Vibrio cholerae ParE2 poisons DNA gyrase via a mechanism distinct from other gyrase inhibitors

Evaluation of different strategies to produce Vibrio cholerae ParE2 toxin

Toxin-antitoxin (TA) systems are small operons that are omnipresent in bacteria and archaea with suggested roles in stabilization of mobile genetic elements, bacteriophage protection, stress response and possibly persister formation. A major bottleneck in the study of TA toxins is the production of sufficient amounts of well-folded, functional protein. Here we examine alternative approaches for obtaining the VcParE2 toxin from Vibrio cholerae. VcParE2 can be successfully produced via bacterial expression in presence of its cognate antitoxin VcParD2, followed by on-column unfolding and refolding. Alternatively, the toxin can be expressed in Spodoptera frugiperda (Sf9) insect cells. The latter requires disruption of the VcparE2 gene via introduction of an insect cell intron. Both methods provide protein with similar structural and functional characteristics.

Toxin release by conditional remodelling of ParDE1 from Mycobacterium tuberculosis leads to gyrase inhibition

Mycobacterium tuberculosis, the causative agent of tuberculosis, is a growing threat to global health, with recent efforts towards its eradication being reversed in the wake of the COVID-19 pandemic. Increasing resistance to gyrase-targeting second-line fluoroquinolone antibiotics indicates the necessity to develop both novel therapeutics and our understanding of M. tuberculosis growth during infection. ParDE toxin-antitoxin systems also target gyrase and are regulated in response to both host-associated and drug-induced stress during infection. Here, we present microbiological, biochemical, structural, and biophysical analyses exploring the ParDE1 and ParDE2 systems of M. tuberculosis H37Rv. The structures reveal conserved modes of toxin-antitoxin recognition, with complex-specific interactions. ParDE1 forms a novel heterohexameric ParDE complex, supported by antitoxin chains taking on two distinct folds. Curiously, ParDE1 exists in solution as a dynamic equilibrium between heterotetrameric and heterohexameric complexes. Conditional remodelling into higher order complexes can be thermally driven in vitro. Remodelling induces toxin release, tracked through concomitant inhibition and poisoning of gyrase activity. Our work aids our understanding of gyrase inhibition, allowing wider exploration of toxin-antitoxin systems as inspiration for potential therapeutic agents.

Friend or Foe: Protein Inhibitors of DNA Gyrase

DNA gyrase is essential for the successful replication of circular chromosomes, such as those found in most bacterial species, by relieving topological stressors associated with unwinding the double-stranded genetic material. This critical central role makes gyrase a valued target for antibacterial approaches, as exemplified by the highly successful fluoroquinolone class of antibiotics. It is reasonable that the activity of gyrase could be intrinsically regulated within cells, thereby helping to coordinate DNA replication with doubling times. Numerous proteins have been identified to exert inhibitory effects on DNA gyrase, although at lower doses, it can appear readily reversible and therefore may have regulatory value. Some of these, such as the small protein toxins found in plasmid-borne addiction modules, can promote cell death by inducing damage to DNA, resulting in an analogous outcome as quinolone antibiotics. Others, however, appear to transiently impact gyrase in a readily reversible and non-damaging mechanism, such as the plasmid-derived Qnr family of DNA-mimetic proteins. The current review examines the origins and known activities of protein inhibitors of gyrase and highlights opportunities to further exert control over bacterial growth by targeting this validated antibacterial target with novel molecular mechanisms. Furthermore, we are gaining new insights into fundamental regulatory strategies of gyrase that may prove important for understanding diverse growth strategies among different bacteria.

Toxin:antitoxin ratio sensing autoregulation of the Vibrio cholerae parDE2 module

The parDE family of toxin-antitoxin (TA) operons is ubiquitous in bacterial genomes and, in Vibrio cholerae, is an essential component to maintain the presence of chromosome II. Here, we show that transcription of the V. cholerae parDE2 (VcparDE) operon is regulated in a toxin:antitoxin ratio-dependent manner using a molecular mechanism distinct from other type II TA systems. The repressor of the operon is identified as an assembly with a 6:2 stoichiometry with three interacting ParD2 dimers bridged by two ParE2 monomers. This assembly docks to a three-site operator containing 5'- GGTA-3' motifs. Saturation of this TA complex with ParE2 toxin results in disruption of the interface between ParD2 dimers and the formation of a TA complex of 2:2 stoichiometry. The latter is operator binding-incompetent as it is incompatible with the required spacing of the ParD2 dimers on the operator.

Type II bacterial toxin-antitoxins: hypotheses, facts, and the newfound plethora of the PezAT system

Toxin-antitoxin (TA) systems are entities found in the prokaryotic genomes, with eight reported types. Type II, the best characterized, is comprised of two genes organized as an operon. Whereas toxins impair growth, the cognate antitoxin neutralizes its activity. TAs appeared to be involved in plasmid maintenance, persistence, virulence, and defence against bacteriophages. Most Type II toxins target the bacterial translational machinery. They seem to be antecessors of Higher Eukaryotes and Prokaryotes Nucleotide-binding (HEPN) RNases, minimal nucleotidyltransferase domains, or CRISPR-Cas systems. A total of four TAs encoded by Streptococcus pneumoniae, RelBE, YefMYoeB, Phd-Doc, and HicAB, belong to HEPN-RNases. The fifth is represented by PezAT/Epsilon-Zeta. PezT/Zeta toxins phosphorylate the peptidoglycan precursors, thereby blocking cell wall synthesis. We explore the body of knowledge (facts) and hypotheses procured for Type II TAs and analyse the data accumulated on the PezAT family. Bioinformatics analyses showed that homologues of PezT/Zeta toxin are abundantly distributed among 14 bacterial phyla mostly in Proteobacteria (48%), Firmicutes (27%), and Actinobacteria (18%), showing the widespread distribution of this TA. The pezAT locus was found to be mainly chromosomally encoded whereas its homologue, the tripartite omega-epsilon-zeta locus, was found mostly on plasmids. We found several orphan pezT/zeta toxins, unaccompanied by a cognate antitoxin.

A Multi-Layer-Controlled Strategy for Cloning and Expression of Toxin Genes in Escherichia coli

Molecular cloning and controlled expression remain challenging when the target gene encodes a protein that is toxic to the host. We developed a set of multi-layer control systems to enable cloning of genes encoding proteins known to be highly toxic in Escherichia coli and other bacteria. The different multi-layer control systems combine a promoter-operator system on a transcriptional level with a riboswitch for translational control. Additionally, replicational control is ensured by using a strain that reduces the plasmid copy number. The use of weaker promoters (such as PBAD or PfdeA) in combination with the effective theophylline riboswitch is essential for cloning genes that encode notoriously toxic proteins that directly target translation and transcription. Controlled overexpression is possible, allowing the system to be used for evaluating in vivo effects of the toxin. Systems with a stronger promoter can be used for successful overexpression and purification of the desired protein but are limited to toxins that are more moderate and do not interfere with their own production.

