Genomic Mining for Substrates of the Type III Secretion System of Pseudomonas syringae pv. tomato DC3000: New Insights into Mechanisms of Pathogenesis

Abstract

The ability of Pseudomonas syringae pv. tomato DC3000 to be pathogenic on plants is dependent upon the type III secretion system (TTSS) and the effector proteins it translocates into plant cells. A draft sequence of DC3000 is currently being analysed for hop (Hrp outer protein) genes that encode TTSS substrates. Several different approaches were used to mine hop genes from the DC3000 genome. These approaches included ones that were based on linkage to other hop genes, the presence of apparent Hrp promoters, indicators of horizontal transfer, and N-terminal export-associated patterns. Many candidate hop genes were confirmed to encode TTSS-secreted proteins based on secretion and/or translocation assays. These approaches have substantially enlarged the inventory of DC3000 TTSS substrates. Hop proteins include extracellular accessory proteins that aid in translocation and effectors that are injected into plant cells. Several novel Hops have a clear potential to modify plant signal transduction pathways, which is consistent with the predicted roles for TTSS effectors. Activities of several Hops and new insights into bacterial pathogenesis of plants will be discussed.