Effect of <i>l</i> <i>acY</i> Expression on Homogeneity of Induction from the P _t _ac and P _trc Promoters by Natural and Synthetic Inducers

Abstract The role of the Escherichia coli lactose permease (LacY) in the homogeneous induction of the lactose‐inducible promoters P tac and P trc by the natural inducer lactose and the synthetic inducer isopropyl‐β‐ d ‐thiogalactopyranoside (IPTG) was investigated. Lactose requires active transport by LacY, whereas IPTG can freely penetrate the cell wall. In E. coli strains lacking a functional LacY, IPTG is required for induction of P tac and P trc . In E. coli strains carrying a functional LacY, induction of P trc and P tac with intermediate concentrations of lactose gave rise to two subpopulations, one fully induced and one uninduced, whereas a single, fully induced population resulted when high inducer concentrations were used. In contrast, induction with IPTG gave rise to a single population of cells at all inducer concentrations in both lacY and lacY + strains.