A ParDE toxin-antitoxin system is responsible for the maintenance of the Yersinia virulence plasmid but not for type III secretion-associated growth inhibition

Many Gram-negative pathogens utilize the type III secretion system (T3SS) to translocate virulence-promoting effector proteins into eukaryotic host cells. The activity of this system results in a severe reduction of bacterial growth and division, summarized as secretion-associated growth inhibition (SAGI). In Yersinia enterocolitica, the T3SS and related proteins are encoded on a virulence plasmid. We identified a ParDE-like toxin-antitoxin system on this virulence plasmid in genetic proximity to yopE, encoding a T3SS effector. Effectors are strongly upregulated upon activation of the T3SS, indicating a potential role of the ParDE system in the SAGI or maintenance of the virulence plasmid. Expression of the toxin ParE in trans resulted in reduced growth and elongated bacteria, highly reminiscent of the SAGI. Nevertheless, the activity of ParDE is not causal for the SAGI. T3SS activation did not influence ParDE activity; conversely, ParDE had no impact on T3SS assembly or activity itself. However, we found that ParDE ensures the presence of the T3SS across bacterial populations by reducing the loss of the virulence plasmid, especially under conditions relevant to infection. Despite this effect, a subset of bacteria lost the virulence plasmid and regained the ability to divide under secreting conditions, facilitating the possible emergence of T3SS-negative bacteria in late acute and persistent infections.

Systematic transcriptome analysis allows the identification of new type I and type II Toxin/Antitoxin systems located in the superintegron of Vibrio cholerae

Vibrio cholerae N16961 genome encodes 18 type II Toxin/Antitoxin (TA) systems, all but one located inside gene cassettes of its chromosomal superintegron (SI). This study aims to investigate additional TA systems in this genome. We screened for all two-genes operons of uncharacterized function by analyzing previous RNAseq data. Assays on nine candidates, revealed one additional functional type II TA encoded by the VCA0497-0498 operon, carried inside a SI cassette. We showed that VCA0498 antitoxin alone and in complex with VCA0497 represses its own operon promoter. VCA0497-0498 is the second element of the recently identified dhiT/dhiA superfamily uncharacterized type II TA system. RNAseq analysis revealed that another SI cassette encodes a novel type I TA system: VCA0495 gene and its two associated antisense non-coding RNAs, ncRNA495 and ncRNA496. Silencing of both antisense ncRNAs lead to cell death, demonstrating the type I TA function. Both VCA0497 and VCA0495 toxins do not show any homology to functionally characterized toxins, however our preliminary data suggest that their activity may end up in mRNA degradation, directly or indirectly. Our findings increase the TA systems number carried in this SI to 19, preferentially located in its distal end, confirming their importance in this large cassette array.

Toxin-antitoxin systems in pathogenic Vibrio species: a mini review from a structure perspective

Toxin-antitoxin (TA) genetic modules have been found to widely exist in bacterial chromosomes and mobile genetic elements. They are composed of stable toxins and less stable antitoxins that can counteract the toxicity of toxins. The interactions between toxins and antitoxins could play critical roles in the virulence and persistence of pathogenic bacteria. There are at least eight types of TA systems which have been identified in a variety of bacteria. Vibrio, a genus of Gram-negative bacteria, is widespread in aquatic environments and can cause various human diseases, such as epidemic cholera. In this review, we mainly explore the structures and functions of TA modules found in common Vibrio pathogens, mainly V. cholerae, for better understanding of TA action mechanisms in pathogenic bacteria.

ParD Antitoxin Hotspot Alters a Disorder-to-Order Transition upon Binding to Its Cognate ParE Toxin, Lessening Its Interaction Affinity and Increasing Its Protease Degradation Kinetics

Type-II toxin-antitoxin (TA) systems are comprised of two tightly interacting proteins, and operons encoding these systems have been identified throughout the genomes of bacteria. In contrast to secretion system effector-immunity pairs, TA systems must remain paired to protect the host cell from toxicity. Continual depletion of the antitoxin results in a shorter half-life than that of the toxin, though it is unclear if antitoxins can be effectively degraded when complexed with toxins. The current work probed the protein-protein interface of the PaParDE1 TA system, guided by an X-ray crystal structure, to determine contributions of antitoxin amino acids to interaction kinetics and affinity. These studies identified a "hotspot" position that alters the binding mode and resulting affinity (K_D) from 152 pM for a 1:1 model for wild type to 25.5 and 626 nM for a 2:1 model with mutated antitoxin. This correlates with an altered induced secondary structure upon complexation with PaParE1 and increased kinetics of Lon protease digestion of the antitoxin despite the toxin presence. However, the decreased affinity at this hotspot was essentially reversed when the antitoxin dimerization region was deleted, yielding insights into complex interactions involved in the tight association. Removal of the antitoxin C-terminal seven amino acids, corresponding to the site of a disorder-to-order transition, completely prevents association. These studies combine to provide a model for the initiation of the TA interaction and highlight how manipulation of the sequence can impact the antitoxin disorder-to-order transition, weakening the affinity and resulting in increased antitoxin susceptibility to degradation.

Entropic pressure controls the oligomerization of the Vibrio cholerae ParD2 antitoxin

ParD2 is the antitoxin component of the parDE2 toxin-antitoxin module from Vibrio cholerae and consists of an ordered DNA-binding domain followed by an intrinsically disordered ParE-neutralizing domain. In the absence of the C-terminal intrinsically disordered protein (IDP) domain, V. cholerae ParD2 (VcParD2) crystallizes as a doughnut-shaped hexadecamer formed by the association of eight dimers. This assembly is stabilized via hydrogen bonds and salt bridges rather than by hydrophobic contacts. In solution, oligomerization of the full-length protein is restricted to a stable, open decamer or dodecamer, which is likely to be a consequence of entropic pressure from the IDP tails. The relative positioning of successive VcParD2 dimers mimics the arrangement of Streptococcus agalactiae CopG dimers on their operator and allows an extended operator to wrap around the VcParD2 oligomer.

Toxin-antitoxin systems and their medical applications: current status and future perspective

Almost all bacteria synthesize two types of toxins-one for its survival by regulating different cellular processes and another as a strategy to interact with host cells for pathogenesis. Usually, "bacterial toxins" are contemplated as virulence factors that harm the host organism. However, toxins produced by bacteria, as a survival strategy against the host, also hamper its cellular processes. To overcome this, the bacteria have evolved with the production of a molecule, referred to as antitoxin, to negate the deleterious effect of the toxin against itself. The toxin and antitoxins are encoded by a two-component toxin-antitoxin (TA) system. The antitoxin, a protein or RNA, sequesters the toxins of the TA system for neutralization within the bacterial cell. In this review, we have described different TA systems of bacteria and their potential medical and biotechnological applications. It is of interest to note that while bacterial toxin-antitoxin systems have been well studied, the TA system in unicellular eukaryotes, though predicted by the investigators, have never been paid the desired attention. In the present review, we have also touched upon the TA system of eukaryotes identified to date. KEY POINTS: Bacterial toxins harm the host and also affect the bacterial cellular processes. The antitoxin produced by bacteria protect it from the toxin's harmful effects. The toxin-antitoxin systems can be targeted for various medical applications.

