Magnesium Transport in Escherichia coli

Mrs2-mediated mitochondrial magnesium uptake is essential for the regulation of MCU-mediated mitochondrial Ca2+ uptake and viability

Mitochondrial Ca2+ uptake is essential in regulating bioenergetics, cell death, and cytosolic Ca2+ transients. Mitochondrial Calcium Uniporter (MCU) mediates the mitochondrial Ca2+ uptake. Though MCU regulation by MICUs is unequivocally established, there needs to be more knowledge of whether divalent cations regulate MCU. Here, we set out to understand the mitochondrial matrix Mg2+-dependent regulation of MCU activity. We showed that decreased matrix [Mg2+] is associated with increased MCU activity and significantly prompted mitochondrial permeability transition pore opening. Our findings support the critical role of mMg2+ in regulating MCU activity.

Cation Homeostasis: Coordinate Regulation of Polyamine and Magnesium Levels in Salmonella

Polyamines are organic cations that are important in all domains of life. Here, we show that in Salmonella, polyamine levels and Mg²⁺ levels are coordinately regulated and that this regulation is critical for viability under both low and high concentrations of polyamines. Upon Mg²⁺ starvation, polyamine synthesis is induced, as is the production of the high-affinity Mg²⁺ transporters MgtA and MgtB. Either polyamine synthesis or Mg²⁺ transport is required to maintain viability. Mutants lacking the polyamine exporter PaeA, the expression of which is induced by PhoPQ in response to low Mg²⁺, lose viability in the stationary phase. This lethality is suppressed by blocking either polyamine synthesis or Mg²⁺ transport, suggesting that once Mg²⁺ levels are reestablished, the excess polyamines must be excreted. Thus, it is the relative levels of both Mg²⁺ and polyamines that are regulated to maintain viability. Indeed, sensitivity to high concentrations of polyamines is proportional to the Mg²⁺ levels in the medium. These results are recapitulated during infection. Polyamine synthesis mutants are attenuated in a mouse model of systemic infection, as are strains lacking the MgtB Mg²⁺ transporter. The loss of MgtB in the synthesis mutant background confers a synthetic phenotype, confirming that Mg²⁺ and polyamines are required for the same process(es). Mutants lacking PaeA are also attenuated, but deleting paeA has no phenotype in a polyamine synthesis mutant background. These data support the idea that the cell coordinately controls both the polyamine and Mg²⁺ concentrations to maintain overall cation homeostasis, which is critical for survival in the macrophage phagosome. IMPORTANCE Polyamines are organic cations that are important in all life forms and are essential in plants and animals. However, their physiological functions and regulation remain poorly understood. We show that polyamines are critical for the adaptation of Salmonella to low Mg²⁺ conditions, including those found in the macrophage phagosome. Polyamines are synthesized upon low Mg²⁺ stress and partially replace Mg²⁺ until cytoplasmic Mg²⁺ levels are restored. Indeed, it is the sum of Mg²⁺ and polyamines in the cell that is critical for viability. While Mg²⁺ and polyamines compensate for one another, too little of both or too much of both is lethal. After cytoplasmic Mg²⁺ levels are reestablished, polyamines must be exported to avoid the toxic effects of excess divalent cations.

Omptin Proteases of Enterobacterales Show Conserved Regulation by the PhoPQ Two-Component System but Exhibit Divergent Protection from Antimicrobial Host Peptides and Complement

Bacteria that colonize eukaryotic surfaces interact with numerous antimicrobial host-produced molecules, including host defense peptides, complement, and antibodies. Bacteria have evolved numerous strategies to both detect and resist these molecules, and in the Enterobacterales order of bacteria these include alterations of the cell surface lipopolysaccharide structure and/or charge and the production of proteases that can degrade these antimicrobial molecules. Here, we show that omptin family proteases from Escherichia coli and Citrobacter rodentium are regulated by the PhoPQ system. Omptin protease activity is induced by growth in low Mg²⁺, and deletion of PhoP dramatically reduces omptin protease activity, transcriptional regulation, and protein levels. We identify conserved PhoP-binding sites in the promoters of the E. coli omptin genes ompT, ompP, and arlC as well as in croP of Citrobacter rodentium and show that mutation of the putative PhoP-binding site in the ompT promoter abrogates PhoP-dependent expression. Finally, we show that although regulation by PhoPQ is conserved, each of the omptin proteins has differential activity toward host defense peptides, complement components, and resistance to human serum, suggesting that each omptin confers unique survival advantages against specific host antimicrobial factors.

OmpR and Prc contribute to switch the Salmonella morphogenetic program in response to phagosome cues

Salmonella enterica serovar Typhimurium infects eukaryotic cells residing within membrane-bound phagosomes. In this compartment, the pathogen replaces the morphogenetic penicillin-binding proteins 2 and 3 (PBP2/PBP3) with PBP2_SAL /PBP3_SAL , two proteins absent in Escherichia coli. The basis for this switch is unknown. Here, we show that PBP3 protein levels drop drastically when S. Typhimurium senses acidity, high osmolarity and nutrient scarcity, cues that activate virulence functions required for intra-phagosomal survival and proliferation. The protease Prc and the transcriptional regulator OmpR contribute to lower PBP3 levels whereas OmpR stimulates PBP2_SAL /PBP3_SAL production. Surprisingly, despite being essential for division in E. coli, PBP3 levels also drop in non-pathogenic and pathogenic E. coli exposed to phagosome cues. Such exposure alters E. coli morphology resulting in very long bent and twisted filaments indicative of failure in the cell division and elongation machineries. None of these aberrant shapes are detected in S. Typhimurium. Expression of PBP3_SAL restores cell division in E. coli exposed to phagosome cues although the cells retain elongation defects in the longitudinal axis. By switching the morphogenetic program, OmpR and Prc allow S. Typhimurium to properly divide and elongate inside acidic phagosomes maintaining its cellular dimensions and the rod shape.

Effect of metal cations on antimicrobial activity and compartmentalization of silver in Shewanella oneidensis MR-1 upon exposure to silver ions

Silver is an antimicrobial agent that is used extensively in consumer products, such as fabrics and humidifiers. Silver ion (Ag⁺) uptake in bacteria represents a crucial phase of antimicrobial activity. However, the uptake mechanism of Ag⁺ in bacteria remains largely unknown. The genus Shewanella drives many geochemical processes of nutrients and pollutants in soils. In the present study, Ag⁺ uptake by Shewanella oneidensis MR-1 was first investigated in a laboratory in defined anaerobic, oligotrophic, and inorganic media with or without cations (potassium ions [K⁺], magnesium ions [Mg²⁺], and zinc ions [Zn²⁺]). Our results revealed variations in antimicrobial activity of Ag⁺ in the presence of Mg²⁺ and Zn²⁺. First, Mg²⁺ significantly decreased antimicrobial activity of Ag⁺ in S. oneidensis MR-1 by inhibiting cellular Ag⁺ uptake when compared with K⁺. The results were consistent with that of Co²⁺ (Mg²⁺ channel blocker) decreased Ag⁺ uptake by S. oneidensis MR-1. Moreover, Mg²⁺ promoted riboflavin secretion and facilitated the formation of metallic Ag nanoparticles on bacterial surfaces, which was beneficial for extracellular electron transfer and consequently reduced antibacterial activity of Ag⁺. Second, Zn²⁺ increased the antimicrobial activity of Ag⁺ in S. oneidensis MR-1, although the effect on Ag⁺ uptake was minimal. A synergistic interaction between Zn²⁺ and Ag⁺ led to an increase in dead cells and decreased ferrihydrite reduction capacity. The findings suggest that Mg²⁺ could reduce the environmental risk of Ag⁺ to soil bacteria, while Zn²⁺ should be of particular concern due to its synergistic antimicrobial effect on bacteria.

