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 cluster in X. campestris pv. vesicatoria spans a 23-kb chromosomal region, comprising 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 hydrophobic domains in the C terminus. An interesting feature is the presence of two imperfect direct repeats in the N-terminal region. Deletion studies showed that one repeat is sufficient for function. Epitope tagging of HrpF allowed detection of the protein in X. campestris pv. vesicatoria. Subcellular localization studies suggest that HrpF is both in the soluble fraction and in the inner membrane. Interestingly, HrpF is 48% identical (67% similar) to the Rhizobium fredii NoIX protein that is part of the host specificity locus. Since several Hrp proteins are believed to be components of the types of III hrp protein secretion apparatus, allowing export of proteins essential for the interaction with the plant, the possible role of hrpF and NoIX in secretion is discussed.