Molecular characterization of enterobacterial pldA genes encoding outer membrane phospholipase A

Critical role of the RpoE stress response pathway in polymyxin resistance of Escherichia coli

OBJECTIVES

Polymyxins, including colistin, are the drugs of last resort to treat MDR bacterial infections in humans. In-depth understanding of the molecular basis and regulation of polymyxin resistance would provide new therapeutic opportunities to combat increasing polymyxin resistance. Here we aimed to identify novel targets that are crucial for polymyxin resistance using Escherichia coli BL21(DE3), a unique colistin-resistant model strain.

METHODS

BL21(DE3) was subjected to random transposon mutagenesis for screening colistin-susceptible mutants. The insertion sites of desired mutants were mapped; the key genes of interest were also inactivated in different strains to examine functional conservation. Specific genes in the known PmrAB and PhoPQ regulatory network were inactivated to examine crosstalk among different pathways. Lipid A species and membrane phospholipids were analysed by normal phase LC/MS.

RESULTS

Among eight mutants with increased susceptibility to colistin, five mutants contained different mutations in three genes (rseP, degS and surA) that belong to the RpoE stress response pathway. Inactivation of rpoE, pmrB, eptA or pmrD led to significantly increased susceptibility to colistin; however, inactivation of phoQ or eptB did not change colistin MIC. RpoE mutation in different E. coli and Salmonella resistant strains all led to significant reduction in colistin MIC (16-32-fold). Inactivation of rpoE did not change the lipid A profile but significantly altered the phospholipid profile.

CONCLUSIONS

Inactivation of the important members of the RpoE regulon in polymyxin-resistant strains led to a drastic reduction in polymyxin MIC and an increase of lysophospholipids with no change in lipid A modifications.

Heme Uptake and Utilization by Gram-Negative Bacterial Pathogens

Iron is a transition metal utilized by nearly all forms of life for essential cellular processes, such as DNA synthesis and cellular respiration. During infection by bacterial pathogens, the host utilizes various strategies to sequester iron in a process termed, nutritional immunity. To circumvent these defenses, Gram-negative pathogens have evolved numerous mechanisms to obtain iron from heme. In this review we outline the systems that exist in several Gram-negative pathogens that are associated with heme transport and utilization, beginning with hemolysis and concluding with heme degradation. In addition, Gram-negative pathogens must also closely regulate the intracellular concentrations of iron and heme, since high levels of iron can lead to the generation of toxic reactive oxygen species. As such, we also provide several examples of regulatory pathways that control heme utilization, showing that co-regulation with other cellular processes is complex and often not completely understood.

An ABC transport system that maintains lipid asymmetry in the gram-negative outer membrane

The outer membranes (OMs) of gram-negative bacteria have an asymmetric lipid distribution with lipopolysaccharides at the outer leaflet and phospholipids (PLs) at the inner leaflet. This lipid arrangement is essential for the barrier function of the OM and for the viability of most gram-negative bacteria. Cells with OM assembly defects or cells exposed to harsh chemical treatments accumulate PLs in the outer leaflet of the OM and this disrupts lipopolysaccharide organization and increases sensitivity to small toxic molecules. We have identified an ABC transport system in Escherichia coli with predicted import function that serves to prevent PL accumulation in the outer leaflet of the OM. This highly conserved pathway, which we have termed the Mla pathway for its role in preserving OM lipid asymmetry, is composed of at least 6 proteins and contains at least 1 component in each cellular compartment. We propose that the Mla pathway constitutes a bacterial intermembrane PL trafficking system.

Characterization of Campylobacter concisus hemolysins

Campylobacter concisus is an opportunistic pathogen commonly found in the human oral cavity. It has also been isolated from clinical sources including gastroenteritis cases. Both secreted and cell-associated hemolytic activities were detected in C. concisus strains isolated from children with gastroenteritis. The secreted hemolytic activity of C. concisus strains was labile and was detected in variable levels from fresh-culture filtrates only. In addition, another secreted hemolysin/cytotoxin with a molecular weight < 10 kDa was detected in a single C. concisus strain (RCH 12). A C. concisus genomic library, constructed from strain RCH 3 in Escherichia coli XL1-Blue, was screened for hemolytic clones. Subcloning and sequence analysis of selected hemolytic clones identified ORFs for genes that enhance hemolytic activity but do not appear to be related to any known hemolysin genes found in Gram-negative bacteria. In a previous study, a stable cell-associated hemolysin was identified as an outer-membrane phospholipase A (OMPLA) encoded by the pldA gene. In this study, we report cloning of the pldA gene of the clinical strain C. concisus RCH 3 and the complementation of phospholipase A activity in an E. coli pldA mutant.

