Fig. 1.

Transcription profiles of four PhoP-regulated promoters in response to extracellular [Mg²⁺]. (a) Each reporter strain contains a chromosomal copy of yfp controlled by a PhoP-regulated promoter, and a copy of cfp, controlled by a constitutive promoter (tetA promoter), at the attachment sites of λ and HK022 phages, respectively. The expression of CFP from the tetA promoter is constant over the range of magnesium concentrations used in our experiments (22). (b) Steady-state transcription of PhoP-regulated promoters as measured by the YFP/CFP fluorescence ratio of single cells. Cultures were grown at 37°C in minimal medium containing the indicated concentrations of MgSO₄. Each point indicates the mean of two independent cultures, and each bar indicates the corresponding range. The dotted lines denote fits to saturating curves. (c) Concentration of magnesium at which YFP/CFP is halfway between the maximal and minimal values, determined for each promoter from the fitted curves in a. Bars denote the errors from the nonlinear fits. (d) Normalized curves from b. Each curve was normalized by shifting and rescaling by constants. For details of the analysis, see SI Text.