Conformational Change in the Catalytic Site of the Ribonuclease YoeB Toxin by YefM Antitoxin

Identification of the first functional toxin-antitoxin system in Streptomyces

Toxin-antitoxin (TA) systems are widespread among the plasmids and genomes of bacteria and archaea. This work reports the first description of a functional TA system in Streptomyces that is identical in two species routinely used in the laboratory: Streptomyces lividans and S. coelicolor. The described system belongs to the YefM/YoeB family and has a considerable similarity to Escherichia coli YefM/YoeB (about 53% identity and 73% similarity). Lethal effect of the S. lividans putative toxin (YoeBsl) was observed when expressed alone in E. coli SC36 (MG1655 ΔyefM-yoeB). However, no toxicity was obtained when co-expression of the antitoxin and toxin (YefM/YoeBsl) was carried out. The toxic effect was also observed when the yoeBsl was cloned in multicopy in the wild-type S. lividans or in a single copy in a S. lividans mutant, in which this TA system had been deleted. The S. lividans YefM/YoeBsl complex, purified from E. coli, binds with high affinity to its own promoter region but not to other three random selected promoters from Streptomyces. In vivo experiments demonstrated that the expression of yoeBsl in E. coli blocks translation initiation processing mRNA at three bases downstream of the initiation codon after 2 minutes of induction. These results indicate that the mechanism of action is identical to that of YoeB from E. coli.

Toxin-antitoxin systems in bacteria and archaea

Almost all bacteria and many archaea contain genes whose expression inhibits cell growth and may lead to cell death when overproduced, reminiscent of apoptotic genes in higher systems. The cellular targets of these toxins are quite diverse and include DNA replication, mRNA stability, protein synthesis, cell-wall biosynthesis, and ATP synthesis. These toxins are co-expressed and neutralized with their cognate antitoxins from a TA (toxin-antitoxin) operon in normally growing cells. Antitoxins are more labile than toxins and are readily degraded under stress conditions, allowing the toxins to exert their toxic effect. Presence of at least 33 TA systems in Escherichia coli and more than 60 TA systems in Mycobacterium tuberculosis suggests that the TA systems are involved not only in normal bacterial physiology but also in pathogenicity of bacteria. The elucidation of their cellular function and regulation is thus crucial for our understanding of bacterial physiology under various stress conditions.

Molecular structure and function of the novel BrnT/BrnA toxin-antitoxin system of Brucella abortus

Type II toxin-antitoxin (TA) systems are expressed from two-gene operons that encode a cytoplasmic protein toxin and its cognate protein antitoxin. These gene cassettes are often present in multiple copies on bacterial chromosomes, where they have been reported to regulate stress adaptation and persistence during antimicrobial treatment. We have identified a novel type II TA cassette in the intracellular pathogen Brucella abortus that consists of the toxin gene, brnT, and its antitoxin, brnA. BrnT is coexpressed and forms a 2:2 tetrameric complex with BrnA, which neutralizes BrnT toxicity. The BrnT(2)-BrnA(2) tetramer binds its own promoter via BrnA, and autorepresses its expression; its transcription is strongly induced in B. abortus by various stressors encountered by the bacterial cell during infection of a mammalian host. Although highly divergent at the primary sequence level, an atomic resolution (1.1 Å) crystal structure of BrnT reveals a secondary topology related to the RelE family of type II ribonuclease toxins. However, overall tertiary structural homology to other RelE family toxins is low. A functional characterization of BrnT by site-directed mutagenesis demonstrates a correspondence between its in vitro activity as a ribonuclease and control of bacteriostasis in vivo. We further present an analysis of the conserved and variable features of structure required for RNA scission in BrnT and the RelE toxin family. This structural investigation informs a model of the RelE-fold as an evolutionarily flexible scaffold that has been selected to bind structurally disparate antitoxins, and exhibit distinct toxin activities including RNA scission and DNA gyrase inhibition.

Characterization of Escherichia coli dinJ-yafQ toxin-antitoxin system using insights from mutagenesis data

Escherichia coli dinJ-yafQ operon codes for a functional toxin-antitoxin (TA) system. YafQ toxin is an RNase which, upon overproduction, specifically inhibits the translation process by cleaving cellular mRNA at specific sequences. DinJ is an antitoxin and counteracts YafQ-mediated toxicity by forming a strong protein complex. In the present study we used site-directed mutagenesis of YafQ to determine the amino acids important for its catalytic activity. His50Ala, His63Ala, Asp67Ala, Trp68Ala, Trp68Phe, Arg83Ala, His87Ala, and Phe91Ala substitutions of the predicted active-site residues of YafQ abolished mRNA cleavage in vivo, whereas Asp61Ala and Phe91Tyr mutations inhibited YafQ RNase activity only moderately. We show that YafQ, upon overexpression, cleaved mRNAs preferably 5' to A between the second and third nucleotides in the codon in vivo. YafQ also showed RNase activity against mRNA, tRNA, and 5S rRNA molecules in vitro, albeit with no strong specificity. The endoribonuclease activity of YafQ was inhibited in the complex with DinJ antitoxin in vitro. DinJ-YafQ protein complex and DinJ antitoxin alone selectively bind to one of the two palindromic sequences present in the intergenic region upstream of the dinJ-yafQ operon, suggesting the autoregulation mode of this TA system.

Toxins-antitoxins: diversity, evolution and function

Genes for toxin-antitoxin (TA) complexes are widespread in prokaryote genomes, and species frequently possess tens of plasmid and chromosomal TA loci. The complexes are categorized into three types based on genetic organization and mode of action. The toxins universally are proteins directed against specific intracellular targets, whereas the antitoxins are either proteins or small RNAs that neutralize the toxin or inhibit toxin synthesis. Within the three types of complex, there has been extensive evolutionary shuffling of toxin and antitoxin genes leading to considerable diversity in TA combinations. The intracellular targets of the protein toxins similarly are varied. Numerous toxins, many of which are sequence-specific endoribonucleases, dampen protein synthesis levels in response to a range of stress and nutritional stimuli. Key resources are conserved as a result ensuring the survival of individual cells and therefore the bacterial population. The toxin effects generally are transient and reversible permitting a set of dynamic, tunable responses that reflect environmental conditions. Moreover, by harboring multiple toxins that intercede in protein synthesis in response to different physiological cues, bacteria potentially sense an assortment of metabolic perturbations that are channeled through different TA complexes. Other toxins interfere with the action of topoisomersases, cell wall assembly, or cytoskeletal structures. TAs also play important roles in bacterial persistence, biofilm formation and multidrug tolerance, and have considerable potential both as new components of the genetic toolbox and as targets for novel antibacterial drugs.

