The role of the pilus in recipient cell recognition during bacterial conjugation mediated by F-like plasmids

Structural insights into the disulfide isomerase and chaperone activity of TrbB of the F plasmid type IV secretion system

Bacteria have evolved elaborate mechanisms to thrive in stressful environments. F-like plasmids in gram-negative bacteria encode for a multi-protein Type IV Secretion System (T4SSF) that is functional for bacterial proliferation and adaptation through the process of conjugation. The periplasmic protein TrbB is believed to have a stabilizing chaperone role in the T4SSF assembly, with TrbB exhibiting disulfide isomerase (DI) activity. In the current report, we demonstrate that the deletion of the disordered N-terminus of TrbBWT, resulting in a truncation construct TrbB37-161, does not affect its catalytic in vitro activity compared to the wild-type protein (p = 0.76). Residues W37-K161, which include the active thioredoxin motif, are sufficient for DI activity. The N-terminus of TrbBWT is disordered as indicated by a structural model of GST-TrbBWT based on ColabFold-AlphaFold2 and Small Angle X-Ray Scattering data and 1H-15N Heteronuclear Single Quantum Correlation (HSQC) spectroscopy of the untagged protein. This disordered region likely contributes to the protein's dynamicity; removal of this region results in a more stable protein based on 1H-15N HSQC and Circular Dichroism Spectroscopies. Lastly, size exclusion chromatography analysis of TrbBWT in the presence of TraW, a T4SSF assembly protein predicted to interact with TrbBWT, does not support the inference of a stable complex forming in vitro. This work advances our understanding of TrbB's structure and function, explores the role of structural disorder in protein dynamics in the context of a T4SSF accessory protein, and highlights the importance of redox-assisted protein folding in the T4SSF.

Systematic investigation of recipient cell genetic requirements reveals important surface receptors for conjugative transfer of IncI2 plasmids

Bacterial conjugation is a major horizontal gene transfer mechanism. While the functions encoded by many conjugative plasmids have been intensively studied, the contribution of recipient chromosome-encoded genes remains largely unknown. Here, we analyzed the genetic requirement of recipient cells for conjugation of IncI2 plasmid TP114, which was recently shown to transfer at high rates in the gut microbiota. We performed transfer assays with ~4,000 single-gene deletion mutants of Escherichia coli. When conjugation occurs on a solid medium, we observed that recipient genes impairing transfer rates were not associated with a specific cellular function. Conversely, transfer assays performed in broth were largely dependent on the lipopolysaccharide biosynthesis pathway. We further identified specific structures in lipopolysaccharides used as recipient cell surface receptors by PilV adhesins associated with the type IVb accessory pilus of TP114. Our strategy is applicable to study other mobile genetic elements and understand important host cell factors for their dissemination.

Plasmids pick a bacterial partner before committing to conjugation

Bacterial conjugation was first described by Lederberg and Tatum in the 1940s following the discovery of the F plasmid. During conjugation a plasmid is transferred unidirectionally from one bacterium (the donor) to another (the recipient), in a contact-dependent manner. Conjugation has been regarded as a promiscuous mechanism of DNA transfer, with host range determined by the recipient downstream of plasmid transfer. However, recent data have shown that F-like plasmids, akin to tailed Caudovirales bacteriophages, can pick their host bacteria prior to transfer by expressing one of at least four structurally distinct isoforms of the outer membrane protein TraN, which has evolved to function as a highly sensitive sensor on the donor cell surface. The TraN sensor appears to pick bacterial hosts by binding compatible outer membrane proteins in the recipient. The TraN variants can be divided into specialist and generalist sensors, conferring narrow and broad plasmid host range, respectively. In this review we discuss recent advances in our understanding of the function of the TraN sensor at the donor-recipient interface, used by F-like plasmids to select bacterial hosts within polymicrobial communities prior to DNA transfer.

The F-pilus biomechanical adaptability accelerates conjugative dissemination of antimicrobial resistance and biofilm formation

Conjugation is used by bacteria to propagate antimicrobial resistance (AMR) in the environment. Central to this process are widespread conjugative F-pili that establish the connection between donor and recipient cells, thereby facilitating the spread of IncF plasmids among enteropathogenic bacteria. Here, we show that the F-pilus is highly flexible but robust at the same time, properties that increase its resistance to thermochemical and mechanical stresses. By a combination of biophysical and molecular dynamics methods, we establish that the presence of phosphatidylglycerol molecules in the F-pilus contributes to the structural stability of the polymer. Moreover, this structural stability is important for successful delivery of DNA during conjugation and facilitates rapid formation of biofilms in harsh environmental conditions. Thus, our work highlights the importance of F-pilus structural adaptations for the efficient spread of AMR genes in a bacterial population and for the formation of biofilms that protect against the action of antibiotics.

Cryo-EM structure of the Agrobacteriumtumefaciens T-pilus reveals the importance of positive charges in the lumen

Agrobacterium tumefaciens is a natural genetic engineer that transfers DNA into plants, which is the most applied process for generation of genetically modified plants. DNA transfer is mediated by a type IV secretion system in the cell envelope and extracellular T-pili. We here report the cryo-electron microscopic structures of the T-pilus at 3.2-Å resolution and of the plasmid pKM101-determined N-pilus at 3-Å resolution. Both pili contain a main pilus protein (VirB2 in A. tumefaciens, TraM in pKM101) and phospholipids arranged in a five-start helical assembly. They contain positively charged amino acids in the lumen, and the lipids are positively charged in the T-pilus (phosphatidylcholine) conferring overall positive charge. Mutagenesis of the lumen-exposed Arg91 in VirB2 results in protein destabilization and loss of pilus formation. Our results reveal that different phospholipids can be incorporated into type IV secretion pili and that the charge of the lumen may be of functional importance.

