Regulatory circuits for plasmid survival

Evolution of a Plasmid Regulatory Circuit Ameliorates Plasmid Fitness Cost

Plasmids play a major role in bacterial evolution and rapid adaptation by facilitating the horizontal transfer of diverse genes. Understanding how plasmids are transferred and maintained in bacterial populations is important, especially given the increasing plasmid-mediated spread of antibiotic-resistance genes to human pathogens. We investigated why broad-host range plasmid pBP136, originally isolated from clinical samples of Bordetella pertussis, quickly became extinct in laboratory Escherichia coli populations. We found that the inactivation of a previously uncharacterized plasmid gene, upf31, drastically improved long-term maintenance of the plasmid in E. coli. Loss of this single gene was associated with decreased transcription of numerous genes in the plasmid korA, korB and korC regulons, as well as changes in many chromosomal genes primarily related to metabolism. This change in transcriptome is likely initiated by Upf31 interacting with one of these major plasmid regulators, KorB. Expression of upf31 in trans not only negatively affected the persistence of a pBP136 upf31 deletion mutant, but also of the closely related archetype IncPβ plasmid R751, which is stable in E. coli and natively encodes an internally truncated upf31 allele. This suggests that whereas the upf31 allele in pBP136 might advantageously modulate gene expression in its original host, B. pertussis, the same function can have harmful effects in E. coli. Thus, using multiple hosts to study the effects of knockouts in broad-host-range plasmid genes of unknown function may reveal unexpected mechanisms that determine the fate of that plasmid in bacterial communities.

Dissemination of IncI plasmid encoding bla CTX-M-1 is not hampered by its fitness cost in the pig's gut

Multiresistance plasmids belonging to the IncI incompatibility group have become one of the most pervasive plasmid types in extended-spectrum beta-lactamase-producing Escherichia coli of animal origin. The extent of the burden imposed on the bacterial cell by these plasmids seems to modulate the emergence of "epidemic" plasmids. However, in vivo data in the natural environment of the strains are scarce. Here, we investigated the cost of a bla CTX-M-1-IncI1 epidemic plasmid in a commensal E. coli animal strain, UB12-RC, before and after oral inoculation of 15 6- to 8-week- old specific-pathogen-free pigs. Growth rate in rich medium was determined on (i) UB12-RC and derivatives, with or without plasmid, in vivo and/or in vitro evolved, and (ii) strains that acquired the plasmid in the gut during the experiment. Although bla CTX-M-1-IncI1 plasmid imposed no measurable burden on the recipient strain after conjugation and during the longitudinal carriage in the pig's gut, we observed a significant difference in the bacterial growth rate between IncI1 plasmid-carrying and plasmid-free isolates collected during in vivo carriage. Only a few mutations on the chromosome of the UB12-RC derivatives were detected by whole-genome sequencing. RNA-Seq analysis of a selected set of these strains showed that transcriptional responses to the bla CTX-M-1-IncI1 acquisition were limited, affecting metabolism, stress response, and motility functions. Our data suggest that the effect of IncI plasmid on host cells is limited, fitness cost being insufficient to act as a barrier to IncI plasmid spread among natural population of E. coli in the gut niche.

A mathematician's guide to plasmids: an introduction to plasmid biology for modellers

Plasmids, extrachromosomal DNA molecules commonly found in bacterial and archaeal cells, play an important role in bacterial genetics and evolution. Our understanding of plasmid biology has been furthered greatly by the development of mathematical models, and there are many questions about plasmids that models would be useful in answering. In this review, we present an introductory, yet comprehensive, overview of the biology of plasmids suitable for modellers unfamiliar with plasmids who want to get up to speed and to begin working on plasmid-related models. In addition to reviewing the diversity of plasmids and the genes they carry, their key physiological functions, and interactions between plasmid and host, we also highlight selected plasmid topics that may be of particular interest to modellers and areas where there is a particular need for theoretical development. The world of plasmids holds a great variety of subjects that will interest mathematical biologists, and introducing new modellers to the subject will help to expand the existing body of plasmid theory.

Biochar Effectively Inhibits the Horizontal Transfer of Antibiotic Resistance Genes via Restraining the Energy Supply for Conjugative Plasmid Transfer

Horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs) through plasmid-mediated conjugation poses a major threat to global public health. Biochar, a widely used environmental remediation material, has remarkable impacts on the fate of ARGs. However, although biochar was reported being able to inhibit the HGT of ARGs via conjugation and transformation, little is known about the intracellular process that mediates the inhibition effects. On the other hand, as typical natural organic matter, fulvic acid is a common environmental influencer, and how it interferes with the effect of biochar on the HGT of ARGs is unknown. Therefore, this study investigated the effects on the conjugative transfer of ARGs between Escherichia coli MG1655 and E. coli HB101 carrying plasmid RP4, with biochars pyrolyzed at three temperatures and with the corresponding biochars coating with fulvic acid. Results showed that biochar with higher pyrolyzed temperature had a more substantial inhibitory effect on the conjugative transfer of the RP4 plasmid. The inhibitory effect of biochar was mainly attributed to (i) down-regulation of plasmid transfer gene expression, including the formation of conjugative transfer channel and plasmid replication, due to restrained adenosine triphosphate (ATP) energy supply and (ii) decreased cell membrane permeability. Conversely, the fulvic acid coating diminished this inhibition effect of biochar, mainly by providing more ATP and strengthening intracellular reactive oxygen species (ROS) defense. Our findings shed light on the intracellular process that mediates the effects of biochar on the conjugative transfer of ARGs, which would provide support for using biochar to reduce the spread of ARGs.

Phenolic compounds promote the horizontal transfer of antibiotic resistance genes in activated sludge

Phenolic compounds are common organic pollutants in wastewater. During the wastewater treatment process, these compounds may influence the microbial community structure and functions. However, the impact of the phenolic compounds in the wastewater treatment plants on the horizontal transfer of antibiotic resistance genes (ARGs) has not been well assessed. In this study, we investigated the horizontal transfer of ARGs under the stress of phenolic compounds. The results showed that in pure culture bacteria system, p-nitrophenol (PNP), p-aminophenol (PAP) and phenol (PhOH) (10-100 mg/L) can significantly increase the horizontal transfer frequency of ARGs by 2.2-4.6, 3.6-9.4 and 1.9-9.0 fold, respectively. And, the RP4 plasmid transfer from Escherichia coli HB101 (E. coli HB101) to the bacteria in activated sludge increased obviously under the stress of phenolic compounds. Further investigation revealed that the PNP and PhOH at the concentration of 10-100 mg/L increased the production of reactive oxygen species and the permeability of cell membrane in the donor and recipient, which could be the causes of horizontal transfer of RP4 plasmid. In addition, it was also found that PNP, PAP and PhOH stress inhibit the expression of the global regulatory genes korB and trbA in the RP4 plasmid, and increase the expression level of the traF gene, thereby promoting the conjugative transfer of the RP4 plasmid. Taken together, these results improved our understanding of the horizontal transfer of ARGs under the stress of phenolic compounds and provided basic information for management of the systems that treat wastewater containing phenolic compounds.

