Identification and characterization of the tolQRA genes of Pseudomonas aeruginosa

Proteomic analysis of ceftazidime and meropenem-exposed Pseudomonas aeruginosa ATCC 9027

BACKGROUND

Pseudomonas aeruginosa is well known for its intrinsic ability to resist a wide range of antibiotics, thus complicates treatment. Thus, understanding the response of the pathogen to antibiotics is important for developing new therapies. In this study, proteomic response of P. aeruginosa to the commonly used anti-pseudomonas antibiotics, ceftazidime (Caz) and meropenem (Mem) was investigated.

METHODS

P. aeruginosa ATCC 9027, an antibiotic-susceptible strain, was exposed to sub-MIC values of antibiotics either Caz or Mem for 14 days to obtain E1 strains and then cultured in antibiotic-free environments for 10 days to obtain E2 strains. Proteomes of the initial and E1, E2 strains were identified and comparatively analyzed using isobaric tags for relative and absolute quantitation (iTRAQ) in cooperation with nano LC-MS/MS. Noted up and down-regulated proteins were confirmed with quantitative reverse transcriptase PCR (qRT-PCR).

RESULTS

Overall, 1039 and 1041 proteins were identified in Caz and Mem-exposed strains, respectively. Upon antibiotic exposure, there were 7-10% up-regulated (Caz: 71, Mem: 85) and down-regulated (Caz: 106, Mem: 69) proteins (1.5-fold change cut-off). For both Caz and Mem, the DEPs were primarily the ones involved in metabolic process, membrane, virulence, protein synthesis, and antibiotic resistance in which proteins involved in antibiotics resistance tended to be up-regulated while proteins involved in protein synthesis and metabolic process were down-regulated. Noted proteins included beta-lactamase AmpC which was up-regulated and OprD which was down-regulated in both the antibiotic-exposed strains. Besides, biofilm formation related proteins TssC1 and Hcp1 in Caz- exposed strains and the membrane/ periplasmic proteins Azu and PagL in Mem-exposed strains were found significantly down-regulated. qRT-PCR results confirmed the expression change of AmpC, Hcp1 and OprD proteins.

CONCLUSION

Exposure of Pseudomonas aeruginosa to sub-MIC values of Caz and Mem resulted in around 10% change in its proteome. Not only proteins with confirmed roles in antibiotic resistance mechanisms changed their expression but also virulence- associated proteins. Both Caz and Mem response involved up-regulation of AmpC and down-regulation of OprD. While TssC1 and Hcp1 were responsible for Caz response, Azu and PagL were more likely involved in Mem response.

Roles of the Tol/Pal System in Bacterial Pathogenesis and Its Application to Antibacterial Therapy

The Tol/Pal system (also written as “The Tol-Pal system”) is a set of protein complexes produced by most Gram-negative bacteria. It comprises the inner membrane-associated and the outer membrane-anchored subunits composed of the TolA, TolQ, and TolR proteins and the TolB and Pal proteins, respectively. Although the Tol/Pal system was first defined as bacterial proteins involved in colicin uptake of Escherichia coli, its global roles have been characterized in several studies as mentioned in this article. Pathogenesis of many Gram-negative pathogens is sustained by the Tol/Pal system. It is also essential for cell growth and fitness in some pathogens. Therefore, the Tol/Pal system is proposed as a potential target for antimicrobial chemotherapy. Although the tol/pal mutants are low in virulence, they still have the ability to stimulate the immune system. The Pal protein is highly immunogenic and induces both adaptive and innate immune responses. Therefore, the tol/pal mutant strains and Pal proteins also have potential vaccine properties. For these reasons, the Tol/Pal system represents a promising research target in the development of antibacterial therapeutic strategies for refractory infections caused by multi-drug-resistant (MDR), Gram-negative pathogens. In this paper, we summarize studies on the Tol/Pal system associated with bacterial pathogenesis and vaccine development.

The role of TolA, TolB, and TolR in cell morphology, OMVs production, and virulence of Salmonella Choleraesuis

The Tol–Pal system of Gram-negative bacteria is necessary for maintaining outer membrane integrity. It is a multiprotein complex of five envelope proteins, TolQ, TolR, TolA, TolB, and Pal. These proteins were first investigated in E. coli, and subsequently been identified in many other bacterial genera. However, the function of the Tol–Pal system in Salmonella Choleraesuis pathogenesis is still unclear. Here, we reported the role of three of these proteins in the phenotype and biology of S. Choleraesuis. We found that mutations in tolA, tolB, and tolR caused severe damage to the cell wall, which was supported by observing the microstructure of spherical forms, long chains, flagella defects, and membrane blebbing. We confirmed that all the mutants significantly decreased S. Choleraesuis survival when exposed to sodium deoxycholate and exhibited a high sensitivity to vancomycin, which may be explained by the disruption of envelope integrity. In addition, tolA, tolB, and tolR mutants displayed attenuated virulence in a mouse infection model. This could be interpreted as a series of defective phenotypes in the mutants, such as severe defects in envelope integrity, growth, and motility. Further investigation showed that all the genes participate in outer membrane vesicles (OMVs) biogenesis. Interestingly, immunization with OMVs from ΔtolB efficiently enhanced murine viability in contrast to OMVs from the wild-type S. Choleraesuis, suggesting its potential use in vaccination strategies. Collectively, this study provides an insight into the biological role of the S. Choleraesuis Tol–Pal system.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Multidrug Adaptive Resistance of Pseudomonas aeruginosa Swarming Cells

Swarming surface motility is a complex adaptation leading to multidrug antibiotic resistance and virulence factor production in Pseudomonas aeruginosa Here, we expanded previous studies to demonstrate that under swarming conditions, P. aeruginosa PA14 is more resistant to multiple antibiotics, including aminoglycosides, β-lactams, chloramphenicol, ciprofloxacin, tetracycline, trimethoprim, and macrolides, than swimming cells, but is not more resistant to polymyxin B. We investigated the mechanism(s) of swarming-mediated antibiotic resistance by examining the transcriptomes of swarming cells and swarming cells treated with tobramycin by transcriptomics (RNA-Seq) and reverse transcriptase quantitative PCR (qRT-PCR). RNA-Seq of swarming cells (versus swimming) revealed 1,581 dysregulated genes, including 104 transcriptional regulators, two-component systems, and sigma factors, numerous upregulated virulence and iron acquisition factors, and downregulated ribosomal genes. Strain PA14 mutants in resistome genes that were dysregulated under swarming conditions were tested for their ability to swarm in the presence of tobramycin. In total, 41 mutants in genes dysregulated under swarming conditions were shown to be more resistant to tobramycin under swarming conditions, indicating that swarming-mediated tobramycin resistance was multideterminant. Focusing on two genes downregulated under swarming conditions, both prtN and wbpW mutants were more resistant to tobramycin, while the prtN mutant was additionally resistant to trimethoprim under swarming conditions; complementation of these mutants restored susceptibility. RNA-Seq of swarming cells treated with subinhibitory concentrations of tobramycin revealed the upregulation of the multidrug efflux pump MexXY and downregulation of virulence factors.

