Question

Tryptophan operon is an example of _______ operon.

• A. Inducible
• B. Repressible
• C. Both A and B
• D. None of the above

Answer 1

The correct option is B Repressible

A repressible operon is the one in which transcription is repressed 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 which has five structural genes. 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. An operon that is switched on by binding of inducer to repressors, is called as in inducible operon. The trp operon is repressed by binding of trptophan to repressor and hence, is not a inducible operon, thus, options A, C, D do not hold true for it. Thus, the correct answer is option B.