Iron uptake from ferrichrome A and iron citrate in Ustilago sphaerogena

Abstract

Double radioactive label transport assays with iron, chromium, and gallium chelates were used to investigate the mechanism of iron uptake by Ustilago sphaerogena. In iron-deficient cells, ferrichrome A iron was taken up without appreciable uptake of the ligand. Iron-sufficient cells partially accumulated the ligand with the metal. The chromiumand gallium-containing analogs of ferrichrome A were transported as intact chelates. Ferrichrome A iron uptake was inhibited by dipyridyl. The data suggest that the intact ferrichrome A chelate binds to a specific receptor, the iron is then separated from the ligand at the membrane by reduction, and the metal is released to the inside of the cell while the ligand is released to the exterior. The reduction step is not transport rate limiting. Iron chelated to citrate was taken up by an energy-dependent process. The citrate ligand was not taken up with the metal. Uptake was sensitive to dipyridyl and ferrozine. Chromic ion chelated to citrate was not transported, suggesting that the iron, rather than the chelate, is recognized by the receptor or that reduction of the metal is required for transport.Under iron-deficient growth conditions, many microorganisms excrete low-molecular-weight iron-chelating agents (known as siderophores) for the purpose of sotubilizing extracellular iron. After complexing iron, the ferric siderophores supply the metal to the cell by binding to specific receptors in the cell membrane (14,15). Siderophore-mediated iron transport has been investigated primarily by following the fate of radioactively labeled iron in transport assays, although recently investigators have followed iron transport by electron paramagnetic resonance and Mossbauer spectroscopy (3,13).Double-label experiments with both the iron and ligand labeled have revealed two basic mechanisms for iron uptake from ferric siderophores. Ferrichrome uptake in Ustilago sphaerogena is the prototype for the first mechanism, known as the iron shuttle mechanism, in which the iron and ligand are taken up at identical rates (6). After the iron is removed from the ligand inside the cell, the free ligand reappears in the medium and may serve for another round of iron transport. In the second mechanism, uptake of iron occurs without uptake of ligand. In this case, the ferric siderophore binds to the cell membrane where the iron is removed from the ligand; the iron is taken into the cell, and the ligand remains extracellular. This is known as t Present address:

Nodulating ability of Rhizobium tropici is conditioned by a plasmid‐encoded citrate synthase

“…6, the dotted line and (A) represent a medium condition where both 300 nM of chelator and 1 |aM of iron were added. Ecker and Emery, 1983). It may also have the same role in R. tropici.…”

SREA is involved in regulation of siderophore biosynthesis, utilization and uptake in Aspergillus nidulans

“…Two different iron uptake systems, a reductive and a non-reductive one, are present in Ustilago as well as in Saccharomyces (Ecker and Emery 1983;Lesuisse and Labbe, 1989;Ardon et al, 1998;Yun et al, 2000b). Up until now, it is not known which mechanism(s) A. nidulans possesses for using siderophore-bound iron.…”

“…The second system depends on members of the major facilitator superfamily and is specialized in the uptake of siderophore-bound iron (Lesuisse et al, 1998;2001;Heymann et al, 1999;2000a,b;Yun et al 2000b,c). The presence of two different iron uptake systems in a fungus, one reductive and another non-reductive, is not unique to Saccharomyces -it was first demonstrated for the basidiomycete Ustilago sphaerogena (Eckert and Emery, 1983). Nevertheless, the utilization of ferri-siderophores by S. cerevisiae is remarkable because this eukaryote model microorganism is an exception, in that it does not excrete siderophores (Neilands et al, 1987).…”

Ferric-reductase activities in Vibrio vulnificus biotypes 1 and 2

“…Ferricyanide (potassium ferricyanide (ICN Biochemicals) added to a ¢nal concentration of 1.3 mM) reduction was measured by following the protocols described previously [10,12].…”

“…The possibility that V. vulni¢cus cells could reduce substrates that are not transported into the bacterial cell, as occurs in Ustilago sphaerogena [12,15] or V. anguillarum [10], was tested by adding 1.3 mM ferricyanide to a cell culture previously washed and resuspended in CM9. Strains from both biotypes were able to reduce ferricyanide (detected as a decrease in absorbance at 420 nm), with the strongest ferricyanide reductase activity corresponding to strain TW1 (data not shown).…”