Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci

How Bacterial Pathogens Coordinate Appetite with Virulence

Cells adjust growth and metabolism to nutrient availability. Having access to a variety of carbon sources during infection of their animal hosts, facultative intracellular pathogens must efficiently prioritize carbon utilization. Here, we discuss how carbon source controls bacterial virulence, with an emphasis on Salmonella enterica serovar Typhimurium, which causes gastroenteritis in immunocompetent humans and a typhoid-like disease in mice, and propose that virulence factors can regulate carbon source prioritization by modifying cellular physiology. On the one hand, bacterial regulators of carbon metabolism control virulence programs, indicating that pathogenic traits appear in response to carbon source availability. On the other hand, signals controlling virulence regulators may impact carbon source utilization, suggesting that stimuli that bacterial pathogens experience within the host can directly impinge on carbon source prioritization. In addition, pathogen-triggered intestinal inflammation can disrupt the gut microbiota and thus the availability of carbon sources. By coordinating virulence factors with carbon utilization determinants, pathogens adopt metabolic pathways that may not be the most energy efficient because such pathways promote resistance to antimicrobial agents and also because host-imposed deprivation of specific nutrients may hinder the operation of certain pathways. We propose that metabolic prioritization by bacteria underlies the pathogenic outcome of an infection.

Pathogenomics of Virulence Traits of Plesiomonas shigelloides That Were Deemed Inconclusive by Traditional Experimental Approaches

One of the major challenges of modern medicine includes the failure of conventional protocols to characterize the pathogenicity of emerging pathogens. This is particularly apparent in the case of Plesiomonas shigelloides. Although a number of infections have been linked to this microorganism, experimental evidence of its virulence factors (VFs), obtained by traditional approaches, is somewhat inconclusive. Hence, it remains unclear whether P. shigelloides is a true or opportunistic one. In the current study, four publicly available whole-genome sequences of P. shigelloides (GN7, NCTC10360, 302-73, and LS1) were profiled using bioinformatics platforms to determine the putative candidate VFs to characterize the bacterial pathogenicity. Overall, 134 unique open reading frames (ORFs) were identified that were homologous or orthologous to virulence genes identified in other pathogens. Of these, 52.24% (70/134) were jointly shared by the strains. The numbers of strain-specific virulence traits were 4 in LS1; 7 in NCTC10360; 10 in 302-73; and 15 in GN7. The pathogenicity islands (PAIs) common to all the strains accounted for 24.07% ORFs. The numbers of PAIs exclusive to each strain were 8 in 302-73; 11 in NCTC10360; 14 in GN7; and 18 in LS1. A PAI encoding Vibrio cholerae ToxR-activated gene d protein was specific to 302-73, GN7, and NCTC10360 strains. Out of 33 antibiotic multi-resistance genes identified, 16 (48.48%) genes were intrinsic to all strains. Further, 17 (22.08%) of 77 antibiotic resistance islands were found in all the strains. Out of 23 identified distinct insertion sequences, 13 were only harbored by strain LS1. The number of intact prophages identified in the strains was 1 in GN7; 2 in 302-73; and 2 in NCTC10360. Further, 1 CRISPR element was identified in LS1; 2 in NCTC10360; and 8 in 302-73. Fifteen (78.95%) of 19 secretion systems and secretion effector variants were identified in all the strains. In conclusion, certain P. shigelloides strains might possess VFs associated with gastroenteritis and extraintestinal infections. However, the role of host factors in the onset of infections should not be undermined.

Terrestrial toxicity model for nickel: Comparison of culture method and modeling approaches

In metal toxicity model studies of soil-plant systems, hydroponic culture is often employed to simulate soil conditions, but the validity of this approach has not been examined. The present study evaluated Ni toxicity to wheat root elongation using models developed in hydroponic and sand cultures and then compared the results with those obtained from real soils. Three mechanism toxicity models were examined: the free ion activity model, the biotic ligand model (BLM), and the Gouy-Chapman-Stern model. After being developed in hydroponic and sand culture bioassays, the models were validated with 18 soils from across China. The results showed that more accurate predictions were obtained with sand rather than hydroponic bioassays. Dissolved organic matter exerted only a limited effect on Ni toxicity, whereas an important role was found for osmotic effects. Of the 3 models, the sand-based BLM best predicted Ni toxicity to wheat root elongation (root-mean-square error = 16.2% and R²  = 0.79) and was as good as the BLM estimated directly from soils. The present study shows that sand is a better matrix than solution for root elongation bioassays of Ni toxicity and that osmotic effects must be taken into account in evaluating soil conditions. Environ Toxicol Chem 2018;37:1349-1358. © 2018 SETAC.