The Facts and Family Secrets of Plasmids That Replicate via the Rolling-Circle Mechanism

Plasmids are self-replicative DNA elements that are transferred between bacteria. Plasmids encode not only antibiotic resistance genes but also adaptive genes that allow their hosts to colonize new niches. Plasmid transfer is achieved by conjugation (or mobilization), phage-mediated transduction, and natural transformation. Thousands of plasmids use the rolling-circle mechanism for their propagation (RCR plasmids). They are ubiquitous, have a high copy number, exhibit a broad host range, and often can be mobilized among bacterial species. Based upon the replicon, RCR plasmids have been grouped into several families, the best known of them being pC194 and pUB110 (Rep_1 family), pMV158 and pE194 (Rep_2 family), and pT181 and pC221 (Rep_trans family). Genetic traits of RCR plasmids are analyzed concerning (i) replication mediated by a DNA-relaxing initiator protein and its interactions with the cognate DNA origin, (ii) lagging-strand origins of replication, (iii) antibiotic resistance genes, (iv) mobilization functions, (v) replication control, performed by proteins and/or antisense RNAs, and (vi) the participating host-encoded functions. The mobilization functions include a relaxase initiator of transfer (Mob), an origin of transfer, and one or two small auxiliary proteins. There is a family of relaxases, the MOB_V family represented by plasmid pMV158, which has been revisited and updated. Family secrets, like a putative open reading frame of unknown function, are reported. We conclude that basic research on RCR plasmids is of importance, and our perspectives contemplate the concept of One Earth because we should incorporate bacteria into our daily life by diminishing their virulence and, at the same time, respecting their genetic diversity.

Contagious Antibiotic Resistance: Plasmid Transfer among Bacterial Residents of the Zebrafish Gut

By characterizing the trajectories of antibiotic resistance gene transfer in bacterial communities such as the gut microbiome, we will better understand the factors that influence this spread of resistance. Our aim was to investigate the host network of a multidrug resistance broad-host-range plasmid in the culturable gut microbiome of zebrafish. This was done through in vitro and in vivo conjugation experiments with Escherichia coli as the donor of the plasmid pB10::gfp When this donor was mixed with the extracted gut microbiome, only transconjugants of Aeromonas veronii were detected. In separate matings between the same donor and four prominent isolates from the gut microbiome, the plasmid transferred to two of these four isolates, A. veronii and Plesiomonas shigelloides, but not to Shewanella putrefaciens and Vibrio mimicus When these A. veronii and P. shigelloides transconjugants were the donors in matings with the same four isolates, the plasmid now also transferred from A. veronii to S. putrefaciens P. shigelloides was unable to donate the plasmid, and V. mimicus was unable to acquire it. Finally, when the E. coli donor was added in vivo to zebrafish through their food, plasmid transfer was observed in the gut, but only to Achromobacter, a rare member of the gut microbiome. This work shows that the success of plasmid-mediated antibiotic resistance spread in a gut microbiome depends on the donor-recipient species combinations and therefore their spatial arrangement. It also suggests that rare gut microbiome members should not be ignored as potential reservoirs of multidrug resistance plasmids from food.IMPORTANCE To understand how antibiotic resistance plasmids end up in human pathogens, it is crucial to learn how, where, and when they are transferred and maintained in members of bacterial communities such as the gut microbiome. To gain insight into the network of plasmid-mediated antibiotic resistance sharing in the gut microbiome, we investigated the transferability and maintenance of a multidrug resistance plasmid among the culturable bacteria of the zebrafish gut. We show that the success of plasmid-mediated antibiotic resistance spread in a gut microbiome can depend on which species are involved, as some are important nodes in the plasmid-host network and others are dead ends. Our findings also suggest that rare gut microbiome members should not be ignored as potential reservoirs of multidrug resistance plasmids from food.

Non-antibiotic pharmaceuticals promote the transmission of multidrug resistance plasmids through intra- and intergenera conjugation

Antibiotic resistance is a global threat to public health. The use of antibiotics at sub-inhibitory concentrations has been recognized as an important factor in disseminating antibiotic resistance via horizontal gene transfer. Although non-antibiotic, human-targeted pharmaceuticals are widely used by society (95% of the pharmaceuticals market), the potential contribution to the spread of antibiotic resistance is not clear. Here, we report that commonly consumed, non-antibiotic pharmaceuticals, including nonsteroidal anti-inflammatories (ibuprofen, naproxen, diclofenac), a lipid-lowering drug (gemfibrozil), and a β-blocker (propranolol), at clinically and environmentally relevant concentrations, significantly accelerated the dissemination of antibiotic resistance via plasmid-borne bacterial conjugation. Various indicators were used to study the bacterial response to these drugs, including monitoring reactive oxygen species (ROS) and cell membrane permeability by flow cytometry, cell arrangement, and whole-genome RNA and protein sequencing. Enhanced conjugation correlated well with increased production of ROS and cell membrane permeability. Additionally, these non-antibiotic pharmaceuticals induced responses similar to those detected when bacteria are exposed to antibiotics, such as inducing the SOS response and enhancing efflux pumps. The findings advance understanding of the transfer of antibiotic resistance genes, emphasizing the concern that non-antibiotic, human-targeted pharmaceuticals enhance the spread of antibiotic resistance among bacterial populations.

Transcriptome Analysis of Zygotic Induction During Conjugative Transfer of Plasmid RP4

Conjugative transfer of bacterial plasmid is one of the major mechanisms of horizontal gene transfer, which is mediated by direct contact between donor and recipient cells. Gene expression of a conjugative plasmid is tightly regulated mostly by plasmid-encoded transcriptional regulators, but it remains obscure how differently plasmid genes are expressed in each cell during the conjugation event. Here, we report a comprehensive analysis of gene expression during conjugative transfer of plasmid RP4, which is transferred between isogenic strains of Pseudomonas putida KT2440 at very high frequency. To discriminate the expression changes in the donor and recipient cells, we took advantage of conjugation in the presence of rifampicin (Rif). Within 10 min of mating, we successfully detected transient transcription of plasmid genes in the resultant transconjugant cells. This phenomenon known as zygotic induction is likely attributed to derepression of multiple RP4-encoded repressors. Interestingly, we also observed that the traJIH operon encoding relaxase and its auxiliary proteins were upregulated specifically in the donor cells. Identification of the 5′ end of the zygotically induced traJ mRNA confirmed that the transcription start site of traJ was located 24-nt upstream of the nick site in the origin of transfer (oriT) as previously reported. Since the traJ promoter is encoded on the region to be transferred first, the relaxase may be expressed in the donor cell after regeneration of the oriT-flanking region, which in itself is likely to displace the autogenous repressors around oriT. This study provides new insights into the regulation of plasmid transfer processes.

Vancomycin resistance plasmids affect persistence of Enterococcus faecium in water

Vancomycin resistant enterococci (VRE) cause 20,000 infections annually in the United States, most of which are nosocomial. Recent findings of VRE in sewage-contaminated surface waters demonstrate an alternate route of human exposure, and a possible setting for horizontal gene exchange facilitated by plasmids and other mobile genetic elements. Maintenance of antibiotic resistance genes and proteins may, however, present a fitness cost in the absence of selective pressure, particularly in habitats such as environmental waters that are not optimal for gut-associated bacteria. Nutrient levels, which are transiently elevated following sewage spills, may also affect survival. We tested the hypotheses that nutrients and/or plasmids conferring vancomycin resistance affect Enterococcus faecium survival in river water by measuring decay of strains that differed only by their plasmid, under natural and augmented nutrient conditions. In natural river water, decay rate (log₁₀ reduction) correlated directly with plasmid size; however, plasmid presence and size had no effect on decay rate when nutrients levels were augmented. Under natural nutrient levels, the vancomycin-resistant strain with the largest plasmid (200 kb) decayed significantly more rapidly than the plasmid-less, susceptible parent strain, in contrast to similar decay rates among strains under augmented nutrient conditions. This work is among the first to show that plasmids conferring antibiotic resistance affect fitness of Enterococcus species in secondary habitats such as surface water. The nutrient-dependent nature of the fitness cost suggests that conveyance of VRE to environmental waters in nutrient-rich sewage may prolong survival of these pathogens, providing greater opportunity for host infection and/or horizontal gene transfer.

