Agrobacterium tumefaciens T-complex transport apparatus: a paradigm for a new family of multifunctional transporters in eubacteria

Virulence factors in brucellosis: implications for aetiopathogenesis and treatment

Brucella species are responsible for the global zoonotic disease brucellosis. These intracellular pathogens express a set of factors - including lipopolysaccharides, virulence regulator proteins and phosphatidylcholine - to ensure their full virulence. Some virulence factors are essential for invasion of the host cell, whereas others are crucial to avoid elimination by the host. They allow Brucella spp. to survive and proliferate within its replicative vacuole and enable the bacteria to escape detection by the host immune system. Several strategies have been used to develop animal vaccines against brucellosis, but no adequate vaccine yet exists to cure the disease in humans. This is probably due to the complicated pathophysiology of human Brucella spp. infection, which is different than in animal models. Here we review Brucella spp. virulence factors and how they control bacterial trafficking within the host cell.

VirB1* promotes T-pilus formation in the vir-Type IV secretion system of Agrobacterium tumefaciens

The vir-type IV secretion system of Agrobacterium is assembled from 12 proteins encoded by the virB operon and virD4. VirB1 is one of the least-studied proteins encoded by the virB operon. Its N terminus is a lytic transglycosylase. The C-terminal third of the protein, VirB1*, is cleaved from VirB1 and secreted to the outside of the bacterial cell, suggesting an additional function. We show that both nopaline and octopine strains produce abundant amounts of VirB1* and perform detailed studies on nopaline VirB1*. Both domains are required for wild-type virulence. We show here that the nopaline type VirB1* is essential for the formation of the T pilus, a subassembly of the vir-T4SS composed of processed and cyclized VirB2 (major subunit) and VirB5 (minor subunit). A nopaline virB1 deletion strain does not produce T pili. Complementation with full-length VirB1 or C-terminal VirB1*, but not the N-terminal lytic transglycosylase domain, restores T pili containing VirB2 and VirB5. T-pilus preparations also contain extracellular VirB1*. Protein-protein interactions between VirB1* and VirB2 and VirB5 were detected in the yeast two-hybrid assay. We propose that VirB1 is a bifunctional protein required for virT4SS assembly. The N-terminal lytic transglycosylase domain provides localized lysis of the peptidoglycan cell wall to allow insertion of the T4SS. The C-terminal VirB1* promotes T-pilus assembly through protein-protein interactions with T-pilus subunits.

Requirements for assembly of PtlH with the pertussis toxin transporter apparatus of Bordetella pertussis

PtlH is an essential component of the Ptl system, the type IV transporter responsible for secretion of pertussis toxin (PT) across the outer membrane of Bordetella pertussis. The nine Ptl proteins are believed to interact to form a membrane-spanning apparatus through which the toxin is secreted. In this study, we monitored the subcellular localization of PtlH in strains of B. pertussis lacking PT, lacking other Ptl proteins, or from which ATP has been depleted in order to gain insight into the requirements for assembly of PtlH with the remainder of the Ptl transporter complex that is thought to be tightly embedded in the membrane. We found that PtlH is exclusively localized to the inner membrane fraction of the cell in a wild-type strain of B. pertussis. In contrast, PtlH localized to both the cytoplasmic and inner membrane fractions of a mutant strain of B. pertussis that does not produce PT. In comparison to how it localized in wild-type strains of B. pertussis, PtlH exhibited aberrant localization in strains lacking PtlD, PtlE, PtlF, and PtlG. We also found that localization of PtlH was perturbed in B. pertussis strains that were treated with carbonyl cyanide m-chlorophenylhydrazone and sodium arsenate, which are capable of depleting cellular ATP levels, and in strains of B. pertussis that produce an altered form of PtlH that lacks ATPase activity. When taken together, these results indicate that tight association of PtlH with the membrane, likely through interactions with components of the transporter-PT complex, requires the toxin substrate, a specific subset of the Ptl proteins, and ATP. Based on these data, a model for the assembly of the Ptl transporter-PT complex is presented.

The Agrobacterium VirE3 effector protein: a potential plant transcriptional activator

During the infection of plants, Agrobacterium tumefaciens introduces several Virulence proteins including VirE2, VirF, VirD5 and VirE3 into plant cells in addition to the T-DNA. Here, we report that double mutation of virF and virE3 leads to strongly diminished tumor formation on tobacco, tomato and sunflower. The VirE3 protein is translated from a polycistronic mRNA containing the virE1, virE2 and virE3 genes, in Agrobacterium. The VirE3 protein has nuclear localization sequences, which suggests that it is transported into the plant cell nucleus upon translocation. Indeed we show here that VirE3 interacts in vitro with importin-α and that a VirE3–GFP fusion protein is localized in the nucleus. VirE3 also interacts with two other proteins, viz. pCsn5, a component of the COP9 signalosome and pBrp, a plant specific general transcription factor belonging to the TFIIB family. We found that VirE3 is able to induce transcription in yeast when bound to DNA through the GAL4-BD. Our data indicate that the translocated effector protein VirE3 is transported into the nucleus and there it may interact with the transcription factor pBrp to induce the expression of genes needed for tumor development.

Systemic movement of Agrobacterium tumefaciens in several plant species

AIMS

The systemic movement of Agrobacterium spp. inside plants of different species was studied to determine the most valuable diagnostic methodology for their detection.