Solution characterization of the dynamic conjugative entry exclusion protein TraG

The R100 plasmid and the secretion system it encodes are representative of F-like conjugative type IV secretion systems for the transmission of mobile DNA elements in gram-negative bacteria, serving as a major contributor to the spread of antibiotic resistance in bacterial pathogens. The TraG protein of F-like systems consists of a membrane-bound N-terminal domain and a periplasmic C-terminal domain, denoted TraG*. TraG* is essential in preventing redundant DNA transfer through a process termed entry exclusion. In the donor cell, it interacts with TraN to facilitate mating pair stabilization; however, if a mating pore forms between bacteria with identical plasmids, TraG* interacts with its cognate TraS in the inner membrane of the recipient bacterium to prevent redundant donor-donor conjugation. Structural studies of TraG* from the R100 plasmid have revealed the presence of a dynamic region between the N- and C-terminal domains of TraG. Thermofluor, circular dichroism, collision-induced unfolding-mass spectrometry, and size exclusion chromatography linked to multiangle light scattering and small angle x-ray scattering experiments indicated an N-terminal truncation mutant displayed higher stability and less disordered content relative to full-length TraG*. The 45 N-terminal residues of TraG* are hypothesized to serve as part of a flexible linker between the two independently functioning domains.

Conjugation of a Hybrid Plasmid Encoding Hypervirulence and Carbapenem Resistance in Klebsiella pneumoniae of Sequence Type 592

Klebsiella pneumoniae simultaneously carrying genes encoding carbapenem resistance and hypervirulence causes fatal infections, representing a severe threat to human health. These carbapenem-resistant and hypervirulent K. pneumoniae (hvCRKP) strains are increasingly reported worldwide and have been found to belong to a variety of sequence types (STs). In this study, we report and characterized an hvCRKP strain of ST592, an uncommon ST, which caused a fatal infection in intensive care unit (ICU) in China and represents a novel type of hvCRKP. We demonstrated that this novel hvCRKP type emerged from the carbapenem-susceptible hypervirulent K. pneumoniae (hvKP) lineage of the K57 capsular type. K57 hvKP contains a pLVPK-like virulence plasmid and then acquired a conjugative bla_KPC–2-carrying plasmid to form hvCRKP. The pLVPK-like virulence plasmid contains no complete conjugation module but was able to be transferred by fusion with the conjugative bla_KPC–2-carrying plasmid during conjugation. This represents a new mechanism of simultaneous transfer genetic determinants of carbapenem resistance and virulence and highlights the undergoing expansion of hvCRKP, which requires rigorous monitoring and novel countermeasures to curb plasmid-mediated transmission.

Mobilization of the nonconjugative virulence plasmid from hypervirulent Klebsiella pneumoniae

Background

Klebsiella pneumoniae, as a global priority pathogen, is well known for its capability of acquiring mobile genetic elements that carry resistance and/or virulence genes. Its virulence plasmid, previously deemed nonconjugative and restricted within hypervirulent K. pneumoniae (hvKP), has disseminated into classic K. pneumoniae (cKP), particularly carbapenem-resistant K. pneumoniae (CRKP), which poses alarming challenges to public health. However, the mechanism underlying its transfer from hvKP to CRKP is unclear.

Methods

A total of 28 sequence type (ST) 11 bloodstream infection-causing CRKP strains were collected from Ruijin Hospital in Shanghai, China, and used as recipients in conjugation assays. Transconjugants obtained from conjugation assays were confirmed by XbaI and S1 nuclease pulsed-field gel electrophoresis, PCR detection and/or whole-genome sequencing. The plasmid stability of the transconjugants was evaluated by serial culture. Genetically modified strains and constructed mimic virulence plasmids were employed to investigate the mechanisms underlying mobilization. The level of extracellular polysaccharides was measured by mucoviscosity assays and uronic acid quantification. An in silico analysis of 2608 plasmids derived from 814 completely sequenced K. pneumoniae strains available in GenBank was performed to investigate the distribution of putative helper plasmids and mobilizable virulence plasmids.

Results

A nonconjugative virulence plasmid was mobilized by the conjugative plasmid belonging to incompatibility group F (IncF) from the hvKP strain into ST11 CRKP strains under low extracellular polysaccharide-producing conditions or by employing intermediate E. coli strains. The virulence plasmid was mobilized via four modes: transfer alone, cotransfer with the conjugative IncF plasmid, hybrid plasmid formation due to two rounds of single-strand exchanges at specific 28-bp fusion sites or homologous recombination. According to the in silico analysis, 31.8% (242) of the putative helper plasmids and 98.8% (84/85) of the virulence plasmids carry the 28-bp fusion site. All virulence plasmids carry the origin of the transfer site.

Conclusions

The nonconjugative virulence plasmid in ST11 CRKP strains is putatively mobilized from hvKP or E. coli intermediates with the help of conjugative IncF plasmids. Our findings emphasize the importance of raising public awareness of the rapid dissemination of virulence plasmids and the consistent emergence of hypervirulent carbapenem-resistant K. pneumoniae (hv-CRKP) strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13073-021-00936-5.

A High-Throughput Method for Screening for Genes Controlling Bacterial Conjugation of Antibiotic Resistance

The rapid transmission of antibiotic resistance genes on conjugative plasmids between bacterial host cells is a major cause of the accelerating antibiotic resistance crisis. There are currently no experimental platforms for fast and cost-efficient screening of genetic effects on antibiotic resistance transmission by conjugation, which prevents understanding and targeting conjugation.

The rapid horizontal transmission of antibiotic resistance genes on conjugative plasmids between bacterial host cells is a major cause of the accelerating antibiotic resistance crisis. There are currently no experimental platforms for fast and cost-efficient screening of genetic effects on antibiotic resistance transmission by conjugation, which prevents understanding and targeting conjugation. We introduce a novel experimental framework to screen for conjugation-based horizontal transmission of antibiotic resistance between >60,000 pairs of cell populations in parallel. Plasmid-carrying donor strains are constructed in high-throughput. We then mix the resistance plasmid-carrying donors with recipients in a design where only transconjugants can reproduce, measure growth in dense intervals, and extract transmission times as the growth lag. As proof-of-principle, we exhaustively explore chromosomal genes controlling F-plasmid donation within Escherichia coli populations, by screening the Keio deletion collection in high replication. We recover all seven known chromosomal gene mutants affecting conjugation as donors and identify many novel mutants, all of which diminish antibiotic resistance transmission. We validate nine of the novel genes’ effects in liquid mating assays and complement one of the novel genes’ effect on conjugation (rseA). The new framework holds great potential for exhaustive disclosing of candidate targets for helper drugs that delay resistance development in patients and societies and improve the longevity of current and future antibiotics. Further, the platform can easily be adapted to explore interspecies conjugation, plasmid-borne factors, and experimental evolution and be used for rapid construction of strains.

