Regulation and function of the synthetic dual operon. (a) Cell growth. E. coli cells carrying the synthetic operon grown under the amino acid-supplied conditions showed a constant growth rate regardless of the induction conditions. No additives, + IPTG and + Dox represent the absence of inducers, the presence of 100 μM IPTG and the presence of 100 nM doxycycline, respectively. (b) Gene expression. Population distributions of relative fluorescence intensity in the absence of inducers (solid line), in the presence of 100 μM IPTG (dotted line), or in the presence of 100 nM Dox (dashed line). Green fluorescence intensity (GFI), red fluorescence intensity (RFI) and forward scattering (FSC) represent the abundances of GFP and RFP expressed in single cells and the relative cell size, respectively. GFI/RFI/FSC indicates the concentration of GFP bias in cells. (c) Microscopic observation. Fluorescence images of cells grown in the absence of inducers (top), in the presence of 100 μM IPTG (middle) or in the presence of 100 nM Dox (bottom) are shown. The scale bars represent 10 μm. (d) Physiological function. E. coli cells carrying the dual operon were cultured under various nutritional and induced conditions. Cell growth was examined after 24 h in culture under each condition. + and − indicate the presence and absence of the additives, respectively. The concentrations of IPTG, Dox, leucine, and histidine were 100 μM, 100 nM, 1 mM, and 1 mM, respectively.