METHODS AND RESULTS

Pathogenic agrobacteria were detected by isolation and PCR in tissue away from primary tumours in tomato plants grown in the presence of Agrobacterium spp. Moreover, this bacterium was also able to induce secondary tumours beyond the inoculation site. In addition, the capacity of agrobacteria to translocate and induce secondary tumours was analysed in rose, grapevine, chrysanthemum, cherry and peach x almond hybrid GF677. No differences among strains of Agrobacterium spp. were detected in secondary tumour development, although some of them induced a significantly higher number of primary tumours in some species. Movement of inoculated pathogenic cells of four strains was also demonstrated in symptomless portions of the plant stems by isolation and PCR. Finally, pathogenic agrobacteria were detected in root, crown and stem portions of naturally infected walnuts. In all assays, PCR was the most efficient technique for detecting the movement of Agrobacterium spp. within the plants.

CONCLUSIONS

Migration of agrobacteria inside plants is a complex phenomenon and more extensive than previously reported. Therefore, efficient and sensitive detection methods such as PCR must be used to select clean plants to avoid latent infections of Agrobacterium spp.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results show that migration of Agrobacterium spp. could be relatively frequent in several cultivated fruit trees, and systemic infections should be taken into account when designing strategies for controlling crown gall disease.

Progress in rickettsial genome analysis from pioneering of Rickettsia prowazekii to the recent Rickettsia typhi

Three rickettsial genomes have been sequenced and annotated. Rickettsia prowazekii and R. typhi have similar gene order and content. The few differences between R. prowazekii and R. typhi include a 12-kb insertion in R. prowazekii, a large inversion close to the origin of replication in R. typhi, and loss of the complete cytochrome c oxidase system by R. typhi. R. prowazekii, R. typhi, and R. conorii have 13, 24, and 560 unique genes, respectively, and share 775 genes, most likely their essential genes. The small genomes contain many pseudogenes and much noncoding DNA, reflecting the process of genome decay. R. typhi contains the largest number of pseudogenes (41), and R. conorii the fewest, in accordance with its larger number of genes and smaller proportion of noncoding DNA. Conversely, typhus rickettsiae contain fewer repetitive sequences. These genomes portray the key themes of rickettsial intracellular survival: lack of enzymes for sugar metabolism, lipid biosynthesis, nucleotide synthesis, and amino acid metabolism, suggesting that rickettsiae depend on the host for nutrition and building blocks; enzymes for the complete TCA cycle and several copies of ATP/ADP translocase genes, suggesting independent synthesis of ATP and acquisition of host ATP; and type IV secretion system. All rickettsiae share two outer membrane proteins (OmpB and Sca 4) and LPS biosynthesis machinery. RickA, unique to spotted fever rickettsiae, plays a role in induction of actin polymerization in R. conorii, but not in R. prowazekii or R. typhi. The genome of R. typhi contains four potentially membranolytic genes (tlyA, tlyC, pldA, and pat-1) and five autotransporter genes, sca 1, sca 2, sca 3, ompA, and ompB. The presence of six 50-amino acid repeat units in Sca 2 suggests function as an adhesin. The high laboratory passage of the sequenced strains raises the issue of the occurrence of laboratory mutations in genes not required for growth in cell culture or eggs. Resequencing revealed that eight annotated pseudogenes of E strain are actually intact genes. Comparative genomics of virulent and avirulent strains of rickettsial species may reveal their virulence factors.

Differential expression of VirB9 and VirB6 during the life cycle of Anaplasma phagocytophilum in human leucocytes is associated with differential binding and avoidance of lysosome pathway

Anaplasma phagocytophilum, an obligate intracellular bacterium, is the aetiologic agent of human granulocytic anaplasmosis (HGA). A. phagocytophilum virB/D operons encoding type IV secretion system are expressed in cell culture and in the blood of HGA patients. In the present study, their expression across the A. phagocytophilum intracellular developmental cycle was investigated. We found that mRNA levels of both virB9 and virB6 were upregulated during infection of human neutrophils in vitro. The antibody against the recombinant VirB9 protein was prepared and immunogold and immunofluorescence labelling were used to determine the VirB9 protein expression by individual organisms. Majority of A. phagocytophilum spontaneously released from the infected host cells poorly expressed VirB9. At 1 h post infection, VirB9 was not detectable on most bacteria associated with neutrophils. However, VirB9 was strongly expressed by A. phagocytophilum during proliferation in neutrophils. In contrast, with HL-60 cells, approximately 80% of A. phagocytophilum organisms associated at 1 h post infection expressed VirB9 protein and were colocalized with lysosome-associated membrane protein-1 (LAMP-1), whereas, VirB9-undetectable bacteria were not colocalized with LAMP-1. These results indicate developmental regulation of expression of components of type IV secretion system during A. phagocytophilum intracellular life cycle and suggest that bacterial developmental stages influence the nature of binding to the hosts and early avoidance of late endosome-lysosome pathway.

Biogenesis, architecture, and function of bacterial type IV secretion systems

Type IV secretion (T4S) systems are ancestrally related to bacterial conjugation machines. These systems assemble as a translocation channel, and often also as a surface filament or protein adhesin, at the envelopes of Gram-negative and Gram-positive bacteria. These organelles mediate the transfer of DNA and protein substrates to phylogenetically diverse prokaryotic and eukaryotic target cells. Many basic features of T4S are known, including structures of machine subunits, steps of machine assembly, substrates and substrate recognition mechanisms, and cellular consequences of substrate translocation. A recent advancement also has enabled definition of the translocation route for a DNA substrate through a T4S system of a Gram-negative bacterium. This review emphasizes the dynamics of assembly and function of model conjugation systems and the Agrobacterium tumefaciens VirB/D4 T4S system. We also summarize salient features of the increasingly studied effector translocator systems of mammalian pathogens.

