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Chapter 16: Problem 4

Contrast the role of the repressor in an inducible system and in a repressible system.

Short Answer

Expert verified
Answer: In an inducible system, the repressor typically binds to the operator, keeping gene expression off until an inducer molecule is present. When the inducer binds to the repressor, it alters its conformation and allows the transcription machinery to initiate the transcription, turning on gene expression. In contrast, in a repressible system, the repressor is unable to bind to the operator by default, keeping gene expression on until a corepressor molecule is present. The corepressor binds to the repressor, enabling it to bind to the operator, and turning off gene expression. The main difference in the role of the repressor between these systems lies in their default state and the molecules that regulate their function - inducer molecules in inducible systems and corepressor molecules in repressible systems.

Step by step solution

01

Define Inducible and Repressible System

Inducible systems are gene regulatory systems where the gene expression is typically off or at low levels under normal conditions. When an inducer molecule is present, the gene expression is turned on. In contrast, repressible systems are gene regulatory systems where gene expression is usually on, but can be turned off or down-regulated when a specific molecule, called a corepressor, is present.
02

Explain the Role of the Repressor in Inducible System

In an inducible system, the repressor is a protein that binds to a specific region of DNA called the operator. When the repressor is bound to the operator, it prevents the transcription machinery (RNA polymerase) from binding and initiating transcription of the genes. However, when an inducer molecule is present, it binds to the repressor, altering its conformation and rendering it unable to bind to the operator. This allows the transcription machinery to bind and initiate the transcription of the target genes, turning on gene expression.
03

Explain the Role of the Repressor in Repressible System

In a repressible system, the repressor is also a protein that binds to the operator region of the DNA. However, in this case, the repressor is inherently unable to bind to the operator in the absence of a corepressor. The gene expression is then “on” by default, as the transcription machinery can freely bind and initiate transcription of the target genes. When a corepressor molecule is present, it binds to the repressor, causing a conformational change, which allows the repressor to bind to the operator region and block transcription, turning off gene expression.
04

Contrast the Role of the Repressor in Inducible and Repressible Systems

The main difference in the role of the repressor between the inducible and repressible systems lies in their default state and the molecules that regulate their function. In an inducible system, the repressor can bind to the operator by default, keeping gene expression off until an inducer molecule is present. In a repressible system, the repressor cannot bind to the operator until a corepressor molecule is present, keeping gene expression on by default. Therefore, the repressor protein plays a crucial role in regulating gene expression in both systems, but its function is modulated differently – by inducer molecules in inducible systems and by corepressor molecules in repressible systems.

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Most popular questions from this chapter

In this chapter, we focused on the regulation of gene expression in prokaryotes. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How do we know that bacteria regulate the expression of certain genes in response to the environment? (b) What evidence established that lactose serves as the inducer of a gene whose product is related to lactose metabolism? (c) What led researchers to conclude that a repressor molecule regulates the lac operon? (d) How do we know that the lac repressor is a protein? (e) How do we know that the trp operon is a repressible con- trol system, in contrast to the lac operon, which is an inducible control system?

Describe the experimental rationale that allowed the lac repressor to be isolated.

Neelaredoxin is a 15 -kDa protein that is a gene product common in anaerobic prokaryotes. It has superoxide-scavenging activity, and it is constitutively expressed. In addition, its expression is not further induced during its exposure to \(\mathrm{O}_{2}\) or \(\mathrm{H}_{2} \mathrm{O}_{2}\) (Silva, G., et al. \(2001 .\) J. Bacteriol. \(183: 4413-4420\) ). What do the terms constitutively expressed and induced mean in terms of neelaredoxin synthesis?

Erythritol, a natural sugar abundant in fruits and fermenting foods, is about 65 percent as sweet as table sugar and has about 95 percent fewer calories. It is "tooth friendly" and generally devoid of negative side effects as a human consumable product. Pathogenic Brucella strains that catabolize erythritol contain four closely spaced genes, all involved in erythritol metabolism. One of the four genes \((e r y D)\) encodes a product that represses the expression of the other three genes. Erythritol catabolism is stimulated by erythritol. Present a simple regulatory model to account for the regulation of erythritol catabolism in Brucella. Does this system appear to be under inducible or repressible control?

One of the most prevalent sexually transmitted diseases is caused by the bacterium Chlamydia trachomatis and leads to blindness if left untreated. Upon infection, metabolically inert cells differentiate, through gene expression, to become metabolically active cells that divide by binary fission. It has been proposed that release from the inert state is dependent on heat-shock proteins that both activate the reproductive cycle and facilitate the binding of chlamydiae to host cells. Researchers made the following observations regarding the heat-shock regulatory system in Chlamydia trachomatis: (1) a regulator protein (call it R) binds to a cis-acting DNA element (call it \(\mathrm{D}\) ); (2) \(\mathrm{R}\) and \(\mathrm{D}\) function as a repressor- operator pair; (3) \(\mathrm{R}\) functions as a negative regulator of transcription; (4) \(\mathrm{D}\) is composed of an inverted-repeat sequence; (5) repression by \(R\) is dependent on \(D\) being supercoiled (Wilson \(\&\) Tan, 2002 ). (a) Based on this information, devise a model to explain the heat-dependent regulation of metabolism in Chlamydia trachomatis. (b) Some bacteria, like \(E .\) coli, use a heat-shock sigma factor to regulate heat-shock transcription. Are the above findings in Chlamydia compatible with use of a heat-sensitive sigma factor?
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