Monothiol Glutaredoxin Is Essential for Oxidative Stress Protection and Virulence in Pseudomonas aeruginosa.

Glutaredoxins (Grxs), ubiquitous redox enzymes belonging to the thioredoxin family, catalyze the reduction of thiol-disulfide exchange reactions in a glutathione-dependent manner. A Pseudomonas aeruginosa ΔgrxD mutant exhibited hypersensitivity to oxidative stress-generating agents, such as paraquat (PQ) and cumene hydroperoxide (CHP). In vitro studies showed that P. aeruginosa GrxD acts as an electron donor for organic hydroperoxide resistance enzyme (Ohr) during CHP degradation. The ectopic expression of iron-sulfur cluster ([Fe-S]) carrier proteins, including ErpA, IscA, and NfuA, complements the function of GrxD in the ΔgrxD mutant under PQ toxicity. Constitutively high expression of iscR, nfuA, tpx, and fprB was observed in the ΔgrxD mutant. These results suggest that GrxD functions as a [Fe-S] cluster carrier protein involved in [Fe-S] cluster maturation. Moreover, the ΔgrxD mutant demonstrates attenuated virulence in a Drosophila melanogaster host model. Altogether, the data shed light on the physiological role of GrxD in oxidative stress protection and virulence of the human pathogen, P. aeruginosa. IMPORTANCE Glutaredoxins (Grxs) are ubiquitous disulfide reductase enzymes. Monothiol Grxs, containing a CXXS motif, play an essential role in iron homeostasis and maturation of [Fe-S] cluster proteins in various organisms. We now establish that the human pathogen Pseudomonas aeruginosa GrxD is crucial for bacterial virulence, maturation of [Fe-S] clusters and facilitation of Ohr enzyme activity. GrxD contains a conserved signature monothiol motif (C29GFS), in which C29 is essential for its function in an oxidative stress protection. Our findings reveal the physiological roles of GrxD in oxidative stress protection and virulence of P. aeruginosa.

Mutation of the gene encoding monothiol glutaredoxin (GrxD) in Pseudomonas aeruginosa increases its susceptibility to polymyxins.

Pseudomonas aeruginosa is a frequent cause of hospital-acquired infections that have a high mortality rate because of its innate drug resistance. Polymyxins are recognised as the last-line antibiotics for the treatment of multidrug-resistant (MDR) P. aeruginosa. In this study, the link between monothiol glutaredoxin (GrxD), which catalyses the reduction of disulphide bonds of various substrates in P. aeruginosa, and antibiotic resistance was examined. A P. aeruginosa ΔgrxD mutant strain was constructed. The ΔgrxD mutant showed significantly increased susceptibility to polymyxin B (PMB) compared with the wild-type P. aeruginosa PAO1. Site-directed mutagenesis was performed to generate amino acid substitutions in GrxD, and the ability of mutated grxD genes to confer resistance to PMB in the ΔgrxD mutant was tested. The results indicated that residue C29 at the active site of GrxD is important for protection against polymyxin killing in the mutant. Polymyxin killing of PAO1 and the ΔgrxD mutant did not appear to involve hydroxyl radicals generated by antibiotic treatment because increased susceptibility of the mutant to PMB was also observed under anaerobic growth as well as aerobically in the presence of the iron chelator 2,2'-dipyridyl. Thus, GrxD could be a target for the development of agents that enhance the effectiveness of PMB in treating clinically important MDR P. aeruginosa infections.

The iron-sulphur cluster biosynthesis regulator IscR contributes to iron homeostasis and resistance to oxidants in Pseudomonas aeruginosa.

IscR is a global transcription regulator responsible for governing various physiological processes during growth and stress responses. The IscR-mediated regulation of the Pseudomonas aeruginosa isc operon, which is involved in iron-sulphur cluster ([Fe-S]) biogenesis, was analysed. The expression of iscR was highly induced through the exposure of the bacteria to various oxidants, such as peroxides, redox-cycling drugs, intracellular iron-chelating agents, and high salts. Two putative type 1 IscR-binding sites were found around RNA polymerase recognition sites, in which IscR-promoter binding could preclude RNA polymerase from binding to the promoter and resulting in repression of the isc operon expression. An analysis of the phenotypes of mutants and cells with altered gene expression revealed the diverse physiological roles of this regulator. High-level IscR strongly inhibited anaerobic, but not aerobic, growth. iscR contributes significantly to the bacteria overall resistance to oxidative stress, as demonstrated through mutants with increased sensitivity to oxidants, such as peroxides and redox-cycling drugs. Moreover, the regulator also plays important roles in modulating intracellular iron homeostasis, potentially through sensing the levels of [Fe-S]. The increased expression of the isc operon in the mutant not only diverts iron away from the available pool but also reduces the total intracellular iron content, affecting many iron metabolism pathways leading to alterations in siderophores and haem levels. The diverse expression patterns and phenotypic changes of the mutant support the role of P. aeruginosa IscR as a global transcriptional regulator that senses [Fe-S] and directly represses or activates the transcription of genes affecting many physiological pathways.

The selectable antibiotic marker, tetA(C), increases Pseudomonas aeruginosa susceptibility to the herbicide/superoxide generator, paraquat.

The presence of the widely used selectable antibiotic marker, tetA(C), unexpectedly increased the sensitivity of Pseudomonas aeruginosa PAO1 to the superoxide-generating herbicide, paraquat. A DNA fragment spanning the first 99 amino acids of TetA(C) was sufficient to confer paraquat sensitivity. The TetA(C)-induced paraquat sensitive phenotype was observed in other Gram-negative bacteria such as Agrobacterium tumefaciens, Salmonella enterica ser. Typhimurium and Xanthomonas campestris suggesting that this is a general property of tetA(C). This finding serves as a cautionary note for those using tetA(C) as a selectable marker for genetic manipulations in studies using paraquat either as a superoxide stress generator or a redox cycling drug.