Citrate-dependent iron transport system in Escherichia coli K-12

Cleavage cascade of the sigma regulator FecR orchestrates TonB-dependent signal transduction

TonB-dependent signal transduction is a versatile mechanism observed in gram-negative bacteria that integrates energy-dependent substrate transport with signal relay. In Escherichia coli, the TonB-ExbBD motor complex energizes the TonB-dependent outer membrane transporter FecA, facilitating ferric citrate import. FecA also acts as a sensor, transmitting signals to the cytoplasmic membrane protein FecR, which eventually activates the cytoplasmic sigma factor FecI, driving transcription of the fec operon. Building on our previous finding that FecR undergoes functional maturation through a three-step cleavage process [T. Yokoyama et al., J. Biol. Chem. 296, 100673 (2021)], we here describe the complete mechanism of FecR-mediated ferric citrate signaling involving FecA and TonB. The cleavage cascade begins with FecR autoproteolysis prior to membrane integration. The soluble C-terminal domain (CTD) fragment of FecR is cotranslocated with the N-terminal domain (NTD) fragment through a twin-arginine translocation (Tat) system-mediated process. In the periplasm, the interaction between the CTD and NTD fragments prevents further cleavage. Binding of ferric citrate induces a conformational change in FecA, exposing its TonB box to the periplasmic space. This structural alteration is transmitted to the interacting FecR CTD via the motor function of TonB, resulting in the release of the CTD blockage from the NTD. Consequently, the successive cleavage of FecR's NTD is initiated, culminating in the ferric citrate signal-induced activation of fec gene expression. Our findings reveal that the regulation of FecR cleavage, controlled by the TonB-FecA axis, plays a central role in the bacterial response to ferric citrate signals.

The fciTABC and feoABI systems contribute to ferric citrate acquisition in Stenotrophomonas maltophilia

Background

Stenotrophomonas maltophilia, a member of γ-proteobacteria, is a ubiquitous environmental bacterium that is recognized as an opportunistic nosocomial pathogen. FecABCD system contributes to ferric citrate acquisition in Escherichia coli. FeoABC system, consisting of an inner membrane transporter (FeoB) and two cytoplasmic proteins (FeoA and FeoC), is a well-known ferrous iron transporter system in γ-proteobacteria. As revealed by the sequenced genome, S. maltophilia appears to be equipped with several iron acquisition systems; however, the understanding of these systems is limited. In this study, we aimed to elucidate the ferric citrate acquisition system of S. maltophilia.

Methods

Candidate genes searching and function validation are the strategy for elucidating the genes involved in ferric citrate acquisition. The candidate genes responsible for ferric citrate acquisition were firstly selected using FecABCD of E. coli as a reference, and then revealed by transcriptome analysis of S. maltophilia KJ with and without 2,2′-dipyridyl (DIP) treatment. Function validation was carried out by deletion mutant construction and ferric citrate utilization assay. The bacterial adenylate cyclase two-hybrid system was used to verify intra-membrane protein–protein interaction.

Results

Smlt2858 and Smlt2356, the homologues of FecA and FecC/D of E. coli, were first considered; however, deletion mutant construction and functional validation ruled out their involvement in ferric citrate acquisition. FciA (Smlt1148), revealed by its upregulation in DIP-treated KJ cells, was the outer membrane receptor for ferric citrate uptake. The fciA gene is a member of the fciTABC operon, in which fciT, fciA, and fciC participated in ferric citrate acquisition. Uniquely, the Feo system of S. maltophilia is composed of a cytoplasmic protein FeoA, an inner membrane transporter FeoB, and a predicted inner membrane protein FeoI. The intra-membrane protein–protein interaction between FeoB and FeoI may extend the substrate profile of FeoB to ferric citrate. FeoABI system functioned as an inner membrane transporter of ferric citrate.

Conclusions

The FciTABC and FeoABI systems contribute to ferric citrate acquisition in S. maltophilia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00809-y.

Transcription regulation of iron carrier transport genes by ECF sigma factors through signaling from the cell surface into the cytoplasm

Bacteria are usually iron-deficient because the Fe³⁺ in their environment is insoluble or is incorporated into proteins. To overcome their natural iron limitation, bacteria have developed sophisticated iron transport and regulation systems. In gram-negative bacteria, these include iron carriers, such as citrate, siderophores, and heme, which when loaded with Fe³⁺ adsorb with high specificity and affinity to outer membrane proteins. Binding of the iron carriers to the cell surface elicits a signal that initiates transcription of iron carrier transport and synthesis genes, referred to as “cell surface signaling”. Transcriptional regulation is not coupled to transport. Outer membrane proteins with signaling functions contain an additional N-terminal domain that in the periplasm makes contact with an anti-sigma factor regulatory protein that extends from the outer membrane into the cytoplasm. Binding of the iron carriers to the outer membrane receptors elicits proteolysis of the anti-sigma factor by two different proteases, Prc in the periplasm, and RseP in the cytoplasmic membrane, inactivates the anti-sigma function or results in the generation of an N-terminal peptide of ∼50 residues with pro-sigma activity yielding an active extracytoplasmic function (ECF) sigma factor. Signal recognition and signal transmission into the cytoplasm is discussed herein.

The Outer Membrane Took Center Stage

My interest in membranes was piqued during a lecture series given by one of the founders of molecular biology, Max Delbrück, at Caltech, where I spent a postdoctoral year to learn more about protein chemistry. That general interest was further refined to my ultimate research focal point-the outer membrane of Escherichia coli-through the influence of the work of Wolfhard Weidel, who discovered the murein (peptidoglycan) layer and biochemically characterized the first phage receptors of this bacterium. The discovery of lipoprotein bound to murein was completely unexpected and demonstrated that the protein composition of the outer membrane and the structure and function of proteins could be unraveled at a time when nothing was known about outer membrane proteins. The research of my laboratory over the years covered energy-dependent import of proteinaceous toxins and iron chelates across the outer membrane, which does not contain an energy source, and gene regulation by iron, including transmembrane transcriptional regulation.

High-Salt Conditions Alter Transcription of Helicobacter pylori Genes Encoding Outer Membrane Proteins

Helicobacter pylori infection and high dietary salt intake are risk factors for the development of gastric adenocarcinoma. One possible mechanism by which a high-salt diet could influence gastric cancer risk is by modulating H. pylori gene expression. In this study, we utilized transcriptome sequencing (RNA-seq) methodology to compare the transcriptional profiles of H. pylori grown in media containing different concentrations of sodium chloride. We identified 118 differentially expressed genes (65 upregulated and 53 downregulated in response to high-salt conditions), including multiple members of 14 operons. Twenty-nine of the differentially expressed genes encode proteins previously shown to undergo salt-responsive changes in abundance, based on proteomic analyses. Real-time reverse transcription (RT)-PCR analyses validated differential expression of multiple genes encoding outer membrane proteins, including adhesins (SabA and HopQ) and proteins involved in iron acquisition (FecA2 and FecA3). Transcript levels of sabA, hopA, and hopQ are increased under high-salt conditions, whereas transcript levels of fecA2 and fecA3 are decreased under high-salt conditions. Transcription of sabA, hopA, hopQ, and fecA3 is derepressed in an arsS mutant strain, but salt-responsive transcription of these genes is not mediated by the ArsRS two-component system, and the CrdRS and FlgRS two-component systems do not have any detectable effects on transcription of these genes. In summary, these data provide a comprehensive view of H. pylori transcriptional alterations that occur in response to high-salt environmental conditions.

