A Secreted Chorismate Mutase from Xanthomonas arboricola pv. juglandis Attenuates Virulence and Walnut Blight Symptoms

Walnut blight is a significant above-ground disease of walnuts caused by Xanthomonas arboricola pv. juglandis (Xaj). The secreted form of chorismate mutase (CM), a key enzyme of the shikimate pathway regulating plant immunity, is highly conserved between plant-associated beta and gamma proteobacteria including phytopathogens belonging to the Xanthomonadaceae family. To define its role in walnut blight disease, a dysfunctional mutant of chorismate mutase was created in a copper resistant strain Xaj417 (XajCM). Infections of immature walnut Juglans regia (Jr) fruit with XajCM were hypervirulent compared with infections with the wildtype Xaj417 strain. The in vitro growth rate, size and cellular morphology were similar between the wild-type and XajCM mutant strains, however the quantification of bacterial cells by dPCR within walnut hull tissues showed a 27% increase in XajCM seven days post-infection. To define the mechanism of hypervirulence, proteome analysis was conducted to compare walnut hull tissues inoculated with the wild type to those inoculated with the XajCM mutant strain. Proteome analysis revealed 3296 Jr proteins (five decreased and ten increased with FDR ≤ 0.05) and 676 Xaj417 proteins (235 increased in XajCM with FDR ≤ 0.05). Interestingly, the most abundant protein in Xaj was a polygalacturonase, while in Jr it was a polygalacturonase inhibitor. These results suggest that this secreted chorismate mutase may be an important virulence suppressor gene that regulates Xaj417 virulence response, allowing for improved bacterial survival in the plant tissues.

The flavin monooxygenase Bs3 triggers cell death in plants, impairs growth in yeast and produces H2O2 in vitro

The pepper resistance gene Bs3 triggers a hypersensitive response (HR) upon transcriptional activation by the corresponding effector protein AvrBs3 from the bacterial pathogen Xanthomonas. Expression of Bs3 in yeast inhibited proliferation, demonstrating that Bs3 function is not restricted to the plant kingdom. The Bs3 sequence shows striking similarity to flavin monooxygenases (FMOs), an FAD- and NADPH-containing enzyme class that is known for the oxygenation of a wide range of substrates and their potential to produce H2O2. Since H2O2 is a hallmark metabolite in plant immunity, we analyzed the role of H2O2 during Bs3 HR. We purified recombinant Bs3 protein from E. coli and confirmed the FMO function of Bs3 with FAD binding and NADPH oxidase activity in vitro. Translational fusion of Bs3 to the redox reporter roGFP2 indicated that the Bs3-dependent HR induces an increase of the intracellular oxidation state in planta. To test if the NADPH oxidation and putative H2O2 production of Bs3 is sufficient to induce HR, we adapted previous studies which have uncovered mutations in the NADPH binding site of FMOs that result in higher NADPH oxidase activity. In vitro studies demonstrated that recombinant Bs3S211A protein has twofold higher NADPH oxidase activity than wildtype Bs3. Translational fusions to roGFP2 showed that Bs3S211A also increased the intracellular oxidation state in planta. Interestingly, while the mutant derivative Bs3S211A had an increase in NADPH oxidase capacity, it did not trigger HR in planta, ultimately revealing that H2O2 produced by Bs3 on its own is not sufficient to trigger HR.

A Glycoside Hydrolase Family 99-Like Domain-Containing Protein Modifies Outer Membrane Proteins to Maintain Xanthomonas Pathogenicity and Viability in Stressful Environments

Protein glycosylation is an essential process that plays an important role in proteome stability, protein structure, and protein function modulation in eukaryotes. However, in bacteria, especially plant pathogenic bacteria, similar studies are lacking. Here, we investigated the relationship between protein glycosylation and pathogenicity by using Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight in rice, as a well-defined example. In our previous work, we identified a virulence-related hypothetical protein, PXO_03177, but how this protein regulates X. oryzae pv. oryzae virulence has remained unclear. BLAST analysis showed that most homologous proteins of PXO_03177 are glycoside hydrolase family 99-like domain-containing proteins. In the current study, we found that the outer membrane integrity of ΔPXO_03177 appeared to be disrupted. Extracting the outer membrane proteins (OMPs) and performing comparative proteomics and sodium dodecyl sulphate-polyacrylamide gel electrophoresis gel staining analyses revealed that PXO_03177 deletion altered the protein levels of 13 OMPs. Western blot analyses showed that the protein level and glycosylation modification of PXO_02523, a related OmpA family-like protein, was changed in the ΔPXO_03177 mutant background strain. Additionally, the interaction between PXO_03177 and PXO_02523 was confirmed by coimmunoprecipitation. Both PXO_03177 and PXO_02523 play important roles in regulating pathogen virulence and viability in stressful environments. This work provides the first evidence that protein glycosylation is necessary for the virulence of plant pathogenic bacteria.

