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. Step 1: Understand the operon system described in the problem. The operon is inducible, meaning its expression is turned on in the presence of a specific environmental signal (oil). The operon is under positive control, which means that the regulatory protein (produced by the reg gene) must interact with the operator region (o) to activate transcription of the structural genes (sg).
2. Step 2: Analyze the wild-type strain. The wild-type strain is inducible, meaning that the operon is only expressed in the presence of oil. This indicates that the reg gene product (regulatory protein) and the operator region (o) are functioning normally in the wild type.
3. Step 3: Examine the constitutive mutant strain. A constitutive mutant expresses the operon continuously, regardless of the presence of oil. This suggests that the mutation disrupts the normal regulation of the operon. The mutation could either be in the reg gene (trans-acting element) or in the operator region (cis-acting element).
4. Step 4: Interpret the partial diploid strains. When the wild-type reg gene is introduced into the mutant strain, the phenotype remains constitutive. This suggests that the mutation is not in the reg gene, as the wild-type reg gene does not restore inducibility. However, when the wild-type operon (including the operator region) is introduced into the mutant strain, the phenotype becomes inducible. This indicates that the mutation is in the operator region (cis-acting element), as the wild-type operator restores normal regulation.
5. Step 5: Conclude that the constitutive mutation is in the cis-acting operator region (o). The operator mutation likely prevents the regulatory protein from properly interacting with the operator, leading to continuous expression of the operon. This conclusion is supported by the fact that the wild-type operator restores inducibility, while the wild-type reg gene does not.

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 inducible operon in the marine bacterium is activated in the presence of oil, demonstrating how bacteria can adapt their metabolic pathways to utilize available resources efficiently.

Trans-acting vs. Cis-acting Elements

Trans-acting elements, such as regulatory genes, produce factors that can diffuse through the cell and affect the expression of genes on different DNA molecules. In contrast, cis-acting elements, like operators, are DNA sequences located near the genes they regulate and only influence the expression of adjacent genes. Understanding this distinction is crucial for analyzing the effects of mutations in the operon.

Constitutive Mutations

Constitutive mutations lead to the continuous expression of genes regardless of environmental conditions. In this scenario, the constitutive mutant strain suggests that the mutation affects the regulation of the operon, potentially indicating a defect in the reg gene or the operator region, which can help determine whether the mutation is in a trans-acting or cis-acting element.