HpaC Controls Substrate Specificity of the Xanthomonas Type III Secretion System

The Gram-negative bacterial plant pathogen Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to inject bacterial effector proteins into the host cell cytoplasm. One essential pathogenicity factor is HrpB2, which is secreted by the T3S system. We show that secretion of HrpB2 is suppressed by HpaC, which was previously identified as a T3S control protein. Since HpaC promotes secretion of translocon and effector proteins but inhibits secretion of HrpB2, HpaC presumably acts as a T3S substrate specificity switch protein. Protein–protein interaction studies revealed that HpaC interacts with HrpB2 and the C-terminal domain of HrcU, a conserved inner membrane component of the T3S system. However, no interaction was observed between HpaC and the full-length HrcU protein. Analysis of HpaC deletion derivatives revealed that the binding site for the C-terminal domain of HrcU is essential for HpaC function. This suggests that HpaC binding to the HrcU C terminus is key for the control of T3S. The C terminus of HrcU also provides a binding site for HrpB2; however, no interaction was observed with other T3S substrates including pilus, translocon and effector proteins. This is in contrast to HrcU homologs from animal pathogenic bacteria suggesting evolution of distinct mechanisms in plant and animal pathogenic bacteria for T3S substrate recognition

Caractérisation de la protéine effectrice AvrBs3 de xanthomonas campestris pv vesicatoria : Injection dans la cellule végétale et localisation nucléaire. Recherche des protéines de piment cibles

PARIS-AgroParisTech Centre Paris (751052302) / SudocSudocFranceF

Mutations in the Regulatory Gene hrpG of Xanthomonas campestris pv. vesicatoria Result in Constitutive Expression of All hrp Genes

hrpG is a key regulatory gene for transcriptional activation of pathogenicity genes (hrp) of Xanthomonas campestris pv. vesicatoria. We identified three mutations in hrpG which render hrp gene expression constitutive in normally suppressing medium. The mutations in hrpG result in novel amino acid substitutions compared to mutations in related proteins, such as OmpR. In addition, mutated hrpG enhances the timing and intensity of plant reactions in infection assays

Two Novel Type III-Secreted Proteins of Xanthomonas campestris pv. vesicatoria Are Encoded within the hrp Pathogenicity Island

The Hrp type III protein secretion system (TTSS) is essential for pathogenicity of gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria. cDNA-amplified fragment length polymorphism and reverse transcription-PCR analyses identified new genes, regulated by key hrp regulator HrpG, in the regions flanking the hrp gene cluster. Sequence analysis revealed genes encoding HpaG, a predicted leucine-rich repeat-containing protein, the lysozyme-like HpaH protein, and XopA and XopD, which are similar in sequence to Hpa1 from Xanthomonas oryzae pv. oryzae and PsvA from Pseudomonas syringae, respectively. XopA and XopD (Xanthomonas outer proteins) are secreted by the Xanthomonas Hrp TTSS and thus represent putative effector proteins. Mutations in xopA, but not in xopD, resulted in reduced bacterial growth in planta and delayed plant reactions in susceptible and resistant host plants. Since the xopD promoter contains a putative hrp box, which is characteristic of hrpL-regulated genes in P. syringae and Erwinia spp., the gene was probably acquired by horizontal gene transfer. Interestingly, the regions flanking the hrp gene cluster also contain insertion sequences and genes for a putative transposase and a tRNA(Arg). These features suggest that the hrp gene cluster of X. campestris pv. vesicatoria is part of a pathogenicity island

Functional Analysis of HrpF, a Putative Type III Translocon Protein from Xanthomonas campestris pv. vesicatoria

Type III secretion systems (TTSSs) are specialized protein transport systems in gram-negative bacteria which target effector proteins into the host cell. The TTSS of the plant pathogen Xanthomonas campestris pv. vesicatoria, encoded by the hrp (hypersensitive reaction and pathogenicity) gene cluster, is essential for the interaction with the plant. One of the secreted proteins is HrpF, which is required for pathogenicity but dispensable for type III secretion of effector proteins in vitro, suggesting a role in translocation. In this study, complementation analyses of an hrpF null mutant strain using various deletion derivatives revealed the functional importance of the C-terminal hydrophobic protein region. Deletion of the N terminus abolished type III secretion of HrpF. Employing the type III effector AvrBs3 as a reporter, we show that the N terminus of HrpF contains a signal for secretion but not a functional translocation signal. Experiments with lipid bilayers revealed a lipid-binding activity of HrpF as well as HrpF-dependent pore formation. These data indicate that HrpF presumably plays a role at the bacterial-plant interface as part of a bacterial translocon which mediates effector protein delivery across the host cell membrane

XopC and XopJ, Two Novel Type III Effector Proteins from Xanthomonas campestris pv. vesicatoria

Pathogenicity of the gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria depends on a type III secretion (TTS) system which translocates bacterial effector proteins into the plant cell. Previous transcriptome analysis identified a genome-wide regulon of putative virulence genes that are coexpressed with the TTS system. In this study, we characterized two of these genes, xopC and xopJ. Both genes encode Xanthomonas outer proteins (Xops) that were shown to be secreted by the TTS system. In addition, type III-dependent translocation of both proteins into the plant cell was demonstrated using the AvrBs3 effector domain as a reporter. XopJ belongs to the AvrRxv/YopJ family of effector proteins from plant and animal pathogenic bacteria. By contrast, XopC does not share significant homology to proteins in the database. Sequence analysis revealed that the xopC locus contains several features that are reminiscent of pathogenicity islands. Interestingly, the xopC region is flanked by 62-bp inverted repeats that are also associated with members of the Xanthomonas avrBs3 effector family. Besides xopC, a second gene of the locus, designated hpaJ, was shown to be coexpressed with the TTS system. hpaJ encodes a protein with similarity to transglycosylases and to the Pseudomonas syringae pv. maculicola protein HopPmaG. HpaJ secretion and translocation by the X. campestris pv. vesicatoria TTS system was not detectable, which is consistent with its predicted Sec signal and a putative function as transglycosylase in the bacterial periplasm