Two distinct catalytic pathways for GH43 xylanolytic enzymes unveiled by X-ray and QM/MM simulations

Xylanolytic enzymes from glycoside hydrolase family 43 (GH43) are involved in the breakdown of hemicellulose, the second most abundant carbohydrate in plants. Here, we kinetically and mechanistically describe the non-reducing-end xylose-releasing exo-oligoxylanase activity and report the crystal structure of a native GH43 Michaelis complex with its substrate prior to hydrolysis. Two distinct calcium-stabilized conformations of the active site xylosyl unit are found, suggesting two alternative catalytic routes. These results are confirmed by QM/MM simulations that unveil the complete hydrolysis mechanism and identify two possible reaction pathways, involving different transition state conformations for the cleavage of xylooligosaccharides. Such catalytic conformational promiscuity in glycosidases is related to the open architecture of the active site and thus might be extended to other exo-acting enzymes. These findings expand the current general model of catalytic mechanism of glycosidases, a main reaction in nature, and impact on our understanding about their interaction with substrates and inhibitors.

Periplasm-enriched fractions from Xanthomonas citri subsp. citri type A and X. fuscans subsp. aurantifolii type B present distinct proteomic profiles under in vitro pathogenicity induction

The causative agent of Asiatic citrus canker, the Gram-negative bacterium Xanthomonas citri subsp. citri (XAC), produces more severe symptoms and attacks a larger number of citric hosts than Xanthomonas fuscans subsp. aurantifolii XauB and XauC, the causative agents of cancrosis, a milder form of the disease. Here we report a comparative proteomic analysis of periplasmic-enriched fractions of XAC and XauB in XAM-M, a pathogenicity- inducing culture medium, for identification of differential proteins. Proteins were resolved by two-dimensional electrophoresis combined with liquid chromatography-mass spectrometry. Among the 12 proteins identified from the 4 unique spots from XAC in XAM-M (p<0.05) were phosphoglucomutase (PGM), enolase, xylose isomerase (XI), transglycosylase, NAD(P)H-dependent glycerol 3-phosphate dehydrogenase, succinyl-CoA synthetase β subunit, 6-phosphogluconate dehydrogenase, and conserved hypothetical proteins XAC0901 and XAC0223; most of them were not detected as differential for XAC when both bacteria were grown in NB medium, a pathogenicity non-inducing medium. XauB showed a very different profile from XAC in XAM-M, presenting 29 unique spots containing proteins related to a great diversity of metabolic pathways. Preponderant expression of PGM and XI in XAC was validated by Western Blot analysis in the periplasmic-enriched fractions of both bacteria. This work shows remarkable differences between the periplasmic-enriched proteomes of XAC and XauB, bacteria that cause symptoms with distinct degrees of severity during citrus infection. The results suggest that some proteins identified in XAC can have an important role in XAC pathogenicity.

Structural and biochemical characterization of a glutathione transferase from the citrus canker pathogen Xanthomonas

The genus Xanthomonas comprises several cosmopolitan plant-pathogenic bacteria that affect more than 400 plant species, most of which are of economic interest. Citrus canker is a bacterial disease that affects citrus species, reducing fruit yield and quality, and is caused by the bacterium Xanthomonas citri subsp. citri (Xac). The Xac3819 gene, which has previously been reported to be important for citrus canker infection, encodes an uncharacterized glutathione S-transferase (GST) of 207 amino-acid residues in length (XacGST). Bacterial GSTs are implicated in a variety of metabolic processes such as protection against chemical and oxidative stresses. XacGST shares high sequence identity (45%) with the GstB dehalogenase from Escherichia coli O6:H1 strain CFT073 (EcGstB). Here, XacGST is reported to be able to conjugate glutathione (GSH) with bromoacetate with a Km of 6.67 ± 0.77 mM, a kcat of 42.69 ± 0.32 s-1 and a kcat/Km of 6.40 ± 0.72 mM-1 s-1 under a saturated GSH concentration (3.6 mM). These values are comparable to those previously reported for EcGstB. In addition, crystal structures of XacGST were determined in the apo form (PDB entry 6nxv) and in a GSH-bound complex (PDB entry 6nv6). XacGST has a canonical GST-like fold with a conserved serine residue (Ser12) at the GSH-binding site near the N-terminus, indicating XacGST to be a serine-type GST that probably belongs to the theta-class GSTs. GSH binding stabilizes a loop of about 20 residues containing a helix that is disordered in the apo XacGST structure.

Overexpression of OsPUB41, a Rice E3 ubiquitin ligase induced by cell wall degrading enzymes, enhances immune responses in Rice and Arabidopsis

BACKGROUND

Cell wall degrading enzymes (CWDEs) induce plant immune responses and E3 ubiquitin ligases are known to play important roles in regulating plant defenses. Expression of the rice E3 ubiquitin ligase, OsPUB41, is enhanced upon treatment of leaves with Xanthomonas oryzae pv. oryzae (Xoo) secreted CWDEs such as Cellulase and Lipase/Esterase. However, it is not reported to have a role in elicitation of immune responses.

RESULTS

Expression of the rice E3 ubiquitin ligase, OsPUB41, is induced when rice leaves are treated with either CWDEs, pathogen associated molecular patterns (PAMPs), damage associated molecular patterns (DAMPs) or pathogens. Overexpression of OsPUB41 leads to induction of callose deposition, enhanced tolerance to Xoo and Rhizoctonia solani infection in rice and Arabidopsis respectively. In rice, transient overexpression of OsPUB41 leads to enhanced expression of PR genes and SA as well as JA biosynthetic and response genes. However, in Arabidopsis, ectopic expression of OsPUB41 results in upregulation of only JA biosynthetic and response genes. Transient overexpression of either of the two biochemically inactive mutants (OsPUB41C40A and OsPUB41V51R) of OsPUB41 in rice and stable transgenics in Arabidopsis ectopically expressing OsPUB41C40A failed to elicit immune responses. This indicates that the E3 ligase activity of OsPUB41 protein is essential for induction of plant defense responses.