Lysocins: Bioengineered Antimicrobials That Deliver Lysins across the Outer Membrane of Gram-Negative Bacteria

The prevalence of multidrug-resistant Pseudomonas aeruginosa has stimulated development of alternative therapeutics. Bacteriophage peptidoglycan hydrolases, termed lysins, represent an emerging antimicrobial option for targeting Gram-positive bacteria. However, lysins against Gram-negatives are generally deterred by the outer membrane and their inability to work in serum. One solution involves exploiting evolved delivery systems used by colicin-like bacteriocins (e.g., S-type pyocins of P. aeruginosa) to translocate through the outer membrane. Following surface receptor binding, colicin-like bacteriocins form Tol- or TonB-dependent translocons to actively import bactericidal domains through outer membrane protein channels. With this understanding, we developed lysocins, which are bioengineered lysin-bacteriocin fusion molecules capable of periplasmic import. In our proof-of-concept studies, components from the P. aeruginosa bacteriocin pyocin S2 (PyS2) responsible for surface receptor binding and outer membrane translocation were fused to the GN4 lysin to generate the PyS2-GN4 lysocin. PyS2-GN4 delivered the GN4 lysin to the periplasm to induce peptidoglycan cleavage and log-fold killing of P. aeruginosa with minimal endotoxin release. While displaying narrow-spectrum antipseudomonal activity in human serum, PyS2-GN4 also efficiently disrupted biofilms, outperformed standard-of-care antibiotics, exhibited no cytotoxicity toward eukaryotic cells, and protected mice from P. aeruginosa challenge in a bacteremia model. In addition to targeting P. aeruginosa, lysocins can be constructed to target other prominent Gram-negative bacterial pathogens.

Proteomic Analysis of Resistance of Gram-Negative Bacteria to Chlorhexidine and Impacts on Susceptibility to Colistin, Antimicrobial Peptides, and Ceragenins

Use of chlorhexidine in clinical settings has led to concerns that repeated exposure of bacteria to sub-lethal doses of chlorhexidine might result in chlorhexidine resistance and cross resistance with other cationic antimicrobials including colistin, endogenous antimicrobial peptides (AMPs) and their mimics, ceragenins. We have previously shown that colistin-resistant Gram-negative bacteria remain susceptible to AMPs and ceragenins. Here, we investigated the potential for cross resistance between chlorhexidine, colistin, AMPs and ceragenins by serial exposure of standard strains of Gram-negative bacteria to chlorhexidine to generate resistant populations of organisms. Furthermore, we performed a proteomics study on the chlorhexidine-resistant strains and compared them to the wild-type strains to find the pathways by which bacteria develop resistance to chlorhexidine. Serial exposure of Gram-negative bacteria to chlorhexidine resulted in four- to eight-fold increases in minimum inhibitory concentrations (MICs). Chlorhexidine-resistant organisms showed decreased susceptibility to colistin (8- to 32-fold increases in MICs) despite not being exposed to colistin. In contrast, chlorhexidine-resistant organisms had the same MICs as the original strains when tested with representative AMPs (LL-37 and magainin I) and ceragenins (CSA-44 and CSA-131). These results imply that there may be a connection between the emergence of highly colistin-resistant Gram-negative pathogens and the prevalence of chlorhexidine usage. Yet, use of chlorhexidine may not impact innate immune defenses (e.g., AMPs) and their mimics (e.g., ceragenins). Here, we also show that chlorhexidine resistance is associated with upregulation of proteins involved in the assembly of LPS for outer membrane biogenesis and virulence factors in Pseudomonas aeruginosa. Additionally, resistance to chlorhexidine resulted in elevated expression levels of proteins associated with chaperones, efflux pumps, flagella and cell metabolism. This study provides a comprehensive overview of the evolutionary proteomic changes in P. aeruginosa following exposure to chlorhexidine and colistin. These results have important clinical implications considering the continuous application of chlorhexidine in hospitals that could influence the emergence of colistin-resistant strains.

Similarities and Differences between Colicin and Filamentous Phage Uptake by Bacterial Cells

Gram-negative bacteria have evolved a complex envelope to adapt and survive in a broad range of ecological niches. This physical barrier is the first line of defense against noxious compounds and viral particles called bacteriophages. Colicins are a family of bactericidal proteins produced by and toxic to Escherichia coli and closely related bacteria. Filamentous phages have a complex structure, composed of at least five capsid proteins assembled in a long thread-shaped particle, that protects the viral DNA. Despite their difference in size and complexity, group A colicins and filamentous phages both parasitize multiprotein complexes of their sensitive host for entry. They first bind to a receptor located at the surface of the target bacteria before specifically recruiting components of the Tol system to cross the outer membrane and find their way through the periplasm. The Tol system is thought to use the proton motive force of the inner membrane to maintain outer membrane integrity during the life cycle of the cell. This review describes the sequential docking mechanisms of group A colicins and filamentous phages during their uptake by their bacterial host, with a specific focus on the translocation step, promoted by interactions with the Tol system.

Outer membrane lipid homeostasis via retrograde phospholipid transport in Escherichia coli

Biogenesis of the outer membrane (OM) in Gram-negative bacteria, which is essential for viability, requires the coordinated transport and assembly of proteins and lipids, including lipopolysaccharides (LPS) and phospholipids (PLs), into the membrane. While pathways for LPS and OM protein assembly are well-studied, how PLs are transported to and from the OM is not clear. Mechanisms that ensure OM stability and homeostasis are also unknown. The trans-envelope Tol-Pal complex, whose physiological role has remained elusive, is important for OM stability. Here, we establish that the Tol-Pal complex is required for PL transport and OM lipid homeostasis in Escherichia coli. Cells lacking the complex exhibit defects in lipid asymmetry and accumulate excess PLs in the OM. This imbalance in OM lipids is due to defective retrograde PL transport in the absence of a functional Tol-Pal complex. Thus, cells ensure the assembly of a stable OM by maintaining an excess flux of PLs to the OM only to return the surplus to the inner membrane. Our findings also provide insights into the mechanism by which the Tol-Pal complex may promote OM invagination during cell division.

Electrostatic interactions between the CTX phage minor coat protein and the bacterial host receptor TolA drive the pathogenic conversion of Vibrio cholerae

Vibrio cholerae is a natural inhabitant of aquatic environments and converts to a pathogen upon infection by a filamentous phage, CTXΦ, that transmits the cholera toxin-encoding genes. This toxigenic conversion of V. cholerae has evident implication in both genome plasticity and epidemic risk, but the early stages of the infection have not been thoroughly studied. CTXΦ transit across the bacterial periplasm requires binding between the minor coat protein named pIII and a bacterial inner-membrane receptor, TolA, which is part of the conserved Tol-Pal molecular motor. To gain insight into the TolA-pIII complex, we developed a bacterial two-hybrid approach, named Oxi-BTH, suited for studying the interactions between disulfide bond-folded proteins in the bacterial cytoplasm of an Escherichia coli reporter strain. We found that two of the four disulfide bonds of pIII are required for its interaction with TolA. By combining Oxi-BTH assays, NMR, and genetic studies, we also demonstrate that two intermolecular salt bridges between TolA and pIII provide the driving forces of the complex interaction. Moreover, we show that TolA residue Arg-325 involved in one of the two salt bridges is critical for proper functioning of the Tol-Pal system. Our results imply that to prevent host evasion, CTXΦ uses an infection strategy that targets a highly conserved protein of Gram-negative bacteria essential for the fitness of V. cholerae in its natural environment.

