Protein tyrosine kinases in bacterial pathogens are associated with virulence and production of exopolysaccharide

Isolation and sequencing of Rhizobium leguminosarum Bv. Trifolii PssN, PssO and PssP genes encoding the proteins involved in polymerization and translocation of exopolysaccharide

Rhizobium leguminosarum bv. trifolii produces an acidic exopolysaccharide (EPS) that plays an important role in symbiotic interaction with clover plants. The sequence of 6.0-kb DNA fragment located upstream of the previously described prsDEorf3 and pssCDE genes involved in exopolysaccharide biosynthesis revealed three new genes designated pssN, pssO and pssP. The predicted protein product of pssP gene shares a significant homology to members of the membrane-periplasmic auxiliary (MPA1) family, that are involved in polymerization of the repeating subunits of EPS. The putative pssN protein product is highly homologous to the family of the outer membrane auxiliary (OMA) proteins engaged in translocation of polysaccharides in bacteria. The PssO did not reveal homology to the known bacterial proteins, but showed characteristic features of outer membrane proteins, and with PssN and PssP, it might be a part of the system involved in polymerization and translocation of EPS across the bacterial membranes.

Analysis of the wee gene cluster responsible for the biosynthesis of the polymeric bioemulsifier from the oil-degrading strain Acinetobacter lwoffii RAG-1

A cluster (27 kbp) of genes responsible for the biosynthesis of the amphipathic, polysaccharide bioemulsifier emulsan from the oil-degrading Acinetobacter lwoffii RAG-1 was isolated and characterized. The complete sequence of this cluster, termed wee, consisted of 20 ORFs. One set of 17 ORFs was transcribed in one direction, while a second set of three ORFs, 607 bp upstream of the first, was transcribed in the opposite direction. Mutations in either of the two regions caused defects in emulsan production, yielding specific activities of 5-14% of parental emulsifying activity. Putative functions could be assigned to proteins involved in production of nucleotide amino sugar precursors, transglycosylation, transacetylation, polymerization and transport. However, no JUMPstart or ops sequences, normally found associated with some polysaccharide biosynthetic gene clusters, were identified. Evidence is presented suggesting that the bioemulsifier may be a member of the group 1 or group 4 polysaccharides.

Establishing a direct role for the Bartonella bacilliformis invasion-associated locus B (IalB) protein in human erythrocyte parasitism

The invasion-associated locus A and B genes (ialAB) of Bartonella bacilliformis were previously shown to confer an erythrocyte-invasive phenotype upon Escherichia coli, indirectly implicating their role in virulence. We report the first direct demonstration of a role for ialB as a virulence factor in B. bacilliformis. The presence of a secretory signal sequence and amino acid sequence similarity to two known outer membrane proteins involved in virulence suggested that IalB was an outer membrane protein. To develop an antiserum for protein localization, the ialB gene was cloned in frame into an expression vector with a six-histidine tag and under control of the lacZ promoter. The IalB fusion protein was purified by nickel affinity chromatography and used to raise polyclonal antibodies. IalB was initially localized to the bacterial membrane fraction. To further localize IalB, B. bacilliformis inner and outer membranes were fractionated by sucrose density gradient centrifugation and identified by appearance, buoyant density (rho), and cytochrome b content. Inner and outer membrane proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and IalB was positively identified by Western blot. Contrary to expectations, IalB was localized to the inner membrane of the pathogen. To directly demonstrate a role for IalB in erythrocyte parasitism, the B. bacilliformis ialB gene was disrupted by insertional mutagenesis. The resulting ialB mutant strain was complemented in trans with a replicative plasmid encoding the full-length ialB gene. PCR and high-stringency DNA hybridization confirmed mutagenesis and transcomplementation events. Abrogation and restoration of ialB expression was verified by SDS-PAGE and immunoblotting. In vitro virulence assays showed that mutagenesis of ialB decreased bacterial association and invasion of human erythrocytes by 47 to 53% relative to controls. Transcomplementation of ialB restored erythrocyte association and invasion rates to levels observed in the parental strain. These data provide direct evidence for IalB's role in erythrocyte parasitism and represent the first demonstration of molecular Koch's postulates for a Bartonella species.