Bacterial persistence by RNA endonucleases

Bacteria form persisters, individual cells that are highly tolerant to different types of antibiotics. Persister cells are genetically identical to nontolerant kin but have entered a dormant state in which they are recalcitrant to the killing activity of the antibiotics. The molecular mechanisms underlying bacterial persistence are unknown. Here, we show that the ubiquitous Lon (Long Form Filament) protease and mRNA endonucleases (mRNases) encoded by toxin-antitoxin (TA) loci are required for persistence in Escherichia coli. Successive deletion of the 10 mRNase-encoding TA loci of E. coli progressively reduced the level of persisters, showing that persistence is a phenotype common to TA loci. In all cases tested, the antitoxins, which control the activities of the mRNases, are Lon substrates. Consistently, cells lacking lon generated a highly reduced level of persisters. Moreover, Lon overproduction dramatically increased the levels of persisters in wild-type cells but not in cells lacking the 10 mRNases. These results support a simple model according to which mRNases encoded by TA loci are activated in a small fraction of growing cells by Lon-mediated degradation of the antitoxins. Activation of the mRNases, in turn, inhibits global cellular translation, and thereby induces dormancy and persistence. Many pathogenic bacteria known to enter dormant states have a plethora of TA genes. Therefore, in the future, the discoveries described here may lead to a mechanistic understanding of the persistence phenomenon in pathogenic bacteria.

The TB Structural Genomics Consortium: a decade of progress

The TB Structural Genomics Consortium is a worldwide organization of collaborators whose mission is the comprehensive structural determination and analyses of Mycobacterium tuberculosis proteins to ultimately aid in tuberculosis diagnosis and treatment. Congruent to the overall vision, Consortium members have additionally established an integrated facilities core to streamline M. tuberculosis structural biology and developed bioinformatics resources for data mining. This review aims to share the latest Consortium developments with the TB community, including recent structures of proteins that play significant roles within M. tuberculosis. Atomic resolution details may unravel mechanistic insights and reveal unique and novel protein features, as well as important protein-protein and protein-ligand interactions, which ultimately lead to a better understanding of M. tuberculosis biology and may be exploited for rational, structure-based therapeutics design.

RatA (YfjG), an Escherichia coli toxin, inhibits 70S ribosome association to block translation initiation

RatA (YfjG) is a toxin encoded by the ratA-ratB (yfjG-yfjF) operon on the Escherichia coli genome. Induction of RatA led to the inhibition of protein synthesis, while DNA and RNA synthesis was not affected. The stability of mRNAs was also unchanged as judged by in vivo primer extension experiments and by Northern blotting analysis. The ribosome profile of the cells overexpressing RatA showed that 70S ribosomes as well as polysomes significantly decreased with concomitant increase of 50S and 30S subunits. The addition of purified RatA to a cell-free system inhibited the formation of 70S ribosomes even in the presence of 6 mM Mg(2+) . RatA was specifically associated with 50S subunits, indicating that it binds to 50S subunits to block its association with 30S subunits leading to the inhibition of formation of 70S ribosomes. However, RatA did not cause dissociation of 70S ribosomes and its anti-association activity was blocked by paromomycin, an inhibitor for IF3, an essential initiation factor, having 21% sequence homology with RatA. Here we demonstrate that RatA is a new E. coli toxin, which effectively blocks the translation initiation step. We propose that this toxin of previously unknown function be renamed as RatA (Ribosome association toxin A).

Functional identification of toxin-antitoxin molecules from Helicobacter pylori 26695 and structural elucidation of the molecular interactions

Bacterial toxin-antitoxin (TA) systems are associated with many important cellular processes including antibiotic resistance and microorganism virulence. Here, we identify and structurally characterize TA molecules from the gastric pathogen, Helicobacter pylori. The HP0894 protein had been previously suggested, through our structural genomics approach, to be a putative toxin molecule. In this study, the intrinsic RNase activity and the bacterial cell growth-arresting activity of HP0894 were established. The RNA-binding surface was identified at three residue clusters: (Lys(8) and Ser(9)), (Lys(50)-Lys(54) and Glu(58)), and (Arg(80) and His(84)-Phe(88)). In particular, the -UA- and -CA- sequences in RNA were preferentially cleaved by HP0894, and residues Lys(52), Trp(53), and Ser(85)-Lys(87) were observed to be the main contributors to sequence recognition. The action of HP0894 could be inhibited by the HP0895 protein, and the HP0894-HP0895 complex formed an oligomer with a binding stoichiometry of 1:1. The N and C termini of HP0894 constituted the binding sites to HP0895. In contrast, the unstructured C-terminal region of HP0895 was responsible for binding to HP0894 and underwent a conformational change in the process. Finally, DNA binding activity was observed for both HP0895 and the HP0894-0895 complex but not for HP0894 alone. Taken together, it is concluded that the HP0894-HP0895 protein couple is a TA system in H. pylori, where HP0894 is a toxin with an RNase function, whereas HP0895 is an antitoxin functioning by binding to both the toxin and DNA.

Balancing at survival's edge: the structure and adaptive benefits of prokaryotic toxin-antitoxin partners

Many prokaryotes express toxin-antitoxin (TA) pairs that are harmful to their hosts if not maintained in delicate balance. The maintenance of potentially lethal toxin-antitoxin pairs could be viewed as a high-risk strategy. However, accumulating evidence suggests that toxin-antitoxin pairs can confer selective evolutionary benefits such as adaptive stress responses, starvation recovery and herd immunity to predation. Many of the known TA pairs interact as proteins, but recent work has identified a new class of antitoxins that are RNA cleavage products. Structural studies have revealed common folds for diverse toxins, highlighting unexpected evolutionary relationships within different toxin classes. TA pairs appear to have diverged in function considerably, to meet the specialised requirements of their varied prokaryotic hosts.

YeeV is an Escherichia coli toxin that inhibits cell division by targeting the cytoskeleton proteins, FtsZ and MreB

Toxin-antitoxin (TA) systems of free-living bacteria have recently demonstrated that these toxins inhibit cell growth by targeting essential functions of cellular metabolism. Here we show that YeeV toxin inhibits cell division, leads to a change in morphology and lysis of Escherichia coli cells. YeeV interacts with two essential cytoskeleton proteins, FtsZ and MreB. Purified YeeV inhibits both the GTPase activity and the GTP-dependent polymerization of FtsZ. YeeV also inhibits ATP-dependent polymerization of MreB. Truncated C-terminal deletions of YeeV result in elongation of cells, and a deletion of the first 15 amino acids from the N-terminus of YeeV caused lemon-shaped cell formation. The YeeV toxin is distinct from other well-studied toxins: it directs the binding of two cytoskeletal proteins and inhibits FtsZ and MreB simultaneously.

