Bacterial type IV secretion: conjugation systems adapted to deliver effector molecules to host cells

Plasticity of repetitive DNA sequences within a bacterial (Type IV) secretion system component

DNA rearrangement permits bacteria to regulate gene content and expression. In Helicobacter pylori, cagY, which contains an extraordinary number of direct DNA repeats, encodes a surface-exposed subunit of a (type IV) bacterial secretory system. Examining potential DNA rearrangements involving the cagY repeats indicated that recombination events invariably yield in-frame open reading frames, producing alternatively expressed genes. In individual hosts, H. pylori cell populations include strains that produce CagY proteins that differ in size, due to the predicted in-frame deletions or duplications, and elicit minimal or no host antibody recognition. Using repetitive DNA, H. pylori rearrangements in a host-exposed subunit of a conserved bacterial secretion system may permit a novel form of antigenic evasion.

Sequence and expression analysis of virB9 of the type IV secretion system of Ehrlichia canis strains in ticks, dogs, and cultured cells

Ehrlichia canis virB9 was cloned and expressed. The sequences of virB9 from six geographic locations were identical. virB9 was transcribed by E. canis in dogs, ticks, and cell culture. Infected dogs had antibodies to recombinant VirB9, indicating that VirB9 was produced by E. canis in dogs and was antigenic.

Site-specific integration of Agrobacterium tumefaciens T-DNA via double-stranded intermediates

Agrobacterium tumefaciens-mediated genetic transformation involves transfer of a single-stranded T-DNA molecule (T strand) into the host cell, followed by its integration into the plant genome. The molecular mechanism of T-DNA integration, the culmination point of the entire transformation process, remains largely obscure. Here, we studied the roles of double-stranded breaks (DSBs) and double-stranded T-DNA intermediates in the integration process. We produced transgenic tobacco (Nicotiana tabacum) plants carrying an I-SceI endonuclease recognition site that, upon cleavage with I-SceI, generates DSB. Then, we retransformed these plants with two A. tumefaciens strains: one that allows transient expression of I-SceI to induce DSB and the other that carries a T-DNA with the I-SceI site and an integration selection marker. Integration of this latter T-DNA as full-length and I-SceI-digested molecules into the DSB site was analyzed in the resulting plants. Of 620 transgenic plants, 16 plants integrated T-DNA into DSB at their I-SceI sites; because DSB induces DNA repair, these results suggest that the invading T-DNA molecules target to the DNA repair sites for integration. Furthermore, of these 16 plants, seven plants incorporated T-DNA digested with I-SceI, which cleaves only double-stranded DNA. Thus, T-strand molecules can be converted into double-stranded intermediates before their integration into the DSB sites within the host cell genome.

Agrobacterium-mediated root transformation is inhibited by mutation of an Arabidopsis cellulose synthase-like gene

Agrobacterium-mediated plant genetic transformation involves a complex interaction between the bacterium and the host plant. Relatively little is known about the role plant genes and proteins play in this process. We previously identified an Arabidopsis mutant, rat4, that is resistant to Agrobacterium transformation. We show here that rat4 contains a T-DNA insertion into the 3'-untranslated region of the cellulose synthase-like gene CSLA9. CSLA9 transcripts are greatly reduced in the rat4 mutant. Genetic complementation of rat4 with wild-type genomic copies of the CSLA9 gene restores both transformation competence and the wild-type level of CSLA9 transcripts. The CSLA9 promoter shows a distinct pattern of expression in Arabidopsis plants. In particular, the promoter is active in the elongation zone of roots, the root tissue that we previously showed is most susceptible to Agrobacterium-mediated transformation. Disruption of the CSLA9 gene in the rat4 mutant results in reduced numbers and rate of growth of lateral roots and reduced ability of the roots to bind A. tumefaciens cells under certain conditions. No major differences in the linkage structure of the non-cellulosic polysaccharides could be traced to the defective CSLA9 gene.

The outs and ins of bacterial type IV secretion substrates

Bacteria use type IV secretion systems (T4SS) to translocate macromolecular substrates destined for bacterial, plant or human target cells. The T4SS are medically important, contributing to virulence-gene spread, genome plasticity and the alteration of host cellular processes during infection. The T4SS are ancestrally related to bacterial conjugation machines, but present-day functions include (i) conjugal transfer of DNA by cell-to-cell contact, (ii) translocation of effector molecules to eukaryotic target cells, and (iii) DNA uptake from or release to the extracellular milieu. Rapid progress has been made toward identification of type IV secretion substrates and the requirements for substrate recognition.

Recognition of the Agrobacterium tumefaciens VirE2 translocation signal by the VirB/D4 transport system does not require VirE1

Agrobacterium tumefaciens uses a type IV secretion system to deliver a nucleoprotein complex and effector proteins directly into plant cells. The single-stranded DNA-binding protein VirE2, the F-box protein VirF and VirE3 are delivered into host cells via this VirB/D4 encoded translocation system. VirE1 functions as a chaperone of VirE2 by regulating its efficient translation and preventing VirE2-VirE2 aggregation in the bacterial cell. We analyzed whether the VirE1 chaperone is also essential for transport recognition of VirE2 by the VirB/D4 encoded type IV secretion system. In addition, we assayed whether translocation of VirF and VirE3, which also forms part of the virE operon, is affected by the absence of VirE1. We employed the earlier developed CRAFT (Cre recombinase Reporter Assay For Translocation) assay to detect transfer of Cre::Vir fusion proteins from A. tumefaciens into plants, monitored by stable reconstitution of a kanamycin resistance marker, and into yeast, screened by loss of the URA3 gene. We show that the C-terminal 50 amino acids of VirE2 and VirE3 are sufficient to mediate Cre translocation into host cells, confirming earlier indications of a C-terminal transport signal. This transfer was independent of the presence or absence of VirE1. Besides, the translocation efficiency of VirF is not altered in a virE1 mutant. The results unambiguously show that the VirE1 chaperone is not essential for the recognition of the VirE2 transport signal by the transport system and the subsequent translocation across the bacterial envelope into host cells.

A cryptic plasmid of Yersinia enterocolitica encodes a conjugative transfer system related to the regions of CloDF13 Mob and IncX Pil

Yersinia enterocolitica 29930 (biotype 1A; O : 7,8), the producing strain of the phage-tail-like bacteriocin enterocoliticin, possesses a plasmid-encoded conjugative type IV transfer system. The genes of the conjugative system were found by screening of a cosmid library constructed from total DNA of strain 29930. The cosmid Cos100 consists of the vector SuperCos1 and an insert DNA of 40 303 bp derived from a cryptic plasmid of strain 29930. The conjugative transfer system consists of genes encoding a DNA transfer and replication system (Dtr) with close relationship to the mob region of the mobilizable plasmid CloDF13 and a gene cluster encoding a mating pair formation system (Mpf) closely related to the Mpf system of the IncX plasmid R6K. However, a gene encoding a homologue of TaxB, the coupling protein of the IncX system, is missing. The whole transfer region has a size of approximately 17 kb. The recombinant plasmid Cos100 was shown to be transferable between Escherichia coli and Yersinia with transfer frequencies up to 0·1 transconjugants per donor. Mutations generated by inserting a tetracycline cassette into putative tri genes yielded a transfer-deficient phenotype. Conjugative transfer of the cryptic plasmid could not be demonstrated in the original host Y. enterocolitica 29930. However, a kanamycin-resistance-conferring derivative of the plasmid was successfully introduced into E. coli K-12 by transformation and was shown to be self-transmissible. Furthermore, Southern blot hybridization and PCR experiments were carried out to elucidate the distribution of the conjugative transfer system in Yersinia. In total, six Y. enterocolitica biotype 1A strains harbouring closely related systems on endogenous plasmids were identified.