Uncovering the basis of protein-protein interaction specificity with a combinatorially complete library

Protein-protein interaction specificity is often encoded at the primary sequence level. However, the contributions of individual residues to specificity are usually poorly understood and often obscured by mutational robustness, sequence degeneracy, and epistasis. Using bacterial toxin-antitoxin systems as a model, we screened a combinatorially complete library of antitoxin variants at three key positions against two toxins. This library enabled us to measure the effect of individual substitutions on specificity in hundreds of genetic backgrounds. These distributions allow inferences about the general nature of interface residues in promoting specificity. We find that positive and negative contributions to specificity are neither inherently coupled nor mutually exclusive. Further, a wild-type antitoxin appears optimized for specificity as no substitutions improve discrimination between cognate and non-cognate partners. By comparing crystal structures of paralogous complexes, we provide a rationale for our observations. Collectively, this work provides a generalizable approach to understanding the logic of molecular recognition.

Recent advancements in the medicinal chemistry of bacterial type II topoisomerase inhibitors

Replication proteins are sought as a potential targets for antimicrobial agents. Despite their promising target characteristics, only topoisomerase II inhibitors targeting DNA gyrase and/or topoisomerase IV have reached clinical use. Topoisomerases are the enzymes that are essential for cellular functions and various biological activities. A wide range of natural and synthetic compounds have been identified as potential topoisomerase inhibitors but the resistance is most commonly found in these drugs. The emergence of FQ resistance has increased the need for the development of novel topoisomerase inhibitors with efficacy and high potency against FQ-resistant strains. Besides structural modifications of existing FQ scaffolds, novel non-quinolone topoisomerase II inhibitors, known as novel bacterial topoisomerase inhibitors, have been developed which showed remarkable inhibitory activity against DNA gyrase/topoisomerase IV or both with an improved spectrum of antibacterial potency including drug-resistant strains. This review aims to summarize various recent advancements in the medicinal chemistry of topoisomerase inhibitors with the following objectives: (1) To represent inclusive data on types of topoisomerases and various marketed topoisomerase inhibitors as drugs; (2) To discuss the recent advances in the medicinal chemistry of various topoisomerase inhibitors (DNA gyrase and topo IV) belonging to different structural classes as potential antibacterial agents; (3) To summarizes the structure activity relationship (SAR) including in silico and mechanistic studies to afford ideas and to provide focused direction for the development of new chemical entities which are effective against drug-resistant bacterial pathogens and biofilms.

Evaluating the Potential for Cross-Interactions of Antitoxins in Type II TA Systems

The diversity of Type-II toxin–antitoxin (TA) systems in bacterial genomes requires tightly controlled interaction specificity to ensure protection of the cell, and potentially to limit cross-talk between toxin–antitoxin pairs of the same family of TA systems. Further, there is a redundant use of toxin folds for different cellular targets and complexation with different classes of antitoxins, increasing the apparent requirement for the insulation of interactions. The presence of Type II TA systems has remained enigmatic with respect to potential benefits imparted to the host cells. In some cases, they play clear roles in survival associated with unfavorable growth conditions. More generally, they can also serve as a “cure” against acquisition of highly similar TA systems such as those found on plasmids or invading genetic elements that frequently carry virulence and resistance genes. The latter model is predicated on the ability of these highly specific cognate antitoxin–toxin interactions to form cross-reactions between chromosomal antitoxins and invading toxins. This review summarizes advances in the Type II TA system models with an emphasis on antitoxin cross-reactivity, including with invading genetic elements and cases where toxin proteins share a common fold yet interact with different families of antitoxins.

The Variety in the Common Theme of Translation Inhibition by Type II Toxin-Antitoxin Systems

Type II Toxin-antitoxin (TA) modules are bacterial operons that encode a toxic protein and its antidote, which form a self-regulating genetic system. Antitoxins put a halter on toxins in many ways that distinguish different types of TA modules. In type II TA modules, toxin and antitoxin are proteins that form a complex which physically sequesters the toxin, thereby preventing its toxic activity. Type II toxins inhibit various cellular processes, however, the translation process appears to be their favorite target and nearly every step of this complex process is inhibited by type II toxins. The structural features, enzymatic activities and target specificities of the different toxin families are discussed. Finally, this review emphasizes that the structural folds presented by these toxins are not restricted to type II TA toxins or to one particular cellular target, and discusses why so many of them evolved to target translation as well as the recent developments regarding the role(s) of these systems in bacterial physiology and evolution.

Expression of different ParE toxins results in conserved phenotypes with distinguishable classes of toxicity

Toxin–antitoxin (TA) systems are found on both chromosomes and plasmids. These systems are unique in that they can confer both fatal and protective effects on bacterial cells—a quality that could potentially be harnessed given further understanding of these TA mechanisms. The current work focuses on the ParE subfamily, which is found throughout proteobacteria and has a sequence identity on average of approximately 12% (similarity at 30%–80%). Our aim is to evaluate the equivalency of chromosomally derived ParE toxin activity depending on its bacterial species of origin. Nine ParE toxins were analyzed, originating from six different bacterial species. Based on the resulting toxicity, three categories can be established: ParE toxins that do not exert toxicity under the experimental conditions, toxins that exert toxicity within the first four hours, and those that exert toxicity only after 10–12 hr of exposure. All tested ParE toxins produce a cellular morphologic change from rods to filaments, consistent with disruption of DNA topology. Analysis of the distribution of filamented cells within a population reveals a correlation between the extent of filamentation and toxicity. No membrane septation is visible along the length of the cell filaments, whereas aberrant lipid blebs are evident. Potent ParE‐mediated toxicity is also correlated with a hallmark signature of abortive DNA replication, consistent with the inhibition of DNA gyrase.

New Strategy on Antimicrobial-resistance: Inhibitors of DNA Replication Enzymes

BACKGROUND

Antimicrobial resistance is found in all microorganisms and has become one of the biggest threats to global health. New antimicrobials with different action mechanisms are effective weapons to fight against antibiotic-resistance.

OBJECTIVE

This review aims to find potential drugs which can be further developed into clinic practice and provide clues for developing more effective antimicrobials.

METHODS

DNA replication universally exists in all living organisms and is a complicated process in which multiple enzymes are involved in. Enzymes in bacterial DNA replication of initiation and elongation phases bring abundant targets for antimicrobial development as they are conserved and indispensable. In this review, enzyme inhibitors of DNA helicase, DNA primase, topoisomerases, DNA polymerase and DNA ligase were discussed. Special attentions were paid to structures, activities and action modes of these enzyme inhibitors.