Studies about the mechanism of internalization by mammary epithelial cells of Escherichia coli isolated from persistent bovine mastitis

The purpose of this study was to investigate the interaction between Escherichia coli and primary mammary epithelial cell cultures derived from cows with persistent intramammary infection (IMI). Two strains of E. coli, isolated from the milk of two different cows suffering from persistent E. coli IMI were tested for adhesion to and invasion of three primary mammary epithelial cell cultures derived from mammary biopsies of the two infected cows. Intracellular E. coli were detected during five days post infection in vitro. Both strains of E. coli adhered to and invaded monolayers of all three primary mammary epithelial cell cultures. One strain adhered less but invaded more than the other. Comparison with other mammary pathogens indicated that E. coli invaded the cells less efficiently than Staphylococcus aureus, about as efficiently as Streptococcus dysgalactiae and more efficiently than Streptococcus uberis. The mechanism of E. coli invasion was studied using the cytoskeleton disrupting agents colchicine and cytochalasin D. These compounds inhibited the invasion of E. coli. Invasion of E. coli could also be inhibited by the phosphokinase inhibitors genistein and staurosporin in a dose-dependent fashion. Phorbol-myristyl-acetate (PMA) had no effect on the invasion of E. coli. Histology of mammary tissue revealed chronic inflammatory changes in quarters that were persistently infected by E. coli. Intracellular bacteria were not detected in mammary tissue sections. Polymerase chain reaction (PCR) analysis suggested that the two strains of E. coli lacked genes encoding for bundle-forming pili (bfpA), intimin (eae) and translocated intimin receptor (tir), which are characteristic for enteropathogenic E. coli (EPEC).

The "catalytic" triad of isocitrate dehydrogenase kinase/phosphatase from E. coli and its relationship with that found in eukaryotic protein kinases

The isocitrate dehydrogenase kinase/phosphatase (IDHK/P) of E. coli is a bifunctional enzyme responsible for the reversible phosphorylation of isocitrate dehydrogenase (IDH) on a seryl residue. As such, it belongs to the serine/threonine protein kinase family. However, only a very limited homology with the well-characterized eukaryotic members of that family was identified so far in its primary structure. In this report, a new region of amino acids including three putative residues involved in the kinase activity of IDHK/P was identified by sequence comparison with eukaryotic protein kinases. In IDHK/P, these residues are Asp-371, Asn-377, and Asp-403. Their counterpart eukaryotic residues have been shown to be involved in either catalysis (former residue) or magnesium binding (the two latter residues). Site-directed mutagenesis was performed on these three IDHK/P residues, and also on the Glu-439 residue equivalent to that of the Ala-Pro-Glu motif found in the eukaryotic protein kinases. Mutations of Asp-371 into either Ala, Glu, or Gln residues drastically lowered the yield and the quality of the purification. Nevertheless, the recovered mutant enzymes were barely able to phosphorylate IDH either in vitro or after expression in an aceK (-) mutant strain. In contrast, mutation of either Asn-377, Asp-403, or Glu-439 into an Ala residue altered neither the yield of purification nor the maximal phosphorylating capacity of the enzyme. However, when IDH was phosphorylated in the presence of increasing concentrations of magnesium ions, the two former mutants displayed a much lower affinity for this cation, with a K(m) value of 0.6 or 0.8 mM, respectively, as compared to 0.1 mM for the wild-type enzyme. On the other hand, the Glu439Ala mutant has an affinity for magnesium essentially unaffected. Therefore, and in contrast to the current opinion, our results suggest that the catalytic mechanism of IDHK/P exhibits some similarities with that found in the eukaryotic members of the protein kinase family.