A bacterial conjugation machinery recruited for pathogenesis

Type IV secretion systems (T4SS) are multicomponent transporters of Gram-negative bacteria adapted to functions as diverse as DNA transfer in bacterial conjugation or the delivery of effector proteins into eukaryotic target cells in pathogenesis. The generally modest sequence conservation between T4SS may reflect their evolutionary distance and/or functional divergence. Here, we show that the establishment of intraerythrocytic parasitism by Bartonella tribocorum requires a putative T4SS, which shares an unprecedented level of sequence identity with the Trw conjugation machinery of the broad-host-range antibiotic resistance plasmid R388 (up to 80% amino acid identity for individual T4SS components). The highly conserved T4SS loci are collinear except for the presence of numerous tandem gene duplications in B. tribocorum, which mostly encode variant forms of presumed surface-exposed pilus subunits. Conservation is not only structural, but also functional: R388 mutated in either trwD or trwH encoding essential T4SS components could be trans-complemented for conjugation by the homologues of the B. tribocorum system. Conservation also includes the transcription regulatory circuit: both T4SS loci encode a highly homologous and interchangeable KorA/KorB repressor system that negatively regulates the expression of all T4SS components. This striking example of adaptive evolution reveals the capacity of T4SS to assume dedicated functions in either DNA transfer or pathogenesis over rather short evolutionary distance and implies a novel role for the conjugation systems of widespread broad-host-range plasmids in the evolution of bacterial pathogens.

A 65-kilobase pathogenicity island is unique to Philadelphia-1 strains of Legionella pneumophila

Nucleotide sequence analysis of an approximately 80-kb genomic region revealed an approximately 65-kb locus that bears hallmarks of a pathogenicity island. This locus includes homologues of a type IV secretion system, mobile genetic elements, and known virulence factors. Comparative studies with other Legionella pneumophila strains and serogroups indicated that this approximately 65-kb locus is unique to L. pneumophila serogroup 1 Philadelphia-1 strains.

F factor conjugation is a true type IV secretion system

The F sex factor of Escherichia coli is a paradigm for bacterial conjugation and its transfer (tra) region represents a subset of the type IV secretion system (T4SS) family. The F tra region encodes eight of the 10 highly conserved (core) gene products of T4SS including TraAF (pilin), the TraBF, -KF (secretin-like), -VF (lipoprotein) and TraCF (NTPase), -EF, -LF and TraGF (N-terminal region) which correspond to TrbCP, -IP, -GP, -HP, -EP, -JP, DP and TrbLP, respectively, of the P-type T4SS exemplified by the IncP plasmid RP4. F lacks homologs of TrbBP (NTPase) and TrbFP but contains a cluster of genes encoding proteins essential for F conjugation (TraFF, -HF, -UF, -WF, the C-terminal region of TraGF, and TrbCF) that are hallmarks of F-like T4SS. These extra genes have been implicated in phenotypes that are characteristic of F-like systems including pilus retraction and mating pair stabilization. F-like T4SS systems have been found on many conjugative plasmids and in genetic islands on bacterial chromosomes. Although few systems have been studied in detail, F-like T4SS appear to be involved in the transfer of DNA only whereas P- and I-type systems appear to transport protein or nucleoprotein complexes. This review examines the similarities and differences among the T4SS, especially F- and P-like systems, and summarizes the properties of the F transfer region gene products.

Cellular prion protein promotes Brucella infection into macrophages

The products of the Brucella abortus virB gene locus, which are highly similar to conjugative DNA transfer system, enable the bacterium to replicate within macrophage vacuoles. The replicative phagosome is thought to be established by the interaction of a substrate of the VirB complex with macrophages, although the substrate and its host cellular target have not yet been identified. We report here that Hsp60, a member of the GroEL family of chaperonins, of B. abortus is capable of interacting directly or indirectly with cellular prion protein (PrPC) on host cells. Aggregation of PrPC tail-like formation was observed during bacterial swimming internalization into macrophages and PrPC was selectively incorporated into macropinosomes containing B. abortus. Hsp60 reacted strongly with serum from human brucellosis patients and was exposed on the bacterial surface via a VirB complex-associated process. Under in vitro and in vivo conditions, Hsp60 of B. abortus bound to PrPC. Hsp60 of B. abortus, expressed on the surface of Lactococcus lactis, promoted the aggregation of PrPC but not PrPC tail formation on macrophages. The PrPC deficiency prevented swimming internalization and intracellular replication of B. abortus, with the result that phagosomes bearing the bacteria were targeted into the endocytic network. These results indicate that signal transduction induced by the interaction between bacterial Hsp60 and PrPC on macrophages contributes to the establishment of B. abortus infection.

A novel sheathed surface organelle of the Helicobacter pylori cag type IV secretion system

Type I strains of Helicobacter pylori (Hp) use a type IV secretion system (T4SS), encoded by the cag pathogenicity island (cag-PAI), to deliver the bacterial protein CagA into eukaryotic cells and to induce interleukin-8 secretion. Translocated CagA is activated by tyrosine phosphorylation involving Src-family kinases. The mechanism and structural basis for type IV protein secretion is not well understood. We describe here, by confocal laser scanning microscopy and field emission scanning electron microscopy, a novel filamentous surface organelle which is part of the Hp T4SS. The organelle is often located at one bacterial pole but can be induced by cell contact also along the lateral side of the bacteria. It consists of a rigid needle, covered focally or completely by HP0527 (Cag7 or CagY), a VirB10-homologous protein. HP0527 is also clustered in the outer membrane. The VirB7-homologous protein HP0532 is found at the base of this organelle. These observations demonstrate for the first time by microscopic techniques a complex T4SS-associated, sheathed surface organelle reminiscent to the needle structures of bacterial type III secretion systems.

