Rhizobactin, a siderophore from Rhizobium meliloti

Abstract

Iron is required at a number of stages in the biological fixation of dinitrogen. Environmental iron occurs in an oxidized and polymerized state which is quantitatively insoluble at biological pH. Consequently, most aerobic and facultative anaerobic microorganisms have evolved specific carriers, termed siderophores, for the dissolution and uptake of iron (III). We have investigated the symbiotic nitrogen fixing bacterium, Rhizobium meliloti DM4, for the presence of a siderophore system of iron assimilation. The bacterium when cultured on low‐iron media was observed to form a substance which corrects iron starvation imposed by the synthetic chelating agent ethylenediamine‐di‐(o‐hydroxyphenyl acetic acid) (EDDA). The compound, designated rhizobactin, was extracted and obtained in crystalline form. A structural analysis of rhizobactin by means of mass and nuclear magnetic resonance spectroscopic techniques indicates that it is not a member of either the catechol or hydroxamate type families of siderophores. Of thirteen wild type isolates of R. meliloti, six vere stimulated by rhizobactin. In addition to rhizobactin, R. meliloti DM4 was stimulated by certain other siderophores, such as ferrichrome and ferrioxamine B, but not by entero‐bactin, aerobactin or mugeneic acid.

Key words:

• Iron assimilation
• siderophore
• nitrogen fixation