Characterization of the nonconserved hpaB-hrpF region in the hrp pathogenicity island from Xanthomonas campestris pv. vesicatoria.

The interaction of the gram-negative phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria with its host plants pepper and tomato is mediated by a type III secretion (T3S) system that translocates bacterial effector proteins into the plant cell. The T3S system is encoded by the chromosomal hrp (hypersensitive response and pathogenicity) gene cluster. Here, we report on the analysis of the hpaB-hrpF region, which encodes the novel virulence factor HpaE, the effector protein XopF1, and two proteins with unknown functions, HpaD and HpaI. Promoter and transcript analyses revealed that the corresponding genes are coexpressed with the hrp genes and that hpaD, hpaI, and xopF1 form a novel operon. In vitro and in vivo assays showed that the efficient T3S and translocation of XopF1 depends on the global T3S chaperone HpaB and the putative lytic transglycosylase HpaH, which specifically contributes to the secretion of a certain set of effectors. Taken together, our data suggest that the efficient secretion of effector proteins in X. campestris pv. vesicatoria requires the contribution of several different Hpa proteins.