Purification and properties of the F sex factor TraD protein, an inner membrane conjugal transfer protein

VirB4- and VirD4-Like ATPases, Components of a Putative Type 4C Secretion System in Clostridioides difficile

The type 4 secretion system (T4SS) represents a bacterial nanomachine capable of trans-cell wall transportation of proteins and DNA and has attracted intense interest due to its roles in the pathogenesis of infectious diseases. In the current investigation, we uncovered three distinct gene clusters in Clostridioides difficile strain 630 encoding proteins structurally related to components of the VirB4/D4 type 4C secretion system from Streptococcus suis strain 05ZYH33 and located within sequences of conjugative transposons (CTn). Phylogenic analysis revealed that VirB4- and VirD4-like proteins of the CTn4 locus, on the one hand, and those of the CTn2 and CTn5 loci, on the other hand, fit into separate clades, suggesting specific roles of identified secretion system variants in the physiology of C. difficile. Our further study on VirB4- and VirD4-like products encoded by CTn4 revealed that both proteins possess Mg²⁺-dependent ATPase activity, form oligomers (most likely hexamers) in aqueous solutions, and rely on potassium but not sodium ions for the highest catalytic rate. VirD4 binds nonspecifically to DNA and RNA. The DNA-binding activity of VirD4 strongly decreased with the W241A variant. Mutations in the nucleotide sequences encoding presumable Walker A and Walker B motifs decreased the stability of the oligomers and significantly but not completely attenuated the enzymatic activity of VirB4. In VirD4, substitutions of amino acid residues in the peptides reminiscent of Walker structural motifs neither attenuated the enzymatic activity of the protein nor influenced the oligomerization state of the ATPase. IMPORTANCE C. difficile is a Gram-positive, anaerobic, spore-forming bacterium that causes life-threatening colitis in humans. Major virulence factors of the microorganism include the toxins TcdA, TcdB, and CDT. However, other bacterial products, including a type 4C secretion system, have been hypothesized to contribute to the pathogenesis of the infection and are considered possible virulence factors of C. difficile. In the current paper, we describe the structural organization of putative T4SS machinery in C. difficile and characterize its VirB4- and VirD4-like components. Our studies, in addition to its significance for basic science, can potentially aid the development of antivirulence drugs suitable for the treatment of C. difficile infection.

Characterization of DtrJ as an IncC plasmid conjugative DNA transfer component

Incompatibility group C (IncC) plasmids are large (50-400 kb), broad host range plasmids that drive the spread of genes conferring resistance to all classes of antibiotics, most notably the bla_NDM gene that confers resistance to last-line carbapenems and the mcr-3 gene that confers resistance to colistin. Several recent studies have improved our understanding of the basic biological mechanisms driving the success of IncC, in particular the identification of multiple novel IncC conjugation genes by transposon directed insertion-site sequencing. Here, one of these genes, dtrJ, was examined in further detail. The dtrJ gene is located in the DNA transfer locus on the IncC backbone, and quantitative reverse-transcriptase PCR analysis revealed it is transcribed in the same operon as the DNA transfer genes traI and traD (encoding the relaxase and coupling protein, respectively) and activated by the AcaDC regulatory complex. We confirmed that DtrJ is not required for pilus biogenesis or mate pair formation. Instead, DtrJ localizes to the membrane, where it interacts with the coupling protein TraD and functions as an IncC DNA transfer protein. Overall, this work has defined the role of DtrJ in DNA transfer of IncC plasmids during conjugation.

