The type III effectors HsvG and HsvB of tumorigenic Pantoea agglomerans determine host specificity and function as transcriptional activators

Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and HR on beet whereas pathovar. betae (Pab) induces galls on beet and gypsophila. The TTSS effector HsvG determines host specificity of both pathovars on gypsophila. A novel HsvG homolog, HsvB, was isolated and showed to determine host specificity of both pathovars on beet. HsvG requires 2 direct amino acids repeats: RI = 74 and RII = 71 for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging the repeat domains between HsvG-Pag and HsvB-Pab resulted in a corresponding switch in host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB are localized to the nucleus of host and nonhost plants. A yeast one-hybrid assay demonstrated that either RI or RII of HsvG or HsvB was sufficient to activate transcription. By random binding-site selection and gel shift assay HsvG was demonstrated as a double-stranded DNA binding protein with ACACC/aAA consensus. Both effectors harbor a region containing helix-turn-helix motifs that might be associated with DNA binding.