Comparative Genomic Analysis of a Clinical Isolate of Klebsiella quasipneumoniae subsp. similipneumoniae, a KPC-2 and OKP-B-6 Beta-Lactamases Producer Harboring Two Drug-Resistance Plasmids from Southeast Brazil

The aim of this study was to unravel the genetic determinants responsible for multidrug (including carbapenems) resistance and virulence in a clinical isolate of Klebsiella quasipneumoniae subsp. similipneumoniae by whole-genome sequencing and comparative analyses. Eighty-three clinical isolates initially identified as carbapenem-resistant K. pneumoniae were collected from nosocomial infections in southeast Brazil. After RAPD screening, the KPC-142 isolate, showing the most divergent DNA pattern, was selected for complete genome sequencing in an Illumina HiSeq 2500 instrument. Reads were assembled into scaffolds, gaps between scaffolds were resolved by in silico gap filling and extensive bioinformatics analyses were performed, using multiple comparative analysis tools and databases. Genome sequencing allowed to correct the classification of the KPC-142 isolate as K. quasipneumoniae subsp. similipneumoniae. To the best of our knowledge this is the first complete genome reported to date of a clinical isolate of this subspecies harboring both class A beta-lactamases KPC-2 and OKP-B-6 from South America. KPC-142 has one 5.2 Mbp chromosome (57.8% G+C) and two plasmids: 190 Kbp pKQPS142a (50.7% G+C) and 11 Kbp pKQPS142b (57.3% G+C). The 3 Kbp region in pKQPS142b containing the bla_KPC−2 was found highly similar to that of pKp13d of K. pneumoniae Kp13 isolated in Southern Brazil in 2009, suggesting the horizontal transfer of this resistance gene between different species of Klebsiella. KPC-142 additionally harbors an integrative conjugative element ICEPm1 that could be involved in the mobilization of pKQPS142b and determinants of resistance to other classes of antimicrobials, including aminoglycoside and silver. We present the completely assembled genome sequence of a clinical isolate of K. quasipneumoniae subsp. similipneumoniae, a KPC-2 and OKP-B-6 beta-lactamases producer and discuss the most relevant genomic features of this important resistant pathogen in comparison to several strains belonging to K. quasipneumoniae subsp. similipneumoniae (phylogroup II-B), K. quasipneumoniae subsp. quasipneumoniae (phylogroup II-A), K. pneumoniae (phylogroup I), and K. variicola (phylogroup III). Our study contributes to the description of the characteristics of a novel K. quasipneumoniae subsp. similipneumoniae strain circulating in South America that currently represent a serious potential risk for nosocomial settings.

A case study in evolutionary contingency

Biological evolution is a fundamentally historical phenomenon in which intertwined stochastic and deterministic processes shape lineages with long, continuous histories that exist in a changing world that has a history of its own. The degree to which these characteristics render evolution historically contingent, and evolutionary outcomes thereby unpredictably sensitive to history has been the subject of considerable debate in recent decades. Microbial evolution experiments have proven among the most fruitful means of empirically investigating the issue of historical contingency in evolution. One such experiment is the Escherichia coli Long-Term Evolution Experiment (LTEE), in which twelve populations founded from the same clone of E. coli have evolved in parallel under identical conditions. Aerobic growth on citrate (Cit(+)), a novel trait for E. coli, evolved in one of these populations after more than 30,000 generations. Experimental replays of this population's evolution from various points in its history showed that the Cit(+) trait was historically contingent upon earlier mutations that potentiated the trait by rendering it mutationally accessible. Here I review this case of evolutionary contingency and discuss what it implies about the importance of historical contingency arising from the core processes of evolution.

New iron acquisition system in Bacteroidetes

Capnocytophaga canimorsus, a dog mouth commensal and a member of the Bacteroidetes phylum, causes rare but often fatal septicemia in humans that have been in contact with a dog. Here, we show that C. canimorsus strains isolated from human infections grow readily in heat-inactivated human serum and that this property depends on a typical polysaccharide utilization locus (PUL), namely, PUL3 in strain Cc5. PUL are a hallmark of Bacteroidetes, and they encode various products, including surface protein complexes that capture and process polysaccharides or glycoproteins. The archetype system is the Bacteroides thetaiotaomicron Sus system, devoted to starch utilization. Unexpectedly, PUL3 conferred the capacity to acquire iron from serotransferrin (STF), and this capacity required each of the seven encoded proteins, indicating that a whole Sus-like machinery is acting as an iron capture system (ICS), a new and unexpected function for Sus-like machinery. No siderophore could be detected in the culture supernatant of C. canimorsus, suggesting that the Sus-like machinery captures iron directly from transferrin, but this could not be formally demonstrated. The seven genes of the ICS were found in the genomes of several opportunistic pathogens from the Capnocytophaga and Prevotella genera, in different isolates of the severe poultry pathogen Riemerella anatipestifer, and in strains of Bacteroides fragilis and Odoribacter splanchnicus isolated from human infections. Thus, this study describes a new type of ICS that evolved in Bacteroidetes from a polysaccharide utilization system and most likely represents an important virulence factor in this group.

Antimicrobial activity of metals: mechanisms, molecular targets and applications

Metals have been used as antimicrobial agents since antiquity, but throughout most of history their modes of action have remained unclear. Recent studies indicate that different metals cause discrete and distinct types of injuries to microbial cells as a result of oxidative stress, protein dysfunction or membrane damage. Here, we describe the chemical and toxicological principles that underlie the antimicrobial activity of metals and discuss the preferences of metal atoms for specific microbial targets. Interdisciplinary research is advancing not only our understanding of metal toxicity but also the design of metal-based compounds for use as antimicrobial agents and alternatives to antibiotics.

Catecholate siderophores protect bacteria from pyochelin toxicity

BACKGROUND

Bacteria produce small molecule iron chelators, known as siderophores, to facilitate the acquisition of iron from the environment. The synthesis of more than one siderophore and the production of multiple siderophore uptake systems by a single bacterial species are common place. The selective advantages conferred by the multiplicity of siderophore synthesis remains poorly understood. However, there is growing evidence suggesting that siderophores may have other physiological roles besides their involvement in iron acquisition.

METHODS AND PRINCIPAL FINDINGS

Here we provide the first report that pyochelin displays antibiotic activity against some bacterial strains. Observation of differential sensitivity to pyochelin against a panel of bacteria provided the first indications that catecholate siderophores, produced by some bacteria, may have roles other than iron acquisition. A pattern emerged where only those strains able to make catecholate-type siderophores were resistant to pyochelin. We were able to associate pyochelin resistance to catecholate production by showing that pyochelin-resistant Escherichia coli became sensitive when biosynthesis of its catecholate siderophore enterobactin was impaired. As expected, supplementation with enterobactin conferred pyochelin resistance to the entE mutant. We observed that pyochelin-induced growth inhibition was independent of iron availability and was prevented by addition of the reducing agent ascorbic acid or by anaerobic incubation. Addition of pyochelin to E. coli increased the levels of reactive oxygen species (ROS) while addition of ascorbic acid or enterobactin reduced them. In contrast, addition of the carboxylate-type siderophore, citrate, did not prevent pyochelin-induced ROS increases and their associated toxicity.

CONCLUSIONS

We have shown that the catecholate siderophore enterobactin protects E. coli against the toxic effects of pyochelin by reducing ROS. Thus, it appears that catecholate siderophores can behave as protectors of oxidative stress. These results support the idea that siderophores can have physiological roles aside from those in iron acquisition.

Host iron binding proteins acting as niche indicators for Neisseria meningitidis

Neisseria meningitidis requires iron, and in the absence of iron alters its gene expression to increase iron acquisition and to make the best use of the iron it has. During different stages of colonization and infection available iron sources differ, particularly the host iron-binding proteins haemoglobin, transferrin, and lactoferrin. This study compared the transcriptional responses of N. meningitidis, when grown in the presence of these iron donors and ferric iron, using microarrays.

Specific transcriptional responses to the different iron sources were observed, including genes that are not part of the response to iron restriction. Comparisons between growth on haemoglobin and either transferrin or lactoferrin identified changes in 124 and 114 genes, respectively, and 33 genes differed between growth on transferrin or lactoferrin. Comparison of gene expression from growth on haemoglobin or ferric iron showed that transcription is also affected by the entry of either haem or ferric iron into the cytoplasm. This is consistent with a model in which N. meningitidis uses the relative availability of host iron donor proteins as niche indicators.