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.

Phospholipase A in Gram-negative bacteria and its role in pathogenesis

Phospholipase A (PLA) is one of the few enzymes present in the outer membrane of Gram-negative bacteria, and is likely to be involved in the membrane disruption processes that occur during host cell invasion. Both secreted and membrane-bound phospholipase A(2) activities have been described in bacteria, fungi and protozoa. Recently there have been increasing reports on the involvement of PLA in bacterial invasion and pathogenesis. This review highlights the latest findings on PLA as a virulence factor in Gram-negative bacteria.

Adenylate cyclase mutations rescue the degP temperature-sensitive phenotype and induce the sigma E and Cpx extracytoplasmic stress regulons in Escherichia coli

Inactivation of the gene encoding the periplasmic protease DegP confers a high-temperature-sensitive phenotype in Escherichia coli. We have previously demonstrated that a degP mutant of E. coli strain CBM (W3110 pldA1) is not temperature sensitive and showed that this was most likely due to constitutive activation of the sigma E and Cpx extracytoplasmic stress regulons in the parent strain. In this study, further characterization of this strain revealed a previously unknown cryptic mutation that rescued the degP temperature-sensitive phenotype by inducing the extracytoplasmic stress regulons. We identified the cryptic mutation as an 11-bp deletion of nucleotides 1884 to 1894 of the adenylate cyclase-encoding cyaA gene (cyaAdelta11). The mechanism in which cyaAdelta11 induces the sigma E and Cpx regulons involves decreased activity of the mutant adenylate cyclase. Addition of exogenous cyclic AMP (cAMP) to the growth medium of a cyaAdelta11 mutant strain that contains a Cpx- and sigma E-inducible degP-lacZ reporter fusion decreased beta-galactosidase expression to levels observed in a cyaA+ strain. We also found that a cyaA null mutant displayed even higher levels of extracytoplasmic stress regulon activation compared to a cyaAdelta11 mutant. Thus, we conclude that the lowered concentration of cAMP in cyaA mutants induces both sigma E and Cpx extracytoplasmic stress regulons and thereby rescues the degP temperature-sensitive phenotype.

Function of neisserial outer membrane phospholipase a in autolysis and assessment of its vaccine potential

Outer membrane phospholipase A (OMPLA) is an outer membrane-localized enzyme, present in many gram-negative bacterial species. It is implicated in the virulence of several pathogens. Here, we investigated the presence, function, and vaccine potential of OMPLA in the human pathogen Neisseria meningitidis. Immunoblot analysis showed the presence of OMPLA in 28 out of 33 meningococcal strains investigated. The OMPLA-negative strains all contained a pldA gene, but these alleles contained premature stop codons. All six Neisseria gonorrhoeae strains tested, but only two out of seven commensal neisserial strains investigated, expressed OMPLA, showing that OMPLA is expressed by, but not limited to, many pathogenic neisserial strains. The function of OMPLA was investigated by assessing the phenotypes of isogenic strains, expressing no OMPLA, expressing wild-type levels of OMPLA, or overexpressing OMPLA. OMPLA exhibited phospholipase activity against endogenous phospholipids. Furthermore, OMPLA was characterized as an autolysin that acted under specific conditions, such as prolonged growth of the bacteria. The vaccine potential of the protein was investigated by immunizing mice with in vitro refolded, recombinant OMPLA. High levels of antibody titers were obtained, but the murine sera were neither bactericidal nor protective. Also, convalescent patients and vaccinee sera did not contain detectable levels of anti-OMPLA antibodies, indicating that OMPLA may not be sufficiently immunogenic to be included in a meningococcal vaccine.

High relative content of lysophospholipids ofHelicobacter pylorimediates increased risk for ulcer disease

Helicobacter pylori phospholipase A (OMPLA) degrades bacterial membrane phospholipids to lysophospholipids. High levels of lysophospholipids are associated with higher hemolytic activity, increased release of urease and vacA and better adherence to epithelial cells in vitro. The phospholipase A gene (pldA) displays phase variation due to a slippage in a homopolymeric tract. The aim of this study was to determine if the relative amount of lysophospholipids in the cell wall is associated with ulcer disease, and to further investigate the significance of pldA phase variation. H. pylori isolates of 40 patients were examined. The relative lysophospholipid content of each isolate was determined and the pldA gene was sequenced. The study indicated that H. pylori can regulate its OMPLA activity by phase variation in the pldA gene or by protein level regulation among phase variants in the pldA 'ON' status. We found a significant difference between the relative amount of lysophospholipids of the ulcer group and the non-ulcer group (p=0.022). When the lysophospholipid/phospholipid ratios were compared with outcome, the OR for ulcer disease was 9.0 (95% CI 1.6-49.4; p=0.014). Isolates with a high OMPLA activity are significantly associated with patients with ulcer disease.

