F-Type Pyocins Are Diverse Noncontractile Phage Tail-Like Weapons for Killing Pseudomonas aeruginosa.

Most Pseudomonas aeruginosa strains produce bacteriocins derived from contractile or noncontractile phage tails known as R- and F-type pyocins, respectively. These bacteriocins possess strain-specific bactericidal activity against P. aeruginosa and likely increase evolutionary fitness through intraspecies competition. R-type pyocins have been studied extensively and show promise as alternatives to antibiotics. Although they have similar therapeutic potential, experimental studies on F-type pyocins are limited. Here, we provide a bioinformatic and experimental investigation of F-type pyocins. We introduce a systematic naming scheme for genes found in R- and F-type pyocin operons and identify 15 genes invariably found in strains producing F-type pyocins. Five proteins encoded at the 3' end of the F-type pyocin cluster are divergent in sequence and likely determine bactericidal specificity. We use sequence similarities among these proteins to define eleven distinct F-type pyocin groups, five of which had not been previously described. The five genes encoding the variable proteins associate in two modules that have clearly reassorted independently during the evolution of these operons. These proteins are considerably more diverse than the specificity-determining tail fibers of R-type pyocins, suggesting that F-type pyocins may have emerged earlier. Experimental studies on six F-type pyocin groups show that each displays a distinct spectrum of bactericidal activity. This activity is strongly influenced by the lipopolysaccharide O-antigen type, but other factors also play a role. F-type pyocins appear to kill as efficiently as R-type pyocins. These studies set the stage for the development of F-type pyocins as antibacterial therapeutics. IMPORTANCE Pseudomonas aeruginosa is an opportunistic pathogen that causes antibiotic-resistant infections with high mortality rates, particularly in immunocompromised individuals and cystic fibrosis patients. Due to the increasing frequency of multidrug-resistant P. aeruginosa infections, there is great need for the development of alternative therapeutics. In this study, we investigate one such potential therapeutic: F-type pyocins, which are bacteriocins naturally produced by P. aeruginosa that resemble noncontractile phage tails. We show that they are potent killers of P. aeruginosa and identify their probable bactericidal specificity determinants, which opens up the possibility of engineering them to precisely target strains of pathogenic bacteria. The resemblance of F-type pyocins to well-characterized phage tails will greatly facilitate their development into effective antibacterials.

Legionella pneumophila CRISPR-Cas Suggests Recurrent Encounters with One or More Phages in the Family Microviridae.

Legionella pneumophila is a ubiquitous freshwater pathogen and the causative agent of Legionnaires' disease. L. pneumophila growth within protists provides a refuge from desiccation, disinfection, and other remediation strategies. One outstanding question has been whether this protection extends to phages. L. pneumophila isolates are remarkably devoid of prophages and to date no Legionella phages have been identified. Nevertheless, many L. pneumophila isolates maintain active CRISPR-Cas defenses. So far, the only known target of these systems is an episomal element that we previously named Legionella mobile element 1 (LME-1). The continued expansion of publicly available genomic data promises to further our understanding of the role of these systems. We now describe over 150 CRISPR-Cas systems across 600 isolates to establish the clearest picture yet of L. pneumophila's adaptive defenses. By searching for targets of 1,500 unique CRISPR-Cas spacers, LME-1 remains the only identified CRISPR-Cas targeted integrative element. We identified 3 additional LME-1 variants-all targeted by previously and newly identified CRISPR-Cas spacers-but no other similar elements. Notably, we also identified several spacers with significant sequence similarity to microviruses, specifically those within the subfamily Gokushovirinae. These spacers are found across several different CRISPR-Cas arrays isolated from geographically diverse isolates, indicating recurrent encounters with these phages. Our analysis of the extended Legionella CRISPR-Cas spacer catalog leads to two main conclusions: current data argue against CRISPR-Cas targeted integrative elements beyond LME-1, and the heretofore unknown L. pneumophila phages are most likely lytic gokushoviruses. IMPORTANCE Legionnaires' disease is an often-fatal pneumonia caused by Legionella pneumophila, which normally grows inside amoebae and other freshwater protists. L. pneumophila trades diminished access to nutrients for the protection and isolation provided by the host. One outstanding question is whether L. pneumophila is susceptible to phages, given the protection provided by its intracellular lifestyle. In this work, we use Legionella CRISPR spacer sequences as a record of phage infection to predict that the "missing" L. pneumophila phages belong to the microvirus subfamily Gokushovirinae. Gokushoviruses are known to infect another intracellular pathogen, Chlamydia. How do gokushoviruses access L. pneumophila (and Chlamydia) inside their "cozy niches"? Does exposure to phages happen during a transient extracellular period (during cell-to-cell spread) or is it indicative of a more complicated environmental lifestyle? One thing is clear, 100 years after their discovery, phages continue to hold important secrets about the bacteria upon which they prey.

