Bacterial cells can take up the amino acid tryptophan from their surroundings, o
ID: 90190 • Letter: B
Question
Bacterial cells can take up the amino acid tryptophan from their surroundings, or, if the external supply is insufficient, they can synthesize tryptophan by using enzymes in the cell. In some bacteria, the control of glutamine synthesis is similar to that off tryptophan synthesis, such that the glutamine repressor inhibits the transcription of the glutamine operon, which contains the genes that code for the enzymes required for glutamine synthesis. On binding to cellular glutamine, the glutamine repressor binds to a site in the promoter of the operon. A. Why is glutamine-dependent binding to the operon a useful property for the glutamine repressor? B. What would you expect to happen to the regulation of the enzymes that synthesize glutamine in cells expressing a mutant form of the glutamine repressor that cannot bind to DNA? C. What would you expect to happen to the regulation of the enzymes that synthesize glutamine in cells the expressing a mutant form of the glutamine repressor that binds to DNA even when no glutamine is bound to it?Explanation / Answer
A. Glutamine-dependent binding site controls the synthesis of glutamine and the expression of glutamine-synthesizing enzymes by turning on and off of transcription. When glutamine is present in sufficient quantity, the glutamine repressor binds to a site in the promoter of the operon, this will block the synthesis of enzymes responsible for making glutamine. In the absence or starvation for glutamine, the unoccupied repressor is unable to bind to the promoter and hence induces the enzymes that would make more glutamine. Thus, glutamine-dependent binding to the operon is a useful property for the glutamine repressor.
B. Mutation in the glutamine repressor such that it cannot bind to DNA results in constitutive transcription of the glutamine synthesizing genes regardless of the level of glutamine in the cells. This means the expression of glutamine-synthesizing enzymes will be permanently switched on.
C. Mutation in the glutamine repressor such that it can bind to DNA permanently stop the transcription. Because repressor is always bound to a site in the promoter which prevents the further process of transcription. This results in the depletion of availability of glutamine to the cells.
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