Isolation of the Actinobacillus pleuropneumoniae haemolysin gene and the activation and secretion of the prohaemolysin by the HlyC, HlyB and HlyD proteins of Escherichia coli

Type 1 secretion necessitates a tight interplay between all domains of the ABC transporter

Type I secretion systems (T1SS) facilitate the secretion of substrates in one step across both membranes of Gram-negative bacteria. A prime example is the hemolysin T1SS which secretes the toxin HlyA. Secretion is energized by the ABC transporter HlyB, which forms a complex together with the membrane fusion protein HlyD and the outer membrane protein TolC. HlyB features three domains: an N-terminal C39 peptidase-like domain (CLD), a transmembrane domain (TMD) and a C-terminal nucleotide binding domain (NBD). Here, we created chimeric transporters by swapping one or more domains of HlyB with the respective domain(s) of RtxB, a HlyB homolog from Kingella kingae. We tested all chimeric transporters for their ability to secrete pro-HlyA when co-expressed with HlyD. The CLD proved to be most critical, as a substitution abolished secretion. Swapping only the TMD or NBD reduced the secretion efficiency, while a simultaneous exchange abolished secretion. These results indicate that the CLD is the most critical secretion determinant, while TMD and NBD might possess additional recognition or interaction sites. This mode of recognition represents a hierarchical and extreme unusual case of substrate recognition for ABC transporters and optimal secretion requires a tight interplay between all domains.

Investigations on the substrate binding sites of hemolysin B, an ABC transporter, of a type 1 secretion system

The ABC transporter hemolysin B (HlyB) is the key protein of the HlyA secretion system, a paradigm of type 1 secretion systems (T1SS). T1SS catalyze the one-step substrate transport across both membranes of Gram-negative bacteria. The HlyA T1SS is composed of the ABC transporter (HlyB), the membrane fusion protein (HlyD), and the outer membrane protein TolC. HlyA is a member of the RTX (repeats in toxins) family harboring GG repeats that bind Ca2+ in the C-terminus upstream of the secretion signal. Beside the GG repeats, the presence of an amphipathic helix (AH) in the C-terminus of HlyA is essential for secretion. Here, we propose that a consensus length between the GG repeats and the AH affects the secretion efficiency of the heterologous RTX secreted by the HlyA T1SS. Our in silico studies along with mutagenesis and biochemical analysis demonstrate that there are two binding pockets in the nucleotide binding domain of HlyB for HlyA. The distances between the domains of HlyB implied to interact with HlyA indicated that simultaneous binding of the substrate to both cytosolic domains of HlyB, the NBD and CLD, is possible and required for efficient substrate secretion.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient 'contact weapons' that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

Identity Determinants of the Translocation Signal for a Type 1 Secretion System

The toxin hemolysin A was first identified in uropathogenic E. coli strains and shown to be secreted in a one-step mechanism by a dedicated secretion machinery. This machinery, which belongs to the Type I secretion system family of the Gram-negative bacteria, is composed of the outer membrane protein TolC, the membrane fusion protein HlyD and the ABC transporter HlyB. The N-terminal domain of HlyA represents the toxin which is followed by a RTX (Repeats in Toxins) domain harboring nonapeptide repeat sequences and the secretion signal at the extreme C-terminus. This secretion signal, which is necessary and sufficient for secretion, does not appear to require a defined sequence, and the nature of the encoded signal remains unknown. Here, we have combined structure prediction based on the AlphaFold algorithm together with functional and in silico data to examine the role of secondary structure in secretion. Based on the presented data, a C-terminal, amphipathic helix is proposed between residues 975 and 987 that plays an essential role in the early steps of the secretion process.

Acyltransferase-mediated selection of the length of the fatty acyl chain and of the acylation site governs activation of bacterial RTX toxins

In a wide range of organisms, from bacteria to humans, numerous proteins have to be posttranslationally acylated to become biologically active. Bacterial repeats in toxin (RTX) cytolysins form a prominent group of proteins that are synthesized as inactive protoxins and undergo posttranslational acylation on ε-amino groups of two internal conserved lysine residues by co-expressed toxin-activating acyltransferases. Here, we investigated how the chemical nature, position, and number of bound acyl chains govern the activities of Bordetella pertussis adenylate cyclase toxin (CyaA), Escherichia coli α-hemolysin (HlyA), and Kingella kingae cytotoxin (RtxA). We found that the three protoxins are acylated in the same E. coli cell background by each of the CyaC, HlyC, and RtxC acyltransferases. We also noted that the acyltransferase selects from the bacterial pool of acyl-acyl carrier proteins (ACPs) an acyl chain of a specific length for covalent linkage to the protoxin. The acyltransferase also selects whether both or only one of two conserved lysine residues of the protoxin will be posttranslationally acylated. Functional assays revealed that RtxA has to be modified by 14-carbon fatty acyl chains to be biologically active, that HlyA remains active also when modified by 16-carbon acyl chains, and that CyaA is activated exclusively by 16-carbon acyl chains. These results suggest that the RTX toxin molecules are structurally adapted to the length of the acyl chains used for modification of their acylated lysine residue in the second, more conserved acylation site.

A bivalent fusion vaccine composed of recombinant Apx proteins shows strong protection against Actinobacillus pleuroneumoniae serovar 1 and 2 in a mouse model

Actinobacillus pleuropneumonia (APP) causes porcine pleuropneumoniae, resulting in severe economic losses in the swine industry. Since there are diverse serotypes of APP, it is necessary for vaccines to induce cross-protection. In this report, we developed a bivalent fusion vaccine, the L vaccine composed of ApxIA and ApxIIA fragments. According to the experimental results of the L vaccine, recombinant protein specific-IgG antibody level increased significantly as well as Apx toxin specific-IgG antibody, suggesting toxin-neutralizing effect. Also, the production of both IgG1 and IgG2a indicates this fusion vaccine induces Th1 and Th2 immune reactions. In addition, lymphocytes were proliferated and immune related-cytokines of TNF-α, IL-12, IFN-γ and IL-5 were detected in the serum after the vaccination. The L vaccine showed a perfect cross-protection against APP serovar 1 and 2 that each secrete different Apx exotoxins. These findings reveal that the fusion L vaccine induces specific humoral and cellular immunity, leading to a perfect cross-protection against A. pleuropneumoniae infections in a murine model.

