Characterization of the hrpJ and hrpU Operons of Pseudomonas syringae pv. syringae Pss61: Similarity with Components of Enteric Bacteria Involved in Flagellar Biogenesis and Demonstration of Their Role in Harpin_Pss Secretion. Michael C. Lidell. Department of Botany, University of Maryland, College Park, MD 20742 U.S.A. Steven W. Hutcheson. Department of Botany, University of Maryland, College Park, MD 20742 U.S.A. MPMI 7:488-497. Accepted 25 March 1994. Copyright 1994 The American Phytopathological Society.

The hrp/hrmA gene cluster of Pseudomonas syringae pv. syringae Pss61 has been shown to form a minimum genetic unit sufficient to enable nonpathogenic bacteria, such as Escherichia coli, to elicit the hypersensitive response associated with disease resistance. The biochemical functions of most of these genes have not been established. The nucleotide sequence of a 4.3-kb Sstl-Bglll fragment carrying hrp apparent translational units V, VI, and VII revealed one partial open reading frame (ORF) and five complete ORFs producing 35,126-, 48,866-, 17,308-, 20,482-, and 26,364-Da gene products (hrp,]3, J4, J5, Ul, 112, respectively). The production of these proteins was confirmed by using T7 RNA polymcrase-directed expression. The partial ORF was found to be identical to the C terminus of HrpJ2. The absence of apparent transcriptional terminators and promoters between hrpl (hrpJ2), hrp,J3, hrp,J4, and hrp,]5 together with the observation that the HrpL-dependent hrp,] promoter directs expression of hrp,]3-J5 indicates that these genes form a single operon controlled by the HrpL-dependent hrpJ promoter. A second HrpL-dependent promoter consensus sequence was also identified upstream of hrpUl and demonstrated to function as a HrpL-dependent promoter, thus indicating that hrpU1, hrpU2, and additional downstream genes may be part of a second operon. The deduced product of hrpJ3 exhibits similarity to FUG of Salmonella typhimurium, a cytoplas-mic protein that regulates flagellar rotation and biogenesis. HrpJ4 shares extensive similarity with the Flil family of ATPase-like proteins and retains the known functional domains conserved among this family of proteins. HrpJ5 has properties similar to the S. typhimurium FliJ. Neither HrpUl nor HrpU2 exhibit significant similarity to known proteins. Secretion of HarpinPss by E. coli MC4100 transformants carrying pHIRll::TnphoA derivatives was blocked in hrp,]4, J5, and U2 mutants. In view of the previously reported similarity of HrpJ2 to the LcrD super-family that includes FlhA, these results predict that the gene products of the hrpJ and hrpV operons form an inner membrane complex for translocation of proteins similar to that used by the flagellar biogenesis system of S. typhimurium.