Xenorhabdus and Photorhabdus spp. are gram negative gamma proteobacteria that form entomopathogenic symbioses with soil nematodes. They undergo a complex life cycle that involves a symbiotic stage, in which the bacteria are carried in the gut of the nematodes, and a pathogenic stage, in which susceptible insect prey are killed by the combined action of the nematode and the bacteria. Both bacteria produce antibiotics, intracellular protein crystals, and numerous other products. These traits change in phase variants, which arise when the bacteria are maintained under stationary phase conditions in the laboratory. Molecular biological studies suggest that Xenorhabdus and Photorhabdus spp. may serve as valuable model systems for studying signal transduction and transcriptional and posttranscriptional regulation of gene expression. Such studies also indicate that these bacterial groups, which had been previously considered to be very similar, may actually be quite different at the molecular level.

EnvZ and OmpR, the regulatory proteins for ompF and ompC expression in Escherichia coli, belong to a modulator-effector family of regulatory proteins which are essential for the response to environmental signals. We show that the soluble cytoplasmic domain of the transmembrane modulator protein EnvZ is phosphorylated in vitro by [7-32P]

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.

The Tar chemoreceptor of Escherichia coli is a membrane-bound sensory protein that facilitates bacterial chemotaxis in response to aspartate. The EnvZ molecule has a membrane topology similar to Tar and is a putative osmosensor that is required for osmoregulation of the genes for the major outer membrane porin proteins, OmpF and OmpC. The cytoplasmic signaling domain of Tar was replaced with the carboxyl portion of EnvZ, and the resulting chimeric receptor activated transcription of the ompC gene in response to aspartate. The activation of ompC by the chimeric receptor was absolutely dependent on OmpR, a transcriptional activator for ompF and ompC.

This paper describes the Vibrio fischeri-squid model system. The various stages of the bacteria-nematode symbiosis are discussed in detail and compared with the bacteria-squid system. Notable differences between the Photorhabdus-Heterorhabditis and Xenorhabdus-Steinernema systems are presented.

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SummaryXenorhabdus nematophila is an emerging model for both mutualism and pathogenicity in different invertebrate hosts. Here we conduct a mutant study of the EnvZ-OmpR two-component system and the flagella sigma factor, FliA (s 28). Both ompR and envZ strains displayed precocious swarming behaviour, elevated flhD and fliA mRNA levels and early production of lipase, protease, haemolysin and antibiotic activity. Inactivation of fliA eliminated exoenzyme production which was restored by complementation with the fliAZ operon. Inactivation of flhA, a gene encoding a component of the flagella export apparatus, eliminated lipase but not protease or haemolysin production indicating these enzymes are secreted by different export pathways. FliA-regulated lipase (xlpA) and protease (xrtA) genes were identified. Their expression and level of production were elevated in the ompR and envZ strains and markedly reduced in the fliA strain while both were expressed normally in the flhA strain. We also found that expression of nrps1 which encodes a non-ribosomal peptide synthetase was elevated in the ompR and envZ strains. The fliA strain was pathogenic towards the insect host indicating that motility and FliA-regulated exoenzyme production were not essential for virulence. These findings support a model in which the EnvZOmpR-FlhDC-FliA regulatory network co-ordinately controls flagella synthesis, and exoenzyme and antibiotic production in X. nematophila.

Two-component signal transduction systems, consisting of histidine kinase (HK) sensors and DNA-binding response regulators, allow bacteria and archaea to respond to diverse environmental stimuli. HKs possess a conserved domain (H-box region) which contains the site of phosphorylation and an ATP-binding kinase domain. In this study, a genomic approach was taken to analyse the HK family in bacteria and archaea. Based on phylogenetic analysis, differences in the sequence and organization of the H-box and kinase domains, and the predicted secondary structure of the H-box region, five major HK types were identified. Of the 336 HKs analysed, 92 % could be assigned to one of the five major HK types. The Type I HKs were found predominantly in bacteria while Type II HKs were not prevalent in bacteria but constituted the major type (13 of 15 HKs) in the archaeon Archaeoglobus fulgidus. Type III HKs were generally more prevalent in Gram-positive bacteria and were the major HK type (14 of 15 HKs) in the archaeon Methanobacterium thermoautotrophicum. Type IV HKs represented a minor type found in bacteria. The fifth HK type was composed of the chemosensor HKs, CheA. Several bacterial genomes contained all five HK types. In contrast, archaeal genomes either contained a specific HK type or lacked HKs altogether. These findings suggest that the different HK types originated in bacteria and that specific HK types were acquired in archaea by horizontal gene transfer.