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

Exotoxins play a central role in the pathologies caused by most major bacterial animal pathogens. The large variety of vertebrate and invertebrate hosts in the animal kingdom is reflected by a large variety of bacterial pathogens and toxins. The group of repeats in the structural toxin (RTX) toxins is particularly abundant among bacterial pathogens of animals. Many of these toxins are described as hemolysins due to their capacity to lyse erythrocytes in vitro. Hemolysis by RTX toxins is due to the formation of cation-selective pores in the cell membrane and serves as an important marker for virulence in bacterial diagnostics. However, their physiologic relevant targets are leukocytes expressing β2 integrins, which act as specific receptors for RTX toxins. For various RTX toxins, the binding to the CD18 moiety of β₂ integrins has been shown to be host specific, reflecting the molecular basis of the host range of RTX toxins expressed by bacterial pathogens. Due to the key role of RTX toxins in the pathogenesis of many bacteria, antibodies directed against specific RTX toxins protect against disease, hence, making RTX toxins valuable targets in vaccine research and development. Due to their specificity, several structural genes encoding for RTX toxins have proven to be essential in modern diagnostic applications in veterinary medicine.

Structure of a bacterial toxin-activating acyltransferase

Secreted pore-forming toxins of pathogenic Gram-negative bacteria such as Escherichia coli hemolysin (HlyA) insert into host-cell membranes to subvert signal transduction and induce apoptosis and cell lysis. Unusually, these toxins are synthesized in an inactive form that requires posttranslational activation in the bacterial cytosol. We have previously shown that the activation mechanism is an acylation event directed by a specialized acyl-transferase that uses acyl carrier protein (ACP) to covalently link fatty acids, via an amide bond, to specific internal lysine residues of the protoxin. We now reveal the 2.15-Å resolution X-ray structure of the 172-aa ApxC, a toxin-activating acyl-transferase (TAAT) from pathogenic Actinobacillus pleuropneumoniae. This determination shows that bacterial TAATs are a structurally homologous family that, despite indiscernible sequence similarity, form a distinct branch of the Gcn5-like N-acetyl transferase (GNAT) superfamily of enzymes that typically use acyl-CoA to modify diverse bacterial, archaeal, and eukaryotic substrates. A combination of structural analysis, small angle X-ray scattering, mutagenesis, and cross-linking defined the solution state of TAATs, with intermonomer interactions mediated by an N-terminal α-helix. Superposition of ApxC with substrate-bound GNATs, and assay of toxin activation and binding of acyl-ACP and protoxin peptide substrates by mutated ApxC variants, indicates the enzyme active site to be a deep surface groove.

Ehrlichia chaffeensis tandem repeat proteins and Ank200 are type 1 secretion system substrates related to the repeats-in-toxin exoprotein family

Ehrlichia chaffeensis has type 1 and 4 secretion systems (T1SS and T4SS), but the substrates have not been identified. Potential substrates include secreted tandem repeat protein (TRP) 47, TRP120, and TRP32, and the ankyrin repeat protein, Ank200, that are involved in molecular host–pathogen interactions including DNA binding and a network of protein–protein interactions with host targets associated with signaling, transcriptional regulation, vesicle trafficking, and apoptosis. In this study we report that E. chaffeensis TRP47, TRP32, TRP120, and Ank200 were not secreted in the Agrobacterium tumefaciens Cre recombinase reporter assay routinely used to identify T4SS substrates. In contrast, all TRPs and the Ank200 proteins were secreted by the Escherichia coli complemented with the hemolysin secretion system (T1SS), and secretion was reduced in a T1SS mutant (ΔTolC), demonstrating that these proteins are T1SS substrates. Moreover, T1SS secretion signals were identified in the C-terminal domains of the TRPs and Ank200, and a detailed bioinformatic analysis of E. chaffeensis TRPs and Ank200 revealed features consistent with those described in the repeats-in-toxins (RTX) family of exoproteins, including glycine- and aspartate-rich tandem repeats, homology with ATP-transporters, a non-cleavable C-terminal T1SS signal, acidic pIs, and functions consistent with other T1SS substrates. Using a heterologous E. coli T1SS, this investigation has identified the first Ehrlichia T1SS substrates supporting the conclusion that the T1SS and corresponding substrates are involved in molecular host–pathogen interactions that contribute to Ehrlichia pathobiology. Further investigation of the relationship between Ehrlichia TRPs, Ank200, and the RTX exoprotein family may lead to a greater understanding of the importance of T1SS substrates and specific functions of T1SS in the pathobiology of obligately intracellular bacteria.

Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 A into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.

Substrate-triggered recruitment of the TolC channel-tunnel during type I export of hemolysin by Escherichia coli

A defining event in type I export of hemolysin by Escherichia coli is the substrate-triggered recruitment of the TolC channel-tunnel by an inner membrane complex. This complex comprises a traffic ATPase (HlyB) and the 478 residue adaptor protein (HlyD), which contacts TolC during recruitment. HlyD has a large periplasmic domain (amino acid residues 81-478) linked by a single transmembrane helix to a small N-terminal cytosolic domain (1-59). Export was disabled by deletion of the ca 60 amino acid residue cytosolic domain of HlyD, even though the truncated HlyD (HlyDDelta45) was, like the wild-type, able to trimerise in the cytosolic membrane, and interact with the traffic ATPase. The mutant HlyB/HlyDDelta45 inner membrane complex engaged the hemolysin substrate, but this substrate-engaged complex failed to trigger recruitment of TolC. Further analyses showed that HlyDDelta45 was specifically unable to bind the substrate. The result suggests that substrate engagement by the traffic ATPase alone is insufficient to trigger TolC recruitment, and that substrate binding to the HlyD cytosolic domain is essential. Analysis of three further N-terminal deletion variants, HlyDDelta26, HlyDDelta26-45 and HlyDDelta34-38, indicated that an extreme N-terminal amphipathic helix and a cytosolic cluster of charged residues are central to the cytosolic domain function. The cytosolic amphipathic helix was not essential for substrate engagement or TolC recruitment, but export was impaired without it. In contrast, when the charged amino acid residues were deleted, the substrate was still engaged by HlyD but engagement was unproductive, i.e. TolC recruitment was not triggered. Our results are compatible with the HlyD cytosolic domain mediating transduction of the substrate binding signal directly, presumably to the HlyD periplasmic domain, to trigger recruitment of TolC and assemble the type I export complex.