CO2 promotes the conjugative transfer of multiresistance genes by facilitating cellular contact and plasmid transfer

The dissemination of antibiotic resistance genes (ARGs), especially via the plasmid-mediated conjugation, is becoming a pervasive global health threat. This study reported that this issue can be worse by CO₂, as increased CO₂ was found to facilitate the conjugative transfer of ARGs carried on plasmid RP4 by 2.4-9.0 and 1.3-3.8 fold within and across genera, respectively. Mechanistic studies revealed that CO₂ benefitted the cell-to-cell contact by increasing cell surface hydrophobicity and decreasing cell surface charge, both of which resulted in the reduced intercellular repulsion. Besides, the transcriptional expression of genes responsible for global regulator (korA, korB and trbA), plasmid transfer and replication system (trfAp), and mating pair formation system (traF and traG) were all influenced by CO₂, facilitating the mobilization and channel transfer of plasmid. Furthermore, the presence of CO₂ induced the release of intracellular Ca²⁺ and increased the transmembrane potential of recipients, which contributed to the increased proton motive force (PMF), providing more power for DNA uptake. This is the first study addressing the potential risks of increased CO₂ on the propagation of ARGs, which provides a new insight into the concerns of anthropogenic CO₂ emissions and CO₂ storage.

Copper nanoparticles and copper ions promote horizontal transfer of plasmid-mediated multi-antibiotic resistance genes across bacterial genera

The spread of antibiotic resistance has become a major concern for public health. As emerging contaminants, various metallic nanoparticles (NPs) and ionic heavy metals have been ubiquitously detected in various environments. Although previous studies have indicated NPs and ionic heavy metals could exhibit co-selection effects for antibiotic resistance, little is known about whether and how they could promote antibiotic resistance spread via horizontal gene transfer across bacterial genera. This study, we report both CuO NPs and copper ions (Cu²⁺) could stimulate the conjugative transfer of multiple-drug resistance genes. When exposing bacteria to CuO NPs or Cu²⁺ at environmental-relevant and sub-inhibitory concentrations (e.g., 1-100 μmol/L), conjugation frequencies of plasmid-encoded antibiotic resistance genes across genera (i.e., from Escherichia coli to Pseudomonas putida) were significantly enhanced (p < 0.05). The over-production of reactive oxygen species played a crucial role in promoting conjugative transfer. Genome-wide RNA and protein sequencing suggested expressional levels of genes and proteins related to oxidative stress, cell membrane permeability, and pilus generation were significantly up-regulated under CuO NPs and Cu²⁺ exposure (p < 0.05). This study provides insights in the contributions of NPs and heavy metals on the spread of antibiotic resistance.

Plasmid Characteristics Modulate the Propensity of Gene Exchange in Bacterial Vesicles

Horizontal gene transfer is responsible for the exchange of many types of genetic elements, including plasmids. Properties of the exchanged genetic element are known to influence the efficiency of transfer via the mechanisms of conjugation, transduction, and transformation. Recently, an alternative general pathway of horizontal gene transfer has been identified, namely, gene exchange by extracellular vesicles. Although extracellular vesicles have been shown to facilitate the exchange of several types of plasmids, the influence of plasmid characteristics on genetic exchange within vesicles is unclear. Here, a set of different plasmids was constructed to systematically test the impact of plasmid properties, specifically, plasmid copy number, size, and origin of replication, on gene transfer in vesicles. The influence of each property on the production, packaging, and uptake of vesicles containing bacterial plasmids was quantified, revealing how plasmid properties modulate vesicle-mediated horizontal gene transfer. The loading of plasmids into vesicles correlates with the plasmid copy number and is influenced by characteristics that help set the number of plasmids within a cell, including size and origin of replication. Plasmid origin also has a separate impact on both vesicle loading and uptake, demonstrating that the origin of replication is a major determinant of the propensity of specific plasmids to transfer within extracellular vesicles.IMPORTANCE Extracellular vesicle formation and exchange are common within bacterial populations. Vesicles package multiple types of biomolecules, including genetic material. The exchange of extracellular vesicles containing genetic material facilitates interspecies DNA transfer and may be a promiscuous mechanism of horizontal gene transfer. Unlike other mechanisms of horizontal gene transfer, it is unclear whether characteristics of the exchanged DNA impact the likelihood of transfer in vesicles. Here, we systematically examine the influence of plasmid copy number, size, and origin of replication on the loading of DNA into vesicles and the uptake of DNA containing vesicles by recipient cells. These results reveal how each plasmid characteristic impacts gene transfer in vesicles and contribute to a greater understanding of the importance of vesicle-mediated gene exchange in the landscape of horizontal gene transfer.

Antiepileptic drug carbamazepine promotes horizontal transfer of plasmid-borne multi-antibiotic resistance genes within and across bacterial genera

Antibiotic resistance is a severe global threat for public health, causing around 700,000 deaths per year. Horizontal gene transfer (HGT) is one of the most significant pathways to disseminate antibiotic resistance. It is commonly acknowledged that sub-minimum inhibition concentrations of antibiotics are major contributors in promoting antibiotic resistance through HGT. Pharmaceuticals are occurring in our environments at increased levels, yet little is known whether non-antibiotic pharmaceuticals cause or accelerate the dissemination of antibiotic resistance. Here, we report for the first time that the antiepileptic drug, carbamazepine, promotes conjugative transfer of antibiotic resistance genes. It was seen that environmentally relevant concentrations of carbamazepine (e.g., 0.05 mg/L) significantly enhanced the conjugative transfer of multiresistance genes carried by plasmid within and across bacterial genera. The underlying mechanisms of the enhanced HGT were revealed by detecting oxidative stress and cell membrane permeability, in combination with MinION DNA sequencing, genome-wide RNA sequencing, and proteomic analysis. Carbamazepine induced a series of acute responses, including increased levels of reactive oxygen species, the SOS response; increased cell membrane permeability, and pilus generation. Expressional levels of genes related to these processes were significantly upregulated during carbamazepine exposure. Given that HGT occurs widely among different species in various environments, these findings are an early warning for a wide assessment of the roles of non-antibiotic pharmaceuticals in the spread of antibiotic resistance.

Fitness Costs of Plasmids: a Limit to Plasmid Transmission

Plasmids mediate the horizontal transmission of genetic information between bacteria, facilitating their adaptation to multiple environmental conditions. An especially important example of the ability of plasmids to catalyze bacterial adaptation and evolution is their instrumental role in the global spread of antibiotic resistance, which constitutes a major threat to public health. Plasmids provide bacteria with new adaptive tools, but they also entail a metabolic burden that, in the absence of selection for plasmid-encoded traits, reduces the competitiveness of the plasmid-carrying clone. Although this fitness reduction can be alleviated over time through compensatory evolution, the initial cost associated with plasmid carriage is the main constraint on the vertical and horizontal replication of these genetic elements. The fitness effects of plasmids therefore have a crucial influence on their ability to associate with new bacterial hosts and consequently on the evolution of plasmid-mediated antibiotic resistance. However, the molecular mechanisms underlying plasmid fitness cost remain poorly understood. Here, we analyze the literature in the field and examine the potential fitness effects produced by plasmids throughout their life cycle in the host bacterium. We also explore the various mechanisms evolved by plasmids and bacteria to minimize the cost entailed by these mobile genetic elements. Finally, we discuss potential future research directions in the field.

Flexibility of KorA, a plasmid-encoded, global transcription regulator, in the presence and the absence of its operator

The IncP (Incompatibility group P) plasmids are important carriers in the spread of antibiotic resistance across Gram-negative bacteria. Gene expression in the IncP-1 plasmids is stringently controlled by a network of four global repressors, KorA, KorB, TrbA and KorC interacting cooperatively. Intriguingly, KorA and KorB can act as co-repressors at varying distances between their operators, even when they are moved to be on opposite sides of the DNA. KorA is a homodimer with the 101-amino acid subunits, folding into an N-terminal DNA-binding domain and a C-terminal dimerization domain. In this study, we have determined the structures of the free KorA repressor and two complexes each bound to a 20-bp palindromic DNA duplex containing its consensus operator sequence. Using a combination of X-ray crystallography, nuclear magnetic resonance spectroscopy, SAXS and molecular dynamics calculations, we show that the linker between the two domains is very flexible and the protein remains highly mobile in the presence of DNA. This flexibility allows the DNA-binding domains of the dimer to straddle the operator DNA on binding and is likely to be important in cooperative binding to KorB. Unexpectedly, the C-terminal domain of KorA is structurally similar to the dimerization domain of the tumour suppressor p53.