Identification of Novel Genomic Islands in Liverpool Epidemic Strain of Pseudomonas aeruginosa Using Segmentation and Clustering

Pseudomonas aeruginosa is an opportunistic pathogen implicated in a myriad of infections and a leading pathogen responsible for mortality in patients with cystic fibrosis (CF). Horizontal transfers of genes among the microorganisms living within CF patients have led to highly virulent and multi-drug resistant strains such as the Liverpool epidemic strain of P. aeruginosa, namely the LESB58 strain that has the propensity to acquire virulence and antibiotic resistance genes. Often these genes are acquired in large clusters, referred to as "genomic islands (GIs)." To decipher GIs and understand their contributions to the evolution of virulence and antibiotic resistance in P. aeruginosa LESB58, we utilized a recursive segmentation and clustering procedure, presented here as a genome-mining tool, "GEMINI." GEMINI was validated on experimentally verified islands in the LESB58 strain before examining its potential to decipher novel islands. Of the 6062 genes in P. aeruginosa LESB58, 596 genes were identified to be resident on 20 GIs of which 12 have not been previously reported. Comparative genomics provided evidence in support of our novel predictions. Furthermore, GEMINI unraveled the mosaic structure of islands that are composed of segments of likely different evolutionary origins, and demonstrated its ability to identify potential strain biomarkers. These newly found islands likely have contributed to the hyper-virulence and multidrug resistance of the Liverpool epidemic strain of P. aeruginosa.

Structural and biophysical analysis of nuclease protein antibiotics

Protein antibiotics (bacteriocins) are a large and diverse family of multidomain toxins that kill specific Gram-negative bacteria during intraspecies competition for resources. Our understanding of the mechanism of import of such potent toxins has increased significantly in recent years, especially with the reporting of several structures of bacteriocin domains. Less well understood is the structural biochemistry of intact bacteriocins and how these compare across bacterial species. Here, we focus on endonuclease (DNase) bacteriocins that target the genomes of Escherichia coli and Pseudomonas aeruginosa, known as E-type colicins and S-type pyocins, respectively, bound to their specific immunity (Im) proteins. First, we report the 3.2 Å structure of the DNase colicin ColE9 in complex with its ultra-high affinity Im protein, Im9. In contrast with Im3, which when bound to the ribonuclease domain of the homologous colicin ColE3 makes contact with the translocation (T) domain of the toxin, we find that Im9 makes no such contact and only interactions with the ColE9 cytotoxic domain are observed. Second, we report small-angle X-ray scattering data for two S-type DNase pyocins, S2 and AP41, into which are fitted recently determined X-ray structures for isolated domains. We find that DNase pyocins and colicins are both highly elongated molecules, even though the order of their constituent domains differs. We discuss the implications of these architectural similarities and differences in the context of the translocation mechanism of protein antibiotics through the cell envelope of Gram-negative bacteria.

Proteomic Analyses of Chlorhexidine Tolerance Mechanisms in Delftia acidovorans Biofilms

Protein expression and fatty acid profiles of biofilm cells of chlorhexidine-tolerant Delftia acidovorans (MIC = 15 µg/ml) and its chlorhexidine-susceptible mutant (MIC = 1 µg/ml) were investigated. The chlorhexidine-susceptible mutant (MT51) was derived from the parental strain (WT15) using Tn5 transposon mutagenesis. The disrupted gene was identified as tolQ, a component of the tolQRAB gene cluster known to be involved in outer membrane stability. Proteomic responses of biofilm cells were compared by differential in-gel electrophoresis following exposure to chlorhexidine at sub-MIC (10 µg/ml) and above-MIC (30 µg/ml) concentrations. Numerous changes in protein abundance were observed in biofilm cells following chlorhexidine exposure, suggesting that molecular changes occurred during adaptation to chlorhexidine. Forty proteins showing significant differences (≥1.5-fold; P < 0.05) were identified by mass spectrometry and were associated with various functions, including amino acid and lipid biosynthesis, protein translation, energy metabolism, and stress-related functions (e.g., GroEL, aspartyl/glutamyl-tRNA amidotransferase, elongation factor Tu, Clp protease, and hydroxymyristoyl-ACP dehydratase). Several proteins involved in fatty acid synthesis were affected by chlorhexidine, in agreement with fatty acid analysis, wherein chlorhexidine-induced shifts in the fatty acid profile were observed in the chlorhexidine-tolerant cells, primarily the cyclic fatty acids. Transmission electron microscopy revealed more prominent changes in the cell envelope of chlorhexidine-susceptible MT51 cells. This study suggests that multiple mechanisms involving both the cell envelope (and likely TolQ) and panmetabolic regulation play roles in chlorhexidine tolerance in D. acidovorans. IMPORTANCE Delftia acidovorans has been associated with a number of serious infections, including bacteremia, empyema, bacterial endocarditis, and ocular and urinary tract infections. It has also been linked with a variety of surface-associated nosocomial infections. Biofilm-forming antimicrobial-resistant D. acidovorans strains have also been isolated, including ones displaying resistance to the common broad-spectrum agent chlorhexidine. The mechanisms of chlorhexidine resistance in D. acidovorans are not known; hence, a chlorhexidine-susceptible mutant of the tolerant wild-type strain was obtained using transposon mutagenesis, and the proteome and ultrastructural changes of both strains were compared under chlorhexidine challenge.

Structure and function of the Escherichia coli Tol-Pal stator protein TolR

TolR is a 15-kDa inner membrane protein subunit of the Tol-Pal complex in Gram-negative bacteria, and its function is poorly understood. Tol-Pal is recruited to cell division sites where it is involved in maintaining the integrity of the outer membrane. TolR is related to MotB, the peptidoglycan (PG)-binding stator protein from the flagellum, suggesting it might serve a similar role in Tol-Pal. The only structure thus far reported for TolR is of the periplasmic domain from Haemophilus influenzae in which N- and C-terminal residues had been deleted (TolR(62–133), Escherichia coli numbering). H. influenzae TolR(62–133) is a symmetrical dimer with a large deep cleft at the dimer interface. Here, we present the 1.7-Å crystal structure of the intact periplasmic domain of E. coli TolR (TolR(36–142)). E. coli TolR(36–142) is also dimeric, but the architecture of the dimer is radically different from that of TolR(62–133) due to the intertwining of its N and C termini. TolR monomers are rotated ∼180° relative to each other as a result of this strand swapping, obliterating the putative PG-binding groove seen in TolR(62–133). We found that removal of the strand-swapped regions (TolR(60–133)) exposes cryptic PG binding activity that is absent in the full-length domain. We conclude that to function as a stator in the Tol-Pal complex dimeric TolR must undergo large scale structural remodeling reminiscent of that proposed for MotB, where the N- and C-terminal sequences unfold in order for the protein to both reach and bind the PG layer ∼90 Å away from the inner membrane.

The periplasmic protein TolB as a potential drug target in Pseudomonas aeruginosa

The Gram-negative bacterium Pseudomonas aeruginosa is one of the most dreaded pathogens in the hospital setting, and represents a prototype of multi-drug resistant "superbug" for which effective therapeutic options are very limited. The identification and characterization of new cellular functions that are essential for P. aeruginosa viability and/or virulence could drive the development of anti-Pseudomonas compounds with novel mechanisms of action. In this study we investigated whether TolB, the periplasmic component of the Tol-Pal trans-envelope protein complex of Gram-negative bacteria, represents a potential drug target in P. aeruginosa. By combining conditional mutagenesis with the analysis of specific pathogenicity-related phenotypes, we demonstrated that TolB is essential for P. aeruginosa growth, both in laboratory and clinical strains, and that TolB-depleted P. aeruginosa cells are strongly defective in cell-envelope integrity, resistance to human serum and several antibiotics, as well as in the ability to cause infection and persist in an insect model of P. aeruginosa infection. The essentiality of TolB for P. aeruginosa growth, resistance and pathogenicity highlights the potential of TolB as a novel molecular target for anti-P. aeruginosa drug discovery.