Actinobacillus species and their role in animal disease

Actinobacillus species are Gram-negative bacteria responsible for several quite distinct disease conditions of animals. The natural habitat of the organisms is primarily the upper respiratory tract and oral cavity. A. lignieresii is the cause of actinomycosis (wooden tongue) in cattle: a sporadic, insidiously-developing granulomatous infection. In sharp contrast is A. pleuropneumoniae which is responsible for a rapidly spreading often fatal pneumonia, common among intensively reared pigs. Detailed investigation of this organism has provided a much clearer picture of the bacterial factors involved in causing disease. A. equuli similarly causes a potent septicaemia in the neonatal foal; growing apparently unrestricted once infection occurs. Other members of the genus induce characteristic pathogenesis in their preferred host, with one, A. actinomycetemcomitans, being a cause of human periodontal disease. This article reviews recent understanding of the taxonomy and bacteriology of the organisms, and the aetiology, pathogenicity, diagnosis and control of animal disease caused by Actinobacillus species.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Endobronchial inoculation with Apx toxins of Actinobacillus pleuropneumoniae leads to pleuropneumonia in pigs

To establish the role of the Apx toxins in the pathogenesis of porcine pleuropneumonia, specific-pathogen-free pigs were inoculated deeply endobronchially with either culture filtrates of Actinobacillus pleuropneumoniae serotype 8 or 9, culture filtrates depleted of the Apx toxins by affinity chromatography, depleted culture filtrate supplemented with purified recombinant Apx toxins (rApx), or purified rApx toxins alone. Results of these experiments indicate that ApxI, ApxIII, and, to a lesser extent, ApxII are the bacterial factors that trigger the development of clinical symptoms and lung lesions typical for porcine pleuropneumonia.

Incomplete activation of Escherichia coli hemolysin (HlyA) due to mutations in the 3' region of hlyC

Mutational analysis of the carboxy-terminal region of Escherichia coli HlyC was performed by site-directed mutagenesis. Replacement of residue Val-127 or Lys-129 reduced the activity of HlyC to about 30 or 60%, respectively, of that of the wild type, while replacement of Gly-128 reduced the activity to less than 1% of the wild-type level. Complete inactivation of HlyC was caused by a double mutation, replacement of Gly-128 with valine and of Lys-129 with isoleucine. Analysis of culture supernatants from mutants with reduced hemolytic activity by two-dimensional gel electrophoresis revealed the production and simultaneous secretion of nonacylated, monoacylated, and fully acylated HlyA forms, demonstrating impairment of the acylation reaction, possibly due to a decreased affinity of HlyC for the individual HlyA acylation sites.

Purification of HlyX, a potential regulator of haemolysin synthesis, and properties of HlyX : FNR hybrids

The hlyX gene of the swine pathogen Actinobacillus pleuropneumoniae is homologous to FNR, an anaerobic transcriptional regulator of Escherichia coli. It endows a haemolytic phenotype upon E. coli, and will complement the anaerobic respiratory deficiencies of fnr mutants of E. coli. The coding region of the hlyX gene was expressed in E. coli and the HlyX protein was purified by using an assay based on its immunological cross-reactivity with anti-FNR antibodies. The HlyX protein had the predicted N-terminal sequence, and resembled the isolated FNR protein in size (Mr 29,000) and monomeric organization. It has no detectable haemolysin activity per se, and is therefore presumed to confer a haemolytic phenotype by activating a latent haemolysin gene in E. coli. Studies with gene fusions showed that HlyX, like FNR, can function as an anaerobic activator and repressor of FNR-regulated genes in vivo. Plasmids that express hybrid HlyX:FNR proteins in which the 189/190-residue N-terminal segments and the remaining 50/60-residue C-terminal segments are exchanged, retained their FNR-specific functions but failed to confer a haemolytic phenotype. This suggests that the specificity for activating the haemolytic response requires the participation of unique features in both the N- and C-terminal segments of HlyX.

Characterisation of haemolytic RTX toxins produced by Australian isolates of Actinobacillus pleuropneumoniae

The haemolytic RTX toxins of 27 isolates of Actinobacillus pleuropneumoniae, representing all serovars that have been isolated in Australia, were characterised. The quantity of protein secreted by these isolates into the media was not significantly different between serovars, but haemolytic activity was detected only in the unconcentrated supernatants from cultures of serovar 1 and 5 isolates. Haemolytic activity in supernatants of serovar 2, 3 and 7 isolates was detected only after the supernatants were concentrated. On Southern hybridisation blots, genomic DNA of serovar 1 and 5 isolates contained regions that were similar to the cloned structural genes for ApxI (apxIA) and for ApxII (apxIIA). In contrast, genomic DNA of serovar 2, 3 and 7 isolates only contained regions similar to, if not identical with, the cloned apxIIA gene. The haemolytic activity of the culture supernatant depends on the type or composition of media and adaptability of the bacteria to in-vitro cultivation. Low passage cultures of A pleuropneumoniae, which were characterised by waxy colonies, produced significantly weaker haemolytic activity than A pleuropneumoniae after several passages in vitro.

Virulence in Actinobacillus pleuropneumoniae and RTX toxins

RTX toxins are pore-forming, cytolytic protein toxins that occur widely among pathogenic Gram-negative bacteria. RTX toxins appear to play a direct role in the virulence of Actinobacillus pleuropneumoniae, the etiological agent of porcine pleuropneumonia. This discovery has led to the development of new diagnostic and epidemiological tools, as well as vaccines, that are useful for a broad variety of serotypes.

The CAMP effect of Actinobacillus pleuropneumoniae is caused by Apx toxins

Actinobacillus pleuropneumoniae shows synergistic haemolysis when cocultured with Staphylococcus aureus on blood agar plates. This CAMP effect has been attributed to a discrete CAMP factor, but also to the A. pleuropneumoniae-RTX-toxins I, II, and III. We examined the CAMP effect of recombinant Escherichia coli strains that secreted each of these toxins, and of A. pleuropneumoniae mutant strains that were devoid of one or more these toxins. We found that the E. coli strains were CAMP positive, whereas the A. pleuropneumoniae strain devoid of functional toxin genes was CAMP negative. This demonstrated that the CAMP effect of A. pleuropneumoniae is caused by the toxins and that no CAMP factor per se exists.

