Pathogenicity of the rice pathogenic bacterium Xanthomonas oryzae pv. oryzae depends on a Hrp (hypersensitive response and pathogenicity) type III secretion system; the expression of which is induced in planta. Expression of the hrp operons is under transcriptional control of two key regulatory proteins, HrpG and HrpX. To identify new proteins that are co-regulated with the type III secretion system, we employed comparative proteomics. Cells of X. oryzae pv. oryzae ectopically expressing hrpX were compared to wild-type cells grown in vitro. Twenty protein spots with different abundances in both samples were identified by 2D-DIGE and LC-MS/MS. Seven spots could be unambiguously identified, corresponding to the HrpB1 protein, two different peptidyl-prolyl cis-trans isomerases, a component of an ATP binding cassette (ABC) transport system, an adenylate kinase, and a secreted protein of unknown function. Interestingly, the isoelectric point of the adenylate kinase was found to be under control of HrpX, most likely due to post-translational modification. Indeed, two glutamate residues of the adenylate kinase were found to be methylated but this modification did not account for the shift in electrophoretic mobility. In summary, we identified new HrpX-regulated proteins of X. oryzae pv. oryzae that might be important for pathogenicity. This article is part of a Special Issue entitled: Trends in microbial proteomics.
BIOLOGICAL SIGNIFICANCE: We use 2D-DIGE to compare the proteomes of rice-pathogenic xanthomonads. We identify seven proteins that are co-regulated with the type III secretion system. We find post-translational glutamate methylation of a bacterial adenylate cyclase. The newly identified HrpX-regulated proteins might be important for pathogenicity.
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