The twin-arginine translocation (Tat) system.

In this Quick guide, Palmer and Berks introduce the twin-arginine translocation (Tat) systems. Tats are found in a variety of microbes and microbe-derived organelles, and are known to translocate folded substrate proteins across biological membranes.

The role of proteinaceous toxins secreted by Staphylococcus aureus in interbacterial competition.

Staphylococcus aureus is highly adapted to colonization of the mammalian host. In humans the primary site of colonization is the epithelium of the nasal cavity. A major barrier to colonization is the resident microbiota, which have mechanisms to exclude S. aureus. As such, S. aureus has evolved mechanisms to compete with other bacteria, one of which is through secretion of proteinaceous toxins. S. aureus strains collectively produce a number of well-characterized Class I, II, and IV bacteriocins as well as several bacteriocin-like substances, about which less is known. These bacteriocins have potent antibacterial activity against several Gram-positive organisms, with some also active against Gram-negative species. S. aureus bacteriocins characterized to date are sporadically produced, and often encoded on plasmids. More recently the type VII secretion system (T7SS) of S. aureus has also been shown to play a role in interbacterial competition. The T7SS is encoded by all S. aureus isolates and so may represent a more widespread mechanism of competition used by this species. T7SS antagonism is mediated by the secretion of large protein toxins, three of which have been characterized to date: a nuclease toxin, EsaD; a membrane depolarizing toxin, TspA; and a phospholipase toxin, TslA. Further study is required to decipher the role that these different types of secreted toxins play in interbacterial competition and colonization of the host.

Characterisation of anhydro-sialic acid transporters from mucosa-associated bacteria.

Sialic acid (Sia) transporters are critical to the capacity of host-associated bacteria to utilise Sia for growth and/or cell surface modification. While N-acetyl-neuraminic acid (Neu5Ac)-specific transporters have been studied extensively, little is known on transporters dedicated to anhydro-Sia forms such as 2,7-anhydro-Neu5Ac (2,7-AN) or 2,3-dehydro-2-deoxy-Neu5Ac (Neu5Ac2en). Here, we used a Sia-transport-null strain of Escherichia coli to investigate the function of members of anhydro-Sia transporter families previously identified by computational studies. First, we showed that the transporter NanG, from the Glycoside-Pentoside-Hexuronide:cation symporter family, is a specific 2,7-AN transporter, and identified by mutagenesis a crucial functional residue within the putative substrate-binding site. We then demonstrated that NanX transporters, of the Major Facilitator Superfamily, also only transport 2,7-AN and not Neu5Ac2en nor Neu5Ac. Finally, we provided evidence that SiaX transporters, of the Sodium-Solute Symporter superfamily, are promiscuous Neu5Ac/Neu5Ac2en transporters able to acquire either substrate equally well. The characterisation of anhydro-Sia transporters expands our current understanding of prokaryotic Sia metabolism within host-associated microbial communities.

Identification of novel tail-anchored membrane proteins integrated by the bacterial twin-arginine translocase.

The twin-arginine protein transport (Tat) system exports folded proteins across the cytoplasmic membranes of prokaryotes and the energy transducing-membranes of plant thylakoids and mitochondria. Proteins are targeted to the Tat machinery by N-terminal signal peptides with a conserved twin-arginine motif, and some substrates are exported as heterodimers where the signal peptide is present on one of the partner proteins. A subset of Tat substrates is found in the membrane. Tat-dependent membrane proteins usually have large globular domains and a single transmembrane helix present at the N- or C-terminus. Five Tat substrates that have C-terminal transmembrane helices have previously been characterized in the model bacterium Escherichia coli. Each of these is an iron-sulfur cluster-containing protein involved in electron transfer from hydrogen or formate. Here we have undertaken a bioinformatic search to identify further tail-anchored Tat substrates encoded in bacterial genomes. Our analysis has revealed additional tail-anchored iron-sulfur proteins associated in modules with either a b-type cytochrome or a quinol oxidase. We also identified further candidate tail-anchored Tat substrates, particularly among members of the actinobacterial phylum, that are not predicted to contain cofactors. Using reporter assays, we show experimentally that six of these have both N-terminal Tat signal peptides and C-terminal transmembrane helices. The newly identified proteins include a carboxypeptidase and a predicted protease, and four sortase substrates for which membrane integration is a prerequisite for covalent attachment to the cell wall.

A type VII-secreted lipase toxin with reverse domain arrangement.