Enhanced succinic acid productivity by expression of mgtCB gene in Escherichia coli mutant

In this study, a novel engineering Escherichia coli strain (CBMG111) with the expression of mgtCB gene was constructed for the enhanced fermentative production of succinic acid by utilizing the synergetic effect of mgtC gene to improve the growth of strains at the environment of low Mg2+ concentration and mgtB to enhance the transport of Mg2+ into cells. After the effect of the expression of the individual genes (mgtA, mgtB, mgtC) on the growth of E. coli was clarified, the fermentative production of succinic acid by CBMG111 was studied with the low-price mixture of Mg(OH)2 and NH3·H2O as the alkaline neutralizer and the biomass hydrolysates as the carbon sources, which demonstrated that the expression of mgtCB gene can significantly increase the productivity of succinic acid (2.97 g L−1 h−1) compared with that by using the engineering strain with the overexpression of mgtA gene.

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.

Effect of magnesium ion on crt gene expression in improving carotenoid yield of Rhodobacter sphaeroides

This study aimed at increasing carotenoid yield of Rhodobacter sphaeroides in wastewater treatment by adding magnesium ion (Mg(2+)). Results showed that Mg(2+) could improve R. sphaeroides biomass and carotenoid yield effectively. The highest carotenoid yield of 4.83 ± 0.14 mg/g biomass and biomass production of 3900 ± 180 mg/L were achieved at optimal Mg(2+) concentration of 15 mmol/L. Mechanism analysis revealed that Mg(2+) could promote carotenoid production by regulating the expressions of crt genes. Up-regulation of crtBDA genes improved carotenoid biosynthesis of R. sphaeroides.

Enterococcus faecalis zinc-responsive proteins mediate bacterial defence against zinc overload, lysozyme and oxidative stress

Two Enterococcus faecalis genes encoding the P-type ATPase EF1400 and the putative SapB protein EF0759 were previously shown to be strongly upregulated in the presence of high concentrations of zinc. In the present work, we showed that a Zn(2+)-responsive DNA-binding motif (zim) is present in the promoter regions of these genes. Both proteins were further studied with respect to their involvement in zinc homeostasis and invasion of the host. EF0759 contributed to intramacrophage survival by an as-yet unknown mechanism(s). EF1400, here renamed ZntAEf, is an ATPase with specificity for zinc and plays a role in dealing with several host defences, i.e. zinc overload, oxidative stress and lysozyme; it provides E. faecalis cells with the ability to survive inside macrophages. As these three host defence mechanisms are important at several sites in the host, i.e. inside macrophages and in saliva, this work suggested that ZntAEf constitutes a crucial E. faecalis defence mechanism that is likely to contribute to the ability of this bacterium to endure life inside its host.

Bacterial Mg2+ homeostasis, transport, and virulence

Organisms must maintain physiological levels of Mg(2+) because this divalent cation is critical for the stabilization of membranes and ribosomes, for the neutralization of nucleic acids, and as a cofactor in a variety of enzymatic reactions. In this review, we describe the mechanisms that bacteria utilize to sense the levels of Mg(2+) both outside and inside the cytoplasm. We examine how bacteria achieve Mg(2+) homeostasis by adjusting the expression and activity of Mg(2+) transporters and by changing the composition of their cell envelope. We discuss the connections that exist between Mg(2+) sensing, Mg(2+) transport, and bacterial virulence. Additionally, we explore the logic behind the fact that bacterial genomes encode multiple Mg(2+) transporters and distinct sensing systems for cytoplasmic and extracytoplasmic Mg(2+). These analyses may be applicable to the homeostatic control of other cations.

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.

ppGpp-mediated stationary phase induction of the genes encoded by horizontally acquired pathogenicity islands and cob/pdu locus in Salmonella enterica serovar Typhimurium

Salmonella enterica is highly diverse in terms of genome structure, which is at least partly due to the horizontal transfer of genetic elements from various sources. In this study, we examined the expression profiles of such genes in Salmonella Pathogenicity Islands (SPIs) and the cob/pdu locus, horizontally acquired large DNA segments, during growth under standard growth conditions. Transcripts from exponentially growing and early stationary phase Salmonellae were compared using various methods including cDNA microarray analysis. Nearly all genes encoded by SPIs and the cob/pdu locus were induced at the onset of the stationary phase in a stringent molecule ppGpp-dependent but stationary phase sigma, sigma38-independent manner. Although, it has been suggested that ppGpp acts in concert with DksA, we found the stationary phase induction of those SPI genes was not DksA dependent. It is suggested that ppGpp stimulates the expression of these stress-inducible genes encoded by horizontally acquired DNA, by itself or in concert with DksA.

Promoter and riboswitch control of the Mg2+ transporter MgtA from Salmonella enterica

The MgtA protein from Salmonella enterica serovar Typhimurium mediates Mg(2+) uptake from the periplasm into the cytoplasm. Here we report that the PhoP/PhoQ two-component regulatory system, which responds to periplasmic Mg(2+), governs mgtA transcription initiation at all investigated Mg(2+) concentrations and that the Mg(2+)-sensing 5' leader region of the mgtA gene controls transcription elongation into the mgtA coding region when Salmonella is grown in media with <50 muM Mg(2+). Overexpression of the Mg(2+) transporter CorA, which is believed to increase cytoplasmic Mg(2+) levels, decreased mgtA transcription in a manner dependent on a functional mgtA 5' leader.