In Vivo and In Vitro Detection of Luminescent and Fluorescent Lactobacillus reuteri and Application of Red Fluorescent mCherry for Assessing Plasmid Persistence

Lactobacillus reuteri is a symbiont that inhabits the gastrointestinal (GI) tract of mammals, and several strains are used as probiotics. After introduction of probiotic strains in a complex ecosystem like the GI tract, keeping track of them is a challenge. The main objectives of this study were to introduce reporter proteins that would enable in vivo and in vitro detection of L. reuteri and increase knowledge about its interactions with the host. We describe for the first time cloning of codon-optimized reporter genes encoding click beetle red luciferase (CBRluc) and red fluorescent protein mCherry in L. reuteri strains ATCC PTA 6475 and R2LC. The plasmid persistence of mCherry-expressing lactobacilli was evaluated by both flow cytometry (FCM) and conventional plate count (PC), and the plasmid loss rates measured by FCM were lower overall than those determined by PC. Neutralization of pH and longer induction duration significantly improved the mCherry signal. The persistency, dose-dependent signal intensity and localization of the recombinant bacteria in the GI tract of mice were studied with an in vivo imaging system (IVIS), which allowed us to detect fluorescence from 6475-CBRluc-mCherry given at a dose of 1×1010 CFU and luminescence signals at doses ranging from 1×105 to 1×1010 CFU. Both 6475-CBRluc-mCherry and R2LC-CBRluc were localized in the colon 1 and 2 h after ingestion, but the majority of the latter were still found in the stomach, possibly reflecting niche specificity for R2LC. Finally, an in vitro experiment showed that mCherry-producing R2LC adhered efficiently to the intra cellular junctions of cultured IPEC-J2 cells. In conclusion, the two reporter genes CBRluc and mCherry were shown to be suitable markers for biophotonic imaging (BPI) of L. reuteri and may provide useful tools for future studies of in vivo and in vitro interactions between the bacteria and the host.

The Interplay between Different Stability Systems Contributes to Faithful Segregation: Streptococcus pyogenes pSM19035 as a Model

The Streptococcus pyogenes pSM19035 low-copy-number θ-replicating plasmid encodes five segregation (seg) loci that contribute to plasmid maintenance. These loci map outside of the minimal replicon. The segA locus comprises β2 recombinase and two six sites, and segC includes segA and also the γ topoisomerase and two ssiA sites. Recombinase β2 plays a role both in maximizing random segregation by resolving plasmid dimers (segA) and in catalyzing inversion between two inversely oriented six sites. segA, in concert with segC, facilitates replication fork pausing at ssiA sites and overcomes the accumulation of "toxic" replication intermediates. The segB1 locus encodes ω, ε, and ζ genes. The short-lived ε2 antitoxin and the long-lived ζ toxin form an inactive ζε2ζ complex. Free ζ toxin halts cell proliferation upon decay of the ε2 antitoxin and enhances survival. If ε2 expression is not recovered, by loss of the plasmid, the toxin raises lethality. The segB2 locus comprises δ and ω genes and six parS sites. Proteins δ2 and ω2, by forming complexes with parS and chromosomal DNA, pair the plasmid copies at the nucleoid, leading to the formation of a dynamic δ2 gradient that separates the plasmids to ensure roughly equal distribution to daughter cells at cell division. The segD locus, which comprises ω2 (or ω2 plus ω22) and parS sites, coordinates expression of genes that control copy number, better-than-random segregation, faithful partition, and antibiotic resistance. The interplay of the seg loci and with the rep locus facilitates almost absolute plasmid stability.

Rebooting the genome: The role of negative feedback in horizontal gene transfer

Horizontal Gene Transfer (HGT) is one of the key mechanisms driving bacterial evolution. Conjugative plasmids are fundamental vehicles for HGT in bacteria, playing an essential role in the spread of antibiotic resistances. Although the classical view has stressed the instrumental role of these mobile genetic elements in the dissemination of antibiotic resistance genes, plasmids contain a rich physiology devoted to horizontal and vertical reproduction. This particular lifestyle imposes specific constrains and trade-offs on plasmid physiology, and plasmids have evolved dedicated circuits to balance the opposing demands of vertical and horizontal reproduction. Recent studies on the transcriptional networks of IncW plasmids and other incompatibility groups have unveiled common architectures in the regulatory networks of different plasmid groups. Comparative studies show that negative feedback loops (NFLs) with strong gains are preferred, opening the question of a possible convergent evolution dictated by certain adaptive properties of this particular network motif. System analysis of NFLs with strong feedback gains indicate that this architecture exhibits transient overshooting after horizontal gene transfer. Since plasmid burden is dependent on the expression of plasmid functions, transcriptional overshooting results in a transient increase of the burden immediately after conjugation. We discuss the possible implications of this phenomenon on plasmid propagation, and the regulatory networks that plasmids have evolved to counteract the detrimental side effects of transient overshooting.

Flow cytometry and real-time quantitative PCR as tools for assessing plasmid persistence

The maintenance of a plasmid in the absence of selection for plasmid-borne genes is not guaranteed. However, plasmid persistence can evolve under selective conditions. Studying the molecular mechanisms behind the evolution of plasmid persistence is key to understanding how plasmids are maintained under nonselective conditions. Given the current crisis of rapid antibiotic resistance spread by multidrug resistance plasmids, this insight is of high medical relevance. The conventional method for monitoring plasmid persistence (i.e., the fraction of plasmid-containing cells in a population over time) is based on cultivation and involves differentiating colonies of plasmid-containing and plasmid-free cells on agar plates. However, this technique is time-consuming and does not easily lend itself to high-throughput applications. Here, we present flow cytometry (FCM) and real-time quantitative PCR (qPCR) as alternative tools for monitoring plasmid persistence. For this, we measured the persistence of a model plasmid, pB10::gfp, in three Pseudomonas hosts and in known mixtures of plasmid-containing and -free cells. We also compared three performance criteria: dynamic range, resolution, and variance. Although not without exceptions, both techniques generated estimates of overall plasmid loss rates that were rather similar to those generated by the conventional plate count (PC) method. They also were able to resolve differences in loss rates between artificial plasmid persistence assays. Finally, we briefly discuss the advantages and disadvantages for each technique and conclude that, overall, both FCM and real-time qPCR are suitable alternatives to cultivation-based methods for routine measurement of plasmid persistence, thereby opening avenues for high-throughput analyses.

Negative feedback and transcriptional overshooting in a regulatory network for horizontal gene transfer

Horizontal gene transfer (HGT) is a major force driving bacterial evolution. Because of their ability to cross inter-species barriers, bacterial plasmids are essential agents for HGT. This ability, however, poses specific requisites on plasmid physiology, in particular the need to overcome a multilevel selection process with opposing demands. We analyzed the transcriptional network of plasmid R388, one of the most promiscuous plasmids in Proteobacteria. Transcriptional analysis by fluorescence expression profiling and quantitative PCR revealed a regulatory network controlled by six transcriptional repressors. The regulatory network relied on strong promoters, which were tightly repressed in negative feedback loops. Computational simulations and theoretical analysis indicated that this architecture would show a transcriptional burst after plasmid conjugation, linking the magnitude of the feedback gain with the intensity of the transcriptional burst. Experimental analysis showed that transcriptional overshooting occurred when the plasmid invaded a new population of susceptible cells. We propose that transcriptional overshooting allows genome rebooting after horizontal gene transfer, and might have an adaptive role in overcoming the opposing demands of multilevel selection.