Growth in the presence of haemoglobin is associated with a response likely to be adaptive to survival within the bloodstream, which is supported by serum killing assays that indicate growth on haemoglobin significantly increases survival, and the response to lactoferrin is associated with increased expression of epithelial cell adhesins and oxidative stress response molecules. The transferrin receptor is the most highly transcribed receptor and has the fewest genes specifically induced in its presence, suggesting this is the favoured iron source for the bacterium. Most strikingly, the responses to haemoglobin, which is associated with unrestricted growth, indicates a low iron transcriptional profile, associated with an aggressive phenotype that may be adaptive to access host iron sources but which may also underlie the lethal features of meningococcal septicaemia, when haemoglobin may become a major source of iron.

Growth phase and metal-dependent transcriptional regulation of the fecA genes in Helicobacter pylori

Balancing metal uptake is essential for maintaining a proper intracellular metal concentration. Here, we report the transcriptional control exerted by the two metal-responsive regulators of Helicobacter pylori, Fur (iron-dependent ferric uptake regulator) and NikR (nickel-responsive regulator), on the three copies of the fecA genes present in this species. By monitoring the patterns of transcription throughout growth and in response to nickel, iron, and a metal chelator, we found that the expression of the three fecA genes is temporally regulated, responds to metals in different ways, and is selectively controlled by either one of the two regulators. fecA1 is expressed at a constant level throughout growth, and its expression is iron sensitive; the expression of fecA2 is mainly off, with minor expression coming up in late exponential phase. In contrast, the expression of fecA3 is maximal in early exponential phase, gradually decreases with time, and is repressed by nickel. The direct roles of Fur and NikR were studied both in vitro, by mapping the binding sites of each regulator on the promoter regions via DNase I footprinting analysis, and in vivo, by using primer extension analyses of the fecA transcripts in fur and nikR deletion strains. Overall, the results show that the expression of each fecA gene is finely tuned in response to metal availability, as well as during the bacterial growth phase, suggesting specific and dedicated functions for the three distinct FecA homologues.

Synthesis of citrate-ciprofloxacin conjugates

Two regioisomeric citrate-functionalized ciprofloxacin conjugates have been synthesized and their antimicrobial activities against a panel of clinically-relevant bacteria have been determined. Cellular uptake mechanisms were investigated using wild-type and ompF deletion strains of Escherichia coli K-12.

Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli

The role of historical contingency in evolution has been much debated, but rarely tested. Twelve initially identical populations of Escherichia coli were founded in 1988 to investigate this issue. They have since evolved in a glucose-limited medium that also contains citrate, which E. coli cannot use as a carbon source under oxic conditions. No population evolved the capacity to exploit citrate for >30,000 generations, although each population tested billions of mutations. A citrate-using (Cit+) variant finally evolved in one population by 31,500 generations, causing an increase in population size and diversity. The long-delayed and unique evolution of this function might indicate the involvement of some extremely rare mutation. Alternately, it may involve an ordinary mutation, but one whose physical occurrence or phenotypic expression is contingent on prior mutations in that population. We tested these hypotheses in experiments that "replayed" evolution from different points in that population's history. We observed no Cit+ mutants among 8.4 x 10(12) ancestral cells, nor among 9 x 10(12) cells from 60 clones sampled in the first 15,000 generations. However, we observed a significantly greater tendency for later clones to evolve Cit+, indicating that some potentiating mutation arose by 20,000 generations. This potentiating change increased the mutation rate to Cit+ but did not cause generalized hypermutability. Thus, the evolution of this phenotype was contingent on the particular history of that population. More generally, we suggest that historical contingency is especially important when it facilitates the evolution of key innovations that are not easily evolved by gradual, cumulative selection.

Ferripyochelin uptake genes are involved in pyochelin-mediated signalling in Pseudomonas aeruginosa

In response to iron starvation, Pseudomonas aeruginosa produces the siderophore pyochelin. When secreted to the extracellular environment, pyochelin chelates iron and transports it to the bacterial cytoplasm via its specific outer-membrane receptor FptA and the inner-membrane permease FptX. Exogenously added pyochelin also acts as a signal which induces the expression of the pyochelin biosynthesis and uptake genes by activating PchR, a cytoplasmic regulatory protein of the AraC/XylS family. The importance of ferripyochelin uptake genes in this regulation was evaluated. The fptA and fptX genes were shown to be part of the fptABCX ferripyochelin transport operon, which is conserved in Burkholderia sp. and Rhodospirillum rubrum. The fptB and fptC genes were found to be dispensable for utilization of pyochelin as an iron source, for signalling and for pyochelin production. By contrast, mutations in fptA and fptX not only interfered with pyochelin utilization, but also affected signalling and diminished siderophore production. It is concluded from this that pyochelin-mediated signalling operates to a large extent via the ferripyochelin transport system.

Gene regulation by transmembrane signaling

Studies of the ferric citrate transport genes in Escherichia coli K-12 have revealed a novel type of transcriptional regulation. The inducer, ferric citrate, binds to an outer membrane protein and must not be transported into the cells to initiate transcription of the ferric citrate transport genes. Rather, a signaling cascade from the cell surface across the outer membrane, the periplasm, and the cytoplasmic membrane into the cytoplasm transmits information on the presence of the inducer in the culture medium into the cytoplasm, where gene transcription occurs. The outer membrane protein FecA serves as a signal receiver and as a signal transmitter across the outer membrane. The FecR protein serves as a signal receiver in the periplasm and as a signal transmitter across the cytoplasmic membrane into the cytoplasm, where the FecI sigma factor is activated to bind RNA polymerase and specifically initiate transcription of the fecABCDE transport genes by binding to the promoter upstream of the fecA gene. Transcription of the fecI fecR regulatory genes is repressed by Fe(2+) bound to the Fur repressor protein. Under iron-limiting conditions, Fur is not loaded with Fe(2+), the fecI and fecR genes are transcribed, and the FecI and FecR proteins are synthesized and respond to the presence of ferric citrate in the medium when ferric citrate binds to the FecA protein. Regulation of the fec genes represents the paradigm of a growing number of gene regulation systems involving transmembrane signaling across three cellular compartments.

Gene Regulation by Transmembrane Signaling

Studies of the ferric citrate transport genes in Escherichia coli K-12 have revealed a novel type of transcriptional regulation. The inducer, ferric citrate, binds to an outer membrane protein and must not be transported into the cells to initiate transcription of the ferric citrate transport genes. Rather, a signaling cascade from the cell surface across the outer membrane, the periplasm, and the cytoplasmic membrane into the cytoplasm transmits information on the presence of the inducer in the culture medium into the cytoplasm, where gene transcription occurs. The outer membrane protein FecA serves as a signal receiver and as a signal transmitter across the outer membrane. The FecR protein serves as a signal receiver in the periplasm and as a signal transmitter across the cytoplasmic membrane into the cytoplasm, where the FecI sigma factor is activated to bind RNA polymerase and specifically initiate transcription of the fecABCDE transport genes by binding to the promoter upstream of the fecA gene. Transcription of the fecI fecR regulatory genes is repressed by Fe2+ bound to the Fur repressor protein. Under iron-limiting conditions, Fur is not loaded with Fe2+, the fecI and fecR genes are transcribed, and the FecI and FecR proteins are synthesized and respond to the presence of ferric citrate in the medium when ferric citrate binds to the FecA protein. Regulation of the fec genes represents the paradigm of a growing number of gene regulation systems involving transmembrane signaling across three cellular compartments.

