Trends in Microbiology
ReviewGenomic mining type III secretion system effectors in Pseudomonas syringae yields new picks for all TTSS prospectors
Section snippets
Features associated with TTSS substrates that can be used in genomic mining
Despite the highly variable structure and function of P. syringae effectors, their coordinated regulation and ability to be secreted or translocated by a TTSS are conserved features that can be exploited for genome-wide searches (Fig. 1). Several features of the Hrp system are notable here. Induction in plant apoplastic fluids or apoplast-mimicking media is activated by the σ54-enhancer-binding proteins HrpR and HrpS 14., 15., 16., which in turn activate the RpoN-dependent expression of
Identifying candidate TTSS effector genes in P. syringae based on regulation
Three of the five recent papers exploiting the P. s. tomato DC3000 draft sequence identify novel virulence genes based on expression in planta or on membership of the Hrp regulon. In vivo expression technology (IVET; pioneered in 1987 with the plant pathogen Xanthomonas campestris pv. campestris [26]) was used by Boch et al. [27] to identify promoters that could restore hrcC expression to P. s. tomato DC3000 in planta. Of the 69 in planta-expressed (ipx) genes that were initially analyzed (from
Identifying candidate TTSS effector genes in P. syringae based on secretion and translocation
Two of the recent papers exploring the P. s. tomato DC3000 genome identify novel effectors on the basis of their secretion or translocation abilities and sequence features predictive of TTSS substrates. Petnicki-Ocwieja et al. used secretion and translocation assays to demonstrate that ten ORFs downstream of HMM-identified Hrp promoters were Hops, and this initial set of confirmed Hops was combined with 18 other established Avr/Hops to identify export signal patterns in the first 50 amino acids
Hop function: helpers versus effectors
One distinction emerging from analyses of the growing pool of Hops is that proteins secreted by the Hrp pathway appear to fall into two functional categories – ‘helpers’ and effectors – with the former proteins aiding the delivery of the effectors (Table 2). HrpA is the clearest example of a helper because it is essential for effector delivery and forms the Hrp pilus through which HrpZ and AvrPto were shown to travel 32., 33., 34.. Harpins might also assist in effector delivery through
Examining the P. s. tomato DC3000 effector inventory for insights into effector function
Now that we have identified many Hops from P. syringae, what are their roles in pathogenesis? We have noted the recent clues about effector targets in plants (for reviews see 43., 44., 45.). Additional insights can be gleaned from the P. s. tomato effector inventory shown in Table 2. Unexpectedly, most of the hop genes are chromosomally encoded, with only four being carried on plasmids. Most striking is the prevalence of Hops with similarities to ADP ribosyltransferases 30., 31.. Although the
Lessons gained from P. s. tomato that could be useful for effector mining in other bacteria
Other plant pathogens could have similarly large inventories of TTSS-secreted proteins. Although only three proteins have been demonstrated to be secreted via a TTSS in R. solanacearum, nearly 50 candidates have been identified in the genome based on homology with known effectors in other pathogens or on structural features such as linkage to PIP boxes (highly conserved plant inducible promoters that activate the Hrp regulon in R. solanacearum and Xanthomonas spp.) and motifs predicting
Acknowledgements
We thank Philip A. Bronstein, C. Robin Buell, Sheng Yang He, Lisa M. Schechter and Glenn M. Young for helpful suggestions and comments pertaining to this review. Research in the authors' laboratories is supported by the National Science Foundation (NSF) Plant Genome Research Program Cooperative Agreement DBI-0077622, NSF Grant MCB-9982646 (to A.C.), NSF Grant IBN-0096348 (to J.R.A.) and the National Research Initiative Competitive Grants Program of the US Dept of Agriculture Grant
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