SURFACE IRON POLYMERS AND HYDROXY-ACIDS - MODEL OF IRON SUPPLY IN SIIDERAMINE-FREE FUNGI

Biochemical and electron microscopic evidence is presented that sideramine-free fungi form iron hydroxide polymer layers on the cell surface when grown in an iron containing medium. Iron hydroxide polymer formation on the cell surface is completely prevented in sideramine producing strains of Neurospora crassa. After feeding a sideramine-free mutant of Neurospora crassa with ornithine in order to restore the sideramine synthesis the iron hydroxide coat is gradually dissolved. The addition of excess citrate and malate to the incubation medium also prevents iron polymer adsorption, suggesting that hydroxy acids may be involved in iron supply, when sideramine-free organisms are grown in iron containing media. In order to study the interaction between iron hydroxide polymer deposition upon the cell surface and iron chelating acids in Neurospora crassa, the amount and the proportion of excreted acids was studied under various experimental conditions. Gas chromatographic analysis of the acids produced under iron deficient conditions revealed that succinate, malate and citrate were present within the cells in the early growth phase. The acids were sequentially excreted into the medium in the order succinate, malate and citrate. The amount of succinate decreased after 2 days of cultivation, whereas the amount of malate and citrate continually increased. Although citrate was present within the cells from the 1st day, excretion occurred very late, generally after the 3rd day. It is suggested that sideramine-free fungi first adsorb iron as a hydroxide polymer on the cell surface, and that it is gradually solubilized by excreted hydroxy acids such as citrate or malate. Thus high local concentrations of iron chelated by hydroxy acids provide sideramine-free fungi with a continuous iron supply. © 1979 Springer-Verlag.