Increased expression of Mg(2+) transport proteins enhances the survival of Salmonella enterica at high temperature

Mg(2+) homeostasis is important for Salmonella pathogenesis. In Salmonella enterica, the transcription of the mgtA gene, which encodes a Mg(2+) transporter, is regulated by a Mg(2+)-sensing riboswitch [Cromie MJ, Shi Y, Latifi T, Groisman EA (2006) Cell 125:71-84]. In a genetic analysis of the determinants of thermotolerance in S. enterica serovar Typhimurium, we isolated the chr-1 mutation that increased the resistance of exponential phase cells to killing by high temperature. This mutation is a single base change in the mgtA riboswitch that causes high-level constitutive expression of mgtA. We showed that another mgtA riboswitch mutation, DeltaUTR(re-100), which had been constructed by Cromie et al., also confers similar increased thermotolerance. Surprisingly, the chr-1 mutation is located at a position that would not be predicted to be important for the regulatory function of the riboswitch. We obtained physiological evidence suggesting that the chr-1 mutation increases the cytosolic free Mg(2+) concentration. High-level expression of the heterologous MgtE Mg(2+) transport protein of Bacillus subtilis also enhanced the thermotolerance of S. enterica. We hypothesize that increased Mg(2+) accumulation might enhance thermotolerance by protecting the integrity of proteins or membranes, by mitigating oxidative damage or acting as an inducer of thermoprotective functions.

The unique nature of mg2+ channels

Considering the biological abundance and importance of Mg2+, there is a surprising lack of information regarding the proteins that transport Mg2+, the mechanisms by which they do so, and their physiological roles within the cell. The best characterized Mg2+ channel to date is the bacterial protein CorA, present in a wide range of bacterial species. The CorA homolog Mrs2 forms the mitochondrial Mg2+ channel in all eukaryotes. Physiologically, CorA is involved in bacterial pathogenesis, and the Mrs2 eukaryotic homolog is essential for cell survival. A second Mg2+ channel widespread in bacteria is MgtE. Its eukaryotic homologs are the SLC41 family of carriers. Physiological roles for MgtE and its homologs have not been established. Recently, the crystal structures for the bacterial CorA and MgtE Mg2+ channels were solved, the first structures of any divalent cation channel. As befits the unique biological chemistry of Mg2+, both structures are unique, unlike that of any other channel or transporter. Although structurally quite different, both CorA and MgtE appear to be gated in a similar manner through multiple Mg2+ binding sites in the cytosolic domain of the channels. These sites essentially serve as Mg2+ "sensors" of cytosolic Mg2+ concentration. Many questions about these channels remain, however, including the molecular basis of Mg2+ selectivity and the physiological role(s) of their eukaryotic homologs.

High stimulus unmasks positive feedback in an autoregulated bacterial signaling circuit

We examined the effect of positive autoregulation on the steady-state behavior of the PhoQ/PhoP two-component signaling system in Escherichia coli. We found that autoregulation has no effect on the steady-state output for a large range of input stimulus, which was modulated by varying the concentration of magnesium in the growth medium. We provide an explanation for this finding with a simple model of the PhoQ/PhoP circuit. The model predicts that even when autoregulation is manifest across a range of stimulus levels, the effects of positive feedback on the steady-state output emerge only in the limit that the system is strongly stimulated. Consistent with this prediction, amplification associated with autoregulation was observed in growth-limiting levels of magnesium, a condition that strongly activates PhoQ/PhoP. In a further test of the model, we found that strains harboring a phosphatase-defective PhoQ showed strong positive feedback and considerable cell-to-cell variability under growth conditions where the wild-type circuit did not show this behavior. Our results demonstrate a simple and general mechanism for regulating the positive feedback associated with autoregulation within a bacterial signaling circuit to boost response range and maintain a relatively uniform and graded output.

The CorA Mg2+ channel is required for the virulence of Salmonella enterica serovar typhimurium

CorA is the primary Mg(2+) channel in Salmonella enterica serovar Typhimurium. A corA mutant is attenuated in mice and defective for invasion of and replication within epithelial cells. Microarray studies show that several virulence effectors are repressed in a corA mutant strain, which ultimately manifests itself as a decrease in virulence.

Regulation of CorA Mg2+ channel function affects the virulence of Salmonella enterica serovar typhimurium

