Heme Transfer to the Bacterial Cell Envelope Occurs via a Secreted Hemophore in the Gram-positive Pathogen Bacillus anthracis

To initiate and sustain an infection in mammals, bacterial pathogens must acquire host iron. However, the host''s compartmentalization of large amounts of iron in heme, which is bound primarily by hemoglobin in red blood cells, acts as a barrier to bacterial iron assimilation. Bacillus anthracis, the causative agent of the disease anthrax, secretes two NEAT (near iron transporter) proteins, IsdX1 and IsdX2, which scavenge heme from host hemoglobin and promote growth under low iron conditions. The mechanism of heme transfer from these hemophores to the bacterial cell is not known. We present evidence that the heme-bound form of IsdX1 rapidly and directionally transfers heme to IsdC, a NEAT protein covalently attached to the cell wall, as well as to IsdX2. In both cases, the transfer of heme is mediated by a physical association between the donor and recipient. Unlike Staphylococcus aureus, whose NEAT proteins acquire heme from hemoglobin directly at the bacterial surface, B. anthracis secretes IsdX1 to capture heme in the extracellular milieu and relies on NEAT-NEAT interactions to deliver the bound heme to the envelope via IsdC. Understanding the mechanism of NEAT-mediated iron transport into pathogenic Gram-positive bacteria may provide an avenue for the development of therapeutics to combat infection.     

Differential Function of Lip Residues in the Mechanism and Biology of an Anthrax Hemophore

To replicate in mammalian hosts, bacterial pathogens must acquire iron. The majority of iron is coordinated to the protoporphyrin ring of heme, which is further bound to hemoglobin. Pathogenic bacteria utilize secreted hemophores to acquire heme from heme sources such as hemoglobin. Bacillus anthracis, the causative agent of anthrax disease, secretes two hemophores, IsdX1 and IsdX2, to acquire heme from host hemoglobin and enhance bacterial replication in iron-starved environments. Both proteins contain NEAr-iron Transporter (NEAT) domains, a conserved protein module that functions in heme acquisition in Gram-positive pathogens. Here, we report the structure of IsdX1, the first of a Gram-positive hemophore, with and without bound heme. Overall, IsdX1 forms an immunoglobin-like fold that contains, similar to other NEAT proteins, a 3₁₀-helix near the heme-binding site. Because the mechanistic function of this helix in NEAT proteins is not yet defined, we focused on the contribution of this region to hemophore and NEAT protein activity, both biochemically and biologically in cultured cells. Site-directed mutagenesis of amino acids in and adjacent to the helix identified residues important for heme and hemoglobin association, with some mutations affecting both properties and other mutations affecting only heme stabilization. IsdX1 with mutations that reduced the ability to associate with hemoglobin and bind heme failed to restore the growth of a hemophore-deficient strain of B. anthracis on hemoglobin as the sole iron source. These data indicate that not only is the 3₁₀-helix important for NEAT protein biology, but also that the processes of hemoglobin and heme binding can be both separate as well as coupled, the latter function being necessary for maximal heme-scavenging activity. These studies enhance our understanding of NEAT domain and hemophore function and set the stage for structure-based inhibitor design to block NEAT domain interaction with upstream ligands.     

IlsA,A Unique Surface Protein of Bacillus cereus Required for Iron Acquisition from Heme,Hemoglobin and Ferritin

The human opportunistic pathogen Bacillus cereus belongs to the B. cereus group that includes bacteria with a broad host spectrum. The ability of these bacteria to colonize diverse hosts is reliant on the presence of adaptation factors. Previously, an IVET strategy led to the identification of a novel B. cereus protein (IlsA, Iron-regulated leucine rich surface protein), which is specifically expressed in the insect host or under iron restrictive conditions in vitro. Here, we show that IlsA is localized on the surface of B. cereus and hence has the potential to interact with host proteins. We report that B. cereus uses hemoglobin, heme and ferritin, but not transferrin and lactoferrin. In addition, affinity tests revealed that IlsA interacts with both hemoglobin and ferritin. Furthermore, IlsA directly binds heme probably through the NEAT domain. Inactivation of ilsA drastically decreases the ability of B. cereus to grow in the presence of hemoglobin, heme and ferritin, indicating that IlsA is essential for iron acquisition from these iron sources. In addition, the ilsA mutant displays a reduction in growth and virulence in an insect model. Hence, our results indicate that IlsA is a key factor within a new iron acquisition system, playing an important role in the general virulence strategy adapted by B. cereus to colonize susceptible hosts.     

