Structural analysis and solution studies of the activated regulatory domain of the response regulator ArcA: a symmetric dimer mediated by the alpha4-beta5-alpha5 face

Insights into DNA-binding motifs and mechanisms of Francisella tularensis novicida two-component system response regulator proteins QseB, KdpE, and BfpR

Two component system bacterial response regulators are typically DNA-binding proteins which enable the genetic regulation of many adaptive bacterial behaviors. Despite structural similarity across response regulator families, there is a diverse array of DNA-binding mechanisms. Bacteria usually encode several dozen two-component system response regulators, but Francisella tularensis only encodes three. Due to their simplified response regulatory network, Francisella species are a model for studying the role of response regulator proteins in virulence. Here, we show that Francisella response regulators QseB, KdpE, and BfpR all utilize different DNA-binding mechanisms. Our evidence suggests that QseB follows a simple mechanism whereby it binds a single inverted repeat sequence with a higher affinity upon phosphorylation. This behavior is independent of whether QseB is a positive or negative regulator of the gene as demonstrated by qseB and priM promoter sequences, respectively. Similarly, KdpE binds DNA more tightly upon phosphorylation, but also exhibits a cooperative binding isotherm. While we propose a KdpE binding site, it is possible that KdpE has a complex DNA-binding mechanism potentially involving multiple copies of KdpE being recruited to a promoter region. Finally, we show that BfpR appears to bind a region of its own promoter sequence with a lower affinity upon phosphorylation. Further structural and enzymatic work will need to be performed to deconvolute the KdpE and BfpR binding mechanisms.

A conserved inhibitory interdomain interaction regulates DNA-binding activities of hybrid two-component systems in Bacteroides

Hybrid two-component systems (HTCSs) comprise a major class of transcription regulators of polysaccharide utilization genes in Bacteroides. Distinct from classical two-component systems in which signal transduction is carried out by intermolecular phosphotransfer between a histidine kinase (HK) and a cognate response regulator (RR), HTCSs contain the membrane sensor HK and the RR transcriptional regulator within a single polypeptide chain. Tethering the DNA-binding domain (DBD) of the RR with the dimeric HK domain in an HTCS could potentially promote dimerization of the DBDs and would thus require a mechanism to suppress DNA-binding activity in the absence of stimulus. Analysis of phosphorylation and DNA-binding activities of several HTCSs from Bacteroides thetaiotaomicron revealed a DBD suppression mechanism in which an inhibitory interaction between the DBD and the phosphoryl group-accepting receiver domain (REC) decreases autophosphorylation rates of HTCS-RECs and represses DNA-binding activities in the absence of phosphorylation. Sequence analyses and structure predictions identified a highly conserved sequence motif correlated with a conserved inhibitory domain arrangement of REC and DBD. The presence of the motif, as in most HTCSs, or its absence, in a small subset of HTCSs, is likely predictive of two distinct regulatory mechanisms evolved for different glycans. Substitutions within the conserved motif relieve the inhibitory interaction and result in elevated DNA-binding activities in the absence of phosphorylation. Our data suggest a fundamental regulatory mechanism shared by most HTCSs to suppress DBD activities using a conserved inhibitory interdomain arrangement to overcome the challenge of the fused HK and RR components.

IMPORTANCE

Different dietary and host-derived complex carbohydrates shape the gut microbial community and impact human health. In Bacteroides, the prevalent gut bacteria genus, utilization of these diverse carbohydrates relies on different gene clusters that are under sophisticated control by various signaling systems, including the hybrid two-component systems (HTCSs). We have uncovered a highly conserved regulatory mechanism in which the output DNA-binding activity of HTCSs is suppressed by interdomain interactions in the absence of stimulating phosphorylation. A consensus amino acid motif is found to correlate with the inhibitory interaction surface while deviations from the consensus can lead to constitutive activation. Understanding of such conserved HTCS features will be important to make regulatory predictions for individual systems as well as to engineer novel systems with substitutions in the consensus to explore the glycan regulation landscape in Bacteroides.

Conserved metabolic regulator ArcA responds to oxygen availability, iron limitation, and cell envelope perturbations during bacteremia

IMPORTANCE

Infections of the bloodstream are life-threatening and can result in sepsis. Gram-negative bacteria cause a significant portion of bloodstream infections, which is also referred to as bacteremia. The long-term goal of our work is to understand how such bacteria establish and maintain infection during bacteremia. We have previously identified the transcription factor ArcA, which promotes fermentation in bacteria, as a likely contributor to the growth and survival of bacteria in this environment. Here, we study ArcA in the Gram-negative species Citrobacter freundii, Klebsiella pneumoniae, and Serratia marcescens. Our findings aid in determining how these bacteria sense their environment, utilize nutrients, and generate energy while countering the host immune system. This information is critical for developing better models of infection to inform future therapeutic development.

Theoretical study of ArcB and its dimerization, interaction with anaerobic metabolites, and activation of ArcA

The complex metabolism of Escherichia coli has been extensively studied, including its response to oxygen availability. The ArcA/B two-component system (TCS) is the key regulator for the transition between these two environmental conditions and has been thoroughly characterized using genetic and biochemical approaches. Still, to date, limited structural data is available. The breakthrough provided by AlphaFold2 in 2021 has brought a reliable tool to the scientific community for assessing the structural features of complex proteins. In this report, we analyzed the structural aspects of the ArcA/B TCS using AlphaFold2 models. The models are consistent with the experimentally determined structures of ArcB kinase. The predicted structure of the dimeric form of ArcB is consistent with the extensive genetic and biochemical data available regarding mechanistic signal perception and regulation. The predicted interaction of the dimeric form of ArcB with its cognate response regulator (ArcA) is also consistent with both the forward and reverse phosphotransfer mechanisms. The ArcB model was used to detect putative binding cavities to anaerobic metabolites, encouraging testing of these predictions experimentally. Finally, the highly accurate models of other ArcB homologs suggest that different experimental approaches are needed to determine signal perception in kinases lacking the PAS domain. Overall, ArcB is a kinase with features that need further testing, especially in determining its crystal structure under different conditions.

Substrate DNA Promoting Binding of Mycobacterium tuberculosis MtrA by Facilitating Dimerization and Interpretation of Affinity by Minor Groove Width

In order to deepen the understanding of the role and regulation mechanisms of prokaryotic global transcription regulators in complex processes, including virulence, the associations between the affinity and binding sequences of Mycobacterium tuberculosis MtrA have been explored extensively. Analysis of MtrA 294 diversified 26 bp binding sequences revealed that the sequence similarity of fragments was not simply associated with affinity. The unique variation patterns of GC content and periodical and sequential fluctuation of affinity contribution curves were observed along the sequence in this study. Furthermore, docking analysis demonstrated that the structure of the dimer MtrA-DNA (high affinity) was generally consistent with other OmpR family members, while Arg 219 and Gly 220 of the wing domain interacted with the minor groove. The results of the binding box replacement experiment proved that box 2 was essential for binding, which implied the differential roles of the two boxes in the binding process. Furthermore, the results of the substitution of the nucleotide at the 20th and/or 21st positions indicated that the affinity was negatively associated with the value of minor groove width precisely at the 21st position. The dimerization of the unphosphorylated MtrA facilitated by a low-affinity DNA fragment was observed for the first time. However, the proportion of the dimer was associated with the affinity of substrate DNA, which further suggested that the affinity was actually one characteristic of the stability of dimers. Based on the finding of 17 inter-molecule hydrogen bonds identified in the interface of the MtrA dimer, including 8 symmetric complementary ones in the conserved α4-β5-α5 face, we propose that hydrogen bonds should be considered just as important as salt bridges and the hydrophobic patch in the dimerization. Our comprehensive study on a large number of binding fragments with quantitative affinity values provided new insight into the molecular mechanism of dimerization, binding specificity and affinity determination of MtrA and clues for solving the puzzle of how global transcription factors regulate a large quantity of target genes.