The CorA Mg(2+) channel is the primary source of intracellular Mg(2+) in Salmonella enterica serovar Typhimurium. In another study, we found that a strain lacking corA was attenuated in mice and also defective for invasion and replication within Caco-2 epithelial cells (K. M. Papp-Wallace, M. Nartea, D. G. Kehres, S. Porwollik, M. McClelland, S. J. Libby, F. C. Fang, and M. E. Maguire, J. Bacteriol. 190:6517-6523, 2008). Therefore, we further examined Salmonella interaction with Caco-2 epithelial cells. Inhibiting CorA acutely or chronically with a high concentration of a selective inhibitor, Co(III) hexaammine, had no effect on S. enterica serovar Typhimurium invasion of Caco-2 epithelial cells. Complementing the corA mutation with corA from various species rescued the invasion defect only if the complementing allele was functional and if it was evolutionarily similar to S. enterica serovar Typhimurium CorA. One explanation for these results could be that regulation of CorA function is needed for optimal virulence. Further experiments examining corA transcription, CorA protein content, CorA transport, and cell Mg(2+) content indicated that both CorA expression and CorA function are differentially regulated. Moreover, the rates of Mg(2+) influx via CorA are not closely correlated with either protein levels or Mg(2+) content. We conclude that loss of the CorA protein disrupts a regulatory network(s) with the ultimate phenotype of decreased virulence. This conclusion is compatible with the microarray results in our other study, which showed that loss of corA resulted in changes in transcription (and protein expression) in multiple metabolic pathways (Papp-Wallace et al., J. Bacteriol. 190:6517-6523, 2008). Further study of the regulation of CorA expression and function provides an opportunity to dissect the complexity of Mg(2+) homeostasis and its ties to virulence within the bacterium.

mgtA Expression is induced by rob overexpression and mediates a Salmonella enterica resistance phenotype

Rob is a member of the Sox/Mar subfamily of AraC/XylS-type transcriptional regulators implicated in bacterial multidrug, heavy metal, superoxide, and organic solvent resistance phenotypes. We demonstrate that, in Salmonella enterica, Rob overexpression upregulates the transcription of mgtA, which codes for the MgtA Mg2+ transporter. mgtA was previously characterized as a member of the Mg2+-modulated PhoPQ regulon. Here we demonstrate that Rob (but not its paralog protein SoxS or MarA) is able to induce mgtA transcription in a PhoP-independent fashion by binding to a conserved Mar/Sox/Rob motif localized downstream of the PhoP-box and overlapping the PhoP-dependent transcriptional start site. We found that Rob-induced mgtA expression confers low-level cyclohexane resistance on Salmonella. Because mgtA intactness is required for Rob-induced cyclohexane resistance, provided the AcrAB multidrug efflux pump can be expressed, we postulate that MgtA is involved in the AcrAB-mediated cyclohexane detoxification mechanism promoted by Rob in Salmonella.

Bacterial homologs of eukaryotic membrane proteins: the 2-TM-GxN family of Mg(2+) transporters

Magnesium is essential for all forms of life. It is the cofactor for many enzymes and plays a key role in many biological processes. Thus, the acquisition of Mg(2+) is crucial for cell survival. The best characterized Mg(2+) transporters to date belong to the 2-TM-GxN type family of transporters. The name indicates the two C-terminal transmembrane (TM) domains and a conserved GxN motif present in all members of this family towards the C-terminal end of TM1. In most members of the family, this conserved motif is generally YGMNF. The prototypical member of this family is CorA. Other characterized members of this family include Mrs2p, Alr, Mnr, AtMGT and ZntB. CorA is widely distributed throughout the prokaryotic world. It is the primary Mg(2+) uptake system in most bacteria and many Archaea. A homolog, Mrs2p, is a eukaryotic mitochondrial Mg(2+) channel. The Mrs2p related AtMGT transporters are found in plants and other eukaryotes. Alr1p and Mnr are Mg(2+) transporters found in the plasma membrane of many fungi. ZntB is a bacterial member of the 2-TM-GxN family but mediates efflux of Zn(2+) instead of influx of Mg(2+). The recent crystal structure of a bacterial CorA shows that the structure of this family is unlike that of any other class of transporter or channel currently known.

A Mg2+-responding RNA that controls the expression of a Mg2+ transporter

Mg2+ is the most abundant divalent cation in biological systems. It is required for ATP-mediated enzymatic reactions and as a stabilizer of ribosomes and membranes. The enteric bacterium Salmonella enterica serovar Typhimurium harbors three Mg2+ transporters and a regulatory system-termed PhoP/PhoQ-whose activity is regulated by the extracytoplasmic levels of Mg2+. We have determined that expression of the PhoP-activated Mg2+ transporter MgtA is also controlled by its 5'-untranslated region (5'UTR). The 5'UTR of the mgtA gene can adopt different stem-loop structures depending on the Mg2+ levels, which determine whether transcription reads through into the mgtA-coding region or stops within the 5'UTR. This makes the mgtA 5'UTR the first example of a cation-responding riboswitch. The initiation of mgtA transcription responds to extracytoplasmic Mg2+, and its elongation into the coding region to cytoplasmic Mg2+, which provides a singular example where the same ligand is sensed in different cellular compartments to regulate disparate steps in gene transcription. The PhoP-activated Mg2+ transporter mgtB is also regulated by Mg2+ in a strain lacking the Mg2+ sensor PhoQ, suggesting the presence of additional Mg2+-responding devices.