RESULTS

Among these enzymes, type II topoisomerase is the most validated target with abundant inhibitors. For type II topoisomerase inhibitors (excluding quinolones), NBTIs and benzimidazole urea derivatives are the most promising inhibitors because of their good antimicrobial activity and physicochemical properties. Simultaneously, DNA gyrase targeted drugs are particularly attractive in the treatment of tuberculosis as DNA gyrase is the sole type II topoisomerase in Mycobacterium tuberculosis. Relatively, exploitation of antimicrobial inhibitors of the other DNA replication enzymes are primeval, in which inhibitors of topo III are even blank so far.

CONCLUSION

This review demonstrates that inhibitors of DNA replication enzymes are abundant, diverse and promising, many of which can be developed into antimicrobials to deal with antibioticresistance.

TASmania: A bacterial Toxin-Antitoxin Systems database

Bacterial Toxin-Antitoxin systems (TAS) are involved in key biological functions including plasmid maintenance, defense against phages, persistence and virulence. They are found in nearly all phyla and classified into 6 different types based on the mode of inactivation of the toxin, with the type II TAS being the best characterized so far. We have herein developed a new in silico discovery pipeline named TASmania, which mines the >41K assemblies of the EnsemblBacteria database for known and uncharacterized protein components of type I to IV TAS loci. Our pipeline annotates the proteins based on a list of curated HMMs, which leads to >2^.10⁶ loci candidates, including orphan toxins and antitoxins, and organises the candidates in pseudo-operon structures in order to identify new TAS candidates based on a guilt-by-association strategy. In addition, we classify the two-component TAS with an unsupervised method on top of the pseudo-operon (pop) gene structures, leading to 1567 “popTA” models offering a more robust classification of the TAs families. These results give valuable clues in understanding the toxin/antitoxin modular structures and the TAS phylum specificities. Preliminary in vivo work confirmed six putative new hits in Mycobacterium tuberculosis as promising candidates. The TASmania database is available on the following server https://shiny.bioinformatics.unibe.ch/apps/tasmania/.

TASmania offers an extensive annotation of TA loci in a very large database of bacterial genomes, which represents a resource of crucial importance for the microbiology community. TASmania supports i) the discovery of new TA families; ii) the design of a robust experimental strategy by taking into account potential interferences in trans; iii) the comparative analysis between TA loci content, phylogeny and/or phenotypes (pathogenicity, persistence, stress resistance, associated host types) by providing a vast repertoire of annotated assemblies. Our database contains TA annotations of a given strain not only mapped to its core genome but also to its plasmids, whenever applicable.

Pseudomonas putida Responds to the Toxin GraT by Inducing Ribosome Biogenesis Factors and Repressing TCA Cycle Enzymes

The potentially self-poisonous toxin-antitoxin modules are widespread in bacterial chromosomes, but despite extensive studies, their biological importance remains poorly understood. Here, we used whole-cell proteomics to study the cellular effects of the Pseudomonas putida toxin GraT that is known to inhibit growth and ribosome maturation in a cold-dependent manner when the graA antitoxin gene is deleted from the genome. Proteomic analysis of P. putida wild-type and ΔgraA strains at 30 °C and 25 °C, where the growth is differently affected by GraT, revealed two major responses to GraT at both temperatures. First, ribosome biogenesis factors, including the RNA helicase DeaD and RNase III, are upregulated in ΔgraA. This likely serves to alleviate the ribosome biogenesis defect of the ΔgraA strain. Secondly, proteome data indicated that GraT induces downregulation of central carbon metabolism, as suggested by the decreased levels of TCA cycle enzymes isocitrate dehydrogenase Idh, α-ketoglutarate dehydrogenase subunit SucA, and succinate-CoA ligase subunit SucD. Metabolomic analysis revealed remarkable GraT-dependent accumulation of oxaloacetate at 25 °C and a reduced amount of malate, another TCA intermediate. The accumulation of oxaloacetate is likely due to decreased flux through the TCA cycle but also indicates inhibition of anabolic pathways in GraT-affected bacteria. Thus, proteomic and metabolomic analysis of the ΔgraA strain revealed that GraT-mediated stress triggers several responses that reprogram the cell physiology to alleviate the GraT-caused damage.

The toxin from a ParDE toxin-antitoxin system found in Pseudomonas aeruginosa offers protection to cells challenged with anti-gyrase antibiotics

Toxin-antitoxin systems are mediators of diverse activities in bacterial physiology. For the ParE-type toxins, their reported role of gyrase inhibition utilized during plasmid-segregation killing indicates they are toxic. However, their location throughout chromosomes leads to questions about function, including potential non-toxic outcomes. The current study has characterized a ParDE system from the opportunistic human pathogen Pseudomonas aeruginosa (Pa). We identified a protective function for this ParE toxin, PaParE, against effects of quinolone and other antibiotics. However, higher concentrations of PaParE are themselves toxic to cells, indicating the phenotypic outcome can vary based on its concentration. Our assays confirmed PaParE inhibition of gyrase-mediated supercoiling of DNA with an IC50 value in the low micromolar range, a species-specificity that resulted in more efficacious inhibition of Escherichia coli derived gyrase versus Pa gyrase, and overexpression in the absence of antitoxin yielded an expected filamentous morphology with multi-foci nucleic acid material. Additional data revealed that the PaParE toxin is monomeric and interacts with dimeric PaParD antitoxin with a KD in the lower picomolar range, yielding a heterotetramer. This work provides novel insights into chromosome-encoded ParE function, whereby its expression can impart partial protection to cultures from selected antibiotics.

Identification of Three Type II Toxin-Antitoxin Systems in Streptococcus suis Serotype 2

Type II toxin-antitoxin (TA) systems are highly prevalent in bacterial genomes and have been extensively studied. These modules involve in the formation of persistence cells, the biofilm formation, and stress resistance, which might play key roles in pathogen virulence. SezAT and yefM-yoeB TA modules in Streptococcus suis serotype 2 (S. suis 2) have been studied, although the other TA systems have not been identified. In this study, we investigated nine putative type II TA systems in the genome of S. suis 2 strain SC84 by bioinformatics analysis and identified three of them (two relBE loci and one parDE locus) that function as typical type II TA systems. Interestingly, we found that the introduction of the two RelBE TA systems into Escherichia coli or the induction of the ParE toxin led to cell filamentation. Promoter activity assays indicated that RelB1, RelB2, ParD, and ParDE negatively autoregulated the transcriptions of their respective TA operons, while RelBE2 positively autoregulated its TA operon transcription. Collectively, we identified three TA systems in S. suis 2, and our findings have laid an important foundation for further functional studies on these TA systems.

Expansion of the SOS regulon of Vibrio cholerae through extensive transcriptome analysis and experimental validation

BACKGROUND

The SOS response is an almost ubiquitous response of cells to genotoxic stresses. The full complement of genes in the SOS regulon for Vibrio species has only been addressed through bioinformatic analyses predicting LexA binding box consensus and in vitro validation. Here, we perform whole transcriptome sequencing from Vibrio cholerae treated with mitomycin C as an SOS inducer to characterize the SOS regulon and other pathways affected by this treatment.

