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

Klug 12th Edition

Ch. 16 - Regulation of Gene Expression in Bacteria

Problem 21

Chapter 16, Problem 21

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?

Verified step by step guidance

Step 1: Understand the operon system described in the problem. The operon is inducible, meaning its expression is turned on in the presence of a specific environmental signal (oil). The operon is under positive control, which means that the regulatory protein (produced by the reg gene) must interact with the operator region (o) to activate transcription of the structural genes (sg).

Step 2: Analyze the wild-type strain. The wild-type strain is inducible, meaning that the operon is only expressed in the presence of oil. This indicates that the reg gene product (regulatory protein) and the operator region (o) are functioning normally in the wild type.

Step 3: Examine the constitutive mutant strain. A constitutive mutant expresses the operon continuously, regardless of the presence of oil. This suggests that the mutation disrupts the normal regulation of the operon. The mutation could either be in the reg gene (trans-acting element) or in the operator region (cis-acting element).

Step 4: Interpret the partial diploid strains. When the wild-type reg gene is introduced into the mutant strain, the phenotype remains constitutive. This suggests that the mutation is not in the reg gene, as the wild-type reg gene does not restore inducibility. However, when the wild-type operon (including the operator region) is introduced into the mutant strain, the phenotype becomes inducible. This indicates that the mutation is in the operator region (cis-acting element), as the wild-type operator restores normal regulation.

Step 5: Conclude that the constitutive mutation is in the cis-acting operator region (o). The operator mutation likely prevents the regulatory protein from properly interacting with the operator, leading to continuous expression of the operon. This conclusion is supported by the fact that the wild-type operator restores inducibility, while the wild-type reg gene does not.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

51s

Was this helpful?

Key Concepts

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

Operon Structure and Function

An operon is a cluster of genes under the control of a single promoter, allowing coordinated expression in response to environmental changes. In this case, the inducible operon in the marine bacterium is activated in the presence of oil, demonstrating how bacteria can adapt their metabolic pathways to utilize available resources efficiently.

Trans-acting vs. Cis-acting Elements

Trans-acting elements, such as regulatory genes, produce factors that can diffuse through the cell and affect the expression of genes on different DNA molecules. In contrast, cis-acting elements, like operators, are DNA sequences located near the genes they regulate and only influence the expression of adjacent genes. Understanding this distinction is crucial for analyzing the effects of mutations in the operon.

Constitutive Mutations

Constitutive mutations lead to the continuous expression of genes regardless of environmental conditions. In this scenario, the constitutive mutant strain suggests that the mutation affects the regulation of the operon, potentially indicating a defect in the reg gene or the operator region, which can help determine whether the mutation is in a trans-acting or cis-acting element.

Recommended video:

Guided course

10:48

Mutations and Phenotypes

Related Practice

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)?

651

views

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

1275

views

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.

1011

views

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.

957

views

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.

883

views

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.

495

views