Genomewide identification of type III substrates in the plant pathogen Pseudomonas syringae

Pseudomonas syringae pv. tomato DC3000 is a Gram-negative bacterial pathogen of plants. It causes disease on tomato, an important agricultural crop, as well as the genetic model plant Arabidopsis thaliana. To cause disease it requires a type III secretion system (TTSS), or Hrp system, to deliver bacterial proteins called effectors into plant cells. To better understand this pathogen I set out to determine its effector inventory using several different strategies. One strategy searched far TTSS substrate genes based on their proximity to the hrp/hrc gene cluster, which encodes the TTSS apparatus. This approach led to the identification of two TTSS substrates: HrpK and HopB1. I determined that HrpK was a member of the translocator class, a TTSS substrate class that aids in the delivery of effectors into plant cells. Another strategy utilized the DC3000 genome sequence. Using several different criteria including presence of putative TTSS promoters, horizontal transfer indicators, and/or biochemical characteristics in the N-termini of predicted products, I searched the DC3000 genome for candidate TTSS substrate genes. The search resulted in a significant increase in the TTSS substrate inventory in DC3000---from five proteins to thirty-eight. Finally, I identified specific type III effectors that were able to travel through a second type III system found in P. syringae, the flagellar TTSS. I also tested the ability of flagellar TTSS substrates to be secreted by the Hrp TTSS. The ability of both TTSSs to secrete each other's substrates suggests the conservation of an ancient TTSS secretion signal at the N-termini of both TTSS substrates.