The pathogenic mechanism of Mycobacterium tuberculosis: implication for new drug development

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is a tenacious pathogen that has latently infected one third of the world's population. However, conventional TB treatment regimens are no longer sufficient to tackle the growing threat of drug resistance, stimulating the development of innovative anti-tuberculosis agents, with special emphasis on new protein targets. The Mtb genome encodes ~4000 predicted proteins, among which many enzymes participate in various cellular metabolisms. For example, more than 200 proteins are involved in fatty acid biosynthesis, which assists in the construction of the cell envelope, and is closely related to the pathogenesis and resistance of mycobacteria. Here we review several essential enzymes responsible for fatty acid and nucleotide biosynthesis, cellular metabolism of lipids or amino acids, energy utilization, and metal uptake. These include InhA, MmpL3, MmaA4, PcaA, CmaA1, CmaA2, isocitrate lyases (ICLs), pantothenate synthase (PS), Lysine-ε amino transferase (LAT), LeuD, IdeR, KatG, Rv1098c, and PyrG. In addition, we summarize the role of the transcriptional regulator PhoP which may regulate the expression of more than 110 genes, and the essential biosynthesis enzyme glutamine synthetase (GlnA1). All these enzymes are either validated drug targets or promising target candidates, with drugs targeting ICLs and LAT expected to solve the problem of persistent TB infection. To better understand how anti-tuberculosis drugs act on these proteins, their structures and the structure-based drug/inhibitor designs are discussed. Overall, this investigation should provide guidance and support for current and future pharmaceutical development efforts against mycobacterial pathogenesis.

The ArcAB Two-Component System: Function in Metabolism, Redox Control, and Infection

ArcAB, also known as the Arc system, is a member of the two-component system family of bacterial transcriptional regulators and is composed of sensor kinase ArcB and response regulator ArcA. In this review, we describe the structure and function of these proteins and assess the state of the literature regarding ArcAB as a sensor of oxygen consumption. The bacterial quinone pool is the primary modulator of ArcAB activity, but questions remain for how this regulation occurs. This review highlights the role of quinones and their oxidation state in activating and deactivating ArcB and compares competing models of the regulatory mechanism. The cellular processes linked to ArcAB regulation of central metabolic pathways and potential interactions of the Arc system with other regulatory systems are also reviewed. Recent evidence for the function of ArcAB under aerobic conditions is challenging the long-standing characterization of this system as strictly an anaerobic global regulator, and the support for additional ArcAB functionality in this context is explored. Lastly, ArcAB-controlled cellular processes with relevance to infection are assessed.

Thiol redox switches regulate the oligomeric state of cyanobacterial Rre1, RpaA, and RpaB response regulators

Cyanobacteria employ two‐component sensor‐response regulator systems to monitor and respond to environmental challenges. The response regulators RpaA, RpaB, Rre1 and RppA are integral to circadian clock function and abiotic stress acclimation in cyanobacteria. RpaA, RpaB and Rre1 are known to interact with ferredoxin or thioredoxin, raising the possibility of their thiol regulation. Here, we report that Synechocystis sp. PCC 6803 Rre1, RpaA and RpaB exist as higher‐order oligomers under oxidising conditions and that reduced thioredoxin A converts them to monomers. We further show that these response regulators contain redox‐responsive cysteine residues with an E_m7 around −300 mV. These findings suggest a direct thiol modulation of the activity of these response regulators, independent of their cognate sensor kinases.

Thiol-based functional mimicry of phosphorylation of the two-component system response regulator ArcA promotes pathogenesis in enteric pathogens

Pathogenic bacteria can rapidly respond to stresses such as reactive oxygen species (ROS) using reversible redox-sensitive oxidation of cysteine thiol (-SH) groups in regulators. Here, we use proteomics to profile reversible ROS-induced thiol oxidation in Vibrio cholerae, the etiologic agent of cholera, and identify two modified cysteines in ArcA, a regulator of global carbon oxidation that is phosphorylated and activated under low oxygen. ROS abolishes ArcA phosphorylation but induces the formation of an intramolecular disulfide bond that promotes ArcA-ArcA interactions and sustains activity. ArcA cysteines are oxidized in cholera patient stools, and ArcA thiol oxidation drives in vitro ROS resistance, colonization of ROS-rich guts, and environmental survival. In other pathogens, such as Salmonella enterica, oxidation of conserved cysteines of ArcA orthologs also promotes ROS resistance, suggesting a common role for ROS-induced ArcA thiol oxidation in modulating ArcA activity, allowing for a balance of expression of stress- and pathogenesis-related genetic programs.

Evaluation of type-B RR dimerization in poplar: A mechanism to preserve signaling specificity?

Plants possess specific signaling pathways, such as the MultiStep Phosphorelay (MSP), which is involved in cytokinin and ethylene sensing, and light, drought or osmotic stress sensing. These MSP comprise histidine-aspartate kinases (HKs) as receptors, histidine phosphotransfer (HPts) proteins acting as phosphorelay proteins, and response regulators (RRs), some of which act as transcription factors (type-B RRs). In previous studies, we identified partners of the poplar osmosensing signaling pathway, composed of two HKs, three main HPts, and six type-B RRs. To date, it is unresolved as to how cytokinin or osmotic stress signal specificity is achieved in the MSP in order to generate specific responses. Here, we present a large-scale interaction study of poplar type-B RR dimerization. Using the two-hybrid assay, we were able to show the homodimerization of type-B RRs, the heterodimerization of duplicated type-B RRs, and surprisingly, a lack of interaction between some type-B RRs belonging to different duplicates. The lack of interaction of the duplicates RR12-14 and RR18-19, which are involved in the osmosensing pathway has been confirmed by BiFC experiments. This study reveals, for the first time, an overview of type-B RR dimerization in poplar and makes way for the hypothesis that signal specificity for cytokinin or osmotic stress could be in part due to the fact that it is impossible for specific type-B RRs to heterodimerize.

Structure of the Acinetobacter baumannii PmrA receiver domain and insights into clinical mutants affecting DNA-binding and promoting colistin resistance

Acinetobacter baumannii is an insidious emerging nosocomial pathogen that has developed resistance to all available antimicrobials, including the last resort antibiotic, colistin. Colistin resistance often occurs due to mutations in the PmrAB two component regulatory system. To better understand the regulatory mechanisms contributing to colistin resistance, we have biochemically characterized the A. baumannii PmrA response regulator. Initial DNA-binding analysis shows that A. baumannii PmrA bound to the Klebsiella pneumoniae PmrA box motif. This prompted analysis of the putative A. baumannii PmrAB regulon which indicated that the A. baumannii PmrA consensus box is 5'- HTTAAD N5 HTTAAD. Additionally, we provide the first structural information for the A. baumannii PmrA N-terminal domain through X-ray crystallography, and we present a full-length model using molecular modeling. From these studies, we were able to infer the effects of two critical PmrA mutations, PmrA::I13M and PmrA::P102R, both of which confer increased colistin resistance. Based on these data, we suggest structural and dynamic reasons for how these mutations can affect PmrA function and hence encourage resistive traits. Understanding these mechanisms will aid in the development of new targeted antimicrobial therapies.

Antibacterial Peptides Resistance in Staphylococcus aureus: Various Mechanisms and the Association with Pathogenicity

Staphylococcus aureus is a bacterium that mainly colonizes the nasal cavity and skin. To colonize the host, it is necessary for S. aureus to resist many antibacterial factors derived from human and commensal bacteria. Among them are the bacteria-derived antimicrobial peptides (AMPs) called bacteriocins. It was reported that some two-component systems (TCSs), which are signal transduction systems specific to bacteria, are involved in the resistance to several bacteriocins in S. aureus. However, the TCS-mediated resistance is limited to relatively low concentrations of bacteriocins, while high concentrations of bacteriocins still exhibit antibacterial activity against S. aureus. To determine whether we could obtain highly bacteriocin-resistant mutants, we tried to isolate highly nisin A-resistant mutants by exposing the cells to sub-minimum inhibitory concentrations (MICs) of nisin A. Nisin A is one of the bacteriocins produced by Lactococcus lactis and is utilized as a food preservative worldwide. Finally, we obtained highly nisin A-resistant mutants with mutations in one TCS, BraRS, and in PmtR, which is involved in the expression of pmtABCD. Notably, some highly resistant strains also showed increased pathogenicity. Based on our findings, this review provides up-to-date information on the role of TCSs in the susceptibility to antibacterial peptides. Additionally, the mechanism for high antimicrobial peptides resistance and its association with pathogenicity in S. aureus is elucidated.