Fatty acylation of two internal lysine residues required for the toxic activity of Escherichia coli hemolysin

Hemolysin of Escherichia coli is activated by fatty acylation of the protoxin, directed by the putative acyl transferase HlyC and by acyl carrier protein (ACP). Mass spectrometry and Edman degradation of proteolytic products from mature toxin activated in vitro with tritium-labeled acylACP revealed two fatty-acylated internal lysine residues, lysine 564 and lysine 690. Resistance of the acylation to chemical treatments suggested that fatty acid was amide linked. Substitution of the two lysines confirmed that they were the only sites of acylation and showed that although each was acylated in the absence of the other, both sites were required for in vivo toxin activity.

The RTX haemolysins ApxI and ApxII are major virulence factors of the swine pathogen Actinobacillus pleuropneumoniae: evidence from mutational analysis

The involvement of the RTX haemolysins (ApxI and ApxII) of the swine pathogen Actinobacillus pleuropneumoniae in virulence was investigated using haemolysin-deficient mutants constructed by a mini-Tn10 mutagenesis procedure. Two types of haemolysin mutant with single insertions of the transposon were obtained from a serotype 1 strain producing both ApxI and ApxII. One presented a complete loss of haemolytic activity because of the absence of ApxI and ApxII production. The other displayed weaker haemolysis than the wild type and produced only ApxII. The chromosomal regions flanking mini-Tn10 were cloned and sequenced. In the non-haemolytic mutant, the transposon had inserted in apxIB, a gene involved in the exportation of ApxI and ApxII toxins. The weakly haemolytic mutant resulted from the disruption of the structural gene for ApxI. Both mutations in the apxI operon were associated with a significant loss of virulence for mice and pigs, demonstrating that haemolysins are involved in A. pleuropneumoniae pathogenicity. The non-haemolytic mutant was apathogenic and the weakly haemolytic mutant retained some virulence for pigs, suggesting that both ApxI and ApxII are needed for full virulence.

Sequence analysis and transcription of the apxI operon (hemolysin I) from Actinobacillus pleuropneumoniae

The DNA sequence of the entire apxI operon from Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 has been determined. This 8292-bp fragment of the chromosomal DNA contains four open reading frames (ORFs) of the strongly hemolytic ApxI toxin. These ORFs correspond to the genes apxIC, apxIA, apxIB and apxID, encoding the activator, the structural toxin protein and the two secretion proteins, respectively. Each of the four ORFs is preceded by a consensus sequence for a putative ribosome-binding site (RBS). The region upstream from apxIC contains several sites that could act as promoters. The transcription start point (tsp) of the apxI operon in A. pleuropneumoniae has been determined by primer extension analysis and was found to be located 133-bp upstream from the translation start codon. The tsp is preceded by sequences matching the -10 and -35 consensus sequence of promoters from Escherichia coli. This is the first promoter identified in A. pleuropneumoniae. The same tsp was used when the expression of apxI was induced by a high concentration of free Ca2+ in the growth medium, as well as when the expression of apxI was not induced by growing the cells in medium depleted of free Ca2+ ions. However, the signal strength of the primer extension was approximately tenfold stronger in Ca(2+)-grown cells. The leader sequence of the transcript is unusually long and very A+U rich (75% A+U).

Unusual routes of protein secretion: the easy way out

Increasing numbers of polypeptides are being discovered that lack a cleavable hydrophobic signal sequence and are released from cells without passing through the classical secretory pathway. This article reviews the current knowledge of these alternative secretion pathways in prokaryotes and eukaryotes and discusses whether the mechanisms described in bacteria and yeast can be used as paradigms to explain unusual secretory phenomena in animal cells.

ABC transporters: bacterial exporters

The ABC transporters (also called traffic ATPases) make up a large superfamily of proteins which share a common function and a common ATP-binding domain. ABC transporters are classified into three major groups: bacterial importers (the periplasmic permeases), eukaryotic transporters, and bacterial exporters. We present a comprehensive review of the bacterial ABC exporter group, which currently includes over 40 systems. The bacterial ABC exporter systems are functionally subdivided on the basis of the type of substrate that each translocates. We describe three main groups: protein exporters, peptide exporters, and systems that transport nonprotein substrates. Prototype exporters from each group are described in detail to illustrate our current understanding of this protein family. The prototype systems include the alpha-hemolysin, colicin V, and capsular polysaccharide exporters from Escherichia coli, the protease exporter from Erwinia chrysanthemi, and the glucan exporters from Agrobacterium tumefaciens and Rhizobium meliloti. Phylogenetic analysis of the ATP-binding domains from 29 bacterial ABC exporters indicates that the bacterial ABC exporters can be divided into two primary branches. One branch contains the transport systems where the ATP-binding domain and the membrane-spanning domain are present on the same polypeptide, and the other branch contains the systems where these domains are found on separate polypeptides. Differences in substrate specificity do not correlate with evolutionary relatedness. A complete survey of the known and putative bacterial ABC exporters is included at the end of the review.

Identification of two components of the Serratia marcescens metalloprotease transporter: protease SM secretion in Escherichia coli is TolC dependent

The Serratia marcescens metalloprotease (protease SM) belongs to a family of proteins secreted from gram-negative bacteria by a signal peptide-independent pathway which requires a specific transporter consisting of three proteins: two in the inner membrane and one in the outer membrane. The prtDSM and prtESM genes encoding the two S. marcescens inner membrane components were cloned and expressed in Escherichia coli. Their nucleotide sequence revealed high overall homology with the two analogous inner membrane components of the Erwinia chrysanthemi protease secretion apparatus and lower, but still significant, homology with the two analogous inner membrane components of the E. coli hemolysin transporter. When expressed in E. coli, these two proteins, PrtDSM and PrtESM, allowed the secretion of protease SM only in the presence of TolC protein, the outer membrane component of the hemolysin transporter.

