Question

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

Short answer

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

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.

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.

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.

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.