Occurrence and regulation of the ferric citrate transport system in Escherichia coli B, Klebsiella pneumoniae, Enterobacter aerogenes, and Photorhabdus luminescens

In Escherichia coli K-12, transcription of the ferric citrate transport genes fecABCDE is initiated by binding of diferric dicitrate to the outer membrane protein FecA which elicits a signaling cascade from the cell surface to the cytoplasm. The FecI sigma factor is only active in the presence of FecR, which transfers the signal across the cytoplasmic membrane. In other bacteria, fecIRA homologues control iron transport gene transcription by siderophores other than citrate. However, in most cases, the FecI homologues are active in the absence of the FecR homologues, which might function as anti-sigma factors. Since not all E. coli strains contain a fec system, we determined the occurrence of fec genes in selected Enterobacteriaceae and the dependence of FecI activity on FecR. Incomplete FecIRA systems were chromosomally encoded in Enterobacter aerogenes strains and plasmid-encoded in K. pneumoniae. E. coli B, Photorhabdus luminescens and one of three Klebsiella pneumoniae strains had a functional FecIRA regulatory system as in E. coli K-12. The cytoplasmic N-terminal FecR fragments caused constitutive FecI activity in the absence of ferric citrate. The PCR-generated mutant FecI(D40G) was inactive and FecI(S15P) was partially active. FecR of E. coli K-12 activated FecI of all tested strains except FecI encoded on the virulence plasmid pLVPK of K. pneumoniae, which differed from E. coli K-12 FecI by having mutations in region 4, which is important for interaction with FecR. The C-terminally truncated FecR homologue of pLVPK was inactive. pLVPK-encoded FecA contains a 38-residue sequence in front of the signal sequence that did not prevent processing and proper integration of FecA into the outer membrane of E. coli and lacks the signaling sequence required for transcription initiation of the fec transport genes, making it induction-incompetent but transport-competent. The evidence indicates that fecIRABCDE genes are acquired by horizontal DNA transfer and can undergo debilitating mutations.

The metal permease ZupT from Escherichia coli is a transporter with a broad substrate spectrum

The Escherichia coli zupT (formerly ygiE) gene encodes a cytoplasmic membrane protein (ZupT) related to members of the eukaryotic ZIP family of divalent metal ion transporters. Previously, ZupT was shown to be responsible for uptake of zinc. In this study, we show that ZupT is a divalent metal cation transporter of broad substrate specificity. An E. coli strain with a disruption in all known iron uptake systems could grow in the presence of chelators only if zupT was expressed. Heterologous expression of Arabidopsis thaliana ZIP1 could also alleviate iron deficiency in this E. coli strain, as could expression of indigenous mntH or feoABC. Transport studies with intact cells showed that ZupT facilitates uptake of 55Fe2+ similarly to uptake of MntH or Feo. Other divalent cations were also taken up by ZupT, as shown using 57Co2+. Expression of zupT rendered E. coli cells hypersensitive to Co2+ and sensitive to Mn2+. ZupT did not appear to be metal regulated: expression of a Phi(zupT-lacZ) operon fusion indicated that zupT is expressed constitutively at a low level.

Transmembrane transcriptional control (surface signalling) of the Escherichia coli Fec type

The ferric citrate transport system of Escherichia coli is the first example of a transcription initiation mechanism that starts at the cell surface. The inducer, ferric citrate, binds to an outer membrane transport protein, and without further transport elicits a signal that is transmitted across the outer membrane, the periplasm, and the cytoplasmic membrane into the cytoplasm. Signal transfer across the three subcellular compartments is mediated by the outer membrane transport protein that interacts in the periplasm with a cytoplasmic transmembrane protein. The latter is required for activation of a sigma factor which belongs to the extracytoplasmic function sigma factor family. A similar kind of transcription regulation has been demonstrated in Pseudomonas putida, P. aeruginosa, Serratia marcescens, Klebsiella pneumoniae, Aerobacter aerogenes, Bordetella pertussis, B. bronchseptica, B. avium, and Ralstonia solanacearum. The genomes of P. putida, P. aeruginosa, Nitrosomonas europaea, Bacteroides thetaiotaomicron and Caulobacter crescentus predict the existence of many more such transcriptional regulatory devices.

Iron uptake by Escherichia coli cultured with antibodies from cows immunized with high-affinity ferric receptors

The synergistic effects of immunoglobulin G (IgG) from cows vaccinated with ferric citrate receptor (FecA) and IgG from cows vaccinated with ferric enterobactin receptor (FepA) were measured in an in vitro iron uptake assay. Serum was isolated and pooled within treatment from five cows each vaccinated with FepA or FecA or not vaccinated. Immunoglobulin G was isolated by ammonium sulfate precipitation and protein G affinity chromatography. Six Escherichia coli isolates from bovine intramammary infections were cultured in an iron-depleted medium to induce high-affinity iron acquisition systems and, in iron-depleted conditions, to specifically induce the expression of FecA. The bacterial cells were mixed with either 3 or 6 mg/mL of purified IgG and 55Fe. The radioactivity of 55Fe taken up by the bacterial cells was measured by a liquid scintillation counter after 5-, 10-, and 15-min incubations at 37 degrees C. The combination of anti-FecA IgG and anti-FepA IgG reduced 55Fe uptake compared with either anti-FecA or anti-FepA alone. Iron uptake was reduced more by anti-FecA IgG than by anti-FepA IgG when the ferric citrate system was induced. Reduction of iron uptake did not differ between anti-FepA alone and anti-FecA alone when citrate was absent from the medium.

Defined inactive FecA derivatives mutated in functional domains of the outer membrane transport and signaling protein of Escherichia coli K-12

The FecA outer membrane protein of Escherichia coli functions as a transporter of ferric citrate and as a signal receiver and signal transmitter for transcription initiation of the fec transport genes. Three FecA regions for which functional roles have been predicted from the crystal structures were mutagenized: (i) loops 7 and 8, which move upon binding of ferric citrate and close the entrance to the ferric citrate binding site; (ii) the dinuclear ferric citrate binding site; and (iii) the interface between the globular domain and the beta-barrel. Deletion of loops 7 and 8 abolished FecA transport and induction activities. Deletion of loops 3 and 11 also inactivated FecA, whereas deletion of loops 9 and 10 largely retained FecA activities. The replacement of arginine residue R365 or R380 and glutamine Q570, which are predicted to serve as binding sites for the negatively charged dinuclear ferric citrate, with alanine resulted in inactive FecA, whereas the binding site mutant R438A retained approximately 50% of the FecA induction and transport activities. Residues R150, E541, and E587, conserved among energy-coupled outer membrane transporters, are predicted to form salt bridges between the globular domain and the beta-barrel and to contribute to the fixation of the globular domain inside the beta-barrel. Mutations E541A and E541R affected FecA induction and transport activity slightly, whereas mutations E587A and E587R more strongly reduced FecA activity. The double mutations R150A E541R and R150A E587R nearly abolished FecA activity. Apparently, the salt bridges are less important than the individual functions these residues seem to have for FecA activity. Comparison of the properties of the FecA, FhuA, FepA, and BtuB transporters indicates that although they have very similar crystal structures, the details of their functional mechanisms differ.

Recognition of iron-free siderophores by TonB-dependent iron transporters

TonB-dependent iron transporters reside in the outer membranes of Gram-negative bacteria, transporting ferric-complexes into the periplasm by a mechanism requiring proton motive force and an integral inner membrane complex, TonB-ExbB-ExbD. Certain TonB-dependent transporters contain an additional domain at the N-terminus, which interacts with an inner membrane regulatory protein and a cytoplasmic sigma factor to induce transcription of iron transport genes when a ferric-ligand is bound at the extracellular surface of the transporter. Transport of the ferric-ligand is apparently not necessary for transcription induction. Recent biophysical and crystallographic experiments have shown that this subclass of TonB-dependent iron transporters can bind iron-free ligands, whereas only the ferric-ligands are transported into the periplasm. This review focuses on the ligand binding properties of these transporters and includes a discussion of the biological function of the additional domain, the mechanism of transcription induction and the mechanism of ferric-ligand transport.

Growth responses of Escherichia coli to immunoglobulin G from cows immunized with ferric citrate receptor, FecA

Effects of purified immunoglobulin (Ig) G from cows immunized with ferric citrate receptor, FecA, on the in vitro growth of Escherichia coli were investigated. Twenty-one cows were assigned to one of 3 treatments: 1) FecA immunization, 2) E. coli J5 bacterin immunization, and 3) unimmunized control. FecA was derived from E. coli UT5600/pSV66. Immunoglobulin G was purified from pooled colostral whey for each treatment group. The IgG from FecA immunized cows had higher titers against FecA compared with other treatment groups. Bacterial isolates tested were 14 E. coli from intramammary infections and E. coli UT5600/pSV66. Iron depletion decreased the growth of E. coli compared with growth in Fe-replete medium. The presence of IgG further decreased the growth compared with the growth under iron restriction alone. Bacterial growth did not differ among IgG sources nor between IgG concentrations. Replenishing media with exogenous iron overrode the inhibitory effects of the Fe-depletion and IgG. Vaccinating cows with FecA had little effect on the growth inhibitory properties of IgG toward E. coli mastitis isolates cultured in Fe-deplete media.

