Expression and localization of HrpA1, a protein of Xanthomonas campestris pv. vesicatoria essential for pathogenicity and induction ofthe hypersensitive reaction

Abstract

The hrp cluster of the pepper and tomato pathogen Xanthomonas campestris pv. vesicatoria is required for both pathogenicity on susceptible host plants and induction of the hypersensitive reaction on resistant plants. The hrpA locus is located at the left end of the 25-kb hrp region and encodes a single 64-kDa Hrp protein, HrpA1, which belongs to the PulD superfamily of proteins involved in type II and type III protein secretion. In this study, we developed a defined medium without any plant-derived molecules that induces expression of hrpA in vitro. The hrpA transcription start site was mapped in the coding region of the hrpB8 gene, which is the last gene of the hrpB operon. The inducible hrpA promoter shows no homology to known promoter elements or other hrp loci of X. campestris pv. vesicatoria. hrpA expression was shown to be independent of the hrp regulatory gene hrpX. The amino acid sequence of the HrpA1 protein is predicted to contain an N-terminal signal sequence and no further transmembrane domains and to be rich in ␤-sheet stretches. Expression of HrpA1 in Escherichia coli cells causes induction of the psp operon like some of its counterparts, suggesting some commonality of function and that HrpA1 forms multimers. The protein product of hrpA1 was identified by using a specific polyclonal antibody. Cell fractionation studies demonstrated that the HrpA1 protein is localized in the outer membrane of X. campestris pv. vesicatoria. HrpA1 is the first component of the Hrp secretion system whose localization has been determined in the original organism.

Virulence of the Phytopathogen Pseudomonas syringae pv. Maculicola Is rpoN Dependent

“…Similarly, in Xanthomonas campestris pv. vesicatoria, hrp gene expression is regulated by an OmpR homolog, HrpG, which in turn activates HrpX, another AraC-like activator that activates the remaining hrp genes (79,92). This latter regulatory cascade is consistent with the fact that rpoN is not required for hrp expression or pathogenicity in X. campestris pv.…”

The Escherichia coli phage‐shock‐protein (psp) operon

“…pIV is an outer membrane protein, one of two non-structural proteins required for filamentous-phage assembly and secretion (Russel, 1994b). The pIV homologues that induce the psp operon are also outer membrane proteins, and are required by both type II (general secretory) and type III (contact-dependent) (Wengelnik et al, 1996) protein-secretion systems. These systems secrete a variety of virulence factors to the exterior of Gram-negative organisms, some into the external milieu, and some into the cytoplasm of eukaryotic target cells, both animal and vegetable (Russel, 1994b).…”

The exbD2 gene as well as the iron-uptake genes tonB, exbB and exbD1 of Xanthomonas campestris pv. campestris are essential for the induction of a hypersensitive response on pepper (Capsicum annuum)

“…On: Thu, 10 May 2018 03:54:04 H.-G. WIGGERICH and A. PU $ HLER products show sequence homology to proteins mediating the type III secretion of virulence factors from animal bacterial pathogens and are therefore predicted to be involved in the secretion of molecules essential for the interaction with the plant (Fenselau et al, 1992 ;Fenselau & Bonas, 1995 ;Wengelnik et al, 1996). Hrpdependent secretion has been demonstrated for the harpin proteins of E. amylovora, Erwinia chrysanthemi and P. syringae pv.…”