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, \(R,\) 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 (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.

Short answer

Answer: The bacterial operon is under negative control. In the presence of tisophane in wild-type cells, enzyme synthesis is inhibited as tisophane binds to the repressor protein, increasing its affinity for the operator. In mutated operator gene (O), the operator is always bound by the repressor, causing constitutive repression of enzyme synthesis, irrespective of the presence or absence of tisophane.

Step-by-step solution

Understanding of the wild-type situation

In the wild-type condition, the bacterial operon responds to the presence or absence of tisophane. When tis is present, no enzymes are made, while in the absence of tis, enzymes are synthesized. This indicates that the synthesis of enzymes is inhibited or repressed when tisophane is present. Thus, the control mechanism appears to be a negative one, as the presence of the substance (tis) hinders the process (enzyme synthesis).

Analyzing the operon in the presence of tisophane in wild-type cells

In the wild-type cells, the presence of tisophane seems to inhibit the expression of the genes responsible for enzyme synthesis. Since the operon is under negative control, we can propose a model in which tisophane binds to the repressor protein (R) encoded by gene \(R\). This binding results in increased affinity of the repressor for the operator site (O), which prevents the transcription of the enzymes needed for tis synthesis.

Analyzing the mutations in the operator gene (O)

Mutations in the operator gene (O) result in repression of enzyme synthesis, regardless of the presence of tisophane. This indicates that the mutated operator gene is insensitive to the repression/release mechanism by the repressor (R) since it is always bound by the repressor. The mutated operator site can no longer distinguish between the presence or absence of tisophane, leading to a constitutive repression of enzyme synthesis.

Conclusion:

Based on the given information, we can determine that the bacterial operon responsible for the enzyme synthesis required for tisophane production is under negative control. In the presence of tisophane in wild-type cells, the repressor binds to tisophane, increasing its affinity for the operator, and inhibiting enzyme synthesis. In mutated operator gene (O), the operator is always bound by the repressor, causing constitutive repression of enzyme synthesis, irrespective of the presence or absence of tisophane.

Key concepts

Negative Control

In the world of bacterial operons, control mechanisms dictate whether certain genes are expressed or suppressed. Negative control is a term used when a gene is turned off in the presence of a specific molecule.
This means that the presence of the molecule inhibits gene expression.

• For the operon producing enzymes for tisophane, when tisophane is present, gene expression is stopped.
• In simpler terms, the presence of tisophane "turns off" the operon.

When trying to understand this mechanism, think of negative control as a system's way of avoiding overproduction when a product is already available. Just like a thermostat might shut off heat when a room reaches a certain temperature, a bacterial operon might shut down enzyme production when the product it synthesizes (like an amino acid) is no longer needed.

Repressor Protein

A repressor protein is a key player in regulating gene expression, particularly in negative control systems. It's a protein molecule that can attach to a specific DNA region, effectively blocking transcription. When we talk about the bacterial operon for tisophane, the role of the repressor protein (encoded by the gene R) is crucial:

• The repressor protein changes shape when it binds to the molecule it's supposed to regulate—in this case, tisophane.
• Once it binds to tisophane, it acquires a conformation that allows it to fit snugly onto the DNA at the operator site.
• This blockage means RNA polymerase cannot access the DNA to initiate the transcription of enzymes for tisophane.

Think of the repressor protein as a security guard at a door: when it detects an important signal (like the presence of tisophane), it quickly moves to block gate access, ensuring no unauthorized entries (or enzyme creations) occur.

Operator Gene Mutations

Mutations in the operator gene can dramatically alter the regulation of an operon, as seen in cases with bacterial operons. Such mutations can render the operon insensitive to normal regulatory mechanisms. For the tisophane operon:

• When the operator gene is mutated, it can't properly "hear" the signals from the repressor protein.
• This means the operator is unable to respond to the presence or absence of tisophane, leading to constant repression.
• As a result, the repressor protein always binds to the operator, and the necessary enzymes are never synthesized, regardless of whether tisophane is needed or not.

The effect of such mutations highlights how crucial the correct function of each component in an operon is. In this case, without proper regulation, the cell wastes energy maintaining repressive conditions, which can impact survival or growth.