The type VII protein secretion system (T7SS) is found in many Gram-positive bacteria and in pathogenic mycobacteria. All T7SS substrate proteins described to date share a common helical domain architecture at the N-terminus that typically interacts with other helical partner proteins, forming a composite signal sequence for targeting to the T7SS. The C-terminal domains are functionally diverse and in Gram-positive bacteria such as Staphylococcus aureus often specify toxic anti-bacterial activity. Here we describe the first example of a class of T7 substrate, TslA, that has a reverse domain organisation. TslA is widely found across Bacillota including Staphylococcus, Enterococcus and Listeria. We show that the S. aureus TslA N-terminal domain is a phospholipase A with anti-staphylococcal activity that is neutralised by the immunity lipoprotein TilA. Two small helical partner proteins, TlaA1 and TlaA2 are essential for T7-dependent secretion of TslA and at least one of these interacts with the TslA C-terminal domain to form a helical stack. Cryo-EM analysis of purified TslA complexes indicate that they share structural similarity with canonical T7 substrates. Our findings suggest that the T7SS has the capacity to recognise a secretion signal present at either end of a substrate.

Interbacterial competition mediated by the type VIIb secretion system.

Successful occupancy of a given niche requires the colonising bacteria to interact extensively with the biotic and abiotic environment, including other resident microbes. Bacteria have evolved a range of protein secretion machines for this purpose with eleven such systems identified to date. The type VIIb secretion system (T7SSb) is utilised by Bacillota to secrete a range of protein substrates, including antibacterial toxins targeting closely related strains, and the system as a whole has been implicated in a range of activities such as iron acquisition, intercellular signalling, host colonisation and virulence. This review covers the components and secretion mechanism of the T7SSb, the substrates of these systems and their roles in Gram-positive bacteria, with a focus on interbacterial competition.

Three small partner proteins facilitate the type VII-dependent secretion of an antibacterial nuclease.

IMPORTANCE: Staphylococcus aureus is an opportunistic human pathogen associated with severe infections and antimicrobial resistance. S. aureus strains utilize a type VII secretion system to secrete toxins targeting competitor bacteria, likely facilitating colonization. EsaD is a nuclease toxin secreted by the type VII secretion system in many strains of S. aureus as well as other related bacterial species. Here, we identify three small proteins of previously unknown function as export factors, required for efficient secretion of EsaD. We show that these proteins bind to the transport domain of EsaD, forming a complex with a striking cane-like conformation.

Three small partner proteins facilitate the type VII-dependent secretion export of an antibacterial nuclease.

The type VIIb protein secretion system (T7SSb) plays a role in interbacterial competition in Gram-positive Firmicute bacteria and secretes various toxic effector proteins. The mechanism of secretion and the roles of numerous conserved genes within T7SSb gene clusters remain unknown. EsaD is a nuclease toxin secreted by the Staphylococcus aureus T7SSb, which forms a complex with its cognate immunity protein, EsaG, and chaperone EsaE. Encoded upstream of EsaD are three small secreted proteins, EsxB, EsxC and EsxD. Here we show that EsxBCD bind to the transport domain of EsaD and function as EsaD export factors. We report the first structural information for a complete T7SSb substrate pre-secretion complex. Cryo-EM of the EsaDEG trimer and the EsaDEG-EsxBCD hexamer shows that incorporation of EsxBCD confers a conformation comprising a flexible globular cargo domain attached to a long narrow shaft that is likely to be crucial for efficient toxin export.

A novel variant of the Listeria monocytogenes type VII secretion system EssC component is associated with an Rhs toxin.

The type VIIb protein secretion system (T7SSb) is found in Bacillota (firmicute) bacteria and has been shown to mediate interbacterial competition. EssC is a membrane-bound ATPase that is a critical component of the T7SSb and plays a key role in substrate recognition. Prior analysis of available genome sequences of the foodborne bacterial pathogen Listeria monocytogenes has shown that although the T7SSb was encoded as part of the core genome, EssC could be found as one of seven different sequence variants. While each sequence variant was associated with a specific suite of candidate substrate proteins encoded immediately downstream of essC, many LXG-domain proteins were encoded across multiple essC sequence variants. Here, we have extended this analysis using a diverse collection of 37 930 L. monocytogenes genomes. We have identified a rare eighth variant of EssC present in ten L. monocytogenes lineage III genomes. These genomes also encode a large toxin of the rearrangement hotspot (Rhs) repeat family adjacent to essC8, along with a probable immunity protein and three small accessory proteins. We have further identified nine novel LXG-domain proteins, and four additional chromosomal hotspots across L. monocytogenes genomes where LXG proteins can be encoded. The eight L. monocytogenes EssC variants were also found in other Listeria species, with additional novel EssC types also identified. Across the genus, species frequently encoded multiple EssC types, indicating that T7SSb diversity is a primary feature of the genus Listeria.

