Figure 3. High and low affinity uptake of iron in filamentous fungi.

Under iron limiting conditions two high affinity iron import mechanisms are involved in uptake of iron from the environment. Via the reductive iron assimilation (RIA) system, ferric iron first becomes reduced to ferrous iron by a reductase (FRE) and then oxidized by a multicopper oxidase (FET) and imported by a plasma membrane transporter (FTR). Many fungi also produce and secrete siderophores (SID) and, after binding of iron (SID Fe) are reimported into the cell. Bound iron is subsequently made accessible to the cell by activity of a specific esterase (ESTB). In S. cerevisiae is only able to take up siderophores (Xenosiderophores) that available in the environment.