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 and (b) the mutations.

Key Concepts

Operon Structure and Function

An operon is a cluster of genes under the control of a single promoter, allowing coordinated regulation of gene expression. In bacteria, operons typically consist of structural genes, a promoter, and an operator. The operator is a regulatory sequence where repressor proteins can bind, influencing whether transcription occurs. Understanding operon structure is crucial for analyzing how genes are turned on or off in response to environmental signals.

Negative Control in Gene Regulation

Negative control refers to the mechanism by which a repressor protein binds to the operator region of an operon, preventing transcription of the downstream genes. In the context of the question, the presence of the amino acid tisophane (tis) leads to the binding of a repressor, inhibiting enzyme synthesis. This mechanism is essential for conserving resources, as the cell only produces enzymes when tis is absent, highlighting the operon's regulatory response to metabolic needs.

Mutations and Their Effects on Gene Regulation

Mutations in the operator gene can alter the binding affinity of the repressor, leading to changes in gene expression. In the case of the O⁻ mutation mentioned, the operator is modified such that the repressor cannot bind, resulting in continuous repression of the operon regardless of tis presence. This illustrates how genetic mutations can disrupt normal regulatory mechanisms, providing insights into the operon's control and the consequences of genetic alterations.