Cloning and characterization of btr, a Bordetella pertussis gene encoding an FNR-like transcriptional regulator

To determine whether hemolytic factors other than the bifunctional hemolysin-adenylate cyclase toxin (cyclolysin) are expressed by Bordetella pertussis, a gene library was constructed from a virulent strain of B. pertussis, BP504, transformed into nonhemolytic Escherichia coli, and screened on blood agar plates. A strongly hemolytic colony which contained the plasmid pHLY1A was isolated. Nucleotide sequencing of pHLY1A revealed an open reading frame that could encode a 27-kDa protein. No similarity was detected between the deduced amino acid sequence of this open reading frame and those of any known bacterial cytolysins. However, significant homology was detected with FNR of E. coli and several other transcriptional regulators including HylX from Actinobacillus pleuropneumoniae, which can also confer a hemolytic phenotype on E. coli. An fnr mutant of E. coli, JRG1728, could be complemented by pHLY1A. Thus, the B. pertussis transcriptional regulator-like gene and the protein which it encoded were named btr and BTR, respectively. A BTR-deficient B. pertussis strain, BJB1, was constructed. The btr::kan mutation had no effect on the expression of hemolytic activity or on phase variation. Northern (RNA) blotting revealed that btr expression was not regulated by the BvgAS two-component sensor-regulator. On the basis of sequence similarity to FNR-like transcriptional regulators and the ability to complement an anaerobically deficient E. coli strain (JRG1728) in growing anaerobically, BTR may regulate B. pertussis gene expression in response to changes in oxygen levels or to changes in the redox potential of the bacterial environment. Its role in virulence remains to be determined.

Transposon mutagenesis in Actinobacillus pleuropneumoniae with a Tn10 derivative

A transposon mutagenesis procedure functional in the gram-negative swine pathogen Actinobacillus pleuropneumoniae was developed for the first time. The technique involved the use of a suicide conjugative plasmid, pLOF/Km, carrying a mini-Tn10 with an isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible transposase located outside the mobile element (M. Herrero, V. de Lorenzo, and K. N. Timmis, J. Bacteriol. 172:6557-6567, 1990). The plasmid was mobilized from Escherichia coli to A. pleuropneumoniae through the RP4-mediated broad-host-range conjugal transfer functions provided by the chromosome of the donor strain. When IPTG was present in the mating medium, A. pleuropneumoniae CM5 transposon mutants were obtained at a frequency of 10(-5), while no mutants were detected in the absence of IPTG. Since the frequency of conjugal transfer of the RP4 plasmid from E. coli to A. pleuropneumoniae CM5 was found to be as low as 10(-4), the above result indicated that the expression level of the transposase was a critical factor for obtaining a workable efficiency of transposon mutagenesis. The transposon insertions occurred at random, as determined by Southern blotting of chromosomal DNA of randomly selected mutants and by the ability to generate mutants defective for the selected phenotypes. Almost all the mutants analyzed resulted from a single insertion of the Tn10 element. About 1.2% of the mutants resulted from the cointegration of pLOF/Km into the A. pleuropneumoniae chromosome. The applicability of this transposon mutagenesis system was verified on other A. pleuropneumoniae strains of different serotypes. The usefulness of this transposon mutagenesis system in genetic studies of A. pleuropneumoniae is discussed.

The RTX cytotoxin-related FrpA protein of Neisseria meningitidis is secreted extracellularly by meningococci and by HlyBD+ Escherichia coli

Neisseria meningitidis produces proteins (FrpA and FrpC) related to the RTX cytotoxin family. In meningococcal strain FAM20 these proteins were both localized in the outer membrane and secreted into the extracellular medium. An Escherichia coli strain with wild-type hemolysin secretion genes hlyB and hlyD and containing a cloned frpA gene secreted FrpA, whereas an isogenic hlyBD mutant strain did not. Low-stringency DNA hybridization revealed hlyBD-like sequences in N. meningitidis FAM20, suggesting that a similar RTX secretion system exists in meningococci. Structural features found at the C termini of other RTX proteins and thought to be important for their secretion were also found at the C terminus of FrpA. The secretion of FrpA from E. coli by heterologous RTX transport proteins further demonstrates the relation of the FrpA protein to RTX toxins.

Cloning and characterization of the Actinobacillus pleuropneumoniae-RTX-toxin III (ApxIII) gene

To study the role of Actinobacillus pleuropneumoniae-RTX-toxin III (ApxIII) in the pathogenesis of porcine pleuropneumonia, we cloned and characterized the gene encoding this toxin. For that purpose, we screened an expression library of genomic DNA of serotype 8 with an ApxIII-specific monoclonal antibody and isolated a 425-bp fragment of an immunoreactive clone. Using this fragment as a probe, we identified and cloned an overlapping chromosomal NsiI restriction fragment of 5.0 kbp. Escherichia coli cells that contained this fragment produced a protein similar to ApxIII. Like ApxIII, the protein had a molecular mass of approximately 120 kDa, was recognized by an ApxIII-specific antibody, killed porcine lung macrophages, and was not lytic for sheep erythrocytes. We concluded from these data that the 5.0-kbp NsiI fragment contained the ApxIII-coding gene. Nucleotide sequence analysis of the 5.0-kbp NsiI fragment revealed the presence of two genes, apxIIIC and apxIIIA. These genes coded for proteins ApxIIIC and ApxIIIA, respectively, which were 53 and 50% identical to the prototypic RTX proteins HlyC and HlyA of E. coli. We assumed that the apxIIIA gene coded for the structural RTX toxin and that the apxIIIC gene coded for its activator. In addition, we found that ApxIII could be secreted from E. coli by the heterologous RTX transporter proteins HlyB and HlyD. The deduced amino acid sequence of ApxIIIA was 50% identical to that of ApxIA and 41% identical to that of ApxIIA. We concluded that, beside ApxI and ApxII, ApxIII is the third RTX toxin produced by A. pleuropneumoniae.