Structural evidence for iron-free citrate and ferric citrate binding to the TonB-dependent outer membrane transporter FecA

Escherichia coli possesses a TonB-dependent transport system, which exploits the iron-binding capacity of citrate and its natural abundance. Here, we describe three structures of the outer membrane ferric citrate transporter FecA: unliganded and complexed with iron-free or diferric dicitrate. We show the structural mechanism for discrimination between the iron-free and ferric siderophore: the binding of diferric dicitrate, but not iron-free dicitrate alone, causes major conformational rearrangements in the transporter. The structure of FecA bound with iron-free dicitrate represents the first structure of a TonB-dependent transporter bound with an iron-free siderophore. Binding of diferric dicitrate to FecA results in changes in the orientation of the two citrate ions relative to each other and in their interactions with FecA, compared to the binding of iron-free dicitrate. The changes in ligand binding are accompanied by conformational changes in three areas of FecA: two extracellular loops, one plug domain loop and the periplasmic TonB-box motif. The positional and conformational changes in the siderophore and transporter initiate two independent events: ferric citrate transport into the periplasm and transcription induction of the fecABCDE transport genes. From these data, we propose a two-step ligand recognition event: FecA binds iron-free dicitrate in the non-productive state or first step, followed by siderophore displacement to form the transport-competent, diferric dicitrate-bound state in the second step.

Molecular epidemiology of the SRL pathogenicity island

The Shigella resistance locus (SRL), which is carried on the SRL pathogenicity island (PAI) in Shigella flexneri 2a YSH6000, mediates resistance to the antibiotics streptomycin, ampicillin, chloramphenicol, and tetracycline. In the present study, we investigated the distribution and structural variation of the SRL and the SRL PAI in 71 Shigella isolates and 28 other enteric pathogens by PCR and Southern analysis. The SRL and SRL-related loci, although absent from the other enteric pathogens evaluated in this study, were found to be present in a number of Shigella isolates. SRL PAI markers were also present in the majority of strains carrying the SRL and SRL-related loci. PCR linkage studies with six of these strains demonstrated that the SRL is carried on elements similar in structure and organization to the YSH6000 SRL PAI, consistent with the hypothesis that the SRL PAI may be involved in the spread of multiple-antibiotic resistance in these strains.

Effect of immunoglobulin G from cows immunized with ferric citrate receptor (FecA) on iron uptake by Escherichia coli

The effects of immunoglobulin (Ig) G from cows immunized with the ferric citrate receptor (FecA) on iron uptake by Escherichia coli were investigated. Receptor FecA was purified from E. coli UT5600/pSV66. Cows were immunized with 400 microg purified FecA three times at 21 d intervals during late lactation and the nonlactating period. Immunoglobulin G was purified by protein G affinity chromatography from colostral whey from cows immunized with FecA and from unimmunized control cows. The purified IgG from FecA immunized cows had higher IgG titers against FecA compared with control IgG. Fifteen E. coli isolated from intramammary infections and E. coli UT5600/pSV66 were grown in an iron-depleted medium containing 1 mM citrate to induce FecA. The bacterial cells were mixed with 0, 2, and 4 mg/ml purified IgG, and 55Fe was added to the assay. After 5, 10, and 15 min incubations at 37 degrees C, samples were passed through 0.45-pm pore size filters. Filters were washed with saline three times, and the radioactivity of 55Fe taken up by the bacterial cells on the filters was measured by a liquid scintillation counter. The measurements were expressed as numbers of 55Fe atoms per colony-forming unit and transformed to log10. The assay was repeated three times for each isolate in a partially balanced incomplete block design. The presence of IgG decreased 55Fe uptake by E. coli mastitis isolates and E. coli UT5600/pSV66. Anti-FecA IgG reduced 55Fe uptake by E. coli greater than IgG from unimmunized cows.

Efficacy of immunization with ferric citrate receptor FecA from Escherichia coli on induced coliform mastitis

The effects of immunization with the ferric citrate receptor FecA on antibody responses and on experimentally induced mastitis following intramammary challenge were investigated. Twenty-one cows were assigned to seven blocks of three cows based on expected parturition. Cows within block were randomly assigned to one of three treatments: 1) FecA immunization, 2) Escherichia coli J5 immunization, and 3) unimmunized controls. Challenge was by infusion of approximately 60 cfu of E. coli 727 into one uninfected mammary gland between 13 and 31 d after parturition. Cows within block were challenged on the same day. Cows immunized with FecA had higher immunoglobulin (Ig)G titers against FecA in serum and in mammary secretions at calving, immediately before challenge, and 7 d after challenge than did cows immunized with E. coli J5 or control cows. Immunization with FecA also increased IgG titers against whole-cell E. coli 727 in serum and in mammary secretions at calving. Serum IgM titers against FecA were higher in FecA immunized cows than in other treatment groups immediately before challenge. Bacterial counts in milk, duration of bacterial isolation in milk, rectal temperature, and milk somatic cell counts following intramammary challenge were similar among treatments. Milk production and dry matter intake did not differ among treatments. The ferric citrate receptor FecA was immunogenic in cows, but immunization had minimal effect on the clinical severity of experimentally induced E. coli mastitis.

Iron deficiency decreases mitochondrial aconitase abundance and citrate concentration without affecting tricarboxylic acid cycle capacity in rat liver

Mitochondrial aconitase (m-acon) is the tricarboxylic acid (TCA) cycle enzyme that converts citrate to isocitrate. m-Acon mRNA is a potential target for regulation by iron regulatory proteins (IRPs), suggesting a link between dietary iron intake, m-acon synthesis, and energy metabolism. Our previous studies indicate that m-acon is one of a limited number of proteins that is down-regulated in iron-deficient liver. Here we use isolated hepatocytes to study the relationships among decreased m-acon abundance, TCA cycle function and cellular citrate concentration in iron deficiency. Rats were fed an iron-deficient (ID) (2 mg Fe/kg diet) diet, or they were pair-fed (PF) or freely fed (C) a control diet (50 mg Fe/kg diet) for up to 21 d. Hepatocyte total IRP activity was greater by d 2 in the ID group than in the C and PF groups and by d 10, the difference was maximal. Liver IRP activity was inversely correlated with m-acon abundance (r = -0.93, P < 0.0001). However, the decrease in m-acon abundance did not affect the ability of hepatocytes to oxidize 2-[(14)C]pyruvate or 1-[(14)C]acetate, indicating that TCA cycle capacity was not affected. Interestingly, by d 21, total liver citrate concentration was 40% lower in ID than in PF rats, suggesting enhanced utilization of citrate. However, the decrease in citrate concentration was not reflected in a change in liver total lipid concentration. Taken together, our results indicate that the iron-dependent regulation of m-acon in liver does not alter TCA cycle capacity but suggest that IRP-mediated changes in m-acon expression may modulate citrate use in other aspects of intermediary or iron metabolism.

Iron metabolism in pathogenic bacteria

The ability of pathogens to obtain iron from transferrins, ferritin, hemoglobin, and other iron-containing proteins of their host is central to whether they live or die. To combat invading bacteria, animals go into an iron-withholding mode and also use a protein (Nramp1) to generate reactive oxygen species in an attempt to kill the pathogens. Some invading bacteria respond by producing specific iron chelators-siderophores-that remove the iron from the host sources. Other bacteria rely on direct contact with host iron proteins, either abstracting the iron at their surface or, as with heme, taking it up into the cytoplasm. The expression of a large number of genes (>40 in some cases) is directly controlled by the prevailing intracellular concentration of Fe(II) via its complexing to a regulatory protein (the Fur protein or equivalent). In this way, the biochemistry of the bacterial cell can accommodate the challenges from the host. Agents that interfere with bacterial iron metabolism may prove extremely valuable for chemotherapy of diseases.