Characterisation of key genotypic and phenotypic traits of clinical cystic fibrosis Staphylococcus aureus isolates.

Introduction. One third of people with CF in the UK are co-infected by both Staphylococcus aureus and Pseudomonas aeruginosa. Chronic bacterial infection in CF contributes to the gradual destruction of lung tissue, and eventually respiratory failure in this group.Gap Statement. The contribution of S. aureus to cystic fibrosis (CF) lung decline in the presence or absence of P. aeruginosa is unclear. Defining the molecular and phenotypic characteristics of a range of S. aureus clinical isolates will help further understand its pathogenic capabilities.Aim. Our objective was to use molecular and phenotypic tools to characterise twenty-five clinical S. aureus isolates collected from mono- and coinfection with P. aeruginosa from people with CF at the Royal Victoria Infirmary, Newcastle upon Tyne.Methodology. Genomic DNA was extracted and sequenced. Multilocus sequence typing was used to construct phylogeny from the seven housekeeping genes. A pangenome was calculated using Roary, and cluster of Orthologous groups were assigned using eggNOG-mapper which were used to determine differences within core, accessory, and unique genomes. Characterisation of sequence type, clonal complex, agr and spa types was carried out using PubMLST, eBURST, AgrVATE and spaTyper, respectively. Antibiotic resistance was determined using Kirby-Bauer disc diffusion tests. Phenotypic testing of haemolysis was carried out using ovine red blood cell agar plates and mucoid phenotypes visualised using Congo red agar.Results. Clinical strains clustered closely based on agr type, sequence type and clonal complex. COG analysis revealed statistically significant enrichment of COG families between core, accessory and unique pangenome groups. The unique genome was significantly enriched for replication, recombination and repair, and defence mechanisms. The presence of known virulence genes and toxins were high within this group, and unique genes were identified in 11 strains. Strains which were isolated from the same patient all surpassed average nucleotide identity thresholds, however, differed in phenotypic traits. Antimicrobial resistance to macrolides was significantly higher in the coinfection group.Conclusion. There is huge variation in genetic and phenotypic capabilities of S. aureus strains. Further studies on how these may differ in relation to other species in the CF lung may give insight into inter-species interactions.

The novel anti-phage system Shield co-opts an RmuC domain to mediate phage defense across Pseudomonas species.

Competitive bacteria-bacteriophage interactions have resulted in the evolution of a plethora of bacterial defense systems preventing phage propagation. In recent years, computational and bioinformatic approaches have underpinned the discovery of numerous novel bacterial defense systems. Anti-phage systems are frequently encoded together in genomic loci termed defense islands. Here we report the identification and characterisation of a novel anti-phage system, that we have termed Shield, which forms part of the Pseudomonas defensive arsenal. The Shield system comprises the core component ShdA, a membrane-bound protein harboring an RmuC domain. Heterologous production of ShdA alone is sufficient to mediate bacterial immunity against several phages. We demonstrate that Shield and ShdA confer population-level immunity and that they can also decrease transformation efficiency. We further show that ShdA homologues can degrade DNA in vitro and, when expressed in a heterologous host, can alter the organisation of the host chromosomal DNA. Use of comparative genomic approaches identified how Shield can be divided into four subtypes, three of which contain additional components that in some cases can negatively affect the activity of ShdA and/or provide additional lines of phage defense. Collectively, our results identify a new player within the Pseudomonas bacterial immunity arsenal that displays a novel mechanism of protection, and reveals a role for RmuC domains in phage defense.

Characterization of a TatA/TatB binding site on the TatC component of the Escherichia coli twin arginine translocase.

