Conservation of xcp genes, involved in the two-step protein secretion process, in different Pseudomonas species and other gram-negative bacteria

The dual GGDEF/EAL domain enzyme PA0285 is a Pseudomonas species housekeeping phosphodiesterase regulating early attachment and biofilm architecture

Bacterial lifestyles depend on conditions encountered during colonization. The transition between planktonic and biofilm growth is dependent on the intracellular second messenger c-di-GMP. High c-di-GMP levels driven by diguanylate cyclases (DGCs) activity favor biofilm formation, while low levels were maintained by phosphodiesterases (PDE) encourage planktonic lifestyle. The activity of these enzymes can be modulated by stimuli-sensing domains such as Per-ARNT-Sim (PAS). In Pseudomonas aeruginosa, more than 40 PDE/DGC are involved in c-di-GMP homeostasis, including 16 dual proteins possessing both canonical DGC and PDE motifs, that is, GGDEF and EAL, respectively. It was reported that deletion of the EAL/GGDEF dual enzyme PA0285, one of five c-di-GMP-related enzymes conserved across all Pseudomonas species, impacts biofilms. PA0285 is anchored in the membrane and carries two PAS domains. Here, we confirm that its role is conserved in various P. aeruginosa strains and in Pseudomonas putida. Deletion of PA0285 impacts the early stage of colonization, and RNA-seq analysis suggests that expression of cupA fimbrial genes is involved. We demonstrate that the C-terminal portion of PA0285 encompassing the GGDEF and EAL domains binds GTP and c-di-GMP, respectively, but only exhibits PDE activity in vitro. However, both GGDEF and EAL domains are important for PA0285 PDE activity in vivo. Complementation of the PA0285 mutant strain with a copy of the gene encoding the C-terminal GGDEF/EAL portion in trans was not as effective as complementation with the full-length gene. This suggests the N-terminal transmembrane and PAS domains influence the PDE activity in vivo, through modulating the protein conformation.

Bacterial protein secretion systems: Game of types

Protein trafficking across the bacterial envelope is a process that contributes to the organisation and integrity of the cell. It is the foundation for establishing contact and exchange between the environment and the cytosol. It helps cells to communicate with one another, whether they establish symbiotic or competitive behaviours. It is instrumental for pathogenesis and for bacteria to subvert the host immune response. Understanding the formation of envelope conduits and the manifold strategies employed for moving macromolecules across these channels is a fascinating playground. The diversity of the nanomachines involved in this process logically resulted in an attempt to classify them, which is where the protein secretion system types emerged. As our knowledge grew, so did the number of types, and their rightful nomenclature started to be questioned. While this may seem a semantic or philosophical issue, it also reflects scientific rigour when it comes to assimilating findings into textbooks and science history. Here I give an overview on bacterial protein secretion systems, their history, their nomenclature and why it can be misleading for newcomers in the field. Note that I do not try to suggest a new nomenclature. Instead, I explore the reasons why naming could have escaped our control and I try to reiterate basic concepts that underlie protein trafficking cross membranes.

Pseudomonas aeruginosa LasB Subverts Alveolar Macrophage Activity by Interfering With Bacterial Killing Through Downregulation of Innate Immune Defense, Reactive Oxygen Species Generation, and Complement Activation

Pseudomonas aeruginosa (P.a) is a pathogen causing significant morbidity and mortality, in particular, in hospital patients undergoing ventilation and in patients with cystic fibrosis. Among the virulence factors secreted or injected into host cells, the physiopathological relevance of type II secretions system (T2SS) is less studied. Although there is extensive literature on the destructive role of LasB in vitro on secreted innate immune components and on some stromal cell receptors, studies on its direct action on myeloid cells are scant. Using a variety of methods, including the use of bacterial mutants, gene-targeted mice, and proteomics technology, we show here, using non-opsonic conditions (thus mimicking resting and naïve conditions in the alveolar space), that LasB, an important component of the P.a T2SS is highly virulent in vivo, and can subvert alveolar macrophage (AM) activity and bacterial killing, in vitro and in vivo by downregulating important secreted innate immune molecules (complement factors, cytokines, etc.) and receptors (IFNAR, Csf1r, etc.). In particular, we show that LasB downregulates the production of C3 and factor B complement molecules, as well as the activation of reactive oxygen species production by AM. In addition, we showed that purified LasB impaired significantly the ability of AM to clear an unrelated bacterium, namely Streptococcus pneumoniae. These data provide a new mechanism of action for LasB, potentially partly explaining the early onset of P.a, alone, or with other bacteria, within the alveolar lumen in susceptible individuals, such as ventilated, chronic obstructive pulmonary disease and cystic fibrosis patients.

Unearthing the genomes of plant-beneficial Pseudomonas model strains WCS358, WCS374 and WCS417

Background

Plant growth-promoting rhizobacteria (PGPR) can protect plants against pathogenic microbes through a diversity of mechanisms including competition for nutrients, production of antibiotics, and stimulation of the host immune system, a phenomenon called induced systemic resistance (ISR). In the past 30 years, the Pseudomonas spp. PGPR strains WCS358, WCS374 and WCS417 of the Willie Commelin Scholten (WCS) collection have been studied in detail in pioneering papers on the molecular basis of PGPR-mediated ISR and mechanisms of biological control of soil-borne pathogens via siderophore-mediated competition for iron.

Results

The genomes of the model WCS PGPR strains were sequenced and analyzed to unearth genetic cues related to biological questions that surfaced during the past 30 years of functional studies on these plant-beneficial microbes. Whole genome comparisons revealed important novel insights into iron acquisition strategies with consequences for both bacterial ecology and plant protection, specifics of bacterial determinants involved in plant-PGPR recognition, and diversity of protein secretion systems involved in microbe-microbe and microbe-plant communication. Furthermore, multi-locus sequence alignment and whole genome comparison revealed the taxonomic position of the WCS model strains within the Pseudomonas genus. Despite the enormous diversity of Pseudomonas spp. in soils, several plant-associated Pseudomonas spp. strains that have been isolated from different hosts at different geographic regions appear to be nearly isogenic to WCS358, WCS374, or WCS417. Interestingly, all these WCS look-a-likes have been selected because of their plant protective or plant growth-promoting properties.

