Article

Iron binding to Azotobacter salinestris melanin, iron mobilization and uptake mediated by siderophores

Department of Microbiology, University of Alberta, Edmonton, Canada.
BioMetals (Impact Factor: 2.5). 02/1995; 8(1):59-64. DOI: 10.1007/BF00156159
Source: PubMed

ABSTRACT

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

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    • "The contribution of melanin to cell wall charge has been studied in Cryptococcus neoformans (Nosanchuk and Casadevall 1997). An investigation of iron binding by the melanin of a bacterium , Azotobacter salinestris (Page and Shivprasad 1995), may provide hints of a similar function for fungal melanins. These authors proposed that the bacterial melanin acts as an iron trap to protect cells from oxidative damage by hydrogen peroxide generated by iron-mediated Fenton reactions. "
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    ABSTRACT: The relationship of polyketide melanogenesis molecular biology to that of nonmelanin-producing pathways in a wide range of fungi and other organisms is discussed. Analytical methods and fundamental properties of melanins are discussed and fungal melanin properties are compared with those of animal and bacterial melanins. The enzymatic degradation of melanins by lignin peroxidases is described.Key words: fungal melanin, polyketide melanin, DHN melanin, melanin degradation, melanin properties, melanin analysis.
    Preview · Article · Feb 2011 · Canadian Journal of Microbiology
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    ABSTRACT: In iron-limited medium, Azotobacter vinelandii strain UW produces three catecholate siderophores: the tricatecholate protochelin, the dicatecholate azotochelin and the monocatecholate aminochelin. Each siderophore was found to bind Fe3+ preferentially to Fe2+, in a ligand:Fe ratio of 1:1, 3:2 and 3:1, respectively. Protochelin had the highest affinity for Fe3+, with a calculated proton-independent solubility coefficient of 1043.9, comparable to ferrioxamine B. Iron-limited wild-type strain UW grown under N2-fixing or nitrogen-sufficient conditions hyper-produced catecholate siderophores in response to oxidative stress caused by high aeration. In addition, superoxide dismutase activity was greatly diminished in iron-limited cells, whereas catalase activity was maintained. The ferredoxin I (FdI)-negative A. vinelandii strain LM100 also hyper-produced catecholates, especially protochelin, under oxidative stress conditions, but had decreased activities of both superoxide dismutase and catalase, and was about 10 times more sensitive to paraquat than strain UW. Protochelin and azotochelin held Fe3+ firmly enough to prevent its reduction by ·O2- and did not promote the generation of hydroxyl radical by the Fenton reaction. Ferric-aminochelin was unable to resist reduction by ·O2- and was a Fenton catalyst. These data suggest that under iron-limited conditions, A. vinelandii suffers oxidative stress caused by ·O2-. The catecholate siderophores azotochelin, and especially protochelin, are hyper-produced to offer chemical protection from oxidative damage catalysed by ·O2- and Fe3+. The results are also consistent with FdI being required for oxidative stress management in A. vinelandii.
    Preview · Article · Jul 1998 · Microbiology
  • [Show abstract] [Hide abstract]
    ABSTRACT: The relationship of polyketide melanogenesis molecular biology to that of nonmelanin-producing pathways in a wide range of fungi and other organisms is discussed. Analytical methods and fundamental properties of melanins are discussed and fungal melanin properties are compared with those of animal and bacterial melanins. The enzymatic degradation of melanins by lignin peroxidases is described.
    No preview · Article · Dec 1998 · Canadian Journal of Microbiology
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