Dps protects cells against multiple stresses during stationary phase

Abstract

Dps, the nonspecific DNA-binding protein from starved cells, is the most abundant protein in stationary-phase Escherichia coli. Dps homologs are found throughout the bacteria and in at least one archaeal species. Dps has been shown to protect cells from oxidative stress during exponential-phase growth. During stationary phase, Dps organizes the chromosome into a highly ordered, stable nucleoprotein complex called the biocrystal. We show here that Dps is required for long-term stationary-phase viability under competitive conditions and that dps mutants have altered lag phases compared to wild-type cells. We also show that during stationary phase Dps protects the cell not only from oxidative stress but also from UV and gamma irradiation, iron and copper toxicity, thermal stress, and acid and base shock. The protective roles of Dps are most likely achieved through a combination of functions associated with the protein-DNA binding and chromosome compaction, metal chelation, ferroxidase activity, and regulation of gene expression.

FIG. 1.

Long-term survival and competition patterns. (A) Superimposition of growth curves of cells grown separately in monoculture. WT, wild type. (B) Competition assay of cells grown in coculture. (C) Competition of dps null mutants containing dps-expressing (pJE106) or control (pBR322) plasmids. Solid symbols, SF2049(pJE106-Dps⁺) plated on kanamycin only (squares) or on kanamycin and ampicillin (circles). Open symbols, SF2043(pBR322-Dps⁻) plated on chloramphenicol only (squares) or on chloramphenicol and ampicillin (circles). Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.

FIG. 2.

Comparison of lag phases of wild-type (WT) and dps mutant cells from 1-day-old stationary-phase cultures. Shown is a superimposition of growth curves of cells grown separately in monoculture.

FIG. 3.

Hydrogen peroxide stress. Overnight cultures of wild-type (WT; squares) or dps (circles) cells were treated with 400 mM H₂O₂. Solid symbols, untreated cells; open symbols, peroxide-treated cells. Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.

FIG. 4.

UV stress. Overnight cultures of wild-type (WT) or dps cells were exposed to UV radiation. Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.

FIG. 5.

Gamma radiation stress. Overnight cultures of wild-type (WT) or dps cells were exposed to gamma irradiation. Representative data are shown.

FIG. 6.

Metal stress during stationary phase and log phase. Wild type (WT; squares) or dps (circles) cells were treated with either iron (A and B) or copper (C and D) during stationary phase (A and C) or log phase (B and D). Open symbols, metal treatment; filled symbols, no-treatment controls. Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.

FIG. 7.

Temperature stress. Overnight cultures of wild-type (WT) or dps cells were incubated at 55°C. Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.

FIG. 8.

pH stress. Overnight cultures of wild-type (solid symbols) or dps (open symbols) cells were incubated at pH 7 (squares), 2 (circles), or 12 (triangles). Asterisks indicate no detectable cells (limit of detection, <1,000 CFU/ml). Representative data are shown.