The Legionella pneumophila effector Ceg4 is a phosphotyrosine phosphatase that attenuates activation of eukaryotic MAPK pathways.

Host colonization by Gram-negative pathogens often involves delivery of bacterial proteins called "effectors" into the host cell. The pneumonia-causing pathogen Legionella pneumophila delivers more than 330 effectors into the host cell via its type IVB Dot/Icm secretion system. The collective functions of these proteins are the establishment of a replicative niche from which Legionella can recruit cellular materials to grow while evading lysosomal fusion inhibiting its growth. Using a combination of structural, biochemical, and in vivo approaches, we show that one of these translocated effector proteins, Ceg4, is a phosphotyrosine phosphatase harboring a haloacid dehalogenase-hydrolase domain. Ceg4 could dephosphorylate a broad range of phosphotyrosine-containing peptides in vitro and attenuated activation of MAPK-controlled pathways in both yeast and human cells. Our findings indicate that L. pneumophila's infectious program includes manipulation of phosphorylation cascades in key host pathways. The structural and functional features of the Ceg4 effector unraveled here provide first insight into its function as a phosphotyrosine phosphatase, paving the way to further studies into L. pneumophila pathogenicity.

Priming in a permissive type I-C CRISPR-Cas system reveals distinct dynamics of spacer acquisition and loss.

CRISPR-Cas is a bacterial and archaeal adaptive immune system that uses short, invader-derived sequences termed spacers to target invasive nucleic acids. Upon recognition of previously encountered invaders, the system can stimulate secondary spacer acquisitions, a process known as primed adaptation. Previous studies of primed adaptation have been complicated by intrinsically high interference efficiency of most systems against bona fide targets. As such, most primed adaptation to date has been studied within the context of imperfect sequence complementarity between spacers and targets. Here, we take advantage of a native type I-C CRISPR-Cas system in Legionella pneumophila that displays robust primed adaptation even within the context of a perfectly matched target. Using next-generation sequencing to survey acquired spacers, we observe strand bias and positional preference that are consistent with a 3'-5' translocation of the adaptation machinery. We show that spacer acquisition happens in a wide range of frequencies across the plasmid, including a remarkable hotspot that predominates irrespective of the priming strand. We systematically characterize protospacer sequence constraints in both adaptation and interference and reveal extensive flexibilities regarding the protospacer adjacent motif in both processes. Lastly, in a strain with a genetically truncated CRISPR array, we observe increased interference efficiency, which, when coupled with forced maintenance of a targeted plasmid, provides a useful experimental system to study spacer loss. Based on these observations, we propose that the Legionella pneumophila type I-C system represents a powerful model to study primed adaptation and the interplay between CRISPR interference and adaptation.

Diverse mechanisms of metaeffector activity in an intracellular bacterial pathogen, Legionella pneumophila.

