Question

Erythritol, a natural sugar abundant in fruits and fermenting foods, is about 65 percent as sweet as table sugar and has about 95 percent fewer calories. It is "tooth friendly" and generally devoid of negative side effects as a human consumable product. Pathogenic Brucella strains that catabolize erythritol contain four closely spaced genes, all involved in erythritol metabolism. One of the four genes (eryD) encodes a product that represses the expression of the other three genes. Erythritol catabolism is stimulated by erythritol. Present a simple regulatory model to account for the regulation of erythritol catabolism in Brucella. Does this system appear to be under inducible or repressible control?

Short answer

Question: Explain the regulation of erythritol catabolism in Brucella and whether it is an inducible or repressible control system. Answer: In Brucella, erythritol catabolism is regulated by four genes, with the eryD gene encoding a repressor protein. Erythritol serves as an inducer, binding to the repressor protein and preventing it from inhibiting the transcription of the other genes involved in the process. The regulatory system is therefore under inducible control, as the presence of erythritol promotes the expression of the catabolic genes.

Step-by-step solution

Understanding the roles of the genes in the process

We have four genes involved in erythritol metabolism in Brucella: three genes responsible for the biochemical reactions, and one gene (eryD) that represses the expression of the other three genes.

Consider the role of erythritol as a regulator

Given that erythritol stimulates its own catabolism, it is likely that erythritol acts as an inducer of the process. In this case, erythritol would bind to the repressor protein encoded by eryD, preventing it from binding to the other genes' regulatory regions.

Presenting a regulatory model

Here is a simple regulatory model for erythritol catabolism in Brucella: 1. In the absence of erythritol, the eryD gene encodes a repressor protein, which binds to the regulatory regions of the other three genes involved in erythritol metabolism, inhibiting their transcription and expression. 2. When erythritol is present, it binds to the repressor protein encoded by eryD, changing its shape and ultimately preventing it from binding to the regulatory regions of the other genes. 3. With the repressor protein not bound to the regulatory regions, the other three genes can now be expressed, and erythritol catabolism proceeds.

Determining the type of control system

Since the presence of erythritol removes the repression of the catabolic genes and allows their expression, the regulatory system is under inducible control.

Key concepts

Gene Regulation in Bacteria

Gene regulation in bacteria is a vital process that ensures genes are turned on or off in response to environmental stimuli. Bacteria, including the pathogenic Brucella strains mentioned in our exercise, have evolved sophisticated mechanisms to conserve energy by only producing proteins when needed.

Central to gene regulation is the DNA in bacteria, which contains segments called operators that often reside next to the genes they regulate. Regulatory proteins can bind to these operators to either inhibit or promote the transcription of the gene. The balance between repression and activation is dynamic and permits bacteria to rapidly adapt to changes.

• In the presence of a specific sugar, such as erythritol, genes for metabolizing that sugar are activated.
• Conversely, in the absence of that sugar, those genes are repressed to conserve resources.

Understanding gene regulation is essential in biology as it holds the key to comprehending how bacteria thrive in diverse environments and how they can be controlled to prevent diseases.

Repressor Protein Mechanism

The repressor protein mechanism plays a crucial role in the regulation of gene expression in bacteria. It involves proteins that can bind to specific DNA sequences and block the transcription of nearby genes. In the context of erythritol catabolism, the repressor protein encoded by the eryD gene showcases this mechanism.

Here's how the repressor protein works:

• It has a shape that allows it to fit snugly onto a specific region of the DNA called the operator.
• When the repressor is bound to the operator, RNA polymerase cannot bind to the DNA or proceed with transcription.
• The result is a decrease in the expression of genes controlled by that operator, conserving the cell's resources.

This mechanism is crucial for bacteria to efficiently manage their energy and respond quickly to environmental changes, such as the availability of a nutrient like erythritol.

Inducible Control System

An inducible control system is a type of gene regulation mechanism where the presence of a particular substance called an 'inducer' activates the transcription of certain genes. In our exercise's case, erythritol functions as an inducer.

Here's how an inducible control system operates:

• The repressor protein is inactive when the inducer is present, as the inducer binds to the repressor and changes its shape, preventing it from attaching to the DNA.
• Once the repressor is unable to bind, the once-repressed genes are now 'turned on' or induced, allowing transcription by RNA polymerase.
• The cell can now produce the enzymes necessary to metabolize the inducer—in this case, to break down erythritol.

The inducible control system allows bacteria like Brucella to adapt to an environment rich in a specific substrate and efficiently use available resources.