Date Published: January 18, 2017
Publisher: Public Library of Science
Author(s): Ameya A. Mashruwala, Jeffrey M. Boyd, Tracey Rouault.
The SrrAB two-component regulatory system (TCRS) positively influences the transcription of genes involved in aerobic respiration in response to changes in respiratory flux. Hydrogen peroxide (H2O2) can arise as a byproduct of spontaneous interactions between dioxygen and components of respiratory pathways. H2O2 damages cellular factors including protein associated iron-sulfur cluster prosthetic groups. We found that a Staphylococcus aureus strain lacking the SrrAB two-component regulatory system (TCRS) is sensitive to H2O2 intoxication. We tested the hypothesis that SrrAB manages the mutually inclusive expression of genes required for aerobic respiration and H2O2 resistance. Consistent with our hypothesis, a ΔsrrAB strain had decreased transcription of genes encoding for H2O2 resistance factors (kat, ahpC, dps). SrrAB was not required for the inducing the transcription of these genes in cells challenged with H2O2. Purified SrrA bound to the promoter region for dps suggesting that SrrA directly influences dps transcription. The H2O2 sensitivity of the ΔsrrAB strain was alleviated by iron chelation or deletion of the gene encoding for the peroxide regulon repressor (PerR). The positive influence of SrrAB upon H2O2 metabolism bestowed protection upon the solvent accessible iron-sulfur (FeS) cluster of aconitase from H2O2 poisoning. SrrAB also positively influenced transcription of scdA (ytfE), which encodes for a FeS cluster repair protein. Finally, we found that SrrAB positively influences H2O2 resistance only during periods of high dioxygen-dependent respiratory activity. SrrAB did not influence H2O2 resistance when cellular respiration was diminished as a result of decreased dioxygen availability, and negatively influenced it in the absence of respiration (fermentative growth). We propose a model whereby SrrAB-dependent regulatory patterns facilitate the adaptation of cells to changes in dioxygen concentrations, and thereby aids in the prevention of H2O2 intoxication during respiratory growth upon dixoygen.
Staphylococcus aureus is a human pathogen that has the ability infect nearly every tissue of the body . The ability of S. aureus to sense various environmental stimuli, and rapidly calibrate its cellular physiology in response, is a cornerstone of its success as a pathogen. S. aureus is a facultative anaerobe and can respire dioxygen. Hydrogen peroxide (H2O2) is a deleterious by-product of aerobic respiration, and can arise as a result of interactions between dioxygen and components of respiratory pathways [2–5]. H2O2 can cause damage to cellular membranes and biological polymers, as well as oxidize protein bound iron-sulfur (FeS) cluster prosthetic groups [6, 7]. Oxidation of FeS clusters by H2O2 results in cluster disintegration and enzyme inactivation [7–9]. Ultimately, H2O2 exposure can result in metabolic standstill and eventually cell death [10, 11]. The importance of reactive oxygen species (ROS) in preventing S. aureus infections is evidenced by the fact that individuals carrying metabolic or genetic defects affecting ROS formation by polymorphonuclear neutrophils, the first line of defense in human innate immunity, often have chronic and reoccurring S. aureus infections .
The goal of this study was to further our understanding of the role of SrrAB in H2O2 resistance. During post-exponential growth a ΔsrrAB strain was sensitive to H2O2 challenge. In S. aureus, AhpC and Kat are required for scavenging H2O2 . The iron-sequestering and DNA-binding protein Dps is utilized to suppress Fenton chemistry [21–23]. ScdA (YtfE) also provides resistance to H2O2 by aiding in the repair of H2O2 damaged FeS proteins [24, 25]. The ΔsrrAB strain has decreased transcription of ahpC, kat, dps and scdA; therefore, the H2O2 sensitivity phenotype of the ΔsrrAB strain is likely to arise, in part, as a result of the combined effect of decreased expression of these genes. In support of this idea we found that a) increasing the expression of H2O2 resistance factors by the introduction of a null perR allele into the ΔsrrAB strain increased H2O2 tolerance, and b) pre-incubation of ΔsrrAB strain with a cell permeable iron chelator prior to H2O2 challenge, alleviated the H2O2 sensitivity phenotype. Further, purified SrrA was capable of binding to the promoter region of dps in EMSA assays, suggesting that SrrAB directly modulates the transcription of at least one H2O2 resistance factor.