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg²⁺ transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Serratia marcescens RamA Expression Is under PhoP-Dependent Control and Modulates Lipid A-Related Gene Transcription and Antibiotic Resistance Phenotypes

Serratia marcescens is an enteric bacterium that can function as an opportunistic pathogen with increasing incidence in clinical settings. This is mainly due to the ability to express a wide range of virulence factors and the acquisition of antibiotic resistance mechanisms. For these reasons, S. marcescens has been declared by the World Health Organization (WHO) as a research priority to develop alternative antimicrobial strategies. In this study, we found a PhoP-binding motif in the promoter region of transcriptional regulator RamA of S. marcescens RM66262. We demonstrated that the expression of ramA is autoregulated and that ramA is also part of the PhoP/PhoQ regulon. We have also shown that PhoP binds directly and specifically to ramA, mgtE1, mgtE2, lpxO1, and lpxO2 promoter regions and that RamA binds to ramA and lpxO1 but not to mgtE1 and lpxO2, suggesting an indirect control for the latter genes. Finally, we have demonstrated that in S. marcescens, RamA overexpression induces the AcrAB-TolC efflux pump, required to reduce the susceptibility of the bacteria to tetracycline and nalidixic acid. In sum, we here provide the first report describing the regulation of ramA under the control of the PhoP/PhoQ regulon and the regulatory role of RamA in S. marcescens. IMPORTANCE We demonstrate that in S. marcescens, the transcriptional regulator RamA is autoregulated and also controlled by the PhoP/PhoQ signal transduction system. We show that PhoP is able to directly and specifically bind to ramA, mgtE1, mgtE2, lpxO1, and lpxO2 promoter regions. In addition, RamA is able to directly interact with the promoter regions of ramA and lpxO1 but indirectly regulates mgtE1 and lpxO2. Finally, we found that in S. marcescens, RamA overexpression induces the AcrAB-TolC efflux pump, required to reduce susceptibility to tetracycline and nalidixic acid. Collectively, these results further our understanding of the PhoP/PhoQ regulon in S. marcescens and demonstrate the involvement of RamA in the protection against antibiotic challenges.

Consequences of producing DNA gyrase from a synthetic gyrBA operon in Salmonella enterica serovar Typhimurium

DNA gyrase is an essential type II topoisomerase that is composed of two subunits, GyrA and GyrB, and has an A₂B₂ structure. Although the A and B subunits are required in equal proportions to form DNA gyrase, the gyrA and gyrB genes that encode them in Salmonella (and in many other bacteria) are at separate locations on the chromosome, are under separate transcriptional control, and are present in different copy numbers in rapidly growing bacteria. In wild‐type Salmonella, gyrA is near the chromosome's replication terminus, while gyrB is near the origin. We generated a synthetic gyrBA operon at the oriC‐proximal location of gyrB to test the significance of the gyrase gene position for Salmonella physiology. Although the strain producing gyrase from an operon had a modest alteration to its DNA supercoiling set points, most housekeeping functions were unaffected. However, its SPI‐2 virulence genes were expressed at a reduced level and its survival was reduced in macrophage. Our data reveal that the horizontally acquired SPI‐2 genes have a greater sensitivity to disturbance of DNA topology than the core genome and we discuss its significance in the context of Salmonella genome evolution and the gyrA and gyrB gene arrangements found in other bacteria.

Novel Template Plasmids pCyaA'-Kan and pCyaA'-Cam for Generation of Unmarked Chromosomal cyaA' Translational Fusion to T3SS Effectors in Salmonella

The type III secretion systems (T3SS) encoded in pathogenicity islands SPI-1 and SPI-2 are key virulence factors of Salmonella. These systems translocate proteins known as effectors into eukaryotic cells during infection. To characterize the functionality of T3SS effectors, gene fusions to the CyaA’ reporter of Bordetella pertussis are often used. CyaA’ is a calmodulin-dependent adenylate cyclase that is only active within eukaryotic cells. Thus, the translocation of an effector fused to CyaA’ can be evaluated by measuring cAMP levels in infected cells. Here, we report the construction of plasmids pCyaA’-Kan and pCyaA’-Cam, which contain the ORF encoding CyaA’ adjacent to a cassette that confers resistance to kanamycin or chloramphenicol, respectively, flanked by Flp recombinase target (FRT) sites. A PCR product from pCyaA’-Kan or pCyaA’-Cam containing these genetic elements can be introduced into the bacterial chromosome to generate gene fusions by homologous recombination using the Red recombination system from bacteriophage λ. Subsequently, the resistance cassette can be removed by recombination between the FRT sites using the Flp recombinase. As a proof of concept, the plasmids pCyaA’-Kan and pCyaA’-Cam were used to generate unmarked chromosomal fusions of 10 T3SS effectors to CyaA’ in S. Typhimurium. Each fusion protein was detected by Western blot using an anti-CyaA’ monoclonal antibody when the corresponding mutant strain was grown under conditions that induce the expression of the native gene. In addition, T3SS-1-dependent secretion of fusion protein SipA-CyaA’ during in vitro growth was verified by Western blot analysis of culture supernatants. Finally, efficient translocation of SipA-CyaA’ into HeLa cells was evidenced by increased intracellular cAMP levels at different times of infection. Therefore, the plasmids pCyaA’-Kan and pCyaA’-Cam can be used to generate unmarked chromosomal cyaA’ translational fusion to study regulated expression, secretion and translocation of Salmonella T3SS effectors into eukaryotic cells.

The PhoPQ Two-Component System Is the Major Regulator of Cell Surface Properties, Stress Responses and Plant-Derived Substrate Utilisation During Development of Pectobacterium versatile-Host Plant Pathosystems

Pectobacterium versatile (formerly P. carotovorum) is a recently defined species of soft rot enterobacteria capable of infecting many plant hosts and damaging different tissues. Complex transcriptional regulation of virulence properties can be expected for such a versatile pathogen. However, the relevant information is available only for related species and is rather limited. The PhoPQ two-component system, originally described in pectobacteria as PehRS, was previously shown to regulate a single gene, pehA. Using an insertional phoP mutant of Pectobacterium versatile (earlier-P. carotovorum), we demonstrate that PhoP regulates at least 115 genes with a majority of them specific for pectobacteria. The functions performed by PhoP-controlled genes include degradation, transport and metabolism of plant-derived carbon sources (polygalacturonate, arabinose-containing polysaccharides and citrate), modification of bacterial cell envelope and stress resistance. We also demonstrated PhoP involvement in establishing the order of plant cell wall decomposition and utilisation of the corresponding breakdown products. Based on experimental data and in silico analysis, we defined a PhoP binding site motif and provided proof for its universality in enteric bacteria. Scanning P. versatile genome for the locations of this motif suggested a much larger PhoP regulon enriched with the genes important for a plant pathogen, which makes PhoP a global virulence regulator. Potential PhoP targets include many regulatory genes and PhoP control over one of them, expI, was confirmed experimentally, highlighting the link between the PhoPQ two-component and quorum sensing systems. High concentrations of calcium and magnesium ions were found to abolish the PhoPQ-dependent transcription activation but did not relieve repression. Reduced PhoP expression and minimisation of PhoP dependence of regulon members' expression in P. versatile cells isolated from potato tuber tissues suggest that PhoPQ system is a key switch of expression levels of multiple virulence-related genes fine-tuned to control the development of P. versatile-host plant pathosystem.

Synthetic dual-function RNA reveals features necessary for target regulation

Small base pairing RNAs (sRNAs) and small proteins comprise two classes of regulators that allow bacterial cells to adapt to a wide variety of growth conditions. A limited number of transcripts encoding both of these activities, regulation of mRNA expression by base pairing and synthesis of a small regulatory protein, have been identified. Given that few have been characterized, little is known about the interplay between the two regulatory functions. To investigate the competition between the two activities, we constructed synthetic dual-function RNAs, hereafter referred to as MgtSR or MgtRS, comprised of the Escherichia coli sRNA MgrR and the open reading frame encoding the small protein MgtS. MgrR is a 98 nt base pairing sRNA that negatively regulates eptB encoding phosphoethanolamine transferase. MgtS is a 31 aa small inner membrane protein that is required for the accumulation of MgtA, a magnesium (Mg2+) importer. Expression of the separate genes encoding MgrR and MgtS is normally induced in response to low Mg2+ by the PhoQP two-component system. By generating various versions of this synthetic dual-function RNA, we probed how the organization of components and the distance between the coding and base pairing sequences contribute to the proper function of both activities of a dual-function RNA. By understanding the features of natural and synthetic dual-function RNAs, future synthetic molecules can be designed to maximize their regulatory impact. IMPORTANCE Dual-function RNAs in bacteria encode a small protein and also base pair with mRNAs to act as small, regulatory RNAs. Given that only a limited number of dual-function RNAs have been characterized, further study of these regulators is needed to increase understanding of their features. This study demonstrates that a functional synthetic dual-regulator can be constructed from separate components and used to study the functional organization of dual-function RNAs, with the goal of exploiting these regulators.

