The xaxAB Genes Encoding a New Apoptotic Toxin from the Insect Pathogen Xenorhabdus nematophila Are Present in Plant and Human Pathogens

Abstract

Xenorhabdus nematophila, a member of the Enterobacteriaceae, kills many species of insects by strongly depressing the immune system and colonizing the entire body. A peptide cytotoxin has been purified from X. nematophila broth growth, and the cytolytic effect on insect immunocytes and hemolytic effect on mammalian red blood cells of this toxin have been described (Ribeiro, C., Vignes, M., and Brehélin, M. (2003) J. Biol. Chem. 278, 3030 -3039). We show here that this toxin, Xenorhabdus ␣-xenorhabdolysin (Xax), triggers apoptosis in both insect and mammalian cells. We also report the cloning and sequencing of two genes, xaxAB, encoding this toxin in X. nematophila. The expression of both genes in recombinant Escherichia coli led to the production of active cytotoxin/hemolysin. However, hemolytic activity was observed only if the two peptides were added in the appropriate order. Furthermore, we report here that inactivation of xaxAB genes in X. nematophila abolished the major cytotoxic activity present in broth growth, called C1. We also show that these genes are present in various entomopathogenic bacteria of the genera Xenorhabdus and Photorhabdus, in Pseudomonas entomophila, in the human pathogens Yersinia enterocolitica and Proteus mirabilis, and in the plant pathogen Pseudomonas syringae. This toxin cannot be classified in any known family of cytotoxins on the basis of amino acid sequences, locus organization, and activity features. It is, therefore, probably the prototype of a new family of binary toxins.

The Diversity of Insect-bacteria Interactions and its Applications for Disease Control

“…Other insecticidal toxins from Photorhabdus are: mcf1 which is apoptotic to insect and mammalian cell lines Dowling et al, 2004;Dowling et al, 2007), PVC (Photorhabdus virulence cassettes) similar to bacteriocins (Yang et al, 2006) and the pirAB toxins, similar to insect hormones (Waterfield et al, 2005b). A number of toxin-encoding genes have been also described from Xenorhabdus species, namely xpt and xaxAB (Sergeant et al, 2003;Vigneux, 2007). The xpt genes are homologues of the Photorhabdus tc, while the xaxAB were discovered in Xenorhabdus as an apoptotic toxin, which Photorhabdus ortholog is phlA (Vigneux, 2007).…”

“…A number of toxin-encoding genes have been also described from Xenorhabdus species, namely xpt and xaxAB (Sergeant et al, 2003;Vigneux, 2007). The xpt genes are homologues of the Photorhabdus tc, while the xaxAB were discovered in Xenorhabdus as an apoptotic toxin, which Photorhabdus ortholog is phlA (Vigneux, 2007). The future publication of the X. nematophilus genome sequence will enable genome-wide comparisons of the two related genera (Latreille et al, 2007).…”

“…Among the most potent toxins produced by Xenorhabdus are the "toxin complex" (Tc/Xpt) (Sergeant et al, 2003) and the XaxAB cytotoxin. Both of them are orally toxic to insects when expressed in E. coli (Sergeant et al, 2003;Vigneux, 2007). The XaxAB cytotoxin, from a novel family of binary toxins, has apoptotic activity both in insect and mammalian cells which constitutes an obstacle for its potential use in insect control.…”

“…The XaxAB cytotoxin, from a novel family of binary toxins, has apoptotic activity both in insect and mammalian cells which constitutes an obstacle for its potential use in insect control. xaxAB homologues have been found in a variety of pathogens including Photorhabdus, P. entomophila, Y. enterocolitica and Proteus mirabilis, and, surprisingly, even in the plant pathogen Pseudomonas syringae (Vigneux, 2007). The xpt genes from X. nematophilus, unlike the Photorhabdus tc, are co-localised and co-expressed.…”

Identification of a new antimicrobial lysine-rich cyclolipopeptide family from Xenorhabdus nematophila

“…4 Bacteria are involved in killing the insect host by producing insecticidal proteins 5 and inhibitors of the insect immune system. [6][7][8] The bacteria proliferate in the killed host and favor the reproduction of the nematode by degrading the insect biomass 9 and by producing antibiotics that inhibits the development of the other microorganisms present in the insects corpse (bacteria, fungi). 10 Boemare et al 11 classified the antibiotic activities of Xenorhabdus into two categories: (i) antimicrobial molecules with broad spectrum and (ii) bacteriocins with very narrow spectrum and active only against bacteria closely related to X. nematophila.…”