The essential virulence protein VirB8 localizes to the inner membrane of Agrobacterium tumefaciens

Pangenome analysis and virulence profiling of Streptococcus intermedius

Background

Streptococcus intermedius, a member of the S. anginosus group, is a commensal bacterium present in the normal microbiota of human mucosal surfaces of the oral, gastrointestinal, and urogenital tracts. However, it has been associated with various infections such as liver and brain abscesses, bacteremia, osteo-articular infections, and endocarditis. Since 2005, high throughput genome sequencing methods enabled understanding the genetic landscape and diversity of bacteria as well as their pathogenic role. Here, in order to determine whether specific virulence genes could be related to specific clinical manifestations, we compared the genomes from 27 S. intermedius strains isolated from patients with various types of infections, including 13 that were sequenced in our institute and 14 available in GenBank.

Results

We estimated the theoretical pangenome size to be of 4,020 genes, including 1,355 core genes, 1,054 strain-specific genes and 1,611 accessory genes shared by 2 or more strains. The pangenome analysis demonstrated that the genomic diversity of S. intermedius represents an “open” pangenome model. We identified a core virulome of 70 genes and 78 unique virulence markers. The phylogenetic clusters based upon core-genome sequences and SNPs were independent from disease types and sample sources. However, using Principal Component analysis based on presence/ absence of virulence genes, we identified the sda histidine kinase, adhesion protein LAP and capsular polysaccharide biosynthesis protein cps4E as being associated to brain abscess or broncho-pulmonary infection. In contrast, liver and abdominal abscess were associated to presence of the fibronectin binding protein fbp54 and capsular polysaccharide biosynthesis protein cap8D and cpsB.

Conclusions

Based on the virulence gene content of 27 S. intermedius strains causing various diseases, we identified putative disease-specific genetic profiles discriminating those causing brain abscess or broncho-pulmonary infection from those causing liver and abdominal abscess. These results provide an insight into S. intermedius pathogenesis and highlights putative targets in a diagnostic perspective.

Monomer-to-dimer transition of Brucella suis type IV secretion system component VirB8 induces conformational changes

Secretion systems are protein complexes essential for bacterial virulence and potential targets for antivirulence drugs. In the intracellular pathogen Brucella suis, a type IV secretion system mediates the translocation of virulence factors into host cells and it is essential for pathogenicity. VirB8 is a core component of the secretion system and dimerization is important for functionality of the protein complex. We set out to study dimerization and possible conformational changes of VirB8 from B. suis (VirB8s) using nuclear magnetic resonance, X-ray crystallography, and differential scanning fluorimetry. We identified changes of the protein induced by a concentration-dependent monomer-to-dimer transition of the periplasmic domain (VirB8sp). We also show that the presence of the detergent CHAPS alters several signals in the heteronuclear single quantum coherence (HSQC) spectra and some of these chemical shift changes correspond to those observed during monomer-dimer transition. X-ray analysis of a monomeric variant (VirB8sp^M102R ) demonstrates that significant structural changes occur in the protein's α-helical regions (α2 and α4). We localized chemical shift changes of residues at the dimer interface as well as to the α1 helix that links this interface to a surface groove that binds dimerization inhibitors. Fragment-based screening identified small molecules that bind to VirB8sp and two of them have differential binding affinity for wild-type and the VirB8sp^M102R variant underlining their different conformations. The observed chemical shift changes suggest conformational changes of VirB8s during monomer-dimer transition that may play a role during secretion system assembly or function and they provide insights into the mechanism of inhibitor action.

DATABASE

BMRB accession no. 26852 and PDB 5JBS.

Type IV Secretion in Agrobacterium tumefaciens and Development of Specific Inhibitors

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) comprises 12 membrane-bound proteins, and it assembles a surface-exposed T-pilus. It is considered to be the archetypical system that is generally used to orient the nomenclature of other T4SS. Whereas the sequence similarities between T4SSs from different organisms are often limited, the general mechanism of action appears to be conserved, and the evolutionary relationship to bacterial conjugation systems and to T4SSs from animal pathogens is well established. Agrobacterium is a natural genetic engineer that is extensively used for the generation of transgenic plants for research and for agro-biotechnological applications. It also served as an early model for the understanding of pathogen-host interactions and for the transfer of macromolecular virulence factors into host cells. The knowledge on the mechanism of its T4SS inspired the search for small molecules that inhibit the virulence of bacterial pathogens and of bacterial conjugation. Inhibitors of bacterial virulence and of conjugation have interesting potential as alternatives to antibiotics and as inhibitors of antimicrobial resistance gene transfer. Mechanistic work on the Agrobacterium T4SS will continue to inspire the search for inhibitor target sites and drug design.

Towards an integrated model of bacterial conjugation

Bacterial conjugation is one of the main mechanisms for horizontal gene transfer. It constitutes a key element in the dissemination of antibiotic resistance and virulence genes to human pathogenic bacteria. DNA transfer is mediated by a membrane-associated macromolecular machinery called Type IV secretion system (T4SS). T4SSs are involved not only in bacterial conjugation but also in the transport of virulence factors by pathogenic bacteria. Thus, the search for specific inhibitors of different T4SS components opens a novel approach to restrict plasmid dissemination. This review highlights recent biochemical and structural findings that shed new light on the molecular mechanisms of DNA and protein transport by T4SS. Based on these data, a model for pilus biogenesis and substrate transfer in conjugative systems is proposed. This model provides a renewed view of the mechanism that might help to envisage new strategies to curb the threating expansion of antibiotic resistance.

Membrane and core periplasmic Agrobacterium tumefaciens virulence Type IV secretion system components localize to multiple sites around the bacterial perimeter during lateral attachment to plant cells

Type IV secretion systems (T4SS) transfer DNA and/or proteins into recipient cells. Here we performed immunofluorescence deconvolution microscopy to localize the assembled T4SS by detection of its native components VirB1, VirB2, VirB4, VirB5, VirB7, VirB8, VirB9, VirB10, and VirB11 in the C58 nopaline strain of Agrobacterium tumefaciens, following induction of virulence (vir) gene expression. These different proteins represent T4SS components spanning the inner membrane, periplasm, or outer membrane. Native VirB2, VirB5, VirB7, and VirB8 were also localized in the A. tumefaciens octopine strain A348. Quantitative analyses of the localization of all the above Vir proteins in nopaline and octopine strains revealed multiple foci in single optical sections in over 80% and 70% of the bacterial cells, respectively. Green fluorescent protein (GFP)-VirB8 expression following vir induction was used to monitor bacterial binding to live host plant cells; bacteria bind predominantly along their lengths, with few bacteria binding via their poles or subpoles. vir-induced attachment-defective bacteria or bacteria without the Ti plasmid do not bind to plant cells. These data support a model where multiple vir-T4SS around the perimeter of the bacterium maximize effective contact with the host to facilitate efficient transfer of DNA and protein substrates.