Mosaic structure of pathogenicity islands in Legionella pneumophila

A gene complex, dot/icm, located in two independent chromosomal loci of L. pneumophila, the causative agent of Legionnaires' disease, is related to virulence. To investigate the evolutionary pattern of these pathogenicity islands of L. pneumophila, portions of four genes in the dot/icm complex, namely, dotA, dotB, icmB, and icmT, were amplified, sequenced, and phylogenetically analyzed, in addition to rpoB, which encodes an RNA polymerase beta-subunit. The nucleotide sequences and phylogenetic analyses of these five genes of 96 L. pneumophila strains revealed that several subgroups of L. pneumophila proliferated clonally. However, incongruent gene tree topologies and the results of statistical testing (Templeton Willcoxon signed-ranked and incongruence length differences tests) indicated that the evolutionary histories of these genes within the pathogenicity islands are not uniform, and that they constitute a mosaic structure. In addition, the nonuniform grouping of some reference strains suggests that intraspecific recombination might be still occurring in nature or in the laboratory.

Transfer of T-DNA and Vir proteins to plant cells by Agrobacterium tumefaciens induces expression of host genes involved in mediating transformation and suppresses host defense gene expression

Agrobacterium tumefaciens is a plant pathogen that incites crown gall tumors by transferring to and expressing a portion of a resident plasmid in plant cells. Currently, little is known about the host response to Agrobacterium infection. Using suppressive subtractive hybridization and DNA macroarrays, we identified numerous plant genes that are differentially expressed during early stages of Agrobacterium-mediated transformation. Expression profiling indicates that Agrobacterium infection induces plant genes necessary for the transformation process while simultaneously repressing host defense response genes, thus indicating successful utilization of existing host cellular machinery for genetic transformation purposes. A comparison of plant responses to different strains of Agrobacterium indicates that transfer of both T-DNA and Vir proteins modulates the expression of host genes during the transformation process.

Topology and membrane interaction of Helicobacter pylori ComB proteins involved in natural transformation competence

The human gastric pathogen Helicobacter pylori is naturally competent for genetic transformation. The H. pylori comB gene duster encodes the VirB4-homologous ATPase ComB4 and the structural proteins ComB7-ComB10, which share significant sequence identity to the Agrobacterium tumefaciens virB-encoded type IV secretion system. To study the topology of the ComB7-10 proteins, we applied TnMax transposon mutagenesis by generating fusions of ComB proteins with mature beta-lactamase (BlaM) or alkaline phosphatase (PhoA). Our data show that the putative lipoprotein ComB7 is secreted and is found membrane-attached, probably by its lipid anchor. According to our topology mapping ComB8 is a bitopic membrane protein with a short N-terminal portion in the cytoplasm and the remainder of the protein expanding into the periplasmic space. ComB9 was verified as a periplasmic protein, tightly attached to the membrane. The N-terminus of ComB10 is anchored in the cytoplasmic membrane and the major portion of the protein, including a putative coiled-coil domain, is located in the periplasm. Limited protease digestion and protein extraction under different salt and pH conditions confirmed the periplasmic localization and the tight membrane association of the ComB protein complex. A hypothetical model of the ComB DNA transformation pore in H. pylori is presented.

Show me the substrates: modulation of host cell function by type IV secretion systems

Evidence for the involvement of type IV protein secretion systems in bacterial virulence is accumulating. Many of the substrate proteins secreted by type IV systems either hijack or interfere with specific host cell pathways. These substrates can be injected directly into host cells via the type IV apparatus or are secreted by the type IV machinery in a state that allows them to gain access to cellular targets without the further assistance of the type IV system. Arguably, the protein substrates of most type IV secretion systems remain undiscovered. Here, we review the activities of known type IV substrates and discuss the putative roles of unidentified substrates.

Conjugative plasmid transfer in gram-positive bacteria

Conjugative transfer of bacterial plasmids is the most efficient way of horizontal gene spread, and it is therefore considered one of the major reasons for the increase in the number of bacteria exhibiting multiple-antibiotic resistance. Thus, conjugation and spread of antibiotic resistance represents a severe problem in antibiotic treatment, especially of immunosuppressed patients and in intensive care units. While conjugation in gram-negative bacteria has been studied in great detail over the last decades, the transfer mechanisms of antibiotic resistance plasmids in gram-positive bacteria remained obscure. In the last few years, the entire nucleotide sequences of several large conjugative plasmids from gram-positive bacteria have been determined. Sequence analyses and data bank comparisons of their putative transfer (tra) regions have revealed significant similarities to tra regions of plasmids from gram-negative bacteria with regard to the respective DNA relaxases and their targets, the origins of transfer (oriT), and putative nucleoside triphosphatases NTP-ases with homologies to type IV secretion systems. In contrast, a single gene encoding a septal DNA translocator protein is involved in plasmid transfer between micelle-forming streptomycetes. Based on these clues, we propose the existence of two fundamentally different plasmid-mediated conjugative mechanisms in gram-positive microorganisms, namely, the mechanism taking place in unicellular gram-positive bacteria, which is functionally similar to that in gram-negative bacteria, and a second type that occurs in multicellular gram-positive bacteria, which seems to be characterized by double-stranded DNA transfer.

Structural definition on the surface of Helicobacter pylori type IV secretion apparatus

Genetic and functional studies have indicated that the type IV secretion system (TFSS) of Helicobacter pylori forms a secretion complex in the cell envelope that protrudes towards the outside in order to inject CagA protein into gastric epithelial cells. However, the proposed structural model is based on partial amino acid homology with the components of the Agrobacterium tumefaciens TFSS. Therefore, we undertook the identification of the structural features of the TFSS exposed on the surface of H. pylori and found that filamentous structures present on the bacterial surface are related to the secretion apparatus. Using immunofluorescence microscopy with antibodies directed to tyrosine-phosphorylated CagA (pY-CagA) and Hp0532 (VirB7) in the infection assay, pY-CagA signals were detected just below the host cell-attached bacteria, where Hp0532 (VirB7) signals were detected as co-localized, suggesting that the CagA injected into the host cell through the TFSS apparatus is still mostly confined to the areas just below the attached bacteria after being phosphorylated. Furthermore, the filamentous structures on bacterium were found to be associated with Hp0532 (VirB7) or Hp0528 (VirB9), the major components of TFSS, by immunogold electron microscopy. These results strongly suggest that the H. pylori TFSS apparatus is a filamentous macromolecular structure protruding from the bacterial envelope.

Identification of Arabidopsis rat mutants

Limited knowledge currently exists regarding the roles of plant genes and proteins in the Agrobacterium tumefaciens-mediated transformation process. To understand the host contribution to transformation, we carried out root-based transformation assays to identify Arabidopsis mutants that are resistant to Agrobacterium transformation (rat mutants). To date, we have identified 126 rat mutants by screening libraries of T-DNA insertion mutants and by using various "reverse genetic" approaches. These mutants disrupt expression of genes of numerous categories, including chromatin structural and remodeling genes, and genes encoding proteins implicated in nuclear targeting, cell wall structure and metabolism, cytoskeleton structure and function, and signal transduction. Here, we present an update on the identification and characterization of these rat mutants.