RESULTS

Comprehensive transcriptional profiling allowed us to define the full landscape of promoters and transcripts active in V. cholerae. We performed extensive transcription start site (TSS) mapping as well as detection/quantification of the coding and non-coding RNA (ncRNA) repertoire in strain N16961. To improve TSS detection, we developed a new technique to treat RNA extracted from cells grown in various conditions. This allowed for identification of 3078 TSSs with an average 5'UTR of 116 nucleotides, and peak distribution between 16 and 64 nucleotides; as well as 629 ncRNAs. Mitomycin C treatment induced transcription of 737 genes and 28 ncRNAs at least 2 fold, while it repressed 231 genes and 17 ncRNAs. Data analysis revealed that in addition to the core genes known to integrate the SOS regulon, several metabolic pathways were induced. This study allowed for expansion of the Vibrio SOS regulon, as twelve genes (ubiEJB, tatABC, smpA, cep, VC0091, VC1190, VC1369-1370) were found to be co-induced with their adjacent canonical SOS regulon gene(s), through transcriptional read-through. Characterization of UV and mitomycin C susceptibility for mutants of these newly identified SOS regulon genes and other highly induced genes and ncRNAs confirmed their role in DNA damage rescue and protection.

CONCLUSIONS

We show that genotoxic stress induces a pervasive transcriptional response, affecting almost 20% of the V. cholerae genes. We also demonstrate that the SOS regulon is larger than previously known, and its syntenic organization is conserved among Vibrio species. Furthermore, this specific co-localization is found in other γ-proteobacteria for genes recN-smpA and rmuC-tatABC, suggesting SOS regulon conservation in this phylum. Finally, we comment on the limitations of widespread NGS approaches for identification of all RNA species in bacteria.

New Shuttle Vectors for Gene Cloning and Expression in Multidrug-Resistant Acinetobacter Species

Understanding bacterial pathogenesis requires adequate genetic tools to assess the role of individual virulence determinants by mutagenesis and complementation assays, as well as for homologous and heterologous expression of cloned genes. Our knowledge of Acinetobacter baumannii pathogenesis has so far been limited by the scarcity of genetic tools to manipulate multidrug-resistant (MDR) epidemic strains, which are responsible for most infections. Here, we report on the construction of new multipurpose shuttle plasmids, namely, pVRL1 and pVRL2, which can efficiently replicate in Acinetobacter spp. and in Escherichia coli The pVRL1 plasmid has been constructed by combining (i) the cryptic plasmid pWH1277 from Acinetobacter calcoaceticus, which provides an origin of replication for Acinetobacter spp.; (ii) a ColE1-like origin of replication; (iii) the gentamicin or zeocin resistance cassette for antibiotic selection; and (iv) a multilinker containing several unique restriction sites. Modification of pVRL1 led to the generation of the pVRL2 plasmid, which allows arabinose-inducible gene transcription with an undetectable basal expression level of cloned genes under uninduced conditions and a high dynamic range of responsiveness to the inducer. Both pVRL1 and pVRL2 can easily be selected in MDR A. baumannii, have a narrow host range and a high copy number, are stably maintained in Acinetobacter spp., and appear to be compatible with indigenous plasmids carried by epidemic strains. Plasmid maintenance is guaranteed by the presence of a toxin-antitoxin system, providing more insights into the mechanism of plasmid stability in Acinetobacter spp.

Novel toxins from type II toxin-antitoxin systems with acetyltransferase activity

Type II toxin-antitoxin (TA) systems are widespread in bacterial and archeal genomes. These modules are very dynamic and participate in bacterial genome evolution through horizontal gene transfer. TA systems are commonly composed of a labile antitoxin and a stable toxin. Toxins appear to preferentially inhibit the protein synthesis process. Toxins use a variety of molecular mechanisms and target nearly every step of translation to achieve their inhibitory function. This review focuses on a recently identified TA family that includes acetyltransferase toxins. The AtaT and TacT toxins are the best-characterized to date in this family. AtaT and TacT both inhibit translation by acetylating the amino acid charged on tRNAs. However, the specificities of these 2 toxins are different as AtaT inhibits translation initiation by acetylation of the initiator tRNA whereas TacT acetylates elongator tRNAs. The molecular mechanisms of these toxins are discussed, as well as the functions and possible evolutionary origins of this diverse toxin family.

Production, biophysical characterization and crystallization of Pseudomonas putida GraA and its complexes with GraT and the graTA operator

The graTA operon from Pseudomonas putida encodes a toxin-antitoxin module with an unusually moderate toxin. Here, the production, SAXS analysis and crystallization of the antitoxin GraA, the GraTA complex and the complex of GraA with a 33 bp operator fragment are reported. GraA forms a homodimer in solution and crystallizes in space group P21, with unit-cell parameters a = 66.9, b = 48.9, c = 62.7 Å, β = 92.6°. The crystals are likely to contain two GraA dimers in the asymmetric unit and diffract to 1.9 Å resolution. The GraTA complex forms a heterotetramer in solution. Crystals of the GraTA complex diffracted to 2.2 Å resolution and are most likely to contain a single heterotetrameric GraTA complex in the asymmetric unit. They belong to space group P41 or P43, with unit-cell parameters a = b = 56.0, c = 128.2 Å. The GraA-operator complex consists of a 33 bp operator region that binds two GraA dimers. It crystallizes in space group P31 or P32, with unit-cell parameters a = b = 105.6, c = 149.9 Å. These crystals diffract to 3.8 Å resolution.

A RelE/ParE superfamily toxin in Vibrio parahaemolyticus has DNA nicking endonuclease activity

Type II toxins in toxin-antitoxin (TA) systems fold into a similar fold and belong to the RelE/ParE superfamily. However, they display two distinct biochemical activities: RelE toxins are mRNA interferases, while ParE toxins are DNA gyrase (Gyr) inhibitors. Previously, we found a TA system, vp1842/vp1843, on the Vibrio parahaemolyticus genome whose toxin Vp1843 belongs to the RelE/ParE toxin superfamily. Vp1843, unlike RelE toxins, has neither protein synthesis inhibitory activity nor ribonuclease activity. In this study, we examined the inhibitory potency of Vp1843 with Escherichia coli Gyr. The result showed that Vp1843, unlike other ParE toxins, had little Gyr inhibitory activity, but rather converted supercoiled DNA to open-circular DNA. Analysis showed further that Vp1843 cleaves a single strand in DNA, and that the antitoxin Vp1842 neutralized the nicking endonuclease activity of Vp1843. Mutations of Lys37 and Pro45 in Vp1843 abolished its nicking activity, suggesting that they play a crucial role in nicking endonuclease activity. To our knowledge, Vp1843 is the first toxin with DNA nicking endonuclease activity among the RelE/ParE toxin superfamily.

