Question

The \(t r p\) operon is considered _______ operon because the protein encoding genes necessary for tryptophan synthesis are not expressed when the level of tryptophan in the cell is high. a. an inducible b. a positive c. a repressible d. a negative e. both \(c\) and \(d\)

Short answer

The \(t r p\) operon is considered both 'c. a repressible and d. a negative' operon.

Step-by-step solution

Understanding Operons

Operons are functional units of DNA that contain a group of genes under the control of a single promoter. The genes within the operon are either expressed altogether or not at all, depending upon the interaction with certain proteins and molecules. One such interaction is called a repressible system.

Associating the Repressible System with the Exercise

A repressible system, like tryptophan (trp) operon, is one where the associated genes are typically in an 'on' state but can be turned 'off'. This switch occurs when a specific molecule, known as a corepressor (in this case tryptophan), binds to a repressor protein and then to the operator of the operon. This prevents the transcription of the genes found downstream and thus halt protein production.

Choosing the Right Answer

From the above information, we see that when the level of tryptophan molecule (corepressor) is high in the cell, the trp operon switches 'off' hindering protein synthesis. Therefore, it is considered a repressible system. This helps conserve energy by preventing the synthesis of proteins that the cell already has in sufficient quantity. Also, it is worth noting that repressible systems typically function in a negative feedback mechanism, where the product of a process inhibits its own production. Therefore, the trp operon is also 'negative'. As a result, the correct answer is both 'repressible' and 'negative'. Hence, the option e. both 'c' and 'd' is correct.

Key concepts

Repressible System

A repressible system is a regulatory genetic mechanism where the default state of gene expression is "on." This means that the genes are actively being transcribed and translated into proteins until a specific signal shuts them down. In the trp operon, this signal is the presence of tryptophan. When tryptophan levels increase, it acts as a signal to turn "off" the operon.
The key components of a repressible system include:

• Structural genes: These are the genes that need to be expressed under normal circumstances, like those involved in synthesizing tryptophan.
• Promoter: A DNA sequence where RNA polymerase attaches to start transcription.
• Operator: A DNA region that controls the access of RNA polymerase to the structural genes.
• Repressor protein: It binds to the operator sequence, blocking RNA polymerase from transcribing the operon.
• Corepressor: A molecule that activates the repressor protein, stabilizing its form so it can bind to the operator.

The trp operon is an excellent example of a repressible system, where the synthesis is regulated according to the cell's needs.

Negative Feedback Mechanism

The negative feedback mechanism is a process used by cells to maintain homeostasis by reducing the effect of fluctuations. It's a way for the cell to regulate various functions efficiently.

In the case of the trp operon, when there is an abundance of tryptophan, this amino acid acts as a signal to stop its further production. The steps in this feedback loop include:

• Accumulation of tryptophan: When the concentration of tryptophan rises, it serves as an indicator that synthesis should stop.
• Binding to the repressor protein: Tryptophan binds to a specific repressor protein.
• Switching off the operon: The hormone-repressor complex binds to the operator region on the operon, inhibiting the transcription activity of RNA polymerase.
• Reduction of tryptophan: This shut-off helps prevent unnecessary synthesis, conserving energy and resources within the cell.

This mechanism ensures that cells do not waistfully over-produce tryptophan when it is already plentiful.

Gene Expression Regulation

Gene expression regulation is the process by which cells control the timing and amount of protein production from their genes. This regulation is essential for cellular function and adapting to various internal and external conditions.

In repressible systems like the trp operon, gene expression is regulated by:

• Regulatory proteins: These proteins, such as repressors, can activate or deactivate gene expression depending on cellular conditions.
• Environmental signals: The presence or absence of certain molecules—such as tryptophan—determine if genes will be expressed.
• Transcription factors: They help RNA polymerase bind to DNA and start transcription.

This regulation optimizes protein production based on the cell’s needs and resource availability. For the trp operon, gene expression is tightly controlled to produce tryptophan only when necessary.

Corepressor Function

The role of a corepressor in genetic regulation is to enhance the function of a repressor protein. In our example, tryptophan acts as a corepressor.

Here's how it works:

• Inactive Repressor: Initially, the repressor protein is in an inactive form and cannot bind to the DNA operator region on its own.
• Corepressor Binding: When tryptophan levels rise, tryptophan binds to the repressor protein.
• Activated Repressor: This binding changes the shape of the repressor protein, activating it.
• Operator Binding: The activated repressor then binds to the operator site on the DNA, blocking transcription by stopping RNA polymerase from moving forward.

As a result, the presence of a corepressor like tryptophan ensures that the operon is switched off when the product is plentiful, thus maintaining cellular efficiency.