Conclusions

The genome sequences of the model WCS strains revealed that they can be considered representatives of universally-present plant-beneficial Pseudomonas spp. With their well-characterized functions in the promotion of plant growth and health, the fully sequenced genomes of the WCS strains provide a genetic framework that allows for detailed analysis of the biological mechanisms of the plant-beneficial traits of these PGPR. Considering the increasing focus on the role of the root microbiome in plant health, functional genomics of the WCS strains will enhance our understanding of the diversity of functions of the root microbiome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1632-z) contains supplementary material, which is available to authorized users.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Type II-dependent secretion of a Pseudomonas aeruginosa DING protein

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that uses a wide range of protein secretion systems to interact with its host. Genes encoding the PAO1 Hxc type II secretion system are linked to genes encoding phosphatases (LapA/LapB). Microarray genotyping suggested that Pseudomonas aeruginosa clinical isolates, including urinary tract (JJ692) and blood (X13273) isolates, lacked the lapA/lapB genes. Instead, we show that they carry a gene encoding a protein of the PstS family. This protein, which we call LapC, also has significant similarities with LapA/LapB. LapC belongs to the family of DING proteins and displays the canonical DINGGG motif within its N terminus. DING proteins are members of a prokaryotic phosphate binding protein superfamily. We show that LapC is secreted in an Hxc-dependent manner and is under the control of the PhoB response regulator. The genetic organization hxc-lapC found in JJ692 and X13273 is similar to PA14, which is the most frequent P. aeruginosa genotype. While the role of LapA, LapB and LapC proteins remains unclear in P. aeruginosa pathogenesis, they are likely to be part of a phosphate scavenging or sensing system needed to survive and thrive when low phosphate environments are encountered within the host.

EcpA, an extracellular protease, is a specific virulence factor required by Xanthomonas oryzae pv. oryzicola but not by X. oryzae pv. oryzae in rice

Previously, 12 protease-deficient mutants of the Xanthomonas oryzae pv. oryzicola (Xoc) RS105 strain were recovered from a Tn5-tagged mutant library. In the current study, the Tn5 insertion site in each mutant was mapped. Mutations in genes encoding components of the type II secretion apparatus, cAMP regulatory protein, integral membrane protease subunit, S-adenosylmethionine decarboxylase proenzyme and extracellular protease (ecpA(Xoc)) either partially or completely abolished extracellular protease activity (ECPA) and reduced virulence in rice. Transcription of ecpA(Xoc) was induced in planta in all the mutants except RΔecpA. Complementation of RΔecpA with ecpA(Xoc) in trans restored ECPA, virulence and bacterial growth in planta. Purified EcpA(Xoc) induced chlorosis- and necrosis-like symptoms similar to those induced by the pathogen when injected into rice leaves. Heterologous expression of ecpA(Xoc) conferred ECPA upon the vascular bacterium X. oryzae pv. oryzae (Xoo) and upon non-pathogenic Escherichia coli. Genetic analysis demonstrated that the C-terminal residues of EcpA in Xoo PXO99(A) and Xoc RS105 are different, and a frame shift in ecpA(Xoo) may explain the absence of EcpA activity in Xoo. Collectively, these results suggest that EcpA(Xoc) is a tissue-specific virulence factor for Xoc but not Xoo, although the two pathovars are closely related bacterial pathogens of rice.

Protein Secretion Systems in Pseudomonas aeruginosa: An Essay on Diversity, Evolution, and Function

Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment. They are used to release enzymes that hydrolyze complex carbon sources into usable compounds, or to release proteins that capture essential ions such as iron. They are also used to colonize and survive within eukaryotic hosts, causing acute or chronic infections, subverting the host cell response and escaping the immune system. In this article, the opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals. This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The striking similarities between secretion systems and type IV pili, flagella, bacteriophage tail, or efflux pumps is a nice illustration of this evolution. Differences are also needed since various secretion configurations call for diversity. For example, some proteins are released in the extracellular medium while others are directly injected into the cytosol of eukaryotic cells. Some proteins are folded before being released and transit into the periplasm. Other proteins cross the whole cell envelope at once in an unfolded state. However, the secretion system requires conserved basic elements or features. For example, there is a need for an energy source or for an outer membrane channel. The structure of this review is thus quite unconventional. Instead of listing secretion types one after each other, it presents a melting pot of concepts indicating that secretion types are in constant evolution and use basic principles. In other words, emergence of new secretion systems could be predicted the way Mendeleïev had anticipated characteristics of yet unknown elements.

Interaction domains in the Pseudomonas aeruginosa type II secretory apparatus component XcpS (GspF)

Pseudomonas aeruginosa is an opportunistic pathogen, which secretes a wide variety of enzymes and toxins into the extracellular medium. Most exoproteins are exported by the type II secretion machinery, the Xcp system, which encompasses 12 different proteins. One of the core components of the Xcp system is the inner-membrane protein XcpS (GspF), homologues of which can be identified in type II secretion machineries as well as in type IV piliation systems. In this study, XcpS was shown to be stabilized by co-expression of the XcpR (GspE) and XcpY (GspL) components of the machinery, demonstrating an interaction between these three proteins. By replacing segments of P. aeruginosa XcpS with the corresponding parts of its Pseudomonas putida counterpart, XcpS domains were identified that are important for species-specific functioning and thus represent putative interaction domains. The cytoplasmic loop of XcpS was found to be involved in the stabilization by XcpR and XcpY.

Type II secretion: from structure to function

Gram-negative bacteria use the type II secretion system to transport a large number of secreted proteins from the periplasmic space into the extracellular environment. Many of the secreted proteins are major virulence factors in plants and animals. The components of the type II secretion system are located in both the inner and outer membranes where they assemble into a multi-protein, cell-envelope spanning, complex. This review discusses recent progress, particularly newly published structures obtained by X-ray crystallography and electron microscopy that have increased our understanding of how the type II secretion apparatus functions and the role that individual proteins play in this complex system.