Dual role of the MgtC virulence factor in host and non-host environments

MgtC is required for intramacrophage replication of intracellular pathogens and growth in low Mg(2+) medium. A link between these two phenotypes has been proposed due to putative Mg(2+) deprivation inside phagosome. MgtC is part of a family of proteins that share a conserved N-terminal transmembrane domain and a variable C-terminal domain. A combination of predictive and experimental approaches indicates that the Salmonella MgtC C-terminal domain is cytoplasmic, adopts a fold also found in metal transporters and RNA interacting domain, and does not bind Mg(2+). MgtC homologues from diverse gamma-proteobacteria, including the extracellular pathogens Yersinia pestis, Photorhabdus luminescens and Pseudomonas aeruginosa, have been expressed in a SalmonellaDeltamgtC strain. The Y. pestis MgtC fully replaced the Salmonella MgtC whereas P. luminescens or P. aeruginosa MgtC complemented only in low Mg(2+) medium, thus dissociating for the first time the two MgtC-related phenotypes. In addition, we identified single amino acids changes that prevent or promote MgtC role in macrophages without affecting MgtC role in low Mg(2+) culture. A SalmonellaDeltamgtC strain showed elongated and autoaggregated bacteria in low Mg(2+) medium but not in macrophages. Taken together our results suggest that MgtC has a dual role when bacteria localize in macrophages or low Mg(2+) environment.

The MgtC virulence factor of Salmonella enterica serovar Typhimurium activates Na(+),K(+)-ATPase

The mgtC gene of Salmonella enterica serovar Typhimurium encodes a membrane protein of unknown function that is important for full virulence in the mouse. Since mgtC is part of an operon with mgtB which encodes a Mg(2+)-transporting P-type ATPase, MgtC was hypothesized to function in ion transport, possibly in Mg(2+) transport. Consequently, MgtC was expressed in Xenopus laevis oocytes, and its effect on ion transport was evaluated using ion selective electrodes. Oocytes expressing MgtC did not exhibit altered currents or membrane potentials in response to changes in extracellular H(+), Mg(2+), or Ca(2+), thus ruling out a previously postulated function as a Mg(2+)/H(+) antiporter. However, addition of extracellular K(+) markedly hyperpolarized membrane potential instead of the expected depolarization. Addition of ouabain to block the oocyte Na(+),K(+)-ATPase completely prevented hyperpolarization and restored the normal K(+)-induced depolarization response. These results suggested that the Na(+),K(+)-ATPase was constitutively activated in the presence of MgtC resulting in a membrane potential largely dependent on Na(+),K(+)-ATPase. Consistent with the involvement of Na(+),K(+)-ATPase, oocytes expressing MgtC exhibited an increased rate of (86)Rb(+) uptake and had increased intracellular free [K(+)] and decreased free [Na(+)] and ATP. The free concentrations of Mg(2+) and Ca(2+) and cytosolic pH were unchanged, although the total intracellular Ca(2+) content was slightly elevated. These results suggest that the serovar Typhimurium MgtC protein may be involved in regulating membrane potential but does not directly transport Mg(2+) or another ion.

An RNA sensor for intracellular Mg(2+)

Most RNA molecules require Mg(2+) for their structure and enzymatic properties. Here we report the first example of an RNA serving as sensor for cytoplasmic Mg(2+). We establish that expression of the Mg(2+) transporter MgtA of Salmonella enterica serovar Typhimurium is controlled by its 5' untranslated region (5'UTR). We show that the 5'UTR of the mgtA gene can adopt different stem-loop structures depending on the Mg(2+) levels, which determine whether transcription reads through into the mgtA coding region or stops within the 5'UTR. We could recapitulate the Mg(2+)-regulated transcription using a defined in vitro transcription system with RNA polymerase as the only protein component. The initiation of mgtA transcription responds to extracytoplasmic Mg(2+) and its elongation into the coding region to cytoplasmic Mg(2+), providing a singular example in which the same ligand is sensed in different cellular compartments to regulate disparate steps in gene transcription.

Transcriptional analysis of long-term adaptation of Yersinia enterocolitica to low-temperature growth

To analyze the transcriptional response of Yersinia enterocolitica cells to prolonged growth at low temperature, a collection of luxCDABE transposon mutants was cultivated in parallel at optimal (30 degrees C) and suboptimal (10 degrees C) temperatures and screened for enhanced promoter activities during growth until entering stationary phase. Among 5,700 Y. enterocolitica mutants, 42 transcriptional units were identified with strongly enhanced or reduced promoter activity at 10 degrees C compared to 30 degrees C, and changes in their transcriptional levels over time were measured. Green fluorescent protein fusions to 10 promoter regions confirmed the data. The temporal order of induction of the temperature-responsive genes of Y. enterocolitica was deduced, starting with the expression of cold shock genes cspA and cspB and the elevated transcription of a glutamate-aspartate symporter. Subsequently, cold-adapted cells drastically up-regulated genes encoding environmental sensors and regulators, such as UhpABC, ArcA, and methyl-accepting chemotaxis protein I (MCPI). Among the most prominent cold-responsive elements that were transcriptionally induced during growth in early and middle exponential phase are the insecticidal toxin genes tcaA and tcaB, as well as genes involved in flagellar synthesis and chemotaxis. The expression pattern of the late-exponential- to early-stationary-growth phase is dominated by factors involved in biodegradative metabolism, namely, a histidine ammonia lyase, three enzymes responsible for uptake and utilization of glycogen, the urease complex, and a subtilisin-like protease. Double-knockout mutants and complementation studies demonstrate inhibitory effects of MCPI and UhpC on the expression of a putative hemolysin transporter. The data partially delineate the spectrum of gene expression of Y. enterocolitica at environmental temperatures, providing evidence that an as-yet-unknown insect phase is part of the life cycle of this human pathogen.

