Multiple methionine sulfoxide reductase genes in Staphylococcus aureus:: expression of activity and roles in tolerance of oxidative stress

Staphylococcus aureus contains three genes encoding MsrA-specific methionine sulfoxide reductase (Msr) activity (msrA1, msrA2 and msrA3) and an additional gene that encodes MsrB-specific Msr activity. Data presented here suggest that MsrA1 is the major contributor of the MsrA activity in S. aureus. In mutational analysis, while the total Msr activity in msrA2 mutant was comparable to that of the parent, Msr activity was significantly up-regulated in the msrA1 or msrA1 msrA2 double mutant. Assessment of substrate specificity together with increased reactivity of the cell-free protein extracts of the msrA1 mutants to anti-MsrB polyclonal antibodies in Western analysis provided evidence that increased Msr activity was due to elevated synthesis of MsrB in the MsrA1 mutants. Previously, it was reported that oxacillin treatment of S. aureus cells led to induced synthesis of MsrA1 and a mutation in msrA1 increased the susceptibility of the organism to H2O2. A mutation in the msrA2 gene, however, was not significant for the bacterial oxidative stress response. In complementation assays, while the msrA2 gene was unable to complement the msrA1 msrA2 double mutant for H2O2 resistance, the same gene restored H2O2 tolerance in the double mutant when placed under the control of the msrA1 promoter. However, msrA1 which was able to complement the oxidative stress response in msrA1 mutants could not restore the tolerance of the msrA1 msrA2 mutants to H2O2 when placed under the control of the msrA2 promoter. Additionally, although the oxacillin minimum inhibitory concentration of the msrA1 mutant was comparable to that of the wild-type parent, in shaking liquid culture, the msrA1 mutant responded more efficiently to sublethal doses of oxacillin. The data suggest complex regulation of Msr proteins and a more significant physiological role for msrA1/msrB in S. aureus.