IMPORTANCE The rapid transmission of antibiotic resistance genes on conjugative plasmids between bacterial host cells is a major cause of the accelerating antibiotic resistance crisis. There are currently no experimental platforms for fast and cost-efficient screening of genetic effects on antibiotic resistance transmission by conjugation, which prevents understanding and targeting conjugation. We introduce a novel experimental framework to screen for conjugation-based horizontal transmission of antibiotic resistance between >60,000 pairs of cell populations in parallel. As proof-of-principle, we exhaustively explore chromosomal genes controlling F-plasmid donation within E. coli populations. We recover all previously known and many novel chromosomal gene mutants that affect conjugation efficiency. The new framework holds great potential for rapid screening of compounds that decrease transmission. Further, the platform can easily be adapted to explore interspecies conjugation, plasmid-borne factors, and experimental evolution and be used for rapid construction of strains.

Plasmid Transfer by Conjugation in Gram-Negative Bacteria: From the Cellular to the Community Level

Bacterial conjugation, also referred to as bacterial sex, is a major horizontal gene transfer mechanism through which DNA is transferred from a donor to a recipient bacterium by direct contact. Conjugation is universally conserved among bacteria and occurs in a wide range of environments (soil, plant surfaces, water, sewage, biofilms, and host-associated bacterial communities). Within these habitats, conjugation drives the rapid evolution and adaptation of bacterial strains by mediating the propagation of various metabolic properties, including symbiotic lifestyle, virulence, biofilm formation, resistance to heavy metals, and, most importantly, resistance to antibiotics. These properties make conjugation a fundamentally important process, and it is thus the focus of extensive study. Here, we review the key steps of plasmid transfer by conjugation in Gram-negative bacteria, by following the life cycle of the F factor during its transfer from the donor to the recipient cell. We also discuss our current knowledge of the extent and impact of conjugation within an environmentally and clinically relevant bacterial habitat, bacterial biofilms.

Protein Dynamics in F-like Bacterial Conjugation

Efficient in silico development of novel antibiotics requires high-resolution, dynamic models of drug targets. As conjugation is considered the prominent contributor to the spread of antibiotic resistance genes, targeted drug design to disrupt vital components of conjugative systems has been proposed to lessen the proliferation of bacterial antibiotic resistance. Advancements in structural imaging techniques of large macromolecular complexes has accelerated the discovery of novel protein-protein interactions in bacterial type IV secretion systems (T4SS). The known structural information regarding the F-like T4SS components and complexes has been summarized in the following review, revealing a complex network of protein-protein interactions involving domains with varying degrees of disorder. Structural predictions were performed to provide insight on the dynamicity of proteins within the F plasmid conjugative system that lack structural information.

Cryoelectron-Microscopic Structure of the pKpQIL Conjugative Pili from Carbapenem-Resistant Klebsiella pneumoniae

Conjugative pili are important in mediating bacterial conjugation and horizontal gene transfer. Since plasmid transfer can include antibiotic-resistance genes, conjugation is an important mechanism in the spread of antibiotic resistance. Filamentous bacteriophages have been shown to exist in two different structural classes: those with a 5-fold rotational symmetry and those with a one-start helix with approximately 5 subunits per turn. Structures for the F and the F-like pED208 conjugation pilus have shown that they have 5-fold rotational symmetry. Here, we report the cryoelectron-microscopic structure of conjugative pili from carbapenem-resistant Klebsiella pneumoniae, encoded on the IncFIIK pKpQIL plasmid, at 3.9 Å resolution and show that it has a one-start helix. These results establish that conjugation pili can exist in at least two structural classes, consistent with other results showing that relatively small perturbations are needed to change the helical symmetry of polymers.

Correlation between Exogenous Compounds and the Horizontal Transfer of Plasmid-Borne Antibiotic Resistance Genes

The global spread of antibiotic resistance has posed a serious threat to public healthcare and undermined decades of progress made in the fight against bacterial infections. It has been demonstrated that the lack of novel effective antibiotics and rapid spread of antibiotic resistance genes via horizontal transfer in the ecosystem are mainly responsible for this crisis. Notably, plasmid-mediated horizontal transfer of antibiotic resistance genes (ARGs) is recognized as the most dominant dissemination pathway of ARGs in humans, animals and environmental settings. Antibiotic selective pressure has always been regarded as one of the crucial contributors to promoting the dissemination of antibiotic resistance through horizontal gene transfer (HGT). However, the roles of exogenous compounds and particularly non-antibiotic drugs in the spread of ARGs are still underappreciated. In this review, we first summarize the major pathways of HGT in bacteria, including conjugation, transformation, transduction and vesiduction. Subsequently, an overview of these compounds capable of promoting the HGT is presented, which guides to the formulation of more reasonable dosing regimens and drug residue standards in clinical practice. By contrast, these compounds that display an inhibition effect on HGT are also highlighted, which provides a unique strategy to minimize the spread of ARGs. Lastly, we discuss the implementations and challenges in bringing these HGT inhibitors into clinical trials.

The Role of the Salmonella spvB IncF Plasmid and Its Resident Entry Exclusion Gene traS on Plasmid Exclusion

Salmonella enterica cause significant illnesses worldwide. There has been a marked increase in resistance to fluoroquinolones and β-lactams/cephalosporins, antibiotics commonly used to treat salmonellosis. However, S. enterica serovars vary in their resistance to these and other antibiotics. The systemic virulence of some Salmonella serovars is due to a low copy number, IncF plasmid (65-100 kb) that contains the ADP-ribosylating toxin, SpvB. This virulence plasmid is present in only nine Salmonella serovars. It is possible that the spvB-virulence plasmid excludes other plasmids and may explain why antibiotic resistance is slow to develop in certain Salmonella serovars such as S. Enteritidis. The distribution of plasmid entry exclusion genes traS/traT and traY/excA are variable in Salmonella IncF and IncI plasmids, respectively and may account for differences in emergent antimicrobial resistance for some Salmonella serovars. The goal of this study is to determine the contribution of the Salmonella spvB-virulence plasmid in F-plasmid exclusion. From conjugation experiments, S. Typhimurium exhibited lower conjugation frequency with incFI and incFII plasmids when the spvB-virulence plasmid is present. Furthermore, introduction of cloned incFI traS into a "plasmidless" S. Typhimurium LT2 strain and Escherichia coli DH5α excluded incFI plasmid. However, deletion of the virulence plasmid traS did not affect plasmid exclusion significantly compared to a spvB control deletion. In addition, differences in F plasmid conjugation in natural Salmonella isolates did not correlate with IncF or SpvB-virulence plasmid genotype. There appear to be other plasmid or chromosomal genes at play in plasmid exclusion that may be responsible for the slow development of antibiotic resistance in certain serovars.