Ribosomally encoded antibacterial proteins and peptides from Pseudomonas

Members of the Pseudomonas genus produce diverse secondary metabolites affecting other bacteria, fungi or predating nematodes and protozoa but are also equipped with the capacity to secrete different types of ribosomally encoded toxic peptides and proteins, ranging from small microcins to large tailocins. Studies with the human pathogen Pseudomonas aeruginosa have revealed that effector proteins of type VI secretion systems are part of the antibacterial armamentarium deployed by pseudomonads. A novel class of antibacterial proteins with structural similarity to plant lectins was discovered by studying antagonism among plant-associated Pseudomonas strains. A genomic perspective on pseudomonad bacteriocinogeny shows that the modular architecture of S pyocins of P. aeruginosa is retained in a large diversified group of bacteriocins, most of which target DNA or RNA. Similar modularity is present in as yet poorly characterized Rhs (recombination hot spot) proteins and CDI (contact-dependent inhibition) proteins. Well-delimited domains for receptor recognition or cytotoxicity enable the design of chimeric toxins with novel functionalities, which has been applied successfully for S and R pyocins. Little is known regarding how these antibacterials are released and ultimately reach their targets. Other remaining issues concern the identification of environmental triggers activating these systems and assessment of their ecological impact in niches populated by pseudomonads.

Colicin import into E. coli cells: a model system for insights into the import mechanisms of bacteriocins

Bacteriocins are a diverse group of ribosomally synthesized protein antibiotics produced by most bacteria. They range from small lanthipeptides produced by lactic acid bacteria to much larger multi domain proteins of Gram negative bacteria such as the colicins from Escherichia coli. For activity bacteriocins must be released from the producing cell and then bind to the surface of a sensitive cell to instigate the import process leading to cell death. For over 50years, colicins have provided a working platform for elucidating the structure/function studies of bacteriocin import and modes of action. An understanding of the processes that contribute to the delivery of a colicin molecule across two lipid membranes of the cell envelope has advanced our knowledge of protein-protein interactions (PPI), protein-lipid interactions and the role of order-disorder transitions of protein domains pertinent to protein transport. In this review, we provide an overview of the arrangement of genes that controls the synthesis and release of the mature protein. We examine the uptake processes of colicins from initial binding and sequestration of binding partners to crossing of the outer membrane, and then discuss the translocation of colicins through the cell periplasm and across the inner membrane to their cytotoxic site of action. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Overexpression of the Escherichia coli TolQ protein leads to a null-FtsN-like division phenotype

Mutations involving the Tol-Pal complex of Escherichia coli result in a subtle phenotype in which cells chain when grown under low-salt conditions. Here, the nonpolar deletion of individual genes encoding the cytoplasmic membrane-associated components of the complex (TolQ, TolR, TolA) produced a similar phenotype. Surprisingly, the overexpression of one of these proteins, TolQ, resulted in a much more overt phenotype in which cells occurred as elongated rods coupled in long chains when grown under normal salt conditions. Neither TolR nor TolA overexpression produced a phenotype, nor was the presence of either protein required for the TolQ-dependent phenotype. Consistent with their native membrane topology, the amino-terminal domain of TolQ specifically associated in vivo with the periplasmic domain of FtsN in a cytoplasm-based two-hybrid analysis. Further, the concomitant overexpression of FtsN rescued the TolQ-dependent phenotype, suggesting a model wherein the overexpression of TolQ sequesters FtsN, depleting this essential protein from the divisome during Gram-negative cell division. The role of the Tol-Pal system in division is discussed.

Over-expression of the cytoplasmic membrane protein TolQ resulted in a division phenotype similar to that seen in cells depleted for FtsN. Two hybrid analysis suggested that TolQ and FtsN physically interact through domains that localize in the periplasmic space; while the concurrent over-expression of FtsN alleviated the TolQ over-expression phenotype. Together these results suggest a model wherein over-expressed TolQ sequesters FtsN, disrupting normal cell division.

Legionella pneumophila LbtU acts as a novel, TonB-independent receptor for the legiobactin siderophore

Gram-negative Legionella pneumophila produces a siderophore (legiobactin) that promotes lung infection. We previously determined that lbtA and lbtB are required for the synthesis and secretion of legiobactin. DNA sequence and reverse transcription-PCR (RT-PCR) analyses now reveal the presence of an iron-repressed gene (lbtU) directly upstream of the lbtAB-containing operon. In silico analysis predicted that LbtU is an outer membrane protein consisting of a 16-stranded transmembrane β-barrel, multiple extracellular domains, and short periplasmic tails. Immunoblot analysis of cell fractions confirmed an outer membrane location for LbtU. Although replicating normally in standard media, lbtU mutants, like lbtA mutants, were impaired for growth on iron-depleted agar media. While producing typical levels of legiobactin, lbtU mutants were unable to use supplied legiobactin to stimulate growth on iron-depleted media and displayed an inability to take up iron. Complemented lbtU mutants behaved as the wild type did. The lbtU mutants were also impaired for infection in a legiobactin-dependent manner. Together, these data indicate that LbtU is involved in the uptake of legiobactin and, based upon its location, is most likely the Legionella siderophore receptor. The sequence and predicted two-dimensional (2D) and 3D structures of LbtU were distinct from those of all known siderophore receptors, which generally contain a 22-stranded β-barrel and an extended N terminus that binds TonB in order to transduce energy from the inner membrane. This observation coupled with the fact that L. pneumophila does not encode TonB suggests that LbtU is a new type of receptor that participates in a form of iron uptake that is mechanistically distinct from the existing paradigm.

Characterization of the orf1-tolQRA operon in Pseudomonas aeruginosa

The tol-pal genes play important roles in maintaining outer membrane integrity, transmembrane transportation, and cell division in Gram-negative bacteria. In Pseudomonas aeruginosa, an important human opportunistic pathogen, the tol-oprL genes are organized uniquely in three operons, orf1-tolQRA, tolB and oprL-orf2, and are regulated by iron availability. Similarity between TolQRA and the iron transport system ExbBD-TonB also exists in P. aeruginosa and they can replace each other imperfectly. It is of importance to investigate the regulation and functions of this membrane complex. In the present study, we characterized the promoters and expression profiles of the orf1-tolQRA operon and investigated the function of Orf1. Primer extension was carried out by using both isotope-labeled and florescence labeled primers and the expression profiles were determined using both lacZ and luxCDABE-based transcriptional fusions. Our results revealed two distinct promoters at the upstream region of tolQRA; the one located in front of orf1 was constitutive whereas the other within the orf1 coding region was iron regulated. Expression profiles indicate the tol genes were also downregulated by the quorum-sensing systems during the late stage of growth. Unlike tolQ and tolA, we were able to construct a viable orf1 knockout strain, and the mutant exhibited altered cell and colony morphology, providing first evidence that Orf1 plays a non-essential role in the Tol-OprL complex in P. aeruginosa.

Toxicogenomic response of Pseudomonas aeruginosa to ortho-phenylphenol

Background

Pseudomonas aeruginosa (P. aeruginosa) is the most common opportunistic pathogen implicated in nosocomial infections and in chronic lung infections in cystic fibrosis patients. Ortho-phenylphenol (OPP) is an antimicrobial agent used as an active ingredient in several EPA registered disinfectants. Despite its widespread use, there is a paucity of information on its target molecular pathways and the cellular responses that it elucidates in bacteria in general and in P. aeruginosa in particular. An understanding of the OPP-driven gene regulation and cellular response it elicits will facilitate more effective utilization of this antimicrobial and possibly lead to the development of more effective disinfectant treatments.

