Examples of operon regulation: trp operon
The lac and trp operons are both found in E. coli.
Both operons are negatively regulated, meaning that the regulator protein binding to the operator sequence inhibits gene expression.
The trp operon is a cluster of genes that produce the amino acid tryptophan.
Tryptophan is essential for protein production.
Under normal conditions, the cell constantly synthesises tryptophan for protein production. The trp operon is usually expressed.
Whenever possible, the prokaryote will try to obtain tryptophan from its environment to save energy. When high levels of tryptophan are present within the cell, gene expression is turned off.