Pathogens deliver complex arsenals of translocated effector proteins to host cells during infection, but the extent to which these proteins are regulated once inside the eukaryotic cell remains poorly defined. Among all bacterial pathogens, Legionella pneumophila maintains the largest known set of translocated substrates, delivering over 300 proteins to the host cell via its Type IVB, Icm/Dot translocation system. Backed by a few notable examples of effector-effector regulation in L. pneumophila, we sought to define the extent of this phenomenon through a systematic analysis of effector-effector functional interaction. We used Saccharomyces cerevisiae, an established proxy for the eukaryotic host, to query > 108,000 pairwise genetic interactions between two compatible expression libraries of ~330 L. pneumophila-translocated substrates. While capturing all known examples of effector-effector suppression, we identify fourteen novel translocated substrates that suppress the activity of other bacterial effectors and one pair with synergistic activities. In at least nine instances, this regulation is direct-a hallmark of an emerging class of proteins called metaeffectors, or "effectors of effectors". Through detailed structural and functional analysis, we show that metaeffector activity derives from a diverse range of mechanisms, shapes evolution, and can be used to reveal important aspects of each cognate effector's function. Metaeffectors, along with other, indirect, forms of effector-effector modulation, may be a common feature of many intracellular pathogens-with unrealized potential to inform our understanding of how pathogens regulate their interactions with the host cell.

Active and adaptive Legionella CRISPR-Cas reveals a recurrent challenge to the pathogen.

Clustered regularly interspaced short palindromic repeats with CRISPR-associated gene (CRISPR-Cas) systems are widely recognized as critical genome defense systems that protect microbes from external threats such as bacteriophage infection. Several isolates of the intracellular pathogen Legionella pneumophila possess multiple CRISPR-Cas systems (type I-C, type I-F and type II-B), yet the targets of these systems remain unknown. With the recent observation that at least one of these systems (II-B) plays a non-canonical role in supporting intracellular replication, the possibility remained that these systems are vestigial genome defense systems co-opted for other purposes. Our data indicate that this is not the case. Using an established plasmid transformation assay, we demonstrate that type I-C, I-F and II-B CRISPR-Cas provide protection against spacer targets. We observe efficient laboratory acquisition of new spacers under 'priming' conditions, in which initially incomplete target elimination leads to the generation of new spacers and ultimate loss of the invasive DNA. Critically, we identify the first known target of L. pneumophila CRISPR-Cas: a 30 kb episome of unknown function whose interbacterial transfer is guarded against by CRISPR-Cas. We provide evidence that the element can subvert CRISPR-Cas by mutating its targeted sequences - but that primed spacer acquisition may limit this mechanism of escape. Rather than generally impinging on bacterial fitness, this element drives a host specialization event - with improved fitness in Acanthamoeba but a reduced ability to replicate in other hosts and conditions. These observations add to a growing body of evidence that host range restriction can serve as an existential threat to L. pneumophila in the wild.

An architectural role for a nuclear noncoding RNA: NEAT1 RNA is essential for the structure of paraspeckles.

NEAT1 RNA, a highly abundant 4 kb ncRNA, is retained in nuclei in approximately 10 to 20 large foci that we show are completely coincident with paraspeckles, nuclear domains implicated in mRNA nuclear retention. Depletion of NEAT1 RNA via RNAi eradicates paraspeckles, suggesting that it controls sequestration of the paraspeckle proteins PSP1 and p54, factors linked to A-I editing. Unlike overexpression of PSP1, NEAT1 overexpression increases paraspeckle number, and paraspeckles emanate exclusively from the NEAT1 transcription site. The PSP-1 RNA binding domain is required for its colocalization with NEAT1 RNA in paraspeckles, and biochemical analyses support that NEAT1 RNA binds with paraspeckle proteins. Unlike other nuclear-retained RNAs, NEAT1 RNA is not A-I edited, consistent with a structural role in paraspeckles. Collectively, results demonstrate that NEAT1 functions as an essential structural determinant of paraspeckles, providing a precedent for a ncRNA as the foundation of a nuclear domain.

Silencing by H-NS potentiated the evolution of Salmonella.

