Question

Which is true for repressible operon?

• A. Off $\underset{Inducer}{⟶}$ on
• B. Inactive repressor + Co-repressor = active repressor
• C. Active repressor + Inducer = inactive repressor
• D. On $\stackrel{Inducer}{⟶}$ off

Answer 1

The correct option is B Inactive repressor + Co-repressor = active repressor

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. If tryptophan is present in the environment, then E. coli does not need to synthesize it; trp operon is turned off. A substance that inhibits the expression of genes not by direct binding to DNA but instead indirectly regulates gene expression by binding to repressors is known as co-repressor. For example, in trp operon, two tryptophan molecules serve as co-repressor and bind to the inacitve trp repressor and activate it. Binding of the tryptophan repressor complex at the operator prevents the RNA polymerase from binding the downstream genes and thus presents their transcription. Here, tryptophan serve as a co-repressor. A molecule that regulates gene expression by binding to repressors or activators to prevent or start the transcription respectively, is known as inducer. Thus, binding of inducer to a repressor always activate the repressor; never inactivate it; option c is incorrect statement. Further, a repressible operon is regulated by repressor and co-repressor; not by inducer, thus option A and D does not hold true for it. Option B is the correct answer.