The structure and interactions of SpoIISA and SpoIISB, a toxin-antitoxin system in Bacillus subtilis

Spore formation in Bacillus subtilis begins with an asymmetric cell division, following which differential gene expression is established by alternative compartment-specific RNA polymerase σ factors. The spoIISAB operon of B. subtilis was identified as a locus whose mutation leads to increased activity of the first sporulation-specific sigma factor, σ^F. Inappropriate spoIISA expression causes lysis of vegetatively growing B. subtilis cells and Escherichia coli cells when expressed heterologously, effects that are countered by co-expression of spoIISB, identifying SpoIISA-SpoIISB as a toxin-antitoxin system. SpoIISA has three putative membrane-spanning segments and a cytoplasmic domain. Here, the crystal structure of a cytoplasmic fragment of SpoIISA (CSpoIISA) in complex with SpoIISB has been determined by selenomethionine-multiwavelength anomalous dispersion phasing to 2.5 Å spacing, revealing a CSpoIISA₂·SpoIISB₂ heterotetramer. CSpoIISA has a single domain α/β structure resembling a GAF domain with an extended α-helix at its N terminus. The two CSpoIISA protomers form extensive interactions through an intermolecular four-helix bundle. Each SpoIISB chain is highly extended and lacking tertiary structure. The SpoIISB chains wrap around the CSpoIISA dimer, forming extensive interactions with both CSpoIISA protomers. CD spectroscopy experiments indicate that SpoIISB is a natively disordered protein that adopts structure only in the presence of CSpoIISA, whereas surface plasmon resonance experiments revealed that the CSpoIISA·SpoIISB complex is stable with a dissociation constant in the nanomolar range. The results are interpreted in relation to sequence conservation and mutational data, and possible mechanisms of cell killing by SpoIISA are discussed.

Post-transcriptional control by bacteriophage T4: mRNA decay and inhibition of translation initiation

Over 50 years of biological research with bacteriophage T4 includes notable discoveries in post-transcriptional control, including the genetic code, mRNA, and tRNA; the very foundations of molecular biology. In this review we compile the past 10 - 15 year literature on RNA-protein interactions with T4 and some of its related phages, with particular focus on advances in mRNA decay and processing, and on translational repression. Binding of T4 proteins RegB, RegA, gp32 and gp43 to their cognate target RNAs has been characterized. For several of these, further study is needed for an atomic-level perspective, where resolved structures of RNA-protein complexes are awaiting investigation. Other features of post-transcriptional control are also summarized. These include: RNA structure at translation initiation regions that either inhibit or promote translation initiation; programmed translational bypassing, where T4 orchestrates ribosome bypass of a 50 nucleotide mRNA sequence; phage exclusion systems that involve T4-mediated activation of a latent endoribonuclease (PrrC) and cofactor-assisted activation of EF-Tu proteolysis (Gol-Lit); and potentially important findings on ADP-ribosylation (by Alt and Mod enzymes) of ribosome-associated proteins that might broadly impact protein synthesis in the infected cell. Many of these problems can continue to be addressed with T4, whereas the growing database of T4-related phage genome sequences provides new resources and potentially new phage-host systems to extend the work into a broader biological, evolutionary context.

Crystal structures of Phd-Doc, HigA, and YeeU establish multiple evolutionary links between microbial growth-regulating toxin-antitoxin systems

Bacterial toxin-antitoxin (TA) systems serve a variety of physiological functions including regulation of cell growth and maintenance of foreign genetic elements. Sequence analyses suggest that TA families are linked by complex evolutionary relationships reflecting likely swapping of functional domains between different TA families. Our crystal structures of Phd-Doc from bacteriophage P1, the HigA antitoxin from Escherichia coli CFT073, and YeeU of the YeeUWV systems from E. coli K12 and Shigella flexneri confirm this inference and reveal additional, unanticipated structural relationships. The growth-regulating Doc toxin exhibits structural similarity to secreted virulence factors that are toxic for eukaryotic target cells. The Phd antitoxin possesses the same fold as both the YefM and NE2111 antitoxins that inhibit structurally unrelated toxins. YeeU, which has an antitoxin-like activity that represses toxin expression, is structurally similar to the ribosome-interacting toxins YoeB and RelE. These observations suggest extensive functional exchanges have occurred between TA systems during bacterial evolution.

Structure of the Escherichia coli antitoxin MqsA (YgiT/b3021) bound to its gene promoter reveals extensive domain rearrangements and the specificity of transcriptional regulation

Bacterial cultures, especially biofilms, produce a small number of persister cells, a genetically identical subpopulation of wild type cells that are metabolically dormant, exhibit multidrug tolerance, and are highly enriched in bacterial toxins. The gene most highly up-regulated in Escherichia coli persisters is mqsR, a ribonuclease toxin that, along with mqsA, forms a novel toxin·antitoxin (TA) system. Like all known TA systems, both the MqsR·MqsA complex and MqsA alone regulate their own transcription. Despite the importance of TA systems in persistence and biofilms, very little is known about how TA modules, and antitoxins in particular, bind and recognize DNA at a molecular level. Here, we report the crystal structure of MqsA bound to a 26-bp fragment from the mqsRA promoter. We show that MqsA binds DNA predominantly via its C-terminal helix-turn-helix domain, with direct binding of recognition helix residues Asn(97) and Arg(101) to the DNA major groove. Unexpectedly, the structure also revealed that the MqsA N-terminal domain interacts with the DNA phosphate backbone. This results in a more than 105° rotation of the N-terminal domains between the free and complexed states, an unprecedented rearrangement for an antitoxin. The structure also shows that MqsA bends the DNA by more than 55° in order to achieve symmetrical binding. Finally, using a combination of biochemical and NMR studies, we show that the DNA sequence specificity of MqsA is mediated by direct readout.

Txe, an endoribonuclease of the enterococcal Axe-Txe toxin-antitoxin system, cleaves mRNA and inhibits protein synthesis

The axe-txe operon encodes a toxin-antitoxin (TA) pair, Axe-Txe, that was initially identified on the multidrug-resistance plasmid pRUM in Enterococcus faecium. In Escherichia coli, expression of the Txe toxin is known to inhibit cell growth, and co-expression of the antitoxin, Axe, counteracts the toxic effect of Txe. Here, we report the nucleotide sequence of pS177, a 39 kb multidrug-resistant plasmid isolated from vancomycin-resistant Ent. faecium, which harbours the axe-txe operon and the vanA gene cluster. RT-PCR analysis revealed that the axe-txe transcript is produced by strain S177 as well as by other vancomycin-resistant enteroccoci. Moreover, we determine the mechanism by which the Txe protein exerts its toxic activity. Txe inhibits protein synthesis in E. coli without affecting DNA or RNA synthesis, and inhibits protein synthesis in a cell-free system. Using in vivo primer extension analysis, we demonstrate that Txe preferentially cleaves single-stranded mRNA at the first base after an AUG start codon. We conclude that Txe is an endoribonuclease which cleaves mRNA and inhibits protein synthesis.

