A bacterial iron exporter for maintenance of iron homeostasis.
Nutritional iron acquisition by bacteria is well described, but almost nothing is known about bacterial iron export even though it is likely to be an important homeostatic mechanism. Here, we show that Bradyrhizobium japonicum MbfA (Blr7895) is an inner membrane protein expressed in cells specifically under high iron conditions. MbfA contains an N-terminal ferritin-like domain (FLD) and a C-terminal domain homologous to the eukaryotic vacuolar membrane Fe(2+)/Mn(2+) transporter CCC1. An mbfA deletion mutant is severely defective in iron export activity, contains >2-fold more intracellular iron than the parent strain, and displays an aberrant iron-dependent gene expression phenotype. B. japonicum is highly resistant to iron and H2O2 stresses, and MbfA contributes substantially to this as determined by phenotypes of the mbfA mutant strain. The N-terminal FLD was localized to the cytoplasmic side of the inner membrane. Substitution mutations in the putative iron-binding amino acid residues E20A and E107A within the N-terminal FLD abrogate iron export activity and stress response function. Purified soluble FLD oxidizes ferrous iron (Fe(2+)) to incorporate ferric iron (Fe(3+)) in a 2:1 iron:protein ratio, which does not occur in the E20A/E107A mutant. The FLD fragment is a dimer in solution, implying that the MbfA exporter functions as a dimer. MbfA belongs to a protein family found in numerous prokaryotic genera. The findings strongly suggest that iron export plays an important role in bacterial iron homeostasis.
From the Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York 14214.
Siva
S
Sankari
From the Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York 14214 mrobrian@buffalo.edu.
Mark R
MR
O'Brian
D020369:Bradyrhizobium / Q000235:genetics / Q000378:metabolism*; D006706:Homeostasis; D007501:Iron / Q000378:metabolism*; D009154:Mutation; D016133:Polymerase Chain Reaction
D007501:Iron
Bacterial Metabolism; Iron Metabolism; Membrane Transport; Metal Homeostasis; Oxidative Stress