The Chromosomal parDE2 Toxin-Antitoxin System of Mycobacterium tuberculosis H37Rv: Genetic and Functional Characterization

Mycobacterium tuberculosis H37Rv escapes host-generated stresses by entering a dormant persistent state. Activation of toxin-antitoxin modules is one of the mechanisms known to trigger such a state with low metabolic activity. M. tuberculosis harbors a large number of TA systems mostly located within discernible genomic islands. We have investigated the parDE2 operon of M. tuberculosis H37Rv encoding MParE2 toxin and MParD2 antitoxin proteins. The parDE2 locus was transcriptionally active from growth phase till late stationary phase in M. tuberculosis. A functional promoter located upstream of parD2 GTG start-site was identified by 5'-RACE and lacZ reporter assay. The MParD2 protein transcriptionally regulated the P parDE2 promoter by interacting through Arg16 and Ser15 residues located in the N-terminus. In Escherichia coli, ectopic expression of MParE2 inhibited growth in early stages, with a drastic reduction in colony forming units. Live-dead analysis revealed that the reduction was not due to cell death alone but due to formation of viable but non-culturable cells (VBNCs) also. The toxic activity of the protein, identified in the C-terminal residues Glu98 and Arg102, was neutralized by the antitoxin MParD2, both in vivo and in vitro. MParE2 inhibited mycobacterial DNA gyrase and interacted with the GyrB subunit without affecting its ATPase activity. Introduction of parE2 gene in the heterologous M. smegmatis host prevented growth and colony formation by the transformed cells. An M. smegmatis strain containing the parDE2 operon also switched to a non-culturable phenotype in response to oxidative stress. Loss in colony-forming ability of a major part of the MParE2 expressing cells suggests its potential role in dormancy, a cellular strategy for adaptation to environmental stresses. Our study has laid the foundation for future investigations to explore the physiological significance of parDE2 operon in mycobacterial pathogenesis.

Desperate times call for desperate measures: benefits and costs of toxin-antitoxin systems

Toxin-antitoxin (TA) loci were first described as killing systems for plasmid maintenance. The surprisingly abundant presence of TA loci in bacterial chromosomes has stimulated an extensive research in the recent decade aimed to understand the biological importance of these potentially deadly systems. Accumulating evidence suggests that the evolutionary success of genomic TA systems could be explained by their ability to increase bacterial fitness under stress conditions. While TA systems remain quiescent under favorable growth conditions, the toxins can be activated in response to stress resulting in growth suppression and development of stress-tolerant dormant state. Yet, several studies suggest that the TA-mediated stress protection is costly and traded off against decreased fitness under normal growth conditions. Here, we give an overview of the fitness benefits of the chromosomal TA systems, and discuss the costs of TA-mediated stress protection.

Keeping the Wolves at Bay: Antitoxins of Prokaryotic Type II Toxin-Antitoxin Systems

In their initial stages of discovery, prokaryotic toxin-antitoxin (TA) systems were confined to bacterial plasmids where they function to mediate the maintenance and stability of usually low- to medium-copy number plasmids through the post-segregational killing of any plasmid-free daughter cells that developed. Their eventual discovery as nearly ubiquitous and repetitive elements in bacterial chromosomes led to a wealth of knowledge and scientific debate as to their diversity and functionality in the prokaryotic lifestyle. Currently categorized into six different types designated types I–VI, type II TA systems are the best characterized. These generally comprised of two genes encoding a proteic toxin and its corresponding proteic antitoxin, respectively. Under normal growth conditions, the stable toxin is prevented from exerting its lethal effect through tight binding with the less stable antitoxin partner, forming a non-lethal TA protein complex. Besides binding with its cognate toxin, the antitoxin also plays a role in regulating the expression of the type II TA operon by binding to the operator site, thereby repressing transcription from the TA promoter. In most cases, full repression is observed in the presence of the TA complex as binding of the toxin enhances the DNA binding capability of the antitoxin. TA systems have been implicated in a gamut of prokaryotic cellular functions such as being mediators of programmed cell death as well as persistence or dormancy, biofilm formation, as defensive weapons against bacteriophage infections and as virulence factors in pathogenic bacteria. It is thus apparent that these antitoxins, as DNA-binding proteins, play an essential role in modulating the prokaryotic lifestyle whilst at the same time preventing the lethal action of the toxins under normal growth conditions, i.e., keeping the proverbial wolves at bay. In this review, we will cover the diversity and characteristics of various type II TA antitoxins. We shall also look into some interesting deviations from the canonical type II TA systems such as tripartite TA systems where the regulatory role is played by a third party protein and not the antitoxin, and a unique TA system encoding a single protein with both toxin as well as antitoxin domains.

A unique hetero-hexadecameric architecture displayed by the Escherichia coli O157 PaaA2-ParE2 antitoxin-toxin complex

Many bacterial pathogens modulate their metabolic activity, virulence and pathogenicity through so-called "toxin-antitoxin" (TA) modules. The genome of the human pathogen Escherichia coli O157 contains two three-component TA modules related to the known parDE module. Here, we show that the toxin EcParE2 maps in a branch of the RelE/ParE toxin superfamily that is distinct from the branches that contain verified gyrase and ribosome inhibitors. The structure of EcParE2 closely resembles that of Caulobacter crescentus ParE but shows a distinct pattern of conserved surface residues, in agreement with its apparent inability to interact with GyrA. The antitoxin EcPaaA2 is characterized by two α-helices (H1 and H2) that serve as molecular recognition elements to wrap itself around EcParE2. Both EcPaaA2 H1 and H2 are required to sustain a high-affinity interaction with EcParE2 and for the inhibition of EcParE2-mediated killing in vivo. Furthermore, evidence demonstrates that EcPaaA2 H2, but not H1, determines specificity for EcParE2. The initially formed EcPaaA2-EcParE2 heterodimer then assembles into a hetero-hexadecamer, which is stable in solution and is formed in a highly cooperative manner. Together these findings provide novel data on quaternary structure, TA interactions and activity of a hitherto poorly characterized family of TA modules.

Conditional Activation of Toxin-Antitoxin Systems: Postsegregational Killing and Beyond

Toxin-antitoxin (TA) systems are small genetic modules formed by a stable toxin and an unstable antitoxin that are widely present in plasmids and in chromosomes of Bacteria and Archaea. Toxins can interfere with cell growth or viability, targeting a variety of key processes. Antitoxin inhibits expression of the toxin, interacts with it, and neutralizes its effect. In a plasmid context, toxins are kept silent by the continuous synthesis of the unstable antitoxins; in plasmid-free cells (segregants), toxins can be activated owing to the faster decay of the antitoxin, and this results in the elimination of these cells from the population (postsegregational killing [PSK]) and in an increase of plasmid-containing cells in a growing culture. Chromosomal TA systems can also be activated in particular circumstances, and the interference with cell growth and viability that ensues contributes in different ways to the physiology of the cell. In this article, we review the conditional activation of TAs in selected plasmidic and chromosomal TA pairs and the implications of this activation. On the whole, the analysis underscores TA interactions involved in PSK and points to the effective contribution of TA systems to the physiology of the cell.

