Nucleoid compaction by MrgAAsp56Ala/Glu60Aladoes not contribute to staphylococcal cell survival against oxidative stress and phagocytic killing by macrophages

Staphylococcus aureus MrgA (encoded by mrgA) belongs to the Dps family of proteins, which play important roles in coping with various stresses. The staphylococcal mrgA gene is specifically expressed under oxidative stress conditions and is one of the most highly induced genes during phagocytic killing by macrophages. We previously reported that mrgA is essential for oxidative stress resistance, and can cause nucleoid compaction. However, whether nucleoid compaction by itself would contribute to oxidative stress resistance was hard to determine, because Dps family proteins generally have ferroxidase activity to prevent hydroxyl radical formation via the Fenton reaction. In this study, we resolved the crystal structure of MrgA and conducted mutation analysis of Asp56 and Glu60, which are located at the expected ferroxidase centre. In the strain expressing Asp56Ala/Glu60Ala MrgA (termed MrgA*), MrgA* retained dodecamer formation and nucleoid compaction ability. By contrast, the ferroxidase activity of MrgA* decreased by about half. Viability of the mrgA* strain was as low as the mrgA null mutant in oxidative stress and phagocytic killing assays. These results suggest that nucleoid compaction by itself is insufficient for oxidative stress resistance, and Asp56 and Glu60 constitute essential molecular sites in MrgA for oxidative stress resistance and survival against phagocytic killing.