Agrobacterium-mediated transformation as a tool for functional genomics in fungi

In the era of functional genomics, the need for tools to perform large-scale targeted and random mutagenesis is increasing. A potential tool is Agrobacterium-mediated fungal transformation. A. tumefaciens is able to transfer a part of its DNA (transferred DNA; T-DNA) to a wide variety of fungi and the number of fungi that can be transformed by Agrobacterium-mediated transformation (AMT) is still increasing. AMT has especially opened the field of molecular genetics for fungi that were difficult to transform with traditional methods or for which the traditional protocols failed to yield stable DNA integration. Because of the simplicity and efficiency of transformation via A. tumefaciens, it is relatively easy to generate a large number of stable transformants. In combination with the finding that the T-DNA integrates randomly and predominantly as a single copy, AMT is well suited to perform insertional mutagenesis in fungi. In addition, in various gene-targeting experiments, high homologous recombination frequencies were obtained, indicating that the T-DNA is also a useful substrate for targeted mutagenesis. In this review, we discuss the potential of the Agrobacterium DNA transfer system to be used as a tool for targeted and random mutagenesis in fungi.

Bacteria-mediated DNA transfer in gene therapy and vaccination

The use of live attenuated bacterial vaccine strains allows the targeted delivery of macromolecules to mammalian cells and tissues via the mucosal route. Depending on their specific virulence mechanisms and inherent metabolic preferences, bacteria invade certain cell types and body niches where they consequently deliver their cargo. Recently, the ability of attenuated strains of Salmonella, Shigella and Yersinia spp., as well as Listeria monocytogenes and invasive Escherichia coli, to deliver eukaryotic expression plasmids into mammalian cells in vitro and in vivo has been discovered. The great potential of bacteria-mediated transfer of plasmid DNA encoding vaccine antigens and/or therapeutic molecules was demonstrated in experimental animal models of infectious diseases, tumours and gene deficiencies. The exact mechanism of DNA transfer from the bacterial vector into the mammalian host is not yet completely known. The understanding of molecular events during bacterial DNA transfer, however, will further the development of bacterial vector systems with great promise for various clinical applications.

Type IV secretion: the Agrobacterium VirB/D4 and related conjugation systems

The translocation of DNA across biological membranes is an essential process for many living organisms. In bacteria, type IV secretion systems (T4SS) are used to deliver DNA as well as protein substrates from donor to target cells. The T4SS are structurally complex machines assembled from a dozen or more membrane proteins in response to environmental signals. In Gram-negative bacteria, the conjugation machines are composed of a cell envelope-spanning secretion channel and an extracellular pilus. These dynamic structures (i) direct formation of stable contacts-the mating junction-between donor and recipient cell membranes, (ii) transmit single-stranded DNA as a nucleoprotein particle, as well as protein substrates, across donor and recipient cell membranes, and (iii) mediate disassembly of the mating junction following substrate transfer. This review summarizes recent progress in our understanding of the mechanistic details of DNA trafficking with a focus on the paradigmatic Agrobacterium tumefaciens VirB/D4 T4SS and related conjugation systems.

Isolation, purification, and identification of the virulence protein VirE2 from Agrobacterium tumefaciens

Bacteria of the genus Agrobacterium can transfer a portion of their Ti plasmid (T-DNA) in complex with the VirE2 and VirD2 proteins into the plant-cell nucleus and cause it to be integrated in the host-cell chromosomes. The mechanism of T-DNA transfer across the plant-cell membrane and cytoplasm is unknown. The aim of this study was to isolate the virulence protein VirE2 in order to explore its role in T-DNA transfer across the eukaryotic-cell membrane and cytoplasm. To obtain VirE2, we cloned the virE2 gene into plasmid pQE31 in Escherichia coli cells. VirE2 protein was isolated from E. coli XL-1 blue cells containing a recombinant plasmid, pQE31-virE2. The cells were ultrasonically disrupted, and the protein containing six histidine residues at the N-terminal end was isolated by affinity chromatography on Ni-NTA agarose. The purified preparation was tested by immunodot, by using polyclonal rabbit antibodies and miniantibodies produced toward VirE2. The capacity of the recombinant protein VirE2 for interacting with single-stranded DNA was tested by the formation of complexes, recorded by agarose-gel electrophoresis. In summary, A. tumefaciens virulence protein VirE2, capable of forming a complex with single-stranded T-DNA during transfer into the plant cell, was isolated, purified, and partially characterized. Anti-VirE2 miniantibodies were obtained, and direct labeling of VirE2 with colloidal gold was done for the first time.

The VirE3 protein of Agrobacterium mimics a host cell function required for plant genetic transformation

To genetically transform plants, Agrobacterium exports its transferred DNA (T-DNA) and several virulence (Vir) proteins into the host cell. Among these proteins, VirE3 is the only one whose biological function is completely unknown. Here, we demonstrate that VirE3 is transferred from Agrobacterium to the plant cell and then imported into its nucleus via the karyopherin alpha-dependent pathway. In addition to binding plant karyopherin alpha, VirE3 interacts with VirE2, a major bacterial protein that directly associates with the T-DNA and facilitates its nuclear import. The VirE2 nuclear import in turn is mediated by a plant protein, VIP1. Our data indicate that VirE3 can mimic this VIP1 function, acting as an 'adapter' molecule between VirE2 and karyopherin alpha and 'piggy-backing' VirE2 into the host cell nucleus. As VIP1 is not an abundant protein, representing one of the limiting factors for transformation, Agrobacterium may have evolved to produce and export to the host cells its own virulence protein that at least partially complements the cellular VIP1 function necessary for the T-DNA nuclear import and subsequent expression within the infected cell.