Cholesteryl-6-O-acyl-α-d-glucopyranoside ofHelicobacter pylorirelate to relative lysophospholipid content

The presence of cholesteryl glucosides and high levels of lysophospholipids are elements making the cell wall of Helicobacter pylori unique. In this study, we have investigated the relationship between lysophospholipid content and cholesteryl glucoside composition of variants of 6 clinical isolates. The samples were characterized by diverse outer membrane phospholipase A activity measured as lysophospholipid content of the cell wall. A pldA negative mutant was also included in the study. Thin-layer chromatography showed that cholesteryl glucosides were present in all samples. However, the distribution of cholesteryl-6-O-acyl-alpha-D-glucopyranoside, cholesteryl-alpha-D-glucopyranoside and cholesteryl-6-O-phosphatidyl-alpha-D-glucopyranoside varied according to lysophospholipid content. Cholesteryl-6-O-acyl-alpha-D-glucopyranoside was exclusively observed in the isolates/variants with an intact pldA and where a significant amount of lysophospholipids could be demonstrated. High lysophospholipid content destabilizes membranes. The balance between cholesteryl-6-O-acyl-alpha-D-glucopyranoside, cholesteryl-alpha-D-glucopyranoside and cholesteryl-6-O-phosphatidyl-alpha-D-glucopyranoside in H. pylori is probably important for the stability of the membrane when the lysophospholipid content varies.

Role of a highly conserved bacterial protein in outer membrane protein assembly

After transport across the cytoplasmic membrane, bacterial outer membrane proteins are assembled into the outer membrane. Meningococcal Omp85 is a highly conserved protein in Gram-negative bacteria, and its homolog Toc75 is a component of the chloroplast protein-import machinery. Omp85 appeared to be essential for viability, and unassembled forms of various outer membrane proteins accumulated upon Omp85 depletion. Immunofluorescence microscopy revealed decreased surface exposure of outer membrane proteins, which was particularly apparent at the cell-division planes. Thus, Omp85 is likely to play a role in outer membrane protein assembly.

Assembly of colicin A in the outer membrane of producing Escherichia coli cells requires both phospholipase A and one porin, but phospholipase A is sufficient for secretion

Three oligomeric forms of colicin A with apparent molecular masses of about 95 to 98 kDa were detected on sodium dodecyl sulfate (SDS)-polyacrylamide gels loaded with unheated samples from colicin A-producing cells of Escherichia coli. These heat-labile forms, called colicins Au, were visualized both on immunoblots probed with monoclonal antibodies against colicin A and by radiolabeling. Cell fractionation studies show that these forms of colicin A were localized in the outer membrane whether or not the producing cells contained the cal gene, which encodes the colicin A lysis protein responsible for colicin A release in the medium. Pulse-chase experiments indicated that their assembly into the outer membrane, as measured by their heat modifiable migration in SDS gels, was an efficient process. Colicins Au were produced in various null mutant strains, each devoid of one major outer membrane protein, except in a mutant devoid of both OmpC and OmpF porins. In cells devoid of outer membrane phospholipase A (OMPLA), colicin A was not expressed. Colicins Au were detected on immunoblots of induced cells probed with either polyclonal antibodies to OmpF or monoclonal antibodies to OMPLA, indicating that they were associated with both OmpF and OMPLA. Similar heat-labile forms were obtained with various colicin A derivatives, demonstrating that the C-terminal domain of colicin A, but not the hydrophobic hairpin present in this domain, was involved in their formation.

Application of high-density array-based signature-tagged mutagenesis to discover novel Yersinia virulence-associated genes

Yersinia pestis, the causative agent of plague, and the enteropathogen Yersinia pseudotuberculosis have nearly identical nucleotide similarity yet cause markedly different diseases. To investigate this conundrum and to study Yersinia pathogenicity, we developed a high-density oligonucleotide array-based modification of signature-tagged mutagenesis (STM). Y. pseudotuberculosis YPIII mutants constructed with the tagged transposons were evaluated in the murine yersiniosis infection model. The DNA tags were amplified using biotinylated primers and hybridized to high-density oligonucleotide arrays containing DNA complementary to the tags. Comparison of the hybridization signals from input pools and output pools identified a mutant whose relative abundance was significantly reduced in the output pool. Sequence data from 31 transposon insertion regions was compared to the complete Y. pestis CO92 genome sequence. The 26 genes present in both species were found to be almost identical, but five Y. pseudotuberculosis genes identified through STM did not have counterparts in the Y. pestis genome and may contribute to the different tropisms in these closely related pathogens. Potential virulence genes identified include those involved in lipopolysaccharide biosynthesis, adhesion, phospholipase activity, iron assimilation, and gene regulation. The phospholipase A (PldA) mutant exhibited reduced phospholipase activity compared to the wild-type strain and in vivo attenuation of the mutant was confirmed. The combination of optimized double tag sequences and high-density array hybridization technology offers improved performance, efficiency, and reliability over classical STM and permits quantitative analysis of data.

