Ferrous iron efflux systems in bacteria

Iron transport and its regulation in plants

Iron is an essential element for plants as well as other organisms, functioning in various cellular processes, including respiration, chlorophyll biosynthesis, and photosynthesis. Plants take up iron from soil in which iron solubility is extremely low especially under aerobic conditions at high-pH range. Therefore, plants have evolved efficient iron-uptake mechanisms. Because iron is prone to being precipitated and excess ionic iron is cytotoxic, plants also have sophisticated internal iron-transport mechanisms. These transport mechanisms comprise iron chelators including nicotianamine, mugineic acid family phytosiderophores and citrate, and various types of transporters of these chelators, iron-chelate complexes, or free iron ions. To maintain iron homeostasis, plants have developed mechanisms for regulating gene expression in response to iron availability. Expression of various genes involved in iron uptake and translocation is induced under iron deficiency by transcription factor networks and is negatively regulated by the ubiquitin ligase HRZ/BTS. This response is deduced to be mediated by cellular iron sensing as well as long-distance iron signaling. The ubiquitin ligase HRZ/BTS is a candidate intracellular iron sensor because it binds to iron and zinc, and its activity is affected by iron availability. The iron-excess response of plants is thought to be partially independent of the iron-deficiency response. In this review, we summarize and discuss extant knowledge of plant iron transport and its regulation.

Sinorhizobium fredii HH103 RirA Is Required for Oxidative Stress Resistance and Efficient Symbiosis with Soybean

Members of Rhizobiaceae contain a homologue of the iron-responsive regulatory protein RirA. In different bacteria, RirA acts as a repressor of iron uptake systems under iron-replete conditions and contributes to ameliorate cell damage during oxidative stress. In Rhizobium leguminosarum and Sinorhizobium meliloti, mutations in rirA do not impair symbiotic nitrogen fixation. In this study, a rirA mutant of broad host range S. fredii HH103 has been constructed (SVQ780) and its free-living and symbiotic phenotypes evaluated. No production of siderophores could be detected in either the wild-type or SVQ780. The rirA mutant exhibited a growth advantage under iron-deficient conditions and hypersensitivity to hydrogen peroxide in iron-rich medium. Transcription of rirA in HH103 is subject to autoregulation and inactivation of the gene upregulates fbpA, a gene putatively involved in iron transport. The S. fredii rirA mutant was able to nodulate soybean plants, but symbiotic nitrogen fixation was impaired. Nodules induced by the mutant were poorly infected compared to those induced by the wild-type. Genetic complementation reversed the mutant's hypersensitivity to H₂O₂, expression of fbpA, and symbiotic deficiency in soybean plants. This is the first report that demonstrates a role for RirA in the Rhizobium-legume symbiosis.

Carbonate-radical-anions, and not hydroxyl radicals, are the products of the Fenton reaction in neutral solutions containing bicarbonate

The Fenton reaction, Fe(H₂O)₆²⁺ + H₂O₂ → Oxidizing product, is of major importance in biology as the major cause of oxidative stress, and in advanced oxidation processes. It is commonly assumed that ·OH is the product of the Fenton reaction. The results presented herein point out that ·OH is indeed the oxidizing product in acidic solutions for [Fe(H₂O)₆²⁺] > [H₂O₂]; Fe^IV_aq is the active oxidizing product in neutral solutions; in slightly acidic solutions for [H₂O₂] > [Fe(H₂O)₆²⁺] a mixture of ·OH and Fe^IV_aq is formed. However CO₃·^- is the active oxidizing product in neutral solutions containing HCO₃^- even at low concentrations, i.e. under physiological conditions. The implications to our understanding of the origins of oxidative stress and of catalytic oxidations in advanced oxidation processes are discussed.