Reconstitution in liposome bilayers enhances nucleotide binding affinity and ATP-specificity of TrwB conjugative coupling protein

Bacterial conjugative systems code for an essential membrane protein that couples the relaxosome to the DNA transport apparatus, called type IV coupling protein (T4CP). TrwB is the T4CP of the conjugative plasmid R388. In earlier work we found that this protein, purified in the presence of detergents, binds preferentially purine nucleotides trisphosphate. In contrast a soluble truncated mutant TrwBΔN70 binds uniformly all nucleotides tested. In this work, TrwB has been successfully reconstituted into liposomes. The non-membranous portion of the protein is almost exclusively oriented towards the outside of the vesicles. Functional analysis of TrwB proteoliposomes demonstrates that when the protein is inserted into the lipid bilayer the affinity for adenine and guanine nucleotides is enhanced as compared to that of the protein purified in detergent or to the soluble deletion mutant, TrwBΔN70. The protein specificity for adenine nucleotides is also increased. No ATPase activity has been found in TrwB reconstituted in proteoliposomes. This result suggests that the N-terminal transmembrane segment of this T4CP interferes with its ATPase activity and can be taken to imply that the TrwB transmembrane domain plays a regulatory role in its biological activity.

In vivo oligomerization of the F conjugative coupling protein TraD

Type IV secretory systems are a group of bacterial transporters responsible for the transport of proteins and nucleic acids directly into recipient cells. Such systems play key roles in the virulence of some pathogenic organisms and in conjugation-mediated horizontal gene transfer. Many type IV systems require conserved "coupling proteins," transmembrane polypeptides that are critical for transporting secreted substrates across the cytoplasmic membrane of the bacterium. In vitro evidence suggests that the functional form of coupling proteins is a homohexameric, ring-shaped complex. Using a library of tagged mutants, we investigated the structural and functional organization of the F plasmid conjugative coupling protein TraD by coimmunoprecipitation, cross-linking, and genetic means. We present direct evidence that coupling proteins form stable oligomeric complexes in the membranes of bacteria and that the formation of some of these complexes requires other F-encoded functions. Our data also show that different regions of TraD play distinct roles in the oligomerization process. We postulate a model for in vivo oligomerization and discuss the probable participation of individual domains of TraD in each step.

Identification of the origin of transfer (oriT) and DNA relaxase required for conjugation of the integrative and conjugative element ICEBs1 of Bacillus subtilis

Integrative and conjugative elements (ICEs), also known as conjugative transposons, are mobile genetic elements that can transfer from one bacterial cell to another by conjugation. ICEBs1 is integrated into the trnS-leu2 gene of Bacillus subtilis and is regulated by the SOS response and the RapI-PhrI cell-cell peptide signaling system. When B. subtilis senses DNA damage or high concentrations of potential mating partners that lack the element, ICEBs1 excises from the chromosome and can transfer to recipients. Bacterial conjugation usually requires a DNA relaxase that nicks an origin of transfer (oriT) on the conjugative element and initiates the 5'-to-3' transfer of one strand of the element into recipient cells. The ICEBs1 ydcR (nicK) gene product is homologous to the pT181 family of plasmid DNA relaxases. We found that transfer of ICEBs1 requires nicK and identified a cis-acting oriT that is also required for transfer. Expression of nicK leads to nicking of ICEBs1 between a GC-rich inverted repeat in oriT, and NicK was the only ICEBs1 gene product needed for nicking. NicK likely mediates conjugation of ICEBs1 by nicking at oriT and facilitating the translocation of a single strand of ICEBs1 DNA through a transmembrane conjugation pore.

The transmembrane domain provides nucleotide binding specificity to the bacterial conjugation protein TrwB

In order to understand the functional significance of the transmembrane domain of TrwB, an integral membrane protein involved in bacterial conjugation, the protein was purified in the native, and also as a truncated soluble form (TrwBDeltaN70). The intact protein (TrwB) binds preferentially purine over pyrimidine nucleotides, NTPs over NDPs, and ribo- over deoxyribonucleotides. In contrast, TrwBDeltaN70 binds uniformly all tested nucleotides. The transmembrane domain has the general effect of making the nucleotide binding site(s) less accessible, but more selective. This is in contrast to other membrane proteins in which most of the protein mass, including the catalytic domain, is outside the membrane, but whose activity is not modified by the presence or absence of the transmembrane segment.