Identification and characterization of a major subgroup of conjugative Campylobacter jejuni plasmids

OBJECTIVES

Enterocyte invasion of Campylobacter jejuni 81-176 has been reported to depend upon the virulence plasmid pVir. The objective of this study was to determine the prevalence of pVir in clinical C. jejuni isolates, to investigate DNA homologies between C. jejuni plasmids and the significance of plasmids for C. jejuni invasiveness.

METHODS

DNA homologies between C. jejuni plasmids were studied by southern blot hybridization. C. jejuni invasion into human intestinal Caco-2 cells was assessed in a gentamicin exclusion assay.

RESULTS

Twenty-nine percent of C. jejuni isolated from patients with bloody or watery diarrhoea harboured plasmids of various sizes. One plasmid (7%) was a pVir homologue whereas, the majority of the plasmids (53%) belonged to a subgroup distinct from pVir. The plasmids of this novel subgroup share extensive DNA sequence homology with each other, including homologues to so-called invasion-promoting genes. However, conjugative transfer of these plasmids clearly did not increase invasiveness of plasmidless recipient C. jejuni strains.

CONCLUSION

This study indicates that only a small proportion of C. jejuni strains carry the virulence factor pVir and that at least one other distinctive group of plasmids in C. jejuni exists, which does not seem to be associated with invasiveness.

Plant proteins that interact with VirB2, the Agrobacterium tumefaciens pilin protein, mediate plant transformation

Agrobacterium tumefaciens uses a type IV secretion system (T4SS) to transfer T-DNA and virulence proteins to plants. The T4SS is composed of two major structural components: the T-pilus and a membrane-associated complex that is responsible for translocating substrates across both bacterial membranes. VirB2 protein is the major component of the T-pilus. We used the C-terminal-processed portion of VirB2 protein as a bait to screen an Arabidopsis thaliana cDNA library for proteins that interact with VirB2 in yeast. We identified three related plant proteins, VirB2-interacting protein (BTI) 1 (BTI1), BTI2, and BTI3 with unknown functions, and a membrane-associated GTPase, AtRAB8. The three BTI proteins also interacted with VirB2 in vitro. Preincubation of Agrobacterium with GST-BTI1 protein decreased the transformation efficiency of Arabidopsis suspension cells by Agrobacterium. Transgenic BTI and AtRAB8 antisense and RNA interference Arabidopsis plants are less susceptible to transformation by Agrobacterium than are wild-type plants. The level of BTI1 protein is transiently increased immediately after Agrobacterium infection. In addition, overexpression of BTI1 protein in transgenic Arabidopsis results in plants that are hypersusceptible to Agrobacterium-mediated transformation. Confocal microscopic data indicate that GFP-BTI proteins preferentially localize to the periphery of root cells in transgenic Arabidopsis plants, suggesting that BTI proteins may contact the Agrobacterium T-pilus. We propose that the three BTI proteins and AtRAB8 are involved in the initial interaction of Agrobacterium with plant cells.

Complete genomic sequence of bacteriophage B3, a Mu-like phage of Pseudomonas aeruginosa

Bacteriophage B3 is a transposable phage of Pseudomonas aeruginosa. In this report, we present the complete DNA sequence and annotation of the B3 genome. DNA sequence analysis revealed that the B3 genome is 38,439 bp long with a G+C content of 63.3%. The genome contains 59 proposed open reading frames (ORFs) organized into at least three operons. Of these ORFs, the predicted proteins from 41 ORFs (68%) display significant similarity to other phage or bacterial proteins. Many of the predicted B3 proteins are homologous to those encoded by the early genes and head genes of Mu and Mu-like prophages found in sequenced bacterial genomes. Only two of the predicted B3 tail proteins are homologous to other well-characterized phage tail proteins; however, several Mu-like prophages and transposable phage D3112 encode approximately 10 highly similar proteins in their predicted tail gene regions. Comparison of the B3 genomic organization with that of Mu revealed evidence of multiple genetic rearrangements, the most notable being the inversion of the proposed B3 immunity/early gene region, the loss of Mu-like tail genes, and an extreme leftward shift of the B3 DNA modification gene cluster. These differences illustrate and support the widely held view that tailed phages are genetic mosaics arising by the exchange of functional modules within a diverse genetic pool.

Tra proteins characteristic of F-like type IV secretion systems constitute an interaction group by yeast two-hybrid analysis

Using yeast two-hybrid screens, we have defined an interaction group of six Tra proteins encoded by the F plasmid and required by F(+) cells to elaborate F pili. The six proteins are TraH, TraF, TraW, TraU, TrbI, and TrbB. Except for TrbI, these proteins were all identified as hallmarks of F-like type IV secretion systems (TFSSs), with no homologues among TFSS genes of P-type or I-type systems (T. Lawley, W. Klimke, M. Gubbins, and L. Frost, FEMS Microbiol. Lett. 224:1-15, 2003). Also with the exception of TrbI, which is an inner membrane protein, the remaining proteins are or are predicted to be periplasmic. TrbI consists of one membrane-spanning segment near its N terminus and an 88-residue, hydrophilic domain that extends into the periplasm. Hence, the proteins of this group probably form a periplasmic cluster in Escherichia coli. The interaction network identifies TraH as the most highly connected node, with two-hybrid links to TrbI, TraU, and TraF. As measured by transcriptional activation of lacZ, the TrbI-TraH interaction in Saccharomyces cerevisiae requires the TraH amino acid segment from residues 193 to 225. The TraU and TraF interactions are localized to C-terminal segments of TraH (amino acids 315 to 458 for TraF and amino acids 341 to 458 for TraU). The TrbI-TraH interaction with full-length (less the signal peptide) TraH is weak but increases 40-fold with N-terminal TraH deletions; the first 50 amino acids appear to be critical for inhibiting TrbI binding in yeast. Previous studies by others have shown that, with the exception of trbB mutations, which do not affect the elaboration or function of F pili under laboratory conditions, a mutation in any of the other genes in this interaction group alters the number or length distribution of F pili. We propose a model whereby one function of the TraH interaction group is to control F-pilus extension and retraction.

