Phenotypic bimodality and hysteresis in the <i>phoQ (T281R)</i> mutant.

<p>(A) Schematic of the PhoQ/PhoP circuit. The <i>phoPQ</i> operon in transcribed and translated to produce PhoP and PhoQ proteins (cyan arrows). Post-tranlational interactions between PhoQ and PhoP generate PhoP-P, which upregulates the expression of the <i>phoPQ</i> operon and the <i>yfp</i> reporter (solid black arrows). The PhoQ phosphatase activity, which is absent in PhoQ (T281R), is depicted with a gray arrow. (B) Bimodal behavior of YFP colony fluorescence. Panel shows CFP and YFP channel images of a minimal medium plate on which an LB overnight culture of the <i>phoQ (T281R)</i> strain was spread. (C and D) Phenotypic hysteresis. Overnight cultures of <i>phoQ (WT)</i>, <i>phoQ (T281R)</i> OFF, and <i>phoQ (T281R)</i> ON strains in LB were diluted 1000-fold into Minimal A medium with 100 µM Mg², grown to mid-exponential phase and imaged by fluorescence microscopy (<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003706#s4" target="_blank">Methods</a>). The PhoP-P state of the cell was determined by measuring YFP expression driven by the PhoP-P responsive <i>mgrB</i> promoter and normalized to CFP expression driven by a constitutive promoter. Cell widths were quantified by fitting phase image masks to ellipses and computing the minor axis length. Representative phase images of the three strains are shown in panel C and reveal morphological differences between OFF and ON cells. Fluorescence and cell width values for each cell are plotted in panel D with indicated colors distinguishing the three strains. Note that <i>phoQ (T281R)</i> OFF cells (maroon) have much lower fluorescence and are wider than <i>phoQ (T281R)</i> ON cells (blue).