Iron specificity of the Fur-dependent regulation of the biosynthesis of the manganese-containing superoxide dismutase in Escherichia coli.

The Fur protein, which regulates iron uptake in Escherichia coli, also represses the biosynthesis of the manganese-containing superoxide dismutase (MnSOD). A strain of E. coli bearing a lacZ fusion to the aerobactin operon was used to compare the metal specificities of the regulation of MnSOD and of aerobactin by Fur. Iron, but not manganese, acted as a corepressor of the Fur-dependent inhibition of MnSOD biosynthesis. The iron-mediated inhibition of MnSOD biosynthesis was dependent upon an intact fur locus, indicating that the effect of iron is mediated by the fur gene product. The suppression of the accumulation of MnSOD by iron, but not by manganese, was not due to destabilization of the MnSOD polypeptide by iron. Thus this effect of iron was also seen in a sodA::lacZ operon fusion in which the production of beta-galactosidase was regulated by the sodA promoter. In contrast, both iron and manganese served as corepressors of aerobactin biosynthesis. It thus appears that the effectiveness of specific metal cations to act as corepressors with Fur varies with the gene being regulated by the Fur-metal complex.