The bacterial H-NS protein silences expression from sequences with higher AT-content than the host genome and is believed to buffer the fitness consequences associated with foreign gene acquisition. Loss of H-NS results in severe growth defects in Salmonella, but the underlying reasons were unclear. An experimental evolution approach was employed to determine which secondary mutations could compensate for the loss of H-NS in Salmonella. Six independently derived S. Typhimurium hns mutant strains were serially passaged for 300 generations prior to whole genome sequencing. Growth rates of all lineages dramatically improved during the course of the experiment. Each of the hns mutant lineages acquired missense mutations in the gene encoding the H-NS paralog StpA encoding a poorly understood H-NS paralog, while 5 of the mutant lineages acquired deletions in the genes encoding the Salmonella Pathogenicity Island-1 (SPI-1) Type 3 secretion system critical to invoke inflammation. We further demonstrate that SPI-1 misregulation is a primary contributor to the decreased fitness in Salmonella hns mutants. Three of the lineages acquired additional loss of function mutations in the PhoPQ virulence regulatory system. Similarly passaged wild type Salmonella lineages did not acquire these mutations. The stpA missense mutations arose in the oligomerization domain and generated proteins that could compensate for the loss of H-NS to varying degrees. StpA variants most able to functionally substitute for H-NS displayed altered DNA binding and oligomerization properties that resembled those of H-NS. These findings indicate that H-NS was central to the evolution of the Salmonellae by buffering the negative fitness consequences caused by the secretion system that is the defining characteristic of the species.

Crystal structure of the Legionella pneumophila Lem10 effector reveals a new member of the HD protein superfamily.

Legionella pneumophila, the intracellular pathogen that can cause severe pneumonia known as Legionnaire's disease, translocates close to 300 effectors inside the host cell using Dot/Icm type IVB secretion system. The structure and function for the majority of these effector proteins remains unknown. Here, we present the crystal structure of the L. pneumophila effector Lem10. The structure reveals a multidomain organization with the largest C-terminal domain showing strong structural similarity to the HD protein superfamily representatives. However, Lem10 lacks the catalytic His-Asp residue pair and does not show any in vitro phosphohydrolase enzymatic activity, typical for HD proteins. While the biological function of Lem10 remains elusive, our analysis shows that similar distinct features are shared by a significant number of HD domains found in Legionella proteins, including the SidE family of effectors known to play an important role during infection. Taken together our data point to the presence of a specific group of non-catalytic Legionella HD domains, dubbed LHDs, which are involved in pathogenesis.

Experimental evolution of Legionella pneumophila in mouse macrophages leads to strains with altered determinants of environmental survival.

The Gram-negative bacterium, Legionella pneumophila, is a protozoan parasite and accidental intracellular pathogen of humans. We propose a model in which cycling through multiple protozoan hosts in the environment holds L. pneumophila in a state of evolutionary stasis as a broad host-range pathogen. Using an experimental evolution approach, we tested this hypothesis by restricting L. pneumophila to growth within mouse macrophages for hundreds of generations. Whole-genome resequencing and high-throughput genotyping identified several parallel adaptive mutations and population dynamics that led to improved replication within macrophages. Based on these results, we provide a detailed view of the population dynamics of an experimentally evolving bacterial population, punctuated by frequent instances of transient clonal interference and selective sweeps. Non-synonymous point mutations in the flagellar regulator, fleN, resulted in increased uptake and broadly increased replication in both macrophages and amoebae. Mutations in multiple steps of the lysine biosynthesis pathway were also independently isolated, resulting in lysine auxotrophy and reduced replication in amoebae. These results demonstrate that under laboratory conditions, host restriction is sufficient to rapidly modify L. pneumophila fitness and host range. We hypothesize that, in the environment, host cycling prevents L. pneumophila host-specialization by maintaining pathways that are deleterious for growth in macrophages and other hosts.

The Legionella pneumophila collagen-like protein mediates sedimentation, autoaggregation, and pathogen-phagocyte interactions.

Although only partially understood, multicellular behavior is relatively common in bacterial pathogens. Bacterial aggregates can resist various host defenses and colonize their environment more efficiently than planktonic cells. For the waterborne pathogen Legionella pneumophila, little is known about the roles of autoaggregation or the parameters which allow cell-cell interactions to occur. Here, we determined the endogenous and exogenous factors sufficient to allow autoaggregation to take place in L. pneumophila. We show that isolates from Legionella species which do not produce the Legionella collagen-like protein (Lcl) are deficient in autoaggregation. Targeted deletion of the Lcl-encoding gene (lpg2644) and the addition of Lcl ligands impair the autoaggregation of L. pneumophila. In addition, Lcl-induced autoaggregation requires divalent cations. Escherichia coli producing surface-exposed Lcl is able to autoaggregate and shows increased biofilm production. We also demonstrate that L. pneumophila infection of Acanthamoeba castellanii and Hartmanella vermiformis is potentiated under conditions which promote Lcl dependent autoaggregation. Overall, this study shows that L. pneumophila is capable of autoaggregating in a process that is mediated by Lcl in a divalent-cation-dependent manner. It also reveals that Lcl potentiates the ability of L. pneumophila to come in contact, attach, and infect amoebae.

