In the case of the trp operon, the ligand tryptophan is required for the repressor to work (repressible negative regulation). But other operons respond to the presence of their small molecule ...

A frequently cited example of a repressible operon under negative control is the trp operon ... evolution has selected a different regulation scheme. If tryptophan is abundant, then the cell ...

The up-regulation of a gene refers to an increase ... An example of an attenuated operon is the trp operon which encodes five enzymes necessary for tryptophan biosynthesis in E.coli.