Structural investigations of calcium binding and its role in activity and activation of outer membrane phospholipase A from Escherichia coli

Outer membrane phospholipase A (OMPLA) is an integral membrane enzyme that catalyses the hydrolysis of phospholipids. Enzymatic activity is regulated by reversible dimerisation and calcium-binding. We have investigated the role of calcium by X-ray crystallography. In monomeric OMPLA, one calcium ion binds between two external loops (L3L4 site) at 10 A from the active site. After dimerisation, a new calcium-binding site (catalytic site) is formed at the dimer interface in the active site of each molecule at 6 A from the L3L4 calcium site. The close spacing and the difference in calcium affinity of both sites suggests that the L3L4 site may function as a storage site for a calcium ion, which relocates to the catalytic site upon dimerisation. A sequence alignment demonstrates conservation of the catalytic calcium site but evolutionary variation of the L3L4 site. The residues in the dimer interface are conserved as well, suggesting that all outer membrane phospholipases require dimerisation and calcium in the catalytic site for activity. For this family of phospholipases, we have characterised a consensus sequence motif (YTQ-X(n)-G-X(2)-H-X-SNG) that contains conserved residues involved in dimerisation and catalysis.

Bacterial phospholipase A: structure and function of an integral membrane phospholipase

Within the large family of lipolytic enzymes, phospholipases constitute a very diverse subgroup with physiological functions such as digestion and signal transduction. Most phospholipases may associate with membranes at the lipid-water interface. However, in many Gram-negative bacteria, a phospholipase is present which is located integrally in the bacterial outer membrane. This phospholipase (outer membrane phospholipase A or OMPLA) is involved in transport across the bacterial outer membrane and has been implicated in bacterial virulence. OMPLA is calcium dependent and its activity is strictly regulated by reversible dimerisation. Recently the crystal structure of this integral membrane enzyme has been elucidated. In this review, we summarise the implications of these structural data for the understanding of the function and regulation of OMPLA, and discuss a mechanism for phospholipase dependent colicin release in Escherichia coli.

Structure and function of bacterial outer membrane proteins: barrels in a nutshell

The outer membrane protects Gram-negative bacteria against a harsh environment. At the same time, the embedded proteins fulfil a number of tasks that are crucial to the bacterial cell, such as solute and protein translocation, as well as signal transduction. Unlike membrane proteins from all other sources, integral outer membrane proteins do not consist of transmembrane alpha-helices, but instead fold into antiparallel beta-barrels. Over recent years, the atomic structures of several outer membrane proteins, belonging to six families, have been determined. They include the OmpA membrane domain, the OmpX protein, phospholipase A, general porins (OmpF, PhoE), substrate-specific porins (LamB, ScrY) and the TonB-dependent iron siderophore transporters FhuA and FepA. These crystallographic studies have yielded invaluable insight into and decisively advanced the understanding of the functions of these intriguing proteins. Our review is aimed at discussing their common principles and peculiarities as well as open questions associated with them.

Simultaneous enhancement of thermostability and catalytic activity of phospholipase A(1) by evolutionary molecular engineering

The thermal stability and catalytic activity of phospholipase A(1) from Serratia sp. strain MK1 were improved by evolutionary molecular engineering. Two thermostable mutants were isolated after sequential rounds of error-prone PCR performed to introduce random mutations and filter-based screening of the resultant mutant library; we determined that these mutants had six (mutant TA3) and seven (mutant TA13) amino acid substitutions. Different types of substitutions were found in the two mutants, and these substitutions resulted in an increase in nonpolar residues (mutant TA3) or in differences between side chains for polar or charged residues (mutant TA13). The wild-type and mutant enzymes were purified, and the effect of temperature on the stability and catalytic activity of the enzymes was investigated. The melting temperatures of the TA3 and TA13 enzymes were increased by 7 and 11 degrees C, respectively, compared with the melting temperature of the wild-type enzyme. Thus, we found that evolutionary molecular engineering was an effective and efficient approach for increasing thermostability without compromising enzyme activity.

