Survival of Escherichia coli cells on solid copper surfaces is increased by glutathione

Generative Landscapes and Dynamics to Design Multidomain Artificial Transmembrane Transporters

Protein design is challenging as it requires simultaneous consideration of interconnected factors, such as fold, dynamics, and function. These evolutionary constraints are encoded in protein sequences and can be learned through the latent generative landscape (LGL) framework to predict functional sequences by leveraging evolutionary patterns, enabling exploration of uncharted sequence space. By simulating designed proteins through molecular dynamics (MD), we gain deeper insights into the interdependencies governing structure and dynamics. We present a synergized workflow combining LGL with MD and biochemical characterization, allowing us to explore the sequence space effectively. This approach has been applied to design and characterize two artificial multidomain ATP-driven transmembrane copper transporters, with native-like functionality. This integrative approach proved effective in unraveling the intricate relationships between sequence, structure, and function.

Bacterial Metallostasis: Metal Sensing, Metalloproteome Remodeling, and Metal Trafficking

Transition metals function as structural and catalytic cofactors for a large diversity of proteins and enzymes that collectively comprise the metalloproteome. Metallostasis considers all cellular processes, notably metal sensing, metalloproteome remodeling, and trafficking (or allocation) of metals that collectively ensure the functional integrity and adaptability of the metalloproteome. Bacteria employ both protein and RNA-based mechanisms that sense intracellular transition metal bioavailability and orchestrate systems-level outputs that maintain metallostasis. In this review, we contextualize metallostasis by briefly discussing the metalloproteome and specialized roles that metals play in biology. We then offer a comprehensive perspective on the diversity of metalloregulatory proteins and metal-sensing riboswitches, defining general principles within each sensor superfamily that capture how specificity is encoded in the sequence, and how selectivity can be leveraged in downstream synthetic biology and biotechnology applications. This is followed by a discussion of recent work that highlights selected metalloregulatory outputs, including metalloproteome remodeling and metal allocation by metallochaperones to both client proteins and compartments. We close by briefly discussing places where more work is needed to fill in gaps in our understanding of metallostasis.

A gold speciation that adds a second layer to synergistic gold-copper toxicity in Cupriavidus metallidurans

The metal-resistant bacterium Cupriavidus metallidurans occurs in metal-rich environments. In auriferous soils, the bacterium is challenged by a mixture of copper ions and gold complexes, which exert synergistic toxicity. The previously used, self-made Au(III) solution caused a synergistic toxicity of copper and gold that was based on the inhibition of the CupA-mediated efflux of cytoplasmic Cu(I) by Au(I) in this cellular compartment. In this publication, the response of the bacterium to gold and copper was investigated by using a commercially available Au(III) solution instead of the self-made solution. The new solution was five times more toxic than the previously used one. Increased toxicity was accompanied by greater accumulation of gold atoms by the cells. The contribution of copper resistance determinants to the commercially available Au(III) solution and synergistic gold-copper toxicity was studied using single- and multiple-deletion mutants. The commercially available Au(III) solution inhibited periplasmic Cu(I) homeostasis, which is required for the allocation of copper ions to copper-dependent proteins in this compartment. The presence of the gene for the periplasmic Cu(I) and Au(I) oxidase, CopA, decreased the cellular copper and gold content. Transcriptional reporter gene fusions showed that up-regulation of gig, encoding a minor contributor to copper resistance, was strictly glutathione dependent. Glutathione was also required to resist synergistic gold-copper toxicity. The new data indicated a second layer of synergistic copper-gold toxicity caused by the commercial Au(III) solution, inhibition of the periplasmic copper homeostasis in addition to the cytoplasmic one.IMPORTANCEWhen living in auriferous soils, Cupriavidus metallidurans is not only confronted with synergistic toxicity of copper ions and gold complexes but also by different gold species. A previously used gold solution made by using aqua regia resulted in the formation of periplasmic gold nanoparticles, and the cells were protected against gold toxicity by the periplasmic Cu(I) and Au(I) oxidase CopA. To understand the role of different gold species in the environment, another Au(III) solution was commercially acquired. This compound was more toxic due to a higher accumulation of gold atoms by the cells and inhibition of periplasmic Cu(I) homeostasis. Thus, the geo-biochemical conditions might influence Au(III) speciation. The resulting Au(III) species may subsequently interact in different ways with C. metallidurans and its copper homeostasis system in the cytoplasm and periplasm. This study reveals that the geochemical conditions may decide whether bacteria are able to form gold nanoparticles or not.