Results

Herein, we performed a genome-wide transcriptome analysis of the cellular responses of P. aeruginosa exposed to 0.82 mM OPP for 20 and 60 minutes. Our data indicated that OPP upregulated the transcription of genes encoding ribosomal, virulence and membrane transport proteins after both treatment times. After 20 minutes of exposure to 0.82 mM OPP, genes involved in the exhibition of swarming motility and anaerobic respiration were upregulated. After 60 minutes of OPP treatment, the transcription of genes involved in amino acid and lipopolysaccharide biosynthesis were upregulated. Further, the transcription of the ribosome modulation factor (rmf) and an alternative sigma factor (rpoS) of RNA polymerase were downregulated after both treatment times.

Conclusion

Results from this study indicate that after 20 minutes of exposure to OPP, genes that have been linked to the exhibition of anaerobic respiration and swarming motility were upregulated. This study also suggests that the downregulation of the rmf and rpoS genes may be indicative of the mechanism by which OPP causes decreases in cell viability in P. aeruginosa. Consequently, a protective response involving the upregulation of translation leading to the increased synthesis of membrane related proteins and virulence proteins is possibly induced after both treatment times. In addition, cell wall modification may occur due to the increased synthesis of lipopolysaccharide after 60 minutes exposure to OPP. This gene expression profile can now be utilized for a better understanding of the target cellular pathways of OPP in P. aeruginosa and how this organism develops resistance to OPP.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Construction of a mutant and characterization of the role of the vaccine antigen P6 in outer membrane integrity of nontypeable Haemophilus influenzae

Outer membrane protein P6 is the subject of investigation as a vaccine antigen to prevent infections caused by nontypeable Haemophilus influenzae, which causes otitis media in children and respiratory tract infections in adults with chronic lung disease. P6 induces protective immune responses in animal models and is the target of potentially protective immune responses in humans. P6 is a 16-kDa lipoprotein that shares homology with the peptidoglycan-associated lipoproteins of gram-negative bacteria and is highly conserved among strains of H. influenzae. To characterize the function of P6, an isogenic mutant was constructed by replacing the P6 gene with a chloramphenicol resistance cassette. The P6 mutant showed altered colony morphology and slower growth in vitro than that of the parent strain. By electron microscopy, the P6 mutant cells demonstrated increased size, variability in size, vesicle formation, and fragility compared to the parent cells. The P6 mutant showed hypersensitivity to selected antibiotics with different mechanisms of action, indicating increased accessibility of the agents to their targets. The P6 mutant was more sensitive to complement-mediated killing by normal human serum. Complementation of the mutation in trans completely or partially restored the phenotypes. We concluded that P6 plays a structural role in maintaining the integrity of the outer membrane by anchoring the outer membrane to the cell wall. The observation that the absence of expression of P6 is detrimental to the cell is a highly desirable feature for a vaccine antigen, supporting further investigation of P6 as a vaccine candidate for H. influenzae.

Tol-Pal proteins are critical cell envelope components of Erwinia chrysanthemi affecting cell morphology and virulence

The tol-pal genes are necessary for maintaining the outer-membrane integrity of Gram-negative bacteria. These genes were first described in Escherichia coli, and more recently in several other species. They are involved in the pathogenesis of E. coli, Haemophilus ducreyi, Vibrio cholerae and Salmonella enterica. The role of the tol-pal genes in bacterial pathogenesis was investigated in the phytopathogenic enterobacterium Erwinia chrysanthemi, assuming that this organism might be a good model for such a study. The whole Er. chrysanthemi tol-pal region was characterized. Tol-Pal proteins, except TolA, showed high identity scores with their E. coli homologues. Er. chrysanthemi mutants were constructed by introducing a uidA-kan cassette in the ybgC, tolQ, tolA, tolB, pal and ybgF genes. All the mutants were hypersensitive to bile salts. Mutations in tolQ, tolA, tolB and pal were deleterious for the bacteria, which required high concentrations of sugars or osmoprotectants for their viability. Consistent with this observation, they were greatly impaired in their cell morphology and division, which was evidenced by observations of cell filaments, spherical forms, membrane blebbing and mislocalized bacterial septa. Moreover, tol-pal mutants showed a reduced virulence in a potato tuber model and on chicory leaves. This could be explained by a combination of impaired phenotypes in the tol-pal mutants, such as reduced growth and motility and a decreased production of pectate lyases, the major virulence factor of Er. chrysanthemi.

Selection for highly biased amino acid frequency in the TolA cell envelope protein of Proteobacteria

The bacterial cell envelope protein TolA functions to maintain the integrity of the cell membrane. This protein contains high levels of alanine and lysine that are used in the formation of alpha helices, which are required for normal protein function. The neutral model of molecular evolution predicts that amino acid composition and nucleotide composition are driven by the underlying GC content, as a result of mutation bias. However, this study shows that selection has acted to maintain high levels of alanine and lysine in the TolA protein of Proteobacteria, which in turn has biased nucleotide composition in the corresponding tolA gene.

Improved methods for producing outer membrane vesicles in Gram-negative bacteria

Outer membrane vesicle formation occurs during Gram-negative bacterial growth. However, natural production of large amounts of outer membrane vesicles has only been described in a few bacterial genera. The purified vesicles of some bacterial pathogens have shown potential applications in vaccinology and in antibiotic therapy. This study focused on the development of a gene expression system able to induce production of large amounts of outer membrane vesicles. The Tol-Pal system of Escherichia coli, required to maintain outer membrane integrity, is composed of five cell envelope proteins, TolA, TolB, TolQ, TolR and Pal. Tol proteins are parasitized by filamentous bacteriophages and by colicins. The phage infection process and colicin import require, respectively, the N-terminal domain of the minor coat g3p protein and the translocation domain of colicins, with both domains interacting with Tol proteins. In this study, we show that the periplasmic production of either Tol, g3p or colicin domains was able to specifically destabilize the E. coli or Shigella flexneri cell envelope and to induce production of high amounts of vesicles. This technique was further found to work efficiently in Salmonella enterica serovar Typhimurium.

Transcriptional organization of the Pseudomonas putida tol-oprL genes

Proteins of the Tol system play a key role in the maintenance of outer membrane integrity and cell morphology in gram-negative bacteria. In Pseudomonas putida, the seven genes, orf1, tolQ, tolR, tolA, tolB, oprL, and orf2, which encode the proteins of this complex, are clustered in a 5.8-kb region of chromosomal DNA. Analysis of polar mutations, reverse transcriptase PCR assays, and transcriptional fusion constructs with a promoterless lacZ gene revealed that the genes are arranged in two operons: orf1 tolQ tolR tolA tolB and oprL orf2. We were also able to find a transcript that was initiated at the orf1 promoter and covered the two operons in a single mRNA. On the basis of the OprL protein level, we surmised that this transcript contributed only about 10 to 15% of the total OprL protein. Primer extension analysis identified the oprL orf2 operon promoter within the tolB gene, and the -10 and -35 regions exhibited some similarity to those of sigma(70)-recognized promoters. The transcription start point of orf1 was located 91 bp upstream of the orf1 start codon, and the -10/-35 region also exhibited sigma(70) -10/-35 recognition sequences. The expression from both promoters in rich and minimal media was constitutive and was very little influenced by the growth phase or iron-deficient conditions. In addition, analyses of the beta-galactosidase activities of different translational fusion constructs revealed that translation of tolA and orf2 genes was dependent on the translation of their corresponding upstream genes (tolR and oprL, respectively).

