The hrpA and hrpC operons of Erwinia amylovora encode components of a type III pathway that secretes harpin

Abstract

A 6.2-kb region of DNA corresponding to complementation groups II and III of the Erwinia amylovora hrp gene cluster was analyzed. Transposon mutagenesis indicated that the two complementation groups are required for secretion of harpin, an elicitor of the hypersensitive reaction. The sequence of the region revealed 10 open reading frames in two putative transcription units: hrpA, hrpB, hrcJ, hrpD, and hrpE in the hrpA operon (group III) and hrpF, hrpG, hrcC, hrpT, and hrpV in the hrpC operon (group II). From promoter regions of the hrpA, hrpC, and hrpN operons, sequences similar to those of the HrpL-dependent promoters of Pseudomonas syringae pathovars were identified with a consensus sequence of 5'-GGAAC-N17-18-CACTNAA-3'. The protein products of seven genes, hrpA, hrcJ, hrpE, hrpF, hrpG, hrcC, and hrpV, were visualized with a T7 polymerase/promoter expression system. HrcC, HrcJ, and HrpT sequences contained potential signal peptides, and HrcC appeared to be envelope associated based on a TnphoA translational fusion. Comparison of deduced amino acid sequences indicated that many of the proteins are homologous to proteins that function in the type III protein secretion pathway. HrcC is a member of the YscC-containing subgroup in the PulD/pIV superfamily of outer membrane proteins. HrcJ is a member of a lipoprotein family that includes YscJ of Yersinia spp., MxiJ of Shigella flexneri, and NolT of Rhizobim fredii. Additional similarities were detected between HrpB and YscI and between HrpE and YscL. HrcJ and HrpE were similar to flagellar biogenesis proteins FliF and FliH, respectively. In addition, HrpA, HrpB, HrcJ, HrpD, HrpE, HrpF, and HrcC showed various degrees of similarity to corresponding proteins of P. syringae. Comparison of hrp clusters with respect to gene organization and similarity of individual proteins confirms that the hrp systems of E. amylovora and P. syringae are closely related to each other and distinct from those of Ralstonia (Pseudomonas) solanacearum and Xanthomonas campestris. Possible implications of extensive similarities between the E. amylovora and P. syringae hrp systems in pathogenesis mechanisms are discussed.