Phosphorylation of Wzc, a tyrosine autokinase, is essential for assembly of group 1 capsular polysaccharides in Escherichia coli

Wzc proteins are tyrosine autokinases. They are found in some important bacterial pathogens of humans and livestock as well as plant-associated bacteria, and are often encoded within gene clusters determining synthesis and assembly of capsular and extracellular polysaccharides. Autophosphorylation of Wzc(cps) is essential for assembly of the serotype K30 group 1 capsule in Escherichia coli O9a:K30, although a genetically unlinked Wzc(cps)-homologue (Etk) can also participate with low efficiency. While autophosphorylation of Wzc(cps) is required for assembly of high molecular weight K30 capsular polysaccharide, it is not essential for either the synthesis of the K30 repeat units or for activity of the K30 polymerase enzyme. Paradoxically, the cognate phosphotyrosine protein phosphatase for Wzc(cps), Wzb(cps), is also required for capsule expression. The tyrosine-rich domain at the C terminus of Wzc(cps) was identified as the site of phosphorylation and autophosphorylation of Wzc requires a functional Walker A motif. Intermolecular transphosphorylation of Wzc(cps) was detected in strains expressing a combination of mutant Wzc(cps) derivatives. The N- and C-terminal domains of Wzc(cps) were expressed independently to mimic the situation found naturally in Gram-positive bacteria. In this format, both domains were required for phosphorylation of the Wzc(cps) C terminus, and for capsule assembly. Regulation by a post-translational phosphorylation event represents a new dimension in the assembly of bacterial cell-surface polysaccharides.

Relationship between exopolysaccharide production and protein-tyrosine phosphorylation in gram-negative bacteria

The phosphorylation of proteins at tyrosine residues is known to play a key role in the control of numerous fundamental processes in animal systems. In contrast, the biological significance of protein-tyrosine phosphorylation in bacteria, which has only been recognised recently, is still unclear. Here, we have analysed the role in Escherichia coli cells of an autophosphorylating protein-tyrosine kinase, Wzc, and a phosphotyrosine-protein phosphatase, Wzb, by performing knock-out experiments on the corresponding genes, wzc and wzb, and looking at the metabolic consequences induced. The results demonstrate that the phosphorylation of Wzc, as regulated by Wzb, is directly connected with the production of a particular capsular polysaccharide, colanic acid. Thus, when Wzc is phosphorylated on tyrosine, no colanic acid is synthesised by bacteria, but when dephosphorylated by Wzb, colanic acid is produced. This process is rather specific to the pair of proteins Wzc/Wzb. Indeed, a much lesser effect, if any, on colanic acid synthesis is observed when knock-out experiments are performed on another pair of genes, etk and etp, which also encode respectively a protein-tyrosine kinase, Etk, and a phosphotyrosine-protein phosphatase, Etp, in E. coli. In addition, the analysis of the phosphorylation reaction at the molecular level reveals differences between Gram-negative and Gram-positive bacteria, namely in the number of protein components required for this reaction to occur.

Erwinia amylovora: the molecular basis of fireblight disease

Summary Taxonomy: Bacteria; Proteobacteria; gamma subdivision; order Enterobacteriales; family Enterobacteriaceae; genus Erwinia. Microbiological properties: Gram-negative, motile rods. Related species:E. carotovora (soft-rot diseases), E. chrysanthemi (soft-rot diseases), E. (Pantoea) stewartii (Stewart's wilt of corn), E. (Pantoea) herbicola (epiphyte).

HOST RANGE

Affects rosaceous plants, primarily members of the Pomoideae. Economically important hosts are apple and pear. The commercial implications of fireblight outbreaks are aggravated by the limited effectiveness of current control measures. Disease symptoms:E. amylovora infection is characterized by water soaking of infected tissue, followed by wilting and tissue necrosis. Necrosis gives tissue a scorched, blackened appearance, giving rise to the name fireblight. Symptoms are often localized to blossom bracts or young shoots but, in highly susceptible hosts, can spread systemically resulting in death of the entire tree. Infections can vary in severity depending on climatic conditions and host susceptibility. Useful web site:http://www.agric.gov.ab.ca.

