[Show abstract][Hide abstract] ABSTRACT: RcsF (regulator of capsule synthesis) is an outer membrane (OM) lipoprotein that functions to sense defects such as changes in LPS. However, LPS is found in the outer leaflet, and RcsF was thought to be tethered to the inner leaflet by its lipidated N terminus, raising the question of how it monitors LPS. We show that RcsF has a transmembrane topology with the lipidated N terminus on the cell surface and the C-terminal signaling domain in the periplasm. Strikingly, the short, unstructured, charged transmembrane domain is threaded through the lumen of β-barrel OM proteins where it is protected from the hydrophobic membrane interior. We present evidence that these unusual complexes, which contain one protein inside another, are formed by the Bam complex that assembles all β-barrel proteins in the OM. The ability of the Bam complex to expose lipoproteins at the cell surface underscores the mechanistic versatility of the β-barrel assembly machine.
Proceedings of the National Academy of Sciences of the United States of America. 09/2014;
[Show abstract][Hide abstract] ABSTRACT: The lipoprotein Lpp is the most numerically abundant protein in Escherichia coli, has been investigated for over 40 years, and has served as the paradigmatic bacterial lipoprotein since its initial discovery. It exists in two distinct forms: a 'bound-form', which is covalently bound to the cell's peptidoglycan layer, and a 'free-form', which is not. Although it is known that the carboxyl-terminus of bound-form Lpp is located in the periplasm, the precise location of free-form Lpp has never been determined. For decades, it has been widely assumed that free-form Lpp is associated with bound-form. In this work, we show that the free and bound forms of Lpp are not largely associated with each other, but are found in distinct subcellular locations. Our results indicate that free-form Lpp spans the outer membrane and is surface-exposed, whereas bound-form Lpp resides in the periplasm. Thus, Lpp represents a novel example of a single lipoprotein that is able to occupy distinct subcellular locations, and challenges models in which the free and bound forms of Lpp are assumed to be associated with each other.
[Show abstract][Hide abstract] ABSTRACT: Members of the genus Xenorhabdus are entomopathogenic bacteria that associate with nematodes. The nematode-bacteria pair infects and kills insects, with both partners contributing to insect pathogenesis and the bacteria providing nutrition to the nematode from available insect-derived nutrients. The nematode provides the bacteria with protection from predators, access to nutrients, and a mechanism of dispersal. Members of the bacterial genus Photorhabdus also associate with nematodes to kill insects, and both genera of bacteria provide similar services to their different nematode hosts through unique physiological and metabolic mechanisms. We posited that these differences would be reflected in their respective genomes. To test this, we sequenced to completion the genomes of Xenorhabdus nematophila ATCC 19061 and Xenorhabdus bovienii SS-2004. As expected, both Xenorhabdus genomes encode many anti-insecticidal compounds, commensurate with their entomopathogenic lifestyle. Despite the similarities in lifestyle between Xenorhabdus and Photorhabdus bacteria, a comparative analysis of the Xenorhabdus, Photorhabdus luminescens, and P. asymbiotica genomes suggests genomic divergence. These findings indicate that evolutionary changes shaped by symbiotic interactions can follow different routes to achieve similar end points.
PLoS ONE 01/2011; 6(11):e27909. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Members of the Steinernema genus of nematodes are colonized mutualistically by members of the Xenorhabdus genus of bacteria. In nature, Steinernema carpocapsae nematodes are always found in association with Xenorhabdus nematophila bacteria. Thus, this interaction, like many microbe-host associations, appears to be species specific. X. nematophila requires the nilA, nilB, and nilC genes to colonize S. carpocapsae. In this work, we showed that of all the Xenorhabdus species examined, only X. nematophila has the nilA, nilB, and nilC genes. By exposing S. carpocapsae to other Xenorhabdus spp., we established that only X. nematophila is able to colonize S. carpocapsae; therefore, the S. carpocapsae-X. nematophila interaction is species specific. Further, we showed that introduction of the nilA, nilB, and nilC genes into other Xenorhabdus species enables them to colonize the same S. carpocapsae host tissue that is normally colonized by X. nematophila. Finally, sequence analysis supported the idea that the nil genes were horizontally acquired. Our findings indicate that a single genetic locus determines host specificity in this bacteria-animal mutualism and that host range expansion can occur through the acquisition of a small genetic element.
Journal of bacteriology 07/2008; 190(12):4121-8. · 3.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Xenorhabdus nematophila is a Gram-negative bacterium that leads both pathogenic and mutualistic lifestyles. In this study, we examine the role of Lrp, the leucine-responsive regulatory protein, in regulating both of these lifestyles. lrp mutants have attenuated virulence towards Manduca sexta insects and are defective in suppression of both cellular and humoral insect immunity. In addition, an lrp mutant is deficient in initiating colonization of and growth within mutualistic host nematodes. Furthermore, nematodes reared on lrp mutant lawns exhibit decreased overall numbers of nematode progeny. To our knowledge, this is the first demonstration of virulence attenuation associated with an lrp mutation in any bacterium, as well as the first report of a factor involved in both X. nematophila symbioses. Protein profiles of wild-type and mutant cells indicate that Lrp is a global regulator of expression in X. nematophila, affecting approximately 65% of 290 proteins. We show that Lrp binds to the promoter regions of genes known to be involved in basic metabolism, mutualism and pathogenesis, demonstrating that the regulation of at least some host interaction factors is likely direct. Finally, we demonstrate that Lrp influences aspects of X. nematophila phenotypic variation, a spontaneous process that occurs during prolonged growth in stationary phase.