Isolation and characterization of mini-Tn5Km2 insertion mutants of Brucella abortus deficient in internalization and intracellular growth in HeLa cells

Brucella spp. are facultative intracellular pathogens that have the ability to survive and multiply in professional and nonprofessional phagocytes and cause abortion in domestic animals and undulant fever in humans. The mechanism and factors of virulence are not fully understood. To identify genes related to internalization and multiplication in host cells, Brucella abortus was mutagenized by mini-Tn5Km2 transposon that carryied the kanamycin resistance gene, 4,400 mutants were screened, and HeLa cells were infected with each mutant. Twenty-three intracellular-growth-defective mutants were screened and were characterized for internalization and intracellular growth. From these results, we divided the mutants into the following three groups: class I, no internalization and intracellular growth within HeLa cells; class II, an internalization similar to that of the wild type but with no intracellular growth; and class III, internalization twice as high as the wild type but with no intracellular growth. Sequence analysis of DNA flanking the site of transposon showed various insertion sites of bacterial genes that are virulence-associated genes, including virB genes, an ion transporter system, and biosynthesis- and metabolism-associated genes. These internalization and intracellular-growth-defective mutants in HeLa cells also showed defective intracellular growth in macrophages. These results suggest that the virulence-associated genes isolated here contributed to the intracellular growth of both nonprofessional and professional phagocytes.

Functional subsets of the virB type IV transport complex proteins involved in the capacity of Agrobacterium tumefaciens to serve as a recipient in virB-mediated conjugal transfer of plasmid RSF1010

The virB-encoded type IV transport complex of Agrobacterium tumefaciens mediates the transfer of DNA and proteins into plant cells, as well as the conjugal transfer of IncQ plasmids, such as RSF1010, between Agrobacterium strains. While several studies have indicated that there are physical interactions among the 11 VirB proteins, the functional significance of the interactions has been difficult to establish since all of the proteins are required for substrate transfer. Our previous studies, however, indicated that although all of the VirB proteins are required for the capacity of a strain to serve as an RSF1010 donor, only a subset of these proteins in the recipient is necessary to increase the conjugal frequency by 3 to 4 logs. The roles of particular groups of VirB proteins in this increased recipient activity were examined in the study reported here. Examination of the expression of subgroups of virB genes revealed that translation of virB6 is necessary for expression of downstream open reading frames. Expression of limited subsets of the VirB proteins in a recipient strain lacking the Ti plasmid revealed that the VirB7 to VirB10 proteins yield a subcomplex that is functional in the recipient assay but that the VirB1 to VirB4 proteins, as a group, dramatically increase this activity in strains expressing VirB7 to VirB10. Finally, the membrane distribution and cross-linking patterns of VirB10, but not of VirB8 or VirB9, in a strain expressing only VirB7 to VirB10 are significantly altered compared to the patterns of the wild type. These characteristics are, however, restored to the wild-type status by coexpression of VirB1 to VirB3. Taken together, these results define subsets of type IV transport complex proteins that are critical in allowing a strain to participate as a recipient in virB-mediated conjugal RSF1010 transfer.

VirB11 ATPases are dynamic hexameric assemblies: new insights into bacterial type IV secretion

The coupling of ATP binding/hydrolysis to macromolecular secretion systems is crucial to the pathogenicity of Gram-negative bacteria. We reported previously the structure of the ADP-bound form of the hexameric traffic VirB11 ATPase of the Helicobacter pylori type IV secretion system (named HP0525), and proposed that it functions as a gating molecule at the inner membrane, cycling through closed and open forms regulated by ATP binding/hydrolysis. Here, we combine crystal structures with analytical ultracentrifugation experiments to show that VirB11 ATPases indeed function as dynamic hexameric assemblies. In the absence of nucleotide, the N-terminal domains exhibit a collection of rigid-body conformations. Nucleotide binding 'locks' the hexamer into a symmetric and compact structure. We propose that VirB11s use the mechanical leverage generated by such nucleotide-dependent conformational changes to facilitate the export of substrates or the assembly of the type IV secretion apparatus. Biochemical characterization of mutant forms of HP0525 coupled with electron microscopy and in vivo assays support such hypothesis, and establish the relevance of VirB11s ATPases as drug targets against pathogenic bacteria.

Xanthomonas citri: breaking the surface

SUMMARY Taxonomy: Bacteria; Proteobacteria, Gammaproteobacteria; Xanthomonadales; Xanthomonadaceae, Xanthomonas. Microbiological properties: Gram-negative, obligately aerobic, straight rods, motile by a single polar flagellum, yellow pigment. Related species: X. campestris, X. axonopodis, X. oryzae, X. albilineans.

HOST RANGE

Affects Rutaceous plants, primarily Citrus spp., Fortunella spp., and Poncirus spp., world-wide. Quarantined pathogen in many countries. Economically important hosts are cultivated orange, grapefruit, lime, lemon, pomelo and citrus rootstock. Disease symptoms: On leaves, first appearance is as oily looking, 2-10 mm, similarly sized, circular spots, usually on the abaxial surface. On leaves, stems, thorns and fruit, circular lesions become raised and blister-like, growing into white or yellow spongy pustules. These pustules then darken and thicken into a light tan to brown corky canker, which is rough to the touch. On stems, pustules may coalesce to split the epidermis along the stem length, and occasionally girdling of young stems may occur. Older lesions on leaves and fruit tend to have more elevated margins and are at times surrounded by a yellow chlorotic halo (that may disappear) and a sunken centre. Sunken craters are especially noticeable on fruit, but the lesions do not penetrate far into the rind. Defoliation and premature abscission of affected fruit occurs on heavily infected trees.

USEFUL WEBSITES

<http://www.biotech.ufl.edu/PlantContainment/canker.htm>; <http://cancer.lbi.ic.unicamp.br/xanthomonas/>

Expression cloning of a periodontitis-associated apoptotic effector, cagE homologue, in Actinobacillus actinomycetemcomitans

To study anti-bacterial immunity and to identify critical bacterial antigens associated with specific periodontal infection, we screened the genomic library of Actinobacillus actinomycetemcomitans, a major Gram(-) anaerobe causing human periodontitis, by expression cloning using disease-associated periodontal CD4(+)T cells derived from HuPBL-engrafted NOD/SCID mice. Here, we report one of the novel genes identified and designated, cagE homologue (in short: cagE) of A. actinomycetemcomitans, which encodes a putative bacterial type IV secretion system with significant homology to Helicobacter pylori CagE and Agrobacterium tumefaciens VirB4. All serum samples from A. actinomycetemcomitans-infected periodontitis patients, but not from the healthy controls, readily recognized CagE by ELISA and Western blot, suggesting its biological and clinical significance. The CagE protein, upon secretion, elicited significant apoptosis on primary human epithelia, endothelia, osteoblasts, and T cells by 4-12h in vitro. Importantly, both cagE(-) mutant strain and N-terminus truncated CagE protein drastically reduced (p<0.001) the induction of apoptosis on human epithelia in vitro. These data strongly suggest that a novel effector protein, CagE in A. actinomycetemcomitans, induces apoptosis of human cells and destructive immunity, thereby it may play an important role in the pathogenesis of A. actinomycetemcomitans-mediated infections.