Role of XcpP in the functionality of the Pseudomonas aeruginosa secreton

In gram-negative bacteria, most signal-peptide-dependent exoproteins are secreted via the type II secretion system (T2SS or secreton). In Pseudomonas aeruginosa, T2SS consists of twelve Xcp proteins (XcpA and XcpP to XcpZ) thought to be organized as a multiproteic complex within the envelope. Although well conserved, T2SS are known to be species-specific, namely for distant organisms, and this characteristic was thought to involve XcpP. To check which domain of XcpP could be involved in the species specificity, hybrid proteins were generated using protein domain swapping between P. aeruginosa XcpP and homolog proteins of either Erwinia chrysanthemi or Pseudomonas alcaligenes. The results obtained with hybrid proteins constructed by exchanging the C-terminal domains of P. aeruginosa and E. chrysanthemi suggested that XcpP interacts with XcpQ, probably via its C-terminal domain. More interestingly, the data obtained with a hybrid protein containing the C-terminal part of the P. alcaligenes XcpP homolog, showed that the wild-type C-terminal end plays a very important role in the function of the protein and is required both for a correct interaction with XcpQ and for modulating the opening of the secreton channel.

Identification of XcpP domains that confer functionality and specificity to the Pseudomonas aeruginosa type II secretion apparatus

Gram-negative bacteria have evolved several types of secretion mechanisms to release proteins into the extracellular medium. One such mechanism, the type II secretory system, is a widely conserved two-step process. The first step is the translocation of signal peptide-bearing exoproteins across the inner membrane. The second step, the translocation across the outer membrane, involves the type II secretory apparatus or secreton. The secretons are made up of 12-15 proteins (Gsp) depending on the organism. Even though the systems are conserved, heterologous secretion is mostly species restricted. Moreover, components of the secreton are not systematically exchangeable, especially with distantly related microorganisms. In closely related species, two components, the GspC and GspD (secretin) family members, confer specificity for substrate recognition and/or secreton assembly. We used Pseudomonas aeruginosa as a model organism to determine which domains of XcpP (GspC member) are involved in specificity. By constructing hybrids between XcpP and OutC, the Erwinia chrysanthemi homologue, we identified a region of 35 residues that was not exchangeable. We showed that this region might influence the stability of the XcpYZ secreton subcomplex. Remarkably, XcpP and OutC have domains, coiled-coil and PDZ, respectively, which exhibit the same function but that are structurally different. Those two domains are exchangeable and we provided evidence that they are involved in the formation of homomultimeric complexes of XcpP.

Identification of XcpZ domains required for assembly of the secreton of Pseudomonas aeruginosa

Most of the exoproteins secreted by Pseudomonas aeruginosa are transported via the type II secretion system. This machinery, which is widely conserved in gram-negative bacteria, consists of 12 Xcp proteins organized as a multiprotein complex, also called the secreton. We previously reported that the mutual stabilization of XcpZ and XcpY plays an important role in the assembly of the secreton. In this study, we engineered variant XcpZ proteins by using linker insertion mutagenesis. We identified three distinct regions of XcpZ required for both the stabilization of XcpY and the functionality of the secreton. Interestingly, we also demonstrated that another component of the machinery, XcpP, can modulate the stabilizing activity of XcpZ on XcpY.

Type II protein secretion in gram-negative pathogenic bacteria: the study of the structure/secretion relationships of the cellulase Cel5 (formerly EGZ) from Erwinia chrysanthemi

Erwinia chrysanthemi, a Gram-negative plant pathogen, secretes the cellulase Cel5 (formerly EGZ) via the type II secretion pathway (referred to as Out). Cel5 is composed of two domains, a large N-terminal catalytic domain (390 amino acid residues) and a small C-terminal cellulose-binding domain (62 amino acid residues) separated by a linker region. A combination of mutagenesis and structural analysis permitted us to investigate the structure/secretion relationships with respect to the catalytic domain of Cel5. The 3D structure of the catalytic domain was solved by molecular replacement at 2.3 A resolution. Cel5 exhibits the (beta/alpha)8 structural fold and two extra-barrel features. Our previous genetic study based upon tRNA-mediated suppression allowed us to predict positions of importance in the molecule in relation to structure and catalysis. Remarkably, all of the predictions proved to be correct when compared with the present structural information. Mutations of Arg57, which is located at the heart of the catalytic domain, allowed us to test the consequences of structural modifications on the secretion efficiency. The results revealed that secretability imposes remarkably strong constraints upon folding. In particular, an Arg-to-His mutation yielded a species that folded to a stable conformation close to, but distinct from the wild-type, which however was not secretable. We discuss the relationships between folding of a protein in the periplasm, en route to the cell exterior, and presentation of secretion information. We propose that different solutions have been selected for type II secreted exoproteins in order to meet the constraints imposed by their interaction with their respective secretion machineries. We propose that evolutionary pressure has led to the adaptation of different secretion motifs for different type II exoproteins.

Maturation of Pseudomonas aeruginosa Elastase

Elastase of Pseudomonas aeruginosa is synthesized as a preproenzyme. After propeptide-mediated folding in the periplasm, the proenzyme is autoproteolytically processed, prior to translocation of both the mature enzyme and the propeptide across the outer membrane. The formation of the two disulfide bonds present in the mature enzyme was examined by studying the expression of the wild-type enzyme and of alanine for cysteine mutant derivatives in the authentic host and in dsb mutants of Escherichia coli. It appeared that the two disulfide bonds are formed successively. First, DsbA catalyzes the formation of the disulfide bond between Cys-270 and Cys-297 within the proenzyme. This step is essential for the subsequent autoproteolytic processing to occur. The second disulfide bond between Cys-30 and Cys-57 is formed more slowly and appears to be formed after processing of the proenzyme, and its formation is catalyzed by DsbA as well. This second disulfide bond appeared to be required for the full proteolytic activity of the enzyme and contributes to its stability.

Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosa and Pseudomonas alcaligenes: species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacteria that secrete proteins using the type II or general secretory pathway, which requires at least 12 xcp gene products (XcpA and XcpP to -Z). Despite strong conservation of this secretion pathway, gram-negative bacteria usually cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs determine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collmer, Mol. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could not be correlated to species-specific recognition of exoproteins, since these bacteria could secrete exoproteins of each other. Moreover, when P. alcaligenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clearly visible. Together, our results indicate that the assembly of a functional Xcp machinery requires species-specific interactions between XcpP and XcpQ and between XcpP or XcpQ and another, as yet uncharacterized component(s).