Functional properties of pro region of Escherichia coli heat-stable enterotoxin

Escherichia coli heat-stable enterotoxin Ip (STp) is synthesized as the 72-amino-acid residue precursor consisting of three regions: pre region (amino acid residues 1 to 19), pro region (amino acid residues 20 to 54), and mature ST (mST) region (amino acid residues 55 to 72). We examined the role of the pro sequence of STp in enterotoxigenicity of a strain by deleting the gene fragment encoding amino acids 22 to 57. This deletion caused a remarkable reduction of its enterotoxic activity of culture supernatant. In order to analyze the sequence responsible for the function of the pro region, two additional deletion mutants were made. The deletion of the sequence covering amino acids 29 to 38, which is conserved in all sequences of ST reported, brought about a significant reduction of enterotoxic activity but the deletion of the non-conserved sequence (amino acids 40 to 53) did not. This result shows that conserved sequence is mainly responsible for the function. Subsequently, to examine the mechanism of action of the pro region, plasmids carrying DNA sequences of hybrid proteins consisting of pre-pro-nuclease, pre-mST-nuclease, pre-pro-mST-nuclease and pre-pro-nuclease-mST were constructed. Amino acid sequence determination and SDS-polyacrylamide gel analysis revealed that these fusion proteins were cleaved between pre sequence and pro sequence during secretion and the cleaved fusion proteins were accumulated in periplasmic space. But the amount of hybrid protein accumulated in the periplasmic space varied among the strains. That is, the amount of the pre-pro-nuclease gene product that accumulated in the periplasmic space was the highest of all fusion gene products. These results indicate that the existence of the mST region strongly interferes with the translocation of the gene product into the periplasmic space and that the pro region functions to guide the mST region into the periplasmic space.

Analysis of hemolysin operons in Actinobacillus pleuropneumoniae

Among the twelve different serotypes of Actinobacillus pleuropneumoniae, the causative agent of swine pleuropneumonia, a strongly active hemolysin I (HlyI) is produced by serotypes which are particularly virulent, and less active hemolysin II (HlyII) is produced by all serotypes except type 10. In the serotypes 1, 5a, 5b, 9, 10 and 11, which produce HlyI, the hemolysin (hly) operon consists of a structural hlyIA gene, encoding pre-HlyI, an activator gene, hlyIC, necessary for the activation of pre-Hly to active Hly, and two genes, hlyIB and hlyID, involved in Hly secretion. These genes are clustered in the order, hlyICABD. This is characteristic to RTX toxin (repeats in the structural toxin) operons. The HlyII operons in all serotypes producing HlyII consist only of the pre-HlyII-encoding gene, appA, and its activator gene, appC. The serotypes, which produce HlyII, but not HlyI, contain a truncated HlyI operon, with the promoter, hlyIB and hlyID, and a small segment of the C terminus of hlyIA. This partial HlyI operon might have been formed by deletion of hlyIC and most of hlyIA. In serotype 3, which produces HlyII, but no HlyI, and which releases only minute amounts of this Hly into the growth medium, none of the hlyI genes and consequently no Hly secretion genes were found. The above results postulate that HlyII is secreted via the products of hlyIB and hlyID, and explain the low amount of HlyII secreted by serotype 3. Cloning and analysis of the structural genes encoding pre-HlyI and pre-HlyII among the different serotypes revealed differences in the hlyIA genes which are highly similar in the serologically related serotypes 1, 9 and 11, and differ from the serotypes, 5a, 5b and 10. The hlyIIA genes, in contrast, seem to be conserved in all serotypes.

An analysis of the codon usage of Pasteurella haemolytica A1

Analysis of approximately 17 kbp of nucleotide sequences from three different regions of the genome of Pasteurella haemolytica A1 showed that the mol% G+C of P. haemolytica A1 DNA is 38.5%. When only the coding sequences (approx. 10 kbp) were analysed, a similar value of 38.8% was obtained. A comparison of the relative synonymous codon usage values of the cloned genes showed that P. haemolytica A1 has a very different codon usage pattern from that of Escherichia coli.

Sequence of a cluster of genes controlling synthesis and secretion of alkaline protease in Pseudomonas aeruginosa: relationships to other secretory pathways

A genetic locus implicated in the synthesis and secretion of alkaline protease (APR) in Pseudomonas aeruginosa has been previously described [Guzzo et al., J. Bacteriol. 172 (1990) 942-948]. The nucleotide sequence of the DNA fragment encoding these functions was determined and revealed the existence of five open reading frames: aprA, the structural gene encoding APR; aprI, which encodes a protease inhibitor; and aprD, aprE, aprF whose products are involved in protease secretion. The AprD, AprE and AprF proteins share significant homology with proteins implicated in secretion of Erwinia chrysanthemi proteases and Escherichia coli alpha-haemolysin. These results provide further evidence for the existence of a specialized secretory system widespread among Gram- bacteria.

Influence of Actinobacillus pleuropneumoniae serotype 2 and its cytolysins on porcine neutrophil chemiluminescence

The effects of Actinobacillus pleuropneumoniae serotype 2 and its metabolites on the oxidative activity of porcine neutrophils were studied by using a chemiluminescence technique. Viable A. pleuropneumoniae stimulated the production of oxygen radicals by neutrophils. After having reached a peak value, the oxidative activity decreased until a total inhibition of the oxidative activity of the neutrophils was achieved. All effects were neutralized with homologous convalescent-phase pig sera which had been adsorbed by heat-inactivated A. pleuropneumoniae. Inactivated bacteria and bacteria in the presence of chloramphenicol each had no influence on the oxidative activity of neutrophils. In contrast, a heat-labile factor in A. pleuropneumoniae culture supernatants stimulated and inhibited the oxidative activity of the neutrophils in a dose-dependent manner. Undiluted and low dilutions of culture supernatants were toxic for the phagocytes, while high dilutions stimulated the oxygen radical production of the neutrophils. These effects were neutralized with homologous convalescent-phase pig sera. In order to investigate whether the heat-labile factors in the culture supernatant could be cytolysins, we repeated the experiments with cytolysin II and cytolysin III produced by recombinant Escherichia coli. It was demonstrated that stimulation and inhibition could be reproduced by both cytolysins. In conclusion, the observations made in this study showed that A. pleuropneumoniae secretes heat-labile metabolites that stimulate neutrophil-oxidative metabolism at relatively low concentrations and kill the neutrophils at higher concentrations. Cytolysins may be responsible, at least in part, for these effects.

Activation of Escherichia coli prohemolysin to the membrane-targetted toxin by HlyC-directed ACP-dependent fatty acylation

Hemolysin (HlyA) and related toxins of Escherichia coli and other Gram-negative pathogenic bacteria form membrane pores in cells of the host immune system, causing cell dysfunction and death. An insight into the mechanism by which HlyA is targetted to mammalian cell membranes was achieved by establishing in vitro activation of the non-toxic precursor proHlyA. By this approach we have discovered that conversion of proHlyA to the post-translational active HlyA toxin is determined by fatty acylation of proHlyA in an apparently novel process directed by the HlyC homodimer activator protein, and dependent upon the cellular acyl carrier protein (ACP). By further exploiting the in vitro activation system it is now possible to obtain direct evidence that HlyC binds to an internal recognition sequence in the proHlyA precursor, in this way providing specificity for the transfer to proHlyA of a fatty acid moiety carried by the ACP. It is possible that the fatty acid modification determines directly the binding of HlyA to mammalian membrane lipids, thus initiating the toxin interaction with the target cells.