IMPORTANCE

Transfer of DNA and/or proteins to host cells through multiprotein type IV secretion system (T4SS) complexes that span the bacterial cell envelope is critical to bacterial pathogenesis. Early reports suggested that T4SS components localized at the cell poles. Now, higher-resolution deconvolution fluorescence microscopy reveals that all structural components of the Agrobacterium tumefaciens vir-T4SS, as well as its transported protein substrates, localize to multiple foci around the cell perimeter. These results lead to a new model of A. tumefaciens attachment to a plant cell, where A. tumefaciens takes advantage of the multiple vir-T4SS along its length to make intimate lateral contact with plant cells and thereby effectively transfer DNA and/or proteins through the vir-T4SS. The T4SS of A. tumefaciens is among the best-studied T4SS, and the majority of its components are highly conserved in different pathogenic bacterial species. Thus, the results presented can be applied to a broad range of pathogens that utilize T4SS.

Agrobacterium tumefaciens type IV secretion protein VirB3 is an inner membrane protein and requires VirB4, VirB7, and VirB8 for stabilization

Agrobacterium tumefaciens VirB proteins assemble a type IV secretion apparatus and a T-pilus for secretion of DNA and proteins into plant cells. The pilin-like protein VirB3, a membrane protein of unknown topology, is required for the assembly of the T-pilus and for T-DNA secretion. Using PhoA and green fluorescent protein (GFP) as periplasmic and cytoplasmic reporters, respectively, we demonstrate that VirB3 contains two membrane-spanning domains and that both the N and C termini of the protein reside in the cytoplasm. Fusion proteins with GFP at the N or C terminus of VirB3 were fluorescent and, like VirB3, localized to a cell pole. Biochemical fractionation studies demonstrated that VirB3 proteins encoded by three Ti plasmids, the octopine Ti plasmid pTiA6NC, the supervirulent plasmid pTiBo542, and the nopaline Ti plasmid pTiC58, are inner membrane proteins and that VirB4 has no effect on membrane localization of pTiA6NC-encoded VirB3 (pTiA6NC VirB3). The pTiA6NC and pTiBo542 VirB2 pilins, like VirB3, localized to the inner membrane. The pTiC58 VirB4 protein was earlier found to be essential for stabilization of VirB3. Stabilization of pTiA6NC VirB3 requires not only VirB4 but also two additional VirB proteins, VirB7 and VirB8. A binary interaction between VirB3 and VirB4/VirB7/VirB8 is not sufficient for VirB3 stabilization. We hypothesize that bacteria use selective proteolysis as a mechanism to prevent assembly of unproductive precursor complexes under conditions that do not favor assembly of large macromolecular structures.

Agrobacterium type IV secretion system and its substrates form helical arrays around the circumference of virulence-induced cells

The genetic transformation of plant cells by Agrobacterium tumefaciens results from the transfer of DNA and proteins via a specific virulence (vir) -induced type IV secretion system (T4SS). To better understand T4SS function, we analyzed the localization of its structural components and substrates by deconvolution fluorescence microscopy. GFP fusions to T4SS proteins with cytoplasmic tails, VirB8 and VirD4, or cytoplasmic T4SS substrate proteins, VirD2, VirE2, and VirF, localize in a helical pattern of fluorescent foci around the perimeter of the bacterial cell. All fusion proteins were expressed at native levels of vir induction. Importantly, most fusion proteins are functional and do not exhibit dominant-negative effects on DNA transfer to plant cells. Further, GFP-VirB8 complements a virB8 deletion strain. We also detect native VirB8 localization as a helical array of foci by immunofluorescence microscopy. T4SS foci likely use an existing helical scaffold during their assembly. Indeed, the bacterial cytoskeletal component MinD colocalizes with GFP-VirB8. Helical arrays of foci are found at all times investigated between 12 and 48 h post vir induction at 19 degrees C. These data lead to a model with multiple T4SSs around the bacterial cell that likely facilitate host cell attachment and DNA transfer. In support, we find multiple T pili around vir-induced bacterial cells.

An anomalous type IV secretion system in Rickettsia is evolutionarily conserved

BACKGROUND

Bacterial type IV secretion systems (T4SSs) comprise a diverse transporter family functioning in conjugation, competence, and effector molecule (DNA and/or protein) translocation. Thirteen genome sequences from Rickettsia, obligate intracellular symbionts/pathogens of a wide range of eukaryotes, have revealed a reduced T4SS relative to the Agrobacterium tumefaciens archetype (vir). However, the Rickettsia T4SS has not been functionally characterized for its role in symbiosis/virulence, and none of its substrates are known.

RESULTS

Superimposition of T4SS structural/functional information over previously identified Rickettsia components implicate a functional Rickettsia T4SS. virB4, virB8 and virB9 are duplicated, yet only one copy of each has the conserved features of similar genes in other T4SSs. An extraordinarily duplicated VirB6 gene encodes five hydrophobic proteins conserved only in a short region known to be involved in DNA transfer in A. tumefaciens. virB1, virB2 and virB7 are newly identified, revealing a Rickettsia T4SS lacking only virB5 relative to the vir archetype. Phylogeny estimation suggests vertical inheritance of all components, despite gene rearrangements into an archipelago of five islets. Similarities of Rickettsia VirB7/VirB9 to ComB7/ComB9 proteins of epsilon-proteobacteria, as well as phylogenetic affinities to the Legionella lvh T4SS, imply the Rickettsiales ancestor acquired a vir-like locus from distantly related bacteria, perhaps while residing in a protozoan host. Modern modifications of these systems likely reflect diversification with various eukaryotic host cells.

CONCLUSION

We present the rvh (Rickettsiales vir homolog) T4SS, an evolutionary conserved transporter with an unknown role in rickettsial biology. This work lays the foundation for future laboratory characterization of this system, and also identifies the Legionella lvh T4SS as a suitable genetic model.