Solution structure and membrane binding of the toxin fst of the par addiction module

The par toxin−antitoxin system is required for the stable inheritance of the plasmid pAD1 in its native host Enterococcus faecalis. It codes for the toxin Fst and a small antisense RNA which inhibits translation of toxin mRNA, and it is the only known antisense regulated toxin−antitoxin system in Gram-positive bacteria. This study presents the structure of the par toxin Fst, the first atomic resolution structure of a component of an antisense regulated toxin−antitoxin system. The mode of membrane binding was determined by relaxation enhancements in a paramagnetic environment and molecular dynamics simulation. Fst forms a membrane-binding α-helix in the N-terminal part and contains an intrinsically disordered region near the C-terminus. It binds in a transmembrane orientation with the C-terminus likely pointing toward the cytosol. Membrane-bound, α-helical peptides are frequently found in higher organisms as components of the innate immune system. Despite similarities to these antimicrobial peptides, Fst shows neither hemolytic nor antimicrobial activity when applied externally to a series of bacteria, fungal cells, and erythrocytes. Moreover, its charge distribution, orientation in the membrane, and structure distinguish it from antimicrobial peptides.

Preliminary crystallographic analysis of the Escherichia coli antitoxin MqsA (YgiT/b3021) in complex with mqsRA promoter DNA

The Escherichia coli proteins MqsR and MqsA comprise a novel toxin-antitoxin (TA) system. MqsA, the antitoxin, defines a new family of antitoxins because unlike other antitoxins MqsA is structured throughout its entire sequence, binds zinc and coordinates DNA via its C-terminal and not its N-terminal domain. In order to understand how bacterial antitoxins, and MqsA in particular, regulate transcription, the MqsA protein was cocrystallized with a 26-mer duplex DNA corresponding to the palindromic region of the mqsRA promoter. The merohedrally twinned crystal belonged to space group P4(1), with unit-cell parameters a=60.99, b=60.99, c=148.60 A. A complete data set was collected to a resolution of 2.1 A. The solvent content of the crystal was consistent with the presence of two MqsA molecules bound to the duplex DNA in the asymmetric unit.

Allostery and intrinsic disorder mediate transcription regulation by conditional cooperativity

Regulation of the phd/doc toxin-antitoxin operon involves the toxin Doc as co- or derepressor depending on the ratio between Phd and Doc, a phenomenon known as conditional cooperativity. The mechanism underlying this observed behavior is not understood. Here we show that monomeric Doc engages two Phd dimers on two unrelated binding sites. The binding of Doc to the intrinsically disordered C-terminal domain of Phd structures its N-terminal DNA-binding domain, illustrating allosteric coupling between highly disordered and highly unstable domains. This allosteric effect also couples Doc neutralization to the conditional regulation of transcription. In this way, higher levels of Doc tighten repression up to a point where the accumulation of toxin triggers the production of Phd to counteract its action. Our experiments provide the basis for understanding the mechanism of conditional cooperative regulation of transcription typical of toxin-antitoxin modules. This model may be applicable for the regulation of other biological systems.

The relBE2Spn toxin-antitoxin system of Streptococcus pneumoniae: role in antibiotic tolerance and functional conservation in clinical isolates

Type II (proteic) chromosomal toxin-antitoxin systems (TAS) are widespread in Bacteria and Archaea but their precise function is known only for a limited number of them. Out of the many TAS described, the relBE family is one of the most abundant, being present in the three first sequenced strains of Streptococcus pneumoniae (D39, TIGR4 and R6). To address the function of the pneumococcal relBE2Spn TAS in the bacterial physiology, we have compared the response of the R6-relBE2Spn wild type strain with that of an isogenic derivative, Delta relB2Spn under different stress conditions such as carbon and amino acid starvation and antibiotic exposure. Differences on viability between the wild type and mutant strains were found only when treatment directly impaired protein synthesis. As a criterion for the permanence of this locus in a variety of clinical strains, we checked whether the relBE2Spn locus was conserved in around 100 pneumococcal strains, including clinical isolates and strains with known genomes. All strains, although having various types of polymorphisms at the vicinity of the TA region, contained a functional relBE2Spn locus and the type of its structure correlated with the multilocus sequence type. Functionality of this TAS was maintained even in cases where severe rearrangements around the relBE2Spn region were found. We conclude that even though the relBE2Spn TAS is not essential for pneumococcus, it may provide additional advantages to the bacteria for colonization and/or infection.

Function inferences from a molecular structural model of bacterial ParE toxin

Toxin-antitoxin (TA) systems contribute to plasmid stability by a mechanism that relies on the differential stabilities of the toxin and antitoxin proteins and leads to the killing of daughter bacteria that did not receive a plasmid copy at the cell division. ParE is the toxic component of a TA system that constitutes along with RelE an important class of bacterial toxin called RelE/ParE superfamily. For ParE toxin, no crystallographic structure is available so far and rare in vitro studies demonstrated that the target of toxin activity is E. coli DNA gyrase. Here, a 3D Model for E. coli ParE toxin by molecular homology modeling was built using MODELLER, a program for comparative modeling. The Model was energy minimized by CHARMM and validated using PROCHECK and VERIFY3D programs. Resulting Ramachandran plot analysis it was found that the portion residues failing into the most favored and allowed regions was 96.8%. Structural similarity search employing DALI server showed as the best matches RelE and YoeB families. The Model also showed similarities with other microbial ribonucleases but in a small score. A possible homologous deep cleft active site was identified in the Model using CASTp program. Additional studies to investigate the nuclease activity in members of ParE family as well as to confirm the inhibitory replication activity are needed. The predicted Model allows initial inferences about the unexplored 3D structure of the ParE toxin and may be further used in rational design of molecules for structure-function studies.

A conserved mode of protein recognition and binding in a ParD-ParE toxin-antitoxin complex

Toxin-antitoxin (TA) systems form a ubiquitous class of prokaryotic proteins with functional roles in plasmid inheritance, environmental stress response, and cell development. ParDE family TA systems are broadly conserved on plasmids and bacterial chromosomes and have been well characterized as genetic elements that promote stable plasmid inheritance. We present a crystal structure of a chromosomally encoded ParD-ParE complex from Caulobacter crescentus at 2.6 A resolution. This TA system forms an alpha(2)beta(2) heterotetramer in the crystal and in solution. The toxin-antitoxin binding interface reveals extensive polar and hydrophobic contacts of ParD antitoxin helices with a conserved recognition and binding groove on the ParE toxin. A cross-species comparison of this complex structure with related toxin structures identified an antitoxin recognition and binding subdomain that is conserved between distantly related members of the RelE/ParE toxin superfamily despite a low level of overall primary sequence identity. We further demonstrate that ParD antitoxin is dimeric, stably folded, and largely helical when not bound to ParE toxin. Thus, the paradigmatic model in which antitoxin undergoes a disorder-to-order transition upon toxin binding does not apply to this chromosomal ParD-ParE TA system.