Agrobacterium rhizogenes GALLS protein substitutes for Agrobacterium tumefaciens single-stranded DNA-binding protein VirE2

Agrobacterium tumefaciens and Agrobacterium rhizogenes transfer plasmid-encoded genes and virulence (Vir) proteins into plant cells. The transferred DNA (T-DNA) is stably inherited and expressed in plant cells, causing crown gall or hairy root disease. DNA transfer from A. tumefaciens into plant cells resembles plasmid conjugation; single-stranded DNA (ssDNA) is exported from the bacteria via a type IV secretion system comprised of VirB1 through VirB11 and VirD4. Bacteria also secrete certain Vir proteins into plant cells via this pore. One of these, VirE2, is an ssDNA-binding protein crucial for efficient T-DNA transfer and integration. VirE2 binds incoming ssT-DNA and helps target it into the nucleus. Some strains of A. rhizogenes lack VirE2, but they still transfer T-DNA efficiently. We isolated a novel gene from A. rhizogenes that restored pathogenicity to virE2 mutant A. tumefaciens. The GALLS gene was essential for pathogenicity of A. rhizogenes. Unlike VirE2, GALLS contains a nucleoside triphosphate binding motif similar to one in TraA, a strand transferase conjugation protein. Despite their lack of similarity, GALLS substituted for VirE2.

Analysis of relative levels of production of pertussis toxin subunits and Ptl proteins in Bordetella pertussis

Pertussis toxin is transported across the outer membrane of Bordetella pertussis by the type IV secretion system known as the Ptl transporter, which is composed of nine different proteins. In order to determine the relative levels of production of pertussis toxin subunits and Ptl proteins in B. pertussis, we constructed translational fusions of the gene for alkaline phosphatase, phoA, with various ptx and ptl genes. Comparison of the alkaline phosphatase activity of strains containing ptx'- or ptl'-phoA fusions indicated that pertussis toxin subunits are produced at higher levels than Ptl proteins, which are encoded by genes located toward the 3' end of the ptx-ptl operon. We also engineered strains of B. pertussis by introducing multiple copies of the ptl genes or subsets of these genes and then examined the ability of each of these strains to secrete pertussis toxin. From these studies, we determined that certain Ptl proteins appear to be limiting in the secretion of pertussis toxin from the bacteria. These results represent an important first step in assessing the stoichiometric relationship of pertussis toxin and its transporter within the bacterial cell.

The versatile bacterial type IV secretion systems

Bacteria use type IV secretion systems for two fundamental objectives related to pathogenesis--genetic exchange and the delivery of effector molecules to eukaryotic target cells. Whereas gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host, interactions between effector and host molecules can suppress defence mechanisms, facilitate intracellular growth and even induce the synthesis of nutrients that are beneficial to bacterial colonization. Rapid progress has been made towards defining the structures and functions of type IV secretion machines, identifying the effector molecules, and elucidating the mechanisms by which the translocated effectors subvert eukaryotic cellular processes during infection.

Multiple substrates of the Legionella pneumophila Dot/Icm system identified by interbacterial protein transfer

Legionella pneumophila is an intracellular pathogen that multiplies in a specialized vacuole within host cells. Biogenesis of this vacuole requires the Dot/Icm type IV protein translocation system. By using a Cre/loxP-based protein translocation assay, we found that proteins translocated by the Dot/Icm complex across the host phagosomal membrane can also be transferred from one bacterial cell to another. The flexibility of this system allowed the identification of several families of proteins translocated by the Dot/Icm complex. When analyzed by immunofluorescence microscopy, a protein identified by this procedure, SidC, was shown to translocate across the phagosomal membranes to the cytoplasmic face of the L. pneumophila phagosome. The identification of large numbers of these substrates, and the fact that the absence of any one substrate rarely results in strong defects in intracellular growth, indicate that there is significant functional redundancy among the Dot/Icm translocation targets.

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.

Plant transformation by coinoculation with a disarmed Agrobacterium tumefaciens strain and an Escherichia coli strain carrying mobilizable transgenes

Transformation of Nicotiana tabacum leaf explants was attempted with Escherichia coli as a DNA donor either alone or in combination with Agrobacterium tumefaciens. We constructed E. coli donor strains harboring either the promiscuous IncP-type or IncN-type conjugal transfer system and second plasmids containing the respective origins of transfer and plant-selectable markers. Neither of these conjugation systems was able to stably transform plant cells at detectable levels, even when VirE2 was expressed in the donor cells. However, when an E. coli strain expressing the IncN-type conjugation system was coinoculated with a disarmed A. tumefaciens strain, plant tumors arose at high frequencies. This was caused by a two-step process in which the IncN transfer system mobilized the entire shuttle plasmid from E. coli to the disarmed A. tumefaciens strain, which in turn processed the T-DNA and transferred it to recipient plant cells. The mobilizable plasmid does not require a broad-host-range replication origin for this process to occur, thus reducing its size and genetic complexity. Tumorigenesis efficiency was further enhanced by incubation of the bacterial strains on medium optimized for bacterial conjugation prior to inoculation of leaf explants. These techniques circumvent the need to construct A. tumefaciens strains containing binary vectors and could simplify the creation of transgenic plants.