Structure of the periplasmic domain of Pseudomonas aeruginosa TolA: evidence for an evolutionary relationship with the TonB transporter protein

The crystal structure of the C-terminal domain III of Pseudomonas aeruginosa TolA has been determined at 1.9 A resolution. The fold is similar to that of the corresponding domain of Escherichia coli TolA, despite the limited amino acid sequence identity of the two proteins (20%). A pattern was discerned that conserves the fold of domain III within the wider TolA family and, moreover, reveals a relationship between TolA domain III and the C-terminal domain of the TonB transporter proteins. We propose that the TolA and TonB C-terminal domains have a common evolutionary origin and are related by means of domain swapping, with interesting mechanistic implications. We have also determined the overall shape of the didomain, domains II + III, of P.aeruginosa TolA by solution X-ray scattering. The molecule is monomeric-its elongated, stalk shape can accommodate the crystal structure of domain III at one end, and an elongated helical bundle within the portion corresponding to domain II. Based on these data, a model for the periplasmic domains of P.aeruginosa TolA is presented that may explain the inferred allosteric properties of members of the TolA family. The mechanisms of TolA-mediated entry of bateriophages in P.aeruginosa and E.coli are likely to be similar.

The pyocins of Pseudomonas aeruginosa

Pyocins are produced by more than 90% of Pseudomonas aeruginosa strains and each strain may synthesise several pyocins. The pyocin genes are located on the P. aeruginosa chromosome and their activities are inducible by mutagenic agents such as mitomycin C. Three types of pyocins are described. (i). R-type pyocins resemble non-flexible and contractile tails of bacteriophages. They provoke a depolarisation of the cytoplasmic membrane in relation with pore formation. (ii). F-type pyocins also resemble phage tails, but with a flexible and non-contractile rod-like structure. (iii). S-type pyocins are colicin-like, protease-sensitive proteins. They are constituted of two components. The large component carries the killing activity (DNase activity for pyocins S1, S2, S3, AP41; tRNase for pyocin S4; channel-forming activity for pyocin S5). It interacts with the small component (immunity protein). The synthesis of pyocins starts when a mutagen increases the expression of the recA gene and activates the RecA protein, which cleaves the repressor PrtR, liberating the expression of the protein activator gene prtN. R and F-pyocins are derived from an ancestral gene, with similarities to the P2 phage family and the lambda phage family, respectively. The killing domains of S1, S2, AP41 pyocins show a close evolutionary relationship with E2 group colicins, S4 pyocin with colicin E5, and S5 pyocin with colicins Ia, and Ib.

The Tol proteins of Escherichia coli and their involvement in the translocation of group A colicins

The Tol proteins are involved in outer membrane stability of Gram-negative bacteria. The TolQRA proteins form a complex in the inner membrane while TolB and Pal interact near the outer membrane. These two complexes are transiently connected by an energy-dependent interaction between Pal and TolA. The Tol proteins have been parasitized by group A colicins for their translocation through the cell envelope. Recent advances in the structure and energetics of the Tol system, as well as the interactions between the N-terminal translocation domain of colicins and the Tol proteins are presented.

Analysis of the Escherichia coli Tol-Pal and TonB systems by periplasmic production of Tol, TonB, colicin, or phage capsid soluble domains

The aim of this review is to describe an in vivo assay of the interactions taking place in the Tol-Pal or TonB-ExbB-ExbD envelope complexes in the periplasm of Escherichia coli and between them and colicins or g3p protein of filamentous bacteriophages. Domains of colicins or periplasmic soluble domains of Tol or TonB proteins can be artificially addressed to the periplasm of bacteria by fusing them to a signal sequence from an exported protein. These domains interact specifically in the periplasm with the Tol or TonB complexes and disturb their function, which can be directly detected by the appearance of specific tol or tonB phenotypes. This technique can be used to detect new interactions, to characterize them biochemically and to map them or to induce tol or tonB phenotypes to study the functions of these two complexes.

Mutational analysis of the TonB1 energy coupler of Pseudomonas aeruginosa

Siderophore-mediated iron transport in Pseudomonas aeruginosa is dependent upon the cytoplasmic membrane-associated TonB1 energy coupling protein for activity. To assess the functional significance of the various regions of this molecule and to identify functionally important residues, the tonB1 gene was subjected to site-directed mutagenesis, and the influence on iron acquisition was determined. The novel N-terminal extension of TonB1, which is absent in all other examples of TonB, was required for TonB1 activity in both P. aeruginosa and Escherichia coli. Appending it to the N terminus of the nonfunctional (in P. aeruginosa) Escherichia coli TonB protein (TonB(Ec)) rendered TonB(Ec) weakly active in P. aeruginosa and did not compromise the activity of this protein in E. coli. Elimination of the membrane-spanning, presumed membrane anchor sequence of TonB1 abrogated TonB1 activity in P. aeruginosa and E. coli. Interestingly, however, a conserved His residue within the membrane anchor sequence, shown to be required for TonB(Ec) function in E. coli, was shown here to be essential for TonB1 activity in E. coli but not in P. aeruginosa. Several mutations within the C-terminal end of TonB1, within a region exhibiting the greatest similarity to other TonB proteins, compromised a TonB1 contribution to iron acquisition in both P. aeruginosa and E. coli, including substitutions at Tyr264, Glu274, Lys278, and Asp304. Mutations at Pro265, Gln293, and Val294 also impacted negatively on TonB1 function in E. coli but not in P. aeruginosa. The Asp304 mutation was suppressed by a second mutation at Glu274 of TonB1 but only in P. aeruginosa. Several TonB1-TonB(Ec) chimeras were constructed, and assessment of their activities revealed that substitutions at the N or C terminus of TonB1 compromised its activity in P. aeruginosa, although chimeras possessing an E. coli C terminus were active in E. coli.

Salmonella enterica serovar typhimurium resistance to bile: identification and characterization of the tolQRA cluster

Salmonella enterica serovar Typhimurium is resistant to the action of bile salts, and resistance to bile is enhanced in strains in which the PhoP-PhoQ (PhoPQ) two-component regulatory system has been activated. To identify genes necessary for bile resistance, MudJ transposon mutagenesis was performed on a strain containing a phoP mutation that results in constitutive expression of PhoP-activated genes. After screening >10,000 mutants for the loss of growth on Luria-Bertani broth-bile plates, 14 bile-sensitive mutants were identified. Of these 14 mutants, 3 were found to retain the bile sensitivity phenotype upon P22 transduction, to possess wild-type growth characteristics, and to contain a smooth lipopolysaccharide. Southern hybridization experiments showed that all three strains contained unique insertions. DNA sequencing of the transposon-chromosomal-DNA fusion junctions of these strains showed all to be linked to the putative Salmonella orf1-tolQRA operon, with insertions in tolQ, orf1, and a gene upstream of the orf1-tolQRA operon not previously associated with Tol function (orfX). Through the use of transcriptional fusions, none of the putative tol (or tol-associated) genes were shown to be regulated by PhoPQ, bile, or the RcsC-RcsB two-component system; however, all of the genes (orfX, orf1, tolQRA) are predicted to be cotranscribed. This is the first identification of Salmonella serovar Typhimurium Tol homologs and the first demonstration of their role in bile resistance in this organism. In addition, the observed regulation, operon arrangement, and phenotypes associated with these tol genes demonstrate significant differences from their Escherichia coli homologs.

The Tol-Pal proteins of the Escherichia coli cell envelope: an energized system required for outer membrane integrity?

The outer membrane of gram-negative bacteria acts as a barrier against harmful lipophilic compounds and larger molecules unable to diffuse freely through the porins. However, outer membrane proteins together with the Tol-Pal and TonB systems have been exploited for the entry of macromolecules such as bacteriocins and phage DNA through the Escherichia coli cell envelope. The TonB system is involved in the active transport of iron siderophores and vitamin B12, while no more precise physiological role of the Tol-Pal system has yet been defined than its requirement for cell envelope integrity. These two systems, containing an energized inner membrane protein interacting with outer membrane proteins, share similarities.