Mutations in the C-terminal region of TraM provide evidence for in vivo TraM-TraD interactions during F-plasmid conjugation

Conjugation is a major mechanism for disseminating genetic information in bacterial populations, but the signal that triggers it is poorly understood in gram-negative bacteria. F-plasmid-mediated conjugation requires TraM, a homotetramer, which binds cooperatively to three binding sites within the origin of transfer. Using in vitro assays, TraM has previously been shown to interact with the coupling protein TraD. Here we present evidence that F conjugation also requires TraM-TraD interactions in vivo. A three-plasmid system was used to select mutations in TraM that are defective for F conjugation but competent for tetramerization and cooperative DNA binding to the traM promoter region. One mutation, K99E, was particularly defective in conjugation and was further characterized by affinity chromatography and coimmunoprecipitation assays that suggested it was defective in interacting with TraD. A C-terminal deletion (S79*, where the asterisk represents a stop codon) and a missense mutation (F121S), which affects tetramerization, also reduced the affinity of TraM for TraD. We propose that the C-terminal region of TraM interacts with TraD, whereas its N-terminal domain is involved in DNA binding. This arrangement of functional domains could in part allow TraM to receive the mating signal generated by donor-recipient contact and transfer it to the relaxosome, thereby triggering DNA transfer.

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.

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.

Purification and properties of TrwB, a hexameric, ATP-binding integral membrane protein essential for R388 plasmid conjugation

TrwB is an integral membrane protein linking the relaxosome to the DNA transport apparatus in plasmid R388 conjugation. Native TrwB has been purified in monomeric and hexameric forms, in the presence of dodecylmaltoside from overexpressing bacterial cells. A truncated protein (TrwBDeltaN70) that lacked the transmembrane domain could be purified only in the monomeric form. Electron microscopy images revealed the hexameric structure and were in fact superimposable to the previously published atomic structure for TrwBDeltaN70. In addition, the electron micrographs showed an appendix, approximately 25 A wide, corresponding to the transmembrane region of TrwB. TrwB was located in the bacterial inner membrane in agreement with its proposed coupling role. Purified TrwB hexamers and monomers bound tightly the fluorescent ATP analogue TNP-ATP. A mutant in the Walker A motif, TrwB-K136T, was equally purified and found to bind TNP-ATP with a similar affinity to that of the wild type. However, the TNP-ATP affinity of TrwBDeltaN70 was significantly reduced in comparison with the TrwB hexamers. Competition experiments in which ATP was used to displace TNP-ATP gave an estimate of ATP binding by TrwB (K(d)((ATP)) = 0.48 mm for hexamers). The transmembrane domain appears to be involved in TrwB protein hexamerization and also influences its nucleotide binding properties.

Bacterial conjugation: a two-step mechanism for DNA transport

Bacterial conjugation is a promiscuous DNA transport mechanism. Conjugative plasmids transfer themselves between most bacteria, thus being one of the main causal agents of the spread of antibiotic resistance among pathogenic bacteria. Moreover, DNA can be transferred conjugatively into eukaryotic host cells. In this review, we aim to address several basic questions regarding the DNA transfer mechanism. Conjugation can be visualized as a DNA rolling-circle replication (RCR) system linked to a type IV secretion system (T4SS), the latter being macromolecular transporters widely involved in pathogenic mechanisms. The scheme 'replication + secretion' suggests how the mechanism would work on the DNA substrate and at the bacterial membrane. But, how do these two parts come into contact? Furthermore, how is the DNA transported? T4SS are known to be involved in protein secretion in different organisms, but DNA is a very different macromolecule. The so-called coupling proteins could be the answer to both questions by performing a dual role in conjugation: coupling the two main components of the machinery (RCR and T4SS) and actively mediating DNA transport. We postulate that the T4SS is responsible for transport of the pilot protein (the relaxase) to the recipient. The DNA that is covalently linked to it is initially transported in a passive manner, trailing on the relaxase. We speculate that the pilus appendage could work as a needle, thrusting the substrate proteins to cross one or several membrane barriers into the recipient cytoplasm. This is the first step in conjugation. The second step is the active pumping of the DNA to the recipient, using the already available T4SS transport conduit. It is proposed that this second step is catalysed by the coupling proteins. Our 'shoot and pump' model solves the protein-DNA transport paradox of T4SS.

