A Novel Toxin Delivery System in Bacteria

Comparative genomics of insect and human pathogenic strains of the bioluminescent bacteria Photorhabdus, identified a novel class of mobile elements present in multiple copies in the genomes of both strains. These elements have been designated Photorhabdus Virulence Cassettes (PVC) as their 3'-ends usually encode homologues of known toxin genes. This region has been designated the payload region of the PVC. Supernatants of cosmid clones containing intact PVC elements alone were shown to be highly toxic to model insects when injected. Expression of selected payload toxins both in E. coli and also inside host cells suggests that they must gain access to the host cell cytoplasm to exert any toxic effect. This suggests that in addition to secretion from the bacterial cell that the PVC elements also mediate delivery into the host. Further, PVC elements appear to be derived from a mobile genetic element similar to bacteriophages and so they may also be responsible for their own horizontal transfer. We will: <OL> <LI> Confirm that the payload proteins are the bona fide toxins responsible for the observed secreted toxic effect.<LI> Determine the roles of the other PVC proteins in toxin secretion and delivery.<LI> Determine how a given payload protein is targeted through the PVC system.