FIG 6.

H₂S induces the OxyR-mediated stress response. (A) Intracellular iron levels induced by H₂S. One-liter cultures grown to mid-log phase (OD₆₀₀ of ∼0.2) were harvested just before (0 min) and 5, 10, and 30 min after the addition of 0.25 mM H₂S or 0.2 mM dipyridyl, and the unincorporated (U) and total (T) intracellular iron concentrations were measured. The experiments were performed at least three times. Error bars show standard deviations. (B) CAT staining analysis. Cells were harvested 30 min after the addition of 0.25 mM H₂S or 0.2 mM dipyridyl along with the untreated control (--). Ten-microgram samples of protein from the cell lysates indicated were separated by native PAGE and stained for CAT activity. The ΔkatB and ΔoxyR mutant strains (constitutive high-level expression) were used as negative and positive controls. In the right panel, two higher concentrations of dipyridyl were tested for enhanced induction of the katB gene. The data are representative of three independent experiments.