FIG 3.

Prestarvation decreases PhoB-regulated promoter activity in both the WT and RU1616 strains. (A) Experimental timeline. Cells were resuspended in MOPS medium (50 μM Pi) with a starting OD of ∼0.2 at time zero, and Pi became limited approximately 30 min later, indicating an ∼30-min period of growth dilution under Pi-replete conditions (area shaded in gray). (B) Comparison of phoA promoter activities in strains containing autoregulated phoBR (WT) and constitutively expressed phoBR (RU1616) when cells were prestarved. (C) Activation kinetics of the phoA promoter in RU1616 with or without prestarvation of Pi. The increased fluorescence upon stimulation is shown in the inset. An IPTG concentration of 150 μM was present in the medium for RU1616 to maintain a steady PhoB level approximately equal to that of fully induced WT. Data are shown as means ± SD of results from 11 individual wells.