CONCLUSION

The results presented here suggest that OsPUB41 is possibly involved in elicitation of CWDE triggered immune responses in rice.

The Role of a Host-Induced Arginase of Xanthomonas oryzae pv. oryzae in Promoting Virulence on Rice

The plant bacterial pathogen Xanthomonas oryzae pv. oryzae causes bacterial blight of rice, which is one of the most destructive rice diseases prevalent in Asia and parts of Africa. Despite many years of research, how X. oryzae pv. oryzae causes bacterial blight of rice is still not completely understood. Here, we show that the loss of the rocF gene caused a significant decrease in the virulence of X. oryzae pv. oryzae in the susceptible rice cultivar IR24. Bioinformatics analysis demonstrated that rocF encodes arginase. Quantitative real-time PCR and Western blot assays revealed that rocF expression was significantly induced by rice and arginine. The rocF deletion mutant strain showed elevated sensitivity to hydrogen peroxide, reduced extracellular polysaccharide (EPS) production, and reduced biofilm formation, all of which are important determinants for the full virulence of X. oryzae pv. oryzae, compared with the wild-type strain. Taken together, the results of this study revealed a mechanism by which a bacterial arginase is required for the full virulence of X. oryzae pv. oryzae on rice because of its contribution to tolerance to reactive oxygen species, EPS production, and biofilm formation.

Crystal Structure-Based Exploration of Arginine-Containing Peptide Binding in the ADP-Ribosyltransferase Domain of the Type III Effector XopAI Protein

Plant pathogens secrete proteins called effectors into the cells of their host to modulate the host immune response against colonization. Effectors can either modify or arrest host target proteins to sabotage the signaling pathway, and therefore are considered potential drug targets for crop disease control. In earlier research, the Xanthomonas type III effector XopAI was predicted to be a member of the arginine-specific mono-ADP-ribosyltransferase family. However, the crystal structure of XopAI revealed an altered active site that is unsuitable to bind the cofactor NAD+, but with the capability to capture an arginine-containing peptide from XopAI itself. The arginine peptide consists of residues 60 through 69 of XopAI, and residue 62 (R62) is key to determining the protein–peptide interaction. The crystal structure and the molecular dynamics simulation results indicate that specific arginine recognition is mediated by hydrogen bonds provided by the backbone oxygen atoms from residues W154, T155, and T156, and a salt bridge provided by the E265 sidechain. In addition, a protruding loop of XopAI adopts dynamic conformations in response to arginine peptide binding and is probably involved in target protein recognition. These data suggest that XopAI binds to its target protein by the peptide-binding ability, and therefore, it promotes disease progression. Our findings reveal an unexpected and intriguing function of XopAI and pave the way for further investigation on the role of XopAI in pathogen invasion.

Sulfone-Based Probes Unraveled Dihydrolipoamide S-Succinyltransferase as an Unprecedented Target in Phytopathogens

Target validation of current drugs remains the major challenge for target-based drug discovery, especially for agrochemical discovery. The bactericide 0 represents a novel lead structure and has shown potent efficacy against those diseases that are extremely difficult to control, such as rice bacterial leaf blight. However, no detailed target analysis of this bactericide has been reported. Here, we developed a panel of 0-derived probes 1-6, in which a conservative modification (alkyne tag) was introduced to keep the antibacterial activity of 0 and provide functionality for target identification via click chemistry. With these cell-permeable probes, we were able to discover dihydrolipoamide S-succinyltransferase (DLST) as an unprecedented target in living cells. The probes showed good preference for DLST, especially probe 1, which demonstrated distinct selectivity and reactivity. Also, we reported 0 as the first covalent DLST inhibitor, which has been used to confirm the involvement of DLST in the regulation of energy production.

Xanthomonas oryzae pv. oryzae Response Regulator TriP Regulates Virulence and Exopolysaccharide Production Via Interacting With c-di-GMP Phosphodiesterase PdeR

PdeR, a response regulator of the two-component system (TCS) with the cognate histidine kinase PdeK, has been shown to be an active phosphodiesterase (PDE) for intracellular cyclic dimeric guanosine monophosphate (c-di-GMP) turnover and positively regulates the virulence of Xanthomonas oryzae pv. oryzae, the causal pathogen of bacterial blight of rice. To further reveal the key components and pathways involved in the PdeR-mediated c-di-GMP regulation of virulence, 16 PdeR-interacting proteins were identified, using the yeast two-hybrid (Y2H) assay. Among them, PXO_04421 (named as TriP, a putative transcriptional regulator interacting with PdeR) was verified via Y2H and glutathione-S-transferase pull-down assays, and its regulatory functions in bacterial virulence and exopolysaccharide (EPS) production were assessed by biochemical and genetic analysis. The REC domain of TriP specifically interacted with the EAL domain of PdeR. TriP promoted the PDE activity of PdeR to degrade c-di-GMP in the presence of PdeK. In-frame deletion in triP abolished the polar localization of PdeR in the cell. Notably, the ∆triP mutant showed significantly reduced virulence on susceptible rice leaves and impaired EPS production compared with wild type, whereas the double mutant ∆triP∆pdeR, like ∆pdeR, caused shorter lesion lengths and produced less EPS than ∆triP. In addition, cross-complementation showed in trans expression of pdeR in ∆triP restored its EPS production to near wild-type levels but not vice versa. Taken together, our results suggest that TriP is a novel regulator that is epistatic to PdeR in positively regulating virulence expression in X. oryzae pv. oryzae.