Genetic evidence for a regulated cysteine protease catalytic triad in LegA7, a Legionella pneumophila protein that impinges on a stress response pathway.

Legionella pneumophila grows within membrane-bound vacuoles in phylogenetically diverse hosts. Intracellular growth requires the function of the Icm/Dot type-IVb secretion system, which translocates more than 300 proteins into host cells. A screen was performed to identify L. pneumophila proteins that stimulate mitogen-activated protein kinase (MAPK) activation, using Icm/Dot translocated proteins ectopically expressed in mammalian cells. In parallel, a second screen was performed to identify L. pneumophila proteins expressed in yeast that cause growth inhibition in MAPK pathway-stimulatory high-osmolarity medium. LegA7 was shared in both screens, a protein predicted to be a member of the bacterial cysteine protease family that has five carboxyl-terminal ankyrin repeats. Three conserved residues in the predicted catalytic triad of LegA7 were mutated. These mutations abolished the ability of LegA7 to inhibit yeast growth. To identify other residues important for LegA7 function, a generalizable selection strategy in yeast was devised to isolate mutants that have lost function and no longer cause growth inhibition on a high-osmolarity medium. Mutations were isolated in the two carboxyl-terminal ankyrin repeats, as well as an inter-domain region located between the cysteine protease domain and the ankyrin repeats. These mutations were predicted by AlphaFold modeling to localize to the face opposite from the catalytic site, arguing that they interfere with the positive regulation of the catalytic activity. Based on our data, we present a model in which LegA7 harbors a cysteine protease domain with an inter-domain and two carboxyl-terminal ankyrin repeat regions that modulate the function of the catalytic domain. IMPORTANCE: Legionella pneumophila grows in a membrane-bound compartment in macrophages during disease. Construction of the compartment requires a dedicated secretion system that translocates virulence proteins into host cells. One of these proteins, LegA7, is shown to activate a stress response pathway in host cells called the mitogen-activated protein kinase (MAPK) pathway. The effects on the mammalian MAPK pathway were reconstructed in yeast, allowing the development of a strategy to identify the role of individual domains of LegA7. A domain similar to cysteine proteases is demonstrated to be critical for impinging on the MAPK pathway, and the catalytic activity of this domain is required for targeting this path. In addition, a conserved series of repeats, called ankyrin repeats, controls this activity. Data are provided that argue the interaction of the ankyrin repeats with unknown targets probably results in activation of the cysteine protease domain.

Genetic evidence for a regulated cysteine protease catalytic triad in LegA7, a Legionella pneumophila protein that impinges on a stress response pathway.

Legionella pneumophila grows within membrane-bound vacuoles in phylogenetically diverse hosts. Intracellular growth requires the function of the Icm/Dot type-IVb secretion system, which translocates more than 300 proteins into host cells. A screen was performed to identify L. pneumophila proteins that stimulate MAPK activation, using Icm/Dot translocated proteins ectopically expressed in mammalian cells. In parallel, a second screen was performed to identify L. pneumophila proteins expressed in yeast that cause growth inhibition in MAPK pathway-stimulatory high osmolarity medium. LegA7 was shared in both screens, a protein predicted to be a member of the bacterial cysteine protease family that has five carboxyl-terminal ankyrin repeats. Three conserved residues in the predicted catalytic triad of LegA7 were mutated. These mutations abolished the ability of LegA7 to inhibit yeast growth. To identify other residues important for LegA7 function, a generalizable selection strategy in yeast was devised to isolate mutants that have lost function and no longer cause growth inhibition on high osmolarity medium. Mutations were isolated in the two carboxyl-terminal ankyrin repeats, as well as an inter-domain region located between the cysteine protease domain and the ankyrin repeats. These mutations were predicted by AlphaFold modeling to localize to the face opposite from the catalytic site, arguing that they interfere with the positive regulation of the catalytic activity. Based on our data, we present a model in which LegA7 harbors a cysteine protease domain with an inter-domain and two carboxyl-terminal ankyrin repeat regions that modulate the function of the catalytic domain.