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.

VirE2, a type IV secretion substrate, interacts with the VirD4 transfer protein at cell poles of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers oncogenic DNA and effector proteins to plant cells during the course of infection. Substrate translocation across the bacterial cell envelope is mediated by a type IV secretion (TFS) system composed of the VirB proteins, as well as VirD4, a member of a large family of inner membrane proteins implicated in the coupling of DNA transfer intermediates to the secretion machine. In this study, we demonstrate with novel cytological screens - a two-hybrid (C2H) assay and bimolecular fluorescence complementation (BiFC) - and by immunoprecipitation of chemically cross-linked protein complexes that the VirE2 effector protein interacts directly with the VirD4 coupling protein at cell poles of A. tumefaciens. Analyses of truncation derivatives showed that VirE2 interacts via its C terminus with VirD4, and, further, an NH2-terminal membrane-spanning domain of VirD4 is dispensable for complex formation. VirE2 interacts with VirD4 independently of the virB-encoded transfer machine and T pilus, the putative periplasmic chaperones AcvB and VirJ, and the T-DNA transfer intermediate. Finally, VirE2 is recruited to polar-localized VirD4 as a complex with its stabilizing secretion chaperone VirE1, yet the effector-coupling protein interaction is not dependent on chaperone binding. Together, our findings establish for the first time that a protein substrate of a type IV secretion system is recruited to a member of the coupling protein superfamily.

TraG-like proteins of type IV secretion systems: functional dissection of the multiple activities of TraG (RP4) and TrwB (R388)

TraG-like proteins are essential components of type IV secretion systems. During secretion, TraG is thought to translocate defined substrates through the inner cell membrane. The energy for this transport is presumably delivered by its potential nucleotide hydrolase (NTPase) activity. TraG of conjugative plasmid RP4 is a membrane-anchored oligomer that binds RP4 relaxase and DNA. TrwB (R388) is a hexameric TraG-like protein that binds ATP. Both proteins, however, lack NTPase activity under in vitro conditions. We characterized derivatives of TraG and TrwB truncated by the N-terminal membrane anchor (TraGdelta2 and TrwBdelta1) and/or containing a point mutation at the putative nucleotide-binding site (TraGdelta2K187T and TraGK187T). Unlike TraG and TrwB, truncated derivatives behaved as monomers without the tendency to form oligomers or aggregates. Surface plasmon resonance analysis with immobilized relaxase showed that mutant TraGK187T was as good a binding partner as the wild-type protein, whereas truncated TraG monomers were unable to bind relaxase. TraGdelta2 and TrwBdelta1 bound ATP and, with similar affinity, ADP. Binding of ATP and ADP was strongly inhibited by the presence of Mg(2+) or single-stranded DNA and was competed for by other nucleotides. Compared to the activity of TraGdelta2, the ATP- and ADP-binding activity of the point mutation derivative TraGdelta2K187T was significantly reduced. Each TraG derivative bound DNA with an affinity similar to that of the native protein. DNA binding was inhibited or competed for by ATP, ADP, and, most prominently, Mg(2+). Thus, both nucleotide binding and DNA binding were sensitive to Mg(2+) and were competitive with respect to each other.

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.

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.

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/>

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

Analysis of Vir protein translocation from Agrobacterium tumefaciens using Saccharomyces cerevisiae as a model: evidence for transport of a novel effector protein VirE3

Agrobacterium tumefaciens causes crown gall disease on a variety of plants. During the infection process Agrobacterium transfers a nucleoprotein complex, the VirD2 T-complex, and at least two Vir proteins, VirE2 and VirF, into the plant cell via the VirB/VirD4 type IV secretion system. Recently, we found that T-DNA could also be transferred from Agrobacterium to Saccharomyces cerevisiae. Here, we describe a novel method to also detect trans-kingdom Vir protein transfer from Agrobacterium to yeast, using the Cre/lox system. Protein fusions between Cre and VirE2 or VirF were expressed in AGROBACTERIUM: Transfer of the Cre-Vir fusion proteins from Agrobacterium to yeast was monitored by a selectable excision event resulting from site-specific recombination mediated by Cre on a lox-flanked transgene in yeast. The VirE2 and VirF proteins were transported to yeast via the virB-encoded transfer system in the presence of coupling factor VirD4, analogous to translocation into plant cells. The yeast system therefore provides a suitable and fast model system to study basic aspects of trans-kingdom protein transport from Agrobacterium into host cells. Using this method we showed that VirE2 and VirF protein transfer was inhibited by the presence of the Osa protein. Besides, we found evidence for a novel third effector protein, VirE3, which has a similar C-terminal signature to VirE2 and VirF.

