The main difference between the mode of regulation of the lac and trp operons in Escherichia coli is that
A) the presence of tryptophan leads to the repression of the trp operon whereas the presence of lactose leads to the expression of the lac operon.
B) the presence of tryptophan leads to the expression of the trp operon whereas the presence of lactose leads to the repression of the lac operon.
C) the lac operon is constitutively expressed whereas the trp operon is regulated.
D) there are no major differences in the mode of regulation, just in the products of the genes involved.
E) the lac gene consists of a promoter region while the trp gene only contains a cis-acting operator sequence.

Question

Quizplus · Accepted Answer

The lac operon is induced (activated) in the presence of lactose, which serves as its substrate, while the trp operon is repressed (inhibited) in the presence of tryptophan, its end product. This reflects a fundamental difference in their regulation: the lac operon is positively regulated by the presence of its substrate, whereas the trp operon is negatively regulated by the presence of its product.

The Main Difference Between the Mode of Regulation of the Lac