Molecular and Evolutionary Analysis of NEAr-Iron Transporter (NEAT) Domains

Iron is essential for bacterial survival, being required for numerous biological processes. NEAr-iron Transporter (NEAT) domains have been studied in pathogenic Gram-positive bacteria to understand how their proteins obtain heme as an iron source during infection. While a 2002 study initially discovered and annotated the NEAT domain encoded by the genomes of several Gram-positive bacteria, there remains a scarcity of information regarding the conservation and distribution of NEAT domains throughout the bacterial kingdom, and whether these domains are restricted to pathogenic bacteria. This study aims to expand upon initial bioinformatics analysis of predicted NEAT domains, by exploring their evolution and conserved function. This information was used to identify new candidate domains in both pathogenic and nonpathogenic organisms. We also searched metagenomic datasets, specifically sequence from the Human Microbiome Project. Here, we report a comprehensive phylogenetic analysis of 343 NEAT domains, encoded by Gram-positive bacteria, mostly within the phylum Firmicutes, with the exception of Eggerthella sp. (Actinobacteria) and an unclassified Mollicutes bacterium (Tenericutes). No new NEAT sequences were identified in the HMP dataset. We detected specific groups of NEAT domains based on phylogeny of protein sequences, including a cluster of novel clostridial NEAT domains. We also identified environmental and soil organisms that encode putative NEAT proteins. Biochemical analysis of heme binding by a NEAT domain from a protein encoded by the soil-dwelling organism Paenibacillus polymyxa demonstrated that the domain is homologous in function to NEAT domains encoded by pathogenic bacteria. Together, this study provides the first global bioinformatics analysis and phylogenetic evidence that NEAT domains have a strong conservation of function, despite group-specific differences at the amino acid level. These findings will provide information useful for future projects concerning the structure and function of NEAT domains, particularly in pathogens where they have yet to be studied.     

The Near-iron Transporter (NEAT) Domains of the Anthrax Hemophore IsdX2 Require a Critical Glutamine to Extract Heme from Methemoglobin

Several Gram-positive pathogenic bacteria employ near-iron transporter (NEAT) domains to acquire heme from hemoglobin during infection. However, the structural requirements and mechanism of action for NEAT-mediated heme extraction remains unknown. Bacillus anthracis exhibits a rapid growth rate during systemic infection, suggesting that the bacterium expresses efficient iron acquisition systems. To understand how B. anthracis acquires iron from heme sources, which account for 80% of mammalian iron stores, we investigated the properties of the five-NEAT domain hemophore IsdX2. Using a combination of bioinformatics and site-directed mutagenesis, we determined that the heme extraction properties of IsdX2 are dependent on an amino acid with an amide side chain within the 3₁₀-helix of the NEAT domain. Additionally, we used a spectroscopic analysis to show that IsdX2 NEAT domains only scavenge heme from methemoglobin (metHb) and that autoxidation of oxyhemoglobin to metHb must occur prior to extraction. We also report the crystal structures of NEAT5 wild type and a Q29T mutant and present surface plasmon resonance data that indicate that the loss of this amide side chain reduces the affinity of the NEAT domain for metHb. We propose a model whereby the amide side chain is first required to drive an interaction with metHb that destabilizes heme, which is subsequently extracted and coordinated in the aliphatic heme-binding environment of the NEAT domain. Because an amino acid with an amide side chain in this position is observed in NEAT domains of several genera of Gram-positive pathogenic bacteria, these results suggest that specific targeting of this or nearby residues may be an entry point for inhibitor development aimed at blocking bacterial iron acquisition during infection.     

The Role of Heme Binding by DNA-protective Protein from Starved Cells (Dps) in the Tolerance of Porphyromonas gingivalis to Heme Toxicity

The widely expressed DNA-protective protein from starved-cells (Dps) family proteins are considered major contributors to prokaryotic resistance to stress. We show here that Porphyromonas gingivalis Dps (PgDps), previously described as an iron-storage and DNA-binding protein, also mediates heme sequestration. We determined that heme binds strongly to PgDps with an apparent K_d of 3.7 × 10⁻⁸ m and is coordinated by a single surface-located cysteine at the fifth axial ligand position. Heme and iron sequestered in separate sites by PgDps provide protection of DNA from H₂O₂-mediated free radical damage and were found to be important for growth of P. gingivalis under excess heme as the only iron source. Conservation of the heme-coordinating cysteine among Dps isoforms from the Bacteroidales order suggests that this function may be a common feature within these anaerobic bacteria.     