Infection and replication sites of Spiroplasma kunkelii (Class: Mollicutes) in midgut and Malpighian tubules of the leafhopper Dalbulus maidis

Spiroplasma kunkelii distribution and infection mechanisms in the intestines and Malpighian tubules of Dalbulus maidis were investigated by transmission electron microscopy. Spiroplasmas were found between microvilli and in endocytic vesicles of the midgut epithelium. At the basal part, cytoplasmic vesicles contained multiple spiroplasmas with tube-like extensions and spiroplasmas accumulated between the laminae rara and densa of the basal lamina. Tip structures of flask-shaped spiroplasmas pierced the lamina densa that was discontinuous in close proximity to spiroplasmas. Spiroplasmas were found in hemolymph, crossed the basal lamina of Malpighian tubule epithelium and accumulated at high numbers in muscle cells that had cytopathogenic changes. S. kunkelii had perithrochous approximately 8nm diameter structures determined to be fimbriae protruding from the cell surface, and similar structures were adhering to the basal lamina of midgut epithelium and to external lamina of muscle cells. Further, spiroplasmas had pili-like appendages at one or both cell poles and appeared to conjugate. This is the first time that fimbriae and pili have been observed in a mollicutes.

Attenuation of Helicobacter pylori CagA x SHP-2 signaling by interaction between CagA and C-terminal Src kinase

Helicobacter pylori (H. pylori) is a causative agent of gastric diseases ranging from gastritis to cancer. The CagA protein is the product of the cagA gene carried among virulent H. pylori strains and is associated with severe disease outcomes, most notably gastric carcinoma. CagA is injected from the attached H. pylori into gastric epithelial cells and undergoes tyrosine phosphorylation. The phosphorylated CagA binds and activates SHP-2 phosphatase and thereby induces a growth factor-like morphological change termed the "hummingbird phenotype." In this work, we demonstrate that CagA is also capable of interacting with C-terminal Src kinase (Csk). As is the case with SHP-2, Csk selectively binds tyrosine-phosphorylated CagA via its SH2 domain. Upon complex formation, CagA stimulates Csk, which in turn inactivates the Src family of protein-tyrosine kinases. Because Src family kinases are responsible for CagA phosphorylation, an essential prerequisite of CagA.SHP-2 complex formation and subsequent induction of the hummingbird phenotype, our results indicate that CagA-Csk interaction down-regulates CagA.SHP-2 signaling by both competitively inhibiting CagA.SHP-2 complex formation and reducing levels of CagA phosphorylation. We further demonstrate that CagA.SHP-2 signaling eventually induces apoptosis in AGS cells. Our results thus indicate that CagA-Csk interaction prevents excess cell damage caused by deregulated activation of SHP-2. Attenuation of CagA activity by Csk may enable cagA-positive H. pylori to persistently infect the human stomach for decades while avoiding excess CagA toxicity to the host.

The Helicobacter pylori CagA protein induces cortactin dephosphorylation and actin rearrangement by c-Src inactivation

The gastric pathogen Helicobacter pylori translocates the CagA protein into epithelial cells by a type IV secretion process. Translocated CagA is tyrosine phosphorylated (CagA(P-Tyr)) on specific EPIYA sequence repeats by Src family tyrosine kinases. Phos phorylation of CagA induces the dephosphorylation of as yet unidentified cellular proteins, rearrangements of the host cell actin cytoskeleton and cell scattering. We show here that CagA(P-Tyr) inhibits the catalytic activity of c-Src in vivo and in vitro. c-Src inactivation leads to tyrosine dephosphorylation of the actin binding protein cortactin. Concomitantly, cortactin is specifically redistributed to actin-rich cellular protrusions. c-Src inactivation and cortactin dephosphorylation are required for rearrangements of the actin cytoskeleton. Moreover, CagA(P-Tyr)-mediated c-Src inhibition downregulates further CagA phosphorylation through a negative feedback loop. This is the first report of a bacterial virulence factor that inhibits signalling of a eukaryotic tyrosine kinase and on a role of c-Src inactivation in host cell cytoskeletal rearrangements.

The VirB4 family of proposed traffic nucleoside triphosphatases: common motifs in plasmid RP4 TrbE are essential for conjugation and phage adsorption

Proteins of the VirB4 family are encoded by conjugative plasmids and by type IV secretion systems, which specify macromolecule export machineries related to conjugation systems. The central feature of VirB4 proteins is a nucleotide binding site. In this study, we asked whether members of the VirB4 protein family have similarities in their primary structures and whether these proteins hydrolyze nucleotides. A multiple-sequence alignment of 19 members of the VirB4 protein family revealed striking overall similarities. We defined four common motifs and one conserved domain. One member of this protein family, TrbE of plasmid RP4, was genetically characterized by site-directed mutagenesis. Most mutations in trbE resulted in complete loss of its activities, which eliminated pilus production, propagation of plasmid-specific phages, and DNA transfer ability in Escherichia coli. Biochemical studies of a soluble derivative of RP4 TrbE and of the full-length homologous protein R388 TrwK revealed that the purified forms of these members of the VirB4 protein family do not hydrolyze ATP or GTP and behave as monomers in solution.

Identification of pathogen-specific and conserved genes expressed in vivo by an avian pathogenic Escherichia coli strain

Escherichia coli is a diverse bacterial species that comprises commensal nonpathogenic strains such as E. coli K-12 and pathogenic strains that cause a variety of diseases in different host species. Avian pathogenic E. coli strain chi7122 (O78:K80:H9) was used in a chicken infection model to identify bacterial genes that are expressed in infected tissues. By using the cDNA selection method of selective capture of transcribed sequences and enrichment for the isolation of pathogen-specific (non-E. coli K-12) transcripts, pathogen-specific cDNAs were identified. Pathogen-specific transcripts corresponded to putative adhesins, lipopolysaccharide core synthesis, iron-responsive, plasmid- and phage-encoded genes, and genes of unknown function. Specific deletion of the aerobactin siderophore system and E. coli iro locus, which were identified by selective capture of transcribed sequences, demonstrated that these pathogen-specific systems contribute to the virulence of strain chi7122. Consecutive blocking to enrich for selection of pathogen-specific genes did not completely eliminate the presence of transcripts that corresponded to sequences also present in E. coli K-12. These E. coli conserved genes are likely to be highly expressed in vivo and contribute to growth or virulence. Overall, the approach we have used simultaneously provided a means to identify novel pathogen-specific genes expressed in vivo and insight regarding the global gene expression and physiology of a pathogenic E. coli strain in a natural animal host during the infectious process.