In the environment, bacteria often evolve by the acquisition of new genes from different species. Plasmids are small DNA molecules that mediate horizontal gene transfer in bacteria, thus they are fundamental agents for the spread of antibiotic resistances. Plasmids replicate inside the bacterial cytoplasm, and propagate infectiously by contact. Plasmids control these two ways of multiplication, but like many other symbionts they suffer from a tradeoff. If plasmids become very infective, they can spread fast and successfully, but this damages the bacterial hosts they depend upon. If, on the contrary, plasmids become very mild, the host is able to grow better but the ability of plasmids to infect new hosts is hampered. We have studied the regulatory mechanisms plasmids use to overcome this paradox. We discovered that negative feedback, a regulatory motif ubiquitous in the plasmid network, allows transient activation of plasmid functions immediately after plasmids invade a new host. This might be an adaptive strategy for plasmids to be highly infective without damaging their hosts, and it illustrates a natural mechanism for DNA transplantation that could be implemented in synthetic genomic transplants.

CRP-cyclic AMP dependent inhibition of the xylene-responsive σ(54)-promoter Pu in Escherichia coli

The expression of σ(54)-dependent Pseudomonas putida Pu promoter is activated by XylR activator when cells are exposed to a variety of aromatic inducers. In this study, the transcriptional activation of the P. putida Pu promoter was recreated in the heterologous host Escherichia coli. Here we show that the cAMP receptor protein (CRP), a well-known carbon utilization regulator, had an inhibitory effect on the expression of Pu promoter in a cAMP-dependent manner. The inhibitory effect was not activator specific. In vivo KMnO4 and DMS footprinting analysis indicated that CRP-cAMP poised the RNA polymerase at Pu promoter, inhibiting the isomerization step of the transcription initiation even in the presence of an activator. Therefore, the presence of PTS-sugar, which eliminates cAMP, could activate the poised RNA polymerase at Pu promoter to transcribe. Moreover, the activation region 1 (AR1) of CRP, which interacts directly with the αCTD (C-terminal domain of α-subunit) of RNA polymerase, was found essential for the CRP-mediated inhibition at Pu promoter. A model for the above observations is discussed.

RepC protein of the octopine-type Ti plasmid binds to the probable origin of replication within repC and functions only in cis

Vegetative replication and partitioning of many plasmids and some chromosomes of alphaproteobacteria are directed by their repABC operons. RepA and RepB proteins direct the partitioning of replicons to daughter cells, while RepC proteins are replication initiators, although they do not resemble any characterized replication initiation protein. Here we show that the replication origin of an Agrobacterium tumefaciens Ti plasmid resides fully within its repC gene. Purified RepC bound to a site within repC with moderate affinity, high specificity and with twofold cooperativity. The binding site was localized to an AT-rich region that contains a large number of GANTC sites, which have been implicated in replication regulation in related organisms. A fragment of RepC containing residues 26-158 was sufficient to bind DNA, although with limited sequence specificity. This portion of RepC is predicted to have structural homology to members of the MarR family of transcription factors. Overexpression of RepC in A. tumefaciens caused large increases in copy number in cis but did not change the copy number of plasmids containing the same oriV sequence in trans, confirming other observations that RepC functions only in cis.

Regulation of bacterial conjugation: balancing opportunity with adversity

Conjugative plasmids are involved in the dissemination of important traits such as antibiotic resistance, virulence determinants and metabolic pathways involved in adapting to environmental niches, a process termed horizontal or lateral gene transfer. Conjugation is the process of transferring DNA from a donor to a recipient cell with the establishment of the incoming DNA and its cargo of genetic traits within the transconjugant. Conjugation is mediated by self-transmissible plasmids as well as phage-like sequences that have been integrated into the bacterial chromosome, such as integrative and conjugative elements (ICEs) that now include conjugative transposons. Both conjugative plasmids and ICEs can mediate the transfer of mobilizable elements by sharing their conjugative machinery. Conjugation can either be induced, usually by small molecules or peptides or by excision of the ICE from the host chromosome, or it can be tightly regulated by plasmid- and host-encoded factors. The transfer potential of these transfer regions depends on the integration of many signals in response to environmental and physiological cues. This review will focus on the mechanisms that influence transfer potential in these systems, particularly those of the IncF incompatibility group.

Plasmid pSM19035, a model to study stable maintenance in Firmicutes

pSM19035 is a low-copy-number theta-replicating plasmid, which belongs to the Inc18 family. Plasmids of this family, which show a modular organization, are functional in evolutionarily diverse bacterial species of the Firmicutes Phylum. This review summarizes our understanding, accumulated during the last 20 years, on the genetics, biochemistry, cytology and physiology of the five pSM19035 segregation (seg) loci, which map outside of the minimal replicon. The segA locus plays a role both in maximizing plasmid random segregation, and in avoiding replication fork collapses in those plasmids with long inverted repeated regions. The segB1 locus, which acts as the ultimate determinant of plasmid maintenance, encodes a short-lived epsilon(2) antitoxin protein and a long-lived zeta toxin protein, which form a complex that neutralizes zeta toxicity. The cells that do not receive a copy of the plasmid halt their proliferation upon decay of the epsilon(2) antitoxin. The segB2 locus, which encodes two trans-acting, ParA- and ParB-like proteins and six cis-acting parS centromeres, actively ensures equal or roughly equal distribution of plasmid copies to daughter cells. The segC locus includes functions that promote the shift from the use of DNA polymerase I to the replicase (PolC-PolE DNA polymerases). The segD locus, which encodes a trans-acting transcriptional repressor, omega(2), and six cis-acting cognate sites, coordinates the expression of genes that control copy number, better-than-random segregation and partition, and assures the proper balance of these different functions. Working in concert the five different loci achieve almost absolute plasmid maintenance with a minimal growth penalty.

Regulation of bacterial conjugation in microaerobiosis by host-encoded functions ArcAB and sdhABCD

The virulence plasmid of Salmonella enterica (pSLT) is an F-like conjugative plasmid. High rates of pSLT transfer occur in the mammalian gut, a microaerobic environment. In this study, we describe genetic screens for host-encoded activators and repressors of the transfer operon (tra) of pSLT. We show that the transcription factor ArcA is an activator of conjugation, especially under microaerobiosis. In turn, succinate dehydrogenase (SdhABCD) is a repressor of mating in aerobiosis. ArcA binds upstream of the main tra promoter (p(traY)) and activates tra transcription, as previously described in F, R1, and R100. In the absence of ArcA, transfer of pSLT decreased 7-fold in aerobiosis and >100-fold in microaerobiosis. In aerobiosis, ArcA activates the traY promoter in an ArcB-independent manner, as described in other F-like plasmids. In microaerobiosis, however, the ArcB sensor is necessary for activation of p(traY). Lack of Sdh causes a >20-fold increase in pSLT transfer in aerobiosis, but has little effect under microaerobiosis. Sdh inhibits conjugal transfer by reducing traJ transcription, probably in an indirect manner. In turn, the sdhCDAB operon is repressed by the ArcAB system under microaerobiosis. Hence, the ArcAB two-component system of S. enterica stimulates pSLT transfer under microaerobiosis by two concerted actions: activation of the tra operon and repression of the sdhCDAB operon.