Structure and role of coupling proteins in conjugal DNA transfer

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

TraG-like proteins of DNA transfer systems and of the Helicobacter pylori type IV secretion system: inner membrane gate for exported substrates?

TraG-like proteins are potential NTP hydrolases (NTPases) that are essential for DNA transfer in bacterial conjugation. They are thought to mediate interactions between the DNA-processing (Dtr) and the mating pair formation (Mpf) systems. TraG-like proteins also function as essential components of type IV secretion systems of several bacterial pathogens such as Helicobacter pylori. Here we present the biochemical characterization of three members of the family of TraG-like proteins, TraG (RP4), TraD (F), and HP0524 (H. pylori). These proteins were found to have a pronounced tendency to form oligomers and were shown to bind DNA without sequence specificity. Standard NTPase assays indicated that these TraG-like proteins do not possess postulated NTP-hydrolyzing activity. Surface plasmon resonance was used to demonstrate an interaction between TraG and relaxase TraI of RP4. Topology analysis of TraG revealed that TraG is a transmembrane protein with cytosolic N and C termini and a short periplasmic domain close to the N terminus. We predict that multimeric inner membrane protein TraG forms a pore. A model suggesting that the relaxosome binds to the TraG pore via TraG-DNA and TraG-TraI interactions is presented.

Interaction between the RP4 coupling protein TraG and the pBHR1 mobilization protein Mob

It is currently believed that interaction between the relaxosome of a mobilizable plasmid and the transfer machinery of the helper conjugative plasmid is mediated by a TraG family coupling protein. The coupling proteins appear as an essential determinant of mobilization specificity and efficiency. Using a two-hybrid system, we demonstrated for the first time the direct in vivo interaction between the coupling protein of a conjugative plasmid (the TraG protein of RP4) and the relaxase of a mobilizable plasmid (the Mob protein of pBHR1, a derivative of the broad host range plasmid pBBR1). This interaction was confirmed in vitro by an overlay assay and was shown to occur even in the absence of the transfer origin of pBHR1. We showed that, among 11 conjugative plasmids tested, pBHR1 is efficiently mobilized only by plasmids encoding an IncP-type transfer system. We also showed that the RP4 TraG coupling protein is essential for mobilization of a pBBR1 derivative and is the element that allows its mobilization by R388 plasmid (IncW) at a detectable frequency.

Mobilization of chimeric oriT plasmids by F and R100-1: role of relaxosome formation in defining plasmid specificity

Cleavage at the F plasmid nic site within the origin of transfer (oriT) requires the F-encoded proteins TraY and TraI and the host-encoded protein integration host factor in vitro. We confirm that F TraY, but not F TraM, is required for cleavage at nic in vivo. Chimeric plasmids were constructed which contained either the entire F or R100-1 oriT regions or various combinations of nic, TraY, and TraM binding sites, in addition to the traM gene. The efficiency of cleavage at nic and the frequency of mobilization were assayed in the presence of F or R100-1 plasmids. The ability of these chimeric plasmids to complement an F traM mutant or affect F transfer via negative dominance was also measured using transfer efficiency assays. In cases where cleavage at nic was detected, R100-1 TraI was not sensitive to the two-base difference in sequence immediately downstream of nic, while F TraI was specific for the F sequence. Plasmid transfer was detected only when TraM was able to bind to its cognate sites within oriT. High-affinity binding of TraY in cis to oriT allowed detection of cleavage at nic but was not required for efficient mobilization. Taken together, our results suggest that stable relaxosomes, consisting of TraI, -M, and -Y bound to oriT are preferentially targeted to the transfer apparatus (transferosome).