Expression of the endogenous type II secretion pathway in Escherichia coli leads to chitinase secretion

Escherichia coli K-12, the most widely used laboratory bacterium, does not secrete proteins into the extracellular medium under standard growth conditions, despite possessing chromosomal genes encoding a putative type II secretion machinery (secreton). We show that in wild-type E.coli K-12, divergent transcription of the two operons in the main chromosomal gsp locus, encoding the majority of the secreton components, is silenced by the nucleoid-structuring protein H-NS. In mutants lacking H-NS, the secreton genes cloned on a moderate-copy-number plasmid are expressed and promote efficient secretion of the endogenous, co-regulated endochitinase ChiA. This is the first time that secretion of an endogenous extracellular protein has been demonstrated in E.coli K-12.

Activation of Pseudomonas aeruginosa elastase in Pseudomonas putida by triggering dissociation of the propeptide-enzyme complex

The propeptide of Pseudomonas aeruginosa elastase functions both as an intramolecular chaperone required for the folding of the enzyme and as an inhibitor that prevents activity of the enzyme before its secretion into the extracellular medium. Since expression of the lasB gene, which encodes elastase, in Pseudomonas putida did not result in extracellular elastase activity, it has been suggested that the enzyme is not recognized by the Xcp secretion machinery of the heterologous host. Here, it is demonstrated that the proenzyme is normally processed in P. putida and that it is indeed not actively secreted by the Xcp machinery. Nevertheless, substantial amounts of the enzyme were detected in the extracellular medium. Co-immunoprecipitations revealed that the extracellular enzyme was associated with the propeptide, which explains the lack of enzymic activity. Since the propeptide-enzyme complex in P. putida apparently does not dissociate spontaneously, it is concluded that a host-specific factor is required to induce this event. Mutants were selected which showed extracellular elastase activity. Two mutations, located within the lasB gene, were further characterized. These mutations, resulting in the substitution of Ala and Thr at positions -15 and -153, respectively, of the propeptide (where position +1 is defined as the first residue of the mature enzyme) destabilized the propeptide-enzyme complex. It is concluded that Ala-15 and Thr-153 are required for the inhibitor function, but not for the chaperone function of the propeptide.

Alteration of the lipopolysaccharide structure affects the functioning of the Xcp secretory system in Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes a wide range of hydrolytic enzymes into the external medium by the Xcp secretion machinery. To better understand the role played by envelope constituents in the functioning of this type II secretory system, we have studied the influence of lipopolysaccharide (LPS) on the secretion of two extracellular enzymes, the elastase LasB and the lipase LipA. Strains with defective LPS decreased production of LasB and altered the secretion processes of both LasB and LipA without any apparent effect on the composition of the Xcp machinery. The PAO1algC strain, defective in the outer core of LPS, was leaky, as shown by the extracellular release of the periplasmic beta-lactamase. Generation of an xcpR mutation in this mutant led only to a partial accumulation of LasB within the cells, indicating that in strain PAO1algC with a functional xcpR gene, LasB was released in the extracellular medium partly by leakage and partly by secretion. The pool of LasB released into the medium by leakage was not recovered in an active form, while extracellular LasB was active when secreted via the secretory machinery. Further analysis revealed that the presence of a functional Xcp machinery is strictly required for the activation process of LasB. Our results provide evidence that the Xcp system is not fully functional when the LPS structure of P. aeruginosa is altered.

Two regions of EpsL involved in species-specific protein-protein interactions with EpsE and EpsM of the general secretion pathway in Vibrio cholerae

Extracellular secretion of proteins via the type II or general secretion pathway in gram-negative bacteria requires the assistance of at least 12 gene products that are thought to form a complex apparatus through which secreted proteins are translocated. Although this apparatus is specifically required only for the outer membrane translocation step during transport across the bacterial cell envelope, it is believed to span both membranes. The EpsE, EpsL, and EpsM proteins of the type II apparatus in Vibrio cholerae are thought to form a trimolecular complex that is required to either control the opening and closing of the secretion pore or to transduce energy to the site of outer membrane translocation. EpsL is likely to play an important role in this relay by interacting with both the cytoplasmic EpsE protein and the cytoplasmic membrane protein EpsM, which is predominantly exposed on the periplasmic side of the membrane. We have now extended this model and mapped the separate regions within EpsL that contain the EpsE and EpsM binding domains. By taking advantage of the species specificity of the type II pathway, we have used chimeric proteins composed of EpsL and its homologue, ExeL, from Aeromonas hydrophila together with either EpsE or its Aeromonas homologue, ExeE, to complement the secretion defect in both epsL and exeL mutant strains. These studies have mapped the species-specific EpsE binding site to the N-terminal cytoplasmic region between residues 57 and 216 of EpsL. In addition, the species-specific EpsM binding site was mapped to the C-terminal half of EpsL by coimmunoprecipitation of EpsM with different EpsL-ExeL chimeras. This site is present in the region between amino acids 216 and 296, which contains the predicted membrane-spanning segment of EpsL.

Molecular organization of the xcp gene cluster in Pseudomonas putida: absence of an xcpX (gspK) homologue

A DNA fragment containing xcp (gsp) gene homologues, required for extracellular protein secretion by the general secretory pathway (GSP) in various Gram-negative bacteria, was cloned from Pseudomonas putida (Pp) strain WCS358 and sequenced. The results presented here and those previously reported (de Groot, A., Krijger, J.-J., Filloux, A., Tommassen, J., 1996. Characterization of type II protein secretion (xcp) genes in the plant growth-stimulating Pseudomonas putida, strain WCS358 Mol. Gen. Genet. 250, 491-504) complete the sequence of the xcp gene cluster of Pp. Unlike that of Pseudomonas aeruginosa (Pa), the xcp gene cluster of Pp contains a gspN homologue. More surprisingly, in contrast to all known gsp gene clusters, the xcpX (gspK) homologue is not found. In addition, genes flanking the xcp cluster of Pp are not related to those flanking the xcp genes of Pa. Overall, the xcp gene products of Pp are as much related to those of Pa as to gsp gene products of enterobacterial species, suggesting that the xcp clusters of Pp and Pa have evolved separately.