Topology of the VirB4 C terminus in the Agrobacterium tumefaciens VirB/D4 type IV secretion system

Gram-negative type IV secretion systems (T4SSs) transfer proteins and DNA to eukaryotic and/or prokaryotic recipients resulting in pathogenesis or conjugative DNA transfer. VirB4, one of the most conserved proteins in these systems, has both energetic and structural roles in substrate translocation. We previously predicted a structural model for the large C-terminal domain (residues 425-789) of VirB4 of Agrobacterium tumefaciens. Here we have defined a homology-based structural model for Agrobacterium VirB11. Both VirB4 and VirB11 models predict hexameric oligomers. Yeast two-hybrid interactions define peptides in the C terminus of VirB4 and the N terminus of VirB11 that interact with each other. These interactions were mapped onto the homology models to predict direct interactions between the hexameric interfaces of VirB4 and VirB11 such that the VirB4 C terminus stacks above VirB11 in the periplasm. In support of this, fractionation and Western blotting show that the VirB4 C terminus is localized to the membrane and periplasm rather than the cytoplasm of cells. Additional high resolution yeast two-hybrid results demonstrate interactions between the C terminus of VirB4 and the periplasmic portions of VirB1, VirB8, and VirB10. Genetic studies reveal dominant negative interactions and thus function of the VirB4 C terminus in vivo. The above data are integrated with the existing body of literature to propose a structural, periplasmic role for the C-terminal half of the Agrobacterium VirB4 protein.

Dimerization and interactions of Brucella suis VirB8 with VirB4 and VirB10 are required for its biological activity

VirB8-like proteins are essential components of type IV secretion systems, bacterial virulence factors that mediate the translocation of effector molecules from many bacterial pathogens into eukaryotic cells. Based on cell biological, genetic, and x-ray crystallographic data, VirB8 was proposed to undergo multiple protein-protein interactions to mediate assembly of the translocation machinery. Here we report the results of a structure-function analysis of the periplasmic domain of VirB8 from the mammalian pathogen Brucella suis, which identifies amino acid residues required for three protein-protein interactions. VirB8 variants changed at residues proposed to be involved in dimerization, and protein-protein interactions were purified and characterized in vitro and in vivo. Changes at M102, Y105, and E214 affected the self-association as measured by analytical ultracentrifugation and gel filtration. The interaction with B. suis VirB10 was reduced by changes at T201, and change at R230 inhibited the interaction with VirB4 in vitro. The in vivo functionality of VirB8 variants was determined by complementation of growth in macrophages by a B. suis virB8 mutant and by using a heterologous assay of type IV secretion system assembly in Agrobacterium tumefaciens. Changes at Y105, T201, R230, and at several other residues impaired the in vivo function of VirB8, suggesting that we have identified interaction sites of relevance in the natural biological context.

The mating pair formation system of conjugative plasmids-A versatile secretion machinery for transfer of proteins and DNA

The mating pair formation (Mpf) system functions as a secretion machinery for intercellular DNA transfer during bacterial conjugation. The components of the Mpf system, comprising a minimal set of 10 conserved proteins, form a membrane-spanning protein complex and a surface-exposed sex pilus, which both serve to establish intimate physical contacts with a recipient bacterium. To function as a DNA secretion apparatus the Mpf complex additionally requires the coupling protein (CP). The CP interacts with the DNA substrate and couples it to the secretion pore formed by the Mpf system. Mpf/CP conjugation systems belong to the family of type IV secretion systems (T4SS), which also includes DNA-uptake and -release systems, as well as effector protein translocation systems of bacterial pathogens such as Agrobacterium tumefaciens (VirB/VirD4) and Helicobacter pylori (Cag). The increased efforts to unravel the molecular mechanisms of type IV secretion have largely advanced our current understanding of the Mpf/CP system of bacterial conjugation systems. It has become apparent that proteins coupled to DNA rather than DNA itself are the actively transported substrates during bacterial conjugation. We here present a unified and updated view of the functioning and the molecular architecture of the Mpf/CP machinery.

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.

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.

Functional analysis of the Agrobacterium tumefaciens T-DNA transport pore protein VirB8

The VirB8 protein of Agrobacterium tumefaciens is essential for DNA transfer to plants. VirB8, a 237-residue polypeptide, is an integral membrane protein with a short N-terminal cytoplasmic domain. It interacts with two transport pore proteins, VirB9 and VirB10, in addition to itself. To study the role of these interactions in DNA transfer and to identify essential amino acids of VirB8, we introduced random mutations in virB8 by the mutagenic PCR method. The putative mutants were tested for VirB8 function by the ability to complement a virB8 deletion mutant in tumor formation assays. After multiple rounds of screening 13 mutants that failed to complement the virB8 deletion mutation were identified. Analysis of the mutant strains by DNA sequence analysis, Western blot assays, and reconstruction of new point mutations led to the identification of five amino acid residues that are essential for VirB8 function. The substitution of glycine-78 to serine, serine-87 to leucine, alanine-100 to valine, arginine-107 to proline or alanine, and threonine-192 to methionine led to the loss of VirB8 activity. When introduced into the wild-type strain, virB8(S87L) partially suppressed the tumor forming ability of the wild-type protein. Analysis of protein-protein interaction by the yeast two-hybrid assay indicated that VirB8(R107P) is defective in interactions with both VirB9 and VirB10. A second mutant VirB8(S87L) is defective in interaction with VirB9.

The N- and C-terminal portions of the Agrobacterium VirB1 protein independently enhance tumorigenesis

Genetic transformation of plants by Agrobacterium tumefaciens is mediated by a virulence (vir)-specific type IV secretion apparatus assembled from 11 VirB proteins and VirD4. VirB1, targeted to the periplasm by an N-terminal signal peptide, is processed to yield VirB1*, comprising the C-terminal 73 amino acids. The N-terminal segment, which shares homology with chicken egg white lysozyme as well as lytic transglycosylases, may provide local lysis of the peptidoglycan cell wall to create channels for transporter assembly. Synthesis of VirB1* followed by its secretion to the exterior of the cell suggests that VirB1* may also have a role in virulence. In the present study, we provide evidence for the dual roles of VirB1 in tumorigenesis as well as the requirements for processing and secretion of VirB1*. Complementation of a virB1 deletion strain with constructs expressing either the N-terminal lysozyme-homologous region or VirB1* results in tumors intermediate in size between those induced by a wild-type strain and a virB1 deletion strain, suggesting that each domain has a unique role in tumorigenesis. The secretion of VirB1* translationally fused to the signal peptide indicates that processing and secretion are not coupled. When expressed independently of all other vir genes, VirB1 was processed and VirB1* was secreted. When restricted to the cytoplasm by deletion of the signal peptide, VirB1 was neither processed nor secreted and did not restore virulence to the virB1 deletion strain. Thus, factors that mediate processing of VirB1 and secretion of VirB1* are localized in the periplasm or outer membrane and are not subject to vir regulation.

