An intracellular iron chelator pleiotropically suppresses enzymatic and growth defects of superoxide dismutase-deficient Escherichia coli

Mutants of Escherichia coli that lack cytoplasmic superoxide dismutase (SOD ) exhibit auxotrophies for sulfur-containing, branched-chain, and aromatic amino acids and cannot catabolize nonfermentable carbon sources. A secondar y-site mutation substantially relieved all of these growth defects. The req uirement for fermentable carbon and the branched-chain auxotrophy occur bec ause superoxide (O-2(-)) leaches iron from the [4Fe-4S] clusters of a famil y of dehydratases, thereby inactivating them; the suppression of these phen otypes was mediated by the restoration of activity to these dehydratases, e vidently without changing the intracellular concentration of O-2(-). Clonin g, complementation, and sequence analysis identified the suppressor mutatio n to be in dapD, which encodes tetrahydrodipicolinate succinylase, an enzym e involved in diaminopimelate and lysine biosynthesis, A block in dapB, whi ch encodes dihydrodipicolinate reductase in the same pathway, conferred sim ilar protection, Genetic analysis indicated that the protection stems from the intracellular accumulation of tetrahydro- or dihydrodipicolinate, Heter ologous expression in the SOD mutants of the dipicolinate synthase of Bacil lus subtilis generated dipicolinate and similarly protected them. Dipicolin ates are excellent iron chelators, and their accumulation in the cell trigg ered derepression of the Fur regulon and a large increase in the intracellu lar pool of free iron, presumably as a dipicolinate chelate. A fur mutation only partially relieved the auxotrophies, indicating that Fur derepression assists but is not sufficient for suppression. It seems plausible that the abundant internal iron permits efficient reactivation of superoxide damage d iron-sulfur clusters. This result provides circumstantial evidence that t he sulfur and aromatic auxotrophies of SOD mutants are also directly or ind irectly linked to iron metabolism.