The gene coding for the Hrp pilus structural protein is required for type III secretion of Hrp and Avr proteins in Pseudomonas syringae pv. tomato

Bacterial surface appendages called pili often are associated with DNA and/ or protein transfer between cells. The exact function of pill in the transf er process is not understood and is a matter of considerable debate. The Hr p pilus is assembled by the Hrp type III protein secretion system of Pseudo monas syringae pv. tomato (Pst) strain DC3000. In this study, we show that the hrpA gene, which encodes the major subunit of the Hrp pilus, is require d for secretion of putative virulence proteins, such as HrpW and AvrPto. In addition, the hrpA gene is required for full expression of genes that enco de regulatory, secretion, and effector proteins of the type III secretion s ystem. hrpA-mediated gene regulation apparently is through effect on the mR NA level of two previously characterized regulatory genes, hrpR and hrpS, E ctopic expression of the hrpRS gene operon restored gene expression, but no t protein secretion, in the hrpA mutant. Three single amino acid mutations at the HrpA carboxyl terminus were identified that affect the secretion or regulatory function of the HrpA protein. These results define an essential role of the Hrp pilus structural gene in protein secretion and coordinate r egulation of the type III secretion system in Pst DC3000.