Subcellular localization of the Agrobacterium tumefaciens T-DNA transport pore proteins: VirB8 is essential for the assembly of the transport pore

Agrobacterium tumefaciens transforms plants by transferring DNA to the plant cell nucleus. The VirB membrane proteins are postulated to form a pore for the transport of the DNA across the bacterial membranes. Immunofluorescence and immunoelectron microscopy were used to study the transport pore complex. Three likely components of the transport pore, VirB8, VirB9 and VirB10, localized primarily to the inner membrane, outer membrane and periplasm respectively. A significant amount of VirB10 was also found associated with the outer membrane. When expressed alone VirB9 and VirB10 were randomly distributed along the cell membrane. Subcellular location of both proteins changed dramatically in the presence of the other VirB proteins. Both proteins localized to fewer sites and most of the gold particles representing protein molecules were found in clusters suggesting that the two proteins are in a protein complex. VirB8, on the other hand, localized to clusters even in the absence of the other VirB proteins. To investigate the role of VirB8 in the formation of VirB9 and VirB10 protein complexes, we studied the effect of deletion of virB8 on the subcellular location of VirB9 and VirB10. In a virB8 deletion mutant both proteins were distributed randomly on the cell membrane indicating that VirB8 is essential for complex assembly.

Components of the RP4 conjugative transfer apparatus form an envelope structure bridging inner and outer membranes of donor cells: implications for related macromolecule transport systems

During bacterial conjugation, the single-stranded DNA molecule is transferred through the cell envelopes of the donor and the recipient cell. A membrane-spanning transfer apparatus encoded by conjugative plasmids has been proposed to facilitate protein and DNA transport. For the IncPalpha plasmid RP4, a thorough sequence analysis of the gene products of the transfer regions Tra1 and Tra2 revealed typical features of mainly inner membrane proteins. We localized essential RP4 transfer functions to Escherichia coli cell fractions by immunological detection with specific polyclonal antisera. Each of the gene products of the RP4 mating pair formation (Mpf) system, specified by the Tra2 core region and by traF of the Tra1 region, was found in the outer membrane fraction with one exception, the TrbB protein, which behaved like a soluble protein. The membrane preparation from Mpf-containing cells had an additional membrane fraction whose density was intermediate between those of the cytoplasmic and outer membranes, suggesting the presence of attachment zones between the two E. coli membranes. The Tra1 region is known to encode the components of the RP4 relaxosome. Several gene products of this transfer region, including the relaxase TraI, were detected in the soluble fraction, but also in the inner membrane fraction. This indicates that the nucleoprotein complex is associated with and/or assembled facing the cytoplasmic site of the E. coli cell envelope. The Tra1 protein TraG was predominantly localized to the cytoplasmic membrane, supporting its potential role as an interface between the RP4 Mpf system and the relaxosome.

Dimerization of the Agrobacterium tumefaciens VirB4 ATPase and the effect of ATP-binding cassette mutations on the assembly and function of the T-DNA transporter

The Agrobacterium tumefaciens VirB4 ATPase functions with other VirB proteins to export T-DNA to susceptible plant cells and other DNA substrates to a variety of prokaryotic and eukaryotic cells. Previous studies have demonstrated that VirB4 mutants with defects in the Walker A nucleotide-binding motif are non-functional and exert a dominant negative phenotype when synthesized in wild-type cells. This study characterized the oligomeric structure of VirB4 and examined the effects of Walker A sequence mutations on complex formation and transporter activity. VirB4 directed dimer formation when fused to the amino-terminal portion of cI repressor protein, as shown by immunity of Escherichia coli cells to lambda phage infection. VirB4 also dimerized in Agrobacterium tumefaciens, as demonstrated by the recovery of a detergent-resistant complex of native protein and a functional, histidine-tagged derivative by precipitation with anti-His6 antibodies and by Co2+ affinity chromatography. Walker A sequence mutants directed repressor dimerization in E. coli and interacted with His-VirB4 in A. tumefaciens, indicating that ATP binding is not required for self-association. A dimerization domain was localized to a proposed N-terminal membrane-spanning region of VirB4, as shown by the dominance of an allele coding for the N-terminal 312 residues and phage immunity of host cells expressing cI repressor fusions to alleles for the first 237 or 312 residues. A recent study reported that the synthesis of a subset of VirB proteins, including VirB4, in agrobacterial recipients has a pronounced stimulatory effect on the virB-dependent conjugal transfer of plasmid RSF1010 by agrobacterial donors. VirB4'312 suppressed the stimulatory effect of VirB proteins for DNA uptake when synthesized in recipient cells. In striking contrast, Walker A sequence mutants contributed to the stimulatory effect of VirB proteins to the same extent as native VirB4. These findings indicate that the oligomeric structure of VirB4, but not its capacity to bind ATP, is important for the assembly of VirB proteins as a DNA uptake system. The results of these studies support a model in which VirB4 dimers or homomultimers contribute structural information for the assembly of a transenvelope channel competent for bidirectional DNA transfer, whereas an ATP-dependent activity is required for configuring this channel as a dedicated export machine.