The twin arginine transport (Tat) pathway exports folded proteins across the cytoplasmic membranes of prokaryotes and the thylakoid membranes of chloroplasts. In Escherichia coli and other Gram-negative bacteria, the Tat machinery comprises TatA, TatB and TatC components. A Tat receptor complex, formed from all three proteins, binds Tat substrates, which triggers receptor organization and recruitment of further TatA molecules to form the active Tat translocon. The polytopic membrane protein TatC forms the core of the Tat receptor and harbours two binding sites for the sequence-related TatA and TatB proteins. A 'polar' cluster binding site, formed by TatC transmembrane helices (TMH) 5 and 6 is occupied by TatB in the resting receptor and exchanges for TatA during receptor activation. The second binding site, lying further along TMH6, is occupied by TatA in the resting state, but its functional relevance is unclear. Here we have probed the role of this second binding site through a programme of random and targeted mutagenesis. Characterization of three stably produced TatC variants, P221R, M222R and L225P, each of which is inactive for protein transport, demonstrated that the substitutions did not affect assembly of the Tat receptor. Moreover, the substitutions that we analysed did not abolish TatA or TatB binding to either binding site. Using targeted mutagenesis we introduced bulky substitutions into the TatA binding site. Molecular dynamics simulations and crosslinking analysis indicated that TatA binding at this site was substantially reduced by these amino acid changes, but TatC retained function. While it is not clear whether TatA binding at the TMH6 site is essential for Tat activity, the isolation of inactivating substitutions indicates that this region of the protein has a critical function.

Homologous recombination between tandem paralogues drives evolution of a subset of type VII secretion system immunity genes in firmicute bacteria.

The type VII secretion system (T7SS) is found in many Gram-positive firmicutes and secretes protein toxins that mediate bacterial antagonism. Two T7SS toxins have been identified in Staphylococcus aureus, EsaD a nuclease toxin that is counteracted by the EsaG immunity protein, and TspA, which has membrane depolarising activity and is neutralised by TsaI. Both toxins are polymorphic, and strings of non-identical esaG and tsaI immunity genes are encoded in all S. aureus strains. To investigate the evolution of esaG repertoires, we analysed the sequences of the tandem esaG genes and their encoded proteins. We identified three blocks of high sequence similarity shared by all esaG genes and identified evidence of extensive recombination events between esaG paralogues facilitated through these conserved sequence blocks. Recombination between these blocks accounts for loss and expansion of esaG genes in S. aureus genomes and we identified evidence of such events during evolution of strains in clonal complex 8. TipC, an immunity protein for the TelC lipid II phosphatase toxin secreted by the streptococcal T7SS, is also encoded by multiple gene paralogues. Two blocks of high sequence similarity locate to the 5' and 3' end of tipC genes, and we found strong evidence for recombination between tipC paralogues encoded by Streptococcus mitis BCC08. By contrast, we found only a single homology block across tsaI genes, and little evidence for intergenic recombination within this gene family. We conclude that homologous recombination is one of the drivers for the evolution of T7SS immunity gene clusters.

The biogenesis of ß-lactamase enzymes.

The discovery of penicillin by Alexander Fleming marked a new era for modern medicine, allowing not only the treatment of infectious diseases, but also the safe performance of life-saving interventions, like surgery and chemotherapy. Unfortunately, resistance against penicillin, as well as more complex ß-lactam antibiotics, has rapidly emerged since the introduction of these drugs in the clinic, and is largely driven by a single type of extra-cytoplasmic proteins, hydrolytic enzymes called ß-lactamases. While the structures, biochemistry and epidemiology of these resistance determinants have been extensively characterized, their biogenesis, a complex process including multiple steps and involving several fundamental biochemical pathways, is rarely discussed. In this review, we provide a comprehensive overview of the journey of ß-lactamases, from the moment they exit the ribosomal channel until they reach their final cellular destination as folded and active enzymes.

Genomic analysis of the progenitor strains of Staphylococcus aureus RN6390.

RN6390 is a commonly used laboratory strain of Staphylococcus aureus derived from NCTC8325. In this study, we sequenced the RN6390 genome and compared it to available genome sequences for NCTC8325. We confirmed that three prophages, Φ11, Φ12 and Φ13, which are present in NCTC8325 are absent from the genome of RN6390, consistent with the successive curing events leading to the generation of this strain. However, we noted that a separate prophage is present in RN6390 that is not found in NCTC8325. Two separate genome sequences have been deposited for the parental strain, NCTC8325. Analysis revealed several differences between these sequences, in particular, between the copy number of esaG genes, which encode immunity proteins to the type VII secreted anti-bacterial toxin, EsaD. Single nucleotide polymorphisms were also detected in ribosomal RNA genes and genes encoding microbial surface components recognizing adhesive matrix molecules (MSCRAMM) between the two NCTC8325 sequences. Comparing each NCTC8325 sequence to other strains in the RN6390 lineage confirmed that sequence assembly errors in the earlier NCTC8325 sequence are the most likely explanation for most of the differences observed.