Genotoxic Effect of Salmonella Paratyphi A Infection on Human Primary Gallbladder Cells

Bacterial infections are increasingly being recognized as risk factors for the development of adenocarcinomas. The strong epidemiological evidence linking Helicobacter pylori infection to stomach cancer has paved the way to the demonstration that bacterial infections cause DNA damage in the host cells, initiating transformation. In this regard, the role of bacterial genotoxins has become more relevant. Salmonella enterica serovars Typhi and Paratyphi A have been clinically associated with gallbladder cancer. By harnessing the stem cell potential of cells from healthy human gallbladder explant, we regenerated and propagated the epithelium of this organ in vitro and used these cultures to model S. Paratyphi A infection. This study demonstrates the importance of the typhoid toxin, encoded only by these specific serovars, in causing genomic instability in healthy gallbladder cells, posing intoxicated cells at risk of malignant transformation.

Carcinoma of the gallbladder (GBC) is the most frequent tumor of the biliary tract. Despite epidemiological studies showing a correlation between chronic infection with Salmonella enterica Typhi/Paratyphi A and GBC, the underlying molecular mechanisms of this fatal connection are still uncertain. The murine serovar Salmonella Typhimurium has been shown to promote transformation of genetically predisposed cells by driving mitogenic signaling. However, insights from this strain remain limited as it lacks the typhoid toxin produced by the human serovars Typhi and Paratyphi A. In particular, the CdtB subunit of the typhoid toxin directly induces DNA breaks in host cells, likely promoting transformation. To assess the underlying principles of transformation, we used gallbladder organoids as an infection model for Salmonella Paratyphi A. In this model, bacteria can invade epithelial cells, and we observed host cell DNA damage. The induction of DNA double-strand breaks after infection depended on the typhoid toxin CdtB subunit and extended to neighboring, non-infected cells. By cultivating the organoid derived cells into polarized monolayers in air-liquid interphase, we could extend the duration of the infection, and we observed an initial arrest of the cell cycle that does not depend on the typhoid toxin. Non-infected intoxicated cells instead continued to proliferate despite the DNA damage. Our study highlights the importance of the typhoid toxin in causing genomic instability and corroborates the epidemiological link between Salmonella infection and GBC.

Diel magnesium fluctuations in chloroplasts contribute to photosynthesis in rice

Photosynthesis provides food, fibre and fuel that support our society; understanding the mechanisms controlling dynamic changes in this process helps identify new options to improve photosynthesis. Photosynthesis shows diel changes, which have been largely attributed to external light/dark conditions, as well as internal gene expression and the post-translational modification of critical enzymes. Here we report diel fluctuations of magnesium (Mg) in rice (Oryza sativa) chloroplasts, which may function as a rhythm regulator contributing to the post-translational regulation of photosynthetic CO₂ assimilation in rice. We found that a chloroplast-localized Mg²⁺ transporter gene, OsMGT3, which is rhythmically expressed in leaf mesophyll cells, partly modulates Mg fluctuations in rice chloroplasts. Knockout of OsMGT3 substantially reduced Mg²⁺ uptake, as well as the amplitude of free Mg²⁺ fluctuations in chloroplasts, which was closely associated with a decrease in ribulose 1,5-bisphosphate carboxylase activity in vivo and a consequent decline in the photosynthetic rate. In addition, the mesophyll-specific overexpression of OsMGT3 remarkably improved photosynthetic efficiency and growth performance in rice. Taken together, these observations demonstrate that OsMGT3-dependent diel Mg fluctuations in chloroplasts may contribute to Mg-dependent enzyme activities for photosynthesis over the daily cycle. Enhancing Mg²⁺ input to chloroplasts could be a potential approach to improving photosynthetic efficiency in plants.

Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import

Magnesium cation (Mg2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg2+ is the central ion in chlorophylls, and Mg2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a 'classical' bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the 'typical' bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

Bactericidal Disruption of Magnesium Metallostasis in Mycobacterium tuberculosis Is Counteracted by Mutations in the Metal Ion Transporter CorA

Antimycobacterial agents might shorten the course of treatment by reducing the number of phenotypically tolerant bacteria if they could kill M. tuberculosis in diverse metabolic states. Here we report two chemically disparate classes of agents that kill M. tuberculosis both when it is replicating and when it is not. Under replicating conditions, the tricyclic 4-hydroxyquinolines and a barbituric acid analogue deplete intrabacterial magnesium as a mechanism of action, and for both compounds, mutations in CorA, a putative Mg²⁺/Co²⁺ transporter, conferred resistance to the compounds when M. tuberculosis was under replicating conditions but not under nonreplicating conditions, illustrating that a given compound can kill M. tuberculosis in different metabolic states by disparate mechanisms. Targeting magnesium metallostasis represents a previously undescribed antimycobacterial mode of action that might cripple M. tuberculosis in a Mg²⁺-deficient intraphagosomal environment of macrophages.

A defining characteristic of treating tuberculosis is the need for prolonged administration of multiple drugs. This may be due in part to subpopulations of slowly replicating or nonreplicating Mycobacterium tuberculosis bacilli exhibiting phenotypic tolerance to most antibiotics in the standard treatment regimen. Confounding this problem is the increasing incidence of heritable multidrug-resistant M. tuberculosis. A search for new antimycobacterial chemical scaffolds that can kill phenotypically drug-tolerant mycobacteria uncovered tricyclic 4-hydroxyquinolines and a barbituric acid derivative with mycobactericidal activity against both replicating and nonreplicating M. tuberculosis. Both families of compounds depleted M. tuberculosis of intrabacterial magnesium. Complete or partial resistance to both chemotypes arose from mutations in the putative mycobacterial Mg²⁺/Co²⁺ ion channel, CorA. Excess extracellular Mg²⁺, but not other divalent cations, diminished the compounds’ cidality against replicating M. tuberculosis. These findings establish depletion of intrabacterial magnesium as an antimicrobial mechanism of action and show that M. tuberculosis magnesium homeostasis is vulnerable to disruption by structurally diverse, nonchelating, drug-like compounds.

Loss of cytosolic Mg2+ binding sites in the Thermotoga maritima CorA Mg2+ channel is not sufficient for channel opening

The CorA Mg²⁺ channel is a homopentamer with five-fold symmetry. Each monomer consists of a large cytoplasmic domain and two transmembrane helices connected via a short periplasmic loop. In the Thermotoga maritima CorA crystal structure, a Mg²⁺ is bound between D89 of one monomer and D253 of the adjacent monomer (M1 binding site). Release of Mg²⁺ from these sites has been hypothesized to cause opening of the channel. We generated mutants to disrupt Mg²⁺ interaction with the M1 site. Crystal structures of the D89K/D253K and D89R/D253R mutants, determined to 3.05 and 3.3 Å, respectively, showed no significant structural differences with the wild type structure despite absence of Mg²⁺ at the M1 sites. Both mutants still appear to be in the closed state. All three mutant CorA proteins exhibited transport of ⁶³Ni²⁺, indicating functionality. Thus, absence of Mg²⁺ from the M1 sites neither causes channel opening nor prevents function. We also provide evidence that the T. maritima CorA is a Mg²⁺ channel and not a Co²⁺ channel.