Infection-blocking genes of a symbiotic Rhizobium leguminosarum strain that are involved in temperature-dependent protein secretion

Rhizobium leguminosarum strain RBL5523 is able to form nodules on pea, but these nodules are ineffective for nitrogen fixation. The impairment in nitrogen fixation appears to be caused by a defective infection of the host plant and is host specific for pea. A Tn5 mutant of this strain, RBL5787, is able to form effective nodules on pea. We have sequenced a 33-kb region around the phage-transductable Tn5 insertion. The Tn5 insertion was localized to the 10th gene of a putative operon of 14 genes that was called the imp (impaired in nitrogen fixation) locus. Several highly similar gene clusters of unknown function are present in Pseudomonas aeruginosa, Vibrio cholerae, Edwardsiella ictaluri, and several other animal pathogens. Homology studies indicate that several genes of the imp locus are involved in protein phosphorylation, either as a kinase or dephosphorylase, or contain a phosphoprotein-binding module called a forkhead-associated domain. Other proteins show similarity to proteins involved in type III protein secretion. Two dimensional gel electrophoretic analysis of the secreted proteins in the supernatant fluid of cultures of RBL5523 and RBL5787 showed the absence in the mutant strain of at least four proteins with molecular masses of approximately 27 kDa and pIs between 5.5 and 6.5. The production of these proteins in the wild-type strain is temperature dependent. Sequencing of two of these proteins revealed that their first 20 amino acids are identical. This sequence showed homology to that of secreted ribose binding proteins (RbsB) from Bacilus subtilis and V. cholerae. Based on this protein sequence, the corresponding gene encoding a close homologue of RbsB was cloned that contains a N-terminal signal sequence that is recognized by type I secretion systems. Inoculation of RBL5787 on pea plants in the presence of supernatant of RBL5523 caused a reduced ability of RBL5787 to nodulate pea and fix nitrogen. Boiling of this supernatant before inoculation restored the formation of effective nodules to the original values, indicating that secreted proteins are indeed responsible for the impaired phenotype. These data suggest that the imp locus is involved in the secretion to the environment of proteins, including periplasmic RbsB protein, that cause blocking of infection specifically in pea plants.

Functional and mutational analysis of conjugative transfer region 2 (Tra2) from the IncHI1 plasmid R27

The transfer 2 region (Tra2) of the conjugative plasmid drR27 (derepressed R27) was analyzed by PSI-BLAST, insertional mutagenesis, genetic complementation, and an H-pilus assay. Tra2 contains 11 mating-pair formation (Mpf) genes that are essential for conjugative transfer, 9 of which are essential for H-pilus production (trhA, -L, -E, -K, -B, -V, -C, -P, and -W). TrhK has similarity to secretin proteins, suggesting a mechanism by which DNA could traverse the outer membrane of donors. The remaining two Mpf genes, trhU and trhN, play an auxiliary role in H-pilus synthesis and are proposed to be involved in DNA transfer and mating-pair stabilization, respectively. Conjugative transfer abilities were restored for each mutant when complemented with the corresponding transfer gene. In addition to the essential Mpf genes, three genes, trhO, trhZ, and htdA, modulate R27 transfer frequency. Disruption of trhO and trhZ severely reduced the transfer frequencies of drR27, whereas disruption of htdA greatly increased the transfer frequency of wild-type R27 to drR27 levels. A comparison of the essential transfer genes encoded by the Tra2 and Tra1 (T. D. Lawley, M. W. Gilmour, J. E. Gunton, L. J. Standeven, and D. E. Taylor, J. Bacteriol. 184:2173-2183, 2002) of R27 to other transfer systems illustrates that the R27 conjugative transfer system is a chimera composed of IncF-like and IncP-like transfer systems. Furthermore, the Mpf/type IV secretion systems encoded by IncH and IncF transfer systems are distinct from that of the IncP transfer system. The phenotypic and ecological significance of these observations is discussed.

Type IV secretion and Brucella virulence

The type IV secretion system, encoded by the virB region, is a key virulence factor for Brucella. The 12 genes of the region form an operon that is specifically induced by phagosome acidification in cells after phagocytosis. We speculate that the system serves to secrete unknown effector molecules, which allow Brucella to pervert the host cell endosomal pathways and to create a novel intracellular compartment in which it can replicate.

Negative transcriptional regulation of virulence and oncogenes of the Ti plasmid by Ros bearing a conserved C2H2-zinc finger motif

The chromosomal ros gene in Agrobacterium tumefaciens encodes a repressor of virulence and oncogenes that are located on a resident Ti plasmid. Mutational inactivation of ros de-represses the expression of the virC and virD operons, causing premature processing and accumulation of T-DNA molecules, and the premature expression of the oncogene, ipt, leading to the synthesis of cytokinin in the bacterium rather than in the plant host cell. Ros is a 15.5 kDa protein containing a novel "eukaryotic" C(2)H(2) zinc finger. Amino acid substitutions in the finger result in the loss of binding of Ros to the ros box, a 40 bp sequence within the operator of virC/D and ipt gene promoters; and the loss of binding of a zinc ion. The ros gene is highly conserved in members of the Rhizobiaceae. Evolutionary distance tree analyses revealed distant ties to the Japanese puffer fish, Fugu rupripes rather than to plants. Interestingly, ros homologues were found in microorganisms derived from marine sources, supporting the hypothesis that ros may have originated from a marine rather than a terrestrial organism.

Membrane sorting during swimming internalization of Brucella is required for phagosome trafficking decisions

Brucella infects macrophages by swimming internalization, after which it is enclosed in macropinosomes. We investigated the role of the uptake pathway in phagosome trafficking, which remains unclear. This study found membrane sorting during swimming internalization and is essential in intracellular replication of Brucella. The B. abortus virB mutant replicated intracellularly when it was in the macropinosome established by wild-type B. abortus that retained its ability to alter phagosome trafficking. Lipid rafts-associated molecules, such as GM1 ganglioside, were selectively included into macropinosomes, but Rab5 effector early endosome autoantigen (EEA1) and lysosomal glycoprotein LAMP-1 were excluded from macropinosomes containing B. abortus induced by swimming internalization. In contrast, when the swimming internalization was bypassed by phorbol myristate acetate (PMA)-induced macropinocytosis, lipid raft-associated molecules were excluded, and EEA1 and LAMP-1 were included into macropinosomes containing bacteria. The phosphatidylinositol 3-kinase inhibitor wortmannin that inhibits PMA-induced macropinocytosis blocked internalization of virB mutant, but not of wild-type of B. abortus and wortmannin treatment did not affect intracellular replication. Our results suggest that membrane sorting requires swimming internalization of B. abortus and decides the intracellular fate of the bacterium, and that Brucella -induced macropinosome formation is a different mechanism from PMA-induced macropinocytosis.

DNA sequence and mutational analyses of the pVir plasmid of Campylobacter jejuni 81-176

The circular pVir plasmid of Campylobacter jejuni strain 81-176 was determined to be 37,468 nucleotides in length with a G+C content of 26%. A total of 83% of the plasmid represented coding information, and all but 2 of the 54 predicted open reading frames were encoded on the same DNA strand. There were seven genes on the plasmid in a continguous region of 8.9 kb that encoded orthologs of type IV secretion proteins found in Helicobacter pylori, including four that have been described previously (D. J. Bacon, R. A. Alm, D. H. Burr, L. Hu, D. J. Kopecko, C. P. Ewing, T. J. Trust, and P. Guerry, Infect. Immun. 68:4384-4390, 2000). There were seven other pVir-encoded proteins that showed significant similarities to proteins encoded by the plasticity zones of either H. pylori J99 or 26695. Mutational analyses of 19 plasmid genes identified 5 additional genes that affect in vitro invasion of intestinal epithelial cells. These included one additional gene encoding a component of a type IV secretion system, an ortholog of Cj0041 from the chromosome of C. jejuni NCTC 11168, two Campylobacter plasmid-specific genes, and an ortholog of HP0996 from the plasticity zone of H. pylori 26695.