Outer-membrane phospholipase A: known structure, unknown biological function

Outer-membrane phospholipase A (OMPLA) is one of the few enzymes present in the outer membrane of Gram-negative bacteria. The enzymatic activity of OMPLA is strictly regulated to prevent uncontrolled breakdown of the surrounding phospholipids. The activity of OMPLA can be induced by membrane perturbation and concurs with dimerization of the enzyme. The recently elucidated crystal structures of the inactive, monomeric and an inhibited dimeric form of the enzyme provide detailed structural insight into the functional properties of the enzyme. OMPLA is a serine hydrolase with a unique Asn-156-His-142-Ser-144 catalytic triad. Only in the dimeric state, complete substrate binding pockets and functional oxyanion holes are formed. A model is proposed for the activation of OMPLA in which membrane perturbation causes the formation of non-bilayer structures, resulting in the presentation of phospholipids to the active site of OMPLA and leading to the formation of the active dimeric species. Possible roles for OMPLA in maintaining the cell envelope integrity and in pathogenicity are discussed.

Characterization of Helicobacter pylori PldA, a phospholipase with a role in colonization of the gastric mucosa

BACKGROUND & AIMS

Phospholipase activity may play a role in the pathogenicity of Helicobacter pylori. Furthermore, some drugs that are effective against H. pylori infection are phospholipase inhibitors. Scrutiny of the H. pylori 26695 genome sequence revealed the presence of a putative protein with homology to Esherichia coli outer membrane phospholipase A (PldA). The aim of this study was to investigate the role of this putative PldA in the pathogenicity of H. pylori.

METHODS

An isogenic pldA mutant was constructed and analyzed for in vitro phospholipase A(2) and hemolytic activity. Adherence of the mutant to human gastric adenocarcinoma cells and the ability to colonize mice were also investigated.

RESULTS

The pldA mutant showed a marked reduction in phospholipase A(2) and hemolytic activity compared with the wild-type strain. The mutant was unable to colonize mice at 2 and 8 weeks, but it did induce a significant immune response. In contrast, the ability of the mutant to adhere to human gastric adenocarcinoma cells was unaffected.

CONCLUSIONS

The results suggest a role for PldA in colonization of the gastric mucosa and possibly tissue damage after colonization.

Cloning and expression of the gene encoding phospholipase A1 from Serratia sp. MK1 in Escherichia coli

The gene encoding extracellular phospholipase A1 of Serratia sp. MK1 was cloned from a genomic DNA library. Formation of transparent halos on the PCY agar plates was used to identify E. coli carrying the phospholipase A1 gene. A 4.2 kb EcoRI fragment was isolated and sequenced. From nucleotide sequences and expression of various plasmids, two open reading frames (plaA and plaS) involved in efficient expression of phospholipase A1 in natural and recombinant host were identified. Extracellular phospholipase A1 activity was identified as the gene product of plaA encoding 321 amino acids with a predicted MW of 33,400. Analysis of the amino acid sequence revealed significant homology (around 70%) to phospholipase A1 of Serratia liquefaciens and Yersinia enterocolitica. The sequence, -Gly-X1-Ser-X2-Gly-, known as a lipase-specific consensus sequence was also found in the bacterial phospholipase A1. PlaS encoding a protein of 224 amino acids showed no enzymatic activity, but might be necessary for the efficient expression of phospholipase A1 in E. coli. To further improve the production of phospholipase A1 as a soluble and active form in E. coli, the effect of some parameters was examined. Surprisingly, a higher yield of soluble and active phospholipase A1 could be obtained under the combined conditions of a lower temperature, an enriched medium, and a lower-strength promoter.

Bacteriocin release protein triggers dimerization of outer membrane phospholipase A in vivo

Bacteriocin release protein is known to activate outer membrane phospholipase A (OMPLA), which results in the release of colicin from Escherichia coli. In vivo chemical cross-linking experiments revealed that the activation coincides with dimerization of OMPLA. Permeabilization of the cell envelope and dimerization were characterized by a lag time of 2 h.

Half-of-the-sites reactivity of outer-membrane phospholipase A against an active-site-directed inhibitor

The reaction of a novel active-site-directed phospholipase A1 inhibitor with the outer-membrane phospholipase A (OMPLA) was investigated. The inhibitor 1-p-nitrophenyl-octylphosphonate-2-tridecylcarbamoyl-3-et hanesulfonyl -amino-3-deoxy-sn-glycerol irreversibly inactivated OMPLA. The inhibition reaction did not require the cofactor calcium or an unprotonated active-site His142. The inhibition of the enzyme solubilized in hexadecylphosphocholine micelles was characterized by a rapid (t1/2 = 20 min) and complete loss of enzymatic activity, concurrent with the covalent modification of 50% of the active-site serines, as judged from the amount of p-nitrophenolate (PNP) released. Modification of the remaining 50% occurred at a much lower rate, indicative of half-of-the-sites reactivity against the inhibitor of this dimeric enzyme. Inhibition of monomeric OMPLA solubilized in hexadecyl-N,N-dimethyl-1-ammonio-3-propanesulfonate resulted in an equimolar monophasic release of PNP, concurrent with the loss of enzymatic activity (t1/2 = 14 min). The half-of-the-sites reactivity is discussed in view of the dimeric nature of this enzyme.

