Fig. 1.
Activation of σB by osmotic upshock occurs rapidly and transiently. Relative transcript levels of sigB and three other σB-dependent genes (opuCA, lmo2230, and lmo2085) were measured in exponential-phase cells in wild-type (open symbols) and ΔsigB (closed symbols) backgrounds grown in BHI broth at 37°C. RNA extracts were prepared either immediately before the addition of 0.5 M NaCl (0 min) or 5 min, 10 min, 15 min, 20 min, or 25 min after osmotic upshock. Real-time determination of gene transcription levels was carried out as previously described (16). All transcript levels were first normalized to the corresponding 16S RNA reference gene levels (internal control) (29), with an efficiency correction included for each primer pair as previously described (25), and then expressed as a percentage of the maximal level of sigB transcript detected in the experiment (thereby allowing the relative transcript levels of the 4 genes to be compared). The values presented on the graphs represent the means of the results of three independent experiments, and error bars indicate the standard deviations (n = 3). Numbers shown above the graphs indicate statistically significant differences in relative gene expression levels (severalfold change) for the wild type between the results seen under stressed (NaCl added at each time point) and nonstressed (time point 0 before adding NaCl) conditions (P < 0.05 by Student's t test). Note that the plot in panel B uses a different scale on the y axis.