Consider the tryptophan operon. Which of the following statements most accurately describes the mechanism by which the repressor protein functions?
A) The repressor protein is produced only when tryptophan is present and binds to the operator to prevent transcription of the genes that code for the enzymes of tryptophan synthesis
B) The repressor protein is produced constitutively (all the time) and is turned off by the binding of tryptophan
C) The repressor protein undergoes a shape change when bound to tryptophan
D) The repressor is an enzyme that degrades mRNA unless tryptophan binds to inhibit the enzyme
The correct answer and explanation is:
The correct answer is C) The repressor protein undergoes a shape change when bound to tryptophan.
In the tryptophan operon, the repressor protein is produced constitutively, meaning it is always present in the cell. However, this repressor is initially inactive and cannot bind to the operator on the DNA to prevent transcription. When tryptophan is abundant in the cell, it binds to the repressor protein. This binding induces a conformational change (shape change) in the repressor, which activates its ability to bind to the operator region of the operon.
Once the repressor is bound to the operator, it physically blocks the RNA polymerase from transcribing the genes necessary for tryptophan synthesis. As a result, the cell halts the production of tryptophan when it is already available in sufficient amounts, conserving energy and resources.
This mechanism is an example of negative feedback regulation. The presence of tryptophan signals the system that the gene products are no longer needed, and this feedback loop prevents the unnecessary synthesis of more tryptophan. It helps the cell maintain a balance of metabolic resources.
In contrast to other mechanisms, such as option B, where the repressor would be turned off by tryptophan binding, the tryptophan operon works by activating the repressor when tryptophan is present. Therefore, the correct explanation is that tryptophan binding results in a shape change that enables the repressor to effectively block transcription.