Sequence comparison of outer membrane phospholipases A: implications for structure and for the catalytic mechanism

In this study, the nucleotide sequence of the Enterobacter agglomerans pldA gene encoding outer membrane phospholipase A (OMPLA; EC 3.1.1.32) was determined. Five other OMPLA amino acid sequences have previously been described, and screening of data bases of whole genome sequencing projects revealed the presence of proteins with homology to OMPLA in Helicobacter pylori, Campylobacter jejuni, Yersinia pestis, Neisseria menigitidis and Neisseria gonorrhoeae. Comparison of these eleven OMPLA amino acid sequences revealed that 30 amino acid residues are completely conserved. Implications of the sequence comparison for the catalytic mechanism of OMPLA are discussed. The presence of proteins homologous to OMPLA even in non-enterobacterial Gram-negative bacteria indicates an important physiological role of this enzyme.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Phospholipase A of Yersinia enterocolitica contributes to pathogenesis in a mouse model

Some isolates of Yersinia enterocolitica exhibit phospholipase activity, which has been linked to lecithin-dependent hemolysis (M. Tsubokura, K. Otsoki, I. Shimohira, and H. Yamamoto, Infect. Immun. 25:939-942, 1979). A gene encoding Y. enterocolitica phospholipase was identified, and analysis of the nucleotide sequence revealed two tandemly transcribed open reading frames. The first, yplA, has 74% identity and 85% similarity to the phospholipase A found in Serratia liquefaciens. Though the other, yplB, was less similar to the downstream accessory protein found in S. liquefaciens, the organization in both species is similar. Subsequently, a yplA-null Y. enterocolitica strain, YEDS10, was constructed and demonstrated to be phospholipase negative by plate and spectrophotometric assays. To ascertain whether the phospholipase has a role in pathogenesis, YEDS10 was tested in the mouse model. In experiments with perorally infected BALB/c mice, fewer YEDS10 organisms were recovered from the mesenteric lymph nodes and Peyer's patches (PP) than the parental strain at 3 or 5 days postinfection. Furthermore, bowel tissue and PP infected with YEDS10 appeared to be less inflamed than those infected with the parental strain. When extremely high doses of both the parental and YEDS10 strains were given, similar numbers of viable bacteria were recovered from the PP and mesenteric lymph nodes on day 3. However, the numbers of foci and the extent of inflammation and necrosis within them were noticeably less for YEDS10 compared to the parental strain. Together these findings suggest that Y. enterocolitica produces a phospholipase A which has a role in pathogenesis.

Topology of the outer membrane phospholipase A of Salmonella typhimurium

The outer membrane phospholipase A (OMPLA) of Enterobacteriaceae has been proposed to span the membrane 14 times as antiparallel amphipathic beta-strands, thereby exposing seven loops to the cell surface. We have employed the epitope insertion method to probe the topology of OMPLA of Salmonella typhimurium. First, missense mutations were introduced at various positions in the pldA gene, encoding OMPLA, to create unique BamHI sites. These BamHI sites were subsequently used to insert linkers, encoding a 16-amino-acid B-cell epitope. Proper assembly of all mutant proteins was revealed by their heat modifiability in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The accessibility of the inserted epitopes was assessed. Immunofluorescence analysis of intact cells with antibodies against the inserted epitope showed that three of seven predicted loops are indeed cell surface exposed. Trypsin accessibility experiments verified the cell surface exposure of two additional loops and provided support for the proposed periplasmic localization of three predicted turns. For two other predicted exposed loops, the results were not conclusive. These results support to a large extent the proposed topology model of OMPLA. Furthermore, the observation that the substitutions Glu66Pro and Glu247Gly virtually abolished enzymatic activity indicates that these residues might play a major role in catalysis.