Full Copper Resistance in Cupriavidus metallidurans Requires the Interplay of Many Resistance Systems

The metal-resistant bacterium Cupriavidus metallidurans uses its copper resistance components to survive the synergistic toxicity of copper ions and gold complexes in auriferous soils. The cup, cop, cus, and gig determinants encode as central component the Cu(I)-exporting P_IB1-type ATPase CupA, the periplasmic Cu(I)-oxidase CopA, the transenvelope efflux system CusCBA, and the Gig system with unknown function, respectively. The interplay of these systems with each other and with glutathione (GSH) was analyzed. Copper resistance in single and multiple mutants up to the quintuple mutant was characterized in dose-response curves, Live/Dead-staining, and atomic copper and glutathione content of the cells. The regulation of the cus and gig determinants was studied using reporter gene fusions and in case of gig also RT-PCR studies, which verified the operon structure of gigPABT. All five systems contributed to copper resistance in the order of importance: Cup, Cop, Cus, GSH, and Gig. Only Cup was able to increase copper resistance of the Δcop Δcup Δcus Δgig ΔgshA quintuple mutant but the other systems were required to increase copper resistance of the Δcop Δcus Δgig ΔgshA quadruple mutant to the parent level. Removal of the Cop system resulted in a clear decrease of copper resistance in most strain backgrounds. Cus cooperated with and partially substituted Cop. Gig and GSH cooperated with Cop, Cus, and Cup. Copper resistance is thus the result of an interplay of many systems. IMPORTANCE The ability of bacteria to maintain homeostasis of the essential-but-toxic "Janus"-faced element copper is important for their survival in many natural environments but also in case of pathogenic bacteria in their respective host. The most important contributors to copper homeostasis have been identified in the last decades and comprise P_IB1-type ATPases, periplasmic copper- and oxygen-dependent copper oxidases, transenvelope efflux systems, and glutathione; however, it is not known how all these players interact. This publication investigates this interplay and describes copper homeostasis as a trait emerging from a network of interacting resistance systems.

The Sensory Histidine Kinase CusS of Escherichia coli Senses Periplasmic Copper Ions

Two-component regulatory systems composed of a membrane-bound sensor/sensory histidine kinase (HK) and a cytoplasmic, DNA-binding response regulator (RR) are often associated with transenvelope efflux systems, which export transition metal cations from the periplasm directly out of the cell. Although much work has been done in this field, more evidence is needed for the hypothesis that the respective two-component regulatory systems are indeed sensing periplasmic ions. If so, a regulatory circuit between the concentration of periplasmic metal cations, sensing of these metals, and control of expression of the genes for transenvelope efflux systems that remove periplasmic cations can be assumed. Escherichia coli possesses only one transenvelope efflux system for metal cations, the Cus system for export of Cu(I) and Ag(I). It is composed of the transenvelope efflux system CusCBA, the periplasmic copper chaperone CusF, and the two-component regulatory system CusS (HK) and CusR (RR). Using phoA- and lacZ-reporter gene fusions, it was verified that an assumed periplasmic part of CusS is located in the periplasm. CusS was more important for copper resistance in E. coli under anaerobic conditions than under aerobic conditions and in complex medium more than in mineral salts medium. Predicted copper-binding sites in the periplasmic part of CusS were identified that, individually, were not essential for copper resistance but were in combination. In summary, evidence was obtained that the two-component regulatory system CusSR that controls expression of cusF and cusCBA does indeed sense periplasmic copper ions. IMPORTANCE Homeostasis of essential-but-toxic transition metal cations such as Zn(II) and Cu(II)/Cu(I) is an important contributor to the fitness of environmental bacteria and pathogenic bacteria during their confrontation with an infected host. Highly efficient removal of threatening concentrations of these metals can be achieved by the combined actions of an inner membrane with a transenvelope efflux system, which removes periplasmic ions after their export from the cytoplasm to this compartment. To understand the resulting metal cation homeostasis in the periplasm, it is important to know if a regulatory circuit exists between periplasmic metal cations, their sensing, and the subsequent control of the expression of the transenvelope efflux system. This publication adds evidence to the hypothesis that two-component regulatory systems in control of the expression of genes for transenvelope efflux systems do indeed sense metal cations in the periplasm.