Carbazole-degradative IncP-7 plasmid pCAR1.2 is structurally unstable in Pseudomonas fluorescens Pf0-1, which accumulates catechol, the intermediate of the carbazole degradation pathway

We determined the effect of the host on the function and structure of the nearly identical IncP-7 carbazole-degradative plasmids pCAR1.1 and pCAR1.2. We constructed Pseudomonas aeruginosa PAO1(pCAR1.2) and P. fluorescens Pf0-1Km(pCAR1.2) and compared their growth on carbazole- and succinate-containing media with that of P. putida KT2440(pCAR1.1). We also assessed the stability of the genetic structures of the plasmids in each of the three hosts. Pf0-1Km(pCAR1.2) showed dramatically delayed growth when carbazole was supplied as the sole carbon source, while the three strains grew at nearly the same rate on succinate. Among the carbazole-grown Pf0-1Km(pCAR1.2) cells, two types of deficient strains appeared and dominated the population; such dominance was not observed in the other two strains or for succinate-grown Pf0-1Km(pCAR1.2). Genetic analysis showed that the two deficient strains possessed pCAR1.2 derivatives in which the carbazole-degradative car operon was deleted or its regulatory gene, antR, was deleted by homologous recombination between insertion sequences. From genomic information and quantitative reverse transcription-PCR analyses of the genes involved in carbazole mineralization by Pf0-1Km(pCAR1.2), we found that the cat genes on the chromosome of Pf0-1Km, which are necessary for the degradation of catechol (a toxic intermediate in the carbazole catabolic pathway), were not induced in the presence of carbazole. The resulting accumulation of catechol may have enabled the strain that lost its carbazole-degrading ability to have overall higher fitness than the wild-type strain. These results suggest that the functions of the chromosomal genes contributed to the selection of plasmid derivatives with altered structures.

The effect of the location of the proteic post-segregational stability system within the replicon of plasmid pTF-FC2 on the fine regulation of plasmid replication

The broad host-range IncQ-2 family plasmid, pTF-FC2, is a mobilizable, medium copy number plasmid that lacks an active partitioning system. Plasmid stability is enhanced by a toxin-antitoxin (TA) system known as pas (plasmid addiction system) that is located within the replicon between the repB (primase) and the repA (helicase) and repC (DNA-binding) genes. The discovery of a closely related IncQ-2 plasmid, pRAS3, with a completely different TA system located between the repB and repAC genes raised the question of whether the location of pas within the replicon had an effect on the plasmid in addition to its ability to act as a TA system. In this work we demonstrate that the presence of the strongly expressed, autoregulated pas operon within the replicon resulted in an increase in the expression of the downstream repAC genes when autoregulation was relieved. While deletion of the pas module did not affect the average plasmid copy number, a pas-containing plasmid exhibited increased stability compared with a pas deletion plasmid even when the TA system was neutralized. It is proposed that the location of a strongly expressed, autoregulated operon within the replicon results in a rapid, but transient, expression of the repAC genes that enables the plasmid to rapidly restore its normal copy number should it fall below a threshold.

Crystal structure of KorA bound to operator DNA: insight into repressor cooperation in RP4 gene regulation

KorA is a global repressor in RP4 which regulates cooperatively the expression of plasmid genes whose products are involved in replication, conjugative transfer and stable inheritance. The structure of KorA bound to an 18-bp DNA duplex that contains the symmetric operator sequence and incorporates 5-bromo-deoxyuridine nucleosides has been determined by multiple-wavelength anomalous diffraction phasing at 1.96-Å resolution. KorA is present as a symmetric dimer and contacts DNA via a helix–turn–helix motif. Each half-site of the symmetric operator DNA binds one copy of the protein in the major groove. As confirmed by mutagenesis, recognition specificity is based on two KorA side chains forming hydrogen bonds to four bases within each operator half-site. KorA has a unique dimerization module shared by the RP4 proteins TrbA and KlcB. We propose that these proteins cooperate with the global RP4 repressor KorB in a similar manner via this dimerization module and thus regulate RP4 inheritance.

A single aromatic residue in transcriptional repressor protein KorA is critical for cooperativity with its co-regulator KorB

A central feature of broad host range IncP-1 plasmids is the set of regulatory circuits that tightly control plasmid core functions under steady-state conditions. Cooperativity between KorB and either KorA or TrbA repressor proteins is a key element of these circuits and deletion analysis has implicated the conserved C-terminal domain of KorA and TrbA in this interaction. By NMR we show that KorA and KorB interact directly and identify KorA amino acids that are affected on KorB binding. Studies on mutants showed that tyrosine 84 (or phenylalanine, in some alleles) is dispensable for repressor activity but critical for the specific interaction with KorB in both in vivo reporter gene assays and in vitro electrophoretic mobility shift and co-purification assays. This confirms that direct and specific protein-protein interactions are responsible for the cooperativity observed between KorB and its corepressors and lays the basis for determining the biological importance of this cooperativity.

Genomic islands: tools of bacterial horizontal gene transfer and evolution

Bacterial genomes evolve through mutations, rearrangements or horizontal gene transfer. Besides the core genes encoding essential metabolic functions, bacterial genomes also harbour a number of accessory genes acquired by horizontal gene transfer that might be beneficial under certain environmental conditions. The horizontal gene transfer contributes to the diversification and adaptation of microorganisms, thus having an impact on the genome plasticity. A significant part of the horizontal gene transfer is or has been facilitated by genomic islands (GEIs). GEIs are discrete DNA segments, some of which are mobile and others which are not, or are no longer mobile, which differ among closely related strains. A number of GEIs are capable of integration into the chromosome of the host, excision, and transfer to a new host by transformation, conjugation or transduction. GEIs play a crucial role in the evolution of a broad spectrum of bacteria as they are involved in the dissemination of variable genes, including antibiotic resistance and virulence genes leading to generation of hospital ‘superbugs’, as well as catabolic genes leading to formation of new metabolic pathways. Depending on the composition of gene modules, the same type of GEIs can promote survival of pathogenic as well as environmental bacteria.

Strong negative self regulation of prokaryotic transcription factors increases the intrinsic noise of protein expression

Background

Many prokaryotic transcription factors repress their own transcription. It is often asserted that such regulation enables a cell to homeostatically maintain protein abundance. We explore the role of negative self regulation of transcription in regulating the variability of protein abundance using a variety of stochastic modeling techniques.

Results

We undertake a novel analysis of a classic model for negative self regulation. We demonstrate that, with standard approximations, protein variance relative to its mean should be independent of repressor strength in a physiological range. Consequently, in that range, the coefficient of variation would increase with repressor strength. However, stochastic computer simulations demonstrate that there is a greater increase in noise associated with strong repressors than predicted by theory. The discrepancies between the mathematical analysis and computer simulations arise because with strong repressors the approximation that leads to Michaelis-Menten-like hyperbolic repression terms ceases to be valid. Because we observe that strong negative feedback increases variability and so is unlikely to be a mechanism for noise control, we suggest instead that negative feedback is evolutionarily favoured because it allows the cell to minimize mRNA usage. To test this, we used in silico evolution to demonstrate that while negative feedback can achieve only a modest improvement in protein noise reduction compared with the unregulated system, it can achieve good improvement in protein response times and very substantial improvement in reducing mRNA levels.

Conclusion

Strong negative self regulation of transcription may not always be a mechanism for homeostatic control of protein abundance, but instead might be evolutionarily favoured as a mechanism to limit the use of mRNA. The use of hyperbolic terms derived from quasi-steady-state approximation should also be avoided in the analysis of stochastic models with strong repressors.

Gene and cell survival: lessons from prokaryotic plasmid R1

Plasmids are units of extrachromosomal genetic inheritance found in all kingdoms of life. They replicate autonomously and undergo stable propagation in their hosts. Despite their small size, plasmid replication and gene expression constitute a metabolic burden that compromises their stable maintenance in host cells. This pressure has driven the evolution of strategies to increase plasmid stability--a process accelerated by the ability of plasmids to transfer horizontally between cells and to exchange genetic material with their host and other resident episomal DNAs. These abilities drive the adaptability and diversity of plasmids and their host cells. Indeed, survival functions found in plasmids have chromosomal homologues that have an essential role in cellular responses to stress. An analysis of these functions in the prokaryotic plasmid R1, and of their intricate interrelationships, reveals remarkable overall similarities with other gene- and cell-survival strategies found within and beyond the prokaryotic world.