Mycobacterium tuberculosis Type II Toxin-Antitoxin Systems: Genetic Polymorphisms and Functional Properties and the Possibility of Their Use for Genotyping

Various genetic markers such as IS-elements, DR-elements, variable number tandem repeats (VNTR), single nucleotide polymorphisms (SNPs) in housekeeping genes and other groups of genes are being used for genotyping. We propose a different approach. We suggest the type II toxin-antitoxin (TA) systems, which play a significant role in the formation of pathogenicity, tolerance and persistence phenotypes, and thus in the survival of Mycobacterium tuberculosis in the host organism at various developmental stages (colonization, infection of macrophages, etc.), as the marker genes. Most genes of TA systems function together, forming a single network: an antitoxin from one pair may interact with toxins from other pairs and even from other families. In this work a bioinformatics analysis of genes of the type II TA systems from 173 sequenced genomes of M. tuberculosis was performed. A number of genes of type II TA systems were found to carry SNPs that correlate with specific genotypes. We propose a minimally sufficient set of genes of TA systems for separation of M. tuberculosis strains at nine basic genotype and for further division into subtypes. Using this set of genes, we genotyped a collection consisting of 62 clinical isolates of M. tuberculosis. The possibility of using our set of genes for genotyping using PCR is also demonstrated.

tRNAs taking charge

Most bacterial toxins derived from chromosomally encoded toxin-antitoxin (TA) systems that have been studied to date appear to protect cells from relatively short pulses of stress by triggering a reversible state of growth arrest. In contrast to many bacterial toxins that are produced as defense mechanisms and secreted from their hosts, TA toxins exert their protective effect from within the cell that produces them. TA toxin-mediated growth arrest is most frequently achieved through their ability to selectively cleave RNA species that participate in protein synthesis. Until very recently, it was thought that the primary conduit for toxin-mediated translation inhibition was cleavage of a single class of RNA, mRNA, thus depleting transcripts and precluding production of essential proteins. This minireview focuses on how the development and implementation of a specialized RNA-seq method to study Mycobacterium tuberculosis TA systems enabled the identification of unexpected RNA targets for toxins, i.e. a handful of tRNAs that are cleaved into tRNA halves. Our result brings to light a new perspective on how these toxins may act in this pathogen and uncovers a striking parallel to signature features of the eukaryotic stress response.

Comprehensive Functional Analysis of the 18 Vibrio cholerae N16961 Toxin-Antitoxin Systems Substantiates Their Role in Stabilizing the Superintegron

The role of chromosomal toxin-antitoxin (TA) systems, which are ubiquitous within the genomes of free-living bacteria, is still debated. We have scanned the Vibrio cholerae N16961 genome for class 2 TA genes and identified 18 gene pair candidates. Interestingly, all but one are located in the chromosome 2 superintegron (SI). The single TA found outside the SI is located on chromosome 1 and is related to the well-characterized HipAB family, which is known to play a role in antibiotic persistence. We investigated this clustering within the SI and its possible biological consequences by performing a comprehensive functional analysis on all of the putative TA systems. We demonstrate that the 18 TAs identified encode functional toxins and that their cognate antitoxins are able to neutralize their deleterious effects when expressed in Escherichia coli. In addition, we reveal that the 17 predicted TA systems of the SI are transcribed and expressed in their native context from their own promoters, a situation rarely found in integron cassettes. We tested the possibility of interactions between noncognate pairs of all toxins and antitoxins and found no cross-interaction between any of the different TAs. Although these observations do not exclude other roles, they clearly strengthen the role of TA systems in stabilizing the massive SI cassette array of V. cholerae.

IMPORTANCE

The chromosomal toxin-antitoxin systems have been shown to play various, sometimes contradictory roles, ranging from genomic stabilization to bacterial survival via persistence. Determining the interactions between TA systems hosted within the same bacteria is essential to understand the hierarchy between these different roles. We identify here the full set of class 2 TAs carried in the Vibrio cholerae N16961 genome and found they are all, with a single exception, located in the chromosome 2 superintegron. Their characterization, in terms of functionality, expression, and possible cross-interactions, supports their main role as being the stabilization of the 176-cassette-long array of the superintegron but does not exclude dual roles, such as stress response elements, persistence, and bacteriophage defense through abortive infection mechanisms.

Modulation of bacterial proliferation as a survival strategy

The cell cycle is one of the most fundamental processes in biology, underlying the proliferation and growth of all living organisms. In bacteria, the cell cycle has been extensively studied since the 1950s. Most of this research has focused on cell cycle regulation in a few model bacteria, cultured under standard growth conditions. However in nature, bacteria are exposed to drastic environmental changes. Recent work shows that by modulating their own growth and proliferation bacteria can increase their survival under stressful conditions, including antibiotic treatment. Here, we review the mechanisms that allow bacteria to integrate environmental information into their cell cycle. In particular, we focus on mechanisms controlling DNA replication and cell division. We conclude this chapter by highlighting the importance of understanding bacterial cell cycle and growth control for future research as well as other disciplines.

Crystallization and preliminary X-ray analysis of two variants of the Escherichia coli O157 ParE2-PaaA2 toxin-antitoxin complex

The paaR2-paaA2-parE2 operon is a three-component toxin-antitoxin module encoded in the genome of the human pathogen Escherichia coli O157. The toxin (ParE2) and antitoxin (PaaA2) interact to form a nontoxic toxin-antitoxin complex. In this paper, the crystallization and preliminary characterization of two variants of the ParE2-PaaA2 toxin-antitoxin complex are described. Selenomethionine-derivative crystals of the full-length ParE2-PaaA2 toxin-antitoxin complex diffracted to 2.8 Å resolution and belonged to space group P41212 (or P43212), with unit-cell parameters a = b = 90.5, c = 412.3 Å. It was previously reported that the full-length ParE2-PaaA2 toxin-antitoxin complex forms a higher-order oligomer. In contrast, ParE2 and PaaA213-63, a truncated form of PaaA2 in which the first 12 N-terminal residues of the antitoxin have been deleted, form a heterodimer as shown by analytical gel filtration, dynamic light scattering and small-angle X-ray scattering. Crystals of the PaaA213-63-ParE2 complex diffracted to 2.7 Å resolution and belonged to space group P6122 (or P6522), with unit-cell parameters a = b = 91.6, c = 185.6 Å.

