Question

Which is true for tryptophan operon?

• A. It is the example of inducible operon.
• B. It is the example of repressible operon.
• C. On $\stackrel{co-repressor}{⟶}$ off.
• D. B and C are correct.

Answer 1

Answer: (D)

B and C are correct.

A repressible operon is the one in which transcription is reduced in the presence of a particular substance. This substance is mostly the end product of a biosynthetic pathway. For example, trp operon is a repressible operon. E. coli can ingest tryptophan from the environment and also synthesize tryptophan. The governing five genes constitute the tryptophan (trp) operon.

If tryptophan is present in the environment, then E. coli does not need to synthesize it; trp operon is turned off. However, when tryptophan availability is low, the switch controlling the operon is turned on. In presence of tryptophan in the cell, two tryptophan molecules serve as co-repressor and bind to the trp repressor. This binding activate the inactive repressor to bind to the trp operator. Binding of the tryptophan-repressor complex at the operator prevents the RNA polymerase from binding the downstream genes and thus, prevents their transcription. Here, tryptophan serve as a corepressor and inhibits the expression of genes not by direct binding to DNA but instead indirectly regulates gene expression by binding to repressors. An operon that is switched on by binding of inducer to repressors, is called as inducible operon. The trp operon is repressed by binding of trptophan to repressor and hence, is not a inducible operon, thus option A does not hold true for it. Option D is the correct answer.