Iron Acquisition in Bacillus cereus: The Roles of IlsA and Bacillibactin in Exogenous Ferritin Iron Mobilization

In host-pathogen interactions, the struggle for iron may have major consequences on the outcome of the disease. To overcome the low solubility and bio-availability of iron, bacteria have evolved multiple systems to acquire iron from various sources such as heme, hemoglobin and ferritin. The molecular basis of iron acquisition from heme and hemoglobin have been extensively studied; however, very little is known about iron acquisition from host ferritin, a 24-mer nanocage protein able to store thousands of iron atoms within its cavity. In the human opportunistic pathogen Bacillus cereus, a surface protein named IlsA (Iron-regulated leucine rich surface protein type A) binds heme, hemoglobin and ferritin in vitro and is involved in virulence. Here, we demonstrate that IlsA acts as a ferritin receptor causing ferritin aggregation on the bacterial surface. Isothermal titration calorimetry data indicate that IlsA binds several types of ferritins through direct interaction with the shell subunits. UV-vis kinetic data show a significant enhancement of iron release from ferritin in the presence of IlsA indicating for the first time that a bacterial protein might alter the stability of the ferritin iron core. Disruption of the siderophore bacillibactin production drastically reduces the ability of B. cereus to utilize ferritin for growth and results in attenuated bacterial virulence in insects. We propose a new model of iron acquisition in B. cereus that involves the binding of IlsA to host ferritin followed by siderophore assisted iron uptake. Our results highlight a possible interplay between a surface protein and a siderophore and provide new insights into host adaptation of B. cereus and general bacterial pathogenesis.     

Differential Contributions of the Outer Membrane Receptors PhuR and HasR to Heme Acquisition in Pseudomonas aeruginosa

Pseudomonas aeruginosa PAO1 encodes two outer membrane receptors, PhuR (Pseudomonas heme uptake) and HasR (heme assimilation system). The HasR and PhuR receptors have distinct heme coordinating ligands and substrate specificities. HasR is encoded in an operon with a secreted hemophore, HasAp. In contrast the non-hemophore-dependent PhuR is encoded within an operon along with proteins required for heme translocation into the cytoplasm. Herein we report on the contributions of the HasR and PhuR receptors to heme uptake and utilization. Employing bacterial genetics and isotopic [¹³C]heme labeling studies we have shown both PhuR and HasR are required for optimal heme utilization. However, the unique His-Tyr-ligated PhuR plays a major role in the acquisition of heme. In contrast the HasR receptor plays a primary role in the sensing of extracellular heme and a supplementary role in heme uptake. We propose PhuR and HasR represent non-redundant heme receptors, capable of accessing heme across a wide range of physiological conditions on colonization of the host.     

Bacillus anthracis secretes proteins that mediate heme acquisition from hemoglobin

Acquisition of iron is necessary for the replication of nearly all bacterial pathogens; however, iron of vertebrate hosts is mostly sequestered by heme and bound to hemoglobin within red blood cells. In Bacillus anthracis, the spore-forming agent of anthrax, the mechanisms of iron scavenging from hemoglobin are unknown. We report here that B. anthracis secretes IsdX1 and IsdX2, two NEAT domain proteins, to remove heme from hemoglobin, thereby retrieving iron for bacterial growth. Unlike other Gram-positive bacteria, which rely on cell wall anchored Isd proteins for heme scavenging, B. anthracis seems to have also evolved NEAT domain proteins in the extracellular milieu and in the bacterial envelope to provide for the passage of heme.     

