Fig 1. Simplified mechanisms of inducer uptake and regulation of target gene expression in common E. coli expression systems.
(A) Lac based gene expression via natural (lactose, galactose) or synthetic (TMG, IPTG) inducers. Uptake basically occurs through GalP (mainly galactose) or LacY (all inducers) transport proteins and by passive diffusion (* only synthetic inducers TMG and IPTG). Inducer binding leads to the release of the LacI repressor from the Plac promoter and thus induces gene expression. (B) Arabinose inducible gene expression upon active uptake via AraE and AraFGH transport proteins. In the presence of arabinose AraC positively regulates PBAD promoter activity, whereas in the absence of arabinose AraC tightly represses target gene expression. (C) Pm/XylS regulated gene expression driven by benzoates that are imported via passive diffusion and initiate the XylS regulator-dependent activation of Pm promoter based expression. Abbreviations: galP: galactose permease gene; lacI: lac repressor gene; lacZYA: lactose metabolization and uptake genes; araFGH: arabinose transporter genes; araE: arabinose transporter genes; araC: ara regulator gene; araBAD: arabinose metabolization genes; xylS: xyl regulator gene; tolX-H: toluene degradation operon.