THE MANGANESE SUPEROXIDE-DISMUTASE OF ESCHERICHIA-COLI K-12 ASSOCIATES WITH DNA

Superoxide dismutases (SODs) are vital components in the resistance of aerobic organisms to the toxicity of oxygen. Escherichia coli contain s two highly homologous cytoplasmic SODs, a manganese- and an iron-con taining enzyme (MnSOD, FeSOD). We previously demonstrated that MnSOD a nd FeSOD have different physiological functions and that MnSOD is more effective in preventing oxidative damage to DNA. In this report, puri fied E. coil MnSOD was shown to bind nonspecifically to DNA by electro phoretic mobility shift assay and nitrocellulose-filter binding method ologies. From electrophoretic mobility shift assay, the equilibrium di ssociation constants for interaction with a variety of double-stranded and single-stranded oligonucleotides ranged from 1.5 +/- 0.2 to 8.4 /- 1.3 mu M at 20 degrees C. This range of concentrations corresponds to MnSOD concentrations in aerobically grown E. coil. In vivo binding of MnSOD to DNA was supported by colocalization of MnSOD and the E. co il nucleoid in immunoelectron microscopy. Both MnSOD and DNA were inho mogeneously distributed in the cytosol, the concentration of each bein g higher in the center of the cell and relatively low near the inner m embrane. In contrast, there was no evidence for physiologically releva nt interaction of FeSOD with DNA. finding to DNA in vitro was weak, K- d > 40-220 mu M, concentrations 7-40 times higher than found in vivo. In addition, the cytoplasmic distribution of FeSOD did not correlate w ith DNA. FeSOD concentration was higher near the inner membrane and lo wer in the center of the cytosol. These results demonstrate that E. co il MnSOD associates with DNA in vitro and in vivo. Combined with prior data demonstrating that MnSOD preferentially protects DNA in vivo whi le an equal enzymatic activity of FeSOD does not (Hopkin, K. A., Papaz ian, M. A., and Steinman, H. M. (1992) J. Biol. Chem. 267, 24253-24258 ), our data suggest that E. coil MnSOD acts as a ''tethered antioxidan t''; association of MnSOD with DNA localizes dismutase activity near a target of oxidative stress and increases protection of DNA from oxida tive damage. This model has implications for the therapeutic use of SO Ds as antioxidants.