Characterization of ATP and DNA binding activities of TrwB, the coupling protein essential in plasmid R388 conjugation

TrwB is the conjugative coupling protein of plasmid R388. TrwBDeltaN70 contains the soluble domain of TrwB. It was constructed by deletion of trwB sequences containing TrwB N-proximal transmembrane segments. Purified TrwBDeltaN70 protein bound tightly the fluorescent ATP analogue TNP-ATP (K(s) = 8.7 microM) but did not show measurable ATPase or GTPase activity. A single ATP binding site was found per TrwB monomer. An intact ATP-binding site was essential for R388 conjugation, since a TrwB mutant with a single amino acid alteration in the ATP-binding signature (K136T) was transfer-deficient. TrwBDeltaN70 also bound DNA nonspecifically. DNA binding enhanced TrwC nic cleavage, providing the first evidence that directly links TrwB with conjugative DNA processing. Since DNA bound by TrwBDeltaN70 also showed increased negative superhelicity (as shown by increased sensitivity to topoisomerase I), nic cleavage enhancement was assumed to be a consequence of the increased single-stranded nature of DNA around nic. The mutant protein TrwB(K136T)DeltaN70 was indistinguishable from TrwBDeltaN70 with respect to the above properties, indicating that TrwB ATP binding activity is not required for them. The reported properties of TrwB suggest potential functions for conjugative coupling proteins, both as triggers of conjugative DNA processing and as motors in the transport process.

Analysis of F factor TraD membrane topology by use of gene fusions and trypsin-sensitive insertions

This report describes a procedure for characterizing membrane protein topology which combines the analysis of reporter protein hybrids and trypsin-sensitive 31-amino-acid insertions generated by using transposons ISphoA/in and ISlacZ/in. Studies of the F factor TraD protein imply that the protein takes on a structure with two membrane-spanning sequences and amino and carboxyl termini facing the cytoplasm. It was possible to assign the subcellular location of one region for which the behavior of fused reporter proteins was ambiguous, based on the trypsin cleavage behavior of a 31-residue insertion.

Genetic analysis of the mobilization and leading regions of the IncN plasmids pKM101 and pCU1

The conjugative IncN plasmids pKM101 and pCU1 have previously been shown to contain identical oriT sequences as well as conserved restriction endonuclease cleavage patterns within their tra regions. Complementation analysis and sequence data presented here indicate that these two plasmids encode essentially identical conjugal DNA-processing proteins. This region contains three genes, traI, traJ, and traK, transcribed in the same orientation from a promoter that probably lies within or near the conjugal transfer origin (oriT). Three corresponding proteins were visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and complementation analysis confirmed that this region contains three tra complementation groups. All three proteins resemble proteins of the IncW plasmid R388 and other plasmids thought to have roles in processing of plasmid DNA during conjugation. The hydropathy profile of TraJ suggests a transmembrane topology similar to that of several homologous proteins. Both traK and traI were required for efficient interplasmid site-specific recombination at oriT, while traJ was not required. The leading region of pKM101 contains three genes (stbA, stbB, and stbC), null mutations in which cause elevated levels of plasmid instability. Plasmid instability was observed only in hosts that are proficient in interplasmid recombination, suggesting that this recombination can potentially lead to plasmid loss and that Stb proteins somehow overcome this, possibly via site-specific multimer resolution.