[Show abstract][Hide abstract] ABSTRACT: Virulence of the insect pathogen Xenorhabdus nematophila is attributed in part to its ability to suppress immunity. For example, X. nematophila suppresses transcripts encoding several antimicrobial proteins, even in the presence of Salmonella enterica, an inducer of these transcripts. We show here that virulence and immune suppression phenotypes can be lost in a subpopulation of X. nematophila. Cells that have undergone 'virulence modulation' (vmo) have attenuated virulence and fail to suppress antimicrobial transcript levels, haemocyte aggregation and nodulation in Manduca sexta insects. When plated on certain media, vmo cells have a higher proportion of translucent (versus opaque) colonies compared with non-vmo cells. Like vmo strains, translucent colony isolates are defective in virulence and immune suppression. The X. nematophila genome encodes two 'opacity' genes with similarity to the Ail/PagC/Rck family of outer membrane proteins involved in adherence, invasion and serum resistance. Quantitative polymerase chain reaction analysis shows that RNA levels of one of these opacity genes, opaB, are higher in opaque relative to translucent colonies. We propose that in X. nematophila opaB may be one of several factors involved in immune suppression during infection, and expression of these factors can be co-ordinately eliminated in a subpopulation, possibly through a phase variation mechanism.
[Show abstract][Hide abstract] ABSTRACT: The bacterial mutualist Xenorhabdus nematophila colonizes a specific region of its nematode host Steinernema carpocapsae. We previously reported the identification of a chromosomal locus encoding three X. nematophila genes of unknown function, nilA, B and C, that are each necessary for colonization. Subsequent work indicated the global regulator Lrp is a repressor of nilC: nilC transcription is elevated in an lrp mutant and Lrp interacts directly with the nilC promoter. In this manuscript, we report the identification of an additional gene, nilR, required for repression of nilC transcription. We show that nilR and lrp mutants also have elevated expression of nilA and nilB, demonstrating that nilA, B and C are co-ordinately regulated. nil gene expression is derepressed most strongly when both nilR and lrp are lacking, suggesting NilR and Lrp synergistically repress nil transcription. NilR contains a helix-turn-helix-type DNA binding domain and likely acts directly at promoters. A comparison of the wild type and nilR proteomes indicates that NilR, unlike Lrp, regulates a small number of genes. Finally, X. nematophila carrying an ectopic copy of nilR colonizes at approximately 60-fold lower levels than the control strain, suggesting that derepression of nil gene expression is necessary for nematode colonization.
[Show abstract][Hide abstract] ABSTRACT: Xenorhabdus nematophila is a gamma-proteobacterial mutualist of an insect-pathogenic nematode, Steinernema carpocapsae. X. nematophila requires nilC, a gene predicted to encode an outer membrane lipoprotein of unknown function, for colonization of its nematode host. Characterization of NilC, described here, demonstrated it is a 28 kDa lipoprotein directed to the periplasm by an N-terminal signal sequence. Lipidation and processing of NilC occurs by a mechanism that is conserved in proteobacteria. This work also showed NilC is membrane associated and oriented towards the periplasm of X. nematophila and is produced as an outer membrane-associated protein when expressed in Escherichia coli. Expression analyses revealed that nilC transcription is directly or indirectly repressed by Lrp, and this regulatory link may explain the nematode mutualism defect of a previously identified lrp::Tn5 mutant. An lrp::Tn5 mutant produces an additional nilC transcript, not observed in wild-type cells growing in vitro, and produces approximately 75-fold more nilC than wild-type cells in late stationary phase. These fundamental characterizations of nilC expression and nilC localization and processing events have provided firm bases for understanding the role of this colonization factor in the X. nematophila/S. carpocapsae microbe-host interaction.
[Show abstract][Hide abstract] ABSTRACT: One stage in the symbiotic interaction between the bacterium Xenorhabdus nematophila and its nematode host, Steinernema carpocapsae, involves the species-specific colonization of the nematode intestinal vesicle by the bacterium. To characterize the bacterial molecular determinants that are essential for vesicle colonization, we adapted and applied a signature-tagged mutagenesis (STM) screen to this system. We identified 15 out of 3000 transposon mutants of X. nematophila with at least a 15-fold reduction in average vesicle colonization. These 15 mutants harbour disruptions in nine separate loci. Three of these loci have predicted open reading frames (ORFs) with similarity to genes (rpoS, rpoE, lrp) encoding regulatory proteins; two have predicted ORFs with similarity to genes (aroA, serC) encoding amino acid biosynthetic enzymes; one, designated nilB (nematode intestine localization), has an ORF with similarity to a gene encoding a putative outer membrane protein (OmpU) in Neisseria; and three, nilA, nilC and nilD, have no apparent homologues in the public database. nilA, nilB and nilC are linked on a single 4 kb locus. nilB and nilC are > 104-fold reduced in their ability to colonize the nematode vesicle and are predicted to encode membrane-localized proteins. The nilD locus contains an extensive repeat region and several small putative ORFs. Other than reduced colonization, the nilB, nilC and nilD mutants did not display alterations in any other phenotype tested, suggesting a specific role for these genes in allowing X. nematophila to associate with the nematode host.