Control of the ferric citrate transport system of Escherichia coli: mutations in region 2.1 of the FecI extracytoplasmic-function sigma factor suppress mutations in the FecR transmembrane regulatory protein

Transcription of the ferric citrate transport genes is initiated by binding of ferric citrate to the FecA protein in the outer membrane of Escherichia coli K-12. Bound ferric citrate does not have to be transported but initiates a signal that is transmitted by FecA across the outer membrane and by FecR across the cytoplasmic membrane into the cytoplasm, where the FecI extracytoplasmic-function (ECF) sigma factor becomes active. In this study, we isolated transcription initiation-negative missense mutants in the cytoplasmic region of FecR that were located at four sites, L13Q, W19R, W39R, and W50R, which are highly conserved in FecR-like open reading frames of the Pseudomonas aeruginosa, Pseudomonas putida, Bordetella pertussis, Bordetella bronchiseptica, and Caulobacter crescentus genomes. The cytoplasmic portion of the FecR mutant proteins, FecR(1-85), did not interact with wild-type FecI, in contrast to wild-type FecR(1-85), which induced FecI-mediated fecB transport gene transcription. Two missense mutations in region 2.1 of FecI, S15A and H20E, partially restored induction of ferric citrate transport gene induction of the fecR mutants by ferric citrate. Region 2.1 of sigma(70) is thought to bind RNA polymerase core enzyme; the residual activity of mutated FecI in the absence of FecR, however, was not higher than that of wild-type FecI. In addition, missense mutations in the fecI promoter region resulted in a twofold increased transcription in fecR wild-type cells and a partial restoration of fec transport gene transcription in the fecR mutants. The mutations reduced binding of the Fe(2+) Fur repressor and as a consequence enhanced fecI transcription. The data reveal properties of the FecI ECF factor distinct from those of sigma(70) and further support the novel transcription initiation model in which the cytoplasmic portion of FecR is important for FecI activity.

Antigenic homology of the inducible ferric citrate receptor (FecA) of coliform bacteria isolated from herds with naturally occurring bovine intramammary infections

Expression of ferric citrate receptor FecA by Escherichia coli and Klebsiella pneumoniae isolated from bovine mastitis was investigated. Transformant E. coli UT5600/pSV66, which produces large quantities of FecA in the presence of citrate, was constructed. The FecA of E. coli UT5600/pSV66 was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used to prepare polyclonal antiserum in rabbits. All coliform isolates of E. coli (n = 18) and K. pneumoniae (n = 17) from naturally occurring bovine intramammary infections in five herds induced iron-regulated outer membrane proteins when grown in Trypticase soy broth containing 200 microM alpha-alpha'-dipyridyl and 1 mM citrate. Polyclonal antiserum against FecA was used in conjunction with an immunoblot technique to determine the degree of antigenic homology of FecA among isolates. In the presence of citrate, each isolate expressed FecA that reacted with the anti-FecA polyclonal antiserum. The molecular mass of FecA ( approximately 80.5 kDa) was also highly conserved among isolates. Therefore, the ferric citrate iron transport may be induced in coliform bacteria and utilized to acquire iron in milk for survival and growth. The FecA is an attractive vaccine component for controlling coliform mastitis during the lactation period.

The anti-sigma factors

A mechanism for regulating gene expression at the level of transcription utilizes an antagonist of the sigma transcription factor known as the anti-sigma (anti-sigma) factor. The cytoplasmic class of anti-sigma factors has been well characterized. The class includes AsiA form bacteriophage T4, which inhibits Escherichia coli sigma 70; FlgM, present in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor sigma 28; SpoIIAB, which inhibits the sporulation-specific sigma factor, sigma F and sigma G, of Bacillus subtilis; RbsW of B. subtilis, which inhibits stress response sigma factor sigma B; and DnaK, a general regulator of the heat shock response, which in bacteria inhibits the heat shock sigma factor sigma 32. In addition to this class of well-characterized cytoplasmic anti-sigma factors, a new class of homologous, inner-membrane-bound anti-sigma factors has recently been discovered in a variety of eubacteria. This new class of anti-sigma factors regulates the expression of so-called extracytoplasmic functions, and hence is known as the ECF subfamily of anti-sigma factors. The range of cell processes regulated by anti-sigma factors is highly varied and includes bacteriophage phage growth, sporulation, stress response, flagellar biosynthesis, pigment production, ion transport, and virulence.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Ferric citrate transport of Escherichia coli: functional regions of the FecR transmembrane regulatory protein

Transcription of the ferric citrate transport genes of Escherichia coli is induced by ferric citrate bound to the outer membrane receptor FecA. Additional ferric citrate-specific regulatory proteins are FecR in the cytoplasmic membrane and the FecI sigma factor in the cytoplasm. To further understand the assumed FecR-mediated signal transduction across the cytoplasmic membrane, the transmembrane topology of FecR (317 amino acids) was determined with hybrid proteins containing portions of FecR and mature BlaM beta-lactamase. BlaM fused to FecR regions extending from residues 107 to 149 and residues 230 to 259 conferred high ampicillin resistance to cells, while BlaM fused to sites between residues 159 and 210 and between residues 265 and 301 conferred low resistance. Cells that synthesized FecR'-BlaM with fusion joints between residues 8 and 81 of FecR were fully sensitive to ampicillin. The ampicillin resistance of the low-resistance FecR'-BlaM hybrids was increased 2- to 10-fold by cosynthesis of plasmid-encoded GroEL GroES and SecB chaperones and in degP and ompT protease mutants, which suggested that the decreased ampicillin resistance level of these hybrids was caused by the formation of inclusion bodies and proteolytic degradation. Replacement of glycine by aspartate residues in the only hydrophobic FecR sequence (residues 85 to 100) abolished the beta-lactamase activity of high-resistance FecR'-BlaM proteins, indicating that there are no other transmembrane regions in FecR that translocate BlaM into the periplasm independent of the hydrophobic sequence. All FecR'-BlaM proteins with at least 61 FecR residues complemented a fecR mutant such that it could grow on ferric citrate as the sole iron source and induced fecA-lacZ transcription independent of ferric citrate. The low resistance mediated by two FecR'-BlaM proteins in a fecA deletion mutant was increased 20-fold by transformation with a fecA-encoding plasmid. We propose that FecR spans the cytoplasmic membrane once, interacts in the periplasm with its C-terminal region with FecA occupied by ferric citrate, and transmits the information through the cytoplasmic membrane into the cytoplasm, where it converts FecI into an active sigma factor.

Signal transduction and transcriptional and posttranscriptional control of iron-regulated genes in bacteria

Iron is an essential element for nearly all living cells. Thus, the ability of bacteria to utilize iron is a crucial survival mechanism independent of the ecological niche in which the microorganism lives, because iron is scarce both in potential biological hosts, where it is bound by high-affinity iron-binding proteins, and in the environment, where it is present as part of insoluble complex hydroxides. Therefore, pathogens attempting to establish an infection and environmental microorganisms must all be able to utilize the otherwise unavailable iron. One of the strategies to perform this task is the possession of siderophore-mediated iron uptake systems that are capable of scavenging the hoarded iron. This metal is, however, a double-edged sword for the cell because it can catalyze the production of deadly free hydroxyl radicals, which are harmful to the cells. It is therefore imperative for the cell to control the concentration of iron at levels that permit key metabolic steps to occur without becoming a messenger of cell death. Early work identified a repressor, Fur, which as a complex with iron repressed the expression of most iron uptake systems as well as other iron-regulated genes when the iron concentration reached a certain level. However, later work demonstrated that this regulation by Fur was not the only answer under low-iron conditions, there was a need for activation of iron uptake genes as well as siderophore biosynthetic genes. Furthermore, it was also realized that in some instances the actual ferric iron-siderophore complex induced the transcription of the cognate receptor and transport genes. It became evident that control of the expression of iron-regulated genes was more complex than originally envisioned. In this review, I analyze the processes of signal transduction, transcriptional control, and posttranscriptional control of iron-regulated genes as reported for the ferric dicitrate system in Escherichia coli; the pyochelin, pyoverdin, and enterobactin systems in Pseudomonas species; the irgB system in Vibrio cholerae; and the plasmid-mediated anguibactin system in Vibrio anguillarum. I hope that by using these diverse paradigms, I will be able to convey a unifying picture of these mechanism and their importance in the maintenance and prosperity of bacteria within their ecological niches.