The Type VII Secretion System of Staphylococcus.

The type VII protein secretion system (T7SS) of Staphylococcus aureus is encoded at the ess locus. T7 substrate recognition and protein transport are mediated by EssC, a membrane-bound multidomain ATPase. Four EssC sequence variants have been identified across S. aureus strains, each accompanied by a specific suite of substrate proteins. The ess genes are upregulated during persistent infection, and the secretion system contributes to virulence in disease models. It also plays a key role in intraspecies competition, secreting nuclease and membrane-depolarizing toxins that inhibit the growth of strains lacking neutralizing immunity proteins. A genomic survey indicates that the T7SS is widely conserved across staphylococci and is encoded in clusters that contain diverse arrays of toxin and immunity genes. The presence of genomic islands encoding multiple immunity proteins in species such as Staphylococcus warneri that lack the T7SS points to a major role for the secretion system in bacterial antagonism.

Multiple evolutionary origins reflect the importance of sialic acid transporters in the colonization potential of bacterial pathogens and commensals.

Located at the tip of cell surface glycoconjugates, sialic acids are at the forefront of host-microbe interactions and, being easily liberated by sialidase enzymes, are used as metabolites by numerous bacteria, particularly by pathogens and commensals living on or near diverse mucosal surfaces. These bacteria rely on specific transporters for the acquisition of host-derived sialic acids. Here, we present the first comprehensive genomic and phylogenetic analysis of bacterial sialic acid transporters, leading to the identification of multiple new families and subfamilies. Our phylogenetic analysis suggests that sialic acid-specific transport has evolved independently at least eight times during the evolution of bacteria, from within four of the major families/superfamilies of bacterial transporters, and we propose a robust classification scheme to bring together a myriad of different nomenclatures that exist to date. The new transporters discovered occur in diverse bacteria, including Spirochaetes, Bacteroidetes, Planctomycetes and Verrucomicrobia, many of which are species that have not been previously recognized to have sialometabolic capacities. Two subfamilies of transporters stand out in being fused to the sialic acid mutarotase enzyme, NanM, and these transporter fusions are enriched in bacteria present in gut microbial communities. Our analysis supports the increasing experimental evidence that competition for host-derived sialic acid is a key phenotype for successful colonization of complex mucosal microbiomes, such that a strong evolutionary selection has occurred for the emergence of sialic acid specificity within existing transporter architectures.

The Carbapenemase BKC-1 from Klebsiella pneumoniae Is Adapted for Translocation by Both the Tat and Sec Translocons.

The cell envelope of Gram-negative bacteria consists of two membranes surrounding the periplasm and peptidoglycan layer. ß-Lactam antibiotics target the periplasmic penicillin-binding proteins that synthesize peptidoglycan, resulting in cell death. The primary means by which bacterial species resist the effects of ß-lactam drugs is to populate the periplasmic space with ß-lactamases. Resistance to ß-lactam drugs is spread by lateral transfer of genes encoding ß-lactamases from one species of bacteria to another. However, the resistance phenotype depends in turn on these "alien" protein sequences being recognized and exported across the cytoplasmic membrane by either the Sec or Tat protein translocation machinery of the new bacterial host. Here, we examine BKC-1, a carbapenemase from an unknown bacterial source that has been identified in a single clinical isolate of Klebsiella pneumoniae. BKC-1 was shown to be located in the periplasm, and functional in both K. pneumoniae and Escherichia coli. Sequence analysis revealed the presence of an unusual signal peptide with a twin arginine motif and a duplicated hydrophobic region. Biochemical assays showed this signal peptide directs BKC-1 for translocation by both Sec and Tat translocons. This is one of the few descriptions of a periplasmic protein that is functionally translocated by both export pathways in the same organism, and we suggest it represents a snapshot of evolution for a ß-lactamase adapting to functionality in a new host. IMPORTANCE Bacteria can readily acquire plasmids via lateral gene transfer (LGT). These plasmids can carry genes for virulence and antimicrobial resistance (AMR). Of growing concern are LGT events that spread ß-lactamases, particularly carbapenemases, and it is important to understand what limits this spread. This study provides insight into the sequence features of BKC-1 that exemplify the limitations on the successful biogenesis of ß-lactamases, which is one factor limiting the spread of AMR phenotypes by LGT. With a very simple evolutionary adaptation, BKC-1 could become a more effective carbapenemase, underscoring the need to understand the evolution, adaptability, and functional assessment of newly reported ß-lactamases rapidly and thoroughly.