VapC from the leptospiral VapBC toxin-antitoxin module displays ribonuclease activity on the initiator tRNA

The prokaryotic ubiquitous Toxin-Antitoxin (TA) operons encode a stable toxin and an unstable antitoxin. The most accepted hypothesis of the physiological function of the TA system is the reversible cessation of cellular growth under stress conditions. The major TA family, VapBC is present in the spirochaete Leptospira interrogans. VapBC modules are classified based on the presence of a predicted ribonucleasic PIN domain in the VapC toxin. The expression of the leptospiral VapC in E. coli promotes a strong bacterial growth arrestment, making it difficult to express the recombinant protein. Nevertheless, we showed that long term induction of expression in E. coli enabled the recovery of VapC in inclusion bodies. The recombinant protein was successfully refolded by high hydrostatic pressure, providing a new method to obtain the toxin in a soluble and active form. The structural integrity of the recombinant VapB and VapC proteins was assessed by circular dichroism spectroscopy. Physical interaction between the VapC toxin and the VapB antitoxin was demonstrated in vivo and in vitro by pull down and ligand affinity blotting assays, respectively, thereby indicating the ultimate mechanism by which the activity of the toxin is regulated in bacteria. The predicted model of the leptospiral VapC structure closely matches the Shigella's VapC X-ray structure. In agreement, the ribonuclease activity of the leptospiral VapC was similar to the activity described for Shigella's VapC, as demonstrated by the cleavage of tRNAfMet and by the absence of unspecific activity towards E. coli rRNA. This finding suggests that the cleavage of the initiator transfer RNA may represent a common mechanism to a larger group of bacteria and potentially configures a mechanism of post-transcriptional regulation leading to the inhibition of global translation.

Structural and biophysical characterization of Staphylococcus aureus SaMazF shows conservation of functional dynamics

The Staphylococcus aureus genome contains three toxin-antitoxin modules, including one mazEF module, SamazEF. Using an on-column separation protocol we are able to obtain large amounts of wild-type SaMazF toxin. The protein is well-folded and highly resistant against thermal unfolding but aggregates at elevated temperatures. Crystallographic and nuclear magnetic resonance (NMR) solution studies show a well-defined dimer. Differences in structure and dynamics between the X-ray and NMR structural ensembles are found in three loop regions, two of which undergo motions that are of functional relevance. The same segments also show functionally relevant dynamics in the distantly related CcdB family despite divergence of function. NMR chemical shift mapping and analysis of residue conservation in the MazF family suggests a conserved mode for the inhibition of MazF by MazE.

Regulating toxin-antitoxin expression: controlled detonation of intracellular molecular timebombs

Genes for toxin-antitoxin (TA) complexes are widely disseminated in bacteria, including in pathogenic and antibiotic resistant species. The toxins are liberated from association with the cognate antitoxins by certain physiological triggers to impair vital cellular functions. TAs also are implicated in antibiotic persistence, biofilm formation, and bacteriophage resistance. Among the ever increasing number of TA modules that have been identified, the most numerous are complexes in which both toxin and antitoxin are proteins. Transcriptional autoregulation of the operons encoding these complexes is key to ensuring balanced TA production and to prevent inadvertent toxin release. Control typically is exerted by binding of the antitoxin to regulatory sequences upstream of the operons. The toxin protein commonly works as a transcriptional corepressor that remodels and stabilizes the antitoxin. However, there are notable exceptions to this paradigm. Moreover, it is becoming clear that TA complexes often form one strand in an interconnected web of stress responses suggesting that their transcriptional regulation may prove to be more intricate than currently understood. Furthermore, interference with TA gene transcriptional autoregulation holds considerable promise as a novel antibacterial strategy: artificial release of the toxin factor using designer drugs is a potential approach to induce bacterial suicide from within.

Toxin-antitoxin systems as multilevel interaction systems

Toxin-antitoxin (TA) systems are small genetic modules usually composed of a toxin and an antitoxin counteracting the activity of the toxic protein. These systems are widely spread in bacterial and archaeal genomes. TA systems have been assigned many functions, ranging from persistence to DNA stabilization or protection against mobile genetic elements. They are classified in five types, depending on the nature and mode of action of the antitoxin. In type I and III, antitoxins are RNAs that either inhibit the synthesis of the toxin or sequester it. In type II, IV and V, antitoxins are proteins that either sequester, counterbalance toxin activity or inhibit toxin synthesis. In addition to these interactions between the antitoxin and toxin components (RNA-RNA, protein-protein, RNA-protein), TA systems interact with a variety of cellular factors, e.g., toxins target essential cellular components, antitoxins are degraded by RNAses or ATP-dependent proteases. Hence, TA systems have the capacity to interact with each other at different levels. In this review, we will discuss the different interactions in which TA systems are involved and their implications in TA system functions and evolution.

A bacterial toxin inhibits DNA replication elongation through a direct interaction with the β sliding clamp

Toxin-antitoxin (TA) systems are ubiquitous on bacterial chromosomes, yet the mechanisms regulating their activity and the molecular targets of toxins remain incompletely defined. Here, we identify SocAB, an atypical TA system in Caulobacter crescentus. Unlike canonical TA systems, the toxin SocB is unstable and constitutively degraded by the protease ClpXP; this degradation requires the antitoxin, SocA, as a proteolytic adaptor. We find that the toxin, SocB, blocks replication elongation through an interaction with the sliding clamp, driving replication fork collapse. Mutations that suppress SocB toxicity map to either the hydrophobic cleft on the clamp that binds DNA polymerase III or a clamp-binding motif in SocB. Our findings suggest that SocB disrupts replication by outcompeting other clamp-binding proteins. Collectively, our results expand the diversity of mechanisms employed by TA systems to regulate toxin activity and inhibit bacterial growth, and they suggest that inhibiting clamp function may be a generalizable antibacterial strategy.

Characterization of the phd-doc and ccd toxin-antitoxin cassettes from Vibrio superintegrons

Toxin-antitoxin (TA) systems have been reported in the genomes of most bacterial species, and their role when located on the chromosome is still debated. TA systems are particularly abundant in the massive cassette arrays associated with chromosomal superintegrons (SI). Here, we describe the characterization of two superintegron cassettes encoding putative TA systems. The first is the phd-doc(SI) system identified in Vibrio cholerae N16961. We determined its distribution in 36 V. cholerae strains and among five V. metschnikovii strains. We show that this cassette, which is in position 72 of the V. cholerae N16961 cassette array, is functional, carries its own promoter, and is expressed from this location. Interestingly, the phd-doc(SI) system is unable to control its own expression, most likely due to the absence of any DNA-binding domain on the antitoxin. In addition, this SI system is able to cross talk with the canonical P1 phage system. The second cassette that we characterized is the ccd(Vfi) cassette found in the V. fischeri superintegron. We demonstrate that CcdB(Vfi) targets DNA-gyrase, as the canonical CcB(F) toxin, and that ccd(Vfi) regulates its expression in a fashion similar to the ccd(F) operon of the conjugative plasmid F. We also establish that this cassette is functional and expressed in its chromosomal context in V. fischeri CIP 103206T. We tested its functional interactions with the ccdAB(F) system and found that CcdA(Vfi) is specific for its associated CcdB(Vfi) and cannot prevent CcdB(F) toxicity. Based on these results, we discuss the possible biological functions of these TA systems in superintegrons.