Surface signaling in transcriptional regulation of the ferric citrate transport system of Escherichia coli: mutational analysis of the alternative sigma factor FecI supports its essential role in fec transport gene transcription

Ferric citrate induces transcription of the ferric citrate transport genes (fec) in escherichia coli by binding to the outer membrane receptor protein FecA without entering the cell. The signal elicited by ferric citrate crosses the outer membrane via TonB, ExbB, and ExbD. FecR transmits the signal across the cytoplasmic membrane and activates FecI located in the cytoplasm. FecI belongs to a subgroup of sigma factors that respond to extracytoplasmic stimuli. Chromosomal insertion and deletion mutations were generated in fecI; the resulting mutants were totally devoid of FecA production and fecB-lacZ expression. Iron starvation did not derepress fec transport gene transcription in fecI mutants. Missense point mutations were generated in the predicted helix-turn-helix motif of FecI to examine its role in transcription initiation. Replacement of glutamate by alanine (E141A) at the third position in the first helix reduced the residual activity of FecI in the absence of ferric citrate to 30% of the wild-type level, but induced fec transcription almost normally n the presence of ferric citrate. Mutant FecI(K145E) displayed 156% of the activity of wild-type FecI in the absence of ferric citrate and conferred full induction by ferric citrate. Mutant FecI(K155E), which has a mutation in the second helix, showed 9% of the wild-type activity in the presence of ferric citrate and 78% in the absence of ferric citrate. The reduced activity of FecI(K155E) was also shown in vitro by DNA binding assays with cell lysates; in gel retardation experiments FecI(K155E) reduced the electrophoretic mobility of fecA promoter-containing DNA less than did wild-type FecI. fecI is not autoregulated, as demonstrated by the lack of FecI-induced fecI-lacZ expression in vivo and by the lack of specific fecI transcription in vitro. Instead, formation of fecI mRNA requires sigma 70. We conclude that transcription of the fec transport genes is regulated by FecI, which responds to ferric citrate via FecR. fecI and fecR co-transcription is inhibited by the iron-loaded Fur repressor, which then results in a low level of transcription of the fec transport genes.

The activity of microcin E492 from Klebsiella pneumoniae is regulated by a microcin antagonist

Microcin E492 is a polypeptide antibiotic that is produced and excreted by Klebsiella pneumoniae. Different growth conditions of the producer strain affect microcin activity. The production of a microcin antagonist is responsible for the changes in microcin activity. The microcin antagonist is induced when cells are iron-deprived, resulting in a low microcin activity. The microcin antagonist was purified using a procedure developed for the isolation of a catechol-type siderophore, and its activity was titrated using purified microcin. The inhibitory effect of the microcin antagonist is not observed when this compound is forming a complex with iron. The same inhibitory effect on microcin activity was obtained using purified enterochelin from Escherichia coli. The microcin antagonist was identified as enterochelin through thin-layer chromatography.

Energy-coupled transport and signal transduction through the gram-negative outer membrane via TonB-ExbB-ExbD-dependent receptor proteins

Iron in the form of ferric siderophore complexes and vitamin B12 are transported through the outer membrane of Gram-negative bacteria by a mechanism which consumes energy. There is no known energy source in the outer membrane or in the adjacent periplasmic space so that energy is provided by the electrochemical potential across the cytoplasmic membrane. Energy flows from the cytoplasmic into the outer membrane via a complex consisting of the TonB, ExbB and ExbD proteins which are anchored in the cytoplasmic membrane. It is proposed that the TonB--ExbB--ExbD complex opens--via an energized conformation of the TonB protein--channels in the outer membrane, formed by proteins which serves as highly specific binding sites for the various ferric siderophores and vitamin B12. In addition, outer membrane receptors together with the TonB--ExbB--ExbD complex are directly involved in induction of the transcription of ferric citrate and pseudobactin transport genes of Escherichia coli and Pseudomonas putida, respectively.

Analysis of the pesticin receptor from Yersinia pestis: role in iron-deficient growth and possible regulation by its siderophore

We have sequenced a region from the pgm locus of Yersinia pestis KIM6+ that confers sensitivity to the bacteriocin pesticin to certain strains of Escherichia coli and Y. pestis. The Y. pestis sequence is 98% identical to the pesticin receptor from Yersinia enterocolitica and is homologous to other TonB-dependent outer membrane proteins. Y. pestis strains with an in-frame deletion in the pesticin receptor gene (psn) were pesticin resistant and no longer expressed a group of iron-regulated outer membrane proteins, IrpB to IrpD. In addition, this strain as well as a Y. pestis strain with a mutation constructed in the gene (irp2) encoding the 190-kDa iron-regulated protein HMWP2 could not grow at 37 degrees C in a defined, iron-deficient medium. However, the irp2 mutant but not the psn mutant could be cross-fed by supernatants from various Yersinia cultures grown under iron-deficient conditions. An analysis of the proteins synthesized by the irp2 mutant suggests that HMWP2 may be indirectly required for maximal expression of the pesticin receptor. HMWP2 likely participates in synthesis of a siderophore which may induce expression of the receptor for pesticin and the siderophore.

Mechanism of tonB-dependent transport of KP-736, a 1,5-dihydroxy-4-pyridone-substituted cephalosporin, into Escherichia coli K-12 cells

The mechanism of transport of KP-736, a novel cephalosporin with a 1,5-dihydroxy-4-pyridone moiety at the C-7 position, into the Escherichia coli K-12 cell was investigated by determining the susceptibilities of iron transport mutants to KP-736. The tonB mutant showed a higher degree of resistance to KP-736, indicating that KP-736 was incorporated into E. coli cells via the tonB-dependent iron transport system. The product of the exbB gene was also necessary for the maximal antibacterial potency of KP-736. Cir-lacking and Fiu-lacking mutants showed a moderate level of resistance to KP-736. However, mutants lacking any one of the proteins FepA, FecA, FhuA, and FhuE did not show any increased resistance to KP-736. Two types of spontaneous mutants (e.g., KT1004 and KT1011) could be isolated from cir and fiu mutants by selection for KP-736 resistance and showed the same level of resistance to KP-736 as a tonB mutant. KT1004 showed tonB phenotypes, resistance to phage phi 80, and loss of FecA, whereas KT1011 did not. KT1011 lost the ability to express both Cir and Fiu proteins. These results indicate that the Cir and Fiu outer membrane proteins are involved specifically in the tonB-dependent transport process of KP-736. Against OmpF- and OmpC-deficient transformants producing various groups of beta-lactamases, KP-736 was more effective than the other cephalosporins tested.

Regulation of citrate-dependent iron transport of Escherichia coli: fecR is required for transcription activation by FecI

Citrate-dependent Fe3+ transport into Escherichia coli K-12 is induced by iron and citrate. The inducer is probably ferric dicitrate which does not have to be taken up into the cytoplasm to induce transcription of the fec transport genes. Two regulatory genes, fecI and fecR, located upstream of the fecABCDE transport genes, are required for induction. We report that in vivo the chromosomally encoded FecI protein activates transcription of the fecA and fecB transport genes in response to ferric citrate and the FecR protein. Cells expressing chromosomally and plasmid-encoded truncated FecR derivatives no longer responded to ferric citrate and expressed the fec transport genes constitutively. The smallest active FecR derivative contained 59 amino acid residues as compared to the 317 residues of wild-type FecR. Constitutive induction was lower than induction of the FecR wild-type strain by ferric citrate. It is concluded that the N-terminal portion of FecR activates FecI and that the C-terminal portion of FecR responds to ferric citrate. Transcription of the fec transport genes is positively regulated by FecI and FecR and negatively regulated by the Fe2(+)-Fur repressor. Transcription activation and repression may occur independently of each other.

