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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 18
Chapter 16, Problem 18

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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Understand the definitions of negative and positive control systems in gene regulation. Negative control involves a repressor protein that inhibits transcription, while positive control involves an activator protein that enhances transcription. Both systems can be inducible (turned on by a specific molecule) or repressible (turned off by a specific molecule).
Analyze the role of the antibiotic in the efflux pump system. The antibiotic may act as an inducer or corepressor, depending on whether it activates or inhibits the pump's expression. This interaction determines whether the system is inducible or repressible.
For negative control, consider if the antibiotic interacts with a repressor protein. If the antibiotic inactivates the repressor, allowing transcription of the efflux pump genes, the system is inducible. If the antibiotic activates the repressor, stopping transcription, the system is repressible.
For positive control, consider if the antibiotic interacts with an activator protein. If the antibiotic activates the activator, enhancing transcription of the efflux pump genes, the system is inducible. If the antibiotic inactivates the activator, reducing transcription, the system is repressible.
Categorize the efflux pump system based on the specific interactions described in the problem. Determine whether the system operates under negative or positive control and whether it is inducible or repressible, depending on the role of the antibiotic in regulating the pump's expression.

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Key Concepts

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

Efflux Pumps

Efflux pumps are membrane proteins in bacteria that actively transport antibiotics and other toxic substances out of the cell. This mechanism helps bacteria survive in the presence of antibiotics by reducing the intracellular concentration of these drugs. The energy-dependent nature of these pumps means that they require ATP or proton motive force to function, which can be a disadvantage when antibiotics are absent.
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Cell-cell interactions

Gene Regulation

Gene regulation refers to the mechanisms that control the expression of genes, determining when and how much of a gene product is made. In the context of efflux pumps, regulation can occur through activator or repressor proteins that respond to the presence of antibiotics. This regulation can be categorized as negative control (where a repressor inhibits gene expression) or positive control (where an activator enhances gene expression), and can be further classified as inducible (activated by the presence of a substance) or repressible (inhibited by the presence of a substance).
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Review of Regulation

Inducible vs. Repressible Systems

Inducible systems are those where gene expression is turned on in response to an external signal, such as the presence of an antibiotic, while repressible systems are turned off when a specific substance is present. In the case of efflux pumps, if the antibiotic induces the expression of the pump, it is considered an inducible system. Conversely, if the antibiotic represses the expression of the pump, it is a repressible system. Understanding these categories helps in predicting bacterial responses to antibiotics.
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Related Practice
Textbook Question

The creation of milk products such as cheeses and yogurts is dependent on the conversion by various anaerobic bacteria, including several Lactobacillus species, of lactose to glucose and galactose, ultimately producing lactic acid. These conversions are dependent on both permease and β-galactosidase as part of the lac operon. After selection for rapid fermentation for the production of yogurt, one Lactobacillus subspecies lost its ability to regulate lac operon expression [Lapierre, L., et al. (2002). J. Bacteriol. 184:928–935]. Would you consider it likely that in this subspecies the lac operon is on or off? What genetic events would likely contribute to the loss of regulation as described above?

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Textbook Question

Both attenuation of the trp operon in E. coli and riboswitches in B. subtilis rely on changes in the secondary structure of the leader regions of mRNA to regulate gene expression. Compare and contrast the specific mechanisms in these two types of regulation with those involving short noncoding RNAs (sRNAs).

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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

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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