Answer:
Repressible operon
Explanation:
In a repressible operon, a corepressor binds to the repressor to activate it. The active repressor then binds to the operator to prevent the expression of the operon. These operons have the genes that code for the enzymes required to make molecules from simpler substances. When the molecule is already present in the cell, it serves as a corepressor to prevent the expression of the genes.
For example, tryptophan operon is expressed only when tryptophan is not present in the cells. Tryptophan serves as a corepressor to prevent the expression of the operon.