Energy-dependent conformational change in the TolA protein of Escherichia coli involves its N-terminal domain, TolQ, and TolR

TolQ, TolR, and TolA inner membrane proteins of Escherichia coli are involved in maintaining the stability of the outer membrane. They share homology with the ExbB, ExbD, and TonB proteins, respectively. The last is involved in energy transduction between the inner and the outer membrane, and its conformation has been shown to depend on the presence of the proton motive force (PMF), ExbB, and ExbD. Using limited proteolysis experiments, we investigated whether the conformation of TolA was also affected by the PMF. We found that dissipation of the PMF by uncouplers led to the formation of a proteinase K digestion fragment of TolA not seen when uncouplers are omitted. This fragment was also detected in Delta tolQ, Delta tolR, and tolA(H22P) mutants but, in contrast to the parental strain, was also seen in the absence of uncouplers. We repeated those experiments in outer membrane mutants such as lpp, pal, and Delta rfa mutants: the behavior of TolA in lpp mutants was similar to that observed with the parental strain. However, the proteinase K-resistant fragment was never detected in the Delta rfa mutant. Altogether, these results suggest that TolA is able to undergo a PMF-dependent change of conformation. This change requires TolQ, TolR, and a functional TolA N-terminal domain. The potential role of this energy-dependent process in the stability of the outer membrane is discussed.

Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

We investigated the involvement of Tol proteins in the surface expression of lipopolysaccharide (LPS). tolQ, -R, -A and -B mutants of Escherichia coli K-12, which do not form a complete LPS-containing O antigen, were transformed with the O7+ cosmid pJHCV32. The tolA and tolQ mutants showed reduced O7 LPS expression compared with the respective isogenic parent strains. No changes in O7 LPS expression were found in the other tol mutants. The O7-deficient phenotype in the tolQ and tolA mutants was complemented with a plasmid encoding the tolQRA operon, but not with a similar plasmid containing a frameshift mutation inactivating tolA. Therefore, the reduction in O7 LPS was attributed to the lack of a functional tolA gene, caused either by a direct mutation of this gene or by a polar effect on tolA gene expression exerted by the tolQ mutation. Reduced surface expression of O7 LPS was not caused by changes in lipid A-core structure or downregulation of the O7 LPS promoter. However, an abnormal accumulation of radiolabelled mannose was detected in the plasma membrane. As mannose is a sugar unique to the O7 subunit, this result suggested the presence of accumulated O7 LPS biosynthesis intermediates. Attempts to construct a tolA mutant in the E. coli O7 wild-type strain VW187 were unsuccessful, suggesting that this mutation is lethal. In contrast, a polar tolQ mutation affecting tolA expression in VW187 caused slow growth rate and serum sensitivity in addition to reduced O7 LPS production. VW187 tolQ cells showed an elongated morphology and became permeable to the membrane-impermeable dye propidium iodide. All these phenotypes were corrected upon complementation with cloned tol genes but were not restored by complementation with the tolQRA operon containing the frameshift mutation in tolA. Our results demonstrate that the TolA protein plays a critical role in the surface expression of O antigen subunits by an as yet uncharacterized involvement in the processing of O antigen.

Mutations in each of the tol genes of Pseudomonas putida reveal that they are critical for maintenance of outer membrane stability

The outer membrane of gram-negative bacteria functions as a permeability barrier that protects cells against a large number of antibacterial agents. OprL protein of Pseudomonas putida has been shown to be crucial to maintain the stability of this cell component (J. J. Rodríguez-Herva, M.-I. Ramos-González, and J. L. Ramos. J. Bacteriol. 178:1699-1706, 1996). In the present study we cloned and mutagenized the orf1, tolQ, tolR, tolA, and tolB genes from P. putida KT2440, which were located upstream of the oprL gene. Polar and nonpolar mutations of the P. putida tolQ, tolR, tolA, and tolB genes were generated in vitro by using the omega-Km(r) interposon, which carries two transcriptional stop signals, or a promoterless xylE cassette, lacking any transcriptional stop signal, respectively. The mutant constructs were used to inactivate, by reverse genetics procedures, the corresponding chromosomal copies of the genes. The phenotype of each mutant strain was analyzed and compared with those of the wild-type strain and the previously characterized P. putida oprL::xylE mutant. All mutant strains exhibited a similar phenotype: altered cell morphology, bleb formation at the cell surface, release of periplasmic and outer membrane proteins to the extracellular medium, increased sensitivity to a variety of compounds (i.e., EDTA, sodium dodecyl sulfate, deoxycholate, and some antibiotics), filament formation, and severely reduced cell motility. Altogether, these results demonstrate the importance of the Tol-OprL system for the maintenance of outer membrane integrity in P. putida and suggest a possible role of these proteins in assembling outer membrane components.

RegA, iron, and growth phase regulate expression of the Pseudomonas aeruginosa tol-oprL gene cluster

The tol-oprL region in Pseudomonas aeruginosa appears to be involved in pyocin uptake and required for cell viability. The complete nucleotide sequences of the tolQRA and oprL genes as well as the incomplete sequences of tolB and orf2 have been previously reported. In addition, the sequence of a P. aeruginosa iron-regulated gene (pig6) has been described and found to share homology with an open reading frame located upstream of the Escherichia coli tolQRA genes (U. A. Ochsner and M. L. Vasil, Proc. Natl. Acad. Sci. USA 93:4409-4414, 1996). In this study, we cloned the remainder of the P. aeruginosa tol-oprL gene cluster and determined its nucleotide sequence. This cluster was found to consist of seven genes in the order orf1 tolQ tolR tolA tolB oprL orf2. Transcriptional analysis of this gene cluster was performed by detecting the presence of mRNAs spanning adjacent genes as well as by using a promoterless lacZ reporter gene fused to each of the seven genes contained in the tol-oprL locus. The results show that there are three major transcriptional units or operons in this region, orf1-tolQRA, tolB, and oprL-orf2, in contrast to the E. coli tol-pal region, where there are only two operons, orf1-tolQRA and tolB-pal-orf2. Analysis of gene expression indicated that the tol-oprL genes of P. aeruginosa are both iron and growth phase modulated. The first operon, orf1-tolQRA, is iron regulated throughout growth, but iron-regulated expression of tolB and oprL fusions occurs only in late log phase. The expression of the three operons was significantly less repressed by iron in fur mutants than in the wild-type strain, suggesting the involvement of Fur in the iron regulation of all three operons. RegA is a positive yet nonessential regulator of tol-oprL expression.

CTXphi infection of Vibrio cholerae requires the tolQRA gene products

CTXphi is a lysogenic filamentous bacteriophage that encodes cholera toxin. Filamentous phages that infect Escherichia coli require both a pilus and the products of tolQRA in order to enter host cells. We have previously shown that toxin-coregulated pilus (TCP), a type IV pilus that is an essential Vibrio cholerae intestinal colonization factor, serves as a receptor for CTXphi. To test whether CTXphi also depends upon tol gene products to infect V. cholerae, we identified and inactivated the V. cholerae tolQRAB orthologues. The predicted amino acid sequences of V. cholerae TolQ, TolR, TolA, and TolB showed significant similarity to the corresponding E. coli sequences. V. cholerae strains with insertion mutations in tolQ, tolR, or tolA were reduced in their efficiency of CTXphi uptake by 4 orders of magnitude, whereas a strain with an insertion mutation in tolB showed no reduction in CTXphi entry. We could detect CTXphi infection of TCP(-) V. cholerae, albeit at very low frequencies. However, strains with mutations in both tcpA and either tolQ, tolR, or tolA were completely resistant to CTXphi infection. Thus, CTXphi, like the E. coli filamentous phages, uses both a pilus and TolQRA to enter its host. This suggests that the pathway for filamentous phage entry into cells is conserved between host bacterial species.