Molecular and functional characterization of the magnesium transporter gene ZmMGT12 in maize

Magnesium (Mg) is an essential mineral element for normal plant growth and development, and the CorA/MRS2/MGT-type Mg transporters play a significant role in maintaining Mg homeostasis in plants. In total, 12 maize CorA-like Mg²⁺ transporters have been identified, but none of them had been functionally characterized. Accordingly, we cloned and functionally characterized ZmMGT12 from the maize CorA-like gene family. ZmMGT12 exhibited the structure typical of Mg²⁺ transporters, i.e., two conserved TM domains and a GMN tripeptide motif. ZmMGT12, Arabidopsis AtMGT6, and rice OsMRS2-6 shared high protein sequence identity and thus clustered in the same phylogenetic branch, suggesting that they could be homologs. A functional complementation assay in the Salmonella typhimurium MM281 mutant indicated that ZmMGT12 possessed Mg²⁺ transport ability. ZmMGT12 was expressed in roots, stems, and leaves, with the highest expression in leaves. Moreover, ZmMGT12 expression was induced by light and exhibited a circadian expression pattern. In addition, the expression level of ZmMGT12 in leaf tissue was related to chlorophyll synthesis. Overexpression of ZmMGT12 in Arabidopsis caused no phenotypic change in transgenic plants, including in fresh shoot weight, chlorophyll content, shoot Mg²⁺ content, and chloroplast Mg²⁺ content. Together, these results suggest that ZmMGT12 is a Mg²⁺ transporter and may play a role in Mg transport into chloroplasts.

Mg Isotope Fractionation during Uptake by a Rock-Inhabiting, Model Microcolonial Fungus Knufia petricola at Acidic and Neutral pH

The model rock-inhabiting microcolonial fungus Knufia petricola fractionates stable Mg isotopes in a time- and pH-dependent manner. During growth, the increase of ²⁶Mg/²⁴Mg in the fungal cells relative to the growth media amounted to 0.65 ± 0.14‰ at pH 6 and 1.11 ± 0.35‰ at pH 3. We suggest a constant equilibrium fractionation factor during incorporation of Mg into ribosomes and ATP as a cause of enrichment of ²⁶Mg in the cells. We suggest too that the proton gradient across the cell wall and cytoplasmic membrane controls Mg²⁺ transport into the fungal cell. As the strength of this gradient is a function of extracellular solution pH, the pH-dependence on Mg isotope fractionation is thus due to differences in fungal cell mass fluxes. Through a mass balance model we show that Mg uptake into the fungal cell is not associated with a unique Mg isotope fractionation factor. This Mg isotope fractionation dependence on pH might also be observed in any organism with cells that follow similar Mg uptake and metabolic pathways and serves to reveal Mg cycling in ecosystems.

Structure and Cooperativity of the Cytosolic Domain of the CorA Mg2+ Channel from Escherichia coli

Structures of the Mg²⁺ bound (closed) and apo (open) states of CorA suggests that channel gating is accomplished by rigid-body motions between symmetric and asymmetric assemblies of the cytosolic portions of the five subunits in response to ligand (Mg²⁺) binding/unbinding at interfacial sites. Here, we structurally and biochemically characterize the isolated cytosolic domain from Escherichia coli CorA. The data reveal an Mg²⁺-ligand binding site located in a novel position between each of the five subunits and two Mg²⁺ ions trapped inside the pore. Soaking experiments show that cobalt hexammine outcompetes Mg²⁺ at the pore site closest to the membrane. This represents the first structural information of how an analog of hexa-hydrated Mg²⁺ (and competitive inhibitor of CorA) associates to the CorA pore. Biochemical data on the isolated cytoplasmic domain and full-length protein suggests that gating of the CorA channel is governed cooperatively.

Internal versus External Dose for Describing Ternary Metal Mixture (Ni, Cu, Cd) Chronic Toxicity to Lemna minor

Simultaneous determinations of internal dose ([M]_tiss) and external doses ([M]_tot, {M²⁺} in solution) were conducted to study ternary mixture (Ni, Cu, Cd) chronic toxicity to Lemna minor in alkaline solution (pH 8.3). Also, concentration addition (CA) based on internal dose was evaluated as a tool for risk assessment of metal mixture. Multiple regression analysis of dose versus root growth inhibition, as well as saturation binding kinetics, provided insight into interactions. Multiple regressions were simpler for [M]_tiss than [M]_tot and {M²⁺}, and along with saturation kinetics to the internal biotic ligand(s) in the cytoplasm, they indicated that Ni-Cu-Cd competed for uptake into plant, but once inside, only Cu-Cd shared a binding site. Copper inorganic complexes (hydroxides, carbonates) played a role in metal bioavailability in single metal exposure but not in mixtures. Regardless of interactions, the current regulatory approach of using CA based on [M]_tot can sufficiently predict mixture toxicity (∑TU close to 1), but CA based on [M]_tiss was closest to unity across a range of doses. Internal dose integrates all metal-metal interactions in solution and during uptake into the organism, thereby providing a more direct metric describing toxicity.

Insertion of Group II Intron-Based Ribozyme Switches into Homing Endonuclease Genes

Fungal mitochondrial genomes act as "reservoirs" for homing endonucleases. These enzymes with their DNA site-specific cleavage activities are attractive tools for genome editing, targeted mutagenesis and gene therapy applications. Herein, we present strategies where homing endonuclease open reading frames (HEases ORFs) are interrupted with group II intron sequences. The ultimate goal is to achieve in vivo expression of HEases that can be regulated by manipulating the splicing efficiency of the HEase ORF-embedded group II introns. That addition of exogenous magnesium chloride (MgCl₂) appears to stimulate splicing of nonnative group II introns in Escherichia coli and the addition of cobalt chloride (CoCl₂) to the growth medium antagonizes the expression of HEase activity (i.e., splicing). Group II introns are potentially autocatalytic self-splicing elements and thus can be used as molecular switches that allow for temporal regulated HEase expression. This should be useful in precision genome engineering, mutagenesis, and minimizing off-target activities.

Outer Membrane Vesicle Production Facilitates LPS Remodeling and Outer Membrane Maintenance in Salmonella during Environmental Transitions

The ability of Gram-negative bacteria to carefully modulate outer membrane (OM) composition is essential to their survival. However, the asymmetric and heterogeneous structure of the Gram-negative OM poses unique challenges to the cell’s successful adaption to rapid environmental transitions. Although mechanisms to recycle and degrade OM phospholipid material exist, there is no known mechanism by which to remove unfavorable lipopolysaccharide (LPS) glycoforms, except slow dilution through cell growth. As all Gram-negative bacteria constitutively shed OM vesicles (OMVs), we propose that cells may utilize OMV formation as a way to selectively remove environmentally disadvantageous LPS species. We examined the native kinetics of OM composition during physiologically relevant environmental changes in Salmonella enterica, a well-characterized model system for activation of PhoP/Q and PmrA/B two-component systems (TCSs). In response to acidic pH, toxic metals, antimicrobial peptides, and lack of divalent cations, these TCSs modify the LPS lipid A and core, lengthen the O antigen, and upregulate specific OM proteins. An environmental change to PhoP/Q- and PmrA/B-activating conditions simultaneously induced the addition of modified species of LPS to the OM, downregulation of previously dominant species of LPS, greater OMV production, and increased OMV diameter. Comparison of the relative abundance of lipid A species present in the OM and the newly budded OMVs following two sets of rapid environmental shifts revealed the retention of lipid A species with modified phosphate moieties in the OM concomitant with the selective loss of palmitoylated species via vesiculation following exposure to moderately acidic environmental conditions.

All Gram-negative bacteria alter the structural composition of LPS present in their OM in response to various environmental stimuli. We developed a system to track the native dynamics of lipid A change in Salmonella enterica serovar Typhimurium following an environmental shift to PhoP/Q- and PmrA/B-inducing conditions. We show that growth conditions influence OMV production, size, and lipid A content. We further demonstrate that the lipid A content of OMVs does not fit a stochastic model of content selection, revealing the significant retention of lipid A species containing covalent modifications that mask their 1- and 4′-phosphate moieties under host-like conditions. Furthermore, palmitoylation of the lipid A to form hepta-acylated species substantially increases the likelihood of its incorporation into OMVs. These results highlight a role for the OMV response in OM remodeling and maintenance processes in Gram-negative bacteria.

