The Concept of Tryptophan Operon

The trp operon codes for a gene responsible for the synthesis of typtophan, an amino acid. Unlike lac operon, it has 5 genes in a cluster. These structural genes are regulated by a repressor protein that binds to the operator sequence. 

When the trp concentration in the bacterial cell is low, the repressor protein cannot bind to the operator. However, when the trp concentration increases to a certain level and no tryptophan synthesis is needed, tryptophan binds to the repressor protein and enhances its ability to bind to the operator to switch off the expression of structural genes. 

Since tryptophan enhances the binding of a repressor protein, it is called co-repressor. And try operon is called repressible operon.

Trp Operon gene	Functions
TrpE	Gene for anthranilate synthetase subunit
TrpD	Gene for anthranilate synthetase subunit
TrpC	Gene for glycerolphosphate synthetase
TrpB	Gene for tryptophan synthetase subunit
TrpA	Gene for tryptophan synthetase subunit