Physiologically relevant small phosphodonors link metabolism to signal transduction

Recent advances in the development of bacterial response regulators inhibitors as antibacterial and/or antibiotic adjuvant agent: A new approach to combat bacterial resistance

The number of new antibacterial agents currently being discovered is insufficient to combat bacterial resistance. It is extremely challenging to find new antibiotics and to introduce them to the pharmaceutical market. Therefore, special attention must be given to find new strategies to combat bacterial resistance and prevent bacteria from developing resistance. Two-component system is a transduction system and the most prevalent mechanism employed by bacteria to respond to environmental changes. This signaling system consists of a membrane sensor histidine kinase that perceives environmental stimuli and a response regulator which acts as a transcription factor. The approach consisting of developing response regulators inhibitors with antibacterial activity or antibiotic adjuvant activity is a novel approach that has never been previously reviewed. In this review we report for the first time, the importance of targeting response regulators and summarizing all existing studies carried out from 2008 until now on response regulators inhibitors as antibacterial agents or / and antibiotic adjuvants. Moreover, we describe the antibacterial activity and/or antibiotic adjuvants activity against the studied bacterial strains and the mechanism of different response regulator inhibitors when it's possible.

Unveiling the molecular mechanisms of the type IX secretion system's response regulator: Structural and functional insights

The type IX secretion system (T9SS) is a nanomachinery utilized by bacterial pathogens to facilitate infection. The system is regulated by a signaling cascade serving as its activation switch. A pivotal member in this cascade, the response regulator protein PorX, represents a promising drug target to prevent the secretion of virulence factors. Here, we provide a comprehensive characterization of PorX both in vitro and in vivo. First, our structural studies revealed PorX harbors a unique enzymatic effector domain, which, surprisingly, shares structural similarities with the alkaline phosphatase superfamily, involved in nucleotide and lipid signaling pathways. Importantly, such pathways have not been associated with the T9SS until now. Enzymatic characterization of PorX's effector domain revealed a zinc-dependent phosphodiesterase activity, with active site dimensions suitable to accommodate a large substrate. Unlike typical response regulators that dimerize via their receiver domain upon phosphorylation, we found that zinc can also induce conformational changes and promote PorX's dimerization via an unexpected interface. These findings suggest that PorX can serve as a cellular zinc sensor, broadening our understanding of its regulatory mechanisms. Despite the strict conservation of PorX in T9SS-utilizing bacteria, we demonstrate that PorX is essential for virulence factors secretion in Porphyromonas gingivalis and affects metabolic enzymes secretion in the nonpathogenic Flavobacterium johnsoniae, but not for the secretion of gliding adhesins. Overall, this study advances our structural and functional understanding of PorX, highlighting its potential as a druggable target for intervention strategies aimed at disrupting the T9SS and mitigating virulence in pathogenic species.

Unveiling the Molecular Mechanisms of the Type-IX Secretion System's Response Regulator: Structural and Functional Insights

The Type-IX secretion system (T9SS) is a nanomachinery utilized by bacterial pathogens to facilitate infection. The system is regulated by a signaling cascade serving as its activation switch. A pivotal member in this cascade, the response regulator protein PorX, represents a promising drug target to prevent the secretion of virulence factors. Here, we provide a comprehensive characterization of PorX both in vitro and in vivo . First, our structural studies revealed PorX harbours a unique enzymatic effector domain, which, surprisingly, shares structural similarities with the alkaline phosphatase superfamily, involved in nucleotide and lipid signaling pathways. Importantly, such pathways have not been associated with the T9SS until now. Enzymatic characterization of PorX's effector domain revealed a zinc-dependent phosphodiesterase activity, with active site dimensions suitable to accommodate a large substrate. Unlike typical response regulators that dimerize via their receiver domain upon phosphorylation, we found that zinc can also induce conformational changes and promote PorX's dimerization via an unexpected interface. These findings suggest that PorX can serve as a cellular zinc sensor, broadening our understanding of its regulatory mechanisms. Despite the strict conservation of PorX in T9SS-utilizing bacteria, we demonstrate that PorX is essential for virulence factors secretion in Porphyromonas gingivalis and affects metabolic enzymes secretion in the non-pathogenic Flavobacterium johnsoniae , but not for the secretion of gliding adhesins. Overall, this study advances our structural and functional understanding of PorX, highlighting its potential as a druggable target for intervention strategies aimed at disrupting the T9SS and mitigating virulence in pathogenic species.

Multifaceted regulation of siderophore synthesis by multiple regulatory systems in Shewanella oneidensis

Siderophore-dependent iron uptake is a mechanism by which microorganisms scavenge and utilize iron for their survival, growth, and many specialized activities, such as pathogenicity. The siderophore biosynthetic system PubABC in Shewanella can synthesize a series of distinct siderophores, yet how it is regulated in response to iron availability remains largely unexplored. Here, by whole genome screening we identify TCS components histidine kinase (HK) BarA and response regulator (RR) SsoR as positive regulators of siderophore biosynthesis. While BarA partners with UvrY to mediate expression of pubABC post-transcriptionally via the Csr regulatory cascade, SsoR is an atypical orphan RR of the OmpR/PhoB subfamily that activates transcription in a phosphorylation-independent manner. By combining structural analysis and molecular dynamics simulations, we observe conformational changes in OmpR/PhoB-like RRs that illustrate the impact of phosphorylation on dynamic properties, and that SsoR is locked in the 'phosphorylated' state found in phosphorylation-dependent counterparts of the same subfamily. Furthermore, we show that iron homeostasis global regulator Fur, in addition to mediating transcription of its own regulon, acts as the sensor of iron starvation to increase SsoR production when needed. Overall, this study delineates an intricate, multi-tiered transcriptional and post-transcriptional regulatory network that governs siderophore biosynthesis.

Pleiotropic Effects of PhaR Regulator in Bradyrhizobium diazoefficiens Microaerobic Metabolism

Bradyrhizobium diazoefficiens can live inside soybean root nodules and in free-living conditions. In both states, when oxygen levels decrease, cells adjust their protein pools by gene transcription modulation. PhaR is a transcription factor involved in polyhydroxyalkanoate (PHA) metabolism but also plays a role in the microaerobic network of this bacterium. To deeply uncover the function of PhaR, we applied a multipronged approach, including the expression profile of a phaR mutant at the transcriptional and protein levels under microaerobic conditions, and the identification of direct targets and of proteins associated with PHA granules. Our results confirmed a pleiotropic function of PhaR, affecting several phenotypes, in addition to PHA cycle control. These include growth deficiency, regulation of carbon and nitrogen allocation, and bacterial motility. Interestingly, PhaR may also modulate the microoxic-responsive regulatory network by activating the expression of fixK2 and repressing nifA, both encoding two transcription factors relevant for microaerobic regulation. At the molecular level, two PhaR-binding motifs were predicted and direct control mediated by PhaR determined by protein-interaction assays revealed seven new direct targets for PhaR. Finally, among the proteins associated with PHA granules, we found PhaR, phasins, and other proteins, confirming a dual function of PhaR in microoxia.

Acetic acid stress response of the acidophilic sulfate reducer Acididesulfobacillus acetoxydans

Acid mine drainage (AMD) waters are a severe environmental threat, due to their high metal content and low pH (pH <3). Current technologies treating AMD utilize neutrophilic sulfate-reducing microorganisms (SRMs), but acidophilic SRM could offer advantages. As AMDs are low in organics these processes require electron donor addition, which is often incompletely oxidized into organic acids (e.g., acetic acid). At low pH, acetic acid is undissociated and toxic to microorganisms. We investigated the stress response of the acetotrophic Acididesulfobacillus acetoxydans to acetic acid. A. acetoxydans was cultivated in bioreactors at pH 5.0 (optimum). For stress experiments, triplicate reactors were spiked until 7.5 mM of acetic acid and compared with (non-spiked) triplicate reactors for physiological, transcriptomic, and membrane lipid changes. After acetic acid spiking, the optical density initially dropped, followed by an adaptation phase during which growth resumed at a lower growth rate. Transcriptome analysis revealed a downregulation of genes involved in glutamate and aspartate synthesis following spiking. Membrane lipid analysis revealed a decrease in iso and anteiso fatty acid relative abundance; and an increase of acetyl-CoA as a fatty acid precursor. These adaptations allow A. acetoxydans to detoxify acetic acid, creating milder conditions for other microorganisms in AMD environments.