Characterization of SlyA in Shigella flexneri Identifies a Novel Role in Virulence

The SlyA transcriptional regulator has important roles in the virulence and pathogenesis of several members of the Enterobacteriaceae family, including Salmonella enterica serovar Typhimurium and Escherichia coli. Despite the identification of the slyA gene in Shigella flexneri nearly 2 decades ago, as well as the significant conservation of SlyA among enteric bacteria, the role of SlyA in Shigella remains unknown. The genes regulated by SlyA in closely related organisms often are absent from or mutated inS. flexneri, and consequently many described SlyA-dependent phenotypes are not present. By characterizing the expression of slyA and determining its ultimate effect in this highly virulent organism, we postulated that novel SlyA-regulated virulence phenotypes would be identified. In this study, we report the first analysis of SlyA in Shigella and show that (i) the slyA gene is transcribed and ultimately translated into protein, (ii) slyA promoter activity is maximal during stationary phase and is negatively autoregulated and positively regulated by the PhoP response regulator, (iii) the exogenous expression of slyA rescues transcription and virulence-associated deficiencies during virulence-repressed conditions, and (iv) the absence of slyA significantly decreases acid resistance, demonstrating a novel and important role in Shigella virulence. Cumulatively, our study illustrates unexpected parallels between the less conserved S. flexneri and S Typhimurium slyA promoters as well as a unique role for SlyA in Shigella virulence that has not been described previously in any closely related organism.

Using Group II Introns for Attenuating the In Vitro and In Vivo Expression of a Homing Endonuclease

In Chaetomium thermophilum (DSM 1495) within the mitochondrial DNA (mtDNA) small ribosomal subunit (rns) gene a group IIA1 intron interrupts an open reading frame (ORF) encoded within a group I intron (mS1247). This arrangement offers the opportunity to examine if the nested group II intron could be utilized as a regulatory element for the expression of the homing endonuclease (HEase). Constructs were generated where the codon-optimized ORF was interrupted with either the native group IIA1 intron or a group IIB type intron. This study showed that the expression of the HEase (in vivo) in Escherichia coli can be regulated by manipulating the splicing efficiency of the HEase ORF-embedded group II introns. Exogenous magnesium chloride (MgCl₂) stimulated the expression of a functional HEase but the addition of cobalt chloride (CoCl₂) to growth media antagonized the expression of HEase activity. Ultimately the ability to attenuate HEase activity might be useful in precision genome engineering, minimizing off target activities, or where pathways have to be altered during a specific growth phase.

Magnesium Transport and Magnesium Homeostasis

This review reviews the properties and regulation of the Salmonella enterica serovar Typhimurium and Escherichia coli transporters that mediate Mg2+ influx: CorA and the Mgt P-type ATPases. In addition, potential Mg2+ regulation of transcription and translation, largely via the PhoPQ two component system, is discussed. CorA proteins are a unique class of transporters and are widespread in the Bacteria and Archaea, with rather distant but functional homologs in eukaryotes. The Mgt transporters are highly homologous to other P-type ATPases but are more closely related to the eukaryotic H+ and Ca2+ ATPases than to most prokaryotic ATPases. Hundreds of homologs of CorA are currently known from genomic sequencing. In contrast, only when extracellular and possibly intracellular Mg2+ levels fall significantly is the expression of mgtA and mgtB induced. Topology studies using blaM and lacZ fusions initially indicated that the Salmonella serovar Typhimurium CorA contained three transmembrane (TM) segments; however, subsequent data obtained using a variety of approaches showed that the CorA superfamily of proteins have only two TMs at the extreme C terminus. PhoP-PhoQ is a two-component system consisting of PhoQ, the sensor/receptor histidine kinase, and PhoP, the response regulator/transcriptional activator. The expression of both mgtA and mgtCB in either E. coli or Salmonella serovar Typhimurium is markedly induced in a PhoPQ-dependent manner by low concentrations of Mg2+ in the medium. phoP and phoQ form an operon with two promoters in both E. coli and Salmonella serovar Typhimurium.

Host-dependent Induction of Transient Antibiotic Resistance: A Prelude to Treatment Failure

Current antibiotic testing does not include the potential influence of host cell environment on microbial susceptibility and antibiotic resistance, hindering appropriate therapeutic intervention. We devised a strategy to identify the presence of host–pathogen interactions that alter antibiotic efficacy in vivo. Our findings revealed a bacterial mechanism that promotes antibiotic resistance in vivo at concentrations of drug that far exceed dosages determined by standardized antimicrobial testing. This mechanism has escaped prior detection because it is reversible and operates within a subset of host tissues and cells. Bacterial pathogens are thereby protected while their survival promotes the emergence of permanent drug resistance. This host-dependent mechanism of transient antibiotic resistance is applicable to multiple pathogens and has implications for the development of more effective antimicrobial therapies.

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Standard MIC testing does not consider the influence of the host milieu, potentially hindering therapeutic intervention.

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Salmonella induce polymyxin resistance during infection at levels of drug that far exceed dosages determined by MIC testing.

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Polymyxin treatment failed to control Salmonella infection and promotes the emergence of drug-resistant mutants.

Physicians rely on laboratory antimicrobial susceptibility testing of clinical isolates to identify a suitable antibiotic for therapy. Although the recommended antibiotics clear most bacterial infections, some patients fail to respond and require prolonged therapy, higher dosing or different antibiotics. Why does this occur and what are the possible implications? By studying antibiotic resistance in the context of infection, we identified a host-dependent mechanism that promotes antibiotic resistance at concentrations of drug that far exceed dosages determined by standardized antimicrobial testing. These findings question current antibiotic testing methods that have guided physician treatment practices and drug development for the last several decades.

Ionic switch controls the DNA state in phage λ

We have recently found that DNA packaged in phage λ undergoes a disordering transition triggered by temperature, which results in increased genome mobility. This solid-to-fluid like DNA transition markedly increases the number of infectious λ particles facilitating infection. However, the structural transition strongly depends on temperature and ionic conditions in the surrounding medium. Using titration microcalorimetry combined with solution X-ray scattering, we mapped both energetic and structural changes associated with transition of the encapsidated λ-DNA. Packaged DNA needs to reach a critical stress level in order for transition to occur. We varied the stress on DNA in the capsid by changing the temperature, packaged DNA length and ionic conditions. We found striking evidence that the intracapsid DNA transition is 'switched on' at the ionic conditions mimicking those in vivo and also at the physiologic temperature of infection at 37°C. This ion regulated on-off switch of packaged DNA mobility in turn affects viral replication. These results suggest a remarkable adaptation of phage λ to the environment of its host bacteria in the human gut. The metastable DNA state in the capsid provides a new paradigm for the physical evolution of viruses.

A FRET-based DNA biosensor tracks OmpR-dependent acidification of Salmonella during macrophage infection

In bacteria, one paradigm for signal transduction is the two-component regulatory system, consisting of a sensor kinase (usually a membrane protein) and a response regulator (usually a DNA binding protein). The EnvZ/OmpR two-component system responds to osmotic stress and regulates expression of outer membrane proteins. In Salmonella, EnvZ/OmpR also controls expression of another two-component system SsrA/B, which is located on Salmonella Pathogenicity Island (SPI) 2. SPI-2 encodes a type III secretion system, which functions as a nanomachine to inject bacterial effector proteins into eukaryotic cells. During the intracellular phase of infection, Salmonella switches from assembling type III secretion system structural components to secreting effectors into the macrophage cytoplasm, enabling Salmonella to replicate in the phagocytic vacuole. Major questions remain regarding how bacteria survive the acidified vacuole and how acidification affects bacterial secretion. We previously reported that EnvZ sensed cytoplasmic signals rather than extracellular ones, as intracellular osmolytes altered the dynamics of a 17-amino-acid region flanking the phosphorylated histidine. We reasoned that the Salmonella cytoplasm might acidify in the macrophage vacuole to activate OmpR-dependent transcription of SPI-2 genes. To address these questions, we employed a DNA-based FRET biosensor ("I-switch") to measure bacterial cytoplasmic pH and immunofluorescence to monitor effector secretion during infection. Surprisingly, we observed a rapid drop in bacterial cytoplasmic pH upon phagocytosis that was not predicted by current models. Cytoplasmic acidification was completely dependent on the OmpR response regulator, but did not require known OmpR-regulated genes such as ompC, ompF, or ssaC (SPI-2). Microarray analysis highlighted the cadC/BA operon, and additional experiments confirmed that it was repressed by OmpR. Acidification was blocked in the ompR null background in a Cad-dependent manner. Acid-dependent activation of OmpR stimulated type III secretion; blocking acidification resulted in a neutralized cytoplasm that was defective for SPI-2 secretion. Based upon these findings, we propose that Salmonella infection involves an acid-dependent secretion process in which the translocon SseB moves away from the bacterial cell surface as it associates with the vacuolar membrane, driving the secretion of SPI-2 effectors such as SseJ. New steps in the SPI-2 secretion process are proposed.