Crucial role of oxidative stress in bactericidal effect of parthenolide against Xanthomonas oryzae pv. oryzae

BACKGROUND

Xanthomonas oryzae pv. oryzae (Xoo) causes rice bacterial blight, which is one of the most devastating diseases on rice. Parthenolide (PTL) is a sesquiterpene lactone possessing multiple bioactivities. In the preliminary study, we found PTL can totally inhibit the growth of Xoo at 10 mg L^-1 in vitro. In this study, we aim to further evaluate the anti-bacterial activity of PTL against Xoo and discern the role of oxidative stress in its bactericidal effect.

RESULTS

PTL was effective against Xoo both in vitro and in vivo. PTL induced reactive oxygen species (ROS) accumulation in Xoo, leading to cell death, while exogenous catalase can fully abolish its bactericidal effect. PTL sensitivity of catalase deletion mutants of Xoo increased significantly compared with that of wild-type Xoo strain. In addition, PTL treatment increased glutathione peroxidase activity and decreased glutathione (GSH) reductase activity in Xoo, but had no effect on its catalase and superoxide dismutase activities. Interestingly, PTL dramatically reduced the GSH level in Xoo, resulting in disturbed GSH/GSSG balance. Moreover, PTL rapidly reacted with GSH by a nucleophilic addition reaction.

CONCLUSION

PTL is a promising lead compound for developing bactericide against Xoo. PTL rapidly reacts with GSH, resulting in disturbed GSH/GSSG balance in Xoo, which causes ROS accumulation, leading to cell death. Oxidative stress plays a critical role in the bactericidal effect of PTL against Xoo. © 2018 Society of Chemical Industry.

Characterization and Thermal Denaturation Kinetic Analysis of Recombinant l-Amino Acid Ester Hydrolase from Stenotrophomonas maltophilia

Stenotrophomonas maltophilia HS1 exhibits l-amino acid ester hydrolase (SmAEH) activity, which can synthesize dipeptides such as Ile-Trp, Val-Gly, and Trp-His from the corresponding amino acid methyl esters and amino acids. The gene encoding SmAEH was cloned and expressed in Escherichia coli and was purified and characterized. SmAEH shared 77% sequence identity with a known amino acid ester hydrolase (AEH) from Xanthomonas citri, which belongs to a class of β-lactam antibiotic acylases. The thermal stability of SmAEH was evaluated using various mathematical models to assess its industrial potential. First-order kinetics provided the best description for the inactivation of the enzyme over a temperature range of 35-50 °C. Decimal reduction time ranged from 212.76 to 3.44 min, with a z value of 8.06 °C, and the deactivation energy was 204.1 kJ mol^-1.

Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi

The rare branched-chain sugar apiose, once thought to only be present in the plant kingdom, was found in two bacterial species: Geminicoccus roseus and Xanthomonas pisi. Glycans with apiose residues were detected in aqueous methanol-soluble fractions as well as in the insoluble pellet fraction of X. pisi. Genes encoding bacterial uridine diphosphate apiose (UDP-apiose) synthases (bUASs) were characterized in these bacterial species, but the enzyme(s) involved in the incorporation of the apiose into glycans remained unknown. In the X. pisi genome two genes flanking the XpUAS were annotated as hypothetical glycosyltransferase (GT) proteins. The first GT (here on named XpApiT) belongs to GT family 90 and has a Leloir type B fold and a putative lipopolysaccharide-modifying (LPS) domain. The second GT (here on XpXylT) belongs to GT family 2 and has a type A fold. The XpXylT and XpApiT genes were cloned and heterologously expressed in E. coli. Analysis of nucleotide sugar extracts from E. coli expressing XpXylT or XpApiT with UAS showed that recombinant XpApiT utilized UDP-apiose and XpXylT utilized UDP-xylose as substrate. Indirect activity assay (UDP-Glo) revealed that XpApiT is an apiosyltransferase (ApiT) able to specifically use UDP-apiose. Further support for the apiosyltransferase activity was demonstrated by in microbe co-expression of UAS and XpApiT in E. coli showing the utilization of UDP-apiose to generate an apioside detectable in the pellet fraction. This work provides evidence that X. pisi developed the ability to synthesize an apioside of indeterminate function; however, the evolution of the bacterial ApiT remains to be determined. From genetic and evolutionary perspectives, the apiose operon may provide a unique opportunity to examine how genomic changes reflect ecological adaptation during the divergence of a bacterial group.