Macrophage plasma membrane cholesterol contributes to Brucella abortus infection of mice

Brucella abortus is a facultative intracellular bacterium capable of surviving inside macrophages. Intracellular replication of B. abortus requires the VirB complex, which is highly similar to conjugative DNA transfer systems. In this study, we show that plasma membrane cholesterol of macrophages is required for the VirB-dependent internalization of B. abortus and also contributes to the establishment of bacterial infection in mice. The internalization of B. abortus was accelerated by treating macrophages with acetylated low-density lipoprotein (acLDL). Treatment of acyl coenzyme A:cholesterol acyltransferase inhibitor, HL-004, to macrophages preloaded with acLDL accelerated the internalization of B. abortus. Ketoconazole, which inhibits cholesterol transport from lysosomes to the cell surface, inhibited the internalization and intracellular replication of B. abortus in macrophages. The Niemann-Pick C1 gene (NPC1), the gene for Niemann-Pick type C disease, characterized by an accumulation of cholesterol in most tissues, promoted B. abortus infection. NPC1-deficient mice were resistant to the bacterial infection. Molecules associated with cholesterol-rich microdomains, "lipid rafts," accumulate in intracellular vesicles of macrophages isolated from NPC1-deficient mice, and the macrophages yielded no intracellular replication of B. abortus. Thus, trafficking of cholesterol-associated microdomains controlled by NPC1 is critical for the establishment of B. abortus infection.

Peptide linkage mapping of the Agrobacterium tumefaciens vir-encoded type IV secretion system reveals protein subassemblies

Numerous bacterial pathogens use type IV secretion systems (T4SS) to deliver virulence factors directly to the cytoplasm of plant, animal, and human host cells. Here, evidence for interactions among components of the Agrobacterium tumefaciens vir-encoded T4SS is presented. The results derive from a high-resolution yeast two-hybrid assay, in which a library of small peptide domains of T4SS components was screened for interactions. The use of small peptides overcomes problems associated with assaying for interactions involving membrane-associated proteins. We established interactions between VirB11 (an inner membrane pore-forming protein), VirB9 (a periplasmic protein), and VirB7 (an outer membrane-associated lipoprotein and putative pilus component). We provide evidence for an interaction pathway, among conserved members of a T4SS, spanning the A. tumefaciens envelope and including a potential pore protein. In addition, we have determined interactions between VirB1 (a lytic transglycosylase likely involved in the local remodeling of the peptidoglycan) and primarily VirB8, but also VirB4, VirB10, and VirB11 (proteins likely to assemble the core structure of the T4SS). VirB4 interacts with VirB8, VirB10, and VirB11, also establishing a connection to the core components. The identification of these interactions suggests a model for assembly of the T4SS.

A single Photorhabdus gene, makes caterpillars floppy (mcf), allows Escherichia coli to persist within and kill insects

Photorhabdus luminescens, a bacterium with alternate pathogenic and symbiotic phases of its lifestyle, represents a source of novel genes associated with both virulence and symbiosis. This entomopathogen lives in a "symbiosis of pathogens" with nematodes that invade insects. Thus the bacteria are symbiotic with entomopathogenic nematodes but become pathogenic on release from the nematode into the insect blood system. Within the insect, the bacteria need to both avoid the peptide- and cellular- (hemocyte) mediated immune response and also to kill the host, which then acts as a reservoir for bacterial and nematode reproduction. However, the mechanisms whereby Photorhabdus evades the insect immune system and kills the host are unclear. Here we show that a single large Photorhabdus gene, makes caterpillars floppy (mcf), is sufficient to allow Esherichia coli both to persist within and kill an insect. The predicted high molecular weight Mcf toxin has little similarity to other known protein sequences but carries a BH3 domain and triggers apoptosis in both insect hemocytes and the midgut epithelium.

The road less traveled: transport of Legionella to the endoplasmic reticulum

Phagosomes containing the bacterial pathogen Legionella pneumophila are transported to the ER after macrophage internalization. To modulate phagosome transport, Legionella use a specialized secretion system that injects bacterial proteins into eukaryotic cells. This review will focus on recent studies that have identified bacterial proteins and host processes that play a concerted role in transporting Legionella to the ER.

Analysis and characterization of the IncFV plasmid pED208 transfer region

pED208 is a transfer-derepressed mutant of the IncFV plasmid, F(0)lac, which has an IS2 element inserted in its traY gene, resulting in constitutive overexpression of its transfer (tra) region. The pED208 transfer region, which encodes proteins responsible for pilus synthesis and conjugative plasmid transfer, was sequenced and found to be very similar to the F tra region in terms of its organization although most pED208 tra proteins share only about 45% amino acid identity. All the essential genes for F transfer had homologs within the pED208 transfer region with the exception of traQ, which encodes the chaperone for stable F-pilin expression. F(0)lac appears to have a fertility inhibition system different than the FinOP system of other F-like plasmids, and its transfer efficiency was increased in the presence of F or R100, suggesting that it could be mobilized by these plasmids. The F-like transfer systems specified by F, R100, and F(0)lac were highly specific for their cognate origins of transfer (oriT) as measured by their abilities to mobilize chimeric oriT-containing plasmids.

Modulation of Brucella-induced macropinocytosis by lipid rafts mediates intracellular replication

Intracellular replication of Brucella requires the VirB complex, which is highly similar to conjugative DNA transfer systems. In this study, we show that Brucella internalizes into macrophages by swimming on the cell surface with generalized membrane ruffling for several minutes, after which the bacteria are enclosed by macropinosomes. Lipid raft-associated molecules such as glycosylphosphatidylinositol (GPI)-anchored proteins, GM1 gangliosides and cholesterol were selectively incorporated into macropinosomes containing Brucella. In contrast, lysosomal glycoprotein LAMP-1 and host cell transmembrane protein CD44 were excluded from the macropinosomes. Removing GPI-anchored proteins from the macrophage surface and cholesterol sequestration markedly inhibited the VirB-dependent macropinocytosis and intracellular replication. Our results suggest that the entry route of Brucella into the macrophage determines the intracellular fate of the bacteria that is modulated by lipid raft microdomains.

Do plant and human pathogens have a common pathogenicity strategy?