The cytoplasmic DNA-binding protein TraM binds to the inner membrane protein TraD in vitro

The cytoplasmic protein TraM is one of four essential gene products of the F factor which are involved in DNA transfer after mating pair formation. TraM binds to three specific sites within the oriT region. Besides regulation of its own synthesis, the precise function of TraM during conjugation is not yet known. In the present work, the affinity of TraM to TraD was studied in vitro by an overlay assay and by affinity chromatography. Whether the interaction between TraM and TraD causes a transient or permanent anchoring of the F factor to the site of transfer is discussed. A 35-kDa host membrane protein of yet unknown function also shows affinity to TraM and may be involved in this anchoring process as well.

VirE1 protein mediates export of the single-stranded DNA-binding protein VirE2 from Agrobacterium tumefaciens into plant cells

Agrobacterium tumefaciens transfers single-stranded DNAs (T strands) into plant cells. VirE1 and VirE2, which is a single-stranded DNA binding protein, are important for tumorigenesis. We show that T strands and VirE2 can enter plant cells independently and that export of VirE2, but not of T strands, depends on VirE1.

Bioenergetic aspects of the translocation of macromolecules across bacterial membranes

Bacteria are extremely versatile in the sense that they have gained the ability to transport all three major classes of biopolymers through their cell envelope: proteins, nucleic acids, and polysaccharides. These macromolecules are translocated across membranes in a large number of cellular processes by specific translocation systems. Members of the ABC (ATP binding cassette) superfamily of transport ATPases are involved in the translocation of all three classes of macromolecules, in addition to unique transport ATPases. An intriguing aspect of these transport processes is that the barrier function of the membrane is preserved despite the fact the dimensions of the translocated molecules by far surpasses the thickness of the membrane. This raises questions like: How are these polar compounds translocated across the hydrophobic interior of the membrane, through a proteinaceous pore or through the lipid phase; what drives these macromolecules across the membrane; which energy sources are used and how is unidirectionality achieved? It is generally believed that macromolecules are translocated in a more or less extended, most likely linear form. A recurring theme in the bioenergetics of these translocation reactions in bacteria is the joint involvement of free energy input in the form of ATP hydrolysis and via proton sym- or antiport, driven by a proton gradient. Important similarities in the bioenergetic mechanisms of the translocation of these biopolymers therefore may exist.

Analysis of a transfer region from the staphylococcal conjugative plasmid pSK41

The nucleotide sequence of a 14.4-kb region (tra) associated with DNA transfer of the staphylococcal conjugative plasmid, pSK41, has been determined. Analysis of the sequence revealed the presence of 15 genes potentially involved in the conjugative process. Polypeptide products likely to correspond to ten of these genes have been identified, of which one was found to be a lipoprotein. Comparison of the deduced tra products to the protein databases revealed several interesting similarities, one of which suggests an evolutionary link between this Gram+ bacterial conjugation system and DNA transfer systems of Gram- bacteria, such as Escherichia coli and Agrobacterium tumefaciens. The nt sequence also provided an insight into the transcriptional organisation and regulation of the region.

The mating pair formation system of plasmid RP4 defined by RSF1010 mobilization and donor-specific phage propagation

Transfer functions of the conjugative plasmid RP4 (IncP alpha) are distributed among distinct regions of the genome, designated Tra1 and Tra2. By deletion analyses, we determined the limits of the Tra1 region, essential for intraspecific Escherichia coli matings. The Tra1 core region encompasses approximately 5.8 kb, including the genes traF, -G, -H, -I, -J, and -K as well as the origin of transfer. The traM gene product, however, is not absolutely required for conjugation but significantly increases transfer efficiency. To determine the transfer phenotype of genes encoded by the Tra2 core region, we generated a series of defined Tra2 mutants. This revealed that at least trbB, -C, -E, -G, and -L are essential for RP4 conjugation. To classify these transfer functions as components of the DNA transfer and replication (Dtr) or of the mating pair formation (Mpf) system, we analyzed the corresponding derivatives with respect to mobilization of IncQ plasmids and donor-specific phage propagation. We found that all of the Tra2 genes listed above and the traG and traF genes of Tra1 are required for RSF1010 mobilization. Expression of traF from Tra1 in conjunction with the Tra2 core was sufficient for phage propagation. This implies that the TraG protein is not directly involved in pilus formation and potentially connects the relaxosome with proteins enabling the membrane passage of the DNA. The proposed roles of the RP4 transfer gene products are discussed in the context of virulence functions encoded by the evolutionarily related Ti T-DNA transfer system of agrobacteria.