Using a synthetic machinery to improve carbon yield with acetylphosphate as the core

In microbial cell factory, CO2 release during acetyl-CoA production from pyruvate significantly decreases the carbon atom economy. Here, we construct and optimize a synthetic carbon conserving pathway named as Sedoheptulose-1,7-bisphosphatase Cycle with Trifunctional PhosphoKetolase (SCTPK) in Escherichia coli. This cycle relies on a generalist phosphoketolase Xfspk and converts glucose into the stoichiometric amounts of acetylphosphate (AcP). Furthermore, genetic circuits responding to AcP positively or negatively are created. Together with SCTPK, they constitute a gene-metabolic oscillator that regulates Xfspk and enzymes converting AcP into valuable chemicals in response to intracellular AcP level autonomously, allocating metabolic flux rationally and improving the carbon atom economy of bioconversion process. Using this synthetic machinery, mevalonate is produced with a yield higher than its native theoretical yield, and the highest titer and yield of 3-hydroxypropionate via malonyl-CoA pathway are achieved. This study provides a strategy for improving the carbon yield of microbial cell factories.

Phosphorylation chemistry of the Bordetella PlrSR TCS and its contribution to bacterial persistence in the lower respiratory tract

Bordetella species cause lower respiratory tract infections in mammals. B. pertussis and B. bronchiseptica are the causative agents of whooping cough and kennel cough, respectively. The current acellular vaccine for B. pertussis protects against disease but does not prevent transmission or colonization. Cases of pertussis are on the rise even in areas of high vaccination. The PlrSR two-component system, is required for persistence in the mouse lung. A partial plrS deletion strain and a plrS H521Q strain cannot survive past 3 days in the lung, suggesting PlrSR works in a phosphorylation-dependent mechanism. We characterized the biochemistry of B. bronchiseptica PlrSR and found that both proteins function as a canonical two-component system. His521 was essential and Glu522 was critical for PlrS autophosphorylation. Asn525 was essential for phosphatase activity. The PAS domain was critical for both PlrS autophosphorylation and phosphatase activities. PlrS could both phosphotransfer to and exert phosphatase activity toward PlrR. Unexpectedly, PlrR formed a tetramer when unphosphorylated and a dimer upon phosphorylation. Finally, we demonstrated the importance of PlrS phosphatase activity for persistence within the murine lung. By characterizing PlrSR we hope to guide future in vivo investigation for development of new vaccines and therapeutics.

Serine-threonine phosphoregulation by PknB and Stp contributes to quiescence and antibiotic tolerance in Staphylococcus aureus

Staphylococcus aureus can cause infections that are often chronic and difficult to treat, even when the bacteria are not antibiotic resistant because most antibiotics act only on metabolically active cells. Subpopulations of persister cells are metabolically quiescent, a state associated with delayed growth, reduced protein synthesis, and increased tolerance to antibiotics. Serine-threonine kinases and phosphatases similar to those found in eukaryotes can fine-tune essential bacterial cellular processes, such as metabolism and stress signaling. We found that acid stress-mimicking conditions that S. aureus experiences in host tissues delayed growth, globally altered the serine and threonine phosphoproteome, and increased threonine phosphorylation of the activation loop of the serine-threonine protein kinase B (PknB). The deletion of stp, which encodes the only annotated functional serine-threonine phosphatase in S. aureus, increased the growth delay and phenotypic heterogeneity under different stress challenges, including growth in acidic conditions, the intracellular milieu of human cells, and abscesses in mice. This growth delay was associated with reduced protein translation and intracellular ATP concentrations and increased antibiotic tolerance. Using phosphopeptide enrichment and mass spectrometry-based proteomics, we identified targets of serine-threonine phosphorylation that may regulate bacterial growth and metabolism. Together, our findings highlight the importance of phosphoregulation in mediating bacterial quiescence and antibiotic tolerance and suggest that targeting PknB or Stp might offer a future therapeutic strategy to prevent persister formation during S. aureus infections.

Response regulator PorX coordinates oligonucleotide signalling and gene expression to control the secretion of virulence factors

The PglZ family of proteins belongs to the alkaline phosphatase superfamily, which consists of metallohydrolases with limited sequence identity but similar metal-coordination architectures in otherwise divergent active sites. Proteins with a well-defined PglZ domain are ubiquitous among prokaryotes as essential components of BREX phage defence systems and two-component systems (TCSs). Whereas other members of the alkaline phosphatase superfamily are well characterized, the activity, structure and biological function of PglZ family proteins remain unclear. We therefore investigated the structure and function of PorX, an orphan response regulator of the Porphyromonas gingivalis TCS containing a putative PglZ effector domain. The crystal structure of PorX revealed a canonical receiver domain, a helical bundle, and an unprecedented PglZ domain, similar to the general organization of the phylogenetically related BREX-PglZ proteins. The PglZ domain of PorX features an active site cleft suitable for large substrates. An extensive search for substrates revealed that PorX is a phosphodiesterase that acts on cyclic and linear oligonucleotides, including signalling molecules such as cyclic oligoadenylates. These results, combined with mutagenesis, biophysical and enzymatic analysis, suggest that PorX coordinates oligonucleotide signalling pathways and indirectly regulates gene expression to control the secretion of virulence factors.

A prebiotic basis for ATP as the universal energy currency

ATP is universally conserved as the principal energy currency in cells, driving metabolism through phosphorylation and condensation reactions. Such deep conservation suggests that ATP arose at an early stage of biochemical evolution. Yet purine synthesis requires 6 phosphorylation steps linked to ATP hydrolysis. This autocatalytic requirement for ATP to synthesize ATP implies the need for an earlier prebiotic ATP equivalent, which could drive protometabolism before purine synthesis. Why this early phosphorylating agent was replaced, and specifically with ATP rather than other nucleoside triphosphates, remains a mystery. Here, we show that the deep conservation of ATP might reflect its prebiotic chemistry in relation to another universally conserved intermediate, acetyl phosphate (AcP), which bridges between thioester and phosphate metabolism by linking acetyl CoA to the substrate-level phosphorylation of ADP. We confirm earlier results showing that AcP can phosphorylate ADP to ATP at nearly 20% yield in water in the presence of Fe3+ ions. We then show that Fe3+ and AcP are surprisingly favoured. A wide range of prebiotically relevant ions and minerals failed to catalyse ADP phosphorylation. From a panel of prebiotic phosphorylating agents, only AcP, and to a lesser extent carbamoyl phosphate, showed any significant phosphorylating potential. Critically, AcP did not phosphorylate any other nucleoside diphosphate. We use these data, reaction kinetics, and molecular dynamic simulations to infer a possible mechanism. Our findings might suggest that the reason ATP is universally conserved across life is that its formation is chemically favoured in aqueous solution under mild prebiotic conditions.

The phytopathogen Dickeya dadantii 3937 cpxR locus gene participates in the regulation of virulence and the global c-di-GMP network

Bacteria use signal transduction systems to sense and respond to their external environment. The two-component system CpxA/CpxR senses misfolded envelope protein stress and responds by up-regulating envelope protein factors and down-regulating virulence factors in several animal pathogens. Dickeya dadantii is a phytopathogen equipped with a type III secretion system (T3SS) for manipulating the host immune response. We found that deletion of cpxR enhanced the expression of the T3SS marker gene hrpA in a designated T3SS-inducing minimal medium (MM). In the ∆cpxR mutant, multiple T3SS and c-di-GMP regulators were also up-regulated. Subsequent analysis revealed that deletion of the phosphodiesterase gene egcpB in ∆cpxR abolished the enhanced T3SS expression. This suggested that CpxR suppresses EGcpB levels, causing low T3SS expression in MM. Furthermore, we found that the ∆cpxR mutant displayed low c-di-GMP phenotypes in biofilm formation and swimming. Increased production of cellular c-di-GMP by in trans expression of the diguanylate cyclase gene gcpA was negated in the ∆cpxR mutant. Here, we propose that CpxA/CpxR regulates T3SS expression by manipulating the c-di-GMP network, in turn modifying the multiple physiological activities involved in the response to environmental stresses in D. dadantii.