The Xanthomonas effector XopK harbours E3 ubiquitin-ligase activity that is required for virulence

Xanthomonas oryzae pv. oryzae is the causative agent of rice bacterial leaf blight. While the type III secretion system of X. oryzae pv. oryzae is essential for virulence, the biochemical activities and virulence mechanisms of non-transcription activator-like (non-TAL) effectors delivered by this system are largely unknown. Here, by screening for non-TAL effectors that contribute to X. oryzae pv. oryzae virulence, we revealed that Xanthomonas outer protein K (XopK) inhibits pathogen-associated molecular pattern-triggered immunity upstream of mitogen-activated protein kinase cascades. Specifically, XopK interacted with and directly ubiquitinated rice somatic embryogenic receptor kinase 2 (OsSERK2), resulting in its degradation. Accordingly, mutation of a putative ubiquitin-conjugation enzyme (E2) binding site abolished XopK-induced degradation of OsSERK2 and compromised XopK-dependent virulence. As crucial immune regulators associated with a multitude of immune receptors, SERKs have been shown to be perturbed by Pseudomonas effectors via different mechanisms. Our study revealed a distinct perturbation mechanism of SERK activity via ubiquitination achieved by Xanthomonas non-TAL effector.

Structural basis of exo-β-mannanase activity in the GH2 family

The classical microbial strategy for depolymerization of β-mannan polysaccharides involves the synergistic action of at least two enzymes, endo-1,4-β-mannanases and β-mannosidases. In this work, we describe the first exo-β-mannanase from the GH2 family, isolated from Xanthomonas axonopodis pv. citri (XacMan2A), which can efficiently hydrolyze both manno-oligosaccharides and β-mannan into mannose. It represents a valuable process simplification in the microbial carbon uptake that could be of potential industrial interest. Biochemical assays revealed a progressive increase in the hydrolysis rates from mannobiose to mannohexaose, which distinguishes XacMan2A from the known GH2 β-mannosidases. Crystallographic analysis indicates that the active-site topology of XacMan2A underwent profound structural changes at the positive-subsite region, by the removal of the physical barrier canonically observed in GH2 β-mannosidases, generating a more open and accessible active site with additional productive positive subsites. Besides that, XacMan2A contains two residue substitutions in relation to typical GH2 β-mannosidases, Gly439 and Gly556, which alter the active site volume and are essential to its mode of action. Interestingly, the only other mechanistically characterized mannose-releasing exo-β-mannanase so far is from the GH5 family, and its mode of action was attributed to the emergence of a blocking loop at the negative-subsite region of a cleft-like active site, whereas in XacMan2A, the same activity can be explained by the removal of steric barriers at the positive-subsite region in an originally pocket-like active site. Therefore, the GH2 exo-β-mannanase represents a distinct molecular route to this rare activity, expanding our knowledge about functional convergence mechanisms in carbohydrate-active enzymes.

Functional characterization of unique enzymes in Xanthomonas euvesicatoria related to degradation of arabinofurano-oligosaccharides on hydroxyproline-rich glycoproteins

In this study, we clarified the functions of three uncharacterized enzymes, XCV2724, XCV2728, and XCV2729, in Xanthomonas euvesicatoria, the causal agent of bacterial spot of tomato and pepper. The genes corresponding to the three enzymes are homologs of hypBA1, hypBA2, and hypAA from Bifidobacterium longum and are unique to Xanthomonas spp. among plant pathogenic bacteria. Functional characterization of the recombinant enzymes expressed using microbial systems revealed that they degrade the arabinofurano-oligosaccharides present on hydroxyproline (Hyp)-rich glycoproteins (HRGPs) such as extensin and solanaceous lectins in plant cell walls. These enzymes work coordinately to degrade the oligosaccharides. First, XeHypAA (XCV2728), belonging to the glycoside hydrolase (GH) 43 family, releases L-arabinose from L-arabinofuranose (Araf)-α1,3-Araf-ß1,2-Araf-ß1,2-Araf-ß-Hyp (Ara4-Hyp), cleaving its α1,3 bond; second, XeHypBA2 (XCV2729), belonging to the GH121 family, releases the disaccharide Araf-ß1,2-Araf from Araf-ß1,2-Araf-ß1,2-Araf-ß-Hyp (Ara3-Hyp); finally, XeHypBA1 (XCV2724), belonging to GH family 127, releases L-arabinose from Araf-ß-Hyp (Ara-Hyp). In summary, the main oligosaccharide structure of Ara4-Hyp on the HRGPs is degraded to Ara3-Hyp, then to Ara-Hyp, and finally to Ara monosaccharides by the action of these three enzymes. HRGPs containing oligosaccharide substrates have been reported to contribute to plant defense, and interestingly, the promoter region of the operon (xehypBA2 and xehypAA) contains the plant-inducible promoter box for binding the regulator protein HrpX involved in pathogenicity. We then analyzed the expression level of the operon gene in hrp-inducing medium and in plants and constructed gene-deletion mutants. However, although the operon was evidently upregulated by HrpX, three single-gene deletion mutants (ΔxehypBA1, ΔxehypBA2, ΔxehypAA) and even a triple-gene deletion mutant (ΔxehypBA1-BA2-AA) remained pathogenic, and had no effect on nonhost resistance, either, indicating that these three enzymes are not involved in either pathogenicity or nonhost resistance reactions. This is the first report of enzymes in plant pathogenic bacteria that catalyze the degradation of Hyp-linked-L-arabinofuranosides in plant cell walls.

