Working diagram of the HSL-inducible system. The luxR gene is constitutively produced by the tetR promoter and the strong ribosome binding site B0034. The gene encoding for mRFP1 is placed downstream of the luxI promoter. LuxR protein is normally unactive, but when HSL is present in the culture it binds LuxR and a complex is formed by two molecules of LuxR and two of HSL. This complex triggers the transcription of the luxI promoter in a concentration-dependent fashion.