Stability of the Agrobacterium tumefaciens VirB10 protein is modulated by growth temperature and periplasmic osmoadaption

Export of oncogenic T-DNA from the phytopathogen Agrobacterium tumefaciens is mediated by the products of the virB operon. It has recently been reported (K. J. Fullner and E. W. Nester, J. Bacteriol. 178:1498-1504, 1996) that DNA transfer does not occur at elevated temperatures; these observations correlate well with much earlier studies on the temperature sensitivity of crown gall tumor development on plants. In testing the hypothesis that this loss of DNA movement reflects a defect in assembly or maintenance of a stable DNA transfer machinery at high temperature, we have found that steady-state levels of VirB10 are sensitive to growth temperature while levels of several other VirB proteins are considerably less affected. This temperature-dependent failure to accumulate VirB10 is exacerbated in an attachment-deficient mutant strain (chvB) which exhibits pleiotropic defects in periplasmic osmoadaption, and virulence of a chvB mutant can be partially restored by lowering the temperature at which the bacteria and the plant tissue are cocultivated. Furthermore, the stability of VirB10 is diminished in cells lacking functional VirB9, but only under conditions of low osmolarity. We propose that newly synthesized VirB10 is inherently labile in the presence of a large osmotic gradient across the inner membrane and is rapidly degraded unless it is stabilized by VirB9-dependent assembly into oligomeric complexes. The possibility that VirB10-containing complexes are not assembled properly at elevated temperatures suggests an explanation for the decades-old observation that tumor formation is exquisitely sensitive to ambient temperature.

Construction of transposon Tn3phoA: its application in defining the membrane topology of the Agrobacterium tumefaciens DNA transfer proteins

Protein fusion with the Escherichia coli alkaline phosphatase is used extensively for the analysis of the topology of membrane proteins. To study the topology of the Agrobacterium T-DNA transfer proteins, we constructed a transposon, Tn3phoA. The transposon mobilizes into plasmids at a high frequency, is stable after transposition, can produce phoA translational fusions and can be used for the analysis of protein topology directly in the bacterium of interest. For studies on the DNA transfer proteins, an Agrobacterium strain deficient in phoA under our experimental conditions was constructed by chemical mutagenesis. A plasmid containing virB and virD4 was used as a target for mutagenesis. Twenty-eight unique phoA-positive clones that mapped to eight virB genes were isolated. Multiple insertions throughout VirB1, VirB5, VirB7, VirB9 and VirB10 indicated that these proteins primarily face the periplasm. Insertions in VirB2, VirB6 and VirB8 allowed the identification of their periplasmic domains. No insertions were found in VirB3, VirB4 and VirB11. These proteins either lack or have a short periplasmic domain. No insertions mapped to VirD4 either. To study VirD4 topology, targeted phoA fusions and random lacZ fusions were constructed. Analysis of the fusion proteins indicated that VirD4 contains a single periplasmic domain near the N-terminus, and most of the protein lies in the cytoplasm. A hypothetical model for the T-DNA transport pore is presented.

Control of genes for conjugative transfer of plasmids and other mobile elements

Conjugative transfer is a primary means of spread of mobile genetic elements (plasmids and transposons) between bacteria.It leads to the dissemination and evolution of the genes (such as those conferring resistance to antibiotics) which are carried by the plasmid. Expression of the plasmid genes needed for conjugative transfer is tightly regulated so as to minimise the burden on the host. For plasmids such as those belonging to the IncP group this results in downregulation of the transfer genes once all bacteria have a functional conjugative apparatus. For F-like plasmids (apart from F itself which is a derepressed mutant) tight control results in very few bacteria having a conjugative apparatus. Chance encounters between the rare transfer-proficient bacteria and a potential recipient initiate a cascade of transfer which can continue until all potential recipients have acquired the plasmid. Other systems express their transfer genes in response to specific stimuli. For the pheromone-responsive plasmids of Enterococcus it is small peptide signals from potential recipients which trigger the conjugative transfer genes. For the Ti plasmids of Agrobacterium it is the presence of wounded plants which are susceptible to infection which stimulates T-DNA transfer to plants. Transfer and integration of T-DNA induces production of opines which the plasmid-positive bacteria can utilise. They multiply and when they reach an appropriate density their plasmid transfer system is switched on to allow transfer of the Ti plasmid to other bacteria. Finally some conjugative transfer systems are induced by the antibiotics to which the elements confer resistance. Understanding these control circuits may help to modify management of microbial communities where plasmid transfer is either desirable or undesirable. z 1998 Published by Elsevier Science B.V.

VirB1, a component of the T-complex transfer machinery of Agrobacterium tumefaciens, is processed to a C-terminal secreted product, VirB1

During genetic transformation of plant cells by Agrobacterium tumefaciens, 11 VirB proteins and VirD4 are proposed to form a transmembrane bridge to transfer a DNA-protein complex (T-complex) into the plant cytoplasm. In this study, the localization of the first product of the virB operon, VirB1, was studied in detail. While full-length VirB1 localized mostly to the inner membrane, an immunoreactive VirB1 product was found as soluble processed form, designated VirB1*. Equal amounts of VirB1* could be detected in concentrated culture supernatants versus associated with the cell. VirB1* was purified from the supernatant of vir-induced cells by ammonium sulfate precipitation and Q-Sepharose chromatography. Sequence analysis of the N terminus of VirB1* localized the processing site after amino acid 172 of VirB1. Cell-associated VirB1* was partly removed by vortexing, suggesting a loose association with the cell or active secretion. However, cross-linking and coimmunoprecipitation showed a close association of cell-bound VirB1* with the VirB9-VirB7 heterodimer, a membrane-associated component of the T-complex transfer machinery. Homologies of the N-terminal part of VirB1 to bacterial transglycosylases suggest that it may assist T-complex transfer by local lysis of the bacterial cell wall, whereas the exposed localization of the C-terminal processing product VirB1* predicts direct interaction with the plant. Thus, VirB1 may be a bifunctional protein where both parts have different functions in T-complex transfer from Agrobacterium to plant cells.

The lipoprotein VirB7 interacts with VirB9 in the membranes of Agrobacterium tumefaciens

VirB9 and VirB7 are essential components of the putative VirB membrane channel required for transfer of the T-complex from Agrobacterium tumefaciens into plants. In this report, we present a biochemical analysis of their interaction and cellular localization. A comparison of relative electrophoretic mobilities under nonreducing and reducing conditions suggested that they form thiol-sensitive complexes with other proteins. Two-dimensional gel electrophoresis identified one complex as a heterodimer of VirB9 and VirB7 covalently linked by a disulfide bond, as well as VirB7 homodimers and monomers. Immunoprecipitation with VirB9-specific antiserum isolated the heterodimeric VirB9-VirB7 complex. Incubation with reducing agent split the complex into its constituent VirB9 and VirB7, which further confirmed linkage via cysteine residues. The interaction between VirB9 and VirB7 also was observed in the yeast two-hybrid system. Membrane attachment of VirB9-VirB7 may be conferred by lipoprotein modification, since labeling with [3H]palmitic acid in A. tumefaciens verified that VirB7 is a lipoprotein associated with VirB9. VirB9 and VirB7 showed equal distribution between inner and outer membranes, in accord with their proposed association with the transmembrane VirB complex.