Structures of full-length VanR from Streptomyces coelicolor in both the inactive and activated states

Vancomycin has historically been used as a last-resort treatment for serious bacterial infections. However, vancomycin resistance has become widespread in certain pathogens, presenting a serious threat to public health. Resistance to vancomycin is conferred by a suite of resistance genes, the expression of which is controlled by the VanR-VanS two-component system. VanR is the response regulator in this system; in the presence of vancomycin, VanR accepts a phosphoryl group from VanS, thereby activating VanR as a transcription factor and inducing expression of the resistance genes. This paper presents the X-ray crystal structures of full-length VanR from Streptomyces coelicolor in both the inactive and activated states at resolutions of 2.3 and 2.0 Å, respectively. Comparison of the two structures illustrates that phosphorylation of VanR is accompanied by a disorder-to-order transition of helix 4, which lies within the receiver domain of the protein. This transition generates an interface that promotes dimerization of the receiver domain; dimerization in solution was verified using analytical ultracentrifugation. The inactive conformation of the protein does not appear intrinsically unable to bind DNA; rather, it is proposed that in the activated form DNA binding is enhanced by an avidity effect contributed by the receiver-domain dimerization.

The role of two-component regulatory systems in environmental sensing and virulence in Salmonella

Adaptation to environments with constant fluctuations imposes challenges that are only overcome with sophisticated strategies that allow bacteria to perceive environmental conditions and develop an appropriate response. The gastrointestinal environment is a complex ecosystem that is home to trillions of microorganisms. Termed microbiota, this microbial ensemble plays important roles in host health and provides colonization resistance against pathogens, although pathogens have evolved strategies to circumvent this barrier. Among the strategies used by bacteria to monitor their environment, one of the most important are the sensing and signalling machineries of two-component systems (TCSs), which play relevant roles in the behaviour of all bacteria. Salmonella enterica is no exception, and here we present our current understanding of how this important human pathogen uses TCSs as an integral part of its lifestyle. We describe important aspects of these systems, such as the stimuli and responses involved, the processes regulated, and their roles in virulence. We also dissect the genomic organization of histidine kinases and response regulators, as well as the input and output domains for each TCS. Lastly, we explore how these systems may be promising targets for the development of antivirulence therapeutics to combat antibiotic-resistant infections.

Structural basis for promoter DNA recognition by the response regulator OmpR

OmpR, a response regulator of the EnvZ/OmpR two-component system (TCS), controls the reciprocal regulation of two porin proteins, OmpF and OmpC, in bacteria. During signal transduction, OmpR (OmpR-FL) undergoes phosphorylation at its conserved Asp residue in the N-terminal receiver domain (OmpRn) and recognizes the promoter DNA from its C-terminal DNA-binding domain (OmpRc) to elicit an adaptive response. Apart from that, OmpR regulates many genes in Escherichia coli and is important for virulence in several pathogens. However, the molecular mechanism of the regulation and the structural basis of OmpR-DNA binding is still not fully clear. In this study, we presented the crystal structure of OmpRc in complex with the F1 region of the ompF promoter DNA from E. coli. Our structural analysis suggested that OmpRc binds to its cognate DNA as a homodimer, only in a head-to-tail orientation. Also, the OmpRc apo-form showed a unique domain-swapped crystal structure under different crystallization conditions. Biophysical experimental data, such as NMR, fluorescent polarization and thermal stability, showed that inactive OmpR-FL (unphosphorylated) could bind to promoter DNA with a weaker binding affinity as compared with active OmpR-FL (phosphorylated) or OmpRc, and also confirmed that phosphorylation may only enhance DNA binding. Furthermore, the dimerization interfaces in the OmpRc-DNA complex structure identified in this study provide an opportunity to understand the regulatory role of OmpR and explore the potential for this "druggable" target.

Identification of Protein Functional Regions

Protein sequence stores the information relative to both functionality and stability, thus making it difficult to disentangle the two contributions. However, the identification of critical residues for function and stability has important implications for the mapping of the proteome interactions, as well as for many pharmaceutical applications, e. g. the identification of ligand binding regions for targeted pharmaceutical protein design. In this work, we propose a computational method to identify critical residues for protein functionality and stability and to further categorise them in strictly functional, structural and intermediate. We evaluate single site conservation and use Direct Coupling Analysis (DCA) to identify co-evolved residues both in natural and artificial evolution processes. We reproduce artificial evolution using protein design and base our approach on the hypothesis that artificial evolution in the absence of any functional constraint would exclusively lead to site conservation and co-evolution events of the structural type. Conversely, natural evolution intrinsically embeds both functional and structural information. By comparing the lists of conserved and co-evolved residues, outcomes of the analysis on natural and artificial evolution, we identify the functional residues without the need of any a priori knowledge of the biological role of the analysed protein.

New Insights into Multistep-Phosphorelay (MSP)/ Two-Component System (TCS) Regulation: Are Plants and Bacteria that Different?

The Arabidopsis multistep-phosphorelay (MSP) is a signaling mechanism based on a phosphorelay that involves three different types of proteins: Histidine kinases, phosphotransfer proteins, and response regulators. Its bacterial equivalent, the two-component system (TCS), is the most predominant device for signal transduction in prokaryotes. The TCS has been extensively studied and is thus generally well-understood. In contrast, the MSP in plants was first described in 1993. Although great advances have been made, MSP is far from being completely comprehended. Focusing on the model organism Arabidopsis thaliana, this review summarized recent studies that have revealed many similarities with bacterial TCSs regarding how TCS/MSP signaling is regulated by protein phosphorylation and dephosphorylation, protein degradation, and dimerization. Thus, comparison with better-understood bacterial systems might be relevant for an improved study of the Arabidopsis MSP.

The dimerization interface in VraR is essential for induction of the cell wall stress response in Staphylococcus aureus: a potential druggable target

Background

Staphylococcus aureus remains a medical challenge in the treatment of bacterial infections. It has acquired resistance to commonly used antibiotics, and to those considered to be the last weapons in treating staphylococcal infections, such as vancomycin. Studies have revealed that S. aureus is capable of mounting a rapid response to antibiotics that target cell wall peptidoglycan biosynthesis, such as β-lactams and vancomycin. The two-component system VraSR has been linked to the coordination of this response. VraS is a histidine kinase that undergoes autophosphorylation in the presence of signals elicited upon cell wall damage and it then transfers its phosphoryl group to VraR. VraR is a response regulator protein that functions as a transcription factor. Phosphorylation of VraR leads to its dimerization, which is required for optimum binding to its target promoters. Two-component systems have been targeted for the development of antibacterial agents. Deletion of the vraS or vraR gene has been shown to re-sensitize S. aureus to β-lactams and vancomycin.

Results

In this study, we explored perturbation of the VraR phosphorylation-induced activation as a means to inhibit the VraSR-mediated signal transduction pathway. We show that dimerization of VraR is essential for the phosphorylation-induced activation of VraR. A single point mutation in the dimerization interface of VraR, in which Met13 was replaced by Ala, led to the inability of VraR to dimerize and to bind optimally to the target promoter. The consequences of these in vitro molecular deficiencies are equally dramatic in vivo. Complementation of a vraR deletion S. aureus strain with the vraRM13Ala mutant gene failed to induce the cell wall stress response.