Conjugation in Escherichia coli: A laboratory exercise

Bacterial conjugation is a genetic transfer that involves cell-to-cell between donor and recipient cells. With the current method used to teach students in genetic courses at the undergraduate level, the transconjugants are identified using bacterial physiology and/or antibiotic resistance. Using physiology, however, is difficult for both first-year undergraduates and special science students at the high school levels, who do not have the basic knowledge. We have developed a laboratory exercise that comprises a simple and rapid technique for transferring bacterial DNA by conjugation and examining the transconjuants using only antibiotic resistance on agar. The identity of the right transconjuants is confirmed by agarose gel electrophoresis. This exercise is designed to help students understand how horizontal gene transfer occurs in bacteria by conjugation using Escherichia coli as a hands-on learning model. Students should be able to draw concept maps of three DNA transfer methods on their own after carrying out the experiment and getting some additional information.

Genomics of IncP-1 antibiotic resistance plasmids isolated from wastewater treatment plants provides evidence for a widely accessible drug resistance gene pool

The dramatic spread of antibiotic resistance is a crisis in the treatment of infectious diseases that affect humans. Several studies suggest that wastewater treatment plants (WWTP) are reservoirs for diverse mobile antibiotic resistance elements. This review summarizes findings derived from genomic analysis of IncP-1 resistance plasmids isolated from WWTP bacteria. Plasmids that belong to the IncP-1 group are self-transmissible, and transfer to and replicate in a wide range of hosts. Their backbone functions are described with respect to their impact on vegetative replication, stable maintenance and inheritance, mobility and plasmid control. Accessory genetic modules, mainly representing mobile genetic elements, are integrated in-between functional plasmid backbone modules. These elements carry determinants conferring resistance to nearly all clinically relevant antimicrobial drug classes, to heavy metals, and quaternary ammonium compounds used as disinfectants. All plasmids analysed here contain integrons that potentially facilitate integration, exchange and dissemination of resistance gene cassettes. Comparative genomics of accessory modules located on plasmids from WWTP and corresponding modules previously identified in other bacterial genomes revealed that animal, human and plant pathogens and other bacteria isolated from different habitats share a common pool of resistance determinants.

Transcription regulatory circuits in bacterial plasmids

Gene regulation circuits control all aspects of the life of plasmids. This review gives an overview of the current orchestration of the circuits that control plasmid replication, plasmid transfer, plasmid segregation and plasmid maintenance.

Increased abundance of IncP-1beta plasmids and mercury resistance genes in mercury-polluted river sediments: first discovery of IncP-1beta plasmids with a complex mer transposon as the sole accessory element

Although it is generally assumed that mobile genetic elements facilitate the adaptation of microbial communities to environmental stresses, environmental data supporting this assumption are rare. In this study, river sediment samples taken from two mercury-polluted (A and B) and two nonpolluted or less-polluted (C and D) areas of the river Nura (Kazakhstan) were analyzed by PCR for the presence and abundance of mercury resistance genes and of broad-host-range plasmids. PCR-based detection revealed that mercury pollution corresponded to an increased abundance of mercury resistance genes and of IncP-1beta replicon-specific sequences detected in total community DNA. The isolation of IncP-1beta plasmids from contaminated sediments was attempted in order to determine whether they carry mercury resistance genes and thus contribute to an adaptation of bacterial populations to Hg pollution. We failed to detect IncP-1beta plasmids in the genomic DNA of the cultured Hg-resistant bacterial isolates. However, without selection for mercury resistance, three different IncP-1beta plasmids (pTP6, pTP7, and pTP8) were captured directly from contaminated sediment slurry in Cupriavidus necator JMP228 based on their ability to mobilize the IncQ plasmid pIE723. These plasmids hybridized with the merRTDeltaP probe and conferred Hg resistance to their host. A broad host range and high stability under conditions of nonselective growth were observed for pTP6 and pTP7. The full sequence of plasmid pTP6 was determined and revealed a backbone almost identical to that of the IncP-1beta plasmids R751 and pB8. However, this is the first example of an IncP-1beta plasmid which had acquired only a mercury resistance transposon but no antibiotic resistance or biodegradation genes. This transposon carries a rather complex set of mer genes and is inserted between Tra1 and Tra2.

Flexibility in repression and cooperativity by KorB of broad host range IncP-1 plasmid RK2

KorB, encoded by plasmid RK2, belongs to the ParB family of active partitioning proteins. It binds to 12 operators on the RK2 genome and was previously known to repress promoters immediately adjacent to operators O(B)1, O(B)10 and O(B)12 (proximal) or up to 154 bp away (distal) from O(B)2, O(B)9 and O(B)11. To achieve strong repression, KorB requires a cooperative interaction with one of two other plasmid-encoded repressors, KorA or TrbA. Reporter gene assays were used in this study to test whether the additional KorB operators may influence transcription and to test how KorB acts at a distance. The distance between O(B)9 and trbBp could be increased to 1.6kb with little reduction in repression or cooperativity with TrbA. KorB was also able to repress the promoter and cooperate with TrbA when the O(B) site was placed downstream of trbBp. This suggested a potential regulatory role for O(B) sites located a long way from any known promoter on RK2. O(B)4, 1.9kb upstream of traGp, was shown to mediate TrbA-potentiated KorB repression of this promoter, but no effect on traJp upstream of O(B)4 was observed, which may be due to the roadblocking or topological influence of the nucleoprotein complex formed at the adjacent transfer origin, oriT. Repression and cooperativity were alleviated significantly when a lac operator was inserted between O(B)9 and trbBp in the context of a LacI+ host, a standard test for spreading of a DNA-binding protein. On the other hand, a standard test for DNA looping, movement of the operator to the opposite face of the DNA helix from the natural binding site, did not significantly affect KorB repression or cooperativity with TrbA and KorA over relatively short distances. While these results are more consistent with spreading as the mechanism by which KorB reaches its target, previous estimates of KorB molecules per cell are not consistent with there being enough to spread up to 1kb from each O(B). A plausible model is therefore that KorB can do both, spreading over relatively short distances and looping over longer distances.

Transcriptional regulation of pWW0 transfer genes in Pseudomonas putida KT2440

The conjugative IncP-9 plasmid pWW0 (TOL) carries transfer genes, many of whose functions can be predicted from sequence similarities to the well-studied IncW and IncP-1 plasmids, and that are clustered with the replication and maintenance genes of the plasmid core. In this study we show that the IncP-9 transfer genes are transcribed from at least three promoter regions. The promoters for traA and traD act divergently from the region found to encode the origin of transfer, oriT. These promoters regulate expression of traA, B, and perhaps traC in one direction and traD in the other, all of whose gene products are predicted to be involved in relaxasome formation and DNA processing during transfer, and they are repressed by TraA. The third promoter region, upstream of mpfR, is responsible for transcription of mpfR and mpfA to mpfJ, encoding proteins involved in mating pair formation. Transcription from this region is negatively autoregulated by MpfR. Thus the pWW0 transfer genes, like those of the IncP-1 plasmids, are expressed at all times, but kept in control by a negative feed back loop to limit the metabolic burden on the host. Although many of the related mating pair formation systems are, as in pWW0, transcribed divergently from an operon for replication and/or stable inheritance functions, MpfR is not related to the known regulatory proteins of these other transfer systems outside those of the IncP-9 family and indeed the regulators tend to be specific for each plasmid family. This suggests that the general pattern of genetic organisation exhibited by these systems has arisen a number of times independently and must therefore be highly favourable to plasmid survival and spread.