Environmental and genetic determinants of plasmid mobility in pathogenic Escherichia coli

Plasmids are key vehicles of horizontal gene transfer (HGT), mobilizing antibiotic resistance, virulence, and other traits among bacterial populations. The environmental and genetic forces that drive plasmid transfer are poorly understood, however, due to the lack of definitive quantification coupled with genomic analysis. Here, we integrate conjugative phenotype with plasmid genotype to provide quantitative analysis of HGT in clinical Escherichia coli pathogens. We find a substantial proportion of these pathogens (>25%) able to readily spread resistance to the most common classes of antibiotics. Antibiotics of varied modes of action had less than a 5-fold effect on conjugation efficiency in general, with one exception displaying 31-fold promotion upon exposure to macrolides and chloramphenicol. In contrast, genome sequencing reveals plasmid incompatibility group strongly correlates with transfer efficiency. Our findings offer new insights into the determinants of plasmid mobility and have implications for the development of treatments that target HGT.

Escherichia coli belonging to ST131 rarely transfers bla ctx-m-15 to fecal Escherichia coli

Background

Extended spectrum beta-lactamase (ESBL)-producing Escherichia coli (E. coli) causing urinary tract infections often belong to sequence type 131 (ST131), serotype O25, carrying bla _CTX-M-15.

Aim

The main aim of this study was to examine the conjugational frequencies of E. coli with plasmids carrying bla _CTX-M-15 to E. coli isolates from the fecal flora of healthy humans to determine whether ST131 is more likely to uptake or donate ESBL resistance compared to other E. coli clones.

Methods

 Donors and recipients were all clinical isolates and did not harbor plasmids with identical incompatibility groups (Inc-groups) based on in silico analyses of Inc-groups and restriction/modification systems (R/M-systems). The in vitro conjugation experiments were performed as filter conjugation with verification of transconjugants by random amplified polymorphic DNA (RAPD) PCR and bla _CTX-M-15 PCR.

Results

The frequencies of conjugation with bla _CTX-M-15-carrying plasmids were found to be very rare with detectable conjugation frequencies in the range of 4x10^-9-7x10^-7 transconjugants/recipient. Recipients of O25/ST131 type yielded significantly lower conjugation frequencies compared to recipients of other O-types (P=0.004). The applied ST131/O25 donors did not yield detectable levels of transconjugants regardless of the applied recipient. Presence of sub-MIC levels of ampicillin increased plasmid transfer frequencies x100 fold (P=0.07).

Conclusion

The results indicate that bla _CTX-M-15 is rarely transferred by conjugation to E. coli isolates of the intestinal flora, even when the gene is plasmid-borne.

Distinct evolutionary dynamics of horizontal gene transfer in drug resistant and virulent clones of Klebsiella pneumoniae

Klebsiella pneumoniae has emerged as an important cause of two distinct public health threats: multi-drug resistant (MDR) healthcare-associated infections and drug susceptible community-acquired invasive infections. These pathotypes are generally associated with two distinct subsets of K. pneumoniae lineages or 'clones' that are distinguished by the presence of acquired resistance genes and several key virulence loci. Genomic evolutionary analyses of the most notorious MDR and invasive community-associated ('hypervirulent') clones indicate differences in terms of chromosomal recombination dynamics and capsule polysaccharide diversity, but it remains unclear if these differences represent generalised trends. Here we leverage a collection of >2200 K. pneumoniae genomes to identify 28 common clones (n ≥ 10 genomes each), and perform the first genomic evolutionary comparison. Eight MDR and 6 hypervirulent clones were identified on the basis of acquired resistance and virulence gene prevalence. Chromosomal recombination, surface polysaccharide locus diversity, pan-genome, plasmid and phage dynamics were characterised and compared. The data showed that MDR clones were highly diverse, with frequent chromosomal recombination generating extensive surface polysaccharide locus diversity. Additional pan-genome diversity was driven by frequent acquisition/loss of both plasmids and phage. In contrast, chromosomal recombination was rare in the hypervirulent clones, which also showed a significant reduction in pan-genome diversity, largely driven by a reduction in plasmid diversity. Hence the data indicate that hypervirulent clones may be subject to some sort of constraint for horizontal gene transfer that does not apply to the MDR clones. Our findings are relevant for understanding the risk of emergence of individual K. pneumoniae strains carrying both virulence and acquired resistance genes, which have been increasingly reported and cause highly virulent infections that are extremely difficult to treat. Specifically, our data indicate that MDR clones pose the greatest risk, because they are more likely to acquire virulence genes than hypervirulent clones are to acquire resistance genes.

The interaction of TraW and TrbC is required to facilitate conjugation in F-like plasmids

Bacterial conjugation, such as that mediated by the E. coli F plasmid, is a main mechanism driving bacterial evolution. Two important proteins required for F-pilus assembly and DNA transfer proficiency are TraW and TrbC. As members of a larger complex, these proteins assemble into a type IV secretion system and are essential components of pore formation and mating pair stabilization between the donor and the recipient cells. In the current report, we demonstrate the physical interaction of TraW and TrbC, show that TraW preferentially interacts with the N-terminal domain of TrbC, and that this interaction is important in restoring conjugation in traW/trbC knockouts.

