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.

Accepted Answer

Step 1: Understand the system and terminology. The operon produces enzymes for making the amino acid tisophane (tis). The regulatory gene R controls the operon. In wild-type cells, enzymes are made only when tis is absent, and not made when tis is present. This suggests that the presence of tis turns off enzyme production. Step 2: Determine the type of control (positive or negative). Since the presence of tis prevents enzyme synthesis, and deletion of R causes loss of enzyme synthesis, R likely acts as an activator that is active only when tis is absent. This indicates positive control, where the regulator R activates transcription in the absence of the effector (tis). Step 3: Model repression in wild-type cells. In the absence of tis, R is active and binds to the operator to promote transcription of the enzymes. When tis is present, it binds to R, causing R to lose its ability to activate transcription, leading to repression of enzyme synthesis. Step 4: Analyze the effect of operator mutations (O⁻). Mutations in the operator cause repression regardless of tis presence, meaning the operon is always off. This suggests that the mutated operator prevents R from binding or functioning properly, blocking activation and thus transcription. Step 5: Summarize the model. (a) In wild-type, R activates transcription when tis is absent; tis binding to R inhibits this activation, repressing the operon. (b) In O⁻ mutants, the operator mutation prevents R-mediated activation, causing constant repression regardless of tis.

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Concetti chiave

Operon Model and Gene Regulation

Negative vs. Positive Control of Gene Expression

Role of Regulatory Genes and Mutations in Operon Function