The five near-iron transporter (NEAT) domain anthrax hemophore, IsdX2, scavenges heme from hemoglobin and transfers heme to the surface protein IsdC

Pathogenic bacteria require iron to replicate inside mammalian hosts. Recent studies indicate that heme acquisition in Gram-positive bacteria is mediated by proteins containing one or more near-iron transporter (NEAT) domains. Bacillus anthracis is a spore-forming, Gram-positive pathogen and the causative agent of anthrax disease. The rapid, extensive, and efficient replication of B. anthracis in host tissues makes this pathogen an excellent model organism for the study of bacterial heme acquisition. B. anthracis secretes two NEAT hemophores, IsdX1 and IsdX2. IsdX1 contains a single NEAT domain, whereas IsdX2 has five, a novel property among hemophores. To understand the functional significance of harboring multiple, non-identical NEAT domains, we purified each individual NEAT domain of IsdX2 as a GST fusion and analyzed the specific function of each domain as it relates to heme acquisition and transport. NEAT domains 1, 3, 4, and 5 all bind heme, with domain 5 having the highest affinity. All NEATs associate with hemoglobin, but only NEAT1 and -5 can extract heme from hemoglobin, seemingly by a specific and active process. NEAT1, -3, and -4 transfer heme to IsdC, a cell wall-anchored anthrax NEAT protein. These results indicate that IsdX2 has all the features required to acquire heme from the host and transport heme to the bacterial cell wall. Additionally, these results suggest that IsdX2 may accelerate iron import rates by acting as a "heme sponge" that enhances B. anthracis replication in iron-starved environments.     

A Bacillus anthracis S-Layer Homology Protein That Binds Heme and Mediates Heme Delivery to IsdC

The sequestration of iron by mammalian hosts represents a significant obstacle to the establishment of a bacterial infection. In response, pathogenic bacteria have evolved mechanisms to acquire iron from host heme. Bacillus anthracis, the causative agent of anthrax, utilizes secreted hemophores to scavenge heme from host hemoglobin, thereby facilitating iron acquisition from extracellular heme pools and delivery to iron-regulated surface determinant (Isd) proteins covalently attached to the cell wall. However, several Gram-positive pathogens, including B. anthracis, contain genes that encode near iron transporter (NEAT) proteins that are genomically distant from the genetically linked Isd locus. NEAT domains are protein modules that partake in several functions related to heme transport, including binding heme and hemoglobin. This finding raises interesting questions concerning the relative role of these NEAT proteins, relative to hemophores and the Isd system, in iron uptake. Here, we present evidence that a B. anthracis S-layer homology (SLH) protein harboring a NEAT domain binds and directionally transfers heme to the Isd system via the cell wall protein IsdC. This finding suggests that the Isd system can receive heme from multiple inputs and may reflect an adaptation of B. anthracis to changing iron reservoirs during an infection. Understanding the mechanism of heme uptake in pathogenic bacteria is important for the development of novel therapeutics to prevent and treat bacterial infections.Pathogenic bacteria need to acquire iron to survive in mammalian hosts (12). However, the host sequesters most iron in the porphyrin heme, and heme itself is often bound to proteins such as hemoglobin (14, 28, 85). Circulating hemoglobin can serve as a source of heme-iron for replicating bacteria in infected hosts, but the precise mechanisms of heme extraction, transport, and assimilation remain unclear (25, 46, 79, 86). An understanding of how bacterial pathogens import heme will lead to the development of new anti-infectives that inhibit heme uptake, thereby preventing or treating infections caused by these bacteria (47, 68).The mechanisms of transport of biological molecules into a bacterial cell are influenced by the compositional, structural, and topological makeup of the cell envelope. Gram-negative bacteria utilize specific proteins to transport heme through the outer membrane, periplasm, and inner membrane (83, 84). Instead of an outer membrane and periplasm, Gram-positive bacteria contain a thick cell wall (59, 60). Proteins covalently anchored to the cell wall provide a functional link between extracellular heme reservoirs and intracellular iron utilization pathways (46). In addition, several Gram-positive and Gram-negative bacterial genera also contain an outermost structure termed the S (surface)-layer (75). The S-layer is a crystalline array of protein that surrounds the bacterial cell and may serve a multitude of functions, including maintenance of cell architecture and protection from host immune components (6, 7, 18, 19, 56). In bacterial pathogens that manifest an S-layer, the “force field” function of this structure raises questions concerning how small molecules such as heme can be successfully passed from the extracellular milieu to cell wall proteins for delivery into the cell cytoplasm.Bacillus anthracis is a Gram-positive, spore-forming bacterium that is the etiological agent of anthrax disease (30, 33). The life cycle of B. anthracis begins after a phagocytosed spore germinates into a vegetative cell inside a mammalian host (2, 40, 69, 78). Virulence determinants produced by the vegetative cells facilitate bacterial growth, dissemination to major organ systems, and eventually host death (76-78). The release of aerosolized spores into areas with large concentrations of people is a serious public health concern (30).Heme acquisition in B. anthracis is mediated by the action of IsdX1 and IsdX2, two extracellular hemophores that extract heme from host hemoglobin and deliver the iron-porphyrin to cell wall-localized IsdC (21, 45). Both IsdX1 and IsdX2 harbor near iron transporter domains (NEATs), a conserved protein module found in Gram-positive bacteria that mediates heme uptake from hemoglobin and contributes to bacterial pathogenesis upon infection (3, 8, 21, 31, 44, 46, 49, 50, 67, 81, 86). Hypothesizing that B. anthracis may contain additional mechanisms for heme transport, we provide evidence that B. anthracis S-layer protein K (BslK), an S-layer homology (SLH) and NEAT protein (32, 43), is surface localized and binds and transfers heme to IsdC in a rapid, contact-dependent manner. These results suggest that the Isd system is not a self-contained conduit for heme trafficking and imply that there is functional cross talk between differentially localized NEAT proteins to promote heme uptake during infection.     