The Cytology of Bacterial Conjugation

This review focuses on the membrane-associated structures present at cell-cell contact sites during bacterial conjugation. These transfer proteins/structures have roles in the formation and stabilization of mating contacts and ultimately the passage of substrate across the cell envelope between two bacterial cells. The review presents evidence for the dynamic interaction between donor and recipient cells, including the assembly of a transmembrane protein complex, and concludes with a refined model for the mechanism of bacterial conjugation. Bacterial conjugation, in addition to being a mechanism for genome evolution, can be considered as a mechanism for macromolecular secretion. In particular, plasmid-conjugative transfer is classified as a type IV secretion (T4S) system and represents the only known bacterial system for secretion of DNA. In all known conjugative transfer systems, a multitude of proteins are required for both plasmid transfer and pilus production. The plasmids discussed in the review include the F factor; the P group of plasmids, including RP4 and R751 (rigid); and the H plasmid group, including R27 (also thick flexible). With the LacI-GFP/lacO system, the F, P, and H plasmids were observed to reside at well-defined positions located at the mid and quarter-cell positions of Escherichia coli throughout the vegetative cycle. In this review, recent observations based on bacterial cell biology techniques, including visualization of plasmid DNA and proteins at the subcellular level, have been combined with electron and light microscopy studies of mating cells to create an integrated overview of gram-negative bacterial conjugation, a concept referred to as the conjugative cycle.

Identification and characterization of traE genes of Spiroplasma kunkelii

Four traE homologs, designated traE1, traE2, traE3 and traE4, were identified and amplified from the genome of the leafhopper-transmitted corn stunt pathogen Spiroplasma kunkelii and were predicted to encode membrane-bound adenine tri-phosphatases (ATPases). Deduced proteins of all traE genes have 62.3% to 89.9% similarity to the conserved VirB4 domain that is frequently a component of type IV secretory pathways involved in intracellular trafficking and secretion of DNA and proteins. In phylogenetic analysis, TraE homologs of S. kunkelii, Mycoplasma pulmonis and Mycoplasma fermentans cluster together and are more similar to TraE proteins of Gram-positive bacteria than to those of Gram-negative bacteria, thereby resembling the 16S rRNA phylogeny. Gene traE2 was most conserved whereas the presence of the three other traE genes varied among S. kunkelii strains, M2, CS-2B, FL-80 and PU8-17. Further, traE1 and traE2 appeared to be located on the chromosome, and traE3 and traE4 genes on plasmids of S. kunkelii strain M2. Transcripts of the spiralin gene and traE2 genes were detected on Northern blots containing total ribonucleic acids (RNA) of S. kunkelii cultures and S. kunkelii-infected plants and insects, in which traE2 appeared to be of a larger transcription unit. Full-length expression products of the other traE genes were not detected. S. kunkelii traE genes could be involved in S. kunkelii cell morphogenesis, adhesion and DNA recombination.

Co-operative interactions control conjugative transfer of broad host-range plasmid RK2: full effect of minor changes in TrbA operator depends on KorB

A network of circuits, with KorB and TrbA as key regulators, controls genes for conjugative transfer of broad host range plasmid RK2. To assess the importance of the TrbA regulon, mutational analysis was applied to the TrbA operator at the trbB promoter and then to other TrbA-regulated promoters in the tra region. All identified TrbA operators are submaximal; in the case of trbBp, a G to A transition that made the operator core a perfect palindrome increased repression by about 50% compared to the wild type. When this change was introduced into the RK2 genome, decreases in transfer frequency of up to three orders of magnitude were observed, with bigger effects when Escherichia coli was the donor compared to Pseudomonas putida. Western blotting showed a significant decrease in Trb protein levels. These effects were much greater than the effect of the mutation on repression by TrbA alone. When KorB was introduced into the reporter system, the effects were closer to those observed in the whole RK2 context. These results indicate that co-operativity, previously observed between TrbA and KorB, allows big changes in transfer gene expression to result from small changes in individual regulator activities.

A rapid screen for functional mutants of TraM, an autoregulatory protein required for F conjugation

TraM is an autoregulatory protein required for conjugative transfer of the F plasmid. A rapid screening procedure was developed to select for traM mutants constructed by random PCR mutagenesis. The mutated traM gene was cloned into pT7-5, without the traM promoters (collectively called P( traM)), such that these mutants were expressed from the downstream traJ promoter, resulting in constitutive, low-level, transcription of traM by polymerases that had circumnavigated the plasmid. P( traM) was cloned into pPR9tt as a translational fusion in which a DNA fragment containing P( traM), the ribosome binding site and first 24 codons of traM was fused to the 5' end of lacZ. To downregulate beta-galactosidase expression, a -1 frameshift mutation was introduced at the junction between traM and lacZ in the fusion. Selected TraM mutants were further characterized for their intracellular levels, electrophoretic mobility on nondenaturing gels, and activity in F conjugation. Point mutations throughout TraM were found to affect both autoregulation and conjugative function.

A LuxR-type regulator from Agrobacterium tumefaciens elevates Ti plasmid copy number by activating transcription of plasmid replication genes

TraR, a LuxR-type quorum-sensing transcription factor in Agrobacterium tumefaciens, activates genes required for conjugal transfer of the Ti plasmid and also enhances the copy number of a nopaline-type Ti plasmid. Here, we show that TraR increases the copy number of an octopine-type Ti plasmid up to eightfold and that TraR activates transcription of the repABC operon up to 25-fold. The ability of TraR to increase copy number was strictly dependent on several TraR-activated promoters of this operon, indicating that TraR affects copy number solely at the level of transcription. Promoter resections and mRNA transcript analysis revealed the presence of three TraR-dependent promoters. Two TraR-dependent transcription start sites are located 45.5 and 65.5 nucleotides downstream of a site called tra box II, whereas the third start site lies 42.5 nucleotides downstream of a site called tra box III. Purified TraR bound to both tra boxes with comparable affinities, causing moderate DNA bending. TraR bound and bent these two sites independently rather than synergistically. Alteration of tra box III to match the consensus sequence dramatically increased TraR-dependent expression of repABC and plasmid copy number. TraR-dependent elevation of Ti plasmid copy number caused a three- to fourfold increase in plant tumorigenesis.

In vitro and in vivo stability of the epsilon2zeta2 protein complex of the broad host-range Streptococcus pyogenes pSM19035 addiction system

Streptococcus pyogenes pSM19035-encoded epsilon (10.7 kDa) and zeta (32.4 kDa) proteins are necessary to secure stable plasmid inheritance in bacteria, with zeta acting as toxin that kills plasmid-deprived cells and epsilon as an antitoxin that neutralises the activity of zeta. The epsilon and zeta proteins co-purify as a stable complex that, according to analytical ultracentrifugation and gel filtration, exists as epsilon2zeta2 heterotetramer in solution. Co-crystals of the epsilon2zeta2 complex contain epsilon and zeta in 1:1 molar ratio. Unfolding studies monitoring circular dichroic and fluorescence changes show that the zeta protein has a significantly lower thermodynamic stability than the epsilon protein both in free state and in the complex. Proteolytic studies indicate that zeta protein is more stable in the epsilon2zeta2 complex than in the free state. In vivo studies reveal a short half-life of the epsilon antitoxin (-18 min) and a long lifetime of the zeta toxin (>60 min). When transcription-translation of a plasmid containing the epsilon and zeta genes was inhibited, cell death was observed after a short lag phase that correlates with the disappearance of the epsilon protein from the background.

Bacterial conjugative transfer: visualization of successful mating pairs and plasmid establishment in live Escherichia coli

We used the LacO/GFP-LacI system to label and visualize the IncP beta plasmid R751 fluorescently during conjugative transfer between live donor and recipient bacteria. Comparisons of R751 in conjugative and non-conjugative conditions have allowed us to identify key localizations and movements associated with the initiation of conjugative transfer in the donor and the establishment of R751 in the recipient. A survey of successful mating pairs demonstrates that close physical contact between donor and recipient bacteria is required for DNA transfer and that regions of intimate contact can occur at any location on the donor or recipient cell membrane. The transferred DNA is positioned at the characteristic centre or quarter-cell position after conversion to a double-stranded molecule in the recipient cell. Initial duplication of plasmids often results in an asymmetric distribution of plasmid foci. Symmetric localization (either at centre or at 1/4 and 3/4 cell lengths) occurs only after a significant lag, presumably reflecting the time required to synthesize the plasmid-encoded partitioning proteins.