Which of the following acts as a corepressor to block transcription of the trp operon?
a. cAMP
b. Allolactose
c. Methionine
d. Tryptophan
The correct answer and explanation is:
Correct Answer: d. Tryptophan
The trp operon is a group of genes in Escherichia coli (E. coli) bacteria that are involved in the synthesis of the amino acid tryptophan. This operon is an example of a repressible operon, which means that it is usually active but can be turned off when the end product (tryptophan) is abundant. The regulation of the trp operon is an excellent example of feedback inhibition at the genetic level.
When tryptophan levels are low in the cell, the trp operon is active. The genes in the operon are transcribed to produce enzymes needed for the synthesis of tryptophan. In this state, the repressor protein encoded by the trpR gene is inactive on its own and cannot bind to the operator region of the operon. This allows RNA polymerase to attach to the promoter and initiate transcription.
However, when tryptophan is plentiful, it acts as a corepressor by binding to the trp repressor protein. This binding changes the shape of the repressor, enabling it to attach to the operator region of the operon. Once bound, the repressor physically blocks RNA polymerase from transcribing the operon’s genes, stopping the production of tryptophan-synthesizing enzymes. This prevents the unnecessary synthesis of tryptophan, conserving the cell’s energy and resources.
To clarify the other choices:
- a. cAMP is involved in the regulation of the lac operon and glucose metabolism.
- b. Allolactose is an inducer of the lac operon, not the trp operon.
- c. Methionine is an amino acid but not involved in trp operon regulation.
Thus, tryptophan is the correct and specific corepressor for the trp operon.