Copper Induces Protein Aggregation, a Toxic Process Compensated by Molecular Chaperones

Copper is well known for its antimicrobial and antiviral properties. Under aerobic conditions, copper toxicity relies in part on the production of reactive oxygen species (ROS), especially in the periplasmic compartment. However, copper is significantly more toxic under anaerobic conditions, in which ROS cannot be produced. This toxicity has been proposed to arise from the inactivation of proteins through mismetallations. Here, using the bacterium Escherichia coli, we discovered that copper treatment under anaerobic conditions leads to a significant increase in protein aggregation. In vitro experiments using E. coli lysates and tightly controlled redox conditions confirmed that treatment with Cu+ under anaerobic conditions leads to severe ROS-independent protein aggregation. Proteomic analysis of aggregated proteins revealed an enrichment of cysteine- and histidine-containing proteins in the Cu+-treated samples, suggesting that nonspecific interactions of Cu+ with these residues are likely responsible for the observed protein aggregation. In addition, E. coli strains lacking the cytosolic chaperone DnaK or trigger factor are highly sensitive to copper stress. These results reveal that bacteria rely on these chaperone systems to protect themselves against Cu-mediated protein aggregation and further support our finding that Cu toxicity is related to Cu-induced protein aggregation. Overall, our work provides new insights into the mechanism of Cu toxicity and the defense mechanisms that bacteria employ to survive. IMPORTANCE With the increase of antibiotic drug resistance, alternative antibacterial treatment strategies are needed. Copper is a well-known antimicrobial and antiviral agent; however, the underlying molecular mechanisms by which copper causes cell death are not yet fully understood. Herein, we report the finding that Cu+, the physiologically relevant copper species in bacteria, causes widespread protein aggregation. We demonstrate that the molecular chaperones DnaK and trigger factor protect bacteria against Cu-induced cell death, highlighting, for the first time, the central role of these chaperones under Cu+ stress. Our studies reveal Cu-induced protein aggregation to be a central mechanism of Cu toxicity, a finding that will serve to guide future mechanistic studies and drug development.

Copper Homeostatic Mechanisms and Their Role in the Virulence of Escherichia coli and Salmonella enterica

Copper is an essential micronutrient that also exerts toxic effects at high concentrations. This review summarizes the current state of knowledge on copper handling and homeostasis systems in Escherichia coli and Salmonella enterica. We describe the mechanisms by which transcriptional regulators, efflux pumps, detoxification enzymes, metallochaperones, and ancillary copper response systems orchestrate cellular response to copper stress. E. coli and S. enterica are important pathogens of humans and animals. We discuss the critical role of copper during killing of these pathogens by macrophages and in nutritional immunity at the bacterial-pathogen-host interface. In closing, we identify opportunities to advance our understanding of the biological roles of copper in these model enteric bacterial pathogens.

Mutant Strains of Escherichia coli and Methicillin-Resistant Staphylococcus aureus Obtained by Laboratory Selection To Survive on Metallic Copper Surfaces

Artificial laboratory evolution was used to produce mutant strains of Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA) able to survive on antimicrobial metallic copper surfaces. These mutants were 12- and 60-fold less susceptible to the copper-mediated contact killing process than their respective parent strains. Growth levels of the mutant and its parent in complex growth medium were similar. Tolerance to copper ions of the mutants was unchanged. The mutant phenotype remained stable over about 250 generations under nonstress conditions. The mutants and their respective parental strains accumulated copper released from the metallic surfaces to similar extents. Nevertheless, only the parental strains succumbed to copper stress when challenged on metallic copper surfaces, suffering complete destruction of the cell structure. Whole-genome sequencing and global transcriptome analysis were used to decipher the genetic alterations in the mutant strains; however, these results did not explain the copper-tolerance phenotypes on the systemic level. Instead, the mutants shared features with those of stressed bacterial subpopulations entering the early or "shallow" persister state. In contrast to the canonical persister state, however, the ability to survive on solid copper surfaces was adopted by the majority of the mutant strain population. This indicated that application of solid copper surfaces in hospitals and elsewhere has to be accompanied by strict cleaning regimens to keep the copper surfaces active and prevent evolution of tolerant mutant strains.IMPORTANCE Microbes are rapidly killed on solid copper surfaces by contact killing. Copper surfaces thus have an important role to play in preventing the spread of nosocomial infections. Bacteria adapt to challenging natural and clinical environments through evolutionary processes, for instance, by acquisition of beneficial spontaneous mutations. We wish to address the question of whether mutants can be selected that have evolved to survive contact killing on solid copper surfaces. We isolated such mutants from Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA) by artificial laboratory evolution. The ability to survive on solid copper surfaces was a stable phenotype of the mutant population and not restricted to a small subpopulation. As a consequence, standard operation procedures with strict hygienic measures are extremely important to prevent the emergence and spread of copper-surface-tolerant persister-like bacterial strains if copper surfaces are to be sustainably used to limit the spread of pathogenic bacteria, e.g., to curb nosocomial infections.