Functional analysis of the Ca(2+)-regulated hemolysin I operon of Actinobacillus pleuropneumoniae serotype 1

The genetic determinant encoding the synthesis and secretion of hemolysin I (HlyI; gene designation, hlyI) by Actinobacillus pleuropneumoniae serotype 1 4074T was cloned in the lambda vector EMBL4. A 10.2-kb fragment that encoded hemolytic activity in the phage lysate was aligned by Southern blot hybridization to genes hlyC, hlyA, hlyB, and hlyD of the Escherichia coli hemolysin operon, and expression of the A. pleuropneumoniae genes in E. coli revealed that they have the same functions as their E. coli analogs: hlyIC encodes a protein that activates inactive 105-kDa prohemolysin I (encoded by hlyIA) to active hemolysin I, while hlyIB and hlyID are necessary for HlyIA secretion. Northern (RNA) hybridization of A. pleuropneumoniae RNA revealed that the gene cluster is transcribed as two RNA species, a major one of 3.5 kb, corresponding to hlyICA, and a second, minor one of 7.5 kb, corresponding to the whole operon, hlyICABD. The level of hlyI mRNA was substantially higher in A. pleuropneumoniae 4074T cells grown in the presence of Ca2+, supporting the view that the expression of the hlyI determinant is Ca2+ regulated. Parallel RNA hybridization with random gene probes suggested that this Ca2+ regulation is specific for the hlyI determinant.

E.coli hemolysin interactions with prokaryotic and eukaryotic cell membranes

The hemolysin toxin (HlyA) is secreted across both the cytoplasmic and outer membranes of pathogenic Escherichia coli and forms membrane pores in cells of the host immune system, causing cell dysfunction and death. The processes underlying the interaction of HlyA with the bacterial and mammalian cell membranes are remarkable. Secretion of HlyA occurs without a periplasmic intermediate and is directed by an uncleaved C-terminal targetting signal and the HlyB and HlyD translocator proteins, the former being a member of a transporter superfamily central to import and export of a wide range of substrates by prokaryotic and eukaryotic cells. The separate process by which HlyA is targetted to mammalian cell membranes is dependent upon fatty acylation of a non-toxic precursor, proHlyA. This is achieved by a novel mechanism directed by the activator protein HlyC, which binds to an internal proHlyA recognition sequence and provides specificity for the transfer of fatty acid from cellular acyl carrier protein.

Molecular cloning and expression of ptxA, the gene encoding the 120-kilodalton cytotoxin of Actinobacillus pleuropneumoniae serotype 2

The genetic determinants of the 120-kDa cytotoxin of Actinobacillus pleuropneumoniae serotype 2 were isolated from a lambda DNA library by a plaque immunoblot technique. Expression of the 120-kDa polypeptide was confirmed by Western immunoblot analysis of infected Escherichia coli cell lysates, which were shown to be toxic for porcine alveolar macrophages in vitro. The genetic determinants of the toxin were subcloned into the plasmid vector pUC18. This plasmid (pPTX1) directed the synthesis and secretion of the active 120-kDa cytotoxin in E. coli. The recombinant toxin was indistinguishable from native cytotoxin from A. pleuropneumoniae serotype 2 with respect to molecular size, reaction in Western blot analysis, heat lability, cytotoxic activity, and neutralization by serum antibody. A restriction endonuclease cleavage map of pPTX1 was prepared, and deletion mutants were used to locate the minimal region of DNA required for production of intracellular toxin; this gene was termed ptxA. Southern hybridization analysis with a 1.7-kb PvuII fragment located within the ptxA gene revealed sequences with a high degree of homology in serotype reference strains 2, 3, 4, 6, and 8. Other reference strains did not contain sequences that were recognized by this probe. However, related sequences (greater than 71% homology) were detected in Actinobacillus actinomycetemcomitans and A. equuli. Weak hybridization was observed between the ptxA probe and pLKT5, which carries the lktAC genes of Pasteurella haemolytica, and between the ptxA probe and pAPH1, which carries the structural gene for type II hemolysin from A. pleuropneumoniae. The isolation of the genetic determinants of this cytotoxin will enable investigations of the structure and organization of the ptx DNA region and further analysis of its role in the pathogenesis of pleuropneumonia.

Association of the RTX proteins of Actinobacillus pleuropneumoniae with hemolytic, CAMP, and neutrophil-cytotoxic activities

The immunoglobulin G from a monospecific rabbit antiserum to the 110-kDa RTX hemolysin of Actinobacillus pleuropneumoniae serotype 1 was used to determine that the related RTX proteins in isolates from serotypes 2 to 12 were also responsible for the hemolytic, CAMP, and neutrophil-cytotoxic activities produced by this bacterium. These proteins share common neutralizing epitopes.

Identification of a second hemolysin (HlyII) in Actinobacillus pleuropneumoniae serotype 1 and expression of the gene in Escherichia coli

Hemolysin genes of the reference strains of Actinobacillus pleuropneumoniae serotypes 1 and 2 were identified, cloned, and expressed in Escherichia coli by using polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of the corresponding appA gene from A. pleuropneumoniae serotype 5. The three genes from serotypes 1, 2, and 5 have identical restriction maps and appear to encode a hemolysin which was previously identified in serotype 2 and designated HlyII. Gene appA is different from hlyIA encoding the major hemolysin type I (HlyI) which was identified earlier in serotype 1. Polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of hlyIA of serotype 1 showed that the gene encoding HlyI is present in serotype 1 but not in serotype 2, in contrast to the gene encoding HlyII that was present in both serotypes. This was confirmed by Western blot (immunoblot) experiments using monoclonal antibodies specific for either recombinant HlyI or recombinant HlyII, which showed that A. pleuropneumoniae serotype 1 strain 4074 produces both HlyI and HlyII, whereas serotype 2 strain S1536 produces only HlyII. The expression of both hemolysins was investigated in all serotypes by the use of monoclonal antibodies. HlyI was shown to be expressed by the reference strains of serotypes 1, 5a, 5b, 9, 10, and 11, whereas HlyII was shown to be expressed by the reference strains of all 12 serotypes tested except serotype 10. A. pleuropneumoniae serotype 1 strain 4074 is the first bacterium which has been shown to contain two different actively expressed RTX toxin genes. Comparison of our data with those from other groups shows that the originally described strongly hemolytic hemolysin type I (HlyI) corresponds to cytolysin I (ClyI) which was recently described by others, while the weakly hemolytic hemolysin type II (HlyII) seems to be identical to ClyII and AppA.

