Fig. 5

Non-incremental tuning of signal output from the major osmosensor of E. coli (EnvZ). a EnvZ is bifunctional and possesses both kinase and phosphatase activity. The ratio of these activities is modulated by several factors including the presence of extracellular osmolarity [66, 67], procaine [68] or MzrA [69, 70]. OmpR serves as the cognate RR of EnvZ and the intracellular level of phosphorylated OmpR (OmpR-P) in governed by EnvZ activity. b OmpR-P levels control transcription of ompF and ompC, which can be monitored by employing an E. coli strain that contains a transcriptional fusion of yfp to ompF (yellow) and of cfp to ompC (blue) [7]. Intracellular levels of OmpR-P (red) can thus be estimated by monitoring the CFP/YFP ratio. c Experimental results demonstrating that increasing sucrose levels in the growth medium resulted in increasing levels of CFP, decreasing levels of YFP and an increase in the CFP/YFP ratio. The black dashed line in Fig. 5B represents signal output from this strain upon growth in medium with no additional sucrose. d When aromatic tuning was performed in EnvZ, a Trp-Leu-Phe triplet (red) was repositioned within the C-terminal region of TM2. e The gray-filled circles on the dashed lines indicate the estimated OmpR-P levels in cells expressing one of the aromatically tuned variants. Aromatic tuning in EnvZ resulted in a pattern of signal output that did not correlate with the absolute vertical position of the aromatic residues, as was the case with Tar, but rather appeared approximately helical in distribution suggesting that the surface of TM2 that the residues were located upon was of greater importance [33]. However, the key outcome is that aromatic tuning was still successful with respect to modulating EnvZ signal output in a stimulus-independent manner