Conclusions

This study highlights the potential of targeting the phosphorylation-induced dimerization of VraR to disrupt the S. aureus cell wall stress response and in turn to re-sensitize S. aureus to β-lactams and vancomycin.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1529-0) contains supplementary material, which is available to authorized users.

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.

Exploring the mode of action of inhibitors targeting the PhoP response regulator of Salmonella enterica through comprehensive pharmacophore approaches

The PhoQ/PhoP two-component system regulates the physiological and virulence functions of Salmonella enterica. However, the mode of action of known PhoP inhibitors is unclear. We systematically constructed a pharmacophore model of inhibitors to probe the interface pharmacophore model of the PhoP dimer, coupling it with Ligplot analysis. We found that these inhibitors bind on the α5-helix, altering the conformation and interfering with PhoP binding on DNA.

Comprehensive pharmacophore approaches explore the mode of action of inhibitors targeting PhoP response regulator of Salmonella enterica.

Single mutations in BraRS confer high resistance against nisin A in Staphylococcus aureus

Nisin A is a lantibiotic produced by Lactococcus lactis that is widely used as a food preservative. In Staphylococcus aureus, the BraRS two‐component system (TCS) senses nisin A and regulates the expression of the ABC transporter VraDE, which is responsible for nisin A resistance. In this study, we exposed S. aureus to a sub‐minimum inhibition concentration of nisin A and obtained three spontaneous mutants that were highly resistant to this lantibiotic, designated as SAN (S. aureus nisin resistant) 1, SAN8, and SAN87. In the wild‐type S. aureus strain, VraDE expression was induced by nisin A. In contrast, SAN8 and SAN87 showed constitutively high VraDE expression, even in the absence of nisin A, while SAN1 showed higher BraRS expression, which resulted in high VraDE expression in the presence of nisin A. We identified a single mutation in the promoter region of braXRS in SAN1, whereas SAN8 and SAN87 had single mutations in braR and braS, respectively. Interestingly, even the unphosphorylated form of the mutant BraR protein induced VraDE expression. These results indicate that conformational changes in BraS or BraR resulting from the point mutations may result in the constitutive expression of VraDE, allowing S. aureus to adapt to high concentrations of nisin A.

Cooperative Function of TraJ and ArcA in Regulating the F Plasmid tra Operon

The F plasmid tra operon encodes most of the proteins required for bacterial conjugation. TraJ and ArcA are known activators of the tra operon promoter P_Y, which is subject to H-NS-mediated silencing. Donor ability and promoter activity assays indicated that P_Y is inactivated by silencers and requires both TraJ and ArcA for activation to support efficient F conjugation. The observed low-level, ArcA-independent F conjugation is caused by tra expression from upstream alternative promoters. Electrophoretic mobility shift assays showed that TraJ alone weakly binds to P_Y regulatory DNA; however, TraJ binding is significantly enhanced by ArcA binding to the same DNA, indicating cooperativity of the two proteins. Analysis of binding affinities between ArcA and various DNA fragments in the P_Y regulatory region defined a 22-bp tandem repeat sequence (from -76 to -55 of P_Y) sufficient for optimal ArcA binding, which is immediately upstream of the predicted TraJ-binding site (from -54 to -34). Deletion analysis of the P_Y promoter in strains deficient in TraJ, ArcA, and/or H-NS determined that sequences upstream of -103 are required by silencers including H-NS for P_Y silencing, whereas sequences downstream of -77 are targeted by TraJ and ArcA for activation. TraJ and ArcA appear not only to counteract P_Y silencers but also to directly activate P_Y in a cooperative manner. Our data reveal the cooperativity of TraJ and ArcA during P_Y activation and provide insights into the regulatory circuit controlling F-family plasmid-mediated bacterial conjugation.IMPORTANCE Conjugation is a major mechanism for dissemination of antibiotic resistance and virulence among bacterial populations. The tra operon in the F family of conjugative plasmids encodes most of the proteins involved in bacterial conjugation. This work reveals that activation of tra operon transcription requires two proteins, TraJ and ArcA, to bind cooperatively to adjacent sites immediately upstream of the major tra promoter P_Y The interaction of TraJ and ArcA with the tra operon not only relieves P_Y from silencers but also directly activates it. These findings provide insights into the regulatory circuit of the F-family plasmid-mediated bacterial conjugation.

RitR is an archetype for a novel family of redox sensors in the streptococci that has evolved from two-component response regulators and is required for pneumococcal colonization

To survive diverse host environments, the human pathogen Streptococcus pneumoniae must prevent its self-produced, extremely high levels of peroxide from reacting with intracellular iron. However, the regulatory mechanism(s) by which the pneumococcus accomplishes this balance remains largely enigmatic, as this pathogen and other related streptococci lack all known redox-sensing transcription factors. Here we describe a two-component-derived response regulator, RitR, as the archetype for a novel family of redox sensors in a subset of streptococcal species. We show that RitR works to both repress iron transport and enable nasopharyngeal colonization through a mechanism that exploits a single cysteine (Cys128) redox switch located within its linker domain. Biochemical experiments and phylogenetics reveal that RitR has diverged from the canonical two-component virulence regulator CovR to instead dimerize and bind DNA only upon Cys128 oxidation in air-rich environments. Atomic structures show that Cys128 oxidation initiates a "helical unravelling" of the RitR linker region, suggesting a mechanism by which the DNA-binding domain is then released to interact with its cognate regulatory DNA. Expanded computational studies indicate this mechanism could be shared by many microbial species outside the streptococcus genus.

Role of CovR phosphorylation in gene transcription in Streptococcus mutans

Streptococcus mutans, the primary aetiological agent of dental caries, is one of the major bacteria of the human oral cavity. The pathogenicity of this bacterium is attributed not only to the expression of virulence factors, but also to its ability to respond and adapt rapidly to the ever-changing conditions of the oral cavity. The two-component signal transduction system (TCS) CovR/S plays a crucial role in virulence and stress response in many streptococci. Surprisingly, in S. mutans the response regulator CovR appears to be an orphan, as the cognate sensor kinase, CovS, is absent in all the strains. We found that acetyl phosphate, an intracellular phosphodonor molecule known to act in signalling, might play a role in CovR phosphorylation in vivo. We also found that in vitro, upon phosphorylation by potassium phosphoramide (a high-energy phophodonor) CovR formed a dimer and showed altered electrophoretic mobility. As expected, we found that the conserved aspartic acid residue at position 53 (D53) was the site of phosphorylation, since neither phosphorylation nor dimerization was seen when an alanine-substituted CovR mutant (D53A) was used. Surprisingly, we found that the ability of CovR to act as a transcriptional regulator does not depend upon its phosphorylation status, since the D53A mutant behaved similarly to the wild-type protein in both in vivo and in vitro DNA-binding assays. This unique phosphorylation-mediated inhibition of CovR function in S. mutans sheds light on an unconventional mechanism of the signal transduction pathway.

The Structure of the Biofilm-controlling Response Regulator BfmR from Acinetobacter baumannii Reveals Details of Its DNA-binding Mechanism

The rise of drug-resistant bacterial infections coupled with decreasing antibiotic efficacy poses a significant challenge to global health care. Acinetobacter baumannii is an insidious, emerging bacterial pathogen responsible for severe nosocomial infections aided by its ability to form biofilms. The response regulator BfmR, from the BfmR/S two-component system, is the master regulator of biofilm initiation in A. baumannii and is a tractable therapeutic target. Here we present the structure of A. baumannii BfmR using a hybrid approach combining X-ray crystallography, nuclear magnetic resonance spectroscopy, chemical crosslinking mass spectrometry, and molecular modeling. We also show that BfmR binds the previously proposed bfmRS promoter sequence with moderate affinity. While BfmR shares many traits with other OmpR/PhoB family response regulators, some unusual properties were observed. Most importantly, we observe that when phosphorylated, BfmR binds this promoter sequence with a lower affinity than when not phosphorylated. All other OmpR/PhoB family members studied to date show an increase in DNA-binding affinity upon phosphorylation. Understanding the structural and biochemical mechanisms of BfmR will aid in the development of new antimicrobial therapies.