Molecular characterization of pldA, the structural gene for a phospholipase A from Campylobacter coli, and its contribution to cell-associated hemolysis

A gene (pldA) encoding a 35.0-kDa protein with significant homology to the Escherichia coli outer membrane phospholipase was identified upstream of an operon encoding an enterochelin transport system in Campylobacter coli. The results of this study suggest that this gene encodes an outer membrane phospholipase A in C. coli. First, expression of the pldA gene product in a PldA-deficient mutant of E. coli led to the restoration of phospholipase A activity. The recombinant product also partitioned to the outer membrane, suggesting that it may be similarly located in C. coli. Second, heterologous overexpression in E. coli, followed by in vitro folding and purification of C. coli PldA, resulted in pure protein which displayed calcium-dependent lysophospholipase and phospholipase A activities in vitro. Finally, mutants of C. coli in which the pldA gene had been inactivated by allelic exchange were deficient in phospholipase A activity. Phospholipases are associated with lysis of erythrocytes by a number of bacterial pathogens. The pldA mutant was shown to have a reduced hemolytic activity compared to the wild-type strain, suggesting a role for the phospholipase A in the lysis of erythrocytes by C. coli. Since hemolysins are intimately associated with the disease-causing potential of a number of bacterial pathogens, it is likely that the phospholipase A plays some role in Campylobacter virulence.

Dimerization regulates the enzymatic activity of Escherichia coli outer membrane phospholipase A

The outer membrane phospholipase A (OMPLA) of Escherichia coli is present in a dormant state in the cell envelope. The enzyme is activated by various processes, which have in common that they perturb the outer membrane. Kinetic experiments, chemical cross-linking, and analytical ultracentrifugation were carried out with purified, detergent-solubilized OMPLA to understand the underlying mechanism that results in activation. Under conditions in which the enzyme displayed full activity, OMPLA was dimeric. High detergent concentrations or very dilute protein concentrations resulted in low specific activity of the enzyme, and under those conditions the enzyme was monomeric. The cofactor Ca2+ was required for dimerization. Covalent modification of the active site serine with hexadecylsulfonylfluoride resulted in stabilization of the dimeric form and a loss of the absolute calcium requirement for dimerization. The results of these experiments provide evidence for dimerization as the molecular mechanism by which the enzymatic activity of OMPLA is regulated. This dimerization probably plays a role in vivo as well. Data from chemical cross-linking on whole cells indicate that OMPLA is present in the outer membrane as a monomer and that activation of the enzyme induces dimerization concurrent with the appearance of enzymatic activity.

In vivo membrane assembly of split variants of the E.coli outer membrane protein OmpA

The two-domain, 325 residue outer membrane protein OmpA of Escherichia coli is a well-established model for the study of membrane assembly. The N-terminal domain, consisting of approximately 170 amino acid residues, is embedded in the membrane, presumably in the form of a beta-barrel consisting of eight antiparallel transmembrane beta-strands. A set of 16 gene variants carrying deletions in the membrane-embedded domain of OmpA was constructed. When pairs of these mutant genes were co-expressed in E.coli, it was found that a functional OmpA protein could be assembled efficiently from two complementary protein fragments. Assembly was found when the polypeptide chain was split at the second or third periplasmic turn. All four protein termini were located in the periplasmic space. Interestingly, duplication of transmembrane strands five and six led to a variant with an unusual topology: the N-terminus of one fragment and the C-terminus of the other fragment were exposed at the cell surface. This is the first demonstration of correct membrane assembly of split beta-structured membrane proteins. These findings are important for a better understanding of their folding/assembly pathway and may have implications for the development of artificial outer membrane proteins and for the cell surface display of heterologous peptides or proteins.

Amplification of a novel gene, sanA, abolishes a vancomycin-sensitive defect in Escherichia coli

We have isolated an Escherichia coli gene which, when overexpressed, is able to complement the permeability defects of a vancomycin-susceptible mutant. This gene, designated sanA, is located at min 47 of the E. coli chromosome and codes for a 20-kDa protein with a highly hydrophobic amino-terminal segment. A strain carrying a null mutation of the sanA gene, transferred to the E. coli chromosome by homologous recombination, is perfectly viable, but after two generations at high temperature (43 degrees C), the barrier function of its envelope towards vancomycin is defective.

A conserved histidine residue of Escherichia coli outer-membrane phospholipase A is important for activity

Escherichia coli outer-membrane phospholipase A (OMPLA) is thought to be a member of the class of serine hydrolases, having a classical Asp-His-Ser catalytic triad [Horrevoets, A. J. G., Verheij, H. M. & de Haas, G. H. (1991) Eur. J. Biochem. 198, 247-253]. To identify the histidine residue that is important for catalytic activity, the four histidine residues in E. coli OMPLA that are conserved in other enterobacterial OMPLA enzymes were replaced by cysteine residues using PCR-directed, site-specific mutagenesis. The resulting mutant proteins were all well expressed and displayed heat modifiability, indicating that they were properly folded. Enzyme assays showed that only the His142Cys mutant protein was lacking enzymatic activity. In addition, a His142Gly mutant protein appeared to be inactive. These results show that His142 is important for the enzymatic activity of OMPLA.