The Agrobacterium tumefaciens virB4 gene product is an essential virulence protein requiring an intact nucleoside triphosphate-binding domain

Products of the approximately 9.5-kb virB operon are proposed to direct the export of T-DNA/protein complexes across the Agrobacterium tumefaciens envelope en route to plant cells. The presence of conserved nucleoside triphosphate (NTP)-binding domains in VirB4 and VirB11 suggests that one or both proteins couple energy, via NTP hydrolysis, to T-complex transport. To assess the importance of VirB4 for virulence, a nonpolar virB4 null mutation was introduced into the pTiA6NC plasmid of strain A348. The 2.37-kb virB4 coding sequence was deleted precisely by oligonucleotide-directed mutagenesis in vitro. The resulting delta virB4 mutation was exchanged for the wild-type allele by two sequential recombination events with the counterselectable Bacillus subtilis sacB gene. Two derivatives, A348 delta B4.4 and A348 delta B4.5, sustained a nonpolar deletion of the wild-type virB4 allele, as judged by Southern blot hybridization and immunoblot analyses with antibodies specific for VirB4, VirB5, VirB10, and VirB11. Transcription of wild-type virB4 from the lac promoter restored virulence to the nonpolar null mutants on a variety of dicotyledonous species, establishing virB4 as an essential virulence gene. A substitution of glutamine for Lys-439 and a deletion of Gly-438, Lys-439, and Thr-440 within the glycine-rich NTP-binding domain (Gly-Pro-Iso-Gly-Arg-Gly-Lys-Thr) abolished complementation of A348 delta B4.4 or A348 delta B4.5, demonstrating that an intact NTP-binding domain is critical for VirB4 function. Merodiploids expressing both the mutant and wild-type virB4 alleles exhibited lower virulence than A348, suggesting that VirB4, a cytoplasmic membrane protein, may contribute as a homo- or heteromultimer to A. tumefaciens virulence.

Sequence alterations affecting F plasmid transfer gene expression: a conjugation system dependent on transcription by the RNA polymerase of phage T7

We constructed derivatives of the Escherichia coli conjugative plasmid F that carry altered sequences in place of the major transfer operon promoter, PY. Replacement of PY with a promoter-deficient sequence resulted in a transfer-deficient, F-pilus-specific phage-resistant plasmid (pOX38-tra701) that could still express TraJ and TraT; TraY, F-pilin, TraD, and TraI were not detectable on Western blots. On a second plasmid (pOX38-tra715) we replaced PY with a phage T7 late promoter sequence. In hosts carrying a lacUV5-promoter-regulated T7 RNA polymerase gene, all transfer-associated properties of pOX38-tra715 could be regulated with IPTG. After induction, pOX38-tra715 transferred at the wild-type frequency, expressed normal numbers of F-pili and conferred sensitivity to pilus-specific phages. No adverse effects on cell viability were apparent, and additional mutations could easily be crossed onto pOX38-tra715. A traJ deletion (pOX38-tra716) had no effect on the IPTG-induced transfer phenotype. Insertion of cam into trbC, resulted in a mutant (pOX38-tra715trbC33) which, after induction, exhibited the same phenotype associated with other trbC mutants; it could also be complemented by expression of trbC in trans. With pOX38-tra715 or its derivatives, we were able to label specifically the products of tra genes located throughout the long tra operon, by using rifampicin. This feature can be used to investigate transfer protein interactions and to follow changes in these proteins that are associated with conjugal mating events.