Diversifying Selection Drives the Evolution of the Type III Secretion System Pilus of Pseudomonas syringae

Abstract

The plant pathogenic bacterium Pseudomonas syringae uses a type III secretion system to inject virulence proteins directly into the cytoplasm of its hosts. The P. syringae type III secretion apparatus is encoded, in part, by the HrpZ operon, which carries the hrpA gene encoding the pilin subunit of the pilus, various components of the structural apparatus, and the HrpZ harpin protein that is believed to produce pores in the host cell membrane. The pilus of the type III system comes into direct contact with the host cell and is, therefore, a likely target of the host's pathogen surveillance systems. We sequenced and analyzed 22 HrpZ operons from P. syringae strains spanning the diversity of the species. Selection analyses, including K(a)/K(s) tests and Tajima's D, revealed strong diversifying selection acting on the hrpA gene. This form of selection enables pathogens to maintain genetic diversity within their populations and is often driven by selection imposed by host defense systems. The HrpZ operon also revealed a single significant recombination event that dramatically changed the evolutionary relationships among P. syringae strains from 2 quite distinct phylogroups. This recombination event appears to have introduced genetic diversity into a clade of strains that may now be undergoing positive selection. The identification of diversifying selection acting on the Hrp pilus across the whole population sample and positive selection within one P. syringae lineage supports a trench warfare coevolutionary model between P. syringae and its plant hosts.

Molecular Evolution of Pathogenicity-Island Genes in Pseudomonas viridiflava

“…Strong selective constraints for conserving the protein structure of HrpA might be reasonable, considering that hrpA encodes a pilus that functions as a conduit for Avr protein delivery ( Jin and He 2001). However, hrpA in P. syringae is reported to be under diversifying selection (Guttman et al 2006), suggesting that modifications in protein structure of HrpA can be adaptive in Pseudomonas. Indeed, unlike within each locus, hrpA is highly divergent between hrpA [T] and hrpA [S] and between species (Table 2).…”

“…hrpA, hrpZ, and hrpW encode outer proteins involved in the type-III secretion system (TTSS). hrpA encodes the pilus, which plays a key role in the secretion of Avr and other type-III effector proteins and is subject to natural selection in P. syringae ( Jin and He 2001;Guttman et al 2006). hrpZ and hrpW encode harpin proteins Preston et al 1995), which also elicit HR in the apoplast (in contrast to Avr proteins that elicit HR in plant cells; Wei et al 1992;He et al 1993).…”

Comparative Genomics of Host-Specific Virulence in Pseudomonas syringae

“…The few studies that have investigated T3SEs in a phylogenetic context have found support for both horizontal gene transfer and pathoadaptation (Prager et al 2000;Lavie et al 2004;Rohmer et al 2004;Lindeberg et al 2005;Guttman et al 2006). While this debate still needs to be resolved, one issue that is clear is that many T3SEs are under intense selective pressures and that evolutionary changes in these genes can profoundly impact pathogen-host interactions (Rohmer et al 2004;Pitman et al 2005;Guttman et al 2006;Ma et al 2006).…”

Pseudomonas syringae naturally lacking the canonical type III secretion system are ubiquitous in nonagricultural habitats, are phylogenetically diverse and can be pathogenic

“…The absence of the canonical T3SS in multiple disparate clades raises interesting questions about the processes leading to the acquisition or the loss of the canonical T3SS in P. syringae. There is strong evidence that the canonical T3SS was acquired by the most recent common ancestor of the P. syringae species complex (Sawada et al, 1999) and has evolved via diversifying selection involving mostly mutations (Guttman et al, 2006). However, those phylogenetic studies ignored the existence of nine clades of P. syringae, five of which are more deeply rooted in the phylogenetic tree than the other previously described clades (Morris et al, 2010), the most deeply rooted being clade UB-246.…”