Escherichia coli HlyT protein, a transcriptional activator of haemolysin synthesis and secretion, is encoded by the rfaH (sfrB) locus required for expression of sex factor and lipopolysaccharide genes

Synthesis and secretion of the 110kDa haemolysin toxin of Escherichia coli and other pathogenic Gram-negative bacteria are governed by the four genes of the hly operon. We have identified, by transposon mutagenesis, an E. coli cellular locus, hlyT, required for the synthesis and secretion of haemolysin encoded in trans by intact hly operons carrying the hly upstream regulatory region. Mutation of the hlyT locus specifically reduced the level of hlyA structural gene transcript 20-100-fold and thus markedly lowered both intracellular and extracellular levels of the HlyA protein. Genetic and structural analysis of the hlyT locus mapped it at co-ordinate 3680 kbp (minute 87) on the chromosome adjacent to the fadBA operon, and identified it specifically as the rfaH (sfrB) locus which is required for transcription of the genes encoding synthesis of the sex pilus and also the lipopolysaccharide core for attachment of the O-antigen of E. coli and Salmonella. Expression of the hly operon in the E. coli hlyT mutant was restored in trans by both the hlyT and rfaH genes, suggesting that the rfaH gene is an important activator of regulon structures that are central to the fertility and virulence of these pathogenic bacteria. DNA sequencing of the hlyT locus identifies the HlyT/RfaH transcriptional activator as a protein of 162 amino acids (Mr 18325) which shows no identity to characterized transcription factors.

Structural and functional relationships among the RTX toxin determinants of gram-negative bacteria

The RTX (repeats in toxin) cytolytic toxins represent a family of important virulence factors that have disseminated widely among Gram-negative bacteria. They are characterised by a series of glycine-rich repeat units at the C-terminal end of each protein. They also have other features in common. Secretion from the cell occurs without a periplasmic intermediate by a novel mechanism which involves recognition of a signal sequence at the C-terminus of the toxin by membrane-associated proteins that export the toxin directly to the outside of the cell. The structural gene for each protein encodes an inactive toxin which is modified post-translationally to an active cytotoxic form by another gene product before secretion. The genes for toxin synthesis, activation and secretion are for the most part grouped together on the chromosome and form an operon. The toxins all create pores in the cell membrane of target cells leading to eventual cell lysis and they appear to require Ca2+ for cytotoxic activity. Although the toxins have a similar mode of action, they vary in target cell specificity. Some are cytotoxic for a wide variety of eukaryotic cell types while others exhibit precise target cell specificity and are only active against leukocytes from certain host species. The characteristic glycine-rich repeat units have been identified in other exoproteins besides those with cytotoxic activity and it is likely that the novel secretory mechanism has been harnessed by a variety of pathogens to release important virulence-associated factors from the cell or to locate them on the cell surface.

Comparison of the cytolysin II genetic determinants of Actinobacillus pleuropneumoniae serotypes

Cytolysins (Cly) I, II, and III are toxins secreted by Actinobacillus pleuropneumoniae. These toxins are thought to play an important role in the pathogenesis of porcine pleuropneumonia. ClyI and ClyII are RTX toxins and in general these toxins are encoded by operons consisting of four genes, C, A, B, and D. Our group recently cloned the C and A genes of the ClyII operon (clyIICA) of serotype 9. We found that this ClyII operon is truncated and lacked intact B and D genes (clyIIBD). B and D genes of the ClyI operon (clyIBD) were present however in serotype 9. In this study we analyzed the ClyII operons of the reference strains of the 12 A. pleuropneumoniae serotypes and compared them with the ClyII operon of serotype 9. We focused on (i) the presence, (ii) the sequence similarity, and (iii) the genomic environment of the clyIICA genes. The presence of the clyIICA sequences was studied by hybridization analysis of genomic DNA. The sequence similarity was studied by restriction fragment analysis on polymerase chain reaction-amplified DNA. The genomic environment was compared by analysis of the sequences that are located 3' of the clyIICA genes. We demonstrated that the clyIICA genes (i) are present in the reference strains of all serotypes, except serotype 10, (ii) have a high degree of sequence similarity, and (iii) are not contiguous with intact clyIIBD genes. We conclude that the organization and nucleotide sequence of the ClyII operons of A. pleuropneumoniae are very similar. We also studied the presence of clyIBD sequences and found them to be present in the reference strains of all serotypes, except serotypes 3 and 6. Thus, in most serotypes the clyIBD genes may complement the absent clyIIBD genes.

Secretion of the Rhizobium leguminosarum nodulation protein NodO by haemolysin-type systems

The Rhizobium leguminosarum biovar viciae nodulation protein NodO is partially homologous to haemolysin of Escherichia coli and, like haemolysin, is secreted into the growth medium. The NodO protein can be secreted by a strain of E. coli carrying the cloned nodO gene plus the haemolysin secretion genes hlyBD, in a process that also requires the outer membrane protein encoded by tolC. The related protease secretion genes, prtDEF, from Erwinia chrysanthemi also enable E. coli to secrete NodO. The Rhizobium genes encoding the proteins required for NodO secretion are unlinked to nodO and are unlike other nod genes, since they do not require flavonoids or NodO for their expression. Although proteins similar to NodO were not found in rhizobia other than R. leguminosarum bv. viciae, several rhizobia and an Agrobacterium strain containing the cloned nodO gene were found to have the ability to secrete NodO. These observations indicate that a wide range of the Rhizobiaceae have a protein secretion mechanism analogous to that which secretes haemolysin and related toxins and proteases in the ENterobacteriaceae.