HRPF OF XANTHOMONAS-CAMPESTRIS PV VESICATORIA ENCODES AN 87-KDA PROTEIN WITH HOMOLOGY TO NOLX OF RHIZOBIUM-FREDII

The gram-negative bacterium Xanthomonas campestris pv. vesicatoria is the causal agent of bacterial spot disease on pepper and tomato plants , The main hrp (hypersensitive reaction and pathogenicity) gene cluste r in X. campestris pv, vesicatoria spans a 23-kb chromosomal region, c omprising six complementation groups designated hrpA to hrpF. Analysis of the hrpF locus revealed a single open reading frame encoding HrpF (86.4 kDa). HrpF is predominantly hydrophilic, and contains two hydrop hobic domains in the C terminus, An interesting feature is the presenc e of two imperfect direct repeats in the N-terminal region, Deletion s tudies showed that one repeat is sufficient for function. Epitope tagg ing of HrpF allowed detection of the protein in X. campestris pv. vesi catoria. Subcellular localization studies suggest that HrpF is both in the soluble fraction and in the inner membrane. Interestingly, HrpF i s 48% identical (67% similar) to the Rhizobium fredii NolX protein tha t is part of the host specificity lotus. Since several Hrp proteins ar e believed to be components of the type III Hrp protein secretion appa ratus, allowing export of proteins essential for the interaction with the plant, the possible role of hrpF and nolX in secretion is discusse d.