Corepressor

- A corepressor is a molecule that cooperates with a repressor protein to switch an operon off​ - FOR EXAMPLE: E. coli can synthesize the amino acid tryptophan when it has insufficient tryptophan​. By default, the trp operon is on and the genes for tryptophan synthesis are transcribed​. When tryptophan is present, it binds to the trp repressor protein, which turns the operon off. The repressor is in the active state only in the presence of its corepressor tryptophan​. Thus the trp operon is turned off (repressed) if tryptophan levels are high​