Threshold response and bistability in gene regulation by small noncoding RNA

In this paper, we study through mathematical modelling the combined effect of transcriptional and translational regulation by proteins and small noncoding RNAs (sRNA) in a genetic feedback motif that has an important role in the survival of E. coli under stress associated with oxygen and energy availability. We show that subtle changes in this motif can bring in drastically different effects on the gene expression. In particular, we show that a threshold response in the gene expression changes to a bistable response as the regulation on sRNA synthesis or degradation is altered. These results are obtained under deterministic conditions. Next, we study how the gene expression is altered by additive and multiplicative noise which might arise due to probabilistic occurrences of different biochemical events. Using the Fokker-Planck formulation, we obtain steady-state probability distributions for sRNA concentration for the network motifs displaying bistability. The probability distributions are found to be bimodal with two peaks at low and high concentrations of sRNAs. We further study the variations in the probability distributions under different values of noise strength and correlations. The results presented here might be of interest for designing synthetic network for artificial control.

Genetic Basis of Exploiting Ecological Opportunity During the Long-Term Diversification of a Bacterial Population

Adaptive diversification is an essential evolutionary process, one that produces phenotypic innovations including the colonization of available ecological niches. Bacteria can diverge in sympatry when ecological opportunities allow, but the underlying genetic mechanisms are often unknown. Perhaps, the longest-lasting adaptive diversification seen in the laboratory occurred during the long-term evolution experiment, in which 12 populations of Escherichia coli have been evolving independently for more than 65,000 generations from a common ancestor. In one population, two lineages, S and L, emerged at ~6500 generations and have dynamically coexisted ever since by negative frequency-dependent interactions mediated, in part, by acetate secretion by L. Mutations in spoT, arcA, and gntR promoted the emergence of the S lineage, although they reproduced only partially its phenotypic traits. Here, we characterize the evolved mechanism of acetate consumption by the S lineage that enabled invasion and coexistence with the L lineage. We identified an additional mutation in acs that, together with the arcA mutation, drove an early restructuring of the transcriptional control of central metabolism in S, leading to improved acetate consumption. Pervasive epistatic interactions within the S genome contributed to the exploitation of this new ecological opportunity. The emergence and maintenance of this long-term polymorphism is a complex multi-step process.

Large-scale identification of coevolution signals across homo-oligomeric protein interfaces by direct coupling analysis

Proteins have evolved to perform diverse cellular functions, from serving as reaction catalysts to coordinating cellular propagation and development. Frequently, proteins do not exert their full potential as monomers but rather undergo concerted interactions as either homo-oligomers or with other proteins as hetero-oligomers. The experimental study of such protein complexes and interactions has been arduous. Theoretical structure prediction methods are an attractive alternative. Here, we investigate homo-oligomeric interfaces by tracing residue coevolution via the global statistical direct coupling analysis (DCA). DCA can accurately infer spatial adjacencies between residues. These adjacencies can be included as constraints in structure prediction techniques to predict high-resolution models. By taking advantage of the ongoing exponential growth of sequence databases, we go significantly beyond anecdotal cases of a few protein families and apply DCA to a systematic large-scale study of nearly 2,000 Pfam protein families with sufficient sequence information and structurally resolved homo-oligomeric interfaces. We find that large interfaces are commonly identified by DCA. We further demonstrate that DCA can differentiate between subfamilies with different binding modes within one large Pfam family. Sequence-derived contact information for the subfamilies proves sufficient to assemble accurate structural models of the diverse protein-oligomers. Thus, we provide an approach to investigate oligomerization for arbitrary protein families leading to structural models complementary to often-difficult experimental methods. Combined with ever more abundant sequential data, we anticipate that this study will be instrumental to allow the structural description of many heteroprotein complexes in the future.

To ∼P or Not to ∼P? Non-canonical activation by two-component response regulators

Bacteria sense and respond to their environment through the use of two-component regulatory systems. The ability to adapt to a wide range of environmental stresses is directly related to the number of two-component systems an organism possesses. Recent advances in this area have identified numerous variations on the archetype systems that employ a sensor kinase and a response regulator. It is now evident that many orphan regulators that lack cognate kinases do not rely on phosphorylation for activation and new roles for unphosphorylated response regulators have been identified. The significance of recent findings and suggestions for further research are discussed.

The Response Regulator BfmR Is a Potential Drug Target for Acinetobacter baumannii

Increasing antibiotic resistance in bacteria, particularly Gram-negative bacilli, has significantly affected the ability of physicians to treat infections, with resultant increased morbidity, mortality, and health care costs. In fact, some strains of bacteria are resistant to all available antibiotics, such as Acinetobacter baumannii, which is the focus of this report. Therefore, the development of new antibiotics active against these resistant strains is urgently needed. In this study, BfmR is further validated as an intriguing target for a novel class of antibiotics. Successful inactivation of BfmR would confer the multiple benefits of a decreased ability of A. baumannii to survive in human body fluids, increased sensitivity to complement-mediated bactericidal activity and, importantly, increased sensitivity to other antibiotics. Structural studies support the potential for this “druggable” target, as they identify the potential for small-molecule binding at functionally relevant sites. Next-phase high-throughput screening studies utilizing BfmR are warranted.

Identification and validation is the first phase of target-based antimicrobial development. BfmR (RstA), a response regulator in a two-component signal transduction system (TCS) in Acinetobacter baumannii, is an intriguing potential antimicrobial target. A unique characteristic of BfmR is that its inhibition would have the dual benefit of significantly decreasing in vivo survival and increasing sensitivity to selected antimicrobials. Studies on the clinically relevant strain AB307-0294 have shown BfmR to be essential in vivo. Here, we demonstrate that this phenotype in strains AB307-0294 and AB908 is mediated, in part, by enabling growth in human ascites fluid and serum. Further, BfmR conferred resistance to complement-mediated bactericidal activity that was independent of capsular polysaccharide. Importantly, BfmR also increased resistance to the clinically important antimicrobials meropenem and colistin. BfmR was highly conserved among A. baumannii strains. The crystal structure of the receiver domain of BfmR was determined, lending insight into putative ligand binding sites. This enabled an in silico ligand binding analysis and a blind docking strategy to assess use as a potential druggable target. Predicted binding hot spots exist at the homodimer interface and the phosphorylation site. These data support pursuing the next step in the development process, which includes determining the degree of inhibition needed to impact growth/survival and the development a BfmR activity assay amenable to high-throughput screening for the identification of inhibitors. Such agents would represent a new class of antimicrobials active against A. baumannii which could be active against other Gram-negative bacilli that possess a TCS with shared homology.

IMPORTANCE Increasing antibiotic resistance in bacteria, particularly Gram-negative bacilli, has significantly affected the ability of physicians to treat infections, with resultant increased morbidity, mortality, and health care costs. In fact, some strains of bacteria are resistant to all available antibiotics, such as Acinetobacter baumannii, which is the focus of this report. Therefore, the development of new antibiotics active against these resistant strains is urgently needed. In this study, BfmR is further validated as an intriguing target for a novel class of antibiotics. Successful inactivation of BfmR would confer the multiple benefits of a decreased ability of A. baumannii to survive in human body fluids, increased sensitivity to complement-mediated bactericidal activity and, importantly, increased sensitivity to other antibiotics. Structural studies support the potential for this “druggable” target, as they identify the potential for small-molecule binding at functionally relevant sites. Next-phase high-throughput screening studies utilizing BfmR are warranted.

Structural characterization of the full-length response regulator spr1814 in complex with a phosphate analogue reveals a novel conformational plasticity of the linker region

Spr1814 of Streptococcus pneumoniae is a response regulator (RR) that belongs to the NarL/FixJ subfamily and has a four-helix helix-turn-helix DNA-binding domain. Here, the X-ray crystal structure of the full-length spr1814 in complex with a phosphate analogue beryllium fluoride (BeF3(-)) was determined at 2.0 Å. This allows for a structural comparison with the previously reported full-length unphosphorylated spr1814. The phosphorylation of conserved aspartic acid residue of N-terminal receiver domain triggers a structural perturbation at the α4-β5-α5 interface, leading to the domain reorganization of spr1814, and this is achieved by a rotational change in the C-terminal DNA-binding domain.