The response of Pseudomonas aeruginosa to iron: genetics, biochemistry and virulence

During the past decade significant progress has been made towards identifying some of the schemes that Pseudomonas aeruginosa uses to obtain iron and towards cataloguing and characterizing many of the genes and gene products that are likely to play a role in these processes. This review will largely recount what we have learned in the past few years about how P. aeruginosa regulates its acquisition, intake and, to some extent, trafficking of iron, and the role of iron acquisition systems in the virulence of this remarkable opportunistic pathogen. More specifically, the genetics, biochemistry and biology of an essential regulator (Ferric uptake regulator - Fur) and a Fur-regulated alternative sigma factor (PvdS), which are central to these processes, will be discussed. These regulatory proteins directly or indirectly regulate a substantial number of other genes encoding proteins with remarkably diverse functions. These genes include: (i) other regulatory genes, (ii) genes involved in basic metabolic processes (e.g. Krebs cycle), (iii) genes required to survive oxidative stress (e.g. superoxide dismutase), (iv) genes necessary for scavenging iron (e.g. siderophores and their cognate receptors) or genes that contribute to the virulence (e.g. exotoxin A) of this opportunistic pathogen. Despite this recent expansion of knowledge about the response of P. aeruginosa to iron, many significant biological issues surrounding iron acquisition still need to be addressed. Virtually nothing is known about which of the distinct iron acquisition mechanisms P. aeruginosa brings to bear on these questions outside the laboratory, whether it be in soil, in a pipeline, on plants or in the lungs of cystic fibrosis patients.

In vitro characterization of peptidoglycan-associated lipoprotein (PAL)-peptidoglycan and PAL-TolB interactions

The Tol-peptidoglycan-associated lipoprotein (PAL) system of Escherichia coli is a multiprotein complex of the envelope involved in maintaining outer membrane integrity. PAL and the periplasmic protein TolB, two components of this complex, are interacting with each other, and they have also been reported to interact with OmpA and the major lipoprotein, two proteins interacting with the peptidoglycan. All these interactions suggest a role of the Tol-PAL system in anchoring the outer membrane to the peptidoglycan. Therefore, we were interested in better understanding the interaction between PAL and the peptidoglycan. We designed an in vitro interaction assay based on the property of purified peptidoglycan to be pelleted by ultracentrifugation. Using this assay, we showed that a purified PAL protein interacted in vitro with pure peptidoglycan. A peptide competition experiment further demonstrated that the region from residues 89 to 130 of PAL was sufficient to bind the peptidoglycan. Moreover, the fact that this same region of PAL was also binding to TolB suggested that these two interactions were exclusive. Indeed, the TolB-PAL complex appeared not to be associated with the peptidoglycan. This led us to the conclusion that PAL may exist in two forms in the cell envelope, one bound to TolB and the other bound to the peptidoglycan.

Immunization of mice with a TolA-like surface protein of Trypanosoma cruzi generates CD4(+) T-cell-dependent parasiticidal activity

The gene family encoding a trypomastigote-specific protein restricted to the part of the flagellum in contact with the cell body of the trypomastigote form of Trypanosoma cruzi has been isolated, characterized, and expressed in a baculovirus expression system. The gene family contains three tandemly repeated members that have 97 to 100% sequence identity. The predicted protein encoded by the gene family has both significant amino acid sequence identity and other physical and biological features in common with the TolA proteins of Escherichia coli and Pseudomonas aeruginosa. Based on these similarities, we have designated this gene family tolT. Immunization of mice with recombinant TolT generates a population of CD4(+) T lymphocytes that recognize T. cruzi-infected macrophages, resulting in the production of gamma interferon (IFN-gamma), which leads to NO production and a 50 to 60% reduction in parasite numbers compared to that seen with infected macrophages incubated with naive T cells. This population of T cells also produces both IFN-gamma and interleukin 2 (IL-2) but not IL-4 or IL-5 when incubated with spleen cells stimulated with TolT antigen, indicating that they are of the T-helper 1 type. T cells from mice chronically infected with T. cruzi also produce significant levels of IFN-gamma when cocultured with macrophages and either TolT protein or paraflagellar rod protein, indicating that both of these flagellar proteins produce positive T-cell responses in mice chronically infected with T. cruzi.

Circular dichroism and molecular modeling of the E. coli TolA periplasmic domains

Colicins are killer proteins that use envelope proteins from the outer and the inner membranes to reach their cellular target in susceptible cells of Escherichia coli. Each group A colicin uses a combination of Tol proteins to cross the outer membrane of gram-negative bacteria and to exert their killing activity. The TolA protein, necessary for the import of all the group A colicins, is a 421-amino acid residue protein composed of three domains (TolAI, TolAII, and TolAIII). TolAIII interacts with the N-terminal domain of colicin A (AT1). Analytical ultracentrifugation reveals that TolAII and TolAIII are monomer structures, TolAII has an elongated structure, and TolAIII is rather globular. Circular dichroism (CD) spectra were done with TolAII-III, TolAII, TolAIII, AT1, and the AT1-TolAII-III complex. TolA CD spectra reveal the presence of alpha-helix structure in aqueous solution and the intensity of the a-helix signal is the highest with TolAII. Few structural changes are observed with the complex AT1-TolAII-III. Molecular modeling was done for TolAII-III, taking into account CD and ultracentrifugation data and show that domain II can adopt a barrel structure made of three twisted alpha-helices similar to coiled coil helices while domain III can adopt a globular structure.

Uptake of pyocin S3 occurs through the outer membrane ferripyoverdine type II receptor of Pseudomonas aeruginosa

Pyocin S3 was found to kill exclusively Pseudomonas aeruginosa isolates producing type II pyoverdine (exemplified by strain ATCC 27853). Killing was specifically inhibited by addition of type II ferripyoverdine. All Tn5 mutants resistant to pyocin S3 were defective for pyoverdine-mediated iron uptake and failed to produce an 85-kDa iron-repressed outer membrane protein. We conclude that this protein is probably the type II ferripyoverdine receptor that is used by pyocin S3 to gain entry into the cell.

Effects of iron and temperature on expression of the Pseudomonas aeruginosa tolQRA genes: role of the ferric uptake regulator

The tolQRA genes have been recently identified in Pseudomonas aeruginosa PAO. In this study, we examined the effect of iron and temperature on tolQRA expression. A promoterless lacZ gene was introduced downstream of plasmid-encoded tolQ and tolA, and expression was monitored by measuring beta-galactosidase activity of cultures. Addition of 25 microM FeCl3 to the culture medium reduced tolQRA expression by 50 to 60% in PAO but by only 25% in the fur mutant PAO A4. Northern hybridization analysis revealed that iron regulation occurs at the level of transcription and involves the P. aeruginosa ferric uptake regulator (Fur). Primer extension analysis was used to identify the proposed transcriptional start site of tolA. Although a putative Fur box was identified 20 bp upstream of the proposed start site, purified Fur did not bind to the tolA or tolQR promoter regions in an in vitro gel retardation assay. Therefore, iron regulation of the tol genes appears to involve an intermediate regulatory gene. Expression of tolQR and tolA was optimal at 37 degrees C and was reduced by 40 to 50% when cultures were grown at either 42 or 25 degreesC. Growth in high-iron medium at 25 degrees C further reduced tolQR and tolA expression.