3-methylcrotonyl Coenzyme A (CoA) carboxylase complex is involved in the Xanthomonas citri subsp. citri lifestyle during citrus infection

Citrus canker is a disease caused by the phytopathogen Xanthomonas citri subsp. citri (Xcc), bacterium which is unable to survive out of the host for extended periods of time. Once established inside the plant, the pathogen must compete for resources and evade the defenses of the host cell. However, a number of aspects of Xcc metabolic and nutritional state, during the epiphytic stage and at different phases of infection, are poorly characterized. The 3-methylcrotonyl-CoA carboxylase complex (MCC) is an essential enzyme for the catabolism of the branched-chain amino acid leucine, which prevents the accumulation of toxic intermediaries, facilitates the generation of branched chain fatty acids and/or provides energy to the cell. The MCC complexes belong to a group of acyl-CoA carboxylases (ACCase) enzymes dependent of biotin. In this work, we have identified two ORFs (XAC0263 and XAC0264) encoding for the α and β subunits of an acyl-CoA carboxylase complex from Xanthomonas and demonstrated that this enzyme has MCC activity both in vitro and in vivo. We also found that this MCC complex is conserved in a group of pathogenic gram negative bacteria. The generation and analysis of an Xcc mutant strain deficient in MCC showed less canker lesions in the interaction with the host plant, suggesting that the expression of these proteins is necessary for Xcc fitness during infection.

A mutation in an exoglucanase of Xanthomonas oryzae pv. oryzae, which confers an endo mode of activity, affects bacterial virulence, but not the induction of immune responses, in rice

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, a serious disease of rice. Xoo secretes a repertoire of cell wall-degrading enzymes, including cellulases, xylanases and pectinases, to degrade various polysaccharide components of the rice cell wall. A secreted Xoo cellulase, CbsA, is not only a key virulence factor of Xoo, but is also a potent inducer of innate immune responses of rice. In this study, we solved the crystal structure of the catalytic domain of the CbsA protein to a resolution of 1.86 Å. The core structure of CbsA shows a central distorted TIM barrel made up of eight β strands with N- and C-terminal loops enclosing the active site, which is a characteristic structural feature of an exoglucanase. The aspartic acid at the 131st position of CbsA was predicted to be important for catalysis and was therefore mutated to alanine to study its role in the catalysis and biological functions of CbsA. Intriguingly, the D131A CbsA mutant protein displayed the enzymatic activity of a typical endoglucanase. D131A CbsA was as proficient as wild-type (Wt) CbsA in inducing rice immune responses, but was deficient in virulence-promoting activity. This indicates that the specific exoglucanase activity of the Wt CbsA protein is required for this protein to promote the growth of Xoo in rice.

The Xanthomonas euvesicatoria type III effector XopAU is an active protein kinase that manipulates plant MAP kinase signaling

The Gram-negative bacterium Xanthomonas euvesicatoria (Xe) is the causal agent of bacterial spot disease of pepper and tomato. Xe delivers effector proteins into host cells through the type III secretion system to promote disease. Here, we show that the Xe effector XopAU, which is conserved in numerous Xanthomonas species, is a catalytically active protein kinase and contributes to the development of disease symptoms in pepper plants. Agrobacterium-mediated expression of XopAU in host and non-host plants activated typical defense responses, including MAP kinase phosphorylation, accumulation of pathogenesis-related (PR) proteins and elicitation of cell death, that were dependent on the kinase activity of the effector. XopAU-mediated cell death was not dependent on early signaling components of effector-triggered immunity and was also observed when the effector was delivered into pepper leaves by Xanthomonas campestris pv. campestris, but not by Xe. Protein-protein interaction studies in yeast and in planta revealed that XopAU physically interacts with components of plant immunity-associated MAP kinase cascades. Remarkably, XopAU directly phosphorylated MKK2 in vitro and enhanced its phosphorylation at multiple sites in planta. Consistent with the notion that MKK2 is a target of XopAU, silencing of the MKK2 homolog or overexpression of the catalytically inactive mutant MKK2K99R in N. benthamiana plants reduced XopAU-mediated cell death and MAPK phosphorylation. Furthermore, yeast co-expressing XopAU and MKK2 displayed reduced growth and this phenotype was dependent on the kinase activity of both proteins. Together, our results support the conclusion that XopAU contributes to Xe disease symptoms in pepper plants and manipulates host MAPK signaling through phosphorylation and activation of MKK2.

Identification of Inhibitors Targeting Ferredoxin-NADP⁺ Reductase from the Xanthomonas citri subsp. citri Phytopathogenic Bacteria

Ferredoxin-NADP(H) reductases (FNRs) deliver NADPH or low potential one-electron donors to redox-based metabolism in plastids and bacteria. Xanthomonas citri subsp. citri (Xcc) is a Gram-negative bacterium responsible for citrus canker disease that affects commercial citrus crops worldwide. The Xcc fpr gene encodes a bacterial type FNR (XccFPR) that contributes to the bacterial response to oxidative stress conditions, usually found during plant colonization. Therefore, XccFPR is relevant for the pathogen survival and its inhibition might represent a strategy to treat citrus canker. Because of mechanistic and structural differences from plastidic FNRs, XccFPR is also a potential antibacterial target. We have optimized an activity-based high-throughput screening (HTS) assay that identifies XccFPR inhibitors. We selected 43 hits from a chemical library and narrowed them down to the four most promising inhibitors. The antimicrobial effect of these compounds was evaluated on Xcc cultures, finding one with antimicrobial properties. Based on the functional groups of this compound and their geometric arrangement, we identified another three XccFPR inhibitors. Inhibition mechanisms and constants were determined for these four XccFPR inhibitors. Their specificity was also evaluated by studying their effect on the plastidic Anabaena PCC 7119 FNR, finding differences that can become interesting tools to discover Xcc antimicrobials.