Role of Glutathione in Buffering Excess Intracellular Copper in Streptococcus pyogenes

Copper (Cu) is an essential metal for bacterial physiology but in excess it is bacteriotoxic. To limit Cu levels in the cytoplasm, most bacteria possess a transcriptionally responsive system for Cu export. In the Gram-positive human pathogen Streptococcus pyogenes (group A Streptococcus [GAS]), this system is encoded by the copYAZ operon. This study demonstrates that although the site of GAS infection represents a Cu-rich environment, inactivation of the copA Cu efflux gene does not reduce virulence in a mouse model of invasive disease. In vitro, Cu treatment leads to multiple observable phenotypes, including defects in growth and viability, decreased fermentation, inhibition of glyceraldehyde-3-phosphate dehydrogenase (GapA) activity, and misregulation of metal homeostasis, likely as a consequence of mismetalation of noncognate metal-binding sites by Cu. Surprisingly, the onset of these effects is delayed by ∼4 h even though expression of copZ is upregulated immediately upon exposure to Cu. Further biochemical investigations show that the onset of all phenotypes coincides with depletion of intracellular glutathione (GSH). Supplementation with extracellular GSH replenishes the intracellular pool of this thiol and suppresses all the observable effects of Cu treatment. These results indicate that GSH buffers excess intracellular Cu when the transcriptionally responsive Cu export system is overwhelmed. Thus, while the copYAZ operon is responsible for Cu homeostasis, GSH has a role in Cu tolerance and allows bacteria to maintain metabolism even in the presence of an excess of this metal ion.IMPORTANCE The control of intracellular metal availability is fundamental to bacterial physiology. In the case of copper (Cu), it has been established that rising intracellular Cu levels eventually fill the metal-sensing site of the endogenous Cu-sensing transcriptional regulator, which in turn induces transcription of a copper export pump. This response caps intracellular Cu availability below a well-defined threshold and prevents Cu toxicity. Glutathione, abundant in many bacteria, is known to bind Cu and has long been assumed to contribute to bacterial Cu handling. However, there is some ambiguity since neither its biosynthesis nor uptake is Cu-regulated. Furthermore, there is little experimental support for this physiological role of glutathione beyond measuring growth of glutathione-deficient mutants in the presence of Cu. Our work with group A Streptococcus provides new evidence that glutathione increases the threshold of intracellular Cu availability that can be tolerated by bacteria and thus advances fundamental understanding of bacterial Cu handling.

Bacterial response mechanism during biofilm growth on different metal material substrates: EPS characteristics, oxidative stress and molecular regulatory network analysis

Overwhelming growth of bacterial biofilms on different metal-based pipeline materials are intractable and pose a serious threat to public health when tap water flows though these pipelines. Indeed, the underlying mechanism of biofilm growth on the surface of different pipeline materials deserves detailed exploration to provide subsequent implementation strategies for biofilm control. Thus, in this study, how bacteria response to their encounters was explored, when they inhabit different metal-based pipeline substrates. Results revealed that bacteria proliferated when they grew on stainless steel (SS) and titanium sheet (Ti), quickly developing into bacterial biofilms. In contrast, the abundance of bacteria on copper (Cu) and nickel foam (Ni) substates decreased sharply by 4-5 logs within 24 h. The morphological shrinkage and shortening of bacterial cells, as well as a sudden 64-fold increase of carbohydrate content in extracellular polymeric substances (EPS), were observed on Cu substrate. Furthermore, generation of reactive oxygen species and fluctuation of enzymatic activity demonstrated the destruction of redox equilibrium in bacteria. Bacteria cultured on Cu substrate showed the strongest response, followed by Ni, SS and Ti. The oxidative stress increased quickly during the growth of bacterial biofilm, and almost all tested metal transporter-related genes were upregulated by 2-11 folds on Cu, which were higher than on other substrates (1-2 folds for SS and Ti, 2-9 folds for Ni). Finally, these behaviors were compared under the biofilm regulatory molecular network. This work may facilitate better understanding different response mechanisms during bacterial biofilm colonization on metal-based pipelines and provide implications for subsequent biofilm control.