Genome-Wide Characterization of the Fur Regulatory Network Reveals a Link between Catechol Degradation and Bacillibactin Metabolism in Bacillus subtilis

Many bacteria synthesize high-affinity iron chelators (siderophores). Siderophore-mediated iron acquisition is an efficient and widely utilized strategy for bacteria to meet their cellular iron requirements. One prominent class of siderophores uses catecholate groups to chelate iron. B. subtilis bacillibactin, structurally similar to enterobactin (made by enteric bacteria), is a triscatecholate siderophore that is hydrolyzed to monomeric units after import to release iron. However, the ultimate fates of these catechol compounds and their potential toxicities have not been defined previously. We performed genome-wide identification of Fur binding sites in vivo and uncovered a connection between catechol degradation and bacillibactin metabolism in B. subtilis. Besides its role in the detoxification of environmental catechols, the catechol 2,3-dioxygenase encoded by catDE also protects cells from intoxication by endogenous bacillibactin-derived catechol metabolites under iron-limited conditions. These findings shed light on the degradation pathway and precursor recycling of the catecholate siderophores.

The ferric uptake regulator (Fur) is the global iron biosensor in many bacteria. Fur functions as an iron-dependent transcriptional repressor for most of its regulated genes. There are a few examples where holo-Fur activates transcription, either directly or indirectly. Recent studies suggest that apo-Fur might also act as a positive regulator and that, besides iron metabolism, the Fur regulon might encompass other biological processes such as DNA synthesis, energy metabolism, and biofilm formation. Here, we obtained a genomic view of the Fur regulatory network in Bacillus subtilis using chromatin immunoprecipitation sequencing (ChIP-seq). Besides the known Fur target sites, 70 putative DNA binding sites were identified, and the vast majority had higher occupancy under iron-sufficient conditions. Among the new sites detected, a Fur binding site in the promoter region of the catDE operon is of particular interest. This operon, encoding catechol 2,3-dioxygenase, is critical for catechol degradation and is under negative regulation of CatR and YodB. These three repressors (Fur, CatR, and YodB) function cooperatively to regulate the transcription of catDE, with Fur functioning as a sensor of iron limitation and CatR as the major sensor of catechol stress. Genetic analysis suggests that CatDE is involved in metabolism of the catecholate siderophore bacillibactin, particularly when bacillibactin is constitutively produced and accumulates intracellularly, potentially generating endogenous toxic catechol derivatives. This study documents a role for catechol degradation in bacillibactin metabolism and provides evidence that catechol 2,3-dioxygenase can detoxify endogenously produced catechol substrates in addition to its more widely studied role in biodegradation of environmental aromatic compounds and pollutants.

Physiological background of the remarkably high Cd2+ tolerance of the Aspergillus fumigatus Af293 strain

The physiological background of the unusually high cadmium tolerance (MIC₅₀  > 2 mM) of Aspergillus fumigatus Af293 was investigated. The cadmium tolerance of the tested environmental and clinical A. fumigatus strains varied over a wide range (0.25 mM < MIC₅₀  < 1 mM). Only the Af293 strain showed a MIC₅₀ value of >2 mM, and this phenotype was accompanied by increased in vivo virulence in mice. A strong correlation was found between the cadmium tolerance and the transcription of the pcaA gene, which encodes a putative cadmium efflux pump. The cadmium tolerance also correlated with the iron tolerance and the extracellular siderophore production of the strains. In addition to these findings, Af293 did not show the synergism between iron toxicity and cadmium toxicity that was detected in the other strains. Based on these results, we suggest that the primary function of PcaA should be acting as a ferrous iron pump and protecting cells from iron overload. Nevertheless, the heterologous expression of pcaA may represent an attractive strain improvement strategy to construct fungal strains for use in biosorption or biomining processes or to prevent accumulation of this toxic metal in crops.

DR1440 is a potential iron efflux protein involved in maintenance of iron homeostasis and resistance of Deinococcus radiodurans to oxidative stress

Iron acquisition by bacteria is well studied, but iron export from bacteria is less understood. Herein, we identified dr1440 with a P-type ATPase motif as a potential exporter of iron from Deinococcus radiodurans, a bacterium known for its extreme resistance to radiation and oxidants. The DR1440 was located in cell membrane as demonstrated by fluorescence labelling analysis. Mutation of dr1440 resulted in cellular accumulation of iron ions, and expression level of dr1440 was up-regulated significantly under iron ion or hydrogen peroxide stress in the wild-type strain, implicating DR1440 as a potential iron efflux protein. The dr1440 mutant displayed higher sensitivity to iron ions and oxidative stresses including hydrogen peroxide, hypochlorous acid, and gamma-ray irradiation compared with the wild-type strain. The high amount of iron in the mutant strain resulted in severe protein carbonylation, suggesting that DR1440 might contribute to intracellular protein protection against reactive oxygen species (ROS) generated from ferrous ion-mediated Fenton-reaction. Mutations of S297A and C299A led to intracellular accumulation of iron, indicating that S297 and C299 might be important functional residues of DR1440. Thus, DR1440 is a potential iron efflux protein involved in iron homeostasis and oxidative stress-resistance of D. radiodurans.