Inhibition of outer membrane proteases of the omptin family by aprotinin

Bacterial proteases are important virulence factors that inactivate host defense proteins and contribute to tissue destruction and bacterial dissemination. Outer membrane proteases of the omptin family, exemplified by Escherichia coli OmpT, are found in some Gram-negative bacteria. Omptins cleave a variety of substrates at the host-pathogen interface, including plasminogen and antimicrobial peptides. Multiple omptin substrates relevant to infection have been identified; nonetheless, an effective omptin inhibitor remains to be found. Here, we purified native CroP, the OmpT ortholog in the murine pathogen Citrobacter rodentium. Purified CroP was found to readily cleave both a synthetic fluorescence resonance energy transfer substrate and the murine cathelicidin-related antimicrobial peptide. In contrast, CroP was found to poorly activate plasminogen into active plasmin. Although classical protease inhibitors were ineffective against CroP activity, we found that the serine protease inhibitor aprotinin displays inhibitory potency in the micromolar range. Aprotinin was shown to act as a competitive inhibitor of CroP activity and to interfere with the cleavage of the murine cathelicidin-related antimicrobial peptide. Importantly, aprotinin was able to inhibit not only CroP but also Yersinia pestis Pla and, to a lesser extent, E. coli OmpT. We propose a structural model of the aprotinin-omptin complex in which Lys15 of aprotinin forms salt bridges with conserved negatively charged residues of the omptin active site.

Identification and regulation of a novel Citrobacter rodentium gut colonization fimbria (Gcf)

The Gram-negative enteric bacterium Citrobacter rodentium is a natural mouse pathogen that has been extensively used as a surrogate model for studying the human pathogens enteropathogenic and enterohemorrhagic Escherichia coli. All three pathogens produce similar attaching and effacing (A/E) lesions in the intestinal epithelium. During infection, these bacteria employ surface structures called fimbriae to adhere and colonize the host intestinal epithelium. For C. rodentium, the roles of only a small number of its genome-carried fimbrial operons have been evaluated. Here, we report the identification of a novel C. rodentium colonization factor, called gut colonization fimbria (Gcf), which is encoded by a chaperone-usher fimbrial operon. A gcfA mutant shows a severe colonization defect within the first 10 days of infection. The gcf promoter is not active in C. rodentium under several in vitro growth conditions; however, it is readily expressed in a C. rodentium Δhns1 mutant lacking the closest ortholog of the Escherichia coli histone-like nucleoid structuring protein (H-NS) but not in mutants with deletion of the other four genes encoding H-NS homologs. H-NS binds to the regulatory region of gcf, further supporting its direct role as a repressor of the gcf promoter that starts transcription 158 bp upstream of the start codon of its first open reading frame. The gcf operon possesses interesting novel traits that open future opportunities to expand our knowledge of the structure, regulation, and function during infection of these important bacterial structures.

IMPORTANCE

Fimbriae are surface bacterial structures implicated in a variety of biological processes. Some have been shown to play a critical role during host colonization and thus in disease. Pathogenic bacteria possess the genetic information for an assortment of fimbriae, but their function and regulation and the interplay between them have not been studied in detail. This work provides new insights into the function and regulation of a novel fimbria called Gcf that is important for early establishment of a successful infection by C. rodentium in mice, despite being poorly expressed under in vitro growth conditions. This discovery offers an opportunity to better understand the individual role and the regulatory mechanisms controlling the expression of specific fimbrial operons that are critical during infection.

An endoplasmic reticulum magnesium transporter is essential for pollen development in Arabidopsis

Magnesium is one of the essential macro-elements for plant growth and development, participated in photosynthesis and various metabolic processes. The Mg-transport abilities of the AtMGT (Magnesium Transporter) genes were identified in bacteria or yeast mutant system. In our previous studies, both the AtMGT5 and AtMGT9 were found essential for pollen development in Arabidopsis. Here we report another AtMGT member, AtMGT4, which was localized to the endoplasmic reticulum, was essential for pollen development as well. AtMGT4 expressed notably in pollen grains from bicellular pollen stage to mature pollen stage. A T-DNA insertional mutant of the gene, named mgt4-1, showed pollen abortive phenotype, thus we could not get any homozygous mutant from progenies of self-crossed +/mgt4-1 plants. Meanwhile, nearly half of pollens in AtMGT4-RNAi transgenic lines were sterile, consistent with the phenotype of +/mgt4-1 mutant. Transgenic plants expressing AtMGT4 in the mgt4-1 background could recover the pollen fertility to the wild type. Together, our findings demonstrated that the disruption of AtMGT4 in Arabidopsis could cause a defect of pollen development. The visible pollen abortion appeared at bicellular pollen stage in +/mgt4-1.

Two-component regulators control hilA expression by controlling fimZ and hilE expression within Salmonella enterica serovar Typhimurium

Salmonellae initiate disease through the invasion of host cells within the intestine. This ability to invade requires the coordinated action of numerous genes, many of which are found within Salmonella pathogenicity island 1 (SPI-1). The key to this process is the ability of the bacteria to respond to the environment, thereby upregulating the necessary genes under optimal conditions. Central to the control of SPI-1 is the transcriptional activator hilA. Work has identified at least 10 different activators and 8 different repressors responsible for the control of hilA. We have previously shown that hilE is a Salmonella-specific negative regulator that is able to repress hilA expression and invasion. Additionally, fimZ, a transcriptional activator responsible for the expression of type I fimbriae as well as flagellar genes, has also been implicated in this process. fimZ is homologous to response regulators from other two-component regulatory systems, although a sensor for the system has not been identified. The phoPQ and phoBR regulons are both two-component systems that negatively affect hilA expression, although the mechanism of action has not been determined. Our results show that PhoBR is capable of inducing fimZ expression, whereas PhoPQ does not affect fimZ expression but does upregulate hilE in an FimZ-dependent manner. Therefore, phosphate (sensed by PhoBR) and magnesium (sensed by PhoPQ) levels are important in controlling hilA expression levels when Salmonella is in the intestinal environment.

Molecular mechanism of Mg2+-dependent gating in CorA

CorA is the major transport system responsible for Mg²⁺ uptake in bacteria and can functionally substitute for its homologue Mrs2p in the yeast inner mitochondrial membrane. Although several CorA crystal structures are available, the molecular mechanism of Mg²⁺ uptake remains to be established. Here we use EPR spectroscopy, electrophysiology and molecular dynamic simulations to show that CorA is regulated by cytoplasmic Mg²⁺ acting as a ligand and elucidate the basic conformational rearrangements responsible for Mg²⁺-dependent gating. Mg²⁺ unbinding at the divalent cation sensor triggers a conformational change that leads to the inward motion of the stalk helix, which propagates to the pore forming transmembrane helix TM1. Helical tilting and rotation in TM1 generates an iris-like motion that increases the diameter of the permeation pathway, triggering ion conduction. This work establishes the molecular basis of a Mg²⁺-driven negative feedback loop in CorA as the key physiological event controlling Mg²⁺ uptake and homeostasis in prokaryotes.