Natural and Artificial Strategies To Control the Conjugative Transmission of Plasmids

Conjugative plasmids are the main carriers of transmissible antibiotic resistance (AbR) genes. For that reason, strategies to control plasmid transmission have been proposed as potential solutions to prevent AbR dissemination. Natural mechanisms that bacteria employ as defense barriers against invading genomes, such as restriction-modification or CRISPR-Cas systems, could be exploited to control conjugation. Besides, conjugative plasmids themselves display mechanisms to minimize their associated burden or to compete with related or unrelated plasmids. Thus, FinOP systems, composed of FinO repressor protein and FinP antisense RNA, aid plasmids to regulate their own transfer; exclusion systems avoid conjugative transfer of related plasmids to the same recipient bacteria; and fertility inhibition systems block transmission of unrelated plasmids from the same donor cell. Artificial strategies have also been designed to control bacterial conjugation. For instance, intrabodies against R388 relaxase expressed in recipient cells inhibit plasmid R388 conjugative transfer; pIII protein of bacteriophage M13 inhibits plasmid F transmission by obstructing conjugative pili; and unsaturated fatty acids prevent transfer of clinically relevant plasmids in different hosts, promoting plasmid extinction in bacterial populations. Overall, a number of exogenous and endogenous factors have an effect on the sophisticated process of bacterial conjugation. This review puts them together in an effort to offer a wide picture and inform research to control plasmid transmission, focusing on Gram-negative bacteria.

Conjugation Inhibitors and Their Potential Use to Prevent Dissemination of Antibiotic Resistance Genes in Bacteria

Antibiotic resistance has become one of the most challenging problems in health care. Bacteria conjugation is one of the main mechanisms whereby bacteria become resistant to antibiotics. Therefore, the search for specific conjugation inhibitors (COINs) is of interest in the fight against the spread of antibiotic resistances in a variety of laboratory and natural environments. Several compounds, discovered as COINs, are promising candidates in the fight against plasmid dissemination. In this review, we survey the effectiveness and toxicity of the most relevant compounds. Particular emphasis has been placed on unsaturated fatty acid derivatives, as they have been shown to be efficient in preventing plasmid invasiveness in bacterial populations. Biochemical and structural studies have provided insights concerning their potential molecular targets and inhibitory mechanisms. These findings open a new avenue in the search of new and more effective synthetic inhibitors. In this pursuit, the use of structure-based drug design methods will be of great importance for the screening of ligands and binding sites of putative targets.

The Mosaic Type IV Secretion Systems

Escherichia coli and other Gram-negative and -positive bacteria employ type IV secretion systems (T4SSs) to translocate DNA and protein substrates, generally by contact-dependent mechanisms, to other cells. The T4SSs functionally encompass two major subfamilies, the conjugation systems and the effector translocators. The conjugation systems are responsible for interbacterial transfer of antibiotic resistance genes, virulence determinants, and genes encoding other traits of potential benefit to the bacterial host. The effector translocators are used by many Gram-negative pathogens for delivery of potentially hundreds of virulence proteins termed effectors to eukaryotic cells during infection. In E. coli and other species of Enterobacteriaceae, T4SSs identified to date function exclusively in conjugative DNA transfer. In these species, the plasmid-encoded systems can be classified as the P, F, and I types. The P-type systems are the simplest in terms of subunit composition and architecture, and members of this subfamily share features in common with the paradigmatic Agrobacterium tumefaciens VirB/VirD4 T4SS. This review will summarize our current knowledge of the E. coli systems and the A. tumefaciens P-type system, with emphasis on the structural diversity of the T4SSs. Ancestral P-, F-, and I-type systems were adapted throughout evolution to yield the extant effector translocators, and information about well-characterized effector translocators also is included to further illustrate the adaptive and mosaic nature of these highly versatile machines.

Conjugative Mating Assays for Sequence-specific Analysis of Transfer Proteins Involved in Bacterial Conjugation

The transfer of genetic material by bacterial conjugation is a process that takes place via complexes formed by specific transfer proteins. In Escherichia coli, these transfer proteins make up a DNA transfer machinery known as the mating pair formation, or DNA transfer complex, which facilitates conjugative plasmid transfer. The objective of this paper is to provide a method that can be used to determine the role of a specific transfer protein that is involved in conjugation using a series of deletions and/or point mutations in combination with mating assays. The target gene is knocked out on the conjugative plasmid and is then provided in trans through the use of a small recovery plasmid harboring the target gene. Mutations affecting the target gene on the recovery plasmid can reveal information about functional aspects of the target protein that result in the alteration of mating efficiency of donor cells harboring the mutated gene. Alterations in mating efficiency provide insight into the role and importance of the particular transfer protein, or a region therein, in facilitating conjugative DNA transfer. Coupling this mating assay with detailed three-dimensional structural studies will provide a comprehensive understanding of the function of the conjugative transfer protein as well as provide a means for identifying and characterizing regions of protein-protein interaction.

Antibiotics as a selective driver for conjugation dynamics

It is generally assumed that antibiotics can promote horizontal gene transfer. However, because of a variety of confounding factors that complicate the interpretation of previous studies, the mechanisms by which antibiotics modulate horizontal gene transfer remain poorly understood. In particular, it is unclear whether antibiotics directly regulate the efficiency of horizontal gene transfer, serve as a selection force to modulate population dynamics after such gene transfer has occurred, or both. Here, we address this question by quantifying conjugation dynamics in the presence and absence of antibiotic-mediated selection. Surprisingly, we find that sublethal concentrations of antibiotics from the most widely used classes do not significantly increase the conjugation efficiency. Instead, our modelling and experimental results demonstrate that conjugation dynamics are dictated by antibiotic-mediated selection, which can both promote and suppress conjugation dynamics. Our findings suggest that the contribution of antibiotics to the promotion of horizontal gene transfer may have been overestimated. These findings have implications for designing effective antibiotic treatment protocols and for assessing the risks of antibiotic use.

HDX-MS and deletion analysis of the type 4 secretion system protein TraF from the Escherichia coli F plasmid

Conjugative DNA transfer by the F-plasmid is achieved through a type IV secretion system (T4SS) encoded within the plasmid's transfer region; TraF is one of several F-T4SS proteins essential for F-pilus assembly. In order to identify regions of the protein important for TraF function, a series of deletion mutants were assessed for their ability to recover conjugative transfer in a traF knockout. Interestingly, modification of any region of TraF abolishes pilus synthesis, resulting in a loss of rescue of conjugative function. Dynamic analysis of TraF by time-resolved hydrogen-deuterium exchange revealed that the C-terminal region containing the predicted thioredoxin-like domain is quite structured, while the N-terminal region, predicted to interact with TraH in the intact F-T4SS, was more dynamic.