Interactions between VirB9 and VirB10 membrane proteins involved in movement of DNA from Agrobacterium tumefaciens into plant cells

The 11 VirB proteins from Agrobacterium tumefaciens are predicted to form a membrane-bound complex that mediates the movement of DNA from the bacterium into plant cells. The studies reported here on the possible VirB protein interactions in such a complex demonstrate that VirB9 and VirB10 can each form high-molecular-weight complexes after treatment with a chemical cross-linker. Analysis of nonpolar virB mutants showed that the formation of the VirB10 complexes does not occur in a virB9 mutant and that VirB9 and VirB10 are not components of the same cross-linked complex. VirB9, when stabilized by the concurrent expression of VirB7, was shown to be sufficient to permit VirB10 to cross-link into its usual high-molecular-weight forms in the absence of other Vir proteins. Randomly introduced single point mutations in virB9 resulted in Agrobacterium strains with severely attenuated virulence. Although some of the mutants contained wild-type levels of VirB9 and displayed an unaltered VirB9 cross-linking pattern, VirB10 cross-linking was drastically reduced. We conclude that specific amino acid residues in VirB9 are necessary for interaction with VirB10 resulting in the capacity of VirB10 to participate in high-molecular-weight complexes that can be visualized by chemical cross-linking.

The Agrobacterium tumefaciens VirB7 lipoprotein is required for stabilization of VirB proteins during assembly of the T-complex transport apparatus

The Agrobacterium tumefaciens virB7 gene product is a lipoprotein whose function is required for the transmission of oncogenic T-DNA to susceptible plant cells. Three lines of study provided evidence that VirB7 interacts with and stabilizes other VirB proteins during the assembly of the putative T-complex transport apparatus. First, a precise deletion of virB7 from the pTiA6NC plasmid of wild-type strain A348 was correlated with significant reductions in the steady-state levels of several VirB proteins, including VirB4, VirB9, VirB10, and VirB11; trans expression of virB7 in the delta virB7 mutant partially restored the levels of these proteins, and trans coexpression of virB7 and virB8 fully restored the levels of these proteins to wild-type levels. Second, modulation of VirB7 levels resulted in corresponding changes in the levels of other VirB proteins in the following cell types: (i) a delta virB7 mutant expressing virB7 and virB8 from isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible Plac and other virB genes from acetosyringone (AS)-inducible PvirB; (ii) a delta virB operon mutant expressing virB7 and virB8 from Plac and virB9, virB10, and virB11 from PvirB; and (iii) a delta virB operon mutant expressing virB7 from IPTG-inducible Pklac and virB9 from an AS-inducible PvirB. Third, the synthesis of a VirB7::PhoA fusion protein in strain A348 was correlated with a significant reduction in the steady-state levels of VirB4, VirB5, and VirB7 through VirB11; these cells also exhibited a severely attenuated virulence phenotype, indicating that synthesis of the fusion protein perturbs the assembly of VirB proteins into a stabilized protein complex required for T-complex transport. Extracts of AS-induced cells electrophoresed under nonreducing conditions possessed undetectable levels of the 32-kDa VirB9 and 4.5-kDa VirB7 monomers and instead possessed a 36-kDa complex that cross-reacted with both VirB7 and VirB9 antisera and accumulated as a function of virB7 expression. Our results are consistent with a model in which VirB7 stabilizes VirB9 by formation of a covalent intermolecular cross-link; in turn, the VirB7-VirB9 heterodimer promotes the assembly of a functional T-complex transport machinery.

The Agrobacterium tumefaciens virB7 gene product, a proposed component of the T-complex transport apparatus, is a membrane-associated lipoprotein exposed at the periplasmic surface

The Agrobacterium tumefaciens virB7 gene product contains a typical signal sequence ending with a consensus signal peptidase II cleavage site characteristic of bacterial lipoproteins. VirB7 was shown to be processed as a lipoprotein by (i) in vivo labeling of native VirB7 and a VirB7::PhoA fusion with [3H]palmitic acid and (ii) inhibition of VirB7 processing by globomycin, a known inhibitor of signal peptidase II. A VirB7 derivative sustaining a Ser substitution for the invariant Cys-15 residue within the signal peptidase II cleavage site could not be visualized immunologically and failed to complement a delta virB7 mutation, establishing the importance of this putative lipid attachment site for VirB7 maturation and function. VirB7 partitioned predominantly with outer membrane fractions from wild-type A348 cells as well as a delta virB operon derivative transformed with a virB7 expression plasmid. Expression of virB7 fused to phoA, the alkaline phosphatase gene of Escherichia coli, gave rise to high alkaline phosphatase activities in E. coli and A. tumefaciens cells, providing genetic evidence for the export of VirB7 in these hosts. VirB7 was shown to be intrinsically resistant to proteinase K; by contrast, a VirB7::PhoA derivative was degraded by proteinase K treatment of A. tumefaciens spheroplasts and remained intact upon treatment of whole cells. Together, the results of these studies favor a model in which VirB7 is topologically configured as a monotopic protein with its amino terminus anchored predominantly to the outer membrane and with its hydrophilic carboxyl domain located in the periplasmic space. Parallel studies of VirB5, VirB8, VirB9, and VirB10 established that each of these membrane-associated proteins also contains a large periplasmic domain whereas VirB11 resides predominantly or exclusively within the interior of the cell.

Temperature affects the T-DNA transfer machinery of Agrobacterium tumefaciens

Early studies on Agrobacterium tumefaciens showed that development of tumors on plants following infection by A. tumefaciens was optimal at temperatures around 22 degrees C and did not occur at temperatures above 29 degrees C. To assess whether this inability to induce tumors is due to a defect in the T-DNA transfer machinery, mobilization of an incompatibility group Q (IncQ) plasmid by the T-DNA transfer machinery of A. tumefaciens was tested at various temperatures. Optimal transfer occurred when matings were performed at 19 degrees C, and transfer was not seen when matings were incubated above 28 degrees C. Transfer of the IncQ plasmid was dependent upon induction of the virB and virD operons by acetosyringone but was not dependent upon induction of the tra genes by octopine. However, alterations in the level of vir gene induction could not account for the decrease in transfer with increasing temperature. A. tumefaciens did successfully mobilize IncQ plasmids at higher temperatures when alternative transfer machineries were provided. Thus, the defect in transfer at high temperature is apparently in the T-DNA transfer machinery itself. As these data correlate with earlier tumorigenesis studies, we propose that tumor suppression at higher temperatures results from a T-DNA transfer machinery which does not function properly.