Recently, a novel 'two-step' model of pathogenicity has been described that suggests host-cell-derived vasculoproliferative factors play a crucial role in the pathogenesis of bacillary angiomatosis, a disease caused by the human pathogenic bacterium Bartonella henselae. The resulting proliferation of endothelial cells could be interpreted as bacterial pathogens triggering the promotion of their own habitat: the host cell. Similar disease mechanisms are well known in the plant pathogen Agrobacterium tumefaciens, which causes crown gall disease. There are notable similarities between the pathogenicity of A. tumefaciens leading to tumourous disease in plants and to the B. henselae-triggered proliferation of endothelial cells in humans. Here, we hypothesize that this pathogenicity strategy might be common to several bacterial species in different hosts owing to shared pathogenicity factors.

The complete nucleotide sequence and environmental distribution of the cryptic, conjugative, broad-host-range plasmid pIPO2 isolated from bacteria of the wheat rhizosphere

Plasmid pIPO2 is a cryptic, conjugative, broad-host-range plasmid isolated from the wheat rhizosphere. It efficiently self-transfers between alpha, beta and gamma Proteobacteria and has a mobilizing/retromobilizing capacity for IncQ plasmids. The complete nucleotide sequence of pIPO2 is presented on the basis of its mini-Tn5::luxABtet-tagged derivative, pIPO2T. The pIPO2 sequence is 39815 bp long and contains at least 43 complete ORFs. Apart from a suite of ORFs with unknown function, all of the genes carried on pIPO2 are predicted to be involved in plasmid replication, maintenance and conjugative transfer. The overall organization of these genes is different from previously described plasmids, but is similar to the genetic organization seen in pSB102, a conjugative plasmid recently isolated from the bacterial community of the alfalfa rhizosphere. The putative conjugative transfer region of pIPO2 covers 23 kb and contains the genes required for DNA processing (Dtr) and mating pair formation (Mpf). The organization of these transfer genes in pIPO2 is highly similar to the genetic organization seen in the environmental plasmid pSB102 and in pXF51 from the plant pathogen Xylella fastidiosa. Plasmids pSB102 and pXF51 have recently been proposed to form a new family of environmental broad-host-range plasmids. Here it is suggested that pIPO2 is a new member of this family. The proposed Mpf system of pIPO2 shares high amino acid sequence similarity with equivalent VirB proteins from the type IV secretion system of Brucella spp. Sequence information was used to design primers specific for the detection of pIPO2. Environmental DNA from a range of diverse habitats was screened by PCR with these primers. Consistently positive signals for the presence of pIPO2 were obtained from a range of soil-related habitats, including the rhizospheres of young wheat plants, of field-grown oats and of grass (all gramineous plants), as well as from the rhizosphere of tomato plants. These data add to the growing evidence that plasmids carry advantageous genes with as yet undefined functions in plant-associated communities.

Agrobacterium-mediated T-DNA transfer and integration into the chromosome of Streptomyces lividans

Summary Agrobacterium tumefaciens is the prototype of a prokaryotic organism transmitting DNA across natural kingdom barriers into higher cells. In nature, a specific segment (T-DNA) of the resident Ti plasmid is transferred from this bacterium into plant cells and integrated into the plant cell genome. Expression of the integrated oncogenes contained in the T-DNA results in the tumour disease known as crown gall. Besides plants, the range of transformable recipients is broad and includes fungi and mammalian cells. We now show further extension of this host range, whereby the actinomycete Streptomyces lividans is also a recipient of the T-DNA. A. tumefaciens cells containing a binary vector system with a vir helper plasmid, pUCD2614, and a T-DNA donor plasmid, pUCD5801, were co-cultured with S. lividans hyphae. A. tumefaciens-S. lividans aggregate when the vir genes are induced with acetosyringone, resulting in the transfer of the T-DNA, as evidenced by the formation of transconjugants containing T-DNA genetic markers and the appearance of the T-DNA in these transconjugants. Close examination of the interacted cells revealed a presumably coiled thread-like interconnection with an average width of approximately 30 nm between A. tumefaciens and S. lividans. This interconnecting structure is dependent on virB genes and appears only under the same conditions as that required for T-pilus formation. Insertion of the T-DNA via A. tumefaciens into the S. lividans genome provides a useful genetic tool for generating novel mutants.

Structure and role of coupling proteins in conjugal DNA transfer

Type IV secretory systems are transmembrane bacterial multiprotein complexes. They are pivotal for conjugation, bacterial-induced plant tumour formation, toxin secretion and mammalian pathogen intracellular activity. These systems are involved in the spread of antibiotic resistance genes among bacteria by enabling conjugative DNA transfer. When such translocons transport DNA, they require the assistance of multimeric integral inner membrane proteins, the type IV coupling proteins. Its structural prototype is plasmid R388 TrwB protein, responsible for coupling the relaxosome with the DNA transport apparatus during bacterial conjugation. Its monomeric molecular structure is reminiscent of ring helicases and AAA ATPases. The quaternary structure is made up by six equivalent protomers featuring a flattened sphere resembling F1-ATPase, with a central channel traversing the particle, thus connecting cytoplasm and periplasm.

Use of colicin-based genetic tools for studying bacterial protein transport

Transport of proteins across the envelope of Gram-negative bacteria is a very challenging domain of investigation, which involves membrane-embedded proteinaceous complexes at which specific targeting occurs. These transporters (translocon or secreton) have been studied both with genetics and biochemistry. In this review we report recent developments that should help to identify novel interactions that exist within these complexes, and to decipher the signals that specifically direct transported proteins to the cognate system. These developments are exclusively based on the re-routing of colicins to these molecular machineries. The re-routing induces a lethal situation in the case of efficient or inefficient transport, depending on the system, thus creating a genetic tool for selection of mutations that correct or generate a transport default.

An essential virulence protein of Brucella abortus, VirB4, requires an intact nucleoside-triphosphate-binding domain

Brucella abortus is a facultative intracellular bacterium capable of surviving inside macrophages. The VirB complex, which is highly similar to conjugative DNA transfer apparatuses, is required for intracellular replication. A conserved NTP-binding domain in VirB4 suggests that one or both proteins couple energy by NTP hydrolysis to transport of putative effector molecule(s). Here it is shown that a mutant strain of B. abortus that contains an in-frame deletion in virB4 is unable to replicate in macrophages and survives in mice. Intracellular replication and virulence in mice are fully restored by expressing virB4 in trans, indicating that VirB4 is essential for intracellular replication and virulence in mice. An alteration within the NTP-binding region of VirB4 by site-directed mutagenesis abolished complementation of a virB4 mutant, demonstrating that an intact NTP-binding domain is critical for VirB4 function. Intracellular replication was inhibited in wild-type B. abortus after introducing a plasmid expressing a mutant VirB4 altered in the NTP-binding region. The dominant negative phenotype suggests that VirB4 either functions as a multimer or interacts with some other component(s) necessary for intracellular replication. Wild-type B. abortus-containing phagosomes lack the glycoprotein LAMP-1, which is an indicator of the normal endocytic pathway. Mutant strains were found in phagosomes that co-localized with LAMP-1, indicating that VirB4 containing the intact NTP-binding region is essential for evasion of fusion with lysosomes.