The Vibrio cholerae master regulator for the activation of biofilm biogenesis genes, VpsR, senses both cyclic di-GMP and phosphate

Vibrio cholerae biofilm formation/maintenance is controlled by myriad factors; chief among these are the regulator VpsR and cyclic di-guanosine monophosphate (c-di-GMP). VpsR has strong sequence similarity to enhancer binding proteins (EBPs) that activate RNA polymerase containing sigma factor σ54. However, we have previously shown that transcription from promoters within the biofilm biogenesis/maintenance pathways uses VpsR, c-di-GMP and RNA polymerase containing the primary sigma factor (σ70). Previous work suggested that phosphorylation of VpsR at a highly conserved aspartate, which is phosphorylated in other EBPs, might also contribute to activation. Using the biofilm biogenesis promoter PvpsL, we show that in the presence of c-di-GMP, either wild type or the phospho-mimic VpsR D59E activates PvpsL transcription, while the phospho-defective D59A variant does not. Furthermore, when c-di-GMP levels are low, acetyl phosphate (Ac∼P) is required for significant VpsR activity in vivo and in vitro. Although these findings argue that VpsR phosphorylation is needed for activation, we show that VpsR is not phosphorylated or acetylated by Ac∼P and either sodium phosphate or potassium phosphate, which are not phosphate donors, fully substitutes for Ac∼P. We conclude that VpsR is an unusual regulator that senses phosphate directly, rather than through phosphorylation, to aid in the decision to form/maintain biofilm.

Advances and prospects in metabolic engineering of Escherichia coli for L-tryptophan production

As an important raw material for pharmaceutical, food and feed industry, highly efficient production of L-tryptophan by Escherichia coli has attracted a considerable attention. However, there are complicated and multiple layers of regulation networks in L-tryptophan biosynthetic pathway and thus have difficulty to rewrite the biosynthetic pathway for producing L-tryptophan with high efficiency in E. coli. This review summarizes the biosynthetic pathway of L-tryptophan and highlights the main regulatory mechanisms in E. coli. In addition, we discussed the latest metabolic engineering strategies achieved in E. coli to reconstruct the L-tryptophan biosynthetic pathway. Moreover, we also review a few strategies that can be used in E. coli to improve robustness and streamline of L-tryptophan high-producing strains. Lastly, we also propose the potential strategies to further increase L-tryptophan production by systematic metabolic engineering and synthetic biology techniques.

Bacterial protein acetylation and its role in cellular physiology and metabolic regulation

Protein acetylation is an evolutionarily conserved posttranslational modification. It affects enzyme activity, metabolic flux distribution, and other critical physiological and biochemical processes by altering protein size and charge. Protein acetylation may thus be a promising tool for metabolic regulation to improve target production and conversion efficiency in fermentation. Here we review the role of protein acetylation in bacterial physiology and metabolism and describe applications of protein acetylation in fermentation engineering and strategies for regulating acetylation status. Although protein acetylation has become a hot topic, the regulatory mechanisms have not been fully characterized. We propose future research directions in protein acetylation.

How the PhoP/PhoQ System Controls Virulence and Mg2+ Homeostasis: Lessons in Signal Transduction, Pathogenesis, Physiology, and Evolution

The PhoP/PhoQ two-component system governs virulence, Mg2+ homeostasis, and resistance to a variety of antimicrobial agents, including acidic pH and cationic antimicrobial peptides, in several Gram-negative bacterial species. Best understood in Salmonella enterica serovar Typhimurium, the PhoP/PhoQ system consists o-regulated gene products alter PhoP-P amounts, even under constant inducing conditions. PhoP-P controls the abundance of hundreds of proteins both directly, by having transcriptional effects on the corresponding genes, and indirectly, by modifying the abundance, activity, or stability of other transcription factors, regulatory RNAs, protease regulators, and metabolites. The investigation of PhoP/PhoQ has uncovered novel forms of signal transduction and the physiological consequences of regulon evolution.

Protein acetyltransferases mediate bacterial adaptation to a diverse environment

Protein lysine acetylation is a conserved post-translational modification that modulates several cellular processes. Protein acetylation and its physiological implications are well understood in eukaryotes; however, its role is emerging in bacteria. Lysine acetylation in bacteria is fine-tuned by the concerted action of lysine acetyltransferases (KATs), protein deacetylases (KDACs), metabolic intermediates- acetyl-coenzyme A (Ac-CoA) and acetyl phosphate (AcP). AcP mediated nonenzymatic acetylation is predominant in bacteria due to its high acetyl transfer potential whereas, enzymatic acetylation by bacterial KATs (bKAT) are considered less abundant. SePat, the first bKAT discovered in Salmonella enterica, regulates the activity of the central metabolic enzyme- acetyl-CoA synthetase, through its acetylation. Recent studies have highlighted the role of bKATs in stress responses like pH tolerance, nutrient stress, persister cell formation, antibiotic resistance and pathogenesis. Bacterial genomes encode many putative bKATs of unknown biological function and significance. Detailed characterization of putative and partially characterized bKATs is important to decipher the acetylation mediated regulation in bacteria. Proper synthesis of information about the diverse roles of bKATs is missing to date, which can lead to the discovery of new antimicrobial targets in future. In this review, we provide an overview of the diverse physiological roles of known bKATs, and their mode of regulation in different bacteria. We also highlight existing gaps in the literature and present questions that may help understand the regulatory mechanisms mediated by bKATs in adaptation to a diverse habitat.

Role of Position K+4 in the Phosphorylation and Dephosphorylation Reaction Kinetics of the CheY Response Regulator

Two-component signaling is a primary method by which microorganisms interact with their environments. A kinase detects stimuli and modulates autophosphorylation activity. The signal propagates by phosphotransfer from the kinase to a response regulator, eliciting a response. Response regulators operate over a range of time scales, corresponding to their related biological processes. Response regulator active site chemistry is highly conserved, but certain variable residues can influence phosphorylation kinetics. An Ala-to-Pro substitution (K+4, residue 113) in the Escherichia coli response regulator CheY triggers a constitutively active phenotype; however, the A113P substitution is too far from the active site to directly affect phosphochemistry. To better understand the activating mechanism(s) of the substitution, we analyzed receiver domain sequences to characterize the evolutionary role of the K+4 position. Although most featured Pro, Leu, Ile, and Val residues, chemotaxis-related proteins exhibited atypical Ala, Gly, Asp, and Glu residues at K+4. Structural and in silico analyses revealed that CheY A113P adopted a partially active configuration. Biochemical data showed that A113P shifted CheY toward a more activated state, enhancing autophosphorylation. By characterizing CheY variants, we determined that this functionality was transmitted through a hydrophobic network bounded by the β5α5 loop and the α1 helix of CheY. This region also interacts with the phosphodonor CheA_P1, suggesting that binding generates an activating perturbation similar to the A113P substitution. Atypical residues like Ala at the K+4 position likely serve two purposes. First, restricting autophosphorylation may minimize background noise generated by intracellular phosphodonors such as acetyl phosphate. Second, optimizing interactions with upstream partners may help prime the receiver domain for phosphorylation.