Activation of the σE-dependent stress pathway by conjugative TraR may anticipate conjugational stress

Horizontal gene transfer by conjugation plays a major role in bacterial evolution, allowing the acquisition of new traits, such as virulence and resistance to antibacterial agents. With the increased antibiotic resistance in bacterial pathogens, a better understanding of how bacteria modulate conjugation under changing environments and the genetic factors involved is needed. Despite the evolutionary advantages conjugation may confer, the process can be quite stressful for the donor cell. Here, we characterize the ability of TraR, encoded on the episomal F' plasmid, to upregulate the σ(E) extracytoplasmic stress pathway in Escherichia coli. TraR, a DksA homolog, modulates transcription initiation through the secondary channel of RNA polymerase. We show here that TraR activates transcription directly; however, unlike DksA, it does so without using ppGpp as a cofactor. TraR expression can stimulate the σ(E) extracytoplasmic stress response independently of the DegS/RseA signal transduction cascade. In the absence of TraR, bacteria carrying conjugative plasmids become more susceptible to external stress. We propose that TraR increases the concentrations of periplasmic chaperones and proteases by directly activating the transcription of σ(E)-dependent promoters; this increased protein folding capacity may prepare the bacterium to endure the periplasmic stress of sex pilus biosynthesis during mating.

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.

Genetic information transfer promotes cooperation in bacteria

Many bacterial species are social, producing costly secreted "public good" molecules that enhance the growth of neighboring cells. The genes coding for these cooperative traits are often propagated via mobile genetic elements and can be virulence factors from a biomedical perspective. Here, we present an experimental framework that links genetic information exchange and the selection of cooperative traits. Using simulations and experiments based on a synthetic bacterial system to control public good secretion and plasmid conjugation, we demonstrate that horizontal gene transfer can favor cooperation. In a well-mixed environment, horizontal transfer brings a direct infectious advantage to any gene, regardless of its cooperation properties. However, in a structured population transfer selects specifically for cooperation by increasing the assortment among cooperative alleles. Conjugation allows cooperative alleles to overcome rarity thresholds and invade bacterial populations structured purely by stochastic dilution effects. Our results provide an explanation for the prevalence of cooperative genes on mobile elements, and suggest a previously unidentified benefit of horizontal gene transfer for bacteria.

Harnessing recombination to speed adaptive evolution in Escherichia coli

Evolutionary engineering typically involves asexual propagation of a strain to improve a desired phenotype. However, asexual populations suffer from extensive clonal interference, a phenomenon where distinct lineages of beneficial clones compete and are often lost from the population given sufficient time. Improved adaptive mutants can likely be generated by genetic exchange between lineages, thereby reducing clonal interference. We present a system that allows continuous in situ recombination by using an Esherichia coli F-based conjugation system lacking surface exclusion. Evolution experiments revealed that Hfr-mediated recombination significantly speeds adaptation in certain circumstances. These results show that our system is stable, effective, and suitable for use in evolutionary engineering applications.

Bacterial contact-dependent delivery systems

Bacteria have developed remarkable systems that sense neighboring target cells upon contact and initiate a series of events that enhance their survival and growth at the expense of the target cells. Four main classes of bacterial cell surface structures have been identified that interact with prokaryotic or eukaryotic target cells to deliver DNA or protein effectors. Type III secretion systems (T3SS) use a flagellum-like tube to deliver protein effectors into eukaryotic host cells, whereas Type IV systems use a pilus-based system to mediate DNA or protein transfer into recipient cells. The contact-dependent growth inhibition system (CDI) is a Type V system, using a long β-helical cell surface protein to contact receptors in target cells and deliver a growth inhibitory signal. Type VI systems utilize a phage-like tube and cell puncturing device to secrete effector proteins into both eukaryotic and prokaryotic target cells.

F plasmid TraF and TraH are components of an outer membrane complex involved in conjugation

F plasmid TraF and TraH are required for F pilus assembly and F plasmid transfer. Using flotation sucrose density gradients, we found that TraF and TraH (as well as TraU and TraW) localized to the outer membrane in the presence of the complete F transfer region, especially TraV, the putative anchor. Mutational analysis of TraH revealed two domains that are important for its function and possible interaction with TrbI, which in turn has a role in stabilizing TraH.

Escherichia coli genes affecting recipient ability in plasmid conjugation: are there any?

Background

How does the recipient cell contribute to bacterial conjugation? To answer this question we systematically analyzed the individual contribution of each Escherichia coli gene in matings using plasmid R388 as a conjugative plasmid. We used an automated conjugation assay and two sets of E. coli mutant collections: the Keio collection (3,908 E. coli single-gene deletion mutants) and a collection of 20,000 random mini-Tn10::Km insertion mutants in E. coli strain DH5α. The combined use of both collections assured that we screened > 99% of the E. coli non-essential genes in our survey.

Results

Results indicate that no non-essential recipient E. coli genes exist that play an essential role in conjugation. Mutations in the lipopolysaccharide (LPS) synthesis pathway had a modest effect on R388 plasmid transfer (6 – 32% of wild type). The same mutations showed a drastic inhibition effect on F-plasmid transfer, but only in liquid matings, suggesting that previously isolated conjugation-defective mutants do in fact impair mating pair formation in liquid mating, but not conjugative DNA processing or transport per se.

Conclusion

We conclude from our genome-wide screen that recipient bacterial cells cannot avoid being used as recipients in bacterial conjugation. This is relevant as an indication of the problems in curbing the dissemination of antibiotic resistance and suggests that conjugation acts as a pure drilling machine, with little regard to the constitution of the recipient cell.

The structure of F-pili

Exchange of DNA between bacteria involves conjugative pili. While the prevailing view has been that F-pili are completely retracted before single-stranded DNA is passed from one cell to another, it has recently been reported that the F-pilus, in addition to establishing the contact between mating cells, serves as a channel for passing DNA between spatially separated cells during conjugation. The structure and function of F-pili are poorly understood. They are built from a single subunit having only 70 residues, and the small size of the subunit has made these filaments difficult to study. Here, we have applied electron cryo-microscopy and single-particle methods to solve the long-existing ambiguity in the packing geometry of F-pilin subunits. We show that the F-pilus has an entirely different symmetry from any of the known bacterial pili as well as any of the filamentous bacteriophages, which have been suggested to be structural homologs. Two subunit packing schemes were identified: one has stacked rings of four subunits axially spaced by approximately 12.8 A, while the other has a one-start helical symmetry with an axial rise of approximately 3.5 A per subunit and a pitch of approximately 12.2 A. Both structures have a central lumen of approximately 30 A diameter that is more than large enough to allow for the passage of single-stranded DNA. Remarkably, both schemes appear to coexist within the same filaments, in contrast to filamentous phages that have been described as belonging to one of two possible symmetry classes. For the segments composed of rings, the twist between adjacent rings is quite variable, while the segments having a one-start helix are in multiple states of both twist and extension. This coexistence of two very different symmetries is similar to what has recently been reported for an archaeal Methanococcus maripaludis pili filament and an archaeal Sulfolobus shibatae flagellar filament.