Production of functionalized oligo-isoprenoids by enzymatic cleavage of rubber

In this study, we show the proof of concept for the production of defined oligo-isoprenoids with terminal functional groups that can be used as starting materials for various purposes including the synthesis of isoprenoid-based plastics. To this end, we used three types of rubber oxygenases for the enzymatic cleavage of rubber [poly(cis-1,4-isoprene)]. Two enzymes, rubber oxygenase RoxAXsp and rubber oxygenase RoxBXsp , originate from Xanthomonas sp. 35Y; the third rubber oxygenase, latex-clearing protein (LcpK30 ), is derived from Gram-positive rubber degraders such as Streptomyces sp. K30. Emulsions of polyisoprene (latex) were treated with RoxAXsp , RoxBXsp , LcpK30 or with combinations of the three proteins. The cleavage products were purified by solvent extraction and FPLC separation. All products had the same general structure with terminal functions (CHO-CH2 - and -CH2 -COCH3 ) but differed in the number of intact isoprene units in between. The composition and m/z values of oligo-isoprenoid products were determined by HPLC-MS analysis. Our results provide a method for the preparation of reactive oligo-isoprenoids that can likely be used to convert polyisoprene latex or rubber waste materials into value-added molecules, biofuels, polyurethanes or other polymers.

RoxB Is a Novel Type of Rubber Oxygenase That Combines Properties of Rubber Oxygenase RoxA and Latex Clearing Protein (Lcp)

Only two types of rubber oxygenases, rubber oxygenase (RoxA) and latex clearing protein (Lcp), have been described so far. RoxA proteins (RoxAs) are c-type cytochromes of ≈70 kDa produced by Gram-negative rubber-degrading bacteria, and they cleave polyisoprene into 12-oxo-4,8-dimethyltrideca-4,8-diene-1-al (ODTD), a C15 oligo-isoprenoid, as the major end product. Lcps are common among Gram-positive rubber degraders and do not share amino acid sequence similarities with RoxAs. Furthermore, Lcps have much smaller molecular masses (≈40 kDa), are b-type cytochromes, and cleave polyisoprene to a mixture of C20, C25, C30, and higher oligo-isoprenoids as end products. In this article, we purified a new type of rubber oxygenase, RoxB Xsp (RoxB of Xanthomonas sp. strain 35Y). RoxB Xsp is distantly related to RoxAs and resembles RoxAs with respect to molecular mass (70.3 kDa for mature protein) and cofactor content (2 c-type hemes). However, RoxB Xsp differs from all currently known RoxAs in having a distinctive product spectrum of C20, C25, C30, and higher oligo-isoprenoids that has been observed only for Lcps so far. Purified RoxB Xsp revealed the highest specific activity of 4.5 U/mg (at 23°C) of all currently known rubber oxygenases and exerts a synergistic effect on the efficiency of polyisoprene cleavage by RoxA Xsp RoxB homologs were identified in several other Gram-negative rubber-degrading species, pointing to a prominent function of RoxB for the biodegradation of rubber in Gram-negative bacteria.IMPORTANCE The enzymatic cleavage of rubber (polyisoprene) is of high environmental importance given that enormous amounts of rubber waste materials are permanently released (e.g., by abrasion of tires). Research from the last decade has discovered rubber oxygenase A, RoxA, and latex clearing protein (Lcp) as being responsible for the primary enzymatic attack on the hydrophobic and water-insoluble biopolymer poly(cis-1,4-isoprene) in Gram-negative and Gram-positive rubber-degrading bacteria, respectively. Here, we provide evidence that a third type of rubber oxygenase is present in Gram-negative rubber-degrading species. Due to its characteristics, we suggest the designation RoxB for the new type of rubber oxygenase. Bioinformatic analysis of genome sequences indicates the presence of roxB homologs in other Gram-negative rubber degraders.

The effector AvrRxo1 phosphorylates NAD in planta

Gram-negative bacterial pathogens of plants and animals employ type III secreted effectors to suppress innate immunity. Most characterized effectors work through modification of host proteins or transcriptional regulators, although a few are known to modify small molecule targets. The Xanthomonas type III secreted avirulence factor AvrRxo1 is a structural homolog of the zeta toxin family of sugar-nucleotide kinases that suppresses bacterial growth. AvrRxo1 was recently reported to phosphorylate the central metabolite and signaling molecule NAD in vitro, suggesting that the effector might enhance bacterial virulence on plants through manipulation of primary metabolic pathways. In this study, we determine that AvrRxo1 phosphorylates NAD in planta, and that its kinase catalytic sites are necessary for its toxic and resistance-triggering phenotypes. A global metabolomics approach was used to independently identify 3'-NADP as the sole detectable product of AvrRxo1 expression in yeast and bacteria, and NAD kinase activity was confirmed in vitro. 3'-NADP accumulated upon transient expression of AvrRxo1 in Nicotiana benthamiana and in rice leaves infected with avrRxo1-expressing strains of X. oryzae. Mutation of the catalytic aspartic acid residue D193 abolished AvrRxo1 kinase activity and several phenotypes of AvrRxo1, including toxicity in yeast, bacteria, and plants, suppression of the flg22-triggered ROS burst, and ability to trigger an R gene-mediated hypersensitive response. A mutation in the Walker A ATP-binding motif abolished the toxicity of AvrRxo1, but did not abolish the 3'-NADP production, virulence enhancement, ROS suppression, or HR-triggering phenotypes of AvrRxo1. These results demonstrate that a type III effector targets the central metabolite and redox carrier NAD in planta, and that this catalytic activity is required for toxicity and suppression of the ROS burst.