Acetylation of Response Regulator Protein MtrA in M. tuberculosis Regulates Its Repressor Activity

MtrA is an essential response regulator (RR) protein in M. tuberculosis, and its activity is modulated after phosphorylation from its sensor kinase MtrB. Interestingly, many regulatory effects of MtrA have been reported to be independent of its phosphorylation, thereby suggesting alternate mechanisms of regulation of the MtrAB two-component system in M. tuberculosis. Here, we show that RR MtrA undergoes non-enzymatic acetylation through acetyl phosphate, modulating its activities independent of its phosphorylation status. Acetylated MtrA shows increased phosphorylation and enhanced interaction with SK MtrB assessed by phosphotransfer assays and FRET analysis. We also observed that acetylated MtrA loses its DNA-binding ability on gene targets that are otherwise enhanced by phosphorylation. More interestingly, acetylation is the dominant post-translational modification, overriding the effect of phosphorylation. Evaluation of the impact of MtrA and its lysine mutant overexpression on the growth of H37Ra bacteria under different conditions along with the infection studies on alveolar epithelial cells further strengthens the importance of acetylated MtrA protein in regulating the growth of M. tuberculosis. Overall, we show that both acetylation and phosphorylation regulate the activities of RR MtrA on different target genomic regions. We propose here that, although phosphorylation-dependent binding of MtrA drives its repressor activity on oriC and rpf, acetylation of MtrA turns this off and facilitates division in mycobacteria. Our findings, thus, reveal a more complex regulatory role of RR proteins in which multiple post-translational modifications regulate the activities at the levels of interaction with SK and the target gene expression.

Regulator RcsB Controls Prodigiosin Synthesis and Various Cellular Processes in Serratia marcescens JNB5-1

Prodigiosin (PG), a red linear tripyrrole pigment normally secreted by Serratia marcescens, has received attention for its reported immunosuppressive, antimicrobial, and anticancer properties. Although several genes have been shown to be important for prodigiosin synthesis, information on the regulatory mechanisms behind this cellular process remains limited. In this work, we identified that the transcriptional regulator RcsB encoding gene BVG90_13250 (rcsB) negatively controlled prodigiosin biosynthesis in S. marcescens Disruption of rcsB conferred a remarkably increased production of prodigiosin. This phenotype corresponded to negative control of transcription of the prodigiosin-associated pig operon by RcsB, probably by binding to the promoter region of the prodigiosin synthesis positive regulator FlhDC. Moreover, using transcriptomics and further experiments, we revealed that RcsB also controlled some other important cellular processes, including swimming and swarming motilities, capsular polysaccharide production, biofilm formation, and acid resistance (AR), in S. marcescens Collectively, this work proposes that RcsB is a prodigiosin synthesis repressor in S. marcescens and provides insight into the regulatory mechanism of RcsB in cell motility, capsular polysaccharide production, and acid resistance in S. marcescens IMPORTANCE RcsB is a two-component response regulator in the Rcs phosphorelay system, and it plays versatile regulatory functions in Enterobacteriaceae However, information on the function of the RcsB protein in bacteria, especially in S. marcescens, remains limited. In this work, we illustrated experimentally that the RcsB protein was involved in diverse cellular processes in S. marcescens, including prodigiosin synthesis, cell motility, capsular polysaccharide production, biofilm formation, and acid resistance. Additionally, the regulatory mechanism of the RcsB protein in these cellular processes was investigated. In conclusion, this work indicated that RcsB could be a regulator for prodigiosin synthesis and provides insight into the function of the RcsB protein in S. marcescens.

Identification of Z nucleotides as an ancient signal for two-component system activation in bacteria

Two-component systems (TCSs) in bacteria are molecular circuits that allow the perception of and response to diverse stimuli. These signaling circuits rely on phosphoryl-group transfers between transmitter and receiver domains of sensor kinase and response regulator proteins, and regulate several cellular processes in response to internal or external cues. Phosphorylation, and thereby activation, of response regulators has been demonstrated to occur by their cognate histidine kinases but also by low molecular weight phosphodonors such as acetyl phosphate and carbamoyl phosphate. Here, we present data indicating that the intermediates of the de novo syntheses of purines and histidine, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-monophosphate (ZMP) and/or 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-triphosphate (ZTP), activate the response regulator UvrY, by promoting its autophosphorylation at the conserved aspartate at position 54. Moreover, these Z nucleotides are shown to also activate the nonrelated response regulators ArcA, CpxR, RcsB, and PhoQ. We propose that ZMP and/or ZTP act as alarmones for a wide range of response regulators in vivo, providing a novel mechanism by which they could impact gene expression in response to metabolic cues.

Regulation of gene expression by protein lysine acetylation in Salmonella

Protein lysine acetylation influences many physiological functions, such as gene regulation, metabolism, and disease in eukaryotes. Although little is known about the role of lysine acetylation in bacteria, several reports have proposed its importance in various cellular processes. Here, we discussed the function of the protein lysine acetylation and the post-translational modifications (PTMs) of histone-like proteins in bacteria focusing on Salmonella pathogenicity. The protein lysine residue in Salmonella is acetylated by the Pat-mediated enzymatic pathway or by the acetyl phosphate-mediated non-enzymatic pathway. In Salmonella, the acetylation of lysine 102 and lysine 201 on PhoP inhibits its protein activity and DNA-binding, respectively. Lysine acetylation of the transcriptional regulator, HilD, also inhibits pathogenic gene expression. Moreover, it has been reported that the protein acetylation patterns significantly differ in the drug-resistant and -sensitive Salmonella strains. In addition, nucleoid-associated proteins such as histone-like nucleoid structuring protein (H-NS) are critical for the gene silencing in bacteria, and PTMs in H-NS also affect the gene expression. In this review, we suggest that protein lysine acetylation and the post-translational modifications of H-NS are important factors in understanding the regulation of gene expression responsible for pathogenicity in Salmonella.

Progress Overview of Bacterial Two-Component Regulatory Systems as Potential Targets for Antimicrobial Chemotherapy

Bacteria adapt to changes in their environment using a mechanism known as the two-component regulatory system (TCS) (also called “two-component signal transduction system” or “two-component system”). It comprises a pair of at least two proteins, namely the sensor kinase and the response regulator. The former senses external stimuli while the latter alters the expression profile of bacterial genes for survival and adaptation. Although the first TCS was discovered and characterized in a non-pathogenic laboratory strain of Escherichia coli, it has been recognized that all bacteria, including pathogens, use this mechanism. Some TCSs are essential for cell growth and fitness, while others are associated with the induction of virulence and drug resistance/tolerance. Therefore, the TCS is proposed as a potential target for antimicrobial chemotherapy. This concept is based on the inhibition of bacterial growth with the substances acting like conventional antibiotics in some cases. Alternatively, TCS targeting may reduce the burden of bacterial virulence and drug resistance/tolerance, without causing cell death. Therefore, this approach may aid in the development of antimicrobial therapeutic strategies for refractory infections caused by multi-drug resistant (MDR) pathogens. Herein, we review the progress of TCS inhibitors based on natural and synthetic compounds.

Modulation of Response Regulator CheY Reaction Kinetics by Two Variable Residues That Affect Conformation

Microorganisms and plants utilize two-component systems to regulate adaptive responses to changing environmental conditions. Sensor kinases detect stimuli and alter their autophosphorylation activity accordingly. Signal propagation occurs via the transfer of phosphoryl groups from upstream kinases to downstream response regulator proteins. Removal of phosphoryl groups from the response regulator typically resets the system. Members of the same protein family may catalyze phosphorylation and dephosphorylation reactions with different efficiencies, exhibiting rate constants spanning many orders of magnitude to accommodate response time scales from milliseconds to days. We previously found that variable positions one or two residues to the C-terminal side of the conserved Asp phosphorylation site (D+2) or Thr/Ser (T+1/T+2) in response regulators alter reaction kinetics by direct interaction with phosphodonor or phosphoacceptor molecules. Here, we explore the kinetic effects of amino acid substitutions at the two positions immediately C-terminal to the conserved Lys (K+1/K+2) in the model Escherichia coli response regulator CheY. We measured CheY autophosphorylation and autodephosphorylation rate constants for 27 pairs of K+1/K+2 residues that represent 84% of naturally occurring response regulators. Effects on autodephosphorylation were modest, but autophosphorylation rate constants varied by 2 orders of magnitude, suggesting that the K+1/K+2 positions influence reaction kinetics by altering the conformational spectrum sampled by CheY at equilibrium. Additional evidence supporting this indirect mechanism includes the following: the effect on autophosphorylation rate constants is independent of the phosphodonor, the autophosphorylation rate constants and dissociation constants for the phosphoryl group analog BeF₃ ^- are inversely correlated, and the K+1/K+2 positions are distant from the phosphorylation site.IMPORTANCE We have identified five variable positions in response regulators that allow the rate constants of autophosphorylation and autodephosporylation reactions each to be altered over 3 orders of magnitude in CheY. The distributions of variable residue combinations across response regulator subfamilies suggest that distinct mechanisms associated with different variable positions allow reaction rates to be tuned independently during evolution for diverse biological purposes. This knowledge could be used in synthetic-biology applications to adjust the properties (e.g., background noise and response duration) of biosensors and may allow prediction of response regulator reaction kinetics from the primary amino acid sequence.

