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Journal of Bacteriology, July 2004, p. 4376-4381, Vol. 186, No. 13
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.13.4376-4381.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Laboratoire EMIP Ecologie Microbienne des Insectes et Interaction Hôte-Pathogène, Université de Montpellier II, UMR1133 INRA-UMII, 34095 Montpellier Cedex 5,1 Laboratoire de Génomique des Microorganismes Pathogènes, Institut Pasteur, 75724 Paris Cedex 15, France,3 The Sanger Institute, Hinxton, Cambridge CB10 1SA, United Kingdom2
Received 20 January 2004/ Accepted 17 March 2004
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Photorhabdus and Xenorhabdus (members of the Enterobacteriaceae family) (2, 12) are bacterial symbionts of entomopathogenic nematodes belonging to the families Heterorhabditidae and Steinernematidae, respectively. These bacteria are transported by their nematode vectors into the hemocoel of the insect host, which is quickly killed by a combination of toxin action and septicemia (12). Three species were defined in the genus Photorhabdus: Photorhabdus luminescens, P. temperata, and P. asymbiotica (11). In addition, P. luminescens and P. temperata are subdivided into subspecies as follows: P. luminescens subsp. luminescens, P. luminescens subsp. akhurstii, and P. luminescens subsp. laumondii and P. temperata subsp. temperata (11). P. asymbiotica was never found associated with entomopathogenic nematodes but is isolated from human infections (8, 19). Recently, Akhurst et al. have proposed two subspecies for P. asymbiotica: P. asymbiotica for an American clinical strain and P. australis for an Australian clinical strain (1).
The recent identification of TTSSs in P. luminescens (7, 28, 31) prompted us to analyze the genomic diversity of TTSS within the genus Photorhabdus.
Comparative genomics of TTSS organization in Photorhabdus. Comparisons of the genomic organization of the Photorhabdus TTSSs (Fig. 1) were performed with three strains: P. luminescens subsp. laumondii (strain TT01) (http://genolist.pasteur.fr/PhotoList/) (7), P. luminescens subsp. akhurstii (strain W14; accession number AY144116) (28), and P. asymbiotica (strain ATCC43949), for which the genome sequence is being determined at the Sanger Center (http://www.sanger.ac.uk/Projects/P_asymbiotica/). We identified a TTSS-encoding locus in all three strains analyzed. Our analyses revealed, first, identical TTSS backbones, including all the genes predicted to encode the injectisome (Sct/Lss proteins): i.e., the basal body, the needle-like structure (sctF), and the translocator (lopB-, lopD-, and lcrV-like genes). As previously reported (28, 31), Photorhabdus TTSSs display many striking similarities to the Yersinia pestis and Pseudomonas aeruginosa TTSSs. P. luminescens subsp. laumondii and P. luminescens subsp. akhurstii as well as P. asymbiotica possess the same genetic organization required for the complete assembly of a functional TTSS machinery. Moreover, these three Photorhabdus species harbor genes similar to the P. aeruginosa exsC and exsD genes encoding transcriptional regulators, which are absent from Yersinia spp., suggesting that the regulation of this system is more related to that performing regulation in P. aeruginosa.
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FIG. 1. Comparative genomics of TTSS organization in Photorhabdus species. Positions of the primer pairs are indicated. ERIC, enterobacterial repetitive intergenic consensus.
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Third, despite the highly conserved organization and protein sequences of the core components of the secretion machinery (TTSS backbone in Fig. 1), P. luminescens is predicted to encode a protein similar to the Yersinia YopT effector whereas P. asymbiotica harbors a gene encoding a protein homologous to the P. aeruginosa ExoU effector (see below).
Distribution of the TTSS among different strains of Photorhabdus and Xenorhabdus species. To determine whether all different Photorhabdus species and subspecies (Table 1) harbor similar TTSSs, PCR amplification was performed on genomic DNA of 11 Photorhabdus strains (Table 1). In addition, seven strains of the more distantly related Xenorhabdus species were included. The oligonucleotide primer sets used (Table 2) were designed in the conserved regions of the known TTSS sequences (Fig. 1). Genes of the delivery system were named according to the nomenclature proposed by Hueck (15). For this study, the sctC/lssC, sctV/lssV/lcrD, and lopB genes were chosen because they are representative of the different parts of the injectisome located in different operons (15). Standard PCR with each primer set was performed in a 50-µl reaction volume with a Gene Amp 2400 thermocycler system (Perkin Elmer), and PCR products were subjected to 0.7% agarose gel electrophoresis for analysis. Genomic DNA from the 11 Photorhabdus strains was successfully amplified using specific primers for these three genes (Table 3). However, amplification results were negative for the seven Xenorhabdus strains, even under lower-level annealing conditions.
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TABLE 1. Bacterial strains used in this study
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TABLE 2. Oligonucleotide primer pairs used
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TABLE 3. PCR assays for sctC, sctV, lopB, lopT-spcT and lopU-spcU in Photorhabdus and Xenorhabdus species
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Phylogenetic analysis. To analyze the relatedness of the Photorhabdus TTSS and to compare it to the taxonomic position of the organism, a phylogenetic study was undertaken. We used the sctV gene (homologous to the lcrD gene of Yersinia spp.) encoding an inner-membrane protein, because sctV is among the best-conserved members of the TTSS. PCR products were isolated using a High Pure PCR purification product kit (Roche), and sequencing was performed on an ABI 3700 sequencer. Sequences were aligned using ClustalW (24), and phylogenetic trees were constructed by using the neighbor-joining method and Kimura distance values (21). A bootstrap confidence analysis was replicated 500 times (9).