Genetic analysis of riboswitch-mediated transcriptional regulation responding to Mn2+ in Salmonella

Riboswitches are a class of cis-acting regulatory RNAs normally characterized from the 5'-UTR of bacterial transcripts that bind a specific ligand to regulate expression of associated genes by forming alternative conformations. Here, we present a riboswitch that contributes to transcriptional regulation through sensing Mn(2+) in Salmonella typhimurium. We characterized a 5'-UTR (UTR1) from the mntH locus encoding a Mn(2+) transporter, which forms a Rho-independent terminator to implement transcription termination with a high Mn(2+) selectivity both in vivo and in vitro. Nucleotide substitutions that cause disruption of the terminator interfere with the regulatory function of UTR1. RNA probing analyses outlined a specific UTR1 conformation that favors the terminator structure in Mn(2+)-replete condition. Switch sequence GCUAUG can alternatively base pair duplicated hexanucleotide CAUAGC to form either a pseudoknot or terminator stem. Mn(2+), but not Mg(2+), and Ca(2+), can enhance cleavage at specific nucleotides in UTR1. We conclude that UTR1 is a riboswitch that senses cytoplasmic Mn(2+) and therefore participates in Mn(2+)-responsive mntH regulation in Salmonella. This riboswitch domain is also conserved in several Gram-negative enteric bacteria, indicating that this Mn(2+)-responsive mechanism could have broader implications in bacterial gene expression. Additionally, a high level of cytoplasmic Mn(2+) can down-regulate transcription of the Salmonella Mg(2+) transporter mgtA locus in a Mg(2+) riboswitch-dependent manner. On the other hand, these two types of cation riboswitches do not share similarity at the primary or secondary structural levels. Taken together, characterization of Mn(2+)-responsive riboswitches should expand the scope of RNA regulatory elements in response to inorganic ions.

The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.

Identification of protoxins and a microbial basis for red maple (Acer rubrum) toxicosis in equines

The leaves of Acer rubrum (red maple), especially when wilted in the fall, cause severe oxidative damage to equine erythrocytes, leading to potentially fatal methemoglobinemia and hemolytic anemia. Gallic acid and tannins from A. rubrum leaves have been implicated as the toxic compounds responsible for red maple toxicosis, but the mechanism of action and toxic principle(s) have not been elucidated to date. In order to investigate further how red maple toxicosis occurs, aqueous solutions of gallic acid, tannic acid, and ground dried A. rubrum leaves were incubated with contents of equine ileum, jejunum, cecum, colon, and liver, and then analyzed for the metabolite pyrogallol, as pyrogallol is a more potent oxidizing agent. Gallic acid was observed to be metabolized to pyrogallol maximally in equine ileum contents in the first 24 hr. Incubation of tannic acid and A. rubrum leaves, individually with ileum contents, produced gallic acid and, subsequently, pyrogallol. Ileum suspensions, when passed through a filter to exclude microbes but not enzymes, formed no pyrogallol, suggesting a microbial basis to the pathway. Bacteria isolated from ileum capable of pyrogallol formation were identified as Klebsiella pneumoniae and Enterobacter cloacae. Therefore, gallotannins and free gallic acid are present in A. rubrum leaves and can be metabolized by K. pneumoniae and E. cloacae found in the equine ileum to form pyrogallol either directly or through a gallic acid intermediate (gallotannins). Identification of these compounds and their physiological effects is necessary for the development of effective treatments for red maple toxicosis in equines.

The PhoP/PhoQ system and its role in Serratia marcescens pathogenesis

Serratia marcescens is able to invade, persist, and multiply inside nonphagocytic cells, residing in nonacidic, nondegradative, autophagosome-like vacuoles. In this work, we have examined the physiological role of the PhoP/PhoQ system and its function in the control of critical virulence phenotypes in S. marcescens. We have demonstrated the involvement of the PhoP/PhoQ system in the adaptation of this bacterium to growth on scarce environmental Mg(2+), at acidic pH, and in the presence of polymyxin B. We have also shown that these environmental conditions constitute signals that activate the PhoP/PhoQ system. We have found that the two S. marcescens mgtE orthologs present a conserved PhoP-binding motif and demonstrated that mgtE1 expression is PhoP dependent, reinforcing the importance of PhoP control in magnesium homeostasis. Finally, we have demonstrated that phoP expression is activated intracellularly and that a phoP mutant strain is defective in survival inside epithelial cells. We have shown that the Serratia PhoP/PhoQ system is involved in prevention of the delivery to degradative/acidic compartments.

CorA, the magnesium/nickel/cobalt transporter, affects virulence and extracellular enzyme production in the soft rot pathogen Pectobacterium carotovorum

Pectobacterium carotovorum (formerly Erwinia carotovora ssp. carotovora) is a phytopathogenic bacterium that causes soft rot disease, characterized by water-soaked soft decay, resulting from the action of cell wall-degrading exoenzymes secreted by the pathogen. Virulence in soft rot bacteria is regulated by environmental factors, host and bacterial chemical signals, and a network of global and gene-specific bacterial regulators. We isolated a mini-Tn5 mutant of P. carotovorum that is reduced in the production of extracellular pectate lyase, protease, polygalacturonase and cellulase. The mutant is also decreased in virulence as it macerates less host tissues than its parent and is severely impaired in multiplication in planta. The inactivated gene responsible for the reduced virulent phenotype was identified as corA. CorA, a magnesium/nickel/cobalt membrane transporter, is the primary magnesium transporter for many bacteria. Compared with the parent, the CorA(-) mutant is cobalt resistant. The mutant phenotype was confirmed in parental strain P. carotovorum by marker exchange inactivation of corA. A functional corA(+) DNA from P. carotovorum restored exoenzyme production and pathogenicity to the mutants. The P. carotovorum corA(+) clone also restored motility and cobalt sensitivity to a CorA(-) mutant of Salmonella enterica. These data indicate that CorA is required for exoenzyme production and virulence in P. carotovorum.

OmpT outer membrane proteases of enterohemorrhagic and enteropathogenic Escherichia coli contribute differently to the degradation of human LL-37

Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) are food-borne pathogens that cause serious diarrheal diseases. To colonize the human intestine, these pathogens must overcome innate immune defenses such as antimicrobial peptides (AMPs). Bacterial pathogens have evolved various mechanisms to resist killing by AMPs, including proteolytic degradation of AMPs. To examine the ability of the EHEC and EPEC OmpT outer membrane (OM) proteases to degrade α-helical AMPs, ompT deletion mutants were generated. Determination of MICs of various AMPs revealed that both mutant strains are more susceptible than their wild-type counterparts to α-helical AMPs, although to different extents. Time course assays monitoring the degradation of LL-37 and C18G showed that EHEC cells degraded both AMPs faster than EPEC cells in an OmpT-dependent manner. Mass spectrometry analyses of proteolytic fragments showed that EHEC OmpT cleaves LL-37 at dibasic sites. The superior protection provided by EHEC OmpT compared to EPEC OmpT against α-helical AMPs was due to higher expression of the ompT gene and, in turn, higher levels of the OmpT protein in EHEC. Fusion of the EPEC ompT promoter to the EHEC ompT open reading frame resulted in decreased OmpT expression, indicating that transcriptional regulation of ompT is different in EHEC and EPEC. We hypothesize that the different contributions of EHEC and EPEC OmpT to the degradation and inactivation of LL-37 may be due to their adaptation to their respective niches within the host, the colon and small intestine, respectively, where the environmental cues and abundance of AMPs are different.

