Iron-containing proteins figure prominently in the nitrogen-fixing symbiosis between soybeans and Bradyrhizobium japonicum. Iron acquisition, however, presents unique challenges both in soil and in planta. Many rhizobial species, including B. japonicum, produce and utilize iron-chelating siderophores as a means of solubilizing and acquiring iron. Siderophore utilization requires a specific uptake system, including a high-affinity outer membrane receptor. Purification of B. japonicum strain 61A152 iron-regulated outer membrane proteins has led to the cloning of two genes with similarity to known siderophore receptors. The fegA gene is predicted to encode a protein with similarity to hydroxamate siderophore receptors (LeVier, Guerinot 1996).
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