The Trp operon is an example of a biosynthetic operon whose expression is regulated by an effector. Operon is part of genome that consists of few genes which are required to do certain process. In this example process is synthesis of tryptophan. The operon consists of 5 structural genes that code for the three enzymes required to convert chorismic acid into tryptophan. If there is tryptophan in enviromt, it is not used.