The Role of Bacillithiol in Gram-Positive Firmicutes

SIGNIFICANCE

Since the discovery and structural characterization of bacillithiol (BSH), the biochemical functions of BSH-biosynthesis enzymes (BshA/B/C) and BSH-dependent detoxification enzymes (FosB, Bst, GlxA/B) have been explored in Bacillus and Staphylococcus species. It was shown that BSH plays an important role in detoxification of reactive oxygen and electrophilic species, alkylating agents, toxins, and antibiotics. Recent Advances: More recently, new functions of BSH were discovered in metal homeostasis (Zn buffering, Fe-sulfur cluster, and copper homeostasis) and virulence control in Staphylococcus aureus. Unexpectedly, strains of the S. aureus NCTC8325 lineage were identified as natural BSH-deficient mutants. Modern mass spectrometry-based approaches have revealed the global reach of protein S-bacillithiolation in Firmicutes as an important regulatory redox modification under hypochlorite stress. S-bacillithiolation of OhrR, MetE, and glyceraldehyde-3-phosphate dehydrogenase (Gap) functions, analogous to S-glutathionylation, as both a redox-regulatory device and in thiol protection under oxidative stress.

CRITICAL ISSUES

Although the functions of the bacilliredoxin (Brx) pathways in the reversal of S-bacillithiolations have been recently addressed, significantly more work is needed to establish the complete Brx reduction pathway, including the major enzyme(s), for reduction of oxidized BSH (BSSB) and the targets of Brx action in vivo.

FUTURE DIRECTIONS

Despite the large number of identified S-bacillithiolated proteins, the physiological relevance of this redox modification was shown for only selected targets and should be a subject of future studies. In addition, many more BSH-dependent detoxification enzymes are evident from previous studies, although their roles and biochemical mechanisms require further study. This review of BSH research also pin-points these missing gaps for future research. Antioxid. Redox Signal. 28, 445-462.

Transcriptome Profiling Reveals Interplay of Multifaceted Stress Response in Escherichia coli on Exposure to Glutathione and Ciprofloxacin

The emergence and spread of multidrug-resistant bacterial strains have serious medical and clinical consequences. In addition, the rate of discovery of new therapeutic antibiotics has been inadequate in last few decades. Fluoroquinolone antibiotics such as ciprofloxacin represent a precious therapeutic resource in the fight against bacterial pathogens. However, these antibiotics have been gradually losing their appeal due to the emergence and buildup of resistance to them. In this report, we shed light on the genome-level expression changes in bacteria with respect to glutathione (GSH) exposure which act as a trigger for fluoroquinolone antibiotic resistance. The knowledge about different bacterial stress response pathways under conditions of exposure to the conditions described above and potential points of cross talk between them could help us in understanding and formulating the conditions under which buildup and spread of antibiotic resistance could be minimized. Our findings are also relevant because GSH-induced genome-level expression changes have not been reported previously for E. coli.

We have previously reported that supplementation of exogenous glutathione (GSH) promotes ciprofloxacin resistance in Escherichia coli by neutralizing antibiotic-induced oxidative stress and by enhancing the efflux of antibiotic. In the present study, we used a whole-genome microarray as a tool to analyze the system-level transcriptomic changes of E. coli on exposure to GSH and/or ciprofloxacin. The microarray data revealed that GSH supplementation affects redox function, transport, acid shock, and virulence genes of E. coli. The data further highlighted the interplay of multiple underlying stress response pathways (including those associated with the genes mentioned above and DNA damage repair genes) at the core of GSH, offsetting the effect of ciprofloxacin in E. coli. The results of a large-scale validation of the transcriptomic data using reverse transcription-quantitative PCR (RT-qPCR) analysis for 40 different genes were mostly in agreement with the microarray results. The altered growth profiles of 12 different E. coli strains carrying deletions in the specific genes mentioned above with GSH and/or ciprofloxacin supplementation implicate these genes in the GSH-mediated phenotype not only at the molecular level but also at the functional level. We further associated GSH supplementation with increased acid shock survival of E. coli on the basis of our transcriptomic data. Taking the data together, it can be concluded that GSH supplementation influences the expression of genes of multiple stress response pathways apart from its effect(s) at the physiological level to counter the action of ciprofloxacin in E. coli.

