Iron Transport in Mycobacterium smegmatis: Uptake of Iron from Ferriexochelin

Abstract

Summary: Exochelins are a group of extracellular iron chelators produced by mycobacteria. Iron uptake by washed suspensions of iron-deficiently grown Mycobacterium smegmatis from 55Fe(III)-exochelin fractions (at about 1 μm) was greatest from the fractions containing the compounds that naturally predominate in culture filtrates. Uptake from the major fraction, as well as from combined exochelins, had a K m of about 6 μm and was unaffected by the presence of a large excess of desferriexochelin; it was inhibited by more than 90% by electron transport inhibitors, uncouplers of oxidative phosphorylation, thiol reagents and by anaero-biosis and low temperature. Uptake of iron from 55Fe-salicylate, which is mediated by myco-bactin, was insensitive to these inhibitors and a 10-fold excess of ferric salicylate did not inhibit 55Fe-exochelin uptake. Thus mycobactin is probably not involved in the transport of iron from ferriexochelin at physiological concentrations. The rate of uptake of iron from 55Fe-exochelin into iron-sufficiently grown cells, which contain less than 0·05% of the concentration of mycobactin found in iron-deficiently grown cells, was only slightly lower than the rate of uptake into iron-deficiently grown cells. Uptake of ferri[3H]exochelin, which could only be carried out at high and probably non-physiological concentrations (about 60 μm), was also extremely sensitive to metabolic inhibitors suggesting that the whole complex was being transported. At these high external concentrations of ferriexochelin a second, non-saturable, inhibitor-insensitive iron uptake process occurred. This process was inhibited in iron-deficiently grown cells by ferric salicylate and may therefore involve mycobactin. A similar but not identical second system which was not sensitive to ferric salicylate was found in iron-sufficiently grown cells; this might indicate yet another pathway of iron uptake from ferriexochelin.