Uptake of iron by Geotrichum candidum, a non-siderophore producer

Geotrichum candidum (isolate 1-9) pathogenic on citrus fruits, appears to lack siderophore production. Iron uptake by G. candidum is mediated by two distinct iron-regulated, energy-and temperature-dependent transport systems that require sulfhydryl groups. One system exhibits specificity for either ferric or ferrous iron, whereas the other exhibits specificity for ferrioxamine-B-mediated iron uptake and presumably other hydroxamate siderophores. Radioactive iron uptake from⁵⁹FeCl₃ showed an optimum at pH 6 and 35° C, and Michaelis-Menten kinetics (apparent K_m = 3 μm, V_max = 0.054 nmol · mg^-1 · min^-1). The maximal rate of Fe²⁺ uptake was higher than Fe³⁺ (V_max = 0.25 nmol · mg^-1 · min^-1) but the K_m was identical. Reduction of ferric to ferrous iron prior to transport could not be detected. The ferrioxamine B system exhibits an optimum at pH 6 and 40° C and saturation kinetics (K_m = 2 μM, V_max = 0.22 nmol · mg^-1 · min^-1). The two systems were distinguished as two separate entities by negative reciprocal competition, and on the basis of differential response to temperature and phenazine methosulfate. Mössbauer studies revealed that cells fed with either⁵⁷FeCl₃ or⁵⁷FeCl₂ accumulated unknown ferric and ferrous binding metabolites.