IMPORTANCE The emergence and spread of multidrug-resistant bacterial strains have serious medical and clinical consequences. In addition, the rate of discovery of new therapeutic antibiotics has been inadequate in last few decades. Fluoroquinolone antibiotics such as ciprofloxacin represent a precious therapeutic resource in the fight against bacterial pathogens. However, these antibiotics have been gradually losing their appeal due to the emergence and buildup of resistance to them. In this report, we shed light on the genome-level expression changes in bacteria with respect to glutathione (GSH) exposure which act as a trigger for fluoroquinolone antibiotic resistance. The knowledge about different bacterial stress response pathways under conditions of exposure to the conditions described above and potential points of cross talk between them could help us in understanding and formulating the conditions under which buildup and spread of antibiotic resistance could be minimized. Our findings are also relevant because GSH-induced genome-level expression changes have not been reported previously for E. coli.

Synergistic gold–copper detoxification at the core of gold biomineralisation inCupriavidus metallidurans

Cupriavidus metalliduransescapes synergistic Cu/Au toxicity by re-oxidation of Au(i) back to Au(iii) using the periplasmic oxidase CopA.

The biological chemistry of the transition metal "transportome" of Cupriavidus metallidurans

This review tries to illuminate how the bacterium Cupriavidus metallidurans CH34 is able to allocate essential transition metal cations to their target proteins although these metals have similar charge-to-surface ratios and chemical features, exert toxic effects, compete with each other, and occur in the bacterial environment over a huge range of concentrations and speciations. Central to this ability is the "transportome", the totality of all interacting metal import and export systems, which, as an emergent feature, transforms the environmental metal content and speciation into the cellular metal mélange. In a kinetic flow equilibrium resulting from controlled uptake and efflux reactions, the periplasmic and cytoplasmic metal content is adjusted in a way that minimizes toxic effects. A central core function of the transportome is to shape the metal ion composition using high-rate and low-specificity reactions to avoid time and/or energy-requiring metal discrimination reactions. This core is augmented by metal-specific channels that may even deliver metals all the way from outside of the cell to the cytoplasm. This review begins with a description of the basic chemical features of transition metal cations and the biochemical consequences of these attributes, and which transition metals are available to C. metallidurans. It then illustrates how the environment influences the metal content and speciation, and how the transportome adjusts this metal content. It concludes with an outlook on the fate of metals in the cytoplasm. By generalization, insights coming from C. metallidurans shed light on multiple transition metal homoeostatic mechanisms in all kinds of bacteria including pathogenic species, where the "battle" for metals is an important part of the host-pathogen interaction.

Metallochaperones and metalloregulation in bacteria

Bacterial transition metal homoeostasis or simply 'metallostasis' describes the process by which cells control the intracellular availability of functionally required metal cofactors, from manganese (Mn) to zinc (Zn), avoiding both metal deprivation and toxicity. Metallostasis is an emerging aspect of the vertebrate host-pathogen interface that is defined by a 'tug-of-war' for biologically essential metals and provides the motivation for much recent work in this area. The host employs a number of strategies to starve the microbial pathogen of essential metals, while for others attempts to limit bacterial infections by leveraging highly competitive metals. Bacteria must be capable of adapting to these efforts to remodel the transition metal landscape and employ highly specialized metal sensing transcriptional regulators, termed metalloregulatory proteins,and metallochaperones, that allocate metals to specific destinations, to mediate this adaptive response. In this essay, we discuss recent progress in our understanding of the structural mechanisms and metal specificity of this adaptive response, focusing on energy-requiring metallochaperones that play roles in the metallocofactor active site assembly in metalloenzymes and metallosensors, which govern the systems-level response to metal limitation and intoxication.

