THE FET4 GENE ENCODES THE LOW-AFFINITY FE(II) TRANSPORT PROTEIN OF SACCHAROMYCES-CEREVISIAE

Previous studies on Fe(II) uptake in Saccharomyces cerevisiae suggeste d the presence of two uptake systems with different affinities for thi s substrate. We demonstrate that the FET3 gene is required for high af finity uptake but not for the low affinity system. This requirement ha s enabled a characterization of the low affinity system. Low affinity uptake is time-, temperature-, and concentration-dependent and prefers Fe(II) over Fe(III) as substrate. We have isolated a new gene, FET4, that is required for low affinity uptake, and our results suggest that FET4 encodes an Fe(II) transporter protein. FET4's predicted amino ac id sequence contains six potential transmembrane domains. Overexpressi ng FET4 increased low affinity uptake, whereas disrupting this gene el iminated that activity. In contrast, overexpressing FET4 decreased hig h affinity activity, while disrupting FET4 increased that activity. Th erefore, the high affinity system may be regulated to compensate for a lterations in low affinity activity. These analyses, and the analysis of the iron-dependent regulation of the plasma membrane Fe(III) reduct ase, demonstrate that the low affinity system is a biologically releva nt mechanism of iron uptake in yeast. Furthermore, our results indicat e that the high and low affinity systems are separate uptake pathways.