Post-translational Protein Acetylation: An Elegant Mechanism for Bacteria to Dynamically Regulate Metabolic Functions

Post-translational modifications (PTM) decorate proteins to provide functional heterogeneity to an existing proteome. The large number of known PTMs highlights the many ways that cells can modify their proteins to respond to diverse stimuli. Recently, PTMs have begun to receive increased interest because new sensitive proteomics workflows and structural methodologies now allow researchers to obtain large-scale, in-depth and unbiased information concerning PTM type and site localization. However, few PTMs have been extensively assessed for functional consequences, leaving a large knowledge gap concerning the inner workings of the cell. Here, we review understanding of N-𝜀-lysine acetylation in bacteria, a PTM that was largely ignored in bacteria until a decade ago. Acetylation is a modification that can dramatically change the function of a protein through alteration of its properties, including hydrophobicity, solubility, and surface properties, all of which may influence protein conformation and interactions with substrates, cofactors and other macromolecules. Most bacteria carry genes predicted to encode the lysine acetyltransferases and lysine deacetylases that add and remove acetylations, respectively. Many bacteria also exhibit acetylation activities that do not depend on an enzyme, but instead on direct transfer of acetyl groups from the central metabolites acetyl coenzyme A or acetyl phosphate. Regardless of mechanism, most central metabolic enzymes possess lysines that are acetylated in a regulated fashion and many of these regulated sites are conserved across the spectrum of bacterial phylogeny. The interconnectedness of acetylation and central metabolism suggests that acetylation may be a response to nutrient availability or the energy status of the cell. However, this and other hypotheses related to acetylation remain untested.

Metabolic intermediate acetyl phosphate modulates bacterial virulence via acetylation

Accumulating evidence indicates that bacterial metabolism plays an important role in virulence. Acetyl phosphate (AcP), the high-energy intermediate of the phosphotransacetylase-acetate kinase pathway, is the major acetyl donor in E. coli. PhoP is an essential transcription factor for bacterial virulence. Here, we show in Salmonella typhimurium that PhoP is non-enzymatically acetylated by AcP, which modifies its transcriptional activity, demonstrating that the acetylation of Lysine 102 (K102) is dependent on the intracellular AcP. The acetylation level of K102 decreases under PhoP-activating conditions including low magnesium, acid stress or following phagocytosis. Notably, in vitro assays show that K102 acetylation affects PhoP phosphorylation and inhibits its transcriptional activity. Both cell and mouse models show that K102 is critical to Salmonella virulence, and suggest acetylation is involved in regulating PhoP activity. Together, the current study highlights the importance of the metabolism in bacterial virulence, and shows AcP might be a key mediator.

Acetate Metabolism and the Inhibition of Bacterial Growth by Acetate

During aerobic growth on glucose, Escherichia coli excretes acetate, a mechanism called "overflow metabolism." At high concentrations, the secreted acetate inhibits growth. Several mechanisms have been proposed for explaining this phenomenon, but a thorough analysis is hampered by the diversity of experimental conditions and strains used in these studies. Here, we describe the construction of a set of isogenic strains that remove different parts of the metabolic network involved in acetate metabolism. Analysis of these strains reveals that (i) high concentrations of acetate in the medium inhibit growth without significantly perturbing central metabolism; (ii) growth inhibition persists even when acetate assimilation is completely blocked; and (iii) regulatory interactions mediated by acetyl-phosphate play a small but significant role in growth inhibition by acetate. The major contribution to growth inhibition by acetate may originate in systemic effects like the uncoupling effect of organic acids or the perturbation of the anion composition of the cell, as previously proposed. Our data suggest, however, that under the conditions considered here, the uncoupling effect plays only a limited role.IMPORTANCE High concentrations of organic acids such as acetate inhibit growth of Escherichia coli and other bacteria. This phenomenon is of interest for understanding bacterial physiology but is also of practical relevance. Growth inhibition by organic acids underlies food preservation and causes problems during high-density fermentation in biotechnology. What causes this phenomenon? Classical explanations invoke the uncoupling effect of acetate and the establishment of an anion imbalance. Here, we propose and investigate an alternative hypothesis: the perturbation of acetate metabolism due to the inflow of excess acetate. We find that this perturbation accounts for 20% of the growth-inhibitory effect through a modification of the acetyl phosphate concentration. Moreover, we argue that our observations are not expected based on uncoupling alone.

Two-Component Sensing and Regulation: How Do Histidine Kinases Talk with Response Regulators at the Molecular Level?

Perceiving environmental and internal information and reacting in adaptive ways are essential attributes of living organisms. Two-component systems are relevant protein machineries from prokaryotes and lower eukaryotes that enable cells to sense and process signals. Implicating sensory histidine kinases and response regulator proteins, both components take advantage of protein phosphorylation and flexibility to switch conformations in a signal-dependent way. Dozens of two-component systems act simultaneously in any given cell, challenging our understanding about the means that ensure proper connectivity. This review dives into the molecular level, attempting to summarize an emerging picture of how histidine kinases and cognate response regulators achieve required efficiency, specificity, and directionality of signaling pathways, properties that rely on protein:protein interactions. α helices that carry information through long distances, the fine combination of loose and specific kinase/regulator interactions, and malleable reaction centers built when the two components meet emerge as relevant universal principles.

Structural Basis of Response Regulator Function

Response regulators function as the output components of two-component systems, which couple the sensing of environmental stimuli to adaptive responses. Response regulators typically contain conserved receiver (REC) domains that function as phosphorylation-regulated switches to control the activities of effector domains that elicit output responses. This modular design is extremely versatile, enabling different regulatory strategies tuned to the needs of individual signaling systems. This review summarizes structural features that underlie response regulator function. An abundance of atomic resolution structures and complementary biochemical data have defined the mechanisms for response regulator enzymatic activities, revealed trends in regulatory strategies utilized by response regulators of different subfamilies, and provided insights into interactions of response regulators with their cognate histidine kinases. Among the hundreds of thousands of response regulators identified, variations abound. This article provides a framework for understanding structural features that enable function of canonical response regulators and a basis for distinguishing noncanonical configurations.

The Yersinia pseudotuberculosis Cpx envelope stress system contributes to transcriptional activation of rovM

The Gram-negative enteropathogen Yersinia pseudotuberculosis possesses a number of regulatory systems that detect cell envelope damage caused by noxious extracytoplasmic stresses. The CpxA sensor kinase and CpxR response regulator two-component regulatory system is one such pathway. Active Cpx signalling upregulates various factors designed to repair and restore cell envelope integrity. Concomitantly, this pathway also down-regulates key determinants of virulence. In Yersinia, cpxA deletion accumulates high levels of phosphorylated CpxR (CpxR~P). Accumulated CpxR~P directly repressed rovA expression and this limited expression of virulence-associated processes. A second transcriptional regulator, RovM, also negatively regulates rovA expression in response to nutrient stress. Hence, this study aimed to determine if CpxR~P can influence rovA expression through control of RovM levels. We determined that the active CpxR~P isoform bound to the promoter of rovM and directly induced its expression, which naturally associated with a concurrent reduction in rovA expression. Site-directed mutagenesis of the CpxR~P binding sequence in the rovM promoter region desensitised rovM expression to CpxR~P. These data suggest that accumulated CpxR~P inversely manipulates the levels of two global transcriptional regulators, RovA and RovM, and this would be expected to have considerable influence on Yersinia pathophysiology and metabolism.