Comparison of the phenotypes of the lpxA and lpxD mutants of Escherichia coli

We compared the phenotype of two thermosensitive Escherichia coli mutants defective in lipid A biosynthesis, i.e. SM101 (lpxA) and CDH23-213 (lpxD). More than 40% of the periplasmic 27-kDa marker enzyme beta-lactamase was released from SM101 at 28 degrees C. At this temperature, the mutant still grew with a generation time (67 min), not much longer than that of the parent control strain (57 min). CDH23-213 released beta-lactamase only at higher temperatures. SM101 and CDH23-213 were both unable to grow in hypo-osmotic conditions. Derivatives of SM101 and CDH23-213 with mdoA::Tn10 had identical phenotypes (including thermosensitivity and defective outer membrane permeability barrier to hydrophobic probes) to those of SM101 and CDH23-213, indicating that the potential loss of membrane-derived oligosaccharides (MDO) did not explain these phenotype properties. A method for the estimation of lipid A synthesis rate was developed.

Crystallization and preliminary X-ray analysis of outer membrane phospholipase A from Escherichia coli

The outer membrane phospholipase A (OMPLA) of Escherichia coli is one of the few integral outer membrane proteins displaying enzymatic activity. It is encoded as a mature protein of 269 amino acids preceded by a signal sequence of 20 amino acids. There is no sequence homology with water-soluble lipases and phospholipases. Crystals of the mature enzyme were obtained at 22 degrees C from 24-28% (v/v) 2-methyl-2,4-pentanediol in Bis-Tris buffer, pH 5.9-6.0, with 1 mM calcium chloride and 1.5% (w/v) beta-octylglucoside. They have the symmetry of the trigonal spacegroup P3(1)21 (or P3(2)21) with cell dimensions of a = b = 79.6 A and c = 102.8 A (alpha = beta = 90 degrees, gamma = 120 degrees). Native crystals diffract to a resolution of 2.6 A.

Effects of temperature and of heat shock on the expression and action of the colicin A lysis protein

At low temperature, the synthesis of the colicin A lysis protein in Escherichia coli was slowed down, and consequently its functioning was retarded. The rates were restored when the bacteria were shifted for 10 min to 42 degrees C, except in an rpoH mutant, suggesting that one or more proteins regulated by sigma 32 is necessary for expression of colicin A lysis protein.

In vitro folding of Escherichia coli outer-membrane phospholipase A

Recombinant outer-membrane phospholipase A (OMPLA) of Escherichia coli was expressed without its signal sequence from the T7 phi 10 promoter. As a result of the cloning strategy the protein had an N-terminal extension of six amino acid residues. The protein accumulated in the cytosol in inclusion bodies. Conditions were established for the efficient folding of OMPLA in vitro in the presence of Triton X-100. After in vitro folding, the protein was present as a mixture of folded and unfolded forms. Ion-exchange chromatography was used for the purification of OMPLA and the separation of correctly folded, enzymically active enzyme from unfolded inactive protein. The final protein preparation was pure and fully heat-modifiable based on SDS/PAGE. The recombinant enzyme had a specific activity of 71 U/mg, which is similar to the value of the wild-type enzyme, purified from the membrane. The final yield of active enzyme was 35 mg protein/l culture of an A600 of 6. Circular dichroism spectroscopy revealed a high content of beta strand, in good agreement with a predicted beta-barrel structure of this outer-membrane protein.

Genetic map of Salmonella typhimurium, edition VIII

We present edition VIII of the genetic map of Salmonella typhimurium LT2. We list a total of 1,159 genes, 1,080 of which have been located on the circular chromosome and 29 of which are on pSLT, the 90-kb plasmid usually found in LT2 lines. The remaining 50 genes are not yet mapped. The coordinate system used in this edition is neither minutes of transfer time in conjugation crosses nor units representing "phage lengths" of DNA of the transducing phage P22, as used in earlier editions, but centisomes and kilobases based on physical analysis of the lengths of DNA segments between genes. Some of these lengths have been determined by digestion of DNA by rare-cutting endonucleases and separation of fragments by pulsed-field gel electrophoresis. Other lengths have been determined by analysis of DNA sequences in GenBank. We have constructed StySeq1, which incorporates all Salmonella DNA sequence data known to us. StySeq1 comprises over 548 kb of nonredundant chromosomal genomic sequences, representing 11.4% of the chromosome, which is estimated to be just over 4,800 kb in length. Most of these sequences were assigned locations on the chromosome, in some cases by analogy with mapped Escherichia coli sequences.