Functional identification of high-affinity iron permeases from Fusarium graminearum

Abstract

The ScFTR1 gene encodes an iron permease in Saccharomyces cerevisiae. Its homologues, FgFtr1 and FgFtr2, were identified from filamentous pathogenic plant fungus, Fusarium graminearum. Homologies between the deduced amino acid sequences of ScFtr1p and FgFtr1 and FgFtr2 were 56 and 54%, respectively, and both had REXXE sequences, which form the conserved amino acid sequence of ScFtr1p. FgFtr1 expression increased under iron depletion, and although FgFtr2 mRNA was not detected in the wild-type strain, it was detected in the Δfgftr1 strain in the iron-depleted condition. When the FgFtr1 and FgFtr2 were deleted, the amount of growth was found not to be different from the wild-type in iron-depleted media. However, the mRNA of FgSid, a homologue of the SIDA of Aspergillus fumigatus, was dramatically increased in the Δfgftr1/ Δfgftr2 strain and in an iron-depleted condition. FgFtr1 and FgFtr2 genes act as functional complements when they are introduced into the S. cerevisiae ΔScftr1 strain. The ΔScftr1 strain, which contains either the FgFtr1 or FgFtr2, grew well in iron-depleted media. Moreover, specific alteration of the REXXE consensus sequence of FgFtr1 and FgFtr2 did not allow for sustained growth of the ΔScftr1 strain on iron-depleted medium. The iron uptake activity was recovered when FgFtr1 and FgFtr2 genes were introduced into the ΔScftr1 strain. Though the Fet3p in S. cerevisiae was found on the intracellular vesicle in the ΔScftr1 strain, Fet3p was found on the plasma membrane when FgFtr1 or FgFtr2 was introduced into the Δftr1 strain. An infection test was carried out with deletion strains; however, no change in the ability of these strains to cause disease was observed. These results suggest that FgFtr1 and FgFtr2 may function as iron permeases in the reductive iron uptake pathway and that they do not play major roles in the pathogenicity of F. graminearum.