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The Hrp type III protein secretion system is essential for pathogenicity of the Gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria. Expression of the hrp gene cluster is controlled by HrpG, a two-component response regulator, and HrpX, an AraC-type transcriptional activator. Using the cDNA-AFLP technique, 30 hrpG-induced (hgi) and five hrpG-repressed (hgr) cDNA fragments were identified, defining a large hrpG-regulon in X. campestris pv. vesicatoria. Expression of most genes in the hrpG-regulon was dependent on hrpX. Seven cDNA fragments map to the known hrp gene cluster and flanking regions. All other genes appear to be scattered over the chromosome and endogenous plasmids. Sequence analysis identified genes encoding putative extracellular proteases, a putative transcriptional regulator and XopJ and XopB (Xanthomonas outer proteins), homologues of YopJ from Yersinia spp. and the avirulence protein AvrPphD of Pseudomonas syringae respectively. XopB is secreted by the Hrp type III secretion system. Analysis of deletion mutants in several hgi genes revealed a new virulence locus. This study demonstrates that cDNA-AFLP is a powerful tool to study prokaryotic transcriptomes and to identify genes contributing to Xanthomonas virulence and putative effector proteins.