Toward a mechanistic understanding of Feo-mediated ferrous iron uptake

Virtually all organisms require iron and have evolved to obtain this element in free or chelated forms. Under anaerobic or low pH conditions commonly encountered by numerous pathogens, iron predominantly exists in the ferrous (Fe2+) form. The ferrous iron transport (Feo) system is the only widespread mechanism dedicated solely to bacterial ferrous iron import, and this system has been linked to pathogenic virulence, bacterial colonization, and microbial survival. The canonical feo operon encodes for three proteins that comprise the Feo system: FeoA, a small cytoplasmic β-barrel protein; FeoB, a large, polytopic membrane protein with a soluble G-protein domain capable of hydrolyzing GTP; and FeoC, a small, cytoplasmic protein containing a winged-helix motif. While previous studies have revealed insight into soluble and fragmentary domains of the Feo system, the chief membrane-bound component FeoB remains poorly studied. However, recent advances have demonstrated that large quantities of intact FeoB can be overexpressed, purified, and biophysically characterized, revealing glimpses into FeoB function. Two models of full-length FeoB have been published, providing starting points for hypothesis-driven investigations into the mechanism of FeoB-mediated ferrous iron transport. Finally, in vivo studies have begun to shed light on how this system functions as a unique multicomponent complex. In light of these new data, this review will summarize what is known about the Feo system, including recent advancements in FeoB structure and function.

Overview on the Bacterial Iron-Riboflavin Metabolic Axis

Redox reactions are ubiquitous in biological processes. Enzymes involved in redox metabolism often use cofactors in order to facilitate electron-transfer reactions. Common redox cofactors include micronutrients such as vitamins and metals. By far, while iron is the main metal cofactor, riboflavin is the most important organic cofactor. Notably, the metabolism of iron and riboflavin seem to be intrinsically related across life kingdoms. In bacteria, iron availability influences expression of riboflavin biosynthetic genes. There is documented evidence for riboflavin involvement in surpassing iron-restrictive conditions in some species. This is probably achieved through increase in iron bioavailability by reduction of extracellular iron, improvement of iron uptake pathways and boosting hemolytic activity. In some cases, riboflavin may also work as replacement of iron as enzyme cofactor. In addition, riboflavin is involved in dissimilatory iron reduction during extracellular respiration by some species. The main direct metabolic relationships between riboflavin and iron in bacterial physiology are reviewed here.

Sequential induction of Fur-regulated genes in response to iron limitation in Bacillus subtilis

Bacterial cells modulate transcription in response to changes in iron availability. The ferric uptake regulator (Fur) senses intracellular iron availability and plays a central role in maintaining iron homeostasis in Bacillus subtilis Here we utilized FrvA, a high-affinity Fe²⁺ efflux transporter from Listeria monocytogenes, as an inducible genetic tool to deplete intracellular iron. We then characterized the responses of the Fur, FsrA, and PerR regulons as cells transition from iron sufficiency to deficiency. Our results indicate that the Fur regulon is derepressed in three distinct waves. First, uptake systems for elemental iron (efeUOB), ferric citrate (fecCDEF), and petrobactin (fpbNOPQ) are induced to prevent iron deficiency. Second, B. subtilis synthesizes its own siderophore bacillibactin (dhbACEBF) and turns on bacillibactin (feuABC) and hydroxamate siderophore (fhuBCGD) uptake systems to scavenge iron from the environment and flavodoxins (ykuNOP) to replace ferredoxins. Third, as iron levels decline further, an "iron-sparing" response (fsrA, fbpAB, and fbpC) is induced to block the translation of abundant iron-utilizing proteins and thereby permit the most essential iron-dependent enzymes access to the limited iron pools. ChIP experiments demonstrate that in vivo occupancy of Fur correlates with derepression of each operon, and the graded response observed here results, at least in part, from higher-affinity binding of Fur to the "late"-induced genes.