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

Identification and biochemical evidence of a medium-chain-length polyhydroxyalkanoate depolymerase in the Bdellovibrio bacteriovorus predatory hydrolytic arsenal

The obligate predator Bdellovibrio bacteriovorus HD100 shows a large set of proteases and other hydrolases as part of its hydrolytic arsenal needed for its predatory life cycle. We present genetic and biochemical evidence that open reading frame (ORF) Bd3709 of B. bacteriovorus HD100 encodes a novel medium-chain-length polyhydroxyalkanoate (mcl-PHA) depolymerase (PhaZ(Bd)). The primary structure of PhaZ(Bd) suggests that this enzyme belongs to the α/β-hydrolase fold family and has a typical serine hydrolase catalytic triad (serine-histidine-aspartic acid) in agreement with other PHA depolymerases and lipases. PhaZ(Bd) has been extracellularly produced using different hypersecretor Tol-pal mutants of Escherichia coli and Pseudomonas putida as recombinant hosts. The recombinant PhaZ(Bd) has been characterized, and its biochemical properties have been compared to those of other PHA depolymerases. The enzyme behaves as a serine hydrolase that is inhibited by phenylmethylsulfonyl fluoride. It is also affected by the reducing agent dithiothreitol and nonionic detergents like Tween 80. PhaZ(Bd) is an endoexohydrolase that cleaves both large and small PHA molecules, producing mainly dimers but also monomers and trimers. The enzyme specifically degrades mcl-PHA and is inactive toward short-chain-length polyhydroxyalkanoates (scl-PHA) like polyhydroxybutyrate (PHB). These studies shed light on the potentiality of these predators as sources of new biocatalysts, such as an mcl-PHA depolymerase, for the production of enantiopure hydroxyalkanoic acids and oligomers as building blocks for the synthesis of biobased polymers.

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: a protein secretion system for all seasons

In Gram-negative bacteria, type II secretion (T2S) is one of five protein secretion systems that permit the export of proteins from within the bacterial cell to the extracellular milieu and/or into target host cells. An analysis of numerous sequenced genomes now reveals that T2S genes are common, but by no means universal, in Gram-negative bacteria. Recent functional studies indicate that T2S can promote the virulence of human, animal and plant pathogens, as well as the physiology of various environmental bacteria. Thus, it is an opportune time to highlight the new and different ways in which T2S serves bacterial function.

Novel virulence-associated type II secretion system unique to high-pathogenicity Yersinia enterocolitica

Yersinia enterocolitica strains comprise an important group of bacterial enteropathogens that cause a broad range of gastrointestinal syndromes. Three groups are distinguishable within this bacterial species, namely, the nonpathogenic group (biotype 1A strains), the low-pathogenicity, non-mouse-lethal group (biotypes 2 to 5), and the high-pathogenicity, mouse-lethal group (biotype 1B). To date, the presence of the high-pathogenicity island (HPI), a chromosomal locus that encodes the yersiniabactin system (involved in iron uptake), defines essentially the difference between low-pathogenicity and high-pathogenicity Y. enterocolitica strains, with the low-pathogenicity strains lacking the HPI. Using the powerful tool of representational difference analysis between the nonpathogenic 1A strain, NF-O, and its high-pathogenicity 1B counterpart, WA-314, we have identified a novel type II secretion gene cluster (yts1C-S) occurring exclusively in the high-pathogenicity group. The encoded secreton, designated Yts1 (for Yersinia type II secretion 1) was shown to be important for virulence in mice. A close examination of the almost completed genome sequence of another high-pathogenicity representative, Y. enterocolitica 8081, revealed a second putative type II secretion cluster uniformly distributed among all Y. enterocolitica isolates. This putative species-specific cluster (designated yts2) differed significantly from yts1, while resembling more closely the putative type II cluster present on the genome of Y. pestis. The Yts1 secreton thus appears to have been additionally acquired by the high-pathogenicity assemblage for a virulence-associated function.

Identification of a novel Gsp-related pathway required for secretion of the manganese-oxidizing factor of Pseudomonas putida strain GB-1

The manganese-oxidizing factor of Pseudomonas putida strain GB-1 is associated with the outer membrane. One of the systems of protein transport across the outer membrane is the general secretory pathway (Gsp). The gsp genes are called xcp in Pseudomonas species. In a previous study, it was shown that mutation of the prepilin peptidase XcpA and of a homologue of the pseudopilin XcpT inhibited transport of the factor. In the present study, we describe the genomic region flanking the xcpT homologue (designated xcmT1). We show that xcmT1 is part of a two-gene operon that includes an xcpS homologue (designated xcmS). No other xcp-like genes are present in the regions flanking the xcmT1/xcmS cluster. We also characterized the site of transposon insertion of another transport mutant of P. putida GB-1. This insertion appeared to be located in a gene (designated xcmX) possibly encoding another pseudopilin-related protein. This xcmX is clustered with two other xcpT-related genes (designated xcmT2 and xcmT3) on one side and homologues of three csg genes (designated csmE, csmF and csmG) on the other side. The csg genes are involved in production of aggregative fibres in Escherichia coli and Salmonella typhimurium. A search for XcmX homologues revealed that the recently published genome of Ralstonia solanacearum and the unannotated genome of P. putida KT2440 contain comparable gene clusters with xcmX and xcp homologues that are different from the well-described 'regular'xcp/gsp clusters. They do contain xcpR and xcpQ homologues but, for example, homologues of xcpP, Y and Z are lacking. The results suggest a novel Xcp-related system for the transport of manganese-oxidizing enzymes to the cell surface.

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).

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.

Influence of deletions within domain II of exotoxin A on its extracellular secretion from Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative bacterium that secretes many proteins into the extracellular medium via the Xcp machinery. This pathway, conserved in gram-negative bacteria, is called the type II pathway. The exoproteins contain information in their amino acid sequence to allow targeting to their secretion machinery. This information may be present within a conformational motif. The nature of this signal has been examined for P. aeruginosa exotoxin A (PE). Previous studies failed to identify a common minimal motif required for Xcp-dependent recognition and secretion of PE. One study identified a motif at the N terminus of the protein, whereas another one found additional information at the C terminus. In this study, we assess the role of the central PE domain II composed of six alpha-helices (A to F). The secretion behavior of PE derivatives, individually deleted for each helix, was analyzed. Helix E deletion has a drastic effect on secretion of PE, which accumulates within the periplasm. The conformational rearrangement induced in this variant is predicted from the three-dimensional PE structure, and the molecular modification is confirmed by gel filtration experiments. Helix E is in the core of the molecule and creates close contact with other domains (I and III). Deletion of the surface-exposed helix F has no effect on secretion, indicating that no secretion information is contained in this helix. Finally, we concluded that disruption of a structured domain II yields an extended form of the molecule and prevents formation of the conformational secretion motif.