Purification and crystallization of Phd, the antitoxin of the phd/doc operon

The antitoxin Phd from the phd/doc module of bacteriophage P1 was crystallized in two distinct crystal forms. Crystals of His-tagged Phd contain a C-terminally truncated version of the protein and diffract to 2.20 A resolution. Crystals of untagged Phd purified from the Phd-Doc complex diffract to 2.25 A resolution. These crystals are partially merohedrally twinned and contain the full-length version of the protein.

The structural basis for mRNA recognition and cleavage by the ribosome-dependent endonuclease RelE

Translational control is widely used to adjust gene expression levels. During the stringent response in bacteria, mRNA is degraded on the ribosome by the ribosome-dependent endonuclease, RelE. The molecular basis for recognition of the ribosome and mRNA by RelE and the mechanism of cleavage are unknown. Here, we present crystal structures of E. coli RelE in isolation (2.5 Å) and bound to programmed Thermus thermophilus 70S ribosomes before (3.3 Å) and after (3.6 Å) cleavage. RelE occupies the A site and causes cleavage of mRNA after the second nucleotide of the codon by reorienting and activating the mRNA for 2′-OH-induced hydrolysis. Stacking of A site codon bases with conserved residues in RelE and 16S rRNA explains the requirement for the ribosome in catalysis and the subtle sequence specificity of the reaction. These structures provide detailed insight into the translational regulation on the bacterial ribosome by mRNA cleavage.

Three dimensional structure of the MqsR:MqsA complex: a novel TA pair comprised of a toxin homologous to RelE and an antitoxin with unique properties

One mechanism by which bacteria survive environmental stress is through the formation of bacterial persisters, a sub-population of genetically identical quiescent cells that exhibit multidrug tolerance and are highly enriched in bacterial toxins. Recently, the Escherichia coli gene mqsR (b3022) was identified as the gene most highly upregulated in persisters. Here, we report multiple individual and complex three-dimensional structures of MqsR and its antitoxin MqsA (B3021), which reveal that MqsR:MqsA form a novel toxin:antitoxin (TA) pair. MqsR adopts an α/β fold that is homologous with the RelE/YoeB family of bacterial ribonuclease toxins. MqsA is an elongated dimer that neutralizes MqsR toxicity. As expected for a TA pair, MqsA binds its own promoter. Unexpectedly, it also binds the promoters of genes important for E. coli physiology (e.g., mcbR, spy). Unlike canonical antitoxins, MqsA is also structured throughout its entire sequence, binds zinc and coordinates DNA via its C- and not N-terminal domain. These studies reveal that TA systems, especially the antitoxins, are significantly more diverse than previously recognized and provide new insights into the role of toxins in maintaining the persister state.

Most bacteria live in biofilms, microbial communities that cause more than 80% of human infections. Biofilms have a genetically identical sub-population of dormant cells, named persister cells, which are the well-recognized source of antibiotic resistance. Recently, it was demonstrated that toxins are highly upregulated in persisters and have therefore been postulated to play a role in the persister state. Using an inter-disciplinary approach, we reveal how mqsR, the gene most highly upregulated in persisters, together with mqsA, function: they are the founding members of a new family of toxin:antitoxin (TA) systems. Unexpectedly, the structure of MqsR reveals that it is a ribonuclease, a protein that controls the production of other essential proteins. Moreover, we identified multiple features of this TA system that are so unique that each is a starting point for drug development. Unlike other antitoxins, MqsA is structured throughout its entire sequence, its structure is unchanged between the free and toxin-bound states and it binds zinc. It also binds DNA via its C- and not N-terminal domain. Finally, MqsA binds both its own promoter and additional genes important for E. coli physiology. Taken together, our data provide fundamental new insights into the role of MqsR and MqsA in bacterial persistence and biofilms.

PemK toxin of Bacillus anthracis is a ribonuclease: an insight into its active site, structure, and function

Bacillus anthracis genome harbors a toxin-antitoxin (TA) module encoding pemI (antitoxin) and pemK (toxin). This study describes the rPemK as a potent ribonuclease with a preference for pyrimidines (C/U), which is consistent with our previous study that demonstrated it as a translational attenuator. The in silico structural modeling of the PemK in conjunction with the site-directed mutagenesis confirmed the role of His-59 and Glu-78 as an acid-base couple in mediating the ribonuclease activity. The rPemK is shown to form a complex with the rPemI, which is in line with its function as a TA module. This rPemI-rPemK complex becomes catalytically inactive when both the proteins interact in a molar stoichiometry of 1. The rPemI displays vulnerability to proteolysis but attains conformational stability only upon rPemK interaction. The pemI-pemK transcript is shown to be up-regulated upon stress induction with a concomitant increase in the amount of PemK and a decline in the PemI levels, establishing the role of these modules in stress. The artificial perturbation of TA interaction could unleash the toxin, executing bacterial cell death. Toward this end, synthetic peptides are designed to disrupt the TA interaction. The peptides are shown to be effective in abrogating TA interaction in micromolar range in vitro. This approach can be harnessed as a potential antibacterial strategy against anthrax in the future.

Three new RelE-homologous mRNA interferases of Escherichia coli differentially induced by environmental stresses

Prokaryotic toxin – antitoxin (TA) loci encode mRNA interferases that inhibit translation, either by cleaving mRNA codons at the ribosomal A site or by cleaving any RNA site-specifically. So far, seven mRNA interferases of Escherichia coli have been identified, four of which cleave mRNA by a translation-dependent mechanism. Here, we experimentally confirmed the presence of three novel TA loci in E. coli. We found that the yafNO, higBA (ygjNM) and ygiUT loci encode mRNA interferases related to RelE. YafO and HigB cleaved translated mRNA only, while YgiU cleaved RNA site-specifically at GC[A/U], independently of translation. Thus, YgiU is the first RelE-related mRNA interferase that cleaves mRNA independently of translation, in vivo. All three loci were induced by amino acid starvation, and inhibition of translation although to different degrees. Carbon starvation induced only two of the loci. The yafNO locus was induced by DNA damage, but the transcription originated from the dinB promoter. Thus, our results showed that the different TA loci responded differentially to environmental stresses. Induction of the three loci depended on Lon protease that may sense the environmental stresses and activate TA loci by cleavage of the antitoxins. Transcription of the three TA operons was autoregulated by the antitoxins.