Novel Metal Cation Resistance Systems from Mutant Fitness Analysis of Denitrifying Pseudomonas stutzeri

Metal ion transport systems have been studied extensively, but the specificity of a given transporter is often unclear from amino acid sequence data alone. In this study, predicted Cu(2+) and Zn(2+) resistance systems in Pseudomonas stutzeri strain RCH2 are compared with those experimentally implicated in Cu(2+) and Zn(2+) resistance, as determined by using a DNA-barcoded transposon mutant library. Mutant fitness data obtained under denitrifying conditions are combined with regulon predictions to yield a much more comprehensive picture of Cu(2+) and Zn(2+) resistance in strain RCH2. The results not only considerably expand what is known about well-established metal ion exporters (CzcCBA, CzcD, and CusCBA) and their accessory proteins (CzcI and CusF), they also reveal that isolates with mutations in some predicted Cu(2+) resistance systems do not show decreased fitness relative to the wild type when exposed to Cu(2+) In addition, new genes are identified that have no known connection to Zn(2+) (corB, corC, Psest_3226, Psest_3322, and Psest_0618) or Cu(2+) resistance (Mrp antiporter subunit gene, Psest_2850, and Psest_0584) but are crucial for resistance to these metal cations. Growth of individual deletion mutants lacking corB, corC, Psest_3226, or Psest_3322 confirmed the observed Zn-dependent phenotypes. Notably, to our knowledge, this is the first time a bacterial homolog of TMEM165, a human gene responsible for a congenital glycosylation disorder, has been deleted and the resulting strain characterized. Finally, the fitness values indicate Cu(2+)- and Zn(2+)-based inhibition of nitrite reductase and interference with molybdenum cofactor biosynthesis for nitrate reductase. These results extend the current understanding of Cu(2+) and Zn(2+) efflux and resistance and their effects on denitrifying metabolism.

IMPORTANCE

In this study, genome-wide mutant fitness data in P. stutzeri RCH2 combined with regulon predictions identify several proteins of unknown function that are involved in resisting zinc and copper toxicity. For zinc, these include a member of the UPF0016 protein family that was previously implicated in Ca(2+)/H(+) antiport and a human congenital glycosylation disorder, CorB and CorC, which were previously linked to Mg(2+) transport, and Psest_3322 and Psest_0618, two proteins with no characterized homologs. Experiments using mutants lacking Psest_3226, Psest_3322, corB, corC, or czcI verified their proposed functions, which will enable future studies of these little-characterized zinc resistance determinants. Likewise, Psest_2850, annotated as an ion antiporter subunit, and the conserved hypothetical protein Psest_0584 are implicated in copper resistance. Physiological connections between previous studies and phenotypes presented here are discussed. Functional and mechanistic understanding of transport proteins improves the understanding of systems in which members of the same protein family, including those in humans, can have different functions.

Mechanism of Attenuation of Uranyl Toxicity by Glutathione in Lactococcus lactis

Both prokaryotic and eukaryotic organisms possess mechanisms for the detoxification of heavy metals, and these mechanisms are found among distantly related species. We investigated the role of intracellular glutathione (GSH), which, in a large number of taxa, plays a role in protection against the toxicity of common heavy metals. Anaerobically grown Lactococcus lactis containing an inducible GSH synthesis pathway was used as a model organism. Its physiological condition allowed study of putative GSH-dependent uranyl detoxification mechanisms without interference from additional reactive oxygen species. By microcalorimetric measurements of metabolic heat during cultivation, it was shown that intracellular GSH attenuates the toxicity of uranium at a concentration in the range of 10 to 150 μM. In this concentration range, no effect was observed with copper, which was used as a reference for redox metal toxicity. At higher copper concentrations, GSH aggravated metal toxicity. Isothermal titration calorimetry revealed the endothermic binding of U(VI) to the carboxyl group(s) of GSH rather than to the reducing thiol group involved in copper interactions. The data indicate that the primary detoxifying mechanism is the intracellular sequestration of carboxyl-coordinated U(VI) into an insoluble complex with GSH. The opposite effects on uranyl and on copper toxicity can be related to the difference in coordination chemistry of the respective metal-GSH complexes, which cause distinct growth phase-specific effects on enzyme-metal interactions.

IMPORTANCE

Understanding microbial metal resistance is of particular importance for bioremediation, where microorganisms are employed for the removal of heavy metals from the environment. This strategy is increasingly being considered for uranium. However, little is known about the molecular mechanisms of uranyl detoxification. Existing studies of different taxa show little systematics but hint at a role of glutathione (GSH). Previous work could not unequivocally demonstrate a GSH function in decreasing the presumed uranyl-induced oxidative stress, nor could a redox-independent detoxifying action of GSH be identified. Combining metabolic calorimetry with cell number-based assays and genetics analysis enables a novel and general approach to quantify toxicity and relate it to molecular mechanisms. The results show that GSH-expressing microorganisms appear advantageous for uranyl bioremediation.