Structure of the Response Regulator NsrR from Streptococcus agalactiae, Which Is Involved in Lantibiotic Resistance

Lantibiotics are antimicrobial peptides produced by Gram-positive bacteria. Interestingly, several clinically relevant and human pathogenic strains are inherently resistant towards lantibiotics. The expression of the genes responsible for lantibiotic resistance is regulated by a specific two-component system consisting of a histidine kinase and a response regulator. Here, we focused on a response regulator involved in lantibiotic resistance, NsrR from Streptococcus agalactiae, and determined the crystal structures of its N-terminal receiver domain and C-terminal DNA-binding effector domain. The C-terminal domain exhibits a fold that classifies NsrR as a member of the OmpR/PhoB subfamily of regulators. Amino acids involved in phosphorylation, dimerization, and DNA-binding were identified and demonstrated to be conserved in lantibiotic resistance regulators. Finally, a model of the full-length NsrR in the active and inactive state provides insights into protein dimerization and DNA-binding.

Two-Component Regulatory Systems of Mycobacteria

Two-component regulatory systems (2CRSs) are widely used by bacteria to sense and respond to environmental stimuli with coordinated changes in gene expression. Systems are normally comprised of a sensory kinase protein that activates a transcriptional regulator by phosphorylation. Mycobacteria have few 2CRSs, but they are of key importance for bacterial survival and play important roles in pathogenicity. Mycobacterium tuberculosis has 12 paired two-component regulatory systems (which include a system with two regulators and one sensor, and a split sensor system), as well as four orphan regulators. Several systems are involved in virulence, and disruption of different systems leads to attenuation or hypervirulence. PhoPR plays a major role in regulating cell wall composition, and its inactivation results in sufficient attenuation of M. tuberculosis that deletion strains are live vaccine candidates. MprAB controls the stress response and is required for persistent infections. SenX3-RegX3 is required for control of aerobic respiration and phosphate uptake, and PrrAB is required for adaptation to intracellular infection. MtrAB is an essential system that controls DNA replication and cell division. The remaining systems (KdpDE, NarL, TrcRS, TcrXY, TcrA, PdtaRS, and four orphan regulators) are less well understood. The structure and binding motifs for several regulators have been characterized, revealing variations in function and operation. The sensors are less well characterized, and stimuli for many remain to be confirmed. This chapter reviews our current understanding of the role of two-component systems in mycobacteria, in particular M. tuberculosis.

Prokaryotic 2-component systems and the OmpR/PhoB superfamily

In bacteria, 2-component regulatory systems (TCSs) are the critical information-processing pathways that link stimuli to specific adaptive responses. Signals perceived by membrane sensors, which are generally histidine kinases, are transmitted by response regulators (RRs) to allow cells to cope rapidly and effectively with environmental challenges. Over the past few decades, genes encoding components of TCSs and their responsive proteins have been identified, crystal structures have been described, and signaling mechanisms have been elucidated. Here, we review recent findings and interesting breakthroughs in bacterial TCS research. Furthermore, we discuss structural features, mechanisms of activation and regulation, and cross-regulation of RRs, with a focus on the largest RR family, OmpR/PhoB, to provide a comprehensive overview of these critically important signaling molecules.

Regulation of transcription by eukaryotic-like serine-threonine kinases and phosphatases in Gram-positive bacterial pathogens

Bacterial eukaryotic-like serine threonine kinases (eSTKs) and serine threonine phosphatases (eSTPs) have emerged as important signaling elements that are indispensable for pathogenesis. Differing considerably from their histidine kinase counterparts, few eSTK genes are encoded within the average bacterial genome, and their targets are pleiotropic in nature instead of exclusive. The growing list of important eSTK/P substrates includes proteins involved in translation, cell division, peptidoglycan synthesis, antibiotic tolerance, resistance to innate immunity and control of virulence factors. Recently it has come to light that eSTK/Ps also directly modulate transcriptional machinery in many microbial pathogens. This novel form of regulation is now emerging as an additional means by which bacteria can alter their transcriptomes in response to host-specific environmental stimuli. Here we focus on the ability of eSTKs and eSTPs in Gram-positive bacterial pathogens to directly modulate transcription, the known mechanistic outcomes of these modifications, and their roles as an added layer of complexity in controlling targeted RNA synthesis to enhance virulence potential.

Structure of the DNA-binding domain of the response regulator SaeR fromStaphylococcus aureus

The SaeR/S two-component regulatory system is essential for controlling the expression of many virulence factors inStaphylococcus aureus. SaeR, a member of the OmpR/PhoB family, is a response regulator with an N-terminal regulatory domain and a C-terminal DNA-binding domain. In order to elucidate how SaeR binds to the promoter regions of target genes, the crystal structure of the DNA-binding domain of SaeR (SaeRDBD) was solved at 2.5 Å resolution. The structure reveals that SaeRDBDexists as a monomer and has the canonical winged helix–turn–helix module. EMSA experiments suggested that full-length SaeR can bind to the P1 promoter and that the binding affinity is higher than that of its C-terminal DNA-binding domain. Five key residues on the winged helix–turn–helix module were verified to be important for binding to the P1 promoterin vitroand for the physiological function of SaeRin vivo.

The Eukaryotic-Like Ser/Thr Kinase PrkC Regulates the Essential WalRK Two-Component System in Bacillus subtilis

Most bacteria contain both eukaryotic-like Ser/Thr kinases (eSTKs) and eukaryotic-like Ser/Thr phosphatases (eSTPs). Their role in bacterial physiology is not currently well understood in large part because the conditions where the eSTKs are active are generally not known. However, all sequenced Gram-positive bacteria have a highly conserved eSTK with extracellular PASTA repeats that bind cell wall derived muropeptides. Here, we report that in the Gram-positive bacterium Bacillus subtilis, the PASTA-containing eSTK PrkC and its cognate eSTP PrpC converge with the essential WalRK two-component system to regulate WalR regulon genes involved in cell wall metabolism. By continuously monitoring gene expression throughout growth, we consistently find a large PrkC-dependent effect on expression of several different WalR regulon genes in early stationary phase, including both those that are activated by WalR (yocH) as well as those that are repressed (iseA, pdaC). We demonstrate that PrkC phosphorylates WalR in vitro and in vivo on a single Thr residue located in the receiver domain. Although the phosphorylated region of the receiver domain is highly conserved among several B. subtilis response regulators, PrkC displays specificity for WalR in vitro. Consistently, strains expressing a nonphosphorylatable WalR point mutant strongly reduce both PrkC dependent activation and repression of yocH, iseA, and pdaC. This suggests a model where the eSTK PrkC regulates the essential WalRK two-component signaling system by direct phosphorylation of WalR Thr101, resulting in the regulation of WalR regulon genes involved in cell wall metabolism in stationary phase. As both the eSTK PrkC and the essential WalRK two-component system are highly conserved in Gram-positive bacteria, these results may be applicable to further understanding the role of eSTKs in Gram-positive physiology and cell wall metabolism.

A central question in bacterial physiology is how bacteria sense and respond to their environment. The archetype of bacterial signaling systems is the two-component signaling system composed of a sensor protein histidine kinase that activates a transcription factor response regulator in response to a specific signal. In addition, bacteria also have signaling systems composed of eukaryotic-like Ser/Thr kinases and phosphatases. Even though these systems do not have dedicated transcription factors, they are capable of affecting gene expression. Here we show that a eukaryotic-like Ser/Thr kinase conserved in all sequenced Gram-positive bacteria converges with an essential two-component signaling system to regulate gene expression in the model organism Bacillus subtilis. We show that this eukaryotic-like Ser/Thr kinase phosphorylates the response regulator of a highly conserved and essential two-component signaling system, thereby increasing its activity. This phosphorylation results in the regulation of genes involved in the essential process of cell wall metabolism. Given that bacterial cell wall metabolism is the target of many known antibiotics, and mutations in both of these signaling systems change the antibiotic sensitivity of a number of important Gram-positive pathogens, we expect that our analysis will suggest novel insight into the emergence of antibiotic resistance.

