Problem 1 - Campbell Biology

Question

If a particular operon encodes enzymes for making an essential amino acid and is regulated like the \(trp\) operon, then \begin{equation} \begin{array}{l}{\text { (A) the amino acid inactivates the repressor. }} \\\ {\text { (B) the repressor is active in the absence of the }} \\ {\text { amino acid. }} \\ {\text { (C) the amino acid acts as a corepressor. }} \\\ {\text { (D) the amino acid turns on transcription of the operon. }}\end{array} \end{equation}

Step-by-Step Solution

Answer

The correct answer is (C) the amino acid acts as a corepressor.

Step 1 - Understand the given operon

The problem states that the operon in question is regulated like the trp operon, which is responsible for the production of tryptophan. Like the trp operon, this operon likely turns off transcription when the amino acid is abundantly available.

Step 2 - Recall the regulation mechanism of the trp operon

The trp operon is regulated through a repressor protein that binds to the operator sequence. The repressor is only active when bound to tryptophan (the corepressor). When tryptophan levels are high, the tryptophan-repressor complex binds to the operator to prevent transcription.

Step 3 - Analyze the provided answer choices

Review each option to see which accurately describes the regulation mechanism similar to the trp operon:(A) The amino acid inactivates the repressor. - This is incorrect because, in the trp operon, tryptophan activates the repressor.(B) The repressor is active in the absence of the amino acid. - This is incorrect because the repressor is inactive without the tryptophan corepressor.(C) The amino acid acts as a corepressor. - This is correct because tryptophan acts as a corepressor in the trp operon system.(D) The amino acid turns on transcription of the operon. - This is incorrect because the presence of the amino acid (tryptophan) turns off transcription.

Step 4 - Select the correct answer

Based on the analysis, the correct answer is (C) the amino acid acts as a corepressor.

Key Concepts

Repressor Protein

A repressor protein plays a crucial role in gene regulation within an operon system, such as the trp operon. The repressor protein is a type of regulatory protein that can bind to an operator sequence in the DNA, hindering RNA polymerase from transcribing the genes downstream.
In the trp operon system:

• The repressor protein becomes active or inactive based on the presence of a specific molecule, in this case, tryptophan.
• When tryptophan (an amino acid) levels are high, it binds to the repressor protein, forming a complex.
• The tryptophan-repressor complex then binds to the operator region of the DNA, effectively blocking transcription.

By understanding how the repressor protein functions, students can better grasp how gene expression is finely tuned in response to cellular conditions.

Corepressor

A corepressor is a small molecule that works together with a repressor protein to inhibit gene transcription.

• In the trp operon, tryptophan acts as a corepressor.
• When tryptophan is present in sufficient quantities, it binds to the repressor protein, altering its shape and allowing it to attach to the operator sequence in the DNA.
• This binding blocks the RNA polymerase from initiating transcription of the downstream genes.

The ability of tryptophan to serve as a corepressor ensures that the cell does not waste energy producing more tryptophan when it is already plentiful. This elegant feedback mechanism is vital for cellular efficiency and resource management.

Gene Transcription

Gene transcription is the process where the DNA sequence of a gene is copied into RNA. This is the first step in gene expression where the information in genes is used to create functional products like proteins.
In the context of the trp operon:

• When tryptophan levels are low, the repressor protein remains inactive without the corepressor (tryptophan).
• As a result, the RNA polymerase can bind to the promoter region and transcribe the genes, producing the necessary enzymes to synthesize tryptophan.
• Conversely, when tryptophan levels are high, the active tryptophan-repressor complex prevents transcription.

Ultimately, the regulation of gene transcription by the trp operon ensures that tryptophan is synthesized only when needed, balancing the cell's metabolic requirements.