A marine bacterium is isolated and shown to contain an inducible operon whose genetic products metabolize oil when it is encountered in the environment. Investigation demonstrates that the operon is under positive control and that there is a reg gene whose product interacts with an operator region (o) to regulate the structural genes, designated sg. In an attempt to understand how the operon functions, a constitutive mutant strain and several partial diploid strains were isolated and tested with the results shown in the following table. Host Chromosome F' Factor Phenotype Wild type None Inducible Wild type reg gene from mutant strain Inducible Wild type Operon from mutant strain Constitutive Mutant strain reg gene from wild type Constitutive Draw all possible conclusions about the mutation as well as the nature of regulation of the operon. Is the constitutive mutation in the trans-acting reg element or in the cis-acting o operator element?

Verified step by step guidance

1. Identify the components of the operon system: the reg gene, the operator (o), and the structural genes (sg).
2. Understand the difference between cis-acting and trans-acting elements: cis-acting elements affect only the DNA molecule they are on, while trans-acting elements can affect other DNA molecules.
3. Analyze the phenotype of the wild type with the mutant reg gene: it remains inducible, suggesting the mutation is not in the reg gene.
4. Examine the phenotype of the wild type with the mutant operon: it becomes constitutive, indicating the mutation is likely in the operator (o), a cis-acting element.
5. Consider the mutant strain with the wild type reg gene: it remains constitutive, supporting the conclusion that the mutation is in the operator, not the reg gene.

Key Concepts

Operon Structure and Function

An operon is a cluster of genes under the control of a single promoter, allowing coordinated expression in response to environmental changes. In this case, the operon is inducible, meaning it is activated in the presence of a specific substrate (oil). Understanding the operon's structure, including regulatory elements like the operator and structural genes, is crucial for analyzing how mutations affect gene expression.

Trans-acting vs. Cis-acting Elements

Trans-acting elements are regulatory proteins that can diffuse through the cell and act on any target gene, while cis-acting elements are DNA sequences located on the same molecule of DNA as the gene they regulate. In this scenario, the reg gene is a trans-acting element that produces a protein interacting with the operator (a cis-acting element) to control the operon's expression. Identifying whether mutations are in trans or cis elements helps determine the nature of the regulatory mechanisms.

Constitutive Mutants

Constitutive mutants are strains that express genes continuously, regardless of environmental conditions. In the context of the operon, a constitutive mutation suggests a failure in regulation, often due to a mutation in the reg gene or the operator. Analyzing the phenotypes of the wild type and mutant strains helps infer whether the mutation affects the trans-acting reg gene or the cis-acting operator, providing insights into the operon's regulatory dynamics.