Transfer of the Salmonella type III effector sopE between unrelated phage families

Salmonella spp. are pathogenic enterobacteria that employ type III secretio n systems to translocate effector proteins and modulate responses of host c ells. The repertoire of translocated effector proteins is thought to define host specificity and epidemic virulence, and varies even between closely r elated Salmonella strains. Therefore, horizontal transfer of effector prote in genes between Salmonella strains plays a key role in shaping the Salmone lla-host interaction. Several effector protein genes are located in tempera te phages. The P2-like phage SopE Phi encodes SopE and the lambda -like GIF SY phages encode several effector proteins of the YopM/IpaH-family. Lysogen ic conversion with these phages is responsible for much of the diversity of the effector protein repertoires observed among Salmonella spp. However, f ree exchange of effector proteins by lysogenic conversion can be restricted by superinfection immunity. To identify genetic mechanisms that may furthe r enhance horizontal transfer of effector genes, we have analyzed sopE loci from Salmonella spp. that do not harbor P2-like sequences of SopEd Phi. In two novel sopE loci that were identified, the 723 nt sopE gene is located in a conserved 1.2 kb cassette present also in SopE Phi. Most strikingly, i n Salmonella enterica subspecies I serovars Gallinarum, Enteritidis, Hadar and Dublin, the sopE-cassette is located in a cryptic lambda -like prophage with similarity to the GIFSY phages. This provides the first evidence for transfer of virulence genes between different phage families. We show that such a mechanism can circumvent restrictions to phage-mediated gene transfe r and thereby enhances reassortment of the effector protein repertoires in Salmonella spp. (C) 2001 Academic Press.