NMR experiments redefine the hemoglobin binding properties of bacterial NEAr‐iron Transporter domains

Iron is a versatile metal cofactor that is used in a wide range of essential cellular processes. During infections, many bacterial pathogens acquire iron from human hemoglobin (Hb), which contains the majority of the body's total iron content in the form of heme (iron protoporphyrin IX). Clinically important Gram‐positive bacterial pathogens scavenge heme using an array of secreted and cell‐wall‐associated receptors that contain NEAr‐iron Transporter (NEAT) domains. Experimentally defining the Hb binding properties of NEAT domains has been challenging, limiting our understanding of their function in heme uptake. Here we show that solution‐state NMR spectroscopy is a powerful tool to define the Hb binding properties of NEAT domains. The utility of this method is demonstrated using the NEAT domains from Bacillus anthracis and Listeria monocytogenes. Our results are compatible with the existence of at least two types of NEAT domains that are capable of interacting with either Hb or heme. These binding properties can be predicted from their primary sequences, with Hb‐ and heme‐binding NEAT domains being distinguished by the presence of (F/Y)YH(Y/F) and S/YXXXY motifs, respectively. The results of this work should enable the functions of a wide range of NEAT domain containing proteins in pathogenic bacteria to be reliably predicted.     

Solution structure of the NEAT (NEAr Transporter) domain from IsdH/HarA: the human hemoglobin receptor in Staphylococcus aureus

During infections the pathogen Staphylococcus aureus procures the essential nutrient iron from its host using iron-regulated surface determinant (Isd) proteins, which scavenge heme bound iron from host hemoproteins. Four Isd proteins are displayed in the cell wall, where they function as receptors for host proteins and heme. Each of the receptors contains one or more copies of a recently discovered domain called NEAT (NEAr Transporter) that has been shown to mediate protein binding. Here we report the three-dimensional solution structure of the NEAT domain from the IsdH/HarA protein, which is the hemoglobin receptor in the Isd system. This is the first structure of a NEAT domain and reveals that they adopt a beta sandwich fold that consists of two five-stranded antiparallel beta sheets. Although unrelated at the primary sequence level, our results indicate that NEAT domains belong to the immunoglobulin superfamily. Binding studies indicate that two IsdH/HarA NEAT domains bind a single molecule of methemoglobin, while the distantly related NEAT domain from the S. aureus IsdC protein binds only heme. A comparison of their primary sequences in light of the new structure is used to predict the hemoglobin and heme binding surfaces on NEAT domains.     

Structure and Function of the PLAA/Ufd3-p97/Cdc48 Complex

PLAA (ortholog of yeast Doa1/Ufd3, also know as human PLAP or phospholipase A2-activating protein) has been implicated in a variety of disparate biological processes that involve the ubiquitin system. It is linked to the maintenance of ubiquitin levels, but the mechanism by which it accomplishes this is unclear. The C-terminal PUL (PLAP, Ufd3p, and Lub1p) domain of PLAA binds p97, an AAA ATPase, which among other functions helps transfer ubiquitinated proteins to the proteasome for degradation. In yeast, loss of Doa1 is suppressed by altering p97/Cdc48 function indicating that physical interaction between PLAA and p97 is functionally important. Although the overall regions of interaction between these proteins are known, the structural basis has been unavailable. We solved the high resolution crystal structure of the p97-PLAA complex showing that the PUL domain forms a 6-mer Armadillo-containing domain. Its N-terminal extension folds back onto the inner curvature forming a deep ridge that is positively charged with residues that are phylogenetically conserved. The C terminus of p97 binds in this ridge, where the side chain of p97-Tyr⁸⁰⁵, implicated in phosphorylation-dependent regulation, is buried. Expressed in doa1Δ null cells, point mutants of the yeast ortholog Doa1 that disrupt this interaction display slightly reduced ubiquitin levels, but unlike doa1Δ null cells, showed only some of the growth phenotypes. These data suggest that the p97-PLAA interaction is important for a subset of PLAA-dependent biological processes and provides a framework to better understand the role of these complex molecules in the ubiquitin system.     