Mechanisms, Detection, and Relevance of Protein Acetylation in Prokaryotes

Posttranslational modification of a protein, either alone or in combination with other modifications, can control properties of that protein, such as enzymatic activity, localization, stability, or interactions with other molecules. N-ε-Lysine acetylation is one such modification that has gained attention in recent years, with a prevalence and significance that rival those of phosphorylation. This review will discuss the current state of the field in bacteria and some of the work in archaea, focusing on both mechanisms of N-ε-lysine acetylation and methods to identify, quantify, and characterize specific acetyllysines. Bacterial N-ε-lysine acetylation depends on both enzymatic and nonenzymatic mechanisms of acetylation, and recent work has shed light into the regulation of both mechanisms. Technological advances in mass spectrometry have allowed researchers to gain insight with greater biological context by both (i) analyzing samples either with stable isotope labeling workflows or using label-free protocols and (ii) determining the true extent of acetylation on a protein population through stoichiometry measurements. Identification of acetylated lysines through these methods has led to studies that probe the biological significance of acetylation. General and diverse approaches used to determine the effect of acetylation on a specific lysine will be covered.

Multiple links connect central carbon metabolism to DNA replication initiation and elongation in Bacillus subtilis

DNA replication is coupled to growth by an unknown mechanism. Here, we investigated this coupling by analyzing growth and replication in 15 mutants of central carbon metabolism (CCM) cultivated in three rich media. In about one-fourth of the condition tested, defects in replication resulting from changes in initiation or elongation were detected. This uncovered 11 CCM genes important for replication and showed that some of these genes have an effect in one, two or three media. Additional results presented here and elsewhere (Jannière, L., Canceill, D., Suski, C., et al. (2007), PLoS One, 2, e447.) showed that, in the LB medium, the CCM genes important for DNA elongation (gapA and ackA) are genetically linked to the lagging strand polymerase DnaE while those important for initiation (pgk and pykA) are genetically linked to the replication enzymes DnaC (helicase), DnaG (primase) and DnaE. Our work thus shows that the coupling between growth and replication involves multiple, medium-dependent links between CCM and replication. They also suggest that changes in CCM may affect initiation by altering the functional recruitment of DnaC, DnaG and DnaE at the chromosomal origin, and may affect elongation by altering the activity of DnaE at the replication fork. The underlying mechanism is discussed.

Cognate sensor kinase-independent activation of Mycobacterium tuberculosis response regulator DevR (DosR) by acetyl phosphate: implications in anti-mycobacterial drug design

The DevRS/DosT two-component system is essential for mycobacterial survival under hypoxia, a prevailing stress within granulomas. DevR (also known as DosR) is activated by an inducing stimulus, such as hypoxia, through conventional phosphorylation by its cognate sensor kinases, DevS (also known as DosS) and DosT. Here, we show that the DevR regulon is activated by acetyl phosphate under 'non-inducing' aerobic conditions when Mycobacterium tuberculosis devS and dosT double deletion strain is cultured on acetate. Overexpression of phosphotransacetylase caused a perturbation of the acetate kinase-phosphotransacetylase pathway, a decrease in the concentration of acetyl phosphate and dampened the aerobic induction response in acetate-grown bacteria. The operation of two pathways of DevR activation, one through sensor kinases and the other by acetyl phosphate, was established by an analysis of wild-type DevS and phosphorylation-defective DevSH395Q mutant strains under conditions partially mimicking a granulomatous-like environment of acetate and hypoxia. Our findings reveal that DevR can be phosphorylated in vivo by acetyl phosphate. Importantly, we demonstrate that acetyl phosphate-dependent phosphorylation can occur in the absence of DevR's cognate kinases. Based on our findings, we conclude that anti-mycobacterial therapy should be targeted to DevR itself and not to DevS/DosT kinases.

Extracellular Acidic pH Inhibits Acetate Consumption by Decreasing Gene Transcription of the Tricarboxylic Acid Cycle and the Glyoxylate Shunt

Escherichia coli produces acetate during aerobic growth on various carbon sources. After consuming the carbon substrate, E. coli can further grow on the acetate. This phenomenon is known as the acetate switch, where cells transition from producing acetate to consuming it. In this study, we investigated how pH governs the acetate switch. When E. coli was grown on a glucose-supplemented medium initially buffered to pH 7, the cells produced and then consumed the acetate. However, when the initial pH was dropped to 6, the cells still produced acetate but were only able to consume it when little (<10 mM) acetate was produced. When significant acetate was produced in acidic medium, which occurs when the growth medium contains magnesium, amino acids, and sugar, the cells were unable to consume the acetate. To determine the mechanism, we characterized a set of metabolic mutants and found that those defective in the tricarboxylic acid (TCA) cycle or glyoxylate shunt exhibited reduced rates of acetate consumption. We further found that the expression of the genes in these pathways was reduced during growth in acidic medium. The expression of the genes involved in the AckA-Pta pathway, which provides the principal route for both acetate production and consumption, was also inhibited in acidic medium but only after glucose was depleted, which correlates with the acetate consumption phase. On the basis of these results, we conclude that growth in acidic environments inhibits the expression of the acetate catabolism genes, which in turn prevents acetate consumption.IMPORTANCE Many microorganisms produce fermentation products during aerobic growth on sugars. One of the best-known examples is the production of acetate by Escherichia coli during aerobic growth on sugars. In E. coli, acetate production is reversible: once the cells consume the available sugar, they can consume the acetate previously produced during aerobic fermentation. We found that pH affects the reversibility of acetate production. When the cells produce significant acetate during growth in acidic environments, they are unable to consume it. Unconsumed acetate may accumulate in the cell and inhibit the expression of pathways required for acetate catabolism. These findings demonstrate how acetate alters cell metabolism; they also may be useful for the design of aerobic fermentation processes.

Measuring the Activities of Two-Component Regulatory System Phosphatases

Two-component regulatory systems (TCSs) are used for signal transduction by organisms from all three phylogenetic domains of the living world. TCSs use transient protein phosphorylation and dephosphorylation reactions to convert stimuli into appropriate responses to changing environmental conditions. Phosphoryl groups flow from ATP to sensor kinases (which detect stimuli) to response regulators (which implement responses) to inorganic phosphate (P_i). The phosphorylation state of response regulators controls their output activity. The rate at which phosphoryl groups are removed from response regulators correlates with the timescale of the corresponding biological function. Dephosphorylation reactions are fastest in chemotaxis TCS and slower in other TCS. Response regulators catalyze their own dephosphorylation, but at least five types of phosphatases are known to enhance dephosphorylation of response regulators. In each case, the phosphatases are believed to stimulate the intrinsic autodephosphorylation reaction. We discuss in depth the properties of TCS (particularly the differences between chemotaxis and nonchemotaxis TCS) relevant to designing in vitro assays for TCS phosphatases. We describe detailed assay methods for chemotaxis TCS phosphatases using loss of ³²P, change in intrinsic fluorescence as a result of dephosphorylation, or release of P_i. The phosphatase activities of nonchemotaxis TCS phosphatases are less well characterized. We consider how the properties of nonchemotaxis TCS affect assay design and suggest suitable modifications for phosphatases from nonchemotaxis TCS, with an emphasis on the P_i release method.

Small phospho-donors phosphorylate MorR without inducing protein conformational changes

Phosphorylation is an essential mechanism of protein control and plays an important role in biology. The two-component system (TCS) is a bacterial regulation mechanism mediated by a response regulator (RR) protein and a kinase protein, which synchronize the regulatory circuit according to the environment. Phosphorylation is a key element in TCS function as it controls RR activity. In the present study, we characterize the behavior of MorR, an RR associated with Mo homeostasis, upon acetylphosphate and phosphoramidate treatment in vitro. Our results show that MorR was phosphorylated by both phospho-donors. Fluorescence experiments showed that MorR tryptophan emission is quenched by phosphoramidate. Furthermore, theoretical and computational results demonstrate that phosphorylation by phosphoramidate is more favorable than that by acetylphosphate. In conclusion, phosphorylated MorR is a monomeric protein and phosphorylation does not appear to induce observable conformational changes in the protein structure.