Iron-responsive gene expression in Pseudomonas fluorescens M114: cloning and characterization of a transcription-activating factor, PbrA

In response to iron limitation. Pseudomonas fluorescens M114 induces a number of genes including an iron-scavenging siderophore termed pseudobactin M114, its cognate receptor, PbuA, and a casein protease. A Tn5lacZ-induced mutant (M114FA1) was isolated that exhibits a pleiotropic phenotype and lacks the ability to express these iron-regulated genes. A cosmid clone was identified which complements this mutation. This clone is capable of activating a number of iron-regulated promoter fusion constructs from P. fluorescens M114 and Pseudomonas putida WCS358 and can also promote expression of these fusions in Escherichia coli. A series of insertion mutants was constructed by homologous recombination which were unable to transcribe the promoter fusions. DNA sequence analysis of the complementing region identified one open reading frame (ORF) termed pbrA (pseudobactin regulation activation) and the deduced amino acid sequence shows domains with significant homology to a number of ECF (extracytoplasmic function) transcriptional regulators of the sigma 70 sigma factor family, including fecl required for expression of the ferric dicitrate outer-membrane receptor protein of E. coli. Sequences upstream of the pbrA gene suggest that transcription of pbrA may also be iron regulated.

The mammalian transferrin-independent iron transport system may involve a surface ferrireductase activity

Mammalian cells accumulate iron from ferric citrate or ferric nitrilotriacetate through the activity of a transferrin-independent iron transport system [Sturrock, Alexander, Lamb, Craven and Kaplan (1990) J. Biol. Chem. 265, 3139-3145]. The uptake system might recognize and transport ferric-anion complexes, or cells may reduce ferric iron at the surface and then transport ferrous iron. To distinguish between these possibilities we exposed cells to either [59Fe]ferric citrate or ferric [14C]citrate and determined whether accumulation of iron was accompanied by the obligatory accumulation of citrate. In HeLa cells and human skin fibroblasts the rate of accumulation of iron was three to five times greater than that of citrate. Incubation of fibroblasts with ferric citrate or ferric ammonium citrate resulted in an enhanced accumulation of iron and citrate; the molar ratio of accumulation approaching unity. A similar rate of citrate accumulation, however, was observed when ferric citrate-incubated cells were exposed to [14C]citrate alone. Further studies demonstrated the independence of iron and citrate accumulation: addition of unlabelled citrate to cells decreased the uptake of labelled citrate without affecting the accumulation of 59Fe; iron uptake was decreased by the addition of ferrous chelators whereas the uptake of citrate was unaffected; reduction of ferric iron by ascorbate increased the uptake of iron but had no effect on the uptake of citrate. When HeLa cells were depleted of calcium, iron uptake decreased, but there was little effect on citrate uptake. These results indicate that transport of iron does not require the obligatory transport of citrate and vice versa. The mammalian transferrin-independent iron transport system appears functionally similar to iron transport systems in both the bacterial and plant kingdoms which require the activities of both a surface reductase and a ferrous metal transporter.

Nodulating ability of Rhizobium tropici is conditioned by a plasmid-encoded citrate synthase

Rhizobium species elicit the formation of nitrogen-fixing root nodules through a complex interaction between bacteria and plants. Various bacterial genes involved in the nodulation and nitrogen-fixation processes have been described and most have been localized on the symbiotic plasmids (pSym). We have found a gene encoding citrate synthase on the pSym plasmid of Rhizobium tropici, a species that forms nitrogen-fixing nodules on the roots of beans (Phaseolus vulgaris) and trees (Leucaena spp.). Citrate synthase is a key metabolic enzyme that incorporates carbon into the tricarboxylic acid cycle by catalysing the condensation of acetyl-CoA and oxaloacetic acid to form citrate. R. tropici pcsA (the plasmid citrate synthase gene) is closely related to the corresponding genes of Proteobacteria. pcsA inactivation by a Tn5-mob insertion causes the bacteria to form fewer nodules (30-50% of the original strain) and to have a decreased citrate synthase activity in minimal medium with sucrose. A clone carrying the pcsA gene complemented all the phenotypic alterations of the pcsA mutant, and conferred Rhizobium leguminosarum bv. phaseoli (which naturally lacks a plasmid citrate synthase gene) a higher nodulation and growth capacity in correlation with a higher citrate synthase activity. We have also found that pcsA gene expression is sensitive to iron availability, suggesting a possible role of pcsA in iron uptake.

Characterization of the ferrous iron uptake system of Escherichia coli

Escherichia coli has an iron(II) transport system (feo) which may make an important contribution to the iron supply of the cell under anaerobic conditions. Cloning and sequencing of the iron(II) transport genes revealed an open reading frame (feoA) possibly coding for a small protein with 75 amino acids and a membrane protein with 773 amino acids (feoB). The upstream region of feoAB contained a binding site for the regulatory protein Fur, which acts with iron(II) as a corepressor in all known iron transport systems of E. coli. In addition, a Fnr binding site was identified in the promoter region. The FeoB protein had an apparent molecular mass of 70 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was localized in the cytoplasmic membrane. The sequence revealed regions of homology to ATPases, which indicates that ferrous iron uptake may be ATP driven. FeoA or FeoB mutants could be complemented by clones with the feoA or feoB gene, respectively.

Identification and characterization of the pupB gene encoding an inducible ferric-pseudobactin receptor of Pseudomonas putida WCS358

Pseudomonas putida WCS358 can transport iron complexed to a wide variety of pseudobactins produced by other Pseudomonas strains. The pupB gene encoding an outer membrane ferric-pseudobactin receptor was isolated from a genomic library of P. putida WCS358. The PupB receptor facilitated iron transport via two distinct heterologous siderophores, i.e. pseudobactin BN8 and pseudobactin BN7. The amino acid sequence deduced from the nucleotide sequence consisted of 804 amino acids (molecular weight 88,369) of which the N-terminal part was very similar to a prokaryotic leader peptide. The mature protein shared significant homology with the receptor for ferric-pseudobactin 358 (PupA) and contained three regions common to TonB-dependent receptor proteins of Escherichia coli. Interestingly, PupB expression was only observed in cells cultured in iron-deficient medium containing pseudobactin BN8 or pseudobactin BN7. This expression required a transcriptional unit, pupR, identified upstream of the structural pupB gene. Transposon Tn5 insertion mutants defective in PupB production still exhibited uptake of iron via pseudobactin BN8, although with reduced efficiency. Apparently, an additional transport system for this ferric-siderophore complex operates in this strain. In addition to pseudobactin BN8 also other heterologous siderophores were capable of inducing synthesis of specific high-molecular-weight outer membrane proteins in strain WCS358, which suggests the existence of multiple siderophore-inducible iron transport systems in this strain.

Escherichia coli K-12 ferrous iron uptake mutants are impaired in their ability to colonize the mouse intestine

The streptomycin-treated mouse colonization model was used to investigate the role of the Fe2+ uptake system (Feo) of Escherichia coli K12 in the colonization of the mouse intestine. Mutants impaired in the uptake of Fe2+ ions were shown to be deficient also in their colonization ability. Both enterochelin-producing and enterochelin-nonproducing Escherichia coli feo mutants were unable to colonize the mouse intestine. These results demonstrated that Fe(II) is an essential source of iron for E. coli grown in the intestine.

A novel purification of ferric citrate receptor (FecA) from Escherichia coli UT5600 and further characterization of its binding activity

In our earlier paper, it was demonstrated that the FecA receptor protein from Escherichia coli UT5600/pBB2 (leu-, proC-, trpE-, entA-, rpsl-, delta (ompT-fepA)-/Ampr, fepA) binds with ferric enterobactin. In order to explore this further the outer membrane receptor protein, FecA, has been isolated from UT5600 (fepA-) and purified to homogeneity by DE-52-cellulose anion exchange chromatography followed by MonoPFPLC chromatofocusing. Partially purified FecA and homogeneous FecA show binding activity to [55Fe]ferric enterobactin and the binding is specific. Binding activity of FecA can be enhanced by ferric citrate. Lipopolysaccharide-free FecA as ascertained by silver staining and the endotoxin test still retains the same activity. In vivo uptake studies using different strains of E. coli suggest that FecA in E. coli plays an important role in ferrienterobactin transport.