Answer: A
Explanation:The Trp operon is responsible for synthesis of the amino acid trytophan when it is not available in the cellular environment.
When the tryptophan is plentiful, a repressor protein binds to two molecules of tryptophan.
Thiss repressor-tryptophan complex binds to the trp operator. This binding prevents the binding of RNA polymerase, so the operon is not transcribed, and no new trptophan is produce.
On the other hand, when tryptophan levels are reduced, the inactive repressor will not bind the tryptophan operator so the operon is transcribed.
Regulation of the trp operon is determined by the concentration of tryptophan; when adequate tryptophan is present in the growth medium, there is no need for tryptophan biosynthesis.
Transcription is turned off when a high concentration of tryptophan is present, and is turned on when tryptophan is absent. The regulatory signal is the concentration of tryptophan itself.
Therefore in the absence of tryptophan, the inactive repressor cannot bind trpO, so operon gene transcription occurs.
