Iron acquisition during growth in an iron-deficient medium by Rhizobium sp. isolated from Cicer arietinum

A catechol-type siderophore was produced extracellularly by Rhizobium sp. strain BICC 651 isolated from Cicer arietinum during growth in iron-deficient medium. Production of the siderophore was fully repressed in the presence of 50 μM Fe3+. The siderophore was purified and characterized as containing 2,3-dihydroxybenzoic acid (DHBA) as the core compound and threonine as its conjugate. The siderophore was able to reverse growth inhibition of the strain induced by ethylenediamine-di(o-hydroxyphenyl-acetic acid) (EDDA). A high-affinity iron-transport system capable of transporting 59Fe-siderophore complex was also induced in Rhizobium BICC 651 grown under iron deficiency. Two protein bands of molecular masses 76 and 82 kDa were also inductively synthesized in the outer membrane of the cells. A partially purified ferrireductase enzyme of Rhizobium BICC 651 catalysed reductive release of iron from the ferric chelate of DHBA. The enzyme had a K m of 0.3 mM for ferri-DHBA, was constitutive in nature, and was present in the cytosolic fraction during growth under both iron-deficient and iron-sufficient conditions. The enzyme occurred as two isoenzymes with R F values of 0.48 and 0.51, respectively, in a nondenaturing polyacrylamide gel.