Genomic Islands of Pseudomonas syringae pv. tabaci 6605: Identification of PtaGI-1 as a Pathogenicity Island With Effector Genes and a Tabtoxin Cluster

Genomic islands (GIs) are 20-500 kb DNA regions that are thought to be acquired by horizontal gene transfer. GIs that confer pathogenicity and environmental adaptation have been reported in Pseudomonas species; however, GIs that enhance bacterial virulence have not. Here, we identified 110 kb and 103 kb GIs in P. syringae pv. tabaci 6605 (Pta6605), the causative agent of tobacco wildfire disease, which has the ability to produce tabtoxin as a phytotoxin. These GIs are partially homologous to known genomic islands in Pseudomonas aeruginosa and P. syringae pv. phaseolicola and were designated PtaGI-1 and PtaGI-2. Both PtaGIs conserve core genes, whereas each GI possesses different accessory genes. PtaGI-1 contains a tabtoxin biosynthetic gene cluster and three type III effector genes among its accessory genes, whereas PtaGI-2 also contains homologous genes to hsvABC, pathogenicity-related genes in Erwinia amylovora. Inoculation revealed that the PtaGI-1 mutant, but not the PtaGI-2 mutant, lost the ability to biosynthesise tabtoxin and to cause disease. Therefore, PtaGI-1 is thought to be a pathogenicity island. Both PtaGI-1 and PtaGI-2 have a pseudogene of tRNALys on the left border and an intact tRNALys gene on the right border. In a colony of Pta6605, both GIs can be excised at tRNALys, and PtaGI-1 and PtaGI-2 exist in a circular form. These results indicate that tabtoxin biosynthesis genes in PtaGI-1 are required for disease development, and PtaGI-1 is necessary for Pta6605 virulence.