HrpXv, an AraC-type regulator, activates expression of five of the six loci in the hrp cluster of Xanthomonas campestris pv vesicatoria

hrp genes, basic pathogenicity genes of the pepper and tomato pathogen Xanthomonas campestris pv, vesicatoria, are regulated dependent on environmental conditions. We isolated the hrp.Xv gene, which was found to be outside the large hrp cluster comprising the six loci hrpA to hrpF. The predicited HrpXv protein is 476 amino acids long and has a molecular mass of 52.5 kDa. HrpX is highly conserved among xanthomonads and is a member of the AraC family of regulatory proteins. An hrpXv insertion mutant has a typical hrp phenotype and no longer allows induction of the five hrp loci hrpB to hrpF in the new hrp induction medium XVM2, indicating that HrpXv is the positive regulator of these loci. An hrpXv mutant could be partially complemented by the related hrpB gene of Burkholderia solanacearum, the protein product of which shows 40 and 58% amino acid identify and similarity, respectively, to HrpXv. The hrpXr gene itself has a low basal level of expression that is enhanced in XVM2. Expression of hrpXv as well as that of the hrpA locus is independent of the hrpXv gene. The transcription start site of hrpXv was mapped. Comparison between the hrpXv promoter and the corresponding region of the hrpXc gene from X. campestris pv. campestris revealed sequence conservation up to position -84. A putative helix-turn-helix motif in the C-terminal region of HrpXv and its possible interaction with a conserved hrp promoter element, the plant-inducible promoter box are discussed.