RelB and RelE of Escherichia coli form a tight complex that represses transcription via the ribbon-helix-helix motif in RelB

RelB, the ribbon–helix–helix (RHH) repressor encoded by the relBE toxin–antitoxin locus of Escherichia coli, interacts with RelE and thereby counteracts the mRNA cleavage activity of RelE. In addition, RelB dimers repress the strong relBE promoter and this repression by RelB is enhanced by RelE; that is, RelE functions as a transcriptional co-repressor. RelB is a Lon protease substrate, and Lon is required both for activation of relBE transcription and for activation of the mRNA cleavage activity of RelE. Here we characterize the molecular interactions important for transcriptional control of the relBE model operon. Using an in vivo screen for relB mutants, we identified multiple nucleotide changes that map to important amino acid positions within the DNA-binding domain formed by the N-terminal RHH motif of RelB. Analysis of DNA binding of a subset of these mutant RHH proteins by gel-shift assays, transcriptional fusion assays and a structure model of RelB–DNA revealed amino acid residues making crucial DNA–backbone contacts within the operator (relO) DNA. Mutational and footprinting analyses of relO showed that RelB dimers bind on the same face of the DNA helix and that the RHH motif recognizes four 6-bp repeats within the bipartite binding site. The spacing between each half-site was found to be essential for cooperative interactions between adjacently bound RelB dimers stabilized by the co-repressor RelE. Kinetic and stoichiometric measurements of the interaction between RelB and RelE confirmed that the proteins form a high-affinity complex with a 2:1 stoichiometry. Lon degraded RelB in vitro and degradation was inhibited by RelE, consistent with the proposal that RelE protects RelB from proteolysis by Lon in vivo.

MqsR, a crucial regulator for quorum sensing and biofilm formation, is a GCU-specific mRNA interferase in Escherichia coli

The mqsR gene has been shown to be positively regulated by the quorum-sensing autoinducer AI-2, which in turn activates a two-component system, the qseB-qseC operon. This operon plays an important role in biofilm formation in Escherichia coli. However, its cellular function has remained unknown. Here, we found that 1 base downstream of mqsR there is a gene, ygiT, that is co-transcribed with mqsR. Induction of mqsR caused cell growth arrest, whereas ygiT co-induction recovered cell growth. We demonstrate that MqsR (98 amino acid residues), which has no homology to the well characterized mRNA interferase MazF, is a potent inhibitor of protein synthesis that functions by degrading cellular mRNAs. In vivo and in vitro primer extension experiments showed that MqsR is an mRNA interferase specifically cleaving mRNAs at GCU. The mRNA interferase activity of purified MqsR was inhibited by purified YgiT (131 residues). MqsR forms a stable 2:1 complex with YgiT, and the complex likely functions as a repressor for the mqsR-ygiT operon by specifically binding to two different palindromic sequences present in the 5'-untranslated region of this operon.

Characterization of YafO, an Escherichia coli toxin

YafO is a toxin encoded by the yafN-yafO antitoxin-toxin operon in the Escherichia coli genome. Our results show that YafO inhibits protein synthesis but not DNA or RNA synthesis. The in vivo [35S]methionine incorporation was inhibited within 5 min after YafO induction. In in vivo primer extension experiments with two different mRNAs, the specific cleavage bands appeared 11-13 bases downstream of the initiation codon, AUG, 2.5 min after the induction of YafO. An identical band was also detected in in vitro toeprinting experiments when YafO was added to the reaction mixture containing 70 S ribosomes and the same mRNAs even in the absence of tRNA(f)(Met). Notably, this band was not detected in the presence of YafO alone, indicating that YafO by itself does not have endoribonuclease activity under the conditions used. The full-length mRNAs almost completely disappeared 30 min after YafO induction in in vivo primer extension experiments, consistent with Northern blotting analysis. Over 84% of [35S]methionine-tRNA(f)(Met) was released from the translation initiation complex at 5.43 microM YafO in vitro. We demonstrated that the 70 S ribosome peak significantly increased upon YafO induction, and when the 70 S ribosomes dissociated into 50 and 30 S subunits, YafO was found to be associated with 50 S subunits. These results demonstrate that YafO is a ribosome-dependent mRNA interferase inhibiting protein synthesis.

Staphylococcus aureus YoeB homologues inhibit translation initiation

YoeB is a bacterial toxin encoded by the yefM-yoeB toxin-antitoxin system found in various bacterial genomes. Here, we show that Staphylococcus aureus contains two YoeB homologues, both of which function as ribosome-dependent mRNA interferases to inhibit translation initiation in a manner identical to that of YoeB-ec from Escherichia coli.

Bacterial toxin HigB associates with ribosomes and mediates translation-dependent mRNA cleavage at A-rich sites

Most pathogenic Proteus species are primarily associated with urinary tract infections, especially in persons with indwelling catheters or functional/anatomic abnormalities of the urinary tract. Urinary tract infections caused by Proteus vulgaris typically form biofilms and are resistant to commonly used antibiotics. The Rts1 conjugative plasmid from a clinical isolate of P. vulgaris carries over 300 predicted open reading frames, including antibiotic resistance genes. The maintenance of the Rts1 plasmid is ensured in part by the HigBA toxin-antitoxin system. We determined the precise mechanism of action of the HigB toxin in vivo, which is distinct from other known toxins. We demonstrate that HigB is an endoribonuclease whose enzymatic activity is dependent on association with ribosomes through the 50 S subunit. Using primer extension analysis of several test mRNAs, we showed that HigB cleaved extensively across the entire length of coding regions only at specific recognition sequences. HigB mediated cleavage of 100% of both in-frame and out-of-frame AAA sequences. In addition, HigB cleaved approximately 20% of AA sequences in coding regions and occasionally cut single As. Remarkably, the cleavage specificity of HigB coincided with one of the most frequently used codons in the AT-rich Proteus spp., AAA (lysine). Therefore, the HigB-mediated plasmid maintenance system for the Rts1 plasmid highlights the intimate relationship between host cells and extrachromosomal DNA that enables the dynamic acquisition of genes that impart a spectrum of survival advantages, including those encoding multidrug resistance and virulence factors.

Inhibitory mechanism of Escherichia coli RelE-RelB toxin-antitoxin module involves a helix displacement near an mRNA interferase active site

In Escherichia coli, RelE toxin participates in growth arrest and cell death by inducing mRNA degradation at the ribosomal A-site under stress conditions. The NMR structures of a mutant of E. coli RelE toxin, RelE(R81A/R83A), with reduced toxicity and its complex with an inhibitory peptide from RelB antitoxin, RelB(C) (Lys(47)-Leu(79)), have been determined. In the free RelE(R81A/R83A) structure, helix alpha4 at the C terminus adopts a closed conformation contacting with the beta-sheet core and adjacent loops. In the RelE(R81A/R83A)-RelB(C) complex, helix alpha3(*) of RelB(C) displaces alpha4 of RelE(R81A/R83A) from the binding site on the beta-sheet core. This helix replacement results in neutralization of a conserved positively charged cluster of RelE by acidic residues from alpha3(*) of RelB. The released helix alpha4 becomes unfolded, adopting an open conformation with increased mobility. The displacement of alpha4 disrupts the geometry of critical residues, including Arg(81) and Tyr(87), in a putative active site of RelE toxin. Our structures indicate that RelB counteracts the toxic activity of RelE by displacing alpha4 helix from the catalytically competent position found in the free RelE structure.