Osa protein encoded by plasmid pSa is located at the inner membrane but does not inhibit membrane association of VirB and VirD virulence proteins in Agrobacterium tumefaciens

The osa gene of IncW plasmid pSa encodes a 21-kDa protein that completely abolishes the oncogenic activity encoded by virulence genes in Agrobacterium tumefaciens. osa is the last gene of a four-gene operon in pSa, the expression of which appears to be highly regulated since the Osa protein is absent when either pSa or the osa operon is present in the Agrobacterium cell. When the osa gene alone or together with upstream genes within the operon are expressed under the control of a constitutive promoter. Osa protein is produced, enabling us to determine its subcellular location. Immunoblot analyses located Osa protein at the inner membrane of both A. tumefaciens and Escherichia coli. Because Osa inhibits oncogenicity of A. tumefaciens, and because alterations of the products of the virB and virD genes affect oncogenicity, studies were conducted to determine if there are changes in their specific association with the membranes in the presence Osa. Immunoblot analyses of VirB2, VirB3, VirB4, VirB9, and VirD4 in the presence and absence of Osa revealed no differences between the two treatments in these Vir protein associations with the membranes. These results indicate that both virB and virD gene products are produced in the presence of Osa; that they appear unaffected in their association with the membranes; and that Osa is associated with the inner membrane, where VirB2, VirB4, and VirD4 proteins are also located.

Inhibition of VirB-mediated transfer of diverse substrates from Agrobacterium tumefaciens by the IncQ plasmid RSF1010

The transfer of DNA from Agrobacterium tumefaciens into a plant cell requires the activities of several virulence (vir) genes that reside on the tumor-inducing (Ti) plasmid. The putative transferred intermediate is a single-stranded DNA (T strand), covalently attached to the VirD2 protein and coated with the single-stranded DNA-binding protein, VirE2. The movement of this intermediate out of Agrobacterium cells and into plant cells requires the expression of the virB operon, which encodes 11 proteins that localize to the membrane system. Our earlier studies showed that the IncQ broad-host-range plasmid RSF1010, which can be transferred from Agrobacterium cells to plant cells, inhibits the transfer of T-DNA from pTiA6 in a fashion that is reversed by overexpression of virB9, virB10, and virB11. Here, we examined the specificity of this inhibition by following the transfer of other T-DNA molecules. By using extracellular complementation assays, the effects of RSF1010 on movement of either VirE2 or an uncoated T strand from A. tumefaciens were also monitored. The RSF1010 derivative plasmid pJW323 drastically inhibited the capacity of strains to serve as VirE2 donors but only partially inhibited T-strand transfer from virE2 mutants. Further, we show that all the virB genes tested are required for the movement of VirE2 and the uncoated T strand as assayed by extracellular complementation. Our results are consistent with a model in which the RSF1010 plasmid, or intermediates from it, compete with the T strand and VirE2 for a common transport site.

Interactions of VirB9, -10, and -11 with the membrane fraction of Agrobacterium tumefaciens: solubility studies provide evidence for tight associations

The eleven predicted gene products of the Agrobacterium tumefaciens virB operon are believed to form a transmembrane pore complex through which T-DNA export occurs. The VirB10 protein is required for virulence and is a component of an aggregate associated with the membrane fraction of A. tumefaciens. Removal of the putative membrane-spanning domain (amino acids 22 through 55) disrupts the membrane topology of VirB10 (J. E. Ward, E. M. Dale, E. W. Nester, and A. N. Binns, J. Bacteriol. 172:5200-5210, 1990). Deletion of the sequences encoding amino acids 22 to 55 abolishes the ability of plasmid-borne virB10 to complement a null mutation in the virB10 gene, suggesting that the proper topology of VirB10 in the membrane may indeed play a crucial role in T-DNA transfer to the plant cell. Western blot (immunoblot) analysis indicated that the observed loss of virulence could not be attributed to a decrease in the steady-state levels of the mutant VirB10 protein. Although the deletion of the single transmembrane domain would be expected to perturb membrane association, VirB10 delta 22-55 was found exclusively in the membrane fraction. Urea extraction studies suggested that this membrane localization might be the result of a peripheral membrane association; however, the mutant protein was found in both inner and outer membrane fractions separated by sucrose density gradient centrifugation. Both wild-type VirB10 and wild-type VirB9 were only partially removed from the membranes by extraction with 1% Triton X-100, while VirB5 and VirB8 were Triton X-100 soluble. VirB11 was stripped from the membranes by 6 M urea but not by a more mild salt extraction. The fractionation patterns of VirB9, VirB10, and VirB11 were not dependent on each other or on VirB8 or VirD4. The observed tight association of VirB9, VirB10, and VirB11 with the membrane fraction support the notion that these proteins may exist as components of multiprotein pore complexes, perhaps spanning both the inner and outer membranes of Agrobacterium cells.

Bacterial conjugation mediated by plasmid RP4: RSF1010 mobilization, donor-specific phage propagation, and pilus production require the same Tra2 core components of a proposed DNA transport complex

DNA transfer by bacterial conjugation requires a mating pair formation (Mpf) system that specifies functions for establishing the physical contact between the donor and the recipient cell and for DNA transport across membranes. Plasmid RP4 (IncP alpha) contains two transfer regions designated Tra1 and Tra2, both of which contribute to Mpf. Twelve components are essential for Mpf, TraF of Tra1 and 11 Tra2 proteins, TrbB, -C, -D, -E, -F, -G, -H, -I, -J, -K, and -L. The phenotype of defined mutants in each of the Tra2 genes was determined. Each of the genes, except trbK, was found to be essential for RP4-specific plasmid transfer and for mobilization of the IncQ plasmid RSF1010. The latter process did not absolutely require trbF, but a severe reduction of the mobilization frequency occurred in its absence. Transfer proficiency of the mutants was restored by complementation with defined Tra2 segments containing single trb genes. Donor-specific phage propagation showed that traF and each of the genes encoded by Tra2 are involved. Phage PRD1, however, still adsorbed to the trbK mutant strain but not to any of the other mutant strains, suggesting the existence of a plasmid-encoded receptor complex. Strains containing the Tra2 plasmid in concert with traF were found to overexpress trb products as well as extracellular filaments visualized by electron microscopy. Each trb gene and traF are needed for the formation of the pilus-like structures. The trbK gene, which is required for PRD1 propagation and for pilus production but not for DNA transfer on solid media, encodes the RP4 entry-exclusion function. The components of the RP4 Mpf system are discussed in the context of related macromolecule export systems.