The aspartate-less receiver (ALR) domains: distribution, structure and function

Two-component signaling systems are ubiquitous in bacteria, Archaea and plants and play important roles in sensing and responding to environmental stimuli. To propagate a signaling response the typical system employs a sensory histidine kinase that phosphorylates a Receiver (REC) domain on a conserved aspartate (Asp) residue. Although it is known that some REC domains are missing this Asp residue, it remains unclear as to how many of these divergent REC domains exist, what their functional roles are and how they are regulated in the absence of the conserved Asp. Here we have compiled all deposited REC domains missing their phosphorylatable Asp residue, renamed here as the Aspartate-Less Receiver (ALR) domains. Our data show that ALRs are surprisingly common and are enriched for when attached to more rare effector outputs. Analysis of our informatics and the available ALR atomic structures, combined with structural, biochemical and genetic data of the ALR archetype RitR from Streptococcus pneumoniae presented here suggest that ALRs have reorganized their active pockets to instead take on a constitutive regulatory role or accommodate input signals other than Asp phosphorylation, while largely retaining the canonical post-phosphorylation mechanisms and dimeric interface. This work defines ALRs as an atypical REC subclass and provides insights into shared mechanisms of activation between ALR and REC domains.

Identification of the amino acids essential for LytSR-mediated signal transduction in Staphylococcus aureus and their roles in biofilm-specific gene expression

Recent studies have demonstrated that expression of the Staphylococcus aureus lrgAB operon is specifically localized within tower structures during biofilm development. To gain a better understanding of the mechanisms underlying this spatial control of lrgAB expression, we carried out a detailed analysis of the LytSR two-component system. Specifically, a conserved aspartic acid (Asp53) of the LytR response regulator was shown to be the target of phosphorylation, which resulted in enhanced binding to the lrgAB promoter and activation of transcription. In addition, we identified His390 of the LytS histidine kinase as the site of autophosphorylation and Asn394 as a critical amino acid involved in phosphatase activity. Interestingly, LytS-independent activation of LytR was observed during planktonic growth, with acetyl phosphate acting as a phosphodonor to LytR. In contrast, mutations disrupting the function of LytS prevented tower-specific lrgAB expression, providing insight into the physiologic environment within these structures. In addition, overactivation of LytR led to increased lrgAB promoter activity during planktonic and biofilm growth and a change in biofilm morphology. Overall, the results of this study are the first to define the LytSR signal transduction pathway, as well as determine the metabolic context within biofilm tower structures that triggers these signaling events.

DNA consensus sequence motif for binding response regulator PhoP, a virulence regulator of Mycobacterium tuberculosis

Tuberculosis has reemerged as a serious threat to human health because of the increasing prevalence of drug-resistant strains and synergetic infection with HIV, prompting an urgent need for new and more efficient treatments. The PhoP–PhoR two-component system of Mycobacterium tuberculosis plays an important role in the virulence of the pathogen and thus represents a potential drug target. To study the mechanism of gene transcription regulation by response regulator PhoP, we identified a high-affinity DNA sequence for PhoP binding using systematic evolution of ligands by exponential enrichment. The sequence contains a direct repeat of two 7 bp motifs separated by a 4 bp spacer, TCACAGC(N₄)TCACAGC. The specificity of the direct-repeat sequence for PhoP binding was confirmed by isothermal titration calorimetry and electrophoretic mobility shift assays. PhoP binds to the direct repeat as a dimer in a highly cooperative manner. We found many genes previously identified to be regulated by PhoP that contain the direct-repeat motif in their promoter sequences. Synthetic DNA fragments at the putative promoter-binding sites bind PhoP with variable affinity, which is related to the number of mismatches in the 7 bp motifs, the positions of the mismatches, and the spacer and flanking sequences. Phosphorylation of PhoP increases the affinity but does not change the specificity of DNA binding. Overall, our results confirm the direct-repeat sequence as the consensus motif for PhoP binding and thus pave the way for identification of PhoP directly regulated genes in different mycobacterial genomes.

Mutations in global regulators lead to metabolic selection during adaptation to complex environments

Adaptation to ecologically complex environments can provide insights into the evolutionary dynamics and functional constraints encountered by organisms during natural selection. Adaptation to a new environment with abundant and varied resources can be difficult to achieve by small incremental changes if many mutations are required to achieve even modest gains in fitness. Since changing complex environments are quite common in nature, we investigated how such an epistatic bottleneck can be avoided to allow rapid adaptation. We show that adaptive mutations arise repeatedly in independently evolved populations in the context of greatly increased genetic and phenotypic diversity. We go on to show that weak selection requiring substantial metabolic reprogramming can be readily achieved by mutations in the global response regulator arcA and the stress response regulator rpoS. We identified 46 unique single-nucleotide variants of arcA and 18 mutations in rpoS, nine of which resulted in stop codons or large deletions, suggesting that subtle modulations of ArcA function and knockouts of rpoS are largely responsible for the metabolic shifts leading to adaptation. These mutations allow a higher order metabolic selection that eliminates epistatic bottlenecks, which could occur when many changes would be required. Proteomic and carbohydrate analysis of adapting E. coli populations revealed an up-regulation of enzymes associated with the TCA cycle and amino acid metabolism, and an increase in the secretion of putrescine. The overall effect of adaptation across populations is to redirect and efficiently utilize uptake and catabolism of abundant amino acids. Concomitantly, there is a pronounced spread of more ecologically limited strains that results from specialization through metabolic erosion. Remarkably, the global regulators arcA and rpoS can provide a “one-step” mechanism of adaptation to a novel environment, which highlights the importance of global resource management as a powerful strategy to adaptation.

Changing environmental conditions are the norm in biology. However, understanding adaptation to complex environments presents many challenges. For example, adaptation to resource-rich environments can potentially have many successful evolutionary trajectories to increased fitness. Even in conditions of plenty, the utilization of numerous but novel resources can require multiple mutations before a benefit is accrued. We evolved two bacterial species isolated from the gut of healthy humans in two different, resource-rich media commonly used in the laboratory. We anticipated that under weak selection the population would evolve tremendous genetic diversity. Despite such a complex genetic background we were able to identify a strong degree of parallel evolution and using a combination of population proteomic and population genomic approaches we show that two global regulators, arcA and rpoS, are the principle targets of selection. Up-regulation of the different metabolic pathways that are controlled by these global regulators in combination with up-regulation of transporters that transport nutrients into the cell revealed increased use of the novel resources. Thus global regulators can provide a one-step model to shift metabolism efficiently and provide rapid a one-step reprogramming of the cell metabolic profile.

Transcriptional regulation of the Aggregatibacter actinomycetemcomitans ygiW-qseBC operon by QseB and integration host factor proteins

The QseBC two-component system plays a pivotal role in regulating virulence and biofilm growth of the oral pathogen Aggregatibacter actinomycetemcomitans. We previously showed that QseBC autoregulates the ygiW-qseBC operon. In this study, we characterized the promoter that drives ygiW-qseBC expression. Using lacZ transcriptional fusion constructs and 5'-rapid amplification of cDNA ends, we showed that ygiW-qseBC expression is driven by a promoter that initiates transcription 53 bases upstream of ygiW and identified putative cis-acting promoter elements, whose function was confirmed using site-specific mutagenesis. Using electrophoretic mobility shift assays, two trans-acting proteins were shown to interact with the ygiW-qseBC promoter. The QseB response regulator bound to probes containing the direct repeat sequence CTTAA-N6-CTTAA, where the CTTAA repeats flank the -35 element of the promoter. The ygiW-qseBC expression could not be detected in A. actinomycetemcomitans ΔqseB or ΔqseBC strains, but was restored to WT levels in the ΔqseBC mutant when complemented by single copy chromosomal insertion of qseBC. Interestingly, qseB partially complemented the ΔqseBC strain, suggesting that QseB could be activated in the absence of QseC. QseB activation required its phosphorylation since complementation did not occur using qseB(pho-), encoding a protein with the active site aspartate substituted with alanine. These results suggest that QseB is a strong positive regulator of ygiW-qseBC expression. In addition, integration host factor (IHF) bound to two sites in the promoter region and an additional site near the 5' end of the ygiW ORF. The expression of ygiW-qseBC was increased by twofold in ΔihfA and ΔihfB strains of A. actinomycetemcomitans, suggesting that IHF is a negative regulator of the ygiW-qseBC operon.

