Anaerobic induction of ProMn-superoxide dismutase in Escherichia coli.

Escherichia coli growing anaerobically respond to NO3- plus PQ2+ with a 20-30-fold induction of an inactive form of the manganese-containing superoxide dismutase (MnSOD). Mutants lacking a functional nitrate reductase fail to show this response. This inactive enzyme can be activated by addition of Mn(II) salts to cell extracts in the presence of acidic guanidinium chloride, followed by dialysis against neutral buffer. Direct addition of Mn(II) to cell extracts does not result in activation. However, addition of Mn(II) to purified apo-MnSOD results in partial activation. Inactive, reconstitutable MnSOD is induced 13-fold within 15 min of exposure to NO3- plus PQ2+. Western blot analysis revealed a 15-fold increase in immunoreactive MnSOD under these conditions, suggestive of de novo synthesis of this protein. A strain of E. coli bearing a multicopy plasmid carrying the MnSOD gene (sodA) overproduces inactive MnSOD 19-fold compared to the parent strain under anaerobic conditions. Strains of E. coli with an inactivating insertion in the sodA gene do not induce inactive, reconstitutable MnSOD in response to NO3- plus PQ2+ and lack the immunoreactive MnSOD band. These results, in toto, suggest that the inactive protein synthesized under anaerobic conditions in the presence of NO3- plus PQ2+, acting as an electron sink, is a product of the sodA gene and is devoid of activity due to occupation of the manganese site by another metal.