Transfer of T-DNA from Agrobacterium to the plant cell

Agrobacterium tumefaciens is the causative agent of crown gall, a disease of dicotyledonous plants characterized by a tumorous phenotype. Earlier in this century, scientific interest in A. tumefaciens was based on the possibility that the study of plant tumors might reveal mechanisms that were also operating in animal neoplasia. In the recent past, the tumorous growth was shown to result from the expression of genes coded for by a DNA segment of bacterial origin that was transferred and became stably integrated into the plant genome. This initial molecular characterization of the infection process suggested that Agrobacterium might be used to deliver genetic material into plants. The potential to genetically engineer plants generated renewed interest in the study of A. tumefaciens. In this review, we concentrate on the most recent advances in the study of Agrobacterium-mediated gene transfer, its relationship to conjugation, DNA processing and transport, and nuclear targeting. In the following discussion, references for earlier work can be found in more comprehensive reviews (Hooykaas and Schilperoort, 1992; Zambryski, 1992; Hooykaas and Beijersbergen, 1994).

Agrobacterium tumefaciens VirB11 protein requires a consensus nucleotide-binding site for function in virulence

virB11, one of the 11 genes of the virB operon, is absolutely required for transport of T-DNA from Agrobacterium tumefaciens into plant cells. Previous studies reported that VirB11 is an ATPase with autophosphorylation activity and localizes to the inner membrane even though the protein does not contain the consensus N-terminal export sequence. In this report, we show that VirB11 localizes to the inner membrane even in the absence of other tumor-inducing (Ti) plasmid-encoded proteins. To facilitate the further characterization of VirB11, we purified this protein from the soluble fraction of an Escherichia coli extract by fusing VirB11 to the maltose-binding protein. The maltose-binding protein-VirB11 fusion was able to complement a virB11 deletion mutant of A. tumefaciens for tumor formation and also localized properly to the inner membrane of A. tumefaciens. The 72-kDa protein, purified from E. coli, exhibited no autophosphorylation, ATPase activity, or ATP-binding activity. To study the importance of the Walker nucleotide-binding site present in VirB11, mutations were generated to replace the conserved lysine residue with either alanine or arginine. Expression of the virB11K175A mutant gene resulted in an avirulent phenotype, and expression of the virB11K175R mutant gene gave rise to an attenuated virulence phenotype. Both mutant proteins were present at levels three to four times higher than that of VirB11 in the wild-type strain. The mutant genes did not exhibit a transdominant phenotype on tumor formation in bacteria that were expressing wild-type virB11. The mutant proteins also localized properly to the inner membrane of A. tumefaciens, but the VirB11K175R protein appeared to be unstable after lysis of the cells.

Detection and subcellular localization of three Ptl proteins involved in the secretion of pertussis toxin from Bordetella pertussis

The ptl locus of Bordetella pertussis contains eight open reading frames which are predicted to encode proteins (PtlA to PtlH) that are essential for secretion of pertussis toxin from the bacterium and which are members of a family of transport proteins found in other types of bacteria. We have detected PtlE, PtlF, and PtlG in immunoblots of extracts of B. pertussis by using antibodies raised to fusion proteins consisting of maltose-binding protein and the individual Ptl proteins. These proteins have apparent molecular weights similar to those predicted by DNA sequence analysis. Cell fractionation studies indicated that all three Ptl proteins are associated with the membranes of B. pertussis, suggesting that the Ptl proteins form a gate or channel which facilitates transport of pertussis toxin. Cell extracts of other Bordetella spp. were probed with antibodies to Ptl proteins for the presence of these transport proteins. Neither Bordetella parapertussis nor Bordetella bronchiseptica contained detectable levels of PtlE or PtlF. This lack of detectable Ptl protein may provide an explanation for previous observations which indicated that introduction of the genes encoding pertussis toxin subunits from B. pertussis into other Bordetella spp. results in production of the toxin but not secretion of the toxin.

Genetic complementation analysis of the Agrobacterium tumefaciens virB operon: virB2 through virB11 are essential virulence genes

The Agrobacterium tumefaciens virB gene products are proposed to assemble into a transport system capable of exporting complexes of DNA and protein across the bacterial envelope en route to plant cells. Nonpolar null mutations were constructed in each of the 11 virB genes of the A. tumefaciens pTiA6NC plasmid. In tumorigenicity assays, delta virB1 mutants exhibited severely attenuated virulence and delta virB2 through delta virB11 mutants exhibited avirulence. NdeI restriction sites introduced at the predicted translational start sites of the virB genes were used to subclone each of the virB genes downstream of the lacZ or virB promoter on broad-host-range plasmids. virB gene expression plasmids were used to define promoter and general sequence requirements for genetic complementation of the deletion mutations. Whereas virB1 and virB2 complemented delta virB1 and delta virB2, respectively, only when expressed in trans from the virB promoter, virB3 through virB11 complemented the corresponding deletion mutations when expressed in trans from either the lacZ or virB promoter. Several virB genes required additional upstream or downstream sequences for complementation: (i) virB2 complemented the delta virB2 mutation only when the complementing plasmid coexpressed virB1 and virB2, (ii) virB6 and virB9 complemented the delta virB6 and delta virB9 mutations only when the complementing plasmids carried at most 55 and 230 bp of sequences residing 5' of these genes, respectively, and (iii) virB7 and virB8 complemented the delta virB7 and delta virB8 mutations only when the complementing plasmid coexpressed virB7 and virB8. These studies established that virB1 is an accessory virulence determinant and virB2 through virB11 are absolutely essential for the A. tumefaciens infection process.