Detection of Bacteria-Specific Metabolism Using Hyperpolarized [2-13C]Pyruvate

The differentiation of bacterial infection from other causes of inflammation is difficult in clinical practice and is critical where patient outcomes rely heavily on early interventions. In addition to physical exam and laboratory markers, several imaging modalities are frequently employed, but these techniques generally target the host immune response, rather than the living microorganisms themselves. Here, we describe a method to detect bacteria-specific metabolism using hyperpolarized (HP) ¹³C magnetic resonance spectroscopy. This technology allows visualization of the real-time conversion of enriched ¹³C substrates to their metabolic products, identified by their distinct chemical shifts. We have identified the rapid metabolism of HP [2-¹³C]pyruvate to [1-¹³C]acetate as a metabolic signature of common bacterial pathogens. We demonstrate this conversion in representative Gram-negative and Gram-positive bacteria, namely, Escherichia coli and Staphylococcus aureus, and its absence in key mammalian cell types. Furthermore, this conversion was successfully modulated in three mutant strains, corresponding to deletions of relevant enzymes.

Stress-Adaptive Responses Associated with High-Level Carbapenem Resistance in KPC-Producing Klebsiella pneumoniae

Carbapenem-resistant Enterobacteriaceae (CRE) organisms have emerged to become a major global public health threat among antimicrobial resistant bacterial human pathogens. Little is known about how CREs emerge. One characteristic phenotype of CREs is heteroresistance, which is clinically associated with treatment failure in patients given a carbapenem. Through in vitro whole-transcriptome analysis we tracked gene expression over time in two different strains (BR7, BR21) of heteroresistant KPC-producing Klebsiella pneumoniae, first exposed to a bactericidal concentration of imipenem followed by growth in drug-free medium. In both strains, the immediate response was dominated by a shift in expression of genes involved in glycolysis toward those involved in catabolic pathways. This response was followed by global dampening of transcriptional changes involving protein translation, folding and transport, and decreased expression of genes encoding critical junctures of lipopolysaccharide biosynthesis. The emerged high-level carbapenem-resistant BR21 subpopulation had a prophage (IS1) disrupting ompK36 associated with irreversible OmpK36 porin loss. On the other hand, OmpK36 loss in BR7 was reversible. The acquisition of high-level carbapenem resistance by the two heteroresistant strains was associated with distinct and shared stepwise transcriptional programs. Carbapenem heteroresistance may emerge from the most adaptive subpopulation among a population of cells undergoing a complex set of stress-adaptive responses.

Lvr, a Signaling System That Controls Global Gene Regulation and Virulence in Pathogenic Leptospira

Leptospirosis is an emerging zoonotic disease with more than 1 million cases annually. Currently there is lack of evidence for signaling pathways involved during the infection process of Leptospira. In our comprehensive genomic analysis of 20 Leptospira spp. we identified seven pathogen-specific Two-Component System (TCS) proteins. Disruption of two these TCS genes in pathogenic Leptospira strain resulted in loss-of-virulence in a hamster model of leptospirosis. Corresponding genes lvrA and lvrB (leptospira virulence regulator) are juxtaposed in an operon and are predicted to encode a hybrid histidine kinase and a hybrid response regulator, respectively. Transcriptome analysis of lvr mutant strains with disruption of one (lvrB) or both genes (lvrA/B) revealed global transcriptional regulation of 850 differentially expressed genes. Phosphotransfer assays demonstrated that LvrA phosphorylates LvrB and predicted further signaling downstream to one or more DNA-binding response regulators, suggesting that it is a branched pathway. Phylogenetic analyses indicated that lvrA and lvrB evolved independently within different ecological lineages in Leptospira via gene duplication. This study uncovers a novel-signaling pathway that regulates virulence in pathogenic Leptospira (Lvr), providing a framework to understand the molecular bases of regulation in this life-threatening bacterium.

Fluorescence Measurement of Kinetics of CheY Autophosphorylation with Small Molecule Phosphodonors

The Escherichia coli chemotaxis protein CheY is a model receiver domain containing a native tryptophan residue that serves as a fluorescent probe for CheY autophosphorylation with small molecule phosphodonors. Here we describe fluorescence measurement of apparent bimolecular rate constants for reaction of wild type and mutant CheY with phosphodonors acetyl phosphate, phosphoramidate, or monophosphoimidazole. Step-by-step protocols to synthesize phosphoramidate (K⁺ salt) and monophosphoimidazole (Na⁺ salt), which are not commercially available, are provided. Key factors to consider in developing autophosphorylation assays for other response regulators are also discussed.

Functional analysis of BAS2108-2109 two component system: Evidence for protease regulation in Bacillus anthracis

BACKGROUND

Bacillus anthracis (BA) is a major bioterrorism concern which has evolved complex regulatory mechanisms for its virulence factors. Secreted proteases play an imperative role in the pathogenesis of BA, however their regulation remains elusive. Two component systems (TCS) are often employed by bacteria to sense and adapt to the environmental perturbations. In several pathogens, TCS are commonly associated with the regulation of virulence factors including proteases. The genome of BA encodes 41 TCS pairs, however, the role of any TCS in regulation of its proteases is not known.

PRINCIPAL FINDINGS

The study established BAS2108-2109 as a prototypical TCS where BAS2108 functions as a histidine kinase and BAS2109 as the response regulator. The expression of BAS2109 was found to be elevated under host simulated conditions and in pellicle forming cells. Electrophoretic mobility shift assay (EMSA) and lacZ reporter assay revealed positive autoregulation of the BAS2108-2109 operon by BAS2109. Collective analysis of ANS assay and EMSA demonstrated Lys167, Thr179 and Thr182 residues are crucial for the DNA binding activity of BAS2109. EMSA analysis further highlighted BAS2109 as the transcriptional regulator for different genes of BA, particularly proteases. Upregulation of proteases in BA overexpressing BAS2109 further strengthen its role in protease regulation.

SIGNIFICANCE

This is the first report to identify a TCS pair for its role in the regulation of proteases of BA. Importance of proteases in the pathogenesis of BA is well documented, therefore, studying the regulatory networks governing their expression will help in identification of new drug targets.

Quantitative Kinetic Analyses of Shutting Off a Two-Component System

Cells rely on accurate control of signaling systems to adapt to environmental perturbations. System deactivation upon stimulus removal is as important as activation of signaling pathways. The two-component system (TCS) is one of the major bacterial signaling schemes. In many TCSs, phosphatase activity of the histidine kinase (HK) is believed to play an essential role in shutting off the pathway and resetting the system to the prestimulus state. Two basic challenges are to understand the dynamic behavior of system deactivation and to quantitatively evaluate the role of phosphatase activity under natural cellular conditions. Here we report a kinetic analysis of the response to shutting off the archetype Escherichia coli PhoR-PhoB TCS pathway using both transcription reporter assays and in vivo phosphorylation analyses. Upon removal of the stimulus, the pathway is shut off by rapid dephosphorylation of the PhoB response regulator (RR) while PhoB-regulated gene products gradually reset to prestimulus levels through growth dilution. We developed an approach combining experimentation and modeling to assess in vivo kinetic parameters of the phosphatase activity with kinetic data from multiple phosphatase-diminished mutants. This enabled an estimation of the PhoR phosphatase activity in vivo, which is much stronger than the phosphatase activity of PhoR cytoplasmic domains analyzed in vitro We quantitatively modeled how strong the phosphatase activity needs to be to suppress nonspecific phosphorylation in TCSs and discovered that strong phosphatase activity of PhoR is required for cross-phosphorylation suppression.IMPORTANCE Activation of TCSs has been extensively studied; however, the kinetics of shutting off TCS pathways is not well characterized. We present comprehensive analyses of the shutoff response for the PhoR-PhoB system that reveal the impact of phosphatase activity on shutoff kinetics. This allows development of a quantitative framework not only to characterize the phosphatase activity in the natural cellular environment but also to understand the requirement for specific strengths of phosphatase activity to suppress nonspecific phosphorylation. Our model suggests that the ratio of the phosphatase rate to the nonspecific phosphorylation rate correlates with TCS expression levels and the ratio of the RR to HK, which may contribute to the great diversity of enzyme levels and activities observed in different TCSs.

