 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.