IRON TRANSPORT IN MYCOBACTERIUM-SMEGMATIS - UPTAKE OF IRON FROM FERRIEXOCHELIN

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 anaerobiosis and low temperature. Uptake of iron from 55Fe-salicylate, which is mediated by mycobactin, 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 nonphysiological 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.