The Xanthomonas Hrp type III system secretes proteins from plant and mammalian bacterial pathogens

Studies of essential pathogenicity determinants in Gram-negative bacteria h ave revealed the conservation of type III protein secretion systems that al low delivery of virulence factors into host cells from plant and animal pat hogens. Ten of 21 Hrp proteins of the plant pathogen Xanthomonas campestris pv, vesicatoria have been suggested to be part of a type III machinery, He re, we report the hrp-dependent secretion of two avirulence proteins, AvrBs 3 and AvrRxv, by X. campestris pv. vesicatoria strains that constitutively express hrp genes. Secretion occurred without leakage of a cytoplasmic mark er in minimal medium containing BSA, at pH 5.4, Secretion was strictly hrp- dependent because a mutant carrying a deletion in hrcV, a conserved hrp gen e, did not secrete AvrBs3 and AvrRxv. Moreover, the Hrp system of X. campes tris pv, vesicatoria was able to secrete proteins from two other plant path ogens: PopA, a protein secreted via the Hrp system in Ralstonia solanacearu m, and AvrB, an avirulence protein from Pseudomonas syringae pv, glycinea. Interestingly, X, campestris pv, vesicatoria also secreted YopE, a type III -secreted cytotoxin of the mammalian pathogen Yersinia pseudotuberculosis i n a hrp-dependent manner. YerA, a YopE-specific chaperone, was required for YopE stability but not for secretion in X. campestris pv. vesicatoria. Our results demonstrate the functional conservation of the type In system of X . campestris for secretion of proteins from both plant and mammalian pathog ens and imply recognition of their respective secretion signals.