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Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 12 - Regulation of Gene Expression in Bacteria and BacteriophageProblem 22c
Chapter 12, Problem 22c

Suppose the lac operon partial diploid cap⁻ I⁺ P⁺ O⁺ Z⁻ Y⁺/ cap⁺ I⁻ P⁺ O⁺ Z⁺ Y⁻ is grown.
Explain how genetic complementation contributes to the growth habit of this strain.

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1
Understand the genetic makeup of the partial diploid strain. The strain has two copies of the lac operon: one copy is cap⁻ I⁺ P⁺ O⁺ Z⁻ Y⁺, and the other copy is cap⁺ I⁻ P⁺ O⁺ Z⁺ Y⁻. Each component of the operon has a specific role: cap (catabolite activator protein), I (repressor), P (promoter), O (operator), Z (β-galactosidase), and Y (permease).
Determine the functionality of each operon copy. The first copy (cap⁻ I⁺ P⁺ O⁺ Z⁻ Y⁺) has a functional repressor (I⁺) and permease (Y⁺), but lacks functional β-galactosidase (Z⁻) and catabolite activator protein (cap⁻). The second copy (cap⁺ I⁻ P⁺ O⁺ Z⁺ Y⁻) has functional β-galactosidase (Z⁺) and catabolite activator protein (cap⁺), but lacks a functional repressor (I⁻) and permease (Y⁻).
Apply the concept of genetic complementation. Complementation occurs when two mutations in different genes are compensated for by the presence of functional alleles in the other copy. In this strain, the functional I⁺ from the first copy can regulate both operons, and the functional Z⁺ and cap⁺ from the second copy can contribute to the operon's activity.
Analyze the growth habit based on the complementation. The functional I⁺ repressor from the first copy can bind to the operator (O⁺) of both operons, regulating their expression. The functional Z⁺ from the second copy allows for β-galactosidase production, and the functional Y⁺ from the first copy allows for permease production. Together, these proteins enable lactose metabolism under appropriate conditions.
Conclude how genetic complementation supports the strain's growth. The combination of functional alleles from both copies of the operon ensures that the strain can metabolize lactose, despite each individual operon copy having deficiencies. This demonstrates the importance of genetic complementation in restoring functionality in partial diploids.

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Key Concepts

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

Lac Operon

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of structural genes (lacZ, lacY, and lacA) and regulatory elements (promoter and operator). The operon is typically off when lactose is absent and activated in its presence, allowing the bacteria to utilize lactose as an energy source. Understanding the lac operon is crucial for analyzing how mutations and genetic complementation affect gene expression and metabolic capabilities.
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Lac Operon Overview

Genetic Complementation

Genetic complementation occurs when two different genetic mutations in a diploid organism can restore a normal phenotype when present together. In the context of the lac operon, if one allele is non-functional (e.g., Z⁻), but a functional allele is present on the homologous chromosome (e.g., Z⁺), the functional gene can compensate for the defective one. This principle is essential for understanding how different mutations can interact and influence the growth and metabolic functions of bacterial strains.
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Complementation

Partial Diploidy

Partial diploidy refers to a genetic condition where an organism has two copies of some genes but only one copy of others, often due to the introduction of a plasmid or a specific genetic construct. In the case of the lac operon, the presence of both cap⁻ and cap⁺ alleles allows for the study of gene interactions and complementation. This concept is vital for understanding how different alleles can affect the operon's function and the overall growth of the bacterial strain.
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Related Practice
Textbook Question

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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Textbook Question

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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Textbook Question

Four independent lac⁻ mutants (mutants A to D) are isolated in haploid strains of E. coli. The strains have the following phenotypic characteristics:

Mutant A is lac⁻, but transcription1 of operon genes is induced by lactose.

Mutant B is lac⁻ and has uninducible2 transcription of operon genes.

Mutant C is lac⁺ and has constitutive3 transcription of operon genes.

Mutant D is lac⁺ and has constitutive3 transcription of operon genes.

A microbiologist develops donor and recipient varieties of each mutant strain and crosses them with the results shown below. The table indicates whether inducible, constitutive, or noninducible transcription occurs, along with lac+ and lac⁻ growth habit for each partial diploid. Assume each strain has a single mutation.

Use this information to identify which lac operon gene is mutated in each strain.

Textbook Question

Suppose the lac operon partial diploid cap⁻ I⁺ P⁺ O⁺ Z⁻ Y⁺/ cap⁺ I⁻ P⁺ O⁺ Z⁺ Y⁻ is grown.

Is transcription of β-galactosidase and permease inducible, constitutive, or noninducible?

Textbook Question

What is a riboswitch? Describe the riboswitch mechanism that regulates transcription of the thi operon in B. subtilus. What parallels can you see between this mechanism and the regulation of transcription of the trp operon in E. coli?

Textbook Question

Suppose the lac operon partial diploid cap⁻ I⁺ P⁺ O⁺ Z⁻ Y⁺/ cap⁺ I⁻ P⁺ O⁺ Z⁺ Y⁻ is grown.

Will this partial diploid strain grow on a lactose medium?