Mutual stabilization of the XcpZ and XcpY components of the secretory apparatus in Pseudomonas aeruginosa

Protein secretion in gram-negative bacteria is often dependent on the general secretory pathway (GSP). In Pseudomonas aeruginosa, this system requires at least 12 Xcp (Gsp) proteins, which are proposed to constitute a multiprotein complex localized in the bacterial envelope. Hitherto, little was known about the mutual interactions between Xcp proteins. In this study, mutants affected in the xcpZ gene encoding a bitopic inner-membrane protein were analysed to investigate the role of this protein in the architecture of the secretory machinery. The absence of XcpZ resulted in a decreased amount of XcpY. Reciprocally, XcpZ was not detectable in a xcpY mutant, demonstrating a mutual stabilization of these two proteins. These results strongly suggest that XcpZ and XcpY interact within the functional secretory apparatus.

The cytochrome c maturation operon is involved in manganese oxidation in Pseudomonas putida GB-1

A Pseudomonas putida strain, strain GB-1, oxidizes Mn2+ to Mn oxide in the early stationary growth phase. It also secretes a siderophore (identified as pyoverdine) when it is subjected to iron limitation. After transposon (Tn5) mutagenesis several classes of mutants with differences in Mn2+ oxidation and/or secretion of the Mn2+-oxidizing activity were identified. Preliminary analysis of the Tn5 insertion site in one of the nonoxidizing mutants suggested that a multicopper oxidase-related enzyme is involved in Mn2+ oxidation. The insertion site in another mutant was preliminarily identified as a gene involved in the general protein secretion pathway. Two mutants defective in Mn2+-oxidizing activity also secreted porphyrins into the medium and appeared to be derepressed for pyoverdine production. These strains were chosen for detailed analysis. Both mutants were shown to contain Tn5 insertions in the ccmF gene, which is part of the cytochrome c maturation operon. They were cytochrome oxidase negative and did not contain c-type cytochromes. Complementation with part of the ccm operon isolated from the wild type restored the phenotype of the parent strain. These results indicate that a functional ccm operon is required for Mn2+ oxidation in P. putida GB-1. A possible relationship between porphyrin secretion resulting from the ccm mutation and stimulation of pyoverdine production is discussed.

GSP-dependent protein secretion in gram-negative bacteria: the Xcp system of Pseudomonas aeruginosa

Bacteria have evolved several secretory pathways to release proteins into the extracellular medium. In Gram-negative bacteria, the exoproteins cross a cell envelope composed of two successive hydrophobic barriers, the cytoplasmic and outer membranes. In some cases, the protein is translocated in a single step across the cell envelope, directly from the cytoplasm to the extracellular medium. In other cases, outer membrane translocation involves an extension of the signal peptide-dependent pathway for translocation across the cytoplasmic membrane via the Sec machinery. By analogy with the so-called general export pathway (GEP), this latter route, including two separate steps across the inner and the outer membrane, was designated as the general secretory pathway (GSP) and is widely conserved among Gram-negative bacteria. In their great majority, exoproteins use the main terminal branch (MTB) of the GSP, namely the Xcp machinery in Pseudomonas aeruginosa, to reach the extracellular medium. In this review, we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requirement and pore formation for driving through the outer membrane.

Identification of an additional member of the secretin superfamily of proteins in Pseudomonas aeruginosa that is able to function in type II protein secretion

Pseudomonas aeruginosa is a prolific exporter of virulence factors and contains three of the four protein secretion systems that have been described in gram-negative bacteria. The P. aeruginosa type II general secretory pathway (GSP) is used to export the largest number of proteins from this organism, including lipase, phospholipase C, alkaline phosphatase, exotoxin A, elastase and LasA. Although these exoproteins contain no sequence similarity, they are specifically and efficiently transported by the secretion apparatus. Bacterial homologues of XcpQ (GspD), the only outer membrane component of this system, have been proposed to play the role of gatekeeper, by presumably interacting and recognizing the exported substrates to allow their passage through the outer membrane. While determining the phenotype of nonpolar deletions in each of the xcp genes, we have shown that a deletion of the P. aeruginosa strain K xcpQ does not completely abolish protein secretion. As the proposed function of XcpQ should be requisite for secretion, we searched for additional factors that could carry out this role. A cosmid DNA library from a PAK strain deleted for xcpP-Z was tested for its ability to increase protein secretion by screening for enhanced growth on lipid agar, a medium that selects for the secretion of lipase. In this manner, we have identified an XcpQ homologue, XqhA, that is solely responsible for the residual export observed in a deltaxcpQ strain, although it is not required for efficient secretion in wild-type P. aeruginosa. We have also demonstrated that this protein is capable of recognizing all of the exoproteins of P. aeruginosa, arguing against the proposed role of members of the secretin family as determinants of specificity.

The secretion apparatus of Pseudomonas aeruginosa: identification of a fifth pseudopilin, XcpX (GspK family)

The xcp gene products in Pseudomonas aeruginosa are required for the secretion of proteins across the outer membrane. Four of the Xcp proteins, XcpT, U, V and W, present sequence homology to the subunits of type IV pili at their N-termini, and they were therefore designated pseudopilins. In this study, we characterized the xcpX gene product, a bitopic cytoplasmic membrane protein. Remarkably, amino acid sequence comparisons also suggested that the XcpX protein resembles the pilins and pseudopilins at the N-terminus. We show that XcpX could be processed by the prepilin peptidase, PilD/XcpA, and that the highly conserved glycine residue preceding the hydrophobic segment could not be mutated without loss of the XcpX function. We, therefore, classified XcpX (GspK) as the fifth pseudopilin of the system.

Lack of functional complementation between Bordetella pertussis filamentous hemagglutinin and Proteus mirabilis HpmA hemolysin secretion machineries

The gram-negative bacterium Bordetella pertussis has adapted specific secretion machineries for each of its major secretory proteins. In particular, the highly efficient secretion of filamentous hemagglutinin (FHA) is mediated by the accessory protein FhaC. FhaC belongs to a family of outer membrane proteins which are involved in the secretion of large adhesins or in the activation and secretion of Ca2+-independent hemolysins by several gram-negative bacteria. FHA shares with these hemolysins a 115-residue-long amino-proximal region essential for its secretion. To compare the secretory pathways of these hemolysins and FHA, we attempted functional transcomplementation between FhaC and the Proteus mirabilis hemolysin accessory protein HpmB. HpmB could not promote the secretion of FHA derivatives. Likewise, FhaC proved to be unable to mediate secretion and activation of HpmA, the cognate secretory partner of HpmB. In contrast, ShlB, the accessory protein of the closely related Serratia marcescens hemolysin, was able to activate and secrete HpmA. Two invariant asparagine residues lying in the region of homology shared by secretory proteins and shown to be essential for the secretion and activation of the hemolysins were replaced in FHA by site-directed mutagenesis. Replacements of these residues indicated that both are involved in, but only the first one is crucial to, FHA secretion. This slight discrepancy together with the lack of functional complementation demonstrates major differences between the hemolysins and FHA secretion machineries.