The resulting trees are shown in Fig. 2. The various taxa could be divided into five distinct groups (Fig. 2A): Ysc, Hrp1, Hrp2, Inv/Mxi/Spa, and EscC/Ssa (13); the Photorhabdus TTSS falls into the Ysc group. We extended this phylogenetic analysis to species of the genus Photorhabdus (Fig. 2B). Both clinical subspecies grouped in a single cluster. The three P. luminescens subspecies P. luminescens subsp. laumondii, P. luminescens subsp. akhurstii, and P. luminescens subsp. luminescens formed a second cluster, and the P. temperata strains formed a third cluster. Remarkably, the resulting sctV tree is similar to the 16S tree (1, 11), indicating that the TTSS phylogeny reflects the phylogeny of the group. Because the same clusters and subclusters describing genetic heterogeneity among strains belonging to the genus Photorhabdus can be distinguished, we therefore suggest that the sctV gene and probably the entire TTSS backbone were present in the Photorhabdus ancestor and were not recently independently acquired.
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FIG. 2. Phylogenetic trees of bacterial sctV genes (A) and Photorhabdus species sctV genes (B). The ClustalW program with default parameters was used for aligning sequences and generating trees (branch strength values after bootstrapping 500 times are shown for some branches). Accession numbers of sequences used in this study are as follows: Chlamydia trachomatis, E0011283; Erwinia amylovora, P35654; Escherichia coli E2348/69 (enteropathogenic E. coli [EPEC]), AF022236; E. coli O157H7, NP_312596; P. luminescens, BX571871; P. aeruginosa, AF010150; P. syringae, P35655; Ralstonia solanacearum, P35656; Rhizobium sp. strain NGR 234, P55726; Salmonella enterica serovar Typhimurium (SPI1), P35657; S. enterica serovar Typhimurium (SPI2), P74856; Shigella flexneri, P35533; Sodalis glossinidius, AF306649; Xanthomonas campestris, P800150; Y. enterocolitica (chromosomal), AF369954; Y. enterocolitica (plasmid), P21210; Y. pestis (chromosomal), NC003143; Y. pestis (plasmid), P31487; strain Hb, AY526326; strain FRG04, AY526327; strain Hm, AY526331; strain HP88, AY526332; strain AU9800946, AY526330; strain AU9800888, AY526333; strain US3265-86, AY526329; strain XlNach, AY526328; strain Meg, AY526334.
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For the sctL-cspI locus, a 2,020-bp fragment containing the lopT and the slcT genes was amplified for all the P. luminescens strains and a 450-bp intergenic region was amplified for the P. asymbiotica and P. temperata strains. For the sctU-lscA region, we amplified a 4.5-kb fragment containing lopU and slcU genes for all the P. asymbiotica strains and a 1,000- to 1,100-bp intergenic region for P. luminescens and P. temperata strains. Intriguingly, no effector-encoding genes were detected in the DNA sequences at these locations in the P. temperata strains Meg and XlNach (Table 3 and Fig. 1); however, we could not rule out the possibility that they were elsewhere on the genome.
P. luminescens lopT is predicted to encode a protein similar to the Yersinia YopT effector. YopT is a cysteine protease (22, 23) that causes cytoskeletal disruption and contributes to the antiphagocytic effect of Yersinia (16, 30). The P. luminescens lopT gene is located at the 3' end of the TTSS and, as in Yersinia, forms a bicistronic operon with a gene predicted to encode a LopT chaperone, namely, SlcT. These similarities with Yersinia Yop/SycT proteins are highly suggestive of a similar role for their P. luminescens counterpart, which is in agreement with the observation of LopT expression concomitant with the in vivo TTSS-dependent inhibition of phagocytosis in the orthopteran Locusta migratoria (Brugirard-Ricaud et al., submitted for publication).
Remarkably, the P. asymbiotica locus does not harbor yopT homologues. However, it contains lopU, a gene predicted to encode a protein similar to the P. aeruginosa ExoU effector. ExoU displays a potent phospholipase activity inducing disruption of epithelial and macrophage cell lines (10). The P. asymbiotica lopU gene is located between sctU and exsA-like genes and forms a probable operon with a gene predicted to encode a LopU chaperone, namely, SlcU. This location corresponds to the inversion point between the Photorhabdus and P. aeruginosa TTSS backbones (15).
In this report we show that all Photorhabdus species contain a remarkably conserved TTSS backbone but that the effectors seem to belong to the flexible gene pool, as they differ considerably among the different species. Moreover, enterobacterial repetitive intergenic consensus sequences were occasionally found in the vicinity of the effector loci, suggesting that they may be recombination hot spots and may account for genome plasticity. It is therefore tempting to speculate that in contrast to the TTSS backbone, the genes encoding the effectors may have been acquired at different steps of the evolution and then have been selected according to the ecological niches and the host ranges of the different Photorhabdus species.
Nucleotide sequence accession number. The nucleotide sequences of Photorhabdus sp. strains Hb, Hm, FRG04, HP88, AU9800946, AU9800888, US3265-86, XlNach, and Meg were deposited in GenBank under accession no. AY526326, AY526331, AY526327, AY526332, AY526330, AY526333, AY656329, AY526328, and AY526334, respectively.
We wish to thank Isabelle Gonçalves for help with bioinformatics and Carmen Buchrieser for critical reading of the manuscript.
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