Pseudomonas syringae Hrp type III secretion system and effector proteins

Abstract

Pseudomonas syringae is a member of an important group of Gram-negative bacterial pathogens of plants and animals that depend on a type III secretion system to inject virulence effector proteins into host cells. In P. syringae, hrp͞hrc genes encode the Hrp (type III secretion) system, and avirulence (avr) and Hrpdependent outer protein (hop) genes encode effector proteins. The hrp͞hrc genes of P. syringae pv syringae 61, P. syringae pv syringae B728a, and P. syringae pv tomato DC3000 are flanked by an exchangeable effector locus and a conserved effector locus in a tripartite mosaic Hrp pathogenicity island (Pai) that is linked to a tRNA Leu gene found also in Pseudomonas aeruginosa but without linkage to Hrp system genes. Cosmid pHIR11 carries a portion of the strain 61 Hrp pathogenicity island that is sufficient to direct Escherichia coli and Pseudomonas fluorescens to inject HopPsyA into tobacco cells, thereby eliciting a hypersensitive response normally triggered only by plant pathogens. Large deletions in strain DC3000 revealed that the conserved effector locus is essential for pathogenicity but the exchangeable effector locus has only a minor role in growth in tomato. P. syringae secretes HopPsyA and AvrPto in culture in a Hrp-dependent manner at pH and temperature conditions associated with pathogenesis. AvrPto is also secreted by Yersinia enterocolitica. The secretion of AvrPto depends on the first 15 codons, which are also sufficient to direct the secretion of an Npt reporter from Y. enterocolitica, indicating that a universal targeting signal is recognized by the type III secretion systems of both plant and animal pathogens. T ype III protein secretion systems underlie the pathogenicity of many Gram-negative bacteria, including important animal pathogens in the genera Yersinia, Salmonella, Shigella, and Escherichia, and plant pathogens in the genera Pseudomonas, Erwinia, Xanthomonas, and Ralstonia (1, 2). The plant pathogens cause diverse diseases in hosts that range from apple trees to the model weed Arabidopsis thaliana, and they all share an ability to colonize the intercellular spaces of plant tissues and to cause death (sometimes delayed) in plant cells. A fundamental difference between this group of plant pathogens and most of the animal pathogens is that the former do not enter living host cells, but rather interact with the host cytoplasm from outside of an approximately 200-nm-thick plant cell wall. The ability to deliver effector proteins across this barrier via the type III secretion system is likely to be unique to plant pathogens, and it is key to their pathogenicity.Pseudomonas syringae is a widespread and representative plant pathogen. It is host specific and elicits leaf spots and other foliar necroses in host plants and the hypersensitive response (HR) in nonhosts (3). In host plants, disease symptoms typically develop after several days of bacterial growth in leaf intercellular spaces. In nonhosts, the defense-associated programmed cell death that characterizes the HR occurs within 24 ...

Modifications to the Arabidopsis Defense Proteome Occur Prior to Significant Transcriptional Change in Response to Inoculation withPseudomonas syringae

“…Gram-negative bacterial pathogens, such as Pseudomonas syringae pv tomato (Pst), introduce effector proteins to the host cell through a type III secretion system (T3SS). Assembly of the T3SS is induced by nutrient conditions typically associated with the apoplastic space (van Dijk et al 1999;Collmer et al, 2000) and occurs rapidly (Thwaites et al, 2004). A functional T3SS is absolutely required for pathogenicity, as demonstrated by the asymptomatic response of the hrpA mutant of the normally highly virulent Pst strain DC3000 on Arabidopsis.…”

Shigella Spa32 Is an Essential Secretory Protein for Functional Type III Secretion Machinery and Uniformity of Its Needle Length

“…The type III protein secretion system is found among various gram-negative animal-and plant-pathogenic bacteria, in which the subsets of effector proteins that are secreted via the system into host cells have crucial roles in the bacterial infection process (8,9,16,38). Genetic and functional studies indicate that the type III secretion system requires proteins encoded by more than 20 genes, which contain the structural components of the secretion complex, secreted proteins, chaperones, and regulators (10,11,19,37,43).…”

Structure of HrcQ B -C, a conserved component of the bacterial type III secretion systems

“…A broadly conserved subset of the hrp genes (designated hrc) appears to encode the core components of the delivery apparatus (2). It has been proposed that these core components are involved in the recognition of the secretion signals carried by the effectors and are thus responsible for the promiscuous character of type III systems for heterologous secreted proteins (8)(9)(10).…”