Secretion of recombinant proteins by Gram-negative bacteria

During the past few years, significant progress has been made towards our understanding of the molecular mechanisms governing the translocation of proteins through bacterial cell membranes. Successful attempts in promoting the secretion of recombinant proteins by employing this knowledge and by empirical efforts have been registered. However, a further in-depth understanding of membrane-translocation mechanisms is required before predictable manipulations of secretion systems can be made to secrete native recombinant proteins that are not naturally targeted to the extracellular compartment.

Membrane topology of three Xcp proteins involved in exoprotein transport by Pseudomonas aeruginosa

Xcp proteins constitute the secretory apparatus of Pseudomonas aeruginosa. Deduced amino acid sequence of xcp genes, expression, and subcellular localization revealed unexpected features. Indeed, most Xcp proteins are found in the cytoplasmic membrane although xcp mutations lead to periplasmic accumulation of exoproteins, indicating that the limiting step is translocation across the outer membrane. To understand the mechanism by which the machinery functions and the interactions between its components, it is valuable to know their membrane organization. We report data demonstrating the N(in)-C(out) topologies of three general secretion pathway components, the XcpP, -Y, and -Z proteins.

Complementation of deletion mutations in a cloned functional cluster of Erwinia chrysanthemi out genes with Erwinia carotovora out homologues reveals OutC and OutD as candidate gatekeepers of species-specific secretion of proteins via the type II pathway

The type II or Sec-dependent secretion system is used by diverse Gram-negative bacteria for secretion of extracellular proteins. Of the 12-15 proteins involved in secretion, the requirement for many has not been demonstrated and little is known about their functions in the secretion process. The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete extra-cellular pectate lyases (Pels) using the type II or Out pathway. However, these two bacteria cannot secrete Pels encoded by heterologously expressed genes from the other species, suggesting the presence of species-specific recognition factors in the Out systems of the two Erwinia species. We previously reported the isolation of a cosmid clone, pCPP2OO6, from E. chrysanthemi EC16, which enables Escherichia coil to secrete heterologously expressed E. chrysanthemi Pels. Sequencing in a region required for secretion revealed the presence of 12 genes, outC-M and outO. We report here the construction of functionally non-polar mutations in each gene in the outC-M operon and outS and outB using a polA(ts) strain of E. coli to facilitate homologous recombination between out genes carrying deletions and their wild-type copies on pCPP2006. By testing for complementation of each deletion with wild-type out genes from E. chrysanthemi EC16 and E. carotovora SCRI193 we have demonstrated that: (i) each out gene is required for secretion of E. chrysanthemi PelE from E. coli with the exception of outH; (ii) each mutation can be complemented by its homologue from E. carotovora, except for outC and outD; (iii) outC and outD from E. carotovora do not confer secretion of Pel1 on the E. chrysanthemi Out system; and (iv) Pel1 secretion can be conferred on the E. chrysanthemi Out system by the presence of outC-M, S and B from E. carotovora. The data suggest that OutC and OutD are gatekeepers of the Out system involved in recognition of Pels targeted for secretion but that OutC and OutD from E. carotovora cannot be successfully assembled into the E. chrysanthemi Out system.

Characterization of type II protein secretion (xcp) genes in the plant growth-stimulating Pseudomonas putida, strain WCS358

In Pseudomonas aeruginosa, the products of the xcp genes are required for the secretion of exoproteins across the outer membrane. Despite structural conservation of the Xcp components, secretion of exoproteins via the Xcp pathway is generally not found in heterologous organisms. To study the specificity of this protein secretion pathway, the xcp genes of another fluorescent pseudomonad, the plant growth-promoting Pseudomonas putida strain WCS358, were cloned and characterized. Nucleotide sequence analysis revealed the presence of at least five genes, i.e., xcpP, Q, R, S, and T, with homology to xcp genes of P. aeruginosa. Unlike the genetic organization in P. aeruginosa, where the xcp cluster consists of two divergently transcribed operons, the xcp genes in P. putida are all oriented in the same direction, and probably comprise a single operon. Upstream of xcpP in P. putida, an additional open reading frame, with no homolog in P. aeruginosa, was identified, which possibly encodes a lipoprotein. Mutational inactivation of xcp genes in P. putida did not affect secretion, indicating that no proteins are secreted via the Xcp system under the growth conditions tested, and that an alternative secretion system is operative. To obtain some insight into the secretory pathway involved, the amino acid sequence of the N-terminus of the major extracellular protein was determined. The protein could be identified as flagellin. Mutations in the xcpQ and R genes of P. aeruginosa could not be complemented by introduction of the corresponding xcp genes of P. putida. However, expression of a hybrid XcpR protein, composed of the N-terminal one-third of P. aeruginosa XcpR and the C-terminal two-thirds of P. putida XcpR, did restore protein secretion in a P. aeruginosa xcpR mutant.

Bacteriocin release proteins: mode of action, structure, and biotechnological application

The mechanism by which Gram-negative bacteria like Escherichia coli secrete bacteriocins into the culture medium is unique and quite different from the mechanism by which other proteins are translocated across the two bacterial membranes, namely through the known branches of the general secretory pathway. The release of bacteriocins requires the expression and activity of a so-called bacteriocin release protein and the presence of the detergent-resistant phospholipase A in the outer membrane. The bacteriocin release proteins are highly expressed small lipoproteins which are synthesized with a signal peptide that remains stable and which accumulates in the cytoplasmic membrane after cleavage. The combined action of these stable, accumulated signal peptides, the lipid-modified mature bacteriocin release proteins (BRPs) and phospholipase A cause the release of bacteriocins. The structure and mode of action of these BRPs as well as their application in the release of heterologous proteins by E. coli is described in this review.