The influence of repressor DNA binding site architecture on transcriptional control

How the architecture of DNA binding sites dictates the extent of repression of promoters is not well understood. Here, we addressed the importance of the number and information content of the three direct repeats (DRs) in the binding and repression of the icdA promoter by the phosphorylated form of the global Escherichia coli repressor ArcA (ArcA-P). We show that decreasing the information content of the two sites with the highest information (DR1 and DR2) eliminated ArcA binding to all three DRs and ArcA repression of icdA. Unexpectedly, we also found that DR3 occupancy functions principally in repression, since mutation of this low-information-content site both eliminated DNA binding to DR3 and significantly weakened icdA repression, despite the fact that binding to DR1 and DR2 was intact. In addition, increasing the information content of any one of the three DRs or addition of a fourth DR increased ArcA-dependent repression but perturbed signal-dependent regulation of repression. Thus, our data show that the information content and number of DR elements are critical architectural features for maintaining a balance between high-affinity binding and signal-dependent regulation of icdA promoter function in response to changes in ArcA-P levels. Optimization of such architectural features may be a common strategy to either dampen or enhance the sensitivity of DNA binding among the members of the large OmpR/PhoB family of regulators as well as other transcription factors.

In Escherichia coli, the response regulator ArcA maintains homeostasis of redox carriers under O₂-limiting conditions through a comprehensive repression of carbon oxidation pathways that require aerobic respiration to recycle redox carriers. Although a binding site architecture comprised of a variable number of sequence recognition elements has been identified within the promoter regions of ArcA-repressed operons, it is unclear how this variable architecture dictates transcriptional regulation. By dissecting the role of multiple sequence elements within the icdA promoter, we provide insight into the design principles that allow ArcA to repress transcription within diverse promoter contexts. Our data suggest that the arrangement of recognition elements is tailored to achieve sufficient repression of a given promoter while maintaining appropriate signal-dependent regulation of repression, providing insight into how diverse binding site architectures link changes in O₂ with the fine-tuning of carbon oxidation pathway levels.

Structural dynamics of the two-component response regulator RstA in recognition of promoter DNA element

The RstA/RstB system is a bacterial two-component regulatory system consisting of the membrane sensor, RstB and its cognate response regulator (RR) RstA. The RstA of Klebsiella pneumoniae (kpRstA) consists of an N-terminal receiver domain (RD, residues 1-119) and a C-terminal DNA-binding domain (DBD, residues 130-236). Phosphorylation of kpRstA induces dimerization, which allows two kpRstA DBDs to bind to a tandem repeat, called the RstA box, and regulate the expression of downstream genes. Here we report the solution and crystal structures of the free kpRstA RD, DBD and DBD/RstA box DNA complex. The structure of the kpRstA DBD/RstA box complex suggests that the two protomers interact with the RstA box in an asymmetric fashion. Equilibrium binding studies further reveal that the two protomers within the kpRstA dimer bind to the RstA box in a sequential manner. Taken together, our results suggest a binding model where dimerization of the kpRstA RDs provides the platform to allow the first kpRstA DBD protomer to anchor protein-DNA interaction, whereas the second protomer plays a key role in ensuring correct recognition of the RstA box.

Atypical response regulator ChxR from Chlamydia trachomatis is structurally poised for DNA binding

ChxR is an atypical two-component signal transduction response regulator (RR) of the OmpR/PhoB subfamily encoded by the obligate intracellular bacterial pathogen Chlamydia trachomatis. Despite structural homology within both receiver and effector domains to prototypical subfamily members, ChxR does not require phosphorylation for dimer formation, DNA binding or transcriptional activation. Thus, we hypothesized that ChxR is in a conformation optimal for DNA binding with limited interdomain interactions. To address this hypothesis, the NMR solution structure of the ChxR effector domain was determined and used in combination with the previously reported ChxR receiver domain structure to generate a full-length dimer model based upon SAXS analysis. Small-angle scattering of ChxR supported a dimer with minimal interdomain interactions and effector domains in a conformation that appears to require only subtle reorientation for optimal major/minor groove DNA interactions. SAXS modeling also supported that the effector domains were in a head-to-tail conformation, consistent with ChxR recognizing tandem DNA repeats. The effector domain structure was leveraged to identify key residues that were critical for maintaining protein - nucleic acid interactions. In combination with prior analysis of the essential location of specific nucleotides for ChxR recognition of DNA, a model of the full-length ChxR dimer bound to its cognate cis-acting element was generated.

Phosphorylation-dependent derepression by the response regulator HnoC in the Shewanella oneidensis nitric oxide signaling network

Nitric oxide (NO) is an important signaling molecule that regulates diverse physiological processes in all domains of life. In many gammaproteobacteria, NO controls behavioral responses through a complex signaling network involving heme-nitric oxide/oxygen binding (H-NOX) domains as selective NO sensors. In Shewanella oneidensis, H-NOX-mediated NO sensing increases biofilm formation, which is thought to serve as a protective mechanism against NO cytotoxicity. The H-NOX/NO-responsive (hno) signaling network involves H-NOX-dependent control of HnoK autophosphorylation and phosphotransfer from HnoK to three response regulators. Two of these response regulators, HnoB and HnoD, regulate cyclic-di-GMP levels and influence biofilm formation. However, the role of the third response regulator in the signaling network, HnoC, has not been determined. Here we describe a role for HnoC as a transcriptional repressor for the signaling genes in the hno network. The genes controlled by HnoC were identified by microarray analysis, and its function as a repressor was confirmed in vivo. HnoC belongs to an uncharacterized family of DNA-binding response regulators. Binding of HnoC to its promoter targets was characterized in vitro, revealing an unprecedented regulation mechanism, which further extends the functional capabilities of DNA-binding response regulators. In the unphosphorylated state HnoC forms a tetramer, which tightly binds to an inverted-repeat target sequence overlapping with the promoter regions. Phosphorylation of HnoC induces dissociation of the response regulator tetramer and detachment of subunits from the promoter DNA, which subsequently leads to transcriptional derepression.

Structural basis of a physical blockage mechanism for the interaction of response regulator PmrA with connector protein PmrD from Klebsiella pneumoniae

In bacteria, the two-component system is the most prevalent for sensing and transducing environmental signals into the cell. The PmrA-PmrB two-component system, responsible for sensing external stimuli of high Fe(3+) and mild acidic conditions, can control the genes involved in lipopolysaccharide modification and polymyxin resistance in pathogens. In Klebsiella pneumoniae, the small basic connector protein PmrD protects phospho-PmrA and prolongs the expression of PmrA-activated genes. We previously determined the phospho-PmrA recognition mode of PmrD. However, how PmrA interacts with PmrD and prevents its dephosphorylation remains unknown. To address this question, we solved the x-ray crystal structure of the N-terminal receiver domain of BeF3(-)-activated PmrA (PmrA(N)) at 1.70 Å. With this structure, we applied the data-driven docking method based on NMR chemical shift perturbation to generate the complex model of PmrD-PmrA(N), which was further validated by site-directed spin labeling experiments. In the complex model, PmrD may act as a blockade to prevent phosphatase from contacting with the phosphorylation site on PmrA.