Staphylococcus aureus MazF specifically cleaves a pentad sequence, UACAU, which is unusually abundant in the mRNA for pathogenic adhesive factor SraP

Escherichia coli mRNA interferases, such as MazF and ChpBK, are sequence-specific endoribonucleases encoded by toxin-antitoxin (TA) systems present in its genome. A MazF homologue in Staphylococcus aureus (MazF(Sa)) has been shown to inhibit cell growth when induced in E. coli. Here, we determined the cleavage site for MazF(Sa) with the use of phage MS2 RNA as a substrate and CspA, an RNA chaperone, which prevents the formation of secondary structures in the RNA substrate. MazF(Sa) specifically cleaves the RNA at a pentad sequence, U downward arrow ACAU. Bioinformatics analysis revealed that this pentad sequence is significantly abundant in several genes, including the sraP gene in the S. aureus N315 strain. This gene encodes a serine-rich protein, which is known to play an important role in adhesion of the pathogen to human tissues and thus in endovascular infection. We demonstrated that the sraP mRNA became extremely unstable in comparison with the ompA mRNA only when MazF(Sa) was induced in E. coli. Further bioinformatics analysis indicated that the pentad sequence is also significantly abundant in the mRNAs for all the pathogenic factors in S. aureus. This observation suggests a possible regulatory relationship between the MazEF(Sa) TA module and the pathogenicity in S. aureus.

Role of vapBC toxin-antitoxin loci in the thermal stress response of Sulfolobus solfataricus

TA (toxin-antitoxin) loci are ubiquitous in prokaryotic micro-organisms, including archaea, yet their physiological function is largely unknown. For example, preliminary reports have suggested that TA loci are microbial stress-response elements, although it was recently shown that knocking out all known chromosomally located TA loci in Escherichia coli did not have an impact on survival under certain types of stress. The hyperthermophilic crenarchaeon Sulfolobus solfataricus encodes at least 26 vapBC (where vap is virulence-associated protein) family TA loci in its genome. VapCs are PIN (PilT N-terminus) domain proteins with putative ribonuclease activity, while VapBs are proteolytically labile proteins, which purportedly function to silence VapCs when associated as a cognate pair. Global transcriptional analysis of S. solfataricus heat-shock-response dynamics (temperature shift from 80 to 90 degrees C) revealed that several vapBC genes were triggered by the thermal shift, suggesting a role in heat-shock-response. Indeed, knocking out a specific vapBC locus in S. solfataricus substantially changed the transcriptome and, in one case, rendered the crenarchaeon heat-shock-labile. These findings indicate that more work needs to be done to determine the role of VapBCs in S. solfataricus and other thermophilic archaea, especially with respect to post-transcriptional regulation.

The inhibitory mechanism of protein synthesis by YoeB, an Escherichia coli toxin

YoeB is a toxin encoded by the yefM-yoeB antitoxin-toxin operon in the Escherichia coli genome. Here we show that YoeB, a highly potent protein synthesis inhibitor, specifically blocks translation initiation. In in vivo primer extension experiments using two different mRNAs, a major band was detected after YoeB induction at three bases downstream of the initiation codon at 2.5 min. An identical band was also detected in in vitro toeprinting experiments after the addition of YoeB to the reaction mixtures containing 70 S ribosomes and the same mRNAs, even in the absence of tRNA(f)(Met). Notably, this band was not detected in the presence of YoeB alone, indicating that YoeB by itself does not have endoribonuclease activity under the conditions used. The 70 S ribosomes increased upon YoeB induction, and YoeB was found to be specifically associated with 50 S subunits. Using tetracycline and hygromycin B, we demonstrated that YoeB binds to the 50 S ribosomal subunit in 70 S ribosomes and interacts with the A site leading to mRNA cleavage at this site. As a result, the 3'-end portion of the mRNA was released from ribosomes, and translation initiation was effectively inhibited. These results demonstrate that YoeB primarily inhibits translation initiation.

mRNA interferases, sequence-specific endoribonucleases from the toxin-antitoxin systems

Escherichia coli contains a large number of suicide or toxin genes, whose expression leads to cell growth arrest and eventual cell death. One such toxin, MazF, is an ACA-specific endoribonuclease, termed "mRNA interferase."E. coli contains other mRNA interferases with different sequence specificities, which are considered to play important roles in growth regulation under stress conditions, and also in eliminating stress-damaged cells from a population. Recently, MazF homologues with 5-base recognition sequences have been identified, for example, those from Mycobacterium tuberculosis. These sequences are significantly underrepresented in the genes for protein families playing a role in the immunity and pathogenesis of M. tuberculosis. An mRNA interferase in Myxococcus xanthus is essential for programmed cell death during fruiting body formation. We propose that mRNA interferases play roles not only in cell growth regulation and programmed cell death, but also in regulation of specific gene expression (either positively or negatively) in bacteria.

RNA processing and decay in bacteriophage T4

Bacteriophage T4 is the archetype of virulent phage. It has evolved very efficient strategies to subvert host functions to its benefit and to impose the expression of its genome. T4 utilizes a combination of host and phage-encoded RNases and factors to degrade its mRNAs in a stage-dependent manner. The host endonuclease RNase E is used throughout the phage development. The sequence-specific, T4-encoded RegB endoribonuclease functions in association with the ribosomal protein S1 to functionally inactivate early transcripts and expedite their degradation. T4 polynucleotide kinase plays a role in this process. Later, the viral factor Dmd protects middle and late mRNAs from degradation by the host RNase LS. T4 codes for a set of eight tRNAs and two small, stable RNA of unknown function that may contribute to phage virulence. Their maturation is assured by host enzymes, but one phage factor, Cef, is required for the biogenesis of some of them. The tRNA gene cluster also codes for a homing DNA endonuclease, SegB, responsible for spreading the tRNA genes to other T4-related phage.

