HPAA MUTANTS OF XANTHOMONAS-CAMPESTRIS PV. VESICATORIA ARE AFFECTED IN PATHOGENICITY BUT RETAIN THE ABILITY TO INDUCE HOST-SPECIFIC HYPERSENSITIVE REACTION

Xanthomonas campestris pv. vesicatoria is the causal agent of bacteria l spot disease on pepper and tomato plants, We reported previously tha t the main hrp(hypersensitive reaction and pathogenicity) gene cluster in X. c. pv, vesicatoria contains six transcription units, designated hrpA to hrpF. We present here the sequence of the hrpD operon and an analysis of non-polar mutants in each of the six genes. Three genes, h rcQ, hrcR and hrcS, are predicted to encode conserved components of ty pe III protein secretion systems in plant and mammalian pathogenic bac teria. For hrpD5 and hrpD6, homologues have only been found in Ralston ia solanacearum. Interestingly, the hrpD operon contains one gene, hpa A (for hrp-associated), which is specifically required for disease dev elopment. hpaA mutants are affected in pathogenicity, but retain in pa rt the ability to induce avirulence gene-mediated, host-specific hyper sensitive reaction (HR). In addition, HpaA was found to contain two fu nctional nuclear localization signals, which are important for the int eraction with the plant. We propose that HpaA is an effector protein t hat may be translocated into the host cell via the Hrp secretion pathw ay,