Specificity of the protein secretory apparatus: secretion of the heat-labile enterotoxin B subunit pentamers by different species of gram- bacteria

The B-subunit pentamer(s) (EtxBp) of Escherichia coli heat-labile enterotoxin (LT) are secreted from Vibrio cholerae via the general secretion pathway (GSP), but remain periplasmic in E. coli. In order to determine if other Gram- bacteria were also able to secrete the ExtBp, the etxB gene, which encodes EtxB was introduced into different bacteria. Of the bacteria examined, most species of Vibrio and Aeromonas were able to secrete this protein through the outer membrane; other Gram- genera, including Erwinia, Klebsiella and Xanthomonas were not, even though they encode GSP genes homologous to those of V. cholerae. Thus, the ability to recognize the EtxBp as a secretable protein is confined to bacteria that were identified as being closely related to V. cholerae by examination of their 5S rRNA [MacDonell and Colwell, Syst. Appl. Microbiol. 6 (1985) 171-182].

A plasmid-encoded prepilin peptidase gene from enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli, a leading agent of infantile diarrhea worldwide, adheres to tissue culture cells in a pattern called "localized adherence." Localized adherence is associated with bundle-forming pili encoded by the plasmid bfpA gene, the product of which is homologous with the major structural subunit proteins of type IV fimbriae in other bacteria. Several of these proteins have been shown to be processed from a precursor by a specific prepilin peptidase. We cloned restriction fragments downstream of the bfpA gene into an E. coli-Pseudomonas aeruginosa shuttle vector and mobilized them into a P. aeruginosa prepilin peptidase (pilD) mutant. A plasmid containing a 1.3-kb PstI-BamHI fragment was able to complement the pilD mutation, as demonstrated by restoration of sensitivity to the pilus-specific bacteriophage PO4. The DNA sequence of this fragment revealed an open reading frame, designated bfpP, the predicted product of which is homologous to other prepilin peptidases, including TcpJ of Vibrio cholerae (30% identical amino acids), PulO of Klebsiella oxytoca (29%), and PilD of P. aeruginosa (28%). A bfpA::TnphoA mutant complemented with a bfpA-containing DNA fragment only partially processes the BfpA protein. When complemented with a larger fragment containing bfpP as well as bfpA, the mutant expresses the fully processed BfpA protein. P. aeruginosa PAK, but not a pilD mutant of PAK, expresses mature BfpA protein when the bfpA gene is mobilized into this strain. Thus, as in other type IV fimbria systems, enteropathogenic E. coli utilizes a specific prepilin peptidase to process the major subunit of the bundle-forming pilus. This prepilin petidase contains sequence and reciprocal functional homologies with the PilD protein of P. aeruginosa.

A superfamily of proteins involved in different secretion pathways in gram-negative bacteria: modular structure and specificity of the N-terminal domain

The family of PulD proteins, which has been characterized in a wide variety of microorganisms, comprises several membrane-associated proteins essential for the transport of macromolecules across bacterial membranes. These proteins are involved in the transport of complex structures (such as phage particles, DNA) or various proteins (such as extracellular enzymes and pathogenicity determinants). Amino acid sequence analysis revealed a possible modular organisation of proteins of this superfamily, with highly conserved C-terminal domains and dissimilar N-terminal domains. In the C-terminal domain, four highly conserved regions have been found, one of them containing a remarkable common motif: (V, I)PXL(S, G)XIPXXGXLF. Structural comparisons between the N-terminal domains indicate that proteins of this superfamily can be divided into at least two subgroups, probably reflecting the existence of distinct secretion mechanisms. This implies that members of the superfamily of PulD-related proteins are independently involved in (1) the general secretory pathway, (2) a new signal-peptide-independent secretion pathway found in several bacterial pathogens, and possibly in (3) the translocation of bacteriophage particles through the bacterial cell envelope.

Characterization of type IV pilus genes in plant growth-promoting Pseudomonas putida WCS358

In a search for factors that could contribute to the ability of the plant growth-stimulating Pseudomonas putida WCS358 to colonize plant roots, the organism was analyzed for the presence of genes required for pilus biosynthesis. The pilD gene of Pseudomonas aeruginosa, which has also been designated xcpA, is involved in protein secretion and in the biogenesis of type IV pili. It encodes a peptidase that processes the precursors of the pilin subunits and of several components of the secretion apparatus. Prepilin processing activity could be demonstrated in P. putida WCS358, suggesting that this nonpathogenic strain may contain type IV pili as well. A DNA fragment containing the pilD (xcpA) gene of P. putida was cloned and found to complement a pilD (xcpA) mutation in P. aeruginosa. Nucleotide sequencing revealed, next to the pilD (xcpA) gene, the presence of two additional genes, pilA and pilC, that are highly homologous to genes involved in the biogenesis of type IV pili. The pilA gene encodes the pilin subunit, and pilC is an accessory gene, required for the assembly of the subunits into pili. In comparison with the pil gene cluster in P. aeruginosa, a gene homologous to pilB is lacking in the P. putida gene cluster. Pili were not detected on the cell surface of P. putida itself, not even when pilA was expressed from the tac promoter on a plasmid, indicating that not all the genes required for pilus biogenesis were expressed under the conditions tested. Expression of pilA of P. putida in P. aeruginosa resulted in the production of pili containing P. putida PilA subunits.

Pseudomonas lipases: molecular genetics and potential industrial applications

Lipases are esterases able to hydrolyze water-insoluble esters such as long-chain triglycerides. These enzymes also catalyze the formation of esters (esterification) and the exchange of ester bonds (transesterification) when present in nonaqueous media. Lipases display a high degree of specificity and enantioselectivity for esterification and transesterification reactions, and thus their potential uses in industry are very wide. These potential industrial applications have been an important driving force for lipase research during the last several years, and in particular for the study of lipases produced by microorganisms. Pseudomonas lipases are very interesting because they display special biochemical characteristics not common among the lipases produced by other microorganisms, such as their thermoresistance and activity at alkaline pHs. Recently, several Pseudomonas genes have been cloned and sequenced, and the regulation of their expression is beginning to be understood. The molecular genetic approach to the study of Pseudomonas lipases will permit the construction of recombinant strains with increased lipase productivity and will provide the opportunity to modify these enzymes to suit particular industrial applications.

