Altered localization of HrpZ in Pseudomonas syringae pv. syringae hrp mutants suggests that different components of the type III secretion pathway control protein translocation across the inner and outer membranes of gram-negative bacteria.

Pseudomonas syringae pv. syringae 61 (Pss61) secretes the HrpZ harpin by a type III protein secretion pathway encoded by a cluster of hrp (hypersensitive response and pathogenicity) and hrc genes. The nine hrc genes represent a subset of hrp genes that are also conserved in the type III virulence protein secretion systems of animal pathogenic Yersinia, Shigella, and Salmonella spp. The hrpJ and hrpU operons contain seven hrc genes (counting hrcQ(A) and hrcQ(B) as one gene), all with additional homologs involved in flagellar biogenesis and secretion, and five of which encode predicted inner membrane proteins. The hrpC and hrpZ operons encode HrcC and HrcJ, respectively, which are associated with the outer membrane. Interposon mutants affected in all of the hrc genes in the hrpJ and hrpU operons and TnphoA-induced hrcC and hrcJ mutants were assayed for altered localization of HrpZ in mid-log-phase cultures by immunoblotting sodium dodecyl sulfate-polyacrylamide gels that were run with various cell fractions. The hrpJ and hrpU operon mutants revealed a novel phenotype of partially reduced accumulation of HrpZ in the total culture (despite wild-type levels of hrpZ operon transcription), all of which was cell bound and equivalent in level to that of cell-bound HrpZ in the wild type. The hrcC and hrcJ mutant cultures accumulated the same total amount of HrpZ as the wild type, but the HrpZ was cell bound. Among all the strains tested, only the hrcC mutant accumulated significant amounts of HrpZ in the periplasm, as indicated by selective release through spheroplasting. Analysis of nonpolar mutations in the hrpU and hrpC operons support the results obtained with polar mutations. These observations indicate that a constant pool of HrpZ is maintained in the cytoplasm of Pss61 despite secretion deficiencies, that the hrpJ and hrpU operons encode an alternative to the Sec (general protein export) pathway for translocation across the inner membrane, that genes in the hrpC operon are necessary for translocation across the outer membrane, and that the Pss61 Hrp system permits study of two genetically distinguishable stages in type III protein secretion.