| Powerful screens forbacterial virulence proteins. |
| Proc. Natl. Acad. Sci., USA 102, 3527-3528 |
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| Summary "Many mammalian and plantpathogenic bacteria injectvirulence effector proteinsinto host cells by means ofthe type III secretion system (TTSS)(1). Effector proteins attack the hostinnate immune system, modify cytoskeletonand membranes, or alter vesicletrafficking (2, 3). The collective actionof these proteins promotes bacterial entryinto, growth and movement within,and dissemination from infected hostcells_tissues. The full complement ofTTSS effectors is not known for mostbacterial pathogens; their identificationremains a crucial step toward a comprehensiveunderstanding of bacterialpathogenesis, host range, and pathogenevolution. In a recent issue of PNAS,Chang et al. (4) describe a powerfulmethod for discovering TTSS effectors.Using this method, those authors identifiedtwo nearly complete repertoires ofTTSS effectors from two pathovars(specific to different plant species) ofthe plant pathogen Pseudomonas syringae.Here, we discuss this article in thecontext of earlier efforts to discoverTTSS effectors in plant pathogens.In both mammalian and plant pathogenicbacteria, the initial method foridentification of proteins secretedthrough the TTSS was to monitor extracellularproteins secreted in bacterialcultures in a TTSS-dependent manner(5-7). Unfortunately, the amounts ofTTSS effector proteins secreted by plantpathogenic bacteria, although detectableby antibodies, are too low to be usefulfor systematic identification of effectors.A breakthrough came in 1996 with thediscovery that bacterial ''avirulence''(avr) genes encode TTSS effector proteinsthat function inside the plant cell(reviewed in ref. 8). These avr genes hadpreviously been cloned based on theirability to convert virulent strains intoavirulent ones in specific plant genotypes(9). This breakthrough, besidessignificantly expanding the number ofknown TTSS effectors in several bacteria,provided an important clue thatwould prove to be pivotal for the developmentof methods, including theone reported by Chang et al. (4), forgenome-scale identification of TTSSeffectors." |