene Expression

Negative control involves a repressor protein binding to the operator to inhibit transcription, while positive control involves an activator protein enhancing transcription. Determining whether the operon is under negative or positive control depends on how the regulatory protein and effector molecules influence RNA polymerase binding and gene expression.

Role of Regulatory Genes and Mutations in Operon Function

Regulatory genes encode proteins that control operon activity, such as repressors or activators. Mutations in regulatory elements like the operator (O⁻) can disrupt normal regulation, causing constitutive repression or expression. Analyzing these mutations helps model how gene expression is altered in mutant strains compared to wild-type.

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Textbook Question

During the reproductive cycle of a temperate bacteriophage, the viral DNA inserts into the bacterial chromosome, where the resultant prophage behaves much like a Trojan horse. It can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. Several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. Insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. As stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?

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Textbook Question

Bacterial strategies to evade natural or human-imposed antibiotics are varied and include membrane-bound efflux pumps that export antibiotics from the cell. A review of efflux pumps [Grkovic, S., et al. (2002)] states that, because energy is required to drive the pumps, activating them in the absence of the antibiotic has a selective disadvantage. The review also states that a given antibiotic may play a role in the regulation of efflux by interacting with either an activator protein or a repressor protein, depending on the system involved. How might such systems be categorized in terms of negative control (inducible or repressible) or positive control (inducible or repressible)?

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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 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:

Describe two different mutations that would result in a uvrA constitutive phenotype. Indicate the actual genotypes involved.

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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 uvrA^C, 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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