A random mutagenesis screen enriched for missense mutations in bacterial effector proteins.

To remodel their hosts and escape immune defenses, many pathogens rely on large arsenals of proteins (effectors) that are delivered to the host cell using dedicated translocation machinery. Effectors hold significant insight into the biology of both the pathogens that encode them and the host pathways that they manipulate. One of the most powerful systems biology tools for studying effectors is the model organism, Saccharomyces cerevisiae. For many pathogens, the heterologous expression of effectors in yeast is growth inhibitory at a frequency much higher than housekeeping genes, an observation ascribed to targeting conserved eukaryotic proteins. Abrogation of yeast growth inhibition has been used to identify bacterial suppressors of effector activity, host targets, and functional residues and domains within effector proteins. We present here a yeast-based method for enriching for informative, in-frame, missense mutations in a pool of random effector mutants. We benchmark this approach against three effectors from Legionella pneumophila, an intracellular bacterial pathogen that injects a staggering >330 effectors into the host cell. For each protein, we show how in silico protein modeling (AlphaFold2) and missense-directed mutagenesis can be combined to reveal important structural features within effectors. We identify known active site residues within the metalloprotease RavK, the putative active site in SdbB, and previously unidentified functional motifs within the C-terminal domain of SdbA. We show that this domain has structural similarity with glycosyltransferases and exhibits in vitro activity consistent with this predicted function.

Coordination of the random asynchronous replication of autosomal loci.

Random monoallelic expression and asynchronous replication define an unusual class of autosomal mammalian genes. We show that every cell has randomly chosen either the maternal or paternal copy of each given autosome pair, such that alleles of these genes scattered across the chosen chromosome replicate earlier than the alleles on the homologous chromosome. Thus, chromosome-pair non-equivalence, rather than being limited to X-chromosome inactivation, is a fundamental property of mouse chromosomes.

Functional diversification despite structural congruence in the HipBST toxin-antitoxin system of Legionella pneumophila.

IMPORTANCE: Toxin-antitoxin (TA) systems are parasitic genetic elements found in almost all bacterial genomes. They are exchanged horizontally between cells and are typically poorly conserved across closely related strains and species. Here, we report the characterization of a tripartite TA system in the bacterial pathogen Legionella pneumophila that is highly conserved across Legionella species genomes. This system (denoted HipBSTLp) is a distant homolog of the recently discovered split-HipA system in Escherichia coli (HipBSTEc). We present bioinformatic, molecular, and structural analyses of the divergence between these two systems and the functionality of this newly described TA system family. Furthermore, we provide evidence to refute previous claims that the toxin in this system (HipTLp) possesses bifunctionality as an L. pneumophila virulence protein. Overall, this work expands our understanding of the split-HipA system architecture and illustrates the potential for undiscovered biology in these abundant genetic elements.

Mobile Element Integration Reveals a Chromosome Dimer Resolution System in Legionellales.

