Bacterial iron homeostasis

2.6 Å resolution crystal structure of the bacterioferritin fromAzotobacter vinelandii

The crystal structure of the bacterioferritin from Azotobacter vinelandii has been determined at 2.6 A resolution. Both the low occupancy of one iron ion in the dinuclear iron center and the deviation of its adjacent residue His130 from the center suggest migration of the iron ion from the dinuclear iron site to the inner nucleation site. The concerted movement of His130 and Glu47 may admit a dynamic gating mechanism for shift of the oxidized iron ion. Ba2+ binding to the fourfold channel implicates that the channel bears Fe2+ conductivity and selectivity to provide a route for iron access to the inner cavity during core formation.

The Fur-like protein Mur of Rhizobium leguminosarum is a Mn(2+)-responsive transcriptional regulator

In wild-type Rhizobium leguminosarum, the sitABCD operon specifies a Mn(2+) transporter whose expression is severely reduced in cells grown in the presence of this metal. Mutations in the R. leguminosarum gene, mur (manganese uptake regulator), whose product resembles the Fur transcriptional regulator, cause high-level expression of sitABCD in the presence of Mn(2+). In gel-shift mobility assays, purified R. leguminosarum Mur protein bound to at least two regions near the sitABCD promoter region, although this DNA has no conventional consensus Fur-binding sequences (fur boxes). Thus, in contrast to gamma-proteobacteria, where Fur binds Fe(2+), the R. leguminosarum Fur homologue, Mur, act as a Mn(2)-responsive transcriptional regulator.

Critical roles of bacterioferritins in iron storage and proliferation of cyanobacteria

Cyanobacteria are key contributors to global photosynthetic productivity, and iron availability is essential for cyanobacterial proliferation. While iron is abundant in the earth's crust, its unique chemical properties render it a limiting factor for photoautotrophic growth. As compared to other nonphotosynthetic organisms, oxygenic photosynthetic organisms such as cyanobacteria, algae, and green plants need large amounts of iron to maintain functional PSI complexes in their photosynthetic apparatus. Ferritins and bacterioferritins are ubiquitously present iron-storage proteins. We have found that in the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803), bacterioferritins are responsible for the storage of as much as 50% of cellular iron. Synechocystis 6803, as well as many other cyanobacterial species, have two bacterioferritins, BfrA and BfrB, in which either the heme binding or di-iron center ligating residues are absent. Purified bacterioferritin complex from Synechocystis 6803 has both BfrA and BfrB proteins. Targeted mutagenesis of each of the two bacterioferritin genes resulted in poor growth under iron-deprived conditions. Inactivation of both genes did not result in a more severe phenotype. These results support the presence of a heteromultimeric structure of Synechocystis bacterioferritin, in which one subunit ligates a di-iron center while the other accommodates heme binding. Notably, the reduced internal iron concentrations in the mutant cells resulted in a lower content of PSI. In addition, they triggered iron starvation responses even in the presence of normal levels of external iron, thus demonstrating a central role of bacterioferritins in iron homeostasis in these photosynthetic organisms.

Transcriptional regulation of the Bacteroides fragilis ferritin gene (ftnA) by redox stress

This study shows that the iron-storage protein ferritin is a component of the redox-stress response in the obligate anaerobe Bacteroides fragilis. It is up-regulated at transcriptional level under aerobic conditions but constitutively expressed at low levels under anaerobic conditions. Northern hybridization and primer extension analysis revealed that ftnA is transcribed as a monocistronic mRNA of approximately 600 nt. Under reduced anaerobic conditions, ftnA mRNA levels were not dependent on the iron content of the culture medium. Following oxygen exposure ftnA message increased about 10-fold in iron-replete medium compared to a fourfold increase under low-iron conditions. Addition of the oxidant potassium ferricyanide induced expression of ftnA mRNA anaerobically, suggesting that the oxidation of the medium affected expression of ftnA. Two transcription initiation start sites were identified. Both transcripts were expressed constitutively under anaerobic conditions but one promoter was induced by oxidative stress or the addition of the oxidant potassium ferricyanide. The effect of redox stress on ftnA expression was further investigated by addition of diamide, a thiol-oxidizing agent, which induced ftnA mRNA levels anaerobically, suggesting that an unbalanced cellular redox state also affects ftnA expression. Induction by hydrogen peroxide and oxygen was decreased in an oxyR deletion mutant but some oxygen induction still occurred. This strongly suggests that ftnA is regulated by both the peroxide response transcriptional activator, OxyR, and another unidentified oxygen-dependent regulator. Taken together, these data show that ftnA mRNA levels are controlled by both iron and oxidative stress; this coordinated regulation may be important for survival in an adverse aerobic environment.