Ni2+-uptake inPseudomonas putida strain S4: a possible role of Mg2+-uptake pump

Essential metal ion homeostasis is based on regulated uptake of metal ions, both during its scarcity and abundance. Pseudomonas putida strain S4, a multimetal resistant bacterium, was employed to investigate Ni(2+) entry into cells. It was observed that Mg(2+) regulates the entry of Ni(2+) and by this plays a protective role to minimize Ni(2+) toxicity in this strain. This protection was evident in both growth as well as viability. Intracellular accumulation of Ni(2+) varied in accordance with Mg(2+) concentrations in the medium. It was hypothesized that Ni(2+) enters the cell using a broad Mg(2+) pump, i.e. the CorA system, as the CorA inhibitor, i.e. Co(III) Hex, also inhibits Ni 2+ uptake. This led to the inference that Mg(2+)-based protection was basically due to competitive inhibition of Ni(2+) uptake. We also show that Zn(2+) can further regulate the entry of Ni(2+).

CorA affects tolerance of Escherichia coli and Salmonella enterica serovar Typhimurium to the lactoperoxidase enzyme system but not to other forms of oxidative stress

The enzyme lactoperoxidase is part of the innate immune system in vertebrates and owes its antimicrobial activity to the formation of oxidative reaction products from various substrates. In a previous study, we have reported that, with thiocyanate as a substrate, the lactoperoxidase system elicits a distinct stress response in Escherichia coli MG1655. This response is different from but partly overlapping with the stress responses to hydrogen peroxide and to superoxide. In the current work, we constructed knockouts in 10 lactoperoxidase system-inducible genes to investigate their role in the tolerance of E. coli MG1655 to this antimicrobial system. Five mutations resulted in a slightly increased sensitivity, but one mutation (corA) caused hypersensitivity to the lactoperoxidase system. This hypersensitive phenotype was specific to the lactoperoxidase system, since neither the sensitivity to hydrogen peroxide nor to the superoxide generator plumbagin was affected in the corA mutant. Salmonella enterica serovar Typhimurium corA had a similar phenotype. Although corA encodes an Mg2+ transporter and at least three other inducible open reading frames belonged to the Mg2+ regulon, repression of the Mg stimulon by Mg2+ did not change the lactoperoxidase sensitivity of either the wild-type or corA mutant. Prior exposure to 0.3 mM Ni2+, which is also transported by CorA, strongly sensitized MG1655 but not the corA mutant to the lactoperoxidase system. Furthermore, this Ni2+-dependent sensitization was suppressed by the CorA-specific inhibitor Co(III) hexaammine. These results indicate that CorA affects the lactoperoxidase sensitivity of E. coli by modulating the cytoplasmic concentrations of transition metals that enhance the toxicity of the lactoperoxidase system.

The CorA Mg2+ transporter does not transport Fe2+

corA encodes the constitutively expressed primary Mg2+ uptake system of most eubacteria and many archaea. Recently, a mutation in corA was reported to make Salmonella enterica serovar Typhimurium markedly resistant to Fe2+-mediated toxicity. Mechanistically, this was hypothesized to be from an ability of CorA to mediate the influx of Fe2+. Consequently, we directly examined Fe2+ transport and toxicity in wild-type versus corA cells. As determined by direct transport assay, CorA cannot transport Fe2+ and Fe2+ does not potently inhibit CorA transport of 63Ni2+. Mg2+ can, relatively weakly, inhibit Fe2+ uptake, but inhibition is not dependent on the presence of a functional corA allele. Although excess Fe2+ was slightly toxic to S. enterica serovar Typhimurium, we were unable to elicit a significant differential sensitivity in a wild-type versus a corA strain. We conclude that CorA does not transport Fe2+ and that the relationship, if any, between iron toxicity and corA is indirect.

