Fig. 7
Effect of the sRNA-based stress circuit on protein production of functional MPs expression. a Confocal scanning light microscopy images with intensity cross-section profiles for both the red FM4-64 membrane dye (purple colour) and the green fluorescent protein signal (GFP, green colour). Yellow bars represent the position used to make a cross-section profile. b Specific growth rate (µmax) for strains (Escherichia coli (E. coli) MG1656 DE3, Additional file 1: Table S2) expressing SohB(TMD)-sfGFP, YidC-sfGFP and GarP-sfGFP. c Total protein production results for overexpression of proteins, SohB(TMD)-sfGFP, YidC-sfGFP and GarP-sfGFP, localized in the membrane. The fluorescence of SohB(TMD)-sfGFP, YidC-sfGFP and GarP-sfGFP producing E. coli MG1656 DE3 cells without stress circuit was arbitrarily set to one and fluorescence of the same strains with respectively stress circuits 1 and 3 were compared to this value. d sfGFP/OD600-values for overexpression of proteins, SohB(TMD)-sfGFP, YidC-sfGFP and GarP-sfGFP with and without stress circuit. sfGFP/OD600 results for an equal number (OD600 = 0.25 for GarP-sfGFP and SohB(TMD)-sfGFP, OD600 = 0.18 for YidC-sfGFP) of E. coli MG1656 DE3 cells were plotted. This ratio was corrected for background fluorescence and optical density of the medium and the cell culture (Escherichia coli MG1656 DE3). All experiments were carried out in triplicates (biological variation) and the error bars represent the standard deviation from the mean value. Statistical difference between strains expressing the MP without and with stress circuit 1 or 3, was calculated using a one-way ANOVA. * = p-value < 0.05, ** = p-value < 0.01, *** = p-value < 0.001, IPTG (0.2 mM) was added to induce protein expression. YidC = membrane protein insertase in E. coli, GarP = TM transporter in E. coli, MP = membrane protein, sfGFP = superfolder green fluorescent protein, SohB(TMD) = transmembrane domain of inner-membrane protein SohB from E. coli, Stress circuit 1 (mut1) = strain expressing MP and sRNA cpxQmut1, Stress circuit 3 (mut3) = strain expressing MP and sRNA cpxQmut3