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. Â 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.