Structural basis for iron binding and release by a novel class of periplasmic iron-binding proteins found in gram-negative pathogens

We have determined the 1.35- and 1.45-A structures, respectively, of closed and open iron-loaded forms of Mannheimia haemolytica ferric ion-binding protein A. M. haemolytica is the causative agent in the economically important and fatal disease of cattle termed shipping fever. The periplasmic iron-binding protein of this gram-negative bacterium, which has homologous counterparts in many other pathogenic species, performs a key role in iron acquisition from mammalian host serum iron transport proteins and is essential for the survival of the pathogen within the host. The ferric (Fe(3+)) ion in the closed structure is bound by a novel asymmetric constellation of four ligands, including a synergistic carbonate anion. The open structure is ligated by three tyrosyl residues and a dynamically disordered solvent-exposed anion. Our results clearly implicate the synergistic anion as the primary mediator of global protein conformation and provide detailed insights into the molecular mechanisms of iron binding and release in the periplasm.

The Binding Mechanism of Pyoverdin with the Outer Membrane Receptor FpvA in Pseudomonas aeruginosa Is Dependent on Its Iron-Loaded Status

In iron-deficient conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophore named pyoverdin (Pvd), which after chelating iron(III) is transported back into the cell via its outer membrane receptor FpvA. FpvA is a TonB-dependent transport protein and has the ability to bind Pvd in its apo- or iron-loaded form. The fluorescence properties of Pvd were used to determine the binding kinetics of metal-free and metal-loaded Pvd to FpvA and showed two major features. First, the kinetics of formation of the FpvA-Pvd complex, in vivo and in vitro, are markedly slower compared to those observed for FpvA-Pvd-metal. Second, apo-Pvd and Pvd-metal absorbed with biphasic kinetics to FpvA: the bimolecular step (association of the ligand with the receptor) is followed by a slower step (t(1/2) values of 5 and 34 min for Pvd-metal and Pvd, respectively) that presumably leads to a more stable complex. The most likely explanation for this second step is that the binding of the ligand to the receptor induces a conformational change on FpvA, which may be different, depending on the loading status of Pvd. Analysis of the dissociation of metal-free Pvd from FpvA revealed an energy and a TonB dependency. The dissociation of iron-free Pvd from FpvA in the absence of the TonB protein occurs with slow kinetics in the range of hours, but it can be highly activated by the protonmotive force and TonB to reach a kinetic with a t(1/2) of 1 min. Apparently, under iron-limited conditions, TonB activates the FpvA receptor, resulting in a fast release of iron-free Pvd and generating an unloaded FpvA receptor, competent for binding extracellular Pvd-Fe.

Zn, Cu and Co in cyanobacteria: selective control of metal availability

Homeostatic systems for essential and non-essential metals create the cellular environments in which the correct metals are acquired by metalloproteins while the incorrect ones are somehow avoided. Cyanobacteria have metal requirements often absent from other bacteria; copper in thylakoidal plastocyanin, zinc in carboxysomal carbonic anhydrase, cobalt in cobalamin but magnesium in chlorophyll, molybdenum in heterocystous nitrogenase, manganese in thylakoidal water-splitting oxygen-evolving complex. This article reviews: an intracellular trafficking pathway for inward copper supply, the sequestration of surplus zinc by metallothionein (also present in other bacteria) and the detection and export of excess cobalt. We consider the influence of homeostatic proteins on selective metal availability.