Pseudomonas aeruginosa MdaB and WrbA are water-soluble two-electron quinone oxidoreductases with the potential to defend against oxidative stress

Water-soluble quinone oxidoreductases capable of reducing quinone substrates via a concerted two-electron mechanism have been implicated in bacterial antioxidant defence. Twoelectron transfer avoids formation of dangerously reactive semi-quinone intermediates, moreover previous work in Pseudomonas putida indicated a direct protective effect for the quinols generated by an over-expressed oxidoreductase. Here, the Pseudomonas aeruginosa orthologs of five quinone oxidoreductases — MdaB, ChrR, WrbA, NfsB, and NQO1 — were tested for their possible role in defending P. aeruginosa against H 2 O 2 challenge. In in vitro assays, each enzyme was shown to reduce quinone substrates with only minimal semiquinone formation. However, when each was individually over-expressed in P. aeruginosa no overt H 2 O 2 -protective phenotype was observed. It was shown that this was due to a masking effect of the P. aeruginosa catalase, KatA; in a katA mutant, H 2 O 2 challenged strains over-expressing the WrbA and MdaB orthologs grew significantly better than the empty plasmid control. A growth advantage was also observed for H 2 O 2 challenged P. putida strains over-expressing P. aeruginosa wrbA , mdaB or katA . Despite not conferring a growth advantage to wild type P. aeruginosa , it is possible that these quinone oxidoreductases defend against H 2 O 2 toxicity at lower concentrations.