Expression of the Arabidopsis histone H2A-1 gene correlates with susceptibility to Agrobacterium transformation

Transformation of plant cells by Agrobacterium tumefaciens involves both bacterial virulence proteins and host proteins. We have previously shown that the Arabidopsis thaliana gene H2A-1 (RAT5), which encodes histone H2A-1, is involved in T-DNA integration into the plant genome. Mutation of RAT5 results in a severely decreased frequency of transformation, whereas overexpression of RAT5 enhances the transformation frequency (Mysore et al., 2000b). We show here that the expression pattern of RAT5 correlates with plant root cells most susceptible to transformation. As opposed to a cyclin-GUS fusion gene whose expression is limited to meristematic tissues, the H2A-1 gene is expressed in many non-dividing cells. Under normal circumstances, the H2A-1 gene is expressed in the elongation zone of the root, the region that is most susceptible to Agrobacterium transformation. In addition, when roots are incubated on medium containing phytohormones, a concomitant shift in H2A-1 expression and transformation susceptibility to the root base is observed. Inoculation of root segments with a transfer-competent, but not a transformation-deficient Agrobacterium strain induces H2A-1 expression. Furthermore, pre-treatment of Arabidopsis root segments with phytohormones both induces H2A-1 expression and increases the frequency of Agrobacterium transformation. Our results suggest that the expression of the H2A-1 gene is both a marker for, and a predictor of, plant cells most susceptible to Agrobacterium transformation.

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.

Characterisation and genetic organisation of a 24-MDa plasmid from the Brazilian Purpuric Fever clone of Haemophilus influenzae biogroup aegyptius

Strains of Haemophilus influenzae biogroup aegyptius causing septicaemia were identified in Brazil in the 1980s, causing the life-threatening illness of Brazilian Purpuric Fever (BPF). The strains were found to fall into a single clonal group, the BPF clone, characterised by their possession of the approximately 24MDa "3031" plasmid. In this work we report the characterisation and genetic organisation of this plasmid. Analysis of the gene content of what appears to be a typical broad host range conjugative plasmid, its presence in non-BPF strains as revealed by Southern hybridisation, and the recent discovery of plasmid-lacking BPF strains, has led us to conclude that it is unlikely to play a critical role in bacterial virulence. Establishing its entire sequence has nonetheless been an important step on the road to delineating, by comparison of BPF and non-BPF strains, chromosomal genetic loci that are involved in the special virulence of the BPF clone.

Roles of internal cysteines in the function, localization, and reactivity of the TraV outer membrane lipoprotein encoded by the F plasmid

We have examined the functional role of two internal cysteine residues of the F-plasmid TraV outer membrane lipoprotein. Each was mutated to a serine separately and together to yield three mutant traV genes: traV(C10S), traV(C18S), and traV(C10S/C18S). All three cysteine mutations complemented a traV mutant for DNA donor activity and for sensitivity to donor-specific bacteriophage; however, when measured by a transduction assay, the donor-specific DNA bacteriophage sensitivities of the traV(C18S) and, especially, traV(C10S/C18S) mutant strains were significantly less than those of the traV(+) and traV(C10S) strains. Thus, unlike the Agrobacterium tumefaciens T-plasmid-encoded VirB7 outer membrane lipoprotein, TraV does not require either internal cysteine to retain significant biological activity. By Western blot analysis, all three mutant TraV proteins were shown to accumulate in the outer membrane. However, by nonreducing gel electrophoresis, wild-type TraV and especially the TraV(C18S) mutant were shown to form mixed disulfides with numerous cell envelope proteins. This was not observed with the TraV(C10S) or TraV(C10S/C18S) proteins. Thus, it appears that TraV C10 is unusually reactive and that this reactivity is reduced by C18, perhaps by intramolecular oxidation. Finally, whereas the TraV(C10S) and TraV(C18S) proteins fractionated primarily with the outer membrane, as did the wild-type protein, the TraV(C10S/C18S) protein was found in osmotic shock fluid and inner membrane fractions as well as outer membrane fractions. Hence, at least one cysteine is required for the efficient localization of TraV to the outer membrane.

Whole-genome analysis of transporters in the plant pathogen Xylella fastidiosa

The transport systems of the first completely sequenced genome of a plant parasite, Xylella fastidiosa, were analyzed. In all, 209 proteins were classified here as constitutive members of transport families; thus, we have identified 69 new transporters in addition to the 140 previously annotated. The analysis lead to several hints on potential ways of controlling the disease it causes on citrus trees. An ADP:ATP translocator, previously found in intracellular parasites only, was found in X. fastidiosa. A P-type ATPase is missing-among the 24 completely sequenced eubacteria to date, only three (including X. fastidiosa) do not have a P-type ATPase, and they are all parasites transmitted by insect vectors. An incomplete phosphotransferase system (PTS) was found, without the permease subunits-we conjecture either that they are among the hypothetical proteins or that the PTS plays a solely metabolic regulatory role. We propose that the Ttg2 ABC system might be an import system eventually involved in glutamate import rather than a toluene exporter, as previously annotated. X. fastidiosa exhibits fewer proteins with > or =4 alpha-helical transmembrane spanners than any other completely sequenced prokaryote to date. X. fastidiosa has only 2.7% of all open reading frames identifiable as major transporters, which puts it as the eubacterium having the lowest percentage of open reading frames involved in transport, closer to two archaea, Methanococcus jannaschii (2.4%) and Methanobacterium thermoautotrophicum (2.4%).

