HRPG, A KEY HRP REGULATORY PROTEIN OF XANTHOMONAS-CAMPESTRIS PV VESICATORIA IS HOMOLOGOUS TO 2-COMPONENT RESPONSE REGULATORS

Xanthomonas campestris pv. vesicatoria is the causal agent of bacteria l spot disease of pepper and tomato plants. Expression of its basic pa thogenicity genes, the hip genes, is induced in planta and in XVM2 med ium and is dependent on the hrp regulatory gene hrpXv for five out of six loci in the 23-kb hrp cluster, Here we describe the isolation of a novel hrp gene, hrpG, that was identified after chemical mutagenesis and that is located next to the hrpXv gene. In a hrpG mutant induction of expression of the seven loci hrpA to hrpF, and hrpXv is abolished, suggesting that hrpG functions at the top of the hip gene regulatory cascade. hrpG is the only gene in the locus and encodes a putative pro tein of 253 amino acids with a molecular mass of 28.9 kDa. The HrpG am ino acid sequence shows similarity to response regulator proteins of t he OmpR subclass of two-component systems, being mostly related to the ChvI proteins of Agrobacterium tumefaciens and Rhizobium spp., and Tc tD of Salmonella typhimurium. Expression of hrpG is low in complex med ium, is increased in XVM2 by a factor of four, and is independent of o ther hrp loci. A model on hrp gene regulation in Xanthomonas campestri s pv. vesicatoria is discussed.