HicA of Escherichia coli defines a novel family of translation-independent mRNA interferases in bacteria and archaea

Toxin-antitoxin (TA) loci are common in free-living bacteria and archaea. TA loci encode a stable toxin that is neutralized by a metabolically unstable antitoxin. The antitoxin can be either a protein or an antisense RNA. So far, six different TA gene families, in which the antitoxins are proteins, have been identified. Recently, Makarova et al. (K. S. Makarova, N. V. Grishin, and E. V. Koonin, Bioinformatics 22:2581-2584, 2006) suggested that the hicAB loci constitute a novel TA gene family. Using the hicAB locus of Escherichia coli K-12 as a model system, we present evidence that supports this inference: expression of the small HicA protein (58 amino acids [aa]) induced cleavage in three model mRNAs and tmRNA. Concomitantly, the global rate of translation was severely reduced. Using tmRNA as a substrate, we show that HicA-induced cleavage does not require the target RNA to be translated. Expression of HicB (145 aa) prevented HicA-mediated inhibition of cell growth. These results suggest that HicB neutralizes HicA and therefore functions as an antitoxin. As with other antitoxins (RelB and MazF), HicB could resuscitate cells inhibited by HicA, indicating that ectopic production of HicA induces a bacteriostatic rather than a bactericidal condition. Nutrient starvation induced strong hicAB transcription that depended on Lon protease. Mining of 218 prokaryotic genomes revealed that hicAB loci are abundant in bacteria and archaea.

Influence of operator site geometry on transcriptional control by the YefM-YoeB toxin-antitoxin complex

YefM-YoeB is among the most prevalent and well-characterized toxin-antitoxin complexes. YoeB toxin is an endoribonuclease whose activity is inhibited by YefM antitoxin. The regions 5' of yefM-yoeB in diverse bacteria possess conserved sequence motifs that mediate transcriptional autorepression. The yefM-yoeB operator site arrangement is exemplified in Escherichia coli: a pair of palindromes with core hexamer motifs and a center-to-center distance of 12 bp overlap the yefM-yoeB promoter. YefM is an autorepressor that initially recognizes a long palindrome containing the core hexamer, followed by binding to a short repeat. YoeB corepressor greatly enhances the YefM-operator interaction. Scanning mutagenesis demonstrated that the short repeat is crucial for correct interaction of YefM-YoeB with the operator site in vivo and in vitro. Moreover, altering the relative positions of the two palindromes on the DNA helix abrogated YefM-YoeB cooperative interactions with the repeats: complex binding to the long repeat was maintained but was perturbed to the short repeat. Although YefM lacks a canonical DNA binding motif, dual conserved arginine residues embedded in a basic patch of the protein are crucial for operator recognition. Deciphering the molecular basis of toxin-antitoxin transcriptional control will provide key insights into toxin-antitoxin activation and function.

Crystallization of Doc and the Phd-Doc toxin-antitoxin complex

The phd/doc addiction system is responsible for the stable inheritance of lysogenic bacteriophage P1 in its plasmidic form in Escherichia coli and is the archetype of a family of bacterial toxin-antitoxin modules. The His66Tyr mutant of Doc (Doc(H66Y)) was crystallized in space group P2(1), with unit-cell parameters a = 53.1, b = 198.0, c = 54.1 A, beta = 93.0 degrees . These crystals diffracted to 2.5 A resolution and probably contained four dimers of Doc in the asymmetric unit. Doc(H66Y) in complex with a 22-amino-acid C-terminal peptide of Phd (Phd(52-73Se)) was crystallized in space group C2, with unit-cell parameters a = 111.1, b = 38.6, c = 63.3 A, beta = 99.3 degrees , and diffracted to 1.9 A resolution. Crystals of the complete wild-type Phd-Doc complex belonged to space group P3(1)21 or P3(2)21, had an elongated unit cell with dimensions a = b = 48.9, c = 354.9 A and diffracted to 2.4 A resolution using synchrotron radiation.

Translation affects YoeB and MazF messenger RNA interferase activities by different mechanisms

Prokaryotic toxin–antitoxin loci encode mRNA cleaving enzymes that inhibit translation. Two types are known: those that cleave mRNA codons at the ribosomal A site and those that cleave any RNA site specifically. RelE of Escherichia coli cleaves mRNA at the ribosomal A site in vivo and in vitro but does not cleave pure RNA in vitro. RelE exhibits an incomplete RNase fold that may explain why RelE requires its substrate mRNA to presented by the ribosome. In contrast, RelE homologue YoeB has a complete RNase fold and cleaves RNA independently of ribosomes in vitro. Here, we show that YoeB cleavage of mRNA is strictly dependent on translation of the mRNA in vivo. Non-translated model mRNAs were not cleaved whereas the corresponding wild-type mRNAs were cleaved efficiently. Model mRNAs carrying frameshift mutations exhibited a YoeB-mediated cleavage pattern consistent with the reading frameshift thus giving strong evidence that YoeB cleavage specificity was determined by the translational reading frame. In contrast, site-specific mRNA cleavage by MazF occurred independently of translation. In one case, translation seriously influenced MazF cleavage efficiency, thus solving a previous apparent paradox. We propose that translation enhances MazF-mediated cleavage of mRNA by destabilization of the mRNA secondary structure.

Doc of prophage P1 is inhibited by its antitoxin partner Phd through fold complementation

Prokaryotic toxin-antitoxin modules are involved in major physiological events set in motion under stress conditions. The toxin Doc (death on curing) from the phd/doc module on phage P1 hosts the C-terminal domain of its antitoxin partner Phd (prevents host death) through fold complementation. This Phd domain is intrinsically disordered in solution and folds into an alpha-helix upon binding to Doc. The details of the interactions reveal the molecular basis for the inhibitory action of the antitoxin. The complex resembles the Fic (filamentation induced by cAMP) proteins and suggests a possible evolutionary origin for the phd/doc operon. Doc induces growth arrest of Escherichia coli cells in a reversible manner, by targeting the protein synthesis machinery. Moreover, Doc activates the endogenous E. coli RelE mRNA interferase but does not require this or any other known chromosomal toxin-antitoxin locus for its action in vivo.

Messenger RNA interferase RelE controls relBE transcription by conditional cooperativity

Prokaryotic toxin-antitoxin (TA) loci consist of two genes in an operon that encodes a metabolically stable toxin and an unstable antitoxin. The antitoxin neutralizes its cognate toxin by forming a tight complex with it. In all cases known, the antitoxin autoregulates TA operon transcription by binding to one or more operators in the promoter region while the toxin functions as a co-repressor of transcription. Interestingly, the toxin can also stimulate TA operon transcription. Here we analyse mechanistic aspects of how RelE of Escherichia coli can function both as a co-repressor and as a derepressor of relBE transcription. When RelB was in excess to RelE, two trimeric RelB(2)*RelE complexes bound cooperatively to two adjacent operator sites in the relBE promoter region and repressed transcription. In contrast, RelE in excess stimulated relBE transcription and released the RelB(2)*RelE complex from operator DNA. A mutational analysis of the operator sites showed that RelE in excess counteracted cooperative binding of the RelB(2)*RelE complexes to the operator sites. Thus, RelE controls relBE transcription by conditional cooperativity.