text: In prokaryotes, the term corepressor is used to denote the activating ligand of a repressor protein.  For example, the E. coli tryptophan repressor (TrpR) is only able to bind to DNA and repress transcription of the trp operon when its corepressor tryptophan is bound to it.  TrpR in the absence of tryptophan is known as an aporepressor and is inactive in repressing gene transcription.  Trp operon encodes enzymes responsible for the synthesis of tryptophan.  Hence TrpR provides a negative feedback mechanism that regulates the biosynthesis of tryptophan.
  In short tryptophan acts as a corepressor for its own biosynthesis.