A role for Dps ferritin activity in long-term survival of Escherichia coli

Escherichia coli expresses three ferritins that store acquired iron by oxidizing soluble Fe(II) to insoluble Fe(III), which can accumulate and later be utilized in cellular processes. Although bacterioferritin (Bfr) and ferritin (FtnA) sequester more Fe(III) atoms per multimeric complex, the abundance of the DNA-binding protein from starved cells (Dps), coupled with its preference for hydrogen peroxide as an oxidant in its ferroxidase activity, makes it a fundamental component in iron homeostasis and long-term stationary phase (LTSP) survival. To investigate the temporal role and mechanisms of action of Dps in parallel with the other ferritins, growth yield, survival, competitive fitness, and siderophore assays were performed under different conditions of iron availability. dps mutant strains exhibit the lowest cell viability and long-term cellular survival rates under all conditions tested, including iron toxicity, iron starvation, and in standard growth medium, in both competition and monoculture. We postulate that Dps is the critical ferritin, since under both monoculture and co-inoculation conditions, ftnA and bfr mutants maintain higher population densities during LTSP, while dps mutant viability decreases by up to four orders of magnitude. Furthermore, the production of siderophores, indicative of cells sensing iron starvation, is ~2,000 times greater in a dps mutant compared to the wild-type, ftnA, or bfr mutant strains, indicating that cells lacking Dps cannot appropriately maintain iron homeostasis.IMPORTANCEIron is a crucial cofactor for many enzymatic processes; however, it can be toxic due to its role in Fenton chemistry and production of reactive oxygen species (ROS). E. coli encodes three differentially expressed iron detoxifying and storage proteins, the ferritins FtnA, Bfr, and Dps, that contribute to maintaining iron homeostasis. While the distinct roles for each ferritin remain poorly elucidated, Dps is postulated to account for the majority of iron storage in stationary phase and later due to its abundance in stationary phase cells. Furthermore, its use of hydrogen peroxide as its preferred oxidant is hypothesized to help modulate the deleterious effects of intracellular iron and ROS production. Given that virtually all bacteria encode at least one dps homolog and most microbes in nature frequently experience a state akin to long-term stationary phase, the data presented here support a model in which Dps plays a fundamental role in long-term bacterial iron homeostasis and survival.