In bacteria, the mechanisms used to repair DNA lesions during genome replication include homologous recombination between sister chromosomes. This can lead to the formation of chromosome dimers if an odd number of crossover events occurs. The dimers must be resolved before cell separation to ensure genomic stability and cell viability. Dimer resolution is achieved by the broadly conserved dif/Xer system, which catalyzes one additional crossover event immediately prior to cell separation. While dif/Xer systems have been characterized or predicted in the vast majority of proteobacteria, no homologs to dif or xer have been identified in the order Legionellales. Here, we report the discovery of a distinct single-recombinase dif/Xer system in the intracellular pathogen Legionella pneumophila. The dif site was uncovered by our analysis of Legionella mobile element-1 (LME-1), which harbors a dif site mimic and integrates into the L. pneumophila genome via site-specific recombination. We demonstrate that lpg1867 (here named xerL) encodes a tyrosine recombinase that is necessary and sufficient for catalyzing recombination at the dif site and that deletion of dif or xerL causes filamentation along with extracellular and intracellular growth defects. We show that the dif/XerL system is present throughout Legionellales and that Coxiella burnetii XerL and its cognate dif site can functionally substitute for the native system in L. pneumophila. Finally, we describe an unexpected link between C. burnetii dif/Xer and the maintenance of its virulence plasmids. IMPORTANCE The maintenance of circular chromosomes depends on the ability to resolve aberrant chromosome dimers after they form. In most proteobacteria, broadly conserved Xer recombinases catalyze single crossovers at short, species-specific dif sites located near the replication terminus. Chromosomal dimerization leads to the formation of two copies of dif within the same molecule, leading to rapid site-specific recombination and conversion back into chromosome monomers. The apparent absence of chromosome dimer resolution mechanisms in Legionellales has been a mystery to date. By studying a phage-like mobile genetic element, LME-1, we have identified a previously unknown single-recombinase dif/Xer system that is not only widespread across Legionellales but whose activity is linked to virulence in two important human pathogens.

Draft Genome Sequence of Staphylococcus chromogenes ATCC 43764, a Coagulase-Negative Staphylococcus Strain with Antibacterial Potential.

Staphylococcus chromogenes can cause subclinical mastitis in cows, and some strains have also demonstrated antibacterial activity against pathogens such as methicillin-resistant Staphylococcus aureus (MRSA). Here, we report the draft genome sequence of the S. chromogenes type strain ATCC 43764, which secretes the prodrug 6-thioguanine (6-TG), which antagonizes MRSA virulence.

Coagulase-negative staphylococci release a purine analog that inhibits Staphylococcus aureus virulence.

Coagulase-negative staphylococci and Staphylococcus aureus colonize similar niches in mammals and conceivably compete for space and nutrients. Here, we report that a coagulase-negative staphylococcus, Staphylococcus chromogenes ATCC43764, synthesizes and secretes 6-thioguanine (6-TG), a purine analog that suppresses S. aureus growth by inhibiting de novo purine biosynthesis. We identify a 6-TG biosynthetic gene cluster in S. chromogenes and other coagulase-negative staphylococci including S. epidermidis, S. pseudintermedius and S. capitis. Recombinant S. aureus strains harbouring this operon produce 6-TG and, when used in subcutaneous co-infections in mice with virulent S. aureus USA300, protect the host from necrotic lesion formation. Used prophylactically, 6-TG reduces necrotic skin lesions in mice infected with USA300, and this effect is mediated by abrogation of toxin production. RNAseq analyses reveal that 6-TG downregulates expression of genes coding for purine biosynthesis, the accessory gene regulator (agr) and ribosomal proteins in S. aureus, providing an explanation for its effect on toxin production.

E3 ubiquitin ligase activity and targeting of BAT3 by multiple Legionella pneumophila translocated substrates.

The intracellular bacterial pathogen Legionella pneumophila modulates a number of host processes during intracellular growth, including the eukaryotic ubiquitination machinery, which dictates the stability, activity, and/or localization of a large number of proteins. A number of L. pneumophila proteins contain eukaryotic-like motifs typically associated with ubiquitination. Central among these is a family of five F-box-domain-containing proteins of Legionella pneumophila. Each of these five proteins is translocated to the host cytosol by the Dot/Icm type IV protein translocation system during infection. We show that three of these proteins, LegU1, LegAU13, and LicA, interact with components of the host ubiquitination machinery in vivo. In addition, LegU1 and LegAU13 are integrated into functional Skp-Cullin-F-box (SCF) complexes that confer E3 ubiquitin ligase activity. LegU1 specifically interacts with and can direct the ubiquitination of the host chaperone protein BAT3. In a screen for additional L. pneumophila proteins that associate with LegU1 in mammalian cells, we identified the bacterial protein Lpg2160. We demonstrate that Lpg2160 also associates with BAT3 independently of LegU1. We show that Lpg2160 is a translocated substrate of the Dot/Icm system and contains a C-terminal translocation signal. We propose a model in which LegU1 and Lpg2160 may function redundantly or in concert to modulate BAT3 activity during the course of infection.