Microbiota-Derived Short-Chain Fatty Acids Modulate Expression of Campylobacter jejuni Determinants Required for Commensalism and Virulence

Campylobacter jejuni promotes commensalism in the intestinal tracts of avian hosts and diarrheal disease in humans, yet components of intestinal environments recognized as spatial cues specific for different intestinal regions by the bacterium to initiate interactions in either host are mostly unknown. By analyzing a C. jejuni acetogenesis mutant defective in converting acetyl coenzyme A (Ac-CoA) to acetate and commensal colonization of young chicks, we discovered evidence for in vivo microbiota-derived short-chain fatty acids (SCFAs) and organic acids as cues recognized by C. jejuni that modulate expression of determinants required for commensalism. We identified a set of C. jejuni genes encoding catabolic enzymes and transport systems for amino acids required for in vivo growth whose expression was modulated by SCFAs. Transcription of these genes was reduced in the acetogenesis mutant but was restored upon supplementation with physiological concentrations of the SCFAs acetate and butyrate present in the lower intestinal tracts of avian and human hosts. Conversely, the organic acid lactate, which is abundant in the upper intestinal tract where C. jejuni colonizes less efficiently, reduced expression of these genes. We propose that microbiota-generated SCFAs and lactate are cues for C. jejuni to discriminate between different intestinal regions. Spatial gradients of these metabolites likely allow C. jejuni to locate preferred niches in the lower intestinal tract and induce expression of factors required for intestinal growth and commensal colonization. Our findings provide insights into the types of cues C. jejuni monitors in the avian host for commensalism and likely in humans to promote diarrheal disease.

Campylobacter jejuni is a commensal of the intestinal tracts of avian species and other animals and a leading cause of diarrheal disease in humans. The types of cues sensed by C. jejuni to influence responses to promote commensalism or infection are largely lacking. By analyzing a C. jejuni acetogenesis mutant, we discovered a set of genes whose expression is modulated by lactate and short-chain fatty acids produced by the microbiota in the intestinal tract. These genes include those encoding catabolic enzymes and transport systems for amino acids that are required by C. jejuni for in vivo growth and intestinal colonization. We propose that gradients of these microbiota-generated metabolites are cues for spatial discrimination between areas of the intestines so that the bacterium can locate niches in the lower intestinal tract for optimal growth for commensalism in avian species and possibly infection of human hosts leading to diarrheal disease.

Cross-regulation between two common ancestral response regulators, HprR and CusR, in Escherichia coli

The uncharacterized two-component system YedVW of Escherichia coli is involved in stress response to hydrogen peroxide. To identify the H2O2-sensing role of YedV, a set of single Cys-to-Ala substitution mutants were constructed. One particular mutant with C165A substitution in the membrane domain rendered YedV inactive in H2O2-dependent transcription of its regulatory target hiuH. We then proposed to rename YedVW to HprSR (hydrogen peroxide response sensor/regulator). One unique characteristic of HprR is the overlapping of its recognition sequence with that of the Cu(II)-response two-component system regulator CusR. Towards understanding this unique regulation system, in this study we analysed the interplay between HprR and CusR with respect to transcription of hiuH, a regulatory target of HprR, and cusC, a target of CusR. Under low protein concentrations in vitro and in vivo, two regulators recognize and transcribe both hiuH and cusC promoters, albeit at different efficiency, apparently in a collaborative fashion. This is a new type of transcription regulation of the common target genes in response to different external signals. Upon increase in protein concentrations, however, HprR and CusR compete with each other in transcription of the common targets, thereby exhibiting a competitive interplay.

Physiological Role of Two-Component Signal Transduction Systems in Food-Associated Lactic Acid Bacteria

Two-component systems (TCSs) are widespread signal transduction pathways mainly found in bacteria where they play a major role in adaptation to changing environmental conditions. TCSs generally consist of sensor histidine kinases that autophosphorylate in response to a specific stimulus and subsequently transfer the phosphate group to their cognate response regulators thus modulating their activity, usually as transcriptional regulators. In this review we present the current knowledge on the physiological role of TCSs in species of the families Lactobacillaceae and Leuconostocaceae of the group of lactic acid bacteria (LAB). LAB are microorganisms of great relevance for health and food production as the group spans from starter organisms to pathogens. Whereas the role of TCSs in pathogenic LAB (most of them belonging to the family Streptococcaceae) has focused the attention, the roles of TCSs in commensal LAB, such as most species of Lactobacillaceae and Leuconostocaceae, have been somewhat neglected. However, evidence available indicates that TCSs are key players in the regulation of the physiology of these bacteria. The first studies in food-associated LAB showed the involvement of some TCSs in quorum sensing and production of bacteriocins, but subsequent studies have shown that TCSs participate in other physiological processes, such as stress response, regulation of nitrogen metabolism, regulation of malate metabolism, and resistance to antimicrobial peptides, among others.

Identification, Heterologous Expression, and Functional Characterization of Bacillus subtilis YutF, a HAD Superfamily 5'-Nucleotidase with Broad Substrate Specificity

5'-nucleotidases (EC 3.1.3.5) catalyze the hydrolytic dephosphorylation of 5'-ribonucleotides and 5'-deoxyribonucleotides as well as complex nucleotides, such as uridine 5'-diphosphoglucose (UDP-glucose), nicotinamide adenine dinucleotide and flavin adenine dinucleotide, to their corresponding nucleosides plus phosphate. These enzymes have been found in diverse species in intracellular and membrane-bound, surface-localized forms. Soluble forms of 5'-nucleotidases belong to the ubiquitous haloacid dehalogenase superfamily (HADSF) and have been shown to be involved in the regulation of nucleotide, nucleoside and nicotinamide adenine dinucleotide (NAD⁺) pools. Despite the important role of 5'-nucleotidases in cellular metabolism, only a few of these enzymes have been characterized in the Gram-positive bacterium Bacillus subtilis, the workhorse industrial microorganism included in the Food and Drug Administration’s GRAS (generally regarded as safe) list. In the present study, we report the identification of a novel 5'-nucleotidase gene from B. subtilis, yutF, which comprises 771 bp encoding a 256-amino-acid protein belonging to the IIA subfamily of the HADSF. The gene product is responsible for the major p-nitrophenyl phosphatase activity in B. subtilis. The yutF gene was overexpressed in Escherichia coli, and its product fused to a polyhistidine tag was purified and biochemically characterized as a soluble 5'-nucleotidase with broad substrate specificity. The recombinant YutF protein was found to hydrolyze various purine and pyrimidine 5'-nucleotides, showing preference for 5'-nucleoside monophosphates and, specifically, 5'-XMP. Recombinant YutF also exhibited phosphohydrolase activity toward nucleotide precursors, ribose-5-phosphate and 5-phosphoribosyl-1-pyrophosphate. Determination of the kinetic parameters of the enzyme revealed a low substrate specificity (K_m values in the mM concentration range) and modest catalytic efficiencies with respect to substrates. An initial study of the regulation of yutF expression showed that the yutF gene is a component of the yutDEF transcription unit and that YutF overproduction positively influences yutDEF expression.

Crystal structure of nonphosphorylated receiver domain of the stress response regulator RcsB from Escherichia coli

RcsB, the transcription-associated response regulator of the Rcs phosphorelay two-component signal transduction system, activates cell stress responses associated with desiccation, cell wall biosynthesis, cell division, virulence, biofilm formation, and antibiotic resistance in enteric bacterial pathogens. RcsB belongs to the FixJ/NarL family of transcriptional regulators, which are characterized by a highly conserved C-terminal DNA-binding domain. The N-terminal domain of RcsB belongs to the family of two-component receiver domains. This receiver domain contains the phosphoacceptor site and participates in RcsB dimer formation; it also contributes to dimer formation with other transcription factor partners. Here, we describe the crystal structure of the Escherichia coli RcsB receiver domain in its nonphosphorylated state. The structure reveals important molecular details of phosphorylation-independent dimerization of RcsB and has implication for the formation of heterodimers.