IRON-BINDING TO AZOTOBACTER-SALINESTRIS MELANIN, IRON MOBILIZATION AND UPTAKE MEDIATED BY SIDEROPHORES

Iron-sufficient Azotobacter salinestris cells bound large amounts of F e-55 to cell-associated catechol melanin in an energy-independent mann er. Iron was mobilized from the cell surface by citric acid acid trans ported into the cell in a process that was inhibited by azide, carbony l cyanide m-chlorophenyl-hydrazone (CCCP), KCl or RbCl, the latter two known to inhibit Na+-dependent activities in A. salinestris. Iron-lim ited cells produced a hydroxamate compound (HDX) which promoted Fe-55- uptake into iron-limited cells in a two step process. Initial uptake w as inhibited by azide or CCCP, but not by KCl, while subsequent uptake was blocked by all inhibitors, Citric acid also mediated energy-depen dent Fe-55-uptake in iron-limited cells, but initial iron-uptake was l ess sensitive to CCCP than HDX-mediated iron-uptake. The results show that melanin serves as an iron trap, probably to protect the cells fro m oxidative damage mediated by H2O2 and the Fenton reaction. A model f or HDX siderophore-mediated iron-uptake is proposed which requires ene rgy to concentrate iron in the periplasm and H+/Na+-dependent events t o bring iron into the cell.