Mechanical fractionation reveals structural requirements for enteropathogenic Escherichia coli Tir insertion into host membranes

Enteropathogenic Escherichia coli (EPEC) inserts its receptor for intimate adherence (Tir) into host cell membranes by using a type III secretion system. Detergents are frequently used to fractionate infected host cells to investigate bacterial protein delivery into mammalian cells. In this study, we found that the Triton X-100-soluble membrane fraction from EPEC-infected HeLa cells was contaminated with bacterial proteins. We therefore applied a mechanical method of cell lysis and ultracentrifugation to fractionate infected HeLa cells to investigate the biology and biochemistry of Tir delivery and translocation. This method demonstrates that the translocation of Tir into the host cell membrane requires its transmembrane domains, but not tyrosine phosphorylation or binding to Tir's ligand, intimin.

Tyrosine phosphorylation of CpsD negatively regulates capsular polysaccharide biosynthesis in streptococcus pneumoniae

In Streptococcus pneumoniae, the first four genes of the capsule locus (cpsA to cpsD) are common to most serotypes. By analysis of various in-frame deletion and site-directed mutants, the function of their gene products in capsular polysaccharide (CPS) biosynthesis was investigated. We found that while CpsB, C and D are essential for encapsulation, CpsA is not. CpsC and CpsD have similarity to the amino-terminal and carboxy-terminal regions, respectively, of the autophosphorylating protein-tyrosine kinase Wzc from Escherichia coli. Alignment of CpsD with Wzc and other related proteins identified conserved Walker A and B sequence motifs and a tyrosine rich domain close to the carboxy-terminus. We have shown that CpsD is also an autophosphorylating protein-tyrosine kinase and that point mutations in cpsD affecting either the ATP-binding domain (Walker A motif) or the carboxy-terminal [YGX]4 repeat domain eliminated tyrosine phosphorylation of CpsD. We describe, for the first time, the phenotypic impact of these two mutations on polysaccharide production and show that they affect CPS production differently. Whereas a mutation in the Walker A motif resulted in loss of encapsulation, mutation of the tyrosines in the [YGX]4 repeat domain resulted in an apparent increase in encapsulation and a mucoid phenotype. These data suggest that autophosphorylation of CpsD at tyrosine attenuates its activity and reduces the level of encapsulation. Additionally, we demonstrated that CpsC is required for CpsD tyrosine phosphorylation and that CpsB influences dephosphorylation of CpsD. These results are consistent with CpsD tyrosine phosphorylation acting to negatively regulate CPS production. This has implications for the function of CpsC/CpsD homologues in both Gram-positive and Gram-negative bacteria and provides a mechanism to explain regulation of CPS production during pathogenesis.

Translocation of Helicobacter pylori CagA into gastric epithelial cells by type IV secretion

The Gram-negative bacterium Helicobacter pylori is a causative agent of gastritis and peptic ulcer disease in humans. Strains producing the CagA antigen (cagA(+)) induce strong gastric inflammation and are strongly associated with gastric adenocarcinoma and MALT lymphoma. We show here that such strains translocate the bacterial protein CagA into gastric epithelial cells by a type IV secretion system, encoded by the cag pathogenicity island. CagA is tyrosine-phosphorylated and induces changes in the tyrosine phosphorylation state of distinct cellular proteins. Modulation of host cells by bacterial protein translocation adds a new dimension to the chronic Helicobacter infection with yet unknown consequences.

Tyrosine phosphorylation of the Helicobacter pylori CagA antigen after cag-driven host cell translocation

Helicobacter pylori strains associated with severe tissue damage and inflammation possess a unique genetic locus, cag, containing 31 genes originating from a distant event of horizontal transfer and retained as a pathogenicity island. The cag system is an Helicobacter-specific type IV secretion engine involved in cellular responses like induction of pedestals, secretion of IL-8, and phosphorylation of proteic targets. It has previously been reported that cocultivation of epithelial cells with Helicobacter pylori triggers signal transduction and tyrosine phosphorylation of a 145-kDa putative host cell protein. Herein, we demonstrate that this protein is not derived from the host but rather is the bacterial immunodominant antigen CagA, a virulence factor commonly expressed in peptic ulcer disease and thought to be an orphan of a specific biological function. Thus, CagA is delivered into the epithelial cells by the cag type IV secretion system where it is phosphorylated on tyrosine residues by an as yet unidentified host cell kinase and wired to eukaryotic signal transduction pathways and cytoskeletal plasticity.