Isogenic mutants of the cag pathogenicity island of Helicobacter pylori in the mouse model of infection: effects on colonization efficiency

Strains of Helicobacter pylori that contain the cag pathogenicity island (cag PAI) are associated with increased virulence and severe clinical outcomes. To evaluate the role of the cag island in infection, isogenic null mutations were generated in two clinical isolates (SS1 and Iris1) with distinct genetic backgrounds. When tested for their ability to establish infection in the stomach of CD1/SPF mice, at the early phase of infection, strains in which cagE, ORF528, ORF527 or ORF525 were inactivated showed a reduced capacity to initiate colonization compared to the wild-type strain. Strains with a mutation in the ORF524 gene were more efficient than the other mutants, but still less efficient than the wild-type strain. Mutation in the effector protein, CagA, which is injected into host cells and tyrosine-phosphorylated, did not change the colonization efficiency. In conclusion, all cag genes analysed, with the exception of the effector protein, CagA, influenced the early phase of colonization in the mouse model of infection. These results suggest that the structure of the H. pylori secretion apparatus itself is involved in this process.

Characterization and transcriptional analysis of gene clusters for a type IV secretion machinery in human granulocytic and monocytic ehrlichiosis agents

Anaplasma (Ehrlichia) phagocytophila and Ehrlichia chaffeensis, the etiologic agents of granulocytic and monocytic ehrlichioses, respectively, are obligatory intracellular bacteria that cause febrile systemic illness in humans. We identified and characterized clusters of genes for a type IV secretion machinery in these two bacteria, and analyzed their gene expression in cell culture and mammalian hosts. Eight virB and virD genes were found in each bacterial genome, and all of the genes were transcribed in cell culture. Although the gene order and orientation were similar to those found in other bacteria, the eight virB and virD genes were clustered at two separate loci in each genome. Five of the genes (virB8, virB9, virB10, virB11, and virD4) were located downstream from a ribA gene. These five genes in both A. phagocytophila and E. chaffeensis were polycistronically transcribed and controlled through at least two tandem promoters located upstream of the virB8 gene in human leukemia cell lines. The virB9 gene of A. phagocytophila was transcriptionally active in peripheral blood leukocytes from human ehrlichiosis patients and experimentally infected animals. Three of the remaining genes (virB3, virB4, and virB6) of both A. phagocytophila and E. chaffeensis were arranged downstream from a sodB gene and cotranscribed with the sodB gene through one or more sodB promoters in human leukocytes. This suggests that transcription of the three virB genes in these two Anaplasma and Ehrlichia spp. is regulated by factors that influence the sodB gene expression. This unique regulation of gene expression for the type IV secretion system may be associated with intracellular survival and replication of Anaplasma and Ehrlichia spp. in granulocytes or monocytes.

Membrane localization of the S1 subunit of pertussis toxin in Bordetella pertussis and implications for pertussis toxin secretion

Pertussis toxin is secreted from Bordetella pertussis with the assistance of the Ptl transport system, a member of the type IV family of macromolecular transporters. The S1 subunit and the B oligomer combine to form the holotoxin prior to export from the bacterial cell, although the site of assembly is not known. To better understand the pathway of pertussis toxin assembly and secretion, we examined the subcellular location of the S1 subunit, expressed with or without the B oligomer and the Ptl proteins. In wild-type B. pertussis, the majority of the S1 subunit that remained cell associated localized to the bacterial membranes. In mutants of B. pertussis that do not express pertussis toxin and/or the Ptl proteins, full-length S1, expressed from a plasmid, partitioned almost entirely to the bacterial membranes. Several lines of evidence strongly suggest that the S1 subunit localizes to the outer membrane of B. pertussis. First, we found that membrane-bound full-length S1 was almost completely insoluble in Triton X-100. Second, recombinant S1 previously has been shown to localize to the outer membrane of Escherichia coli (J. T. Barbieri, M. Pizza, G. Cortina, and R. Rappuoli, Infect. Immun. 58:999-1003, 1990). Third, the S1 subunit possesses a distinctive amino acid motif at its carboxy terminus, including a terminal phenylalanine, which is highly conserved among bacterial outer membrane proteins. By using site-directed mutagenesis, we determined that the terminal phenylalanine is critical for stable expression of the S1 subunit. Our findings provide evidence that prior to assembly with the B oligomer and independent of the Ptl proteins, the S1 subunit localizes to the outer membrane of B. pertussis. Thus, outer membrane-bound S1 may serve as a nucleation site for assembly with the B oligomer and for interactions with the Ptl transport system.

Type IVB secretion by intracellular pathogens

A growing number of pathogens are being found to possess specialized secretion systems which they use in various ways to subvert host defenses. One class, called type IV, are defined as having homology to the conjugal transfer systems of naturally occurring plasmids. It has been proposed that pathogens with type IV secretion systems have acquired and adapted the conjugal transfer systems of plasmids and now use them to export toxins. Several well-characterized intracellular pathogens, including Legionella pneumophila, Coxiella burnetii, Brucella abortus, and Rickettsia prowazekii, contain type IV systems which are known or suspected to be of critical importance in their ability to cause disease. Specifically, these systems are believed to be the key factors determining intracellular fate, and thus the ability to replicate and cause disease.

Partners-in-infection: host proteins involved in the transformation of plant cells by Agrobacterium

Genetic modification of plant cells by Agrobacterium is the only known natural example of DNA transport between kingdoms. While the bacterial factors involved in Agrobacterium infection have been relatively well characterized, studies of their host cellular partners are just beginning. Here, we describe the plant cell factors that might participate in Agrobacterium-mediated genetic transformation and discuss their possible roles in this process. Because Agrobacterium probably adapts existing cellular processes for its life cycle, identifying the host factors participating in Agrobacterium infection might contribute to a better understanding of such basic biological processes as cell communication, intracellular transport and DNA repair and recombination as well as help expand the host range of Agrobacterium as a genetic engineering tool.