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

Abstract

SummaryPantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) induces galls on both beet and gypsophila. The pathogenicity of both pathovars is dependent on the presence of a plasmid harbouring type III secretion system (TTSS) components and effectors. The HsvG TTSS effectors of Pag (HsvG-Pag) and Pab (HsvGPab) determine the host specificity of both pathovars on gypsophila. Here we describe a novel HsvG homologue, HsvB, which determines the host specificity of Pag and Pab on beet. HsvG requires two direct amino acid repeats for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB were localized to the nuclei of host and non-host plants. A yeast one-hybrid assay revealed that a single repeat of HsvG or HsvB was sufficient to activate transcription. By employing random binding-site selection and gel-shift assay HsvG was demonstrated to be a double-stranded DNA-binding protein with an ACACC/aAA consensus binding site. These results suggest that HsvG and HsvB are host-specificity determinants and bear the potential to affect the host transcriptional machinery.

Characterization of nuclear localization signals in the type III effectors HsvG and HsvB of the gall-forming bacterium Pantoea agglomerans

“…On: Sat, 12 May 2018 18:26:09 transcription (Nissan et al, 2006). Recent preliminary results demonstrated that also in planta HsvG and HsvB acted as TAL effectors in a host-specific manner (Nissan et al, 2009).…”

“…Transient expression of GFP-HsvG or GFP-HsvB fusions in leaves of gypsophila, beet or melon (a non-host) showed that HsvG and HsvB localized to the nuclei of both host and non-host plants. By means of random binding-site selection (Okuno et al, 2001) and gel-shift assay, HsvG was demonstrated to be a double-stranded DNA-binding protein with a consensus DNA-binding site (Nissan et al, 2006). The localization inside the nucleus, on the one hand, and the DNA binding, on the other hand, led to the hypothesis that HsvG and HsvB act as transcription activator-like (TAL) effectors.…”

“…We have previously shown that HsvG and HsvB are paralogous type III effectors (78 % amino acid identity), present in both Pag and Pab, and determine host specificity on gypsophila and beet, respectively (Nissan et al, 2006). Sequence analysis of the predicted HsvG and HsvB proteins from both pathovars revealed a T3SS recognition domain, a helix-turn-helix (HTH) region, and a direct repeat domain.…”

“…Their structures were distinguished mainly by the presence of two direct repeats (71 and 74 aa) in HsvG as opposed to one repeat in HsvB. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities (Nissan et al, 2006). Transient expression of GFP-HsvG or GFP-HsvB fusions in leaves of gypsophila, beet or melon (a non-host) showed that HsvG and HsvB localized to the nuclei of both host and non-host plants.…”

“…Because the predicted proteins of HsvG and HsvB have molecular masses of about 70 kDa (Nissan et al, 2006;Valinsky et al, 1998), active import into the nucleus and the presence of NLS signals are expected. The only reported type III effectors harbouring NLSs belong to the AvrBs3/PthA family of Xanthomonas spp.…”