Question

question 12
synthesis of a repressible enzyme is inhibited by ______.
a substrate bound to repressor
b substrate bound to promoter
c corepressor and repressor binding to operator
d repressor alone bound to operator

Explanation:

Repressible enzymes are typically involved in anabolic pathways. Their synthesis is inhibited when a corepressor (often the end product of the pathway) binds to the repressor protein, activating it. The activated repressor-corepressor complex then binds to the operator region of the operon, blocking RNA polymerase from transcribing the enzyme-coding genes. Option A is incorrect because substrate binding to repressor is not the mechanism for repressible systems. Option B is wrong as substrate does not bind to the promoter. Option D is incorrect because repressor alone (without corepressor) is inactive and cannot bind the operator to inhibit synthesis.

Answer:

C. corepressor and repressor binding to operator