Two-Component System Cross-Regulation Integrates Bacillus anthracis Response to Heme and Cell Envelope Stress

Two-component signaling systems (TCSs) are one of the mechanisms that bacteria employ to sense and adapt to changes in the environment. A prototypical TCS functions as a phosphorelay from a membrane-bound sensor histidine kinase (HK) to a cytoplasmic response regulator (RR) that controls target gene expression. Despite significant homology in the signaling domains of HKs and RRs, TCSs are thought to typically function as linear systems with little to no cross-talk between non-cognate HK-RR pairs. Here we have identified several cell envelope acting compounds that stimulate a previously uncharacterized Bacillus anthracis TCS. Furthermore, this TCS cross-signals with the heme sensing TCS HssRS; therefore, we have named it HssRS interfacing TCS (HitRS). HssRS reciprocates cross-talk to HitRS, suggesting a link between heme toxicity and cell envelope stress. The signaling between HssRS and HitRS occurs in the parental B. anthracis strain; therefore, we classify HssRS-HitRS interactions as cross-regulation. Cross-talk between HssRS and HitRS occurs at both HK-RR and post-RR signaling junctions. Finally, HitRS also regulates a previously unstudied ABC transporter implicating this transporter in the response to cell envelope stress. This chemical biology approach to probing TCS signaling provides a new model for understanding how bacterial signaling networks are integrated to enable adaptation to complex environments such as those encountered during colonization of the vertebrate host.     

Structure of the Hemoglobin-IsdH Complex Reveals the Molecular Basis of Iron Capture by Staphylococcus aureus

Staphylococcus aureus causes life-threatening disease in humans. The S. aureus surface protein iron-regulated surface determinant H (IsdH) binds to mammalian hemoglobin (Hb) and extracts heme as a source of iron, which is an essential nutrient for the bacteria. However, the process of heme transfer from Hb is poorly understood. We have determined the structure of IsdH bound to human Hb by x-ray crystallography at 4.2 Å resolution, revealing the structural basis for heme transfer. One IsdH molecule is bound to each α and β Hb subunit, suggesting that the receptor acquires iron from both chains by a similar mechanism. Remarkably, two near iron transporter (NEAT) domains in IsdH perform very different functions. An N-terminal NEAT domain binds α/β globin through a site distant from the globin heme pocket and, via an intervening structural domain, positions the C-terminal heme-binding NEAT domain perfectly for heme transfer. These data, together with a 2.3 Å resolution crystal structure of the isolated N-terminal domain bound to Hb and small-angle x-ray scattering of free IsdH, reveal how multiple domains of IsdH cooperate to strip heme from Hb. Many bacterial pathogens obtain iron from human hemoglobin using proteins that contain multiple NEAT domains and other domains whose functions are poorly understood. Our results suggest that, rather than acting as isolated units, NEAT domains may be integrated into higher order architectures that employ multiple interaction interfaces to efficiently extract heme from host proteins.     

Aspergillus fumigatus SidJ Mediates Intracellular Siderophore Hydrolysis

Siderophore-mediated iron handling is crucial for the virulence of Aspergillus fumigatus. Here we identified a new component of its siderophore metabolism, termed SidJ, which is encoded by AFUA_3G03390. The encoding gene is localized in a siderophore biosynthetic gene cluster that is conserved in a variety of fungi. During iron starvation, SidJ deficiency resulted in decreased growth and increased intracellular accumulation of hydrolysis products of the siderophore fusarinine C. The implied role in siderophore hydrolysis is consistent with a putative esterase domain in SidJ, which now represents the first functionally characterized member of the DUF1749 (domain of unknown function) protein family, with members found exclusively in fungi and plants.