Xcp-mediated protein secretion in Pseudomonas aeruginosa: identification of two additional genes and evidence for regulation of xcp gene expression

In Pseudomonas aeruginosa, several exoproteins synthesized with a signal sequence (elastase, lipase, phospholipases, alkaline phosphatase and exotoxin A) are secreted by a two-step mechanism. They first cross the inner membrane in a signal sequence-dependent way, and are further translocated across the outer membrane in a second step requiring secretion functions encoded by several xcp genes. Ten xcp genes have already been characterized (Bally et al., 1992a). In this study, two additional xcp genes, xcpP and xcpQ, are described. They are located in the 40 min region of the chromosome where they probably define an operon, divergent from the xcpR-Z operon previously characterized in this region. These two genes encode two proteins, XcpP and XcpQ, similar to PulC and PulD of the pul system of Klebsiella oxytoca. Moreover, the two divergent operons share a common regulation which is growth-phase dependent.

Common components in the assembly of type 4 fimbriae, DNA transfer systems, filamentous phage and protein-secretion apparatus: a general system for the formation of surface-associated protein complexes

The Pseudomonas aeruginosa genes pilB-D and pilQ are necessary for the assembly of type 4 fimbriae. Homologues of these genes and of the subunit (pilin) gene have been described in various different bacterial species, but not always in association with type 4 fimbrial biosynthesis and function. Pil-like proteins are also involved in protein secretion, DNA transfer by conjugation and transformation, and morphogensis of filamentous bacteriophages. It seems likely that the Pil homologues function in the processing and export of proteins resembling type 4 fimbrial subunits, and in their organization into fimbrial-like structures. These may either be true type 4 fimbriae, or components of protein complexes which act in the transport of macromolecules (DNA or protein) into or out of the cell. Some PilB-like and PilQ-like proteins are apparently also involved in the assembly of non-type 4 polymeric structures (filamentous phage virions and conjugative pili). The diverse studies summarized in this review are providing insight into an extensive infrastructural system which appears to be utilized in the formation of a variety of cell surface-associated complexes.

Role of the lipB gene product in the folding of the secreted lipase of Pseudomonas glumae

The LipB protein of Pseudomonas glumae is essential for the production of active extracellular lipase encoded by the lipA gene. When lipase is overproduced in P. glumae in the absence of a functional lipB gene, the enzyme accumulates intracellularly in an inactive conformation. Heterologous expression of the lipase in Pseudomonas aeruginosa, Bacillus subtilis and Escherichia coli indicated that LipB is not directly involved in the translocation of the lipase across the inner or outer membrane. However, the presence of LipB was essential for obtaining active lipase and had a profound influence on the stability of the protein to proteolytic degradation. Inactive lipase, produced in the absence of LipB could be activated in vitro by unfolding and refolding, which demonstrates that LipB activity is not responsible for an essential covalent modification of the enzyme. We propose that LipB is a lipase-specific foldase. Furthermore, proper folding of the lipase in the periplasm appears to be essential for Xcp-mediated translocation across the outer membrane.

Conservation of genes encoding components of a type IV pilus assembly/two-step protein export pathway in Neisseria gonorrhoeae

Three gonococcal genes have been identified which encode proteins with substantial similarities to known components of the type IV pilus biogenesis pathway in Pseudomonas aeruginosa. Two of the genes were identified based on their hybridization with a DNA probe derived from the pilB gene of P. aeruginosa under conditions of reduced stringency. The product of the gonococcal pilF gene is most closely related to the pilus assembly protein PilB of P. aeruginosa while the product of the gonococcal pilT gene is most similar to the PilT protein of P. aeruginosa which is involved in pilus-associated twitching motility and colony morphology. The products of both of these genes display canonical nucleoside triphosphate binding sites and are predicted to be to cytoplasmically localized based on their overall hydrophilicity. The gonococcal pilD gene, identified by virtue of its linkage to the pilF gene, is homologous to a family of prepilin leader peptidase genes. When expressed in Escherichia coli, the gonococcal PilD protein functions to process gonococcal prepilin in a manner consistent with its being gonococcal prepilin peptidase. These results suggest that Neisseria gonorrhoeae is capable of expressing many of the essential elements of a highly conserved protein translocation system and that these gene products are probably involved in pilus biogenesis.

Membrane traffic wardens and protein secretion in gram-negative bacteria

Recent progress in the genetic analysis of protein secretion in diverse Gram-negative bacteria has revealed three major, highly conserved but functionally independent pathways that involve accessory apparatus proteins. Protein secretion via the Type I pathway is signal sequence-independent with no free periplasmic intermediate. Secretion by the Type II pathway is signal sequence-dependent and via the periplasm. Recent results also suggest that a third (Type III) secretory pathway exists in which protein secretion is signal sequence-independent.

Analysis of eight out genes in a cluster required for pectic enzyme secretion by Erwinia chrysanthemi: sequence comparison with secretion genes from other gram-negative bacteria

Many extracellular proteins produced by Erwinia chrysanthemi require the out gene products for transport across the outer membrane. In a previous report (S. Y. He, M. Lindeberg, A. K. Chatterjee, and A. Collmer, Proc. Natl. Acad. Sci. USA 88:1079-1083, 1991) cosmid pCPP2006, sufficient for secretion of Erwinia chrysanthemi extracellular proteins by Escherichia coli, was partially sequenced, revealing four out genes sharing high homology with pulH through pulK from Klebsiella oxytoca. The nucleotide sequence of eight additional out genes reveals homology with pulC through pulG, pulL, pulM, pulO, and other genes involved in secretion by various gram-negative bacteria. Although signal sequences and hydrophobic regions are generally conserved between Pul and Out proteins, four out genes contain unique inserts, a pulN homolog is not present, and outO appears to be transcribed separately from outC through outM. The sequenced region was subcloned, and an additional 7.6-kb region upstream was identified as being required for secretion in E. coli. out gene homologs were found on Erwinia carotovora cosmid clone pAKC651 but were not detected in E. coli. The outC-through-outM operon is weakly induced by polygalacturonic acid and strongly expressed in the early stationary phase. The out and pul genes are highly similar in sequence, hydropathic properties, and overall arrangement but differ in both transcriptional organization and the nature of their induction.

Secretion across the bacterial outer membrane

Many bacteria secrete extracellular proteins such as hydrolytic enzymes or toxins. In Gram-negative bacteria, secreted proteins must cross the two membranes that constitute the cell envelope. Recent studies have identified several specific secretion systems that can be classified in three distinct pathways, and related systems have been discovered in a wide range of prokaryotic and eukaryotic cells.