The CorA Mg2+ transporter is a homotetramer

CorA is a primary Mg2+ transporter for Bacteria and Archaea. The C-terminal domain of approximately 80 amino acids forms three transmembrane (TM) segments, which suggests that CorA is a homo-oligomer. A Cys residue was added to the cytoplasmic C terminus (C317) of Salmonella enterica serovar Typhimurium CorA with or without mutation of the single periplasmic Cys191 to Ser; each mutant retained function. Oxidation of the Cys191Ser Cys317 CorA gave a dimer. Oxidation of Cys317 CorA showed a dimer plus an additional band, apparently cross-linked via both Cys317 and C191. To determine oligomer order, intact cells or purified membranes were treated with formaldehyde or carbon disulfide. Higher-molecular-mass bands formed, consistent with the presence of a tetramer. Cross-linking of the Bacillus subtilis CorA expressed in Salmonella serovar Typhimurium similarly indicated a tetramer. CorA periplasmic soluble domains from both Salmonella serovar Typhimurium and the archaeon Methanococcus jannaschii were purified and shown to retain structure. Formaldehyde treatment showed formation of a tetramer. Finally, previous mutagenesis of the CorA membrane domain identified six intramembrane residues forming an apparent pore that interacts with Mg2+ during transport. Each was mutated to Cys. In mutants carrying a single intramembrane Cys residue, spontaneous disulfide bond formation that was enhanced by oxidation with Cu(II)-1,10-phenanthroline was observed between monomers, indicating that these Mg2+-interacting residues within the membrane are very close to their cognate residue on another monomer. Thus, CorA appears to be a homotetramer with a TM segment of one monomer physically close to the same TM segment of another monomer.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Unravelling the biology of macrophage infection by gene expression profiling of intracellular Salmonella enterica

For intracellular pathogens such as Salmonellae, Mycobacteriae and Brucellae, infection requires adaptation to the intracellular environment of the phagocytic cell. The transition from extracellular to intravacuolar environment has been expected to involve a global modulation of bacterial gene expression, but the precise events have been difficult to determine. We now report the complete transcriptional profile of intracellular Salmonella enterica sv. Typhimurium following macrophage infection. During replication in murine macrophage-like J774-A.1 cells, 919 of 4451 S. Typhimurium genes showed significant changes in transcription. The expression profile identified alterations in numerous virulence and SOS response genes and revealed unexpected findings concerning the biology of the Salmonella-macrophage interaction. We observed that intracellular Salmonella are not starved for amino acids or iron (Fe2+), and that the intravacuolar environment is low in phosphate and magnesium but high in potassium. S. Typhimurium appears to be using the Entner-Douderoff pathway to use gluconate and related sugars as a carbon source within macrophages. Almost half the in vivo-regulated genes were of unknown function, suggesting that intracellular growth involves novel macrophage-associated functions. This is the first report that identifies the whole set of in vivo-regulated genes for any bacterial pathogen during infection of mammalian cells.

Cation hexaammines are selective and potent inhibitors of the CorA magnesium transport system

Cation hexaammines and related compounds are chemically stable analogs of the hydrated form of cations, particularly Mg(2+). We tested the ability of several of these compounds to inhibit transport by the CorA or MgtB Mg(2+) transport systems or the PhoQ receptor kinase for Mg(2+) in Salmonella typhimurium. Cobalt(III)-, ruthenium(II)-, and ruthenium(III)-hexaammines were potent inhibitors of CorA-mediated influx. Cobalt(III)- and ruthenium(III)chloropentaammines were slightly less potent inhibitors of CorA. The compounds inhibited uptake by the bacterial S. typhimurium CorA and by the archaeal Methanococcus jannaschii CorA, which bear only 12% identity in the extracellular periplasmic domain. Cation hexaammines also inhibited growth of S. typhimurium strains dependent on CorA for Mg(2+) uptake but not of isogenic strains carrying a second Mg(2+) uptake system. In contrast, hexacyano-cobaltate(III) and ruthenate(II)- and nickel(II)hexaammine had little effect on uptake. The inhibition by the cation hexaammines was selective for CorA because none of the compounds had any effect on transport by the MgtB P-type ATPase Mg(2+) transporter or the PhoQ Mg(2+) receptor kinase. These results demonstrate that cation hexaammines are potent and highly selective inhibitors of the CorA Mg(2+) transport system and further indicate that the initial interaction of the CorA transporter is with a fully hydrated Mg(2+) cation.

The CorA Mg2+ transport protein of Salmonella typhimurium. Mutagenesis of conserved residues in the third membrane domain identifies a Mg2+ pore

The CorA transport system is the major Mg2+ influx pathway for bacteria and the Archaea. CorA contains three C-terminal transmembrane segments. No conserved charged residues are apparent within the membrane, suggesting that Mg2+ influx does not involve electrostatic interactions. We have mutated conserved residues within the third transmembrane segment to identify sites involved in transport. Mutation of conserved aromatic residues at either end of the membrane segment to alternative aromatic amino acids did not affect total cation uptake or cation affinity. Mutation to alanine greatly diminished uptake with little change in cation affinity implying that the conserved aromatic residues play a structural role in stabilizing this membrane segment of CorA at the interface between the bilayer and the aqueous environment. In contrast, mutation of Tyr292, Met299, and Tyr307 greatly altered the transport properties of CorA. Y292F, Y292S, Y292C, or Y292I mutations essentially abolished transport, without effect on expression or membrane insertion. M299C and M299A mutants exhibited a decrease in cation affinity for Mg2+, Co2+, or Ni2+ of 10-50-fold without a significant change in uptake capacity. Mutations at Tyr307 had no significant effect on cation uptake capacity; however, the affinity of Y307F and Y307A mutations for Mg2+ and Co2+ was decreased 3-10-fold, while affinity for Ni2+ was unchanged compared with the wild type CorA. In contrast, the affinity of the Y307S mutant for all three cations was decreased 2-5-fold. Projection of the third transmembrane segment as an alpha-helix suggests that Tyr292, Met299, and Tyr307 all reside on the same face of the alpha-helix. We interpret the transport data to suggest that a hydroxyl group is important at Tyr307, and that these three residues interact with Mg2+ during transport, forming part of the cation pore or channel within CorA.

