Growth in iron-enriched medium partially compensates Escherichia coli for the lack of manganese and iron superoxide dismutase

Abstract

Enrichment of the growth medium with iron partially relieves the phenotypic deficits imposed on Escherichia coli by lack of both manganese and iron superoxide dismutases. Thus iron supplementation increased the aerobic growth rate, decreased the leakage of sulfite, and diminished sensitivity toward paraquat. Iron supplementation increased the activities of several [4Fe-4S]-containing dehydratases, and this was seen even in the presence of 50 microg/ml of rifampicin, an amount which completely inhibited growth. Assessing the O-2 scavenging activity by means of lucigenin luminescence indicated that the iron-enriched sodAsodB cells had gained some means of eliminating O-2, which was not detectable as superoxide dismutase activity in cell extracts. It is noteworthy that iron-enriched cells were not more sensitive toward the lethality of H2O2 despite having the usual amount of catalase activity. This indicates that iron taken into the cells from the medium is not available for Fenton chemistry, but is available for reconstitution of iron-sulfur clusters. We suppose that oxidation of the [4Fe-4S] clusters of dehydratases by O-2 and their subsequent reductive reconstitution provides a mechanism for scavenging O-2 and that speeding this reductive reconstitution by iron enrichment both spared other targets from O-2 attack and maintained adequate levels of these enzymes to meet the metabolic needs of the cells.