Repression by TraR depends on the structure and concentration of the acyl-HSL. An early exponential-phase culture of E. coli DH5α(pPBL1) harboring pZLQR in A medium containing IPTG was split into 10 subcultures. To 9 of these, synthetic acyl-HSL (▾, AAI; ▵, VAI) was added to the concentrations indicated, incubation was continued until the cultures reached late exponential phase, and the cells were assayed for β-galactosidase activity. An activity of 100% was determined as the level of β-galactosidase activity in cells not exposed to an acyl-HSL.