Question

Contrast positive versus negative regulation of gene expression. Describe the role of the repressor in an inducible system and in a repressible system.

Short answer

Question: Describe the difference between positive and negative regulation of gene expression and explain the role of a repressor in inducible and repressible systems, providing examples for each case. Answer: Positive regulation of gene expression involves activators that enhance gene expression, while negative regulation involves repressors that decrease gene expression. In an inducible system, such as the lac operon, the repressor remains bound to the DNA until the inducer (e.g., lactose) is present, allowing gene expression. In a repressible system, such as the trp operon, the repressor remains inactive until the corepressor (e.g., tryptophan) is present, inhibiting gene expression.

Step-by-step solution

Understand Positive and Negative Regulation of Gene Expression

Positive regulation of gene expression involves the activation of genes, while negative regulation involves their repression. Positive regulators, known as activators, enhance the transcription of a gene, leading to increased gene expression. In contrast, negative regulators or repressors bind to specific DNA sequences and prevent the binding of RNA polymerase, thereby decreasing gene expression.

Describe the Role of the Repressor in an Inducible System

In an inducible system, the gene is usually turned off, and the presence of a specific molecule (the inducer) is required to turn it on. The inducer binds to the repressor, causing it to detach from the DNA sequence and allowing transcription of the gene. A classic example of an inducible system is the lac operon in bacteria, where the presence of lactose induces the expression of genes needed for lactose metabolism.

Describe the Role of the Repressor in a Repressible System

In a repressible system, the gene is usually turned on, and the presence of a specific molecule (the corepressor) is required to turn it off. The corepressor binds to the repressor, causing it to bind to the DNA sequence and inhibiting transcription of the gene. A classic example of a repressible system is the trp operon in bacteria, where the presence of tryptophan suppresses the expression of genes required for tryptophan synthesis.

Contrast Positive and Negative Regulation with Examples

In summary, positive regulation of gene expression involves activators that enhance gene expression, while negative regulation involves repressors that decrease gene expression. The role of a repressor in an inducible system, such as the lac operon, is to remain bound to the DNA until the inducer (e.g., lactose) is present. In contrast, the role of a repressor in a repressible system, such as the trp operon, is to remain inactive until the corepressor (e.g., tryptophan) is present, inhibiting gene expression.