Towards a systems biology approach to study type II/IV secretion systems

Many gram-negative bacteria produce thin protein filaments, named pili, which extend beyond the confines of the outer membrane. The importance of these pili is illustrated by the fact that highly complex, multi-protein pilus-assembly machines have evolved, not once, but several times. Their many functions include motility, adhesion, secretion, and DNA transfer, all of which can contribute to the virulence of bacterial pathogens or to the spread of virulence factors by horizontal gene transfer. The medical importance has stimulated extensive biochemical and genetic studies but the assembly and function of pili remains an enigma. It is clear that progress in this field requires a more holistic approach where the entire molecular apparatus that forms the pilus is studied as a system. In recent years systems biology approaches have started to complement classical studies of pili and their assembly. Moreover, continued progress in structural biology is building a picture of the components that make up the assembly machine. However, the complexity and multiple-membrane spanning nature of these secretion systems pose formidable technical challenges, and it will require a concerted effort before we can create comprehensive and predictive models of these remarkable molecular machines.

The VirB5 protein localizes to the T-pilus tips in Agrobacterium tumefaciens

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) mediates the transfer of single-stranded DNA and protein virulence factors into plant cells, and also determines the assembly of the T-pilus, which is believed to play a role in host recognition. The T-pilus is composed of the major component VirB2 and the minor component VirB5. Using immuno-electron microscopy we detected the major component VirB2 along the entire length of detached T-pili, but not on cell-bound T-pili or on the cell surface. In contrast, the minor T-pilus component VirB5 was detected on the tips of cell-bound T-pili as well as on the ends of detached T-pili and on the cell surface. To gain further insights into the role of VirB5 we introduced changes at its C terminus. C-terminal deletions of up to four amino acids and alanine replacements did not abolish T-pilus formation and incorporation of the VirB5 variants at the tip, although they did impact the length of T-pili. Also, these changes differentially affected the ability of the T4SS to transfer DNA into plant and bacterial recipients, suggesting differential effects on host-cell specificity. The data presented here suggest that VirB5 localizes at the T-pilus tip, and provide novel insights into its role during the type IV secretion process.

Hfq is a regulator of F-plasmid TraJ and TraM synthesis in Escherichia coli

The F plasmid of Escherichia coli allows horizontal DNA transfer between an F(+) donor cell and an F(-) recipient. Expression of the transfer genes is tightly controlled by a number of factors, including the following plasmid-encoded regulatory proteins: TraJ, the primary activator of the 33-kb tra operon, and the autoregulators TraM and TraY. Here, we demonstrate that the host RNA binding protein, Hfq, represses TraJ and TraM synthesis by destabilizing their respective mRNAs. Mating assays and immunoblot analyses for TraM and TraJ showed that transfer efficiency and protein levels increased in host cells containing a disruption in hfq compared to wild-type cells in stationary phase. The stability of transcripts containing a putative Hfq binding site located in the intergenic untranslated region between traM and traJ was increased in hfq mutant donor cells, suggesting that Hfq destabilizes these transcripts. Electrophoretic mobility shift assays demonstrated that Hfq specifically binds this region but not the antisense RNA, FinP, encoded on the opposite strand. Together, these findings indicate that Hfq regulates traM and traJ transcript stability by a mechanism separate from FinOP-mediated repression.

Characterization of the opposing roles of H-NS and TraJ in transcriptional regulation of the F-plasmid tra operon

The transfer (tra) operon of the conjugative F plasmid of Escherichia coli is a polycistronic 33-kb operon which encodes most of the proteins necessary for F-plasmid transfer. Here, we report that transcription from PY, the tra operon promoter, is repressed by the host nucleoid-associated protein, H-NS. Electrophoretic mobility shift assays indicate that H-NS binds preferentially to the tra promoter region, while Northern blot and transcriptional fusion analyses indicate that transcription of traY, the first gene in the tra operon, is derepressed in an hns mutant throughout growth. The plasmid-encoded regulatory protein TraJ is essential for transcription of the tra operon in wild-type Escherichia coli; however, TraJ is not necessary for plasmid transfer or traY operon transcription in an hns mutant. This indicates that H-NS represses transcription from PY directly and not indirectly via its effects on TraJ levels. These results suggest that TraJ functions to disrupt H-NS silencing at PY, allowing transcription of the tra operon.

Immobilization induces alterations in the outer membrane protein pattern of Yersinia ruckeri

We compared the outer membrane protein (OMP) pattern of 2-day-old immobilized Yersinia ruckericells (IC) with that of early (FC24) and late (FC48) stationary-phase planktonic counterparts. Fifty-five OMPs were identified. Principal component analysis discriminated between the protein maps of FC and IC. Some OMPs involved in bacterial adaptation were accumulated by both FC48 and IC but the expression of other proteins was controlled by the sessile mode of growth.