Magnesium and the role of MgtC in growth of Salmonella typhimurium

Salmonella typhimurium has three distinct transport systems for Mg2+: CorA, MgtA, and MgtB. The mgtCB operon encodes two proteins, MgtC, a hydrophobic protein with a predicted molecular mass of 22.5 kDa, and MgtB, a 102-kDa P-type ATPase Mg2+ transport protein. The mgtCB locus has been identified as part of a new Salmonella pathogenicity island, SPI-3. Transcription of mgtCB is regulated by extracellular Mg2+ via the two-component PhoPQ regulatory system important for virulence. To elucidate MgtC's role in a low-Mg2+ environment, we looked at growth and transport in strains lacking the CorA and MgtA Mg2+ transporters but expressing MgtB, MgtC, or both. mgtC mgtB+ and mgtC+ mgtB+ strains exhibited growth in N minimal medium without added Mg2+ with a 1- to 2-h lag phase. An mgtC+ mgtB strain was also able to grow in N minimal medium without added Mg2+ but only after a 24-h lag phase. In N minimal medium containing 10 mM Mg2+, all strains grew after a short lag phase; the mgtC+ mgtB strain grew to a higher optical density at 600 nm than an mgtC+ mgtB+ strain and was comparable to wild type. The lengthy lag phase before growth in an mgtC+ mgtB strain was not due to lack of expression of MgtC. Western blot analysis indicated that substantial MgtC protein is present by 2 h after suspension in N minimal medium. Surprisingly, in an mgtC+ mgtB+ strain, MgtC was undetectable during Mg2+ starvation, although large amounts of MgtB were observed. The lack of expression of MgtC is not dependent on functional MgtB, since a strain carrying a nonfunctional MgtB with a mutation (D379A) also did not make MgtC. Since, during invasion of eukaryotic cells, S. typhimurium appears to be exposed to a low-pH as well as a low-Mg2+ environment, the growth of an mgtC+ mgtB strain was tested at low pH with and without added Mg2+. While significant quantities of MgtC could be detected after suspension at pH 5.2, the mgtC+ mgtB strain was unable to grow at pH 5.2 whether or not Mg2+ was present. Finally, using 63Ni2+ and 57Co2+ as alternative substrates for the unavailable 28Mg2+, cation uptake could not be detected in an mgtC+ mgtB strain after Mg2+ starvation. We conclude that MgtC is not a Mg2+ transporter and that it does not have a primary role in the survival of S. typhimurium at low pH.

Magnesium transport in Salmonella typhimurium: regulation of mgtA and mgtCB during invasion of epithelial and macrophage cells

Salmonella typhimurium contains two inducible Mg2+ transport systems, MgtA and MgtB, the latter encoded by a two-gene operon, mgtCB. Mg2+ deprivation of S. typhimurium increases transcription of both mgtA and mgtCB over a thousandfold and a similar increase occurs upon S. typhimurium invasion of epithelial cells. These increases are mediated by the phoPQ two-component signal transduction system, an essential system for S. typhimurium virulence. It was therefore hypothesized that expression of MgtA and MgtCB is increased upon invasion of eukaryotic cells because of a lack of intravacuolar Mg2+. However, when S. typhimurium was grown at pH 5.2, the capacity of the constitutive CorA transporter in mediating Mg2+ was greater than that at pH 7.4. Furthermore, induction of mgtA and mgtCB transcription was greater in the presence of a wild-type corA allele than in its absence. This implies that intravacuolar S. typhimurium could obtain sufficient Mg2+ via the CorA system. The effect of acid pH on mgtA and mgtCB transcription was also measured. Compared to induction at pH 7.4, exposure to pH 5.2 almost completely abolished induction of mgtA at low Mg2+ concentrations but diminished induction of mgtCB only twofold. Adaptation of cells to acid pH by overnight growth resulted in normal levels of induction of mgtA and mgtCB at low Mg2+ concentrations. These results imply an additional level of regulation for mgtA that is not present for mgtCB. Conversely, repression of mgtA and mgtCB expression by increased extracellular Mg2+ was relatively insensitive to acid. Transcription of both loci was strongly induced upon invasion of the Hep-2 or CMT-93 epithelial-like or J774 macrophage-like cell lines. However, the presence or absence of functional alleles of either or both mgtA or mgtCB had no effect on invasion efficiency or short-term survival of S. typhimurium within the eukaryotic cells. It was concluded that the strong Mg(2+)-dependent induction of mgtA and mgtCB upon invasion of eukaryotic cells is not required because S. typhimurium lacks sufficient Mg2+ during eukaryotic cell invasion and initial intravacuolar growth.