Question

A bacterial operon is responsible for production of the biosynthetic enzymes needed to make the theoretical amino acid tisophane (tis). The operon is regulated by a separate gene, \(R,\) deletion of which 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 \(\left(O^{-}\right)\) 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.

Short answer

Answer: The bacterial operon for tisophane synthesis is under negative control. In the presence of mutations in the operator gene, enzyme synthesis is repressed regardless of the presence of tis, as the mutated operator is unable to respond to tis and leads to constant binding of the repressor protein.

Step-by-step solution

To determine if the operon is under positive or negative control, we can analyze the conditions in which the enzymes are synthesized. In the wild-type condition, when tis is present, no enzymes are made, and in the absence of tis, the enzymes are made. This suggests that the operon is under negative control, as the presence of tis prevents enzyme synthesis. #Step 2: Propose a model for the wild-type condition#

In the wild-type condition, when tis is present, no enzymes are synthesized. We know that gene \(R\) is essential for enzyme synthesis. Since enzymes are not synthesized when tis is present, we can propose a model where the gene R product (repressor protein) binds to the operator gene when tis is present. This binding prevents RNA polymerase from transcribing the genes necessary for enzyme synthesis, leading to the repression of enzyme production. #Step 3: Analyze the mutations in the operator gene#

In the case of mutations in the operator gene \((O^{-})\), repression occurs regardless of the presence of tis. This suggests that the mutated operator gene is no longer responsive to the presence of tis. As a result, the repressor protein encoded by gene R will bind to the mutated operator even when tis is absent, causing the repression of enzyme synthesis. #Step 4: Propose a model for the mutations#

Based on our analysis of the mutated operator gene, we can propose a model where the mutated operator is unable to respond to tis. In this case, the repressor protein encoded by gene R will bind to the mutated operator and prevent RNA polymerase from transcribing the genes needed for enzyme synthesis. As a result, the enzyme synthesis will be repressed even in the absence of tis. In conclusion, the operon is under negative control. In wild-type cells, repression occurs due to the binding of the gene R product (repressor protein) to the operator gene when tis is present. For the mutated operator gene, repression occurs regardless of the presence of tis as the mutated operator is unable to respond to tis, leading to constant binding of the repressor protein and preventing enzyme synthesis.

Key concepts

Negative Control of Operon

In the context of bacterial genetics, negative control of an operon is a regulatory mechanism that prevents gene transcription in particular conditions. The problem described involves a bacterial operon that synthesizes enzymes for tisophane (tis) production. When tis is abundant, the operon is inactive, whereas a lack of tis activates the operon. This exemplifies negative control, where the product of the gene R acts as a repressor, inhibiting the operon's function in the presence of tis. This repressor binds to the operator sequence, blocking RNA polymerase from initiating transcription.

Understanding this negative control is crucial because it highlights the operon's ability to conserve resources by producing enzymes only when necessary. This efficiency is significant in bacterial growth and survival, especially in environments where resources are scarce and competition is high.

Biosynthetic Enzyme Regulation

Regulation of biosynthetic enzymes, such as those involved in the production of amino acids, is a fundamental process in bacteria. In our exercise, the operon that produces enzymes for tis synthesis exhibits such regulation. The system conserves energy and resources by producing enzymes only when the product, tis, is absent. This regulation not only prevents the wasteful synthesis of enzymes when their product is readily available but also ensures the bacteria's survival by producing essential compounds in times of need.

Enzyme regulation often occurs at the transcriptional level, where the operon may be induced or repressed based on the cell's metabolic requirements. This form of regulation is a classic example of the bacterial cell's ability to adapt to its nutritional environment, demonstrating the elegance of cellular control mechanisms.

Repressor Protein Binding

Repressor protein binding plays a pivotal role in the regulation of gene expression. The repressor protein, produced by the R gene in our exercise, is designed to recognize and bind to the operator sequence of the operon. When tis is present, it binds to the repressor protein, which then attaches to the operator, preventing transcription. The exercise illustrates a common scenario where the repressor protein's binding is crucial for turning off the operon's activity.

It's essential to grasp that this binding is a reversible process; should tis become scarce, the repressor will no longer bind to the operator, allowing gene expression to resume. Such dynamic binding lays the foundation for the operon’s responsiveness to environmental cues, maintaining the balance between gene expression and metabolic demands.

Transcriptional Regulation in Bacteria

Transcriptional regulation in bacteria is an essential process that allows these organisms to respond swiftly to environmental changes. The operon model demonstrates how bacterial genes are turned on or off at the transcription level. In the case of the tis operon, gene R and the operator play crucial roles. The repressor protein, encoded by gene R, is the transcriptional regulatory protein that interacts with the DNA at the operator site.

This interaction decides if the adjacent genes are transcribed into RNA, which ultimately translates into the biosynthetic enzymes needed for tis. Mutations, like those in the operator gene \(O^{-}\), can lead to constitutive repression, a state where the enzyme is constantly repressed, illustrating the fine-tuned nature of transcriptional regulation and its impact on cellular function under varying conditions.