F-like type IV secretion systems encode proteins with thioredoxin folds that are putative DsbC homologues

F and R27 are conjugative plasmids of enteric bacteria belonging to the IncF and IncHI1 plasmid incompatibility groups, respectively. Based on sequence analysis, two genes of the F transfer region, traF and trbB, and three genes of the R27 transfer region, trhF, dsbC, and htdT, are predicted to encode periplasmic proteins containing a C-terminal thioredoxin fold. The C-X-X-C active-site motif of thioredoxins is present in all of these proteins except TraF(F). Escherichia coli carrying a dsbA mutation, which is deficient in disulfide bond formation, cannot synthesize pili and exhibits hypersensitivity to dithiothreitol (DTT) as monitored by mating ability. Overproduction of the E. coli disulfide bond isomerase DsbC, TrbB(F), DsbC(R27), or HtdT(R27), but not TraF(F) or TrhF(R27), reverses this hypersensitivity to DTT. Site-directed mutagenesis established that the C-X-X-C motif was necessary for this activity. Secretion into the periplasm of the C-terminal regions of TrbB(F) and DsbC(R27), containing putative thioredoxin folds, but not TrhF(R27), partially complemented the host dsbA mutation. A trbB(F) deletion mutant showed a 10-fold-lower mating efficiency in an E. coli dsbC null strain but had no phenotype in wild-type E. coli, suggesting redundancy in function between TrbB(F) and E. coli DsbC. Our results indicate that TrbB(F), DsbC(R27), and HtdT(R27) are putative disulfide bond isomerases for their respective transfer systems. TraF(F) is essential for conjugation but appears to have a function other than disulfide bond chemistry.

Fluorescence assays for F-pili and their application

Conjugative pili are extracellular filaments elaborated by Gram-negative bacteria expressing certain type IV secretion systems. They are required at the earliest stages of conjugal DNA transfer to establish specific and secure cell-cell contacts. Conjugative pili also serve as adsorption organelles for both RNA and DNA bacteriophages. Beyond these facts, the structure, formation and function of these filaments are poorly understood. This paper describes a rapid, quantitative assay for F-pili encoded by the F plasmid type IV secretion system. The assay is based on the specific lateral adsorption of icosahedral RNA bacteriophage R17 by F-pili. Bacteriophage particles conjugated with a fluorescent dye, Alexa 488, and bound to F-pili defined filaments visible by immunofluorescence microscopy. F-pili attached to F+ cells and free F-pili were both visible by this method. For quantification, cell-bound bacteriophage were separated from free bacteriophage particles by sedimentation and released by suspending cell pellets in 0.1 % SDS. Fluorescence in cell-free supernatant fractions was measured by fluorometry. The authors present a characterization of this assay and its application to F-pilus formation by cells carrying mutations in the gene for the F-pilus subunit F-pilin. Each mutation introduced a cysteine, which F-pilin normally lacks, at a different position in its primary structure. Cysteine residues in the N-terminal domain I abolished filament formation as measured by fluorescent R17 binding. This was confirmed by measurements of DNA donor activity and filamentous DNA bacteriophage infection. With one exception (G53C), cysteines elsewhere in the F-pilin primary structure did not abolish filament formation, although some mutations differentially affected F-pilus functions.

Differential binding of Escherichia coli O157:H7 to alfalfa, human epithelial cells, and plastic is mediated by a variety of surface structures

Escherichia coli O157:H7 carried on plant surfaces, including alfalfa sprouts, has been implicated in food poisoning and outbreaks of disease in the United States. Adhesion to cell surfaces is a key component for bacterial establishment and colonization on many types of surfaces. Several E. coli O157:H7 surface proteins are thought to be important for adhesion and/or biofilm formation. Therefore, we examined whether mutations in several genes encoding potential adhesins and regulators of adherence have an effect on bacterial binding to plants and also examined the role of these genes during adhesion to Caco-2 cells and during biofilm formation on plastic in vitro. The genes tested included those encoding adhesins (cah, aidA1, and ompA) and mediators of hyperadherence (tdcA, yidE, waaI, and cadA) and those associated with fimbria formation (csgA, csgD, and lpfD2). The introduction of some of these genes (cah, aidA1, and csg loci) into an E. coli K-12 strain markedly increased its ability to bind to alfalfa sprouts and seed coats. The addition of more than one of these genes did not show an additive effect. In contrast, deletion of one or more of these genes in a strain of E. coli O157:H7 did not affect its ability to bind to alfalfa. Only the absence of the ompA gene had a significant effect on binding, and the plant-bacterium interaction was markedly reduced in a tdcA ompA double mutant. In contrast, the E. coli O157:H7 ompA and tdcA ompA mutant strains were only slightly affected in adhesion to Caco-2 cells and during biofilm formation. These findings suggest that some adhesins alone are sufficient to promote binding to alfalfa and that they may exist in E. coli O157:H7 as redundant systems, allowing it to compensate for the loss of one or more of these systems. Binding to the three types of surfaces appeared to be mediated by overlapping but distinct sets of genes. The only gene which appeared to be irreplaceable for binding to plant surfaces was ompA.

The mating pair stabilization protein, TraN, of the F plasmid is an outer-membrane protein with two regions that are important for its function in conjugation

F plasmid TraN (602 aa, processed to 584 aa with 22 conserved cysteines), which is essential for F plasmid conjugation, is an outer-membrane protein involved in mating pair stabilization (MPS). Unlike R100 TraN, F TraN requires OmpA in the recipient cell for efficient MPS. The authors have identified three external loops (aa 172–187, 212–220 and 281–284) in the highly divergent region from aa 164 to aa 333 as candidates for interaction with OmpA. These loops were identified using both site-directed and random TnphoA/in mutagenesis to insert epitopes (31-aa or c-myc) into TraN and monitor their effect on sensitivity to external proteases and on mating ability. TraN is a hallmark protein of F-type IV secretion systems as demonstrated byblastsearches of the databases. The C-terminal region is highly conserved and contains five of the six completely conserved cysteines. Mutation of these residues to serine demonstrated their importance in TraN function. TraN appears to require both intra- and intermolecular disulfide bond formation for its stability and structure as demonstrated by its instability in adsbAmutant and its aberrant migration on SDS-polyacrylamide gels under non-reducing conditions or by cross-linking with bis(sulfosuccinimidyl)suberate (BS3). Thus, F TraN appears to have two domains: the N-terminal region is involved in OmpA interaction with OmpA during MPS; and the C-terminal region, which is rich in conserved cysteine residues, is essential for conjugation.

Mechanisms of, and barriers to, horizontal gene transfer between bacteria

Bacteria evolve rapidly not only by mutation and rapid multiplication, but also by transfer of DNA, which can result in strains with beneficial mutations from more than one parent. Transformation involves the release of naked DNA followed by uptake and recombination. Homologous recombination and DNA-repair processes normally limit this to DNA from similar bacteria. However, if a gene moves onto a broad-host-range plasmid it might be able to spread without the need for recombination. There are barriers to both these processes but they reduce, rather than prevent, gene acquisition.

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.