A mutation in the poxA gene of Salmonella enterica serovar Typhimurium alters protein production, elevates susceptibility to environmental challenges, and decreases swine colonization

Control of foodborne Salmonella within the farm-retail continuum is a complex issue since over 2500 serovars of Salmonella exist, the host range of Salmonella spp. varies greatly, and Salmonella is environmentally ubiquitous. To identify Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) genes important for pathogen survival, our research group previously screened a signature-tagged mutagenesis bank in an ex vivo swine stomach content assay. A mutation in the poxA gene, a member of the gene family encoding class-II aminoacyl-tRNA synthetases, decreased survival of Salmonella Typhimurium in the ex vivo swine stomach content assay. In the current study, complementation with a plasmid-encoded poxA gene restored survival of the poxA mutant to the level of the parental, wild-type strain. In vivo analysis of the poxA mutant in the natural porcine host revealed significantly reduced fecal shedding of Salmonella, decreased colonization of the tonsils, and decreased detection of the mutant strain in the cecal contents of the pigs at 7 days postinoculation (p < 0.05). Body temperature (fever) of the pigs inoculated with wild-type Salmonella Typhimurium was significantly higher than that of pigs inoculated with the poxA mutant (p < 0.05). Two-dimensional gel electrophoresis revealed characteristic differences in the protein profile of the poxA mutant relative to the wild-type strain, indicating that deletion of poxA in Salmonella Typhimurium exerts selective effects on translation and/or posttranslational modifications of mRNA species that are necessary for stress survival and colonization of the natural swine host.

Temperature and Mg2+ sensing by a novel PhoP-PhoQ two-component system for regulation of virulence in Edwardsiella tarda

The PhoP-PhoQ two-component system is commonly used by bacteria to sense environmental factors. Here we show that the PhoP-PhoQ system of Edwardsiella tarda detects changes in environmental temperature and Mg(2+) concentration as well as regulates the type III and VI secretion systems through direct activation of esrB. Protein secretion is activated from 23 to 35 °C or at low Mg(2+) concentrations, but it is suppressed at or below 20 °C, at or above 37 °C, or at high Mg(2+) concentrations. The effects of temperature and Mg(2+) concentration are additive. The PhoQ sensor domain has a low T(m) of 37.9 °C, and it detects temperatures through a conformational change of its secondary structure. Mutation of specific Pro or Thr residues increased the stability of the PhoQ sensor drastically, altering its temperature-sensing ability. The PhoQ sensor detects Mg(2+) concentration through the direct binding of Mg(2+) to a cluster of acidic residues (DDDSAD) and through changes that likely affect its tertiary structure. Here, we describe for the first time the use of PhoP-PhoQ as a temperature sensor for bacterial virulence control.

Cation selectivity by the CorA Mg2+ channel requires a fully hydrated cation

The CorA Mg(2+) channel is the primary uptake system in about half of all bacteria and archaea. However, the basis for its Mg(2+) selectivity is unknown. Previous data suggested that CorA binds a fully hydrated Mg(2+) ion, unlike other ion channels. The crystal structure of Thermotoga maritima CorA shows a homopentamer with two transmembrane segments per monomer connected by a short periplasmic loop. This highly conserved loop, (281)EFMPELKWS(289) in Salmonella enterica serovar Typhimurium CorA, is the only portion of the channel outside of the cell, suggesting a role in cation selectivity. Mutation of charged residues in the loop, E281 and K287, to any of several amino acids had little effect, demonstrating that despite conservation electrostatic interactions with these residues are not essential. While mutation of the universally conserved E285 gave a minimally functional channel, E285A and E285K mutants were the most functional, again indicating that the negative charge at this position is not a determining factor. Several mutations at K287 and W288 behaved anomalously in a transport assay. Analysis indicated that mutation of K287 and W288 disrupts cooperative interactions between distinct Mg(2+) binding sites. Overall, these results are not compatible with electrostatic interaction of the Mg(2+) ion with the periplasmic loop. Instead, the loop appears to form an initial binding site for hydrated Mg(2+), not for the dehydrated cation. The loop residues may function to accelerate dehydration of the before entry of Mg(2+) into the pore of the channel.

Phage display as a novel screening tool for primary toxicological targets

In the present study the use of phage display as a screening tool to determine primary toxicological targets was investigated. These primary toxicological targets are the targets in the cell with which a chemical compound initially interacts and that are responsible for consecutive (toxic) effects. Nickel was used as model compound for the present study. By selection of Ni-binding peptides out of a 12-mer peptide phage library, it was possible to identify primary toxicological targets of Ni (and other metals). The selected Ni-binding peptides showed similarities to important primary toxicological targets of Ni, such as the hydrogenase nickel incorporation protein (hypB) and the Mg/Ni/Co transporter (corA). This shows that phage display, which is already widely used in other research fields, also has potential in ecotoxicology, as a novel screening tool with which to determine primary toxicological targets of chemical compounds.

Magnesium transporter AtMGT9 is essential for pollen development in Arabidopsis

Magnesium (Mg(2+)) is abundant in plant cells and plays a critical role in many physiological processes. A 10-member gene family AtMGT (also known as AtMRS2) was identified in Arabidopsis, which belongs to a eukaryote subset of the CorA superfamily, functioning as Mg(2+) transporters. Some family members (AtMGT1 and AtMGT10) function as high-affinity Mg(2+) transporter and could complement bacterial mutant or yeast mutant lacking Mg(2+) transport capability. Here we report an AtMGT family member, AtMGT9, that functions as a low-affinity Mg(2+) transporter, and is essential for pollen development. The functional complementation assay in Salmonella mutant strain MM281 showed that AtMGT9 is capable of mediating Mg(2+) uptake in the sub-millimolar range of Mg(2+). The AtMGT9 gene was expressed most strongly in mature anthers and was also detectable in vascular tissues of the leaves, and in young roots. Disruption of AtMGT9 gene expression resulted in abortion of half of the mature pollen grains in heterozygous mutant +/mgt9, and no homozygous mutant plant was obtained in the progeny of selfed +/mgt9 plants. Transgenic plants expressing AtMGT9 in these heterozygous plants can recover the pollen phenotype to the wild type. In addition, AtMGT9 RNAi transgenic plants also showed similar abortive pollen phenotype to mutant +/mgt9. Together, our results demonstrate that AtMGT9 functions as a low-affinity Mg(2+) transporter that plays a crucial role in male gametophyte development and male fertility.

Mg(2+)-dependent gating of bacterial MgtE channel underlies Mg(2+) homeostasis

The MgtE family of Mg(2+) transporters is ubiquitously distributed in all phylogenetic domains. Recent crystal structures of the full-length MgtE and of its cytosolic domain in the presence and absence of Mg(2+) suggested a Mg(2+)-homeostasis mechanism, in which the MgtE cytosolic domain acts as a 'Mg(2+) sensor' to regulate the gating of the ion-conducting pore in response to the intracellular Mg(2+) concentration. However, complementary functional analyses to confirm the proposed model have been lacking. Moreover, the limited resolution of the full-length structure precluded an unambiguous characterization of these regulatory divalent-cation-binding sites. Here, we showed that MgtE is a highly Mg(2+)-selective channel gated by Mg(2+) and elucidated the Mg(2+)-dependent gating mechanism of MgtE, using X-ray crystallographic, genetic, biochemical, and electrophysiological analyses. These structural and functional results have clarified the control of Mg(2+) homeostasis through cooperative Mg(2+) binding to the MgtE cytosolic domain.

An outer membrane protease of the omptin family prevents activation of the Citrobacter rodentium PhoPQ two-component system by antimicrobial peptides

The PhoPQ two-component system of the intracellular pathogen Salmonella enterica senses and controls resistance to alpha-helical antimicrobial peptides (AMPs) by regulating covalent modifications of lipid A. A homologue of the phoPQ operon was found in the genome of the murine enteric extracellular pathogen, Citrobacter rodentium. Here we report that C. rodentium PhoPQ was apparently unable to mediate activation of target genes in the presence of alpha-helical AMPs. However, these AMPs activated C. rodentium PhoPQ expressed in a S. entericaDeltaphoPQ mutant. Analysis of the outer membrane (OM) fractions of the C. rodentium wild-type and DeltaphoPQ strains led to the identification of an omptin family protease (CroP) that was absent in DeltaphoPQ. Deletion of croP in C. rodentium resulted in higher susceptibility to alpha-helical AMPs, indicating a direct role of CroP in AMP resistance. CroP greatly contributed to the protection of the OM from AMP damage by actively degrading alpha-helical AMPs before they reach the periplasmic space. Accordingly, transcriptional activation of PhoP-regulated genes by alpha-helical AMPs was restored in the DeltacroP mutant. This study shows that resistance to alpha-helical AMPs by the extracellular pathogen C. rodentium relies primarily on the CroP OM protease.