CatB is Critical for Total Catalase Activity and Reduces Bactericidal Effects of Phenazine-1-Carboxylic Acid on Xanthomonas oryzae pv. oryzae and X. oryzae pv. oryzicola

Rice bacterial leaf blight, caused by Xanthomonas oryzae pv. oryzae, and rice bacterial leaf streak, caused by X. oryzae pv. oryzicola, are major diseases of rice. Phenazine-1-carboxylic acid (PCA) is a natural product that is isolated from Pseudomonas spp. and is used to control many important rice diseases in China. We previously reported that PCA disturbs the redox balance, which results in the accumulation of reactive oxygen species in X. oryzae pv. oryzae. In this study, we found that PCA significantly upregulated the transcript levels of catB and katE, which encode catalases, and that PCA sensitivity was reduced when X. oryzae pvs. oryzae and oryzicola were cultured with exogenous catalase. Furthermore, catB deletion mutants of X. oryzae pvs. oryzae and oryzicola showed dramatically decreased total catalase activity, increased sensitivity to PCA, and reduced virulence in rice. In contrast, deletion mutants of srpA and katG, which also encode catalases, exhibited little change in PCA sensitivity. The results indicate that catB in both X. oryzae pvs. oryzae and oryzicola encodes a catalase that helps protect the bacteria against PCA-induced stress.

Identification of Pectin Degrading Enzymes Secreted by Xanthomonas oryzae pv. oryzae and Determination of Their Role in Virulence on Rice

Xanthomonas oryzae pv.oryzae (Xoo) causes the serious bacterial blight disease of rice. Xoo secretes a repertoire of plant cell wall degrading enzymes (CWDEs) like cellulases, xylanases, esterases etc., which act on various components of the rice cell wall. The major cellulases and xylanases secreted by Xoo have been identified and their role in virulence has been determined. In this study, we have identified some of the pectin degrading enzymes of Xoo and assessed their role in virulence. Bioinformatics analysis indicated the presence of four pectin homogalacturonan (HG) degrading genes in the genome of Xoo. The four HG degrading genes include one polygalacturonase (pglA), one pectin methyl esterase (pmt) and two pectate lyases (pel and pelL). There was no difference in the expression of pglA, pmt and pel genes by laboratory wild type Xoo strain (BXO43) grown in either nutrient rich PS medium or in plant mimic XOM2 medium whereas the expression of pelL gene was induced in XOM2 medium as indicated by qRT-PCR experiments. Gene disruption mutations were generated in each of these four genes. The polygalacturonase mutant pglA- was completely deficient in degrading the substrate Na-polygalacturonicacid (PGA). Strains carrying mutations in the pmt, pel and pelL genes were as efficient as wild type Xoo (BXO43) in cleaving PGA. These observations clearly indicate that PglA is the major pectin degrading enzyme produced by Xoo. The pectin methyl esterase, Pmt, is the pectin de-esterifying enzyme secreted by Xoo as evident from the enzymatic activity assay performed using pectin as the substrate. Mutations in the pglA, pmt, pel and pelL genes have minimal effects on virulence. This suggests that, as compared to cellulases and xylanases, the HG degrading enzymes may not have a major role in the pathogenicity of Xoo.

Crystal Structures of Peptide Deformylase from Rice Pathogen Xanthomonas oryzae pv. oryzae in Complex with Substrate Peptides, Actinonin, and Fragment Chemical Compounds

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight on rice; this species is one of the most destructive pathogenic bacteria in rice cultivation worldwide. Peptide deformylase (PDF) catalyzes the removal of the N-formyl group from the N-terminus of newly synthesized polypeptides in bacterial cells and is an important target to develop antibacterial agents. We determined crystal structures of Xoo PDF (XoPDF) at up to 1.9 Å resolution, which include apo, two substrate-bound (methionine-alanine or methionine-alanine-serine), an inhibitor-bound (actinonin), and six fragment chemical-bound structures. Six fragment chemical compounds were bound in the substrate-binding pocket. The fragment chemical-bound structures were compared to the natural PDF inhibitor actinonin-bound structure. The fragment chemical molecules will be useful to design an inhibitor specific to XoPDF and a potential pesticide against Xoo.

treA Codifies for a Trehalase with Involvement in Xanthomonas citri subsp. citri Pathogenicity

Citrus canker, caused by the bacterium Xanthomonas citri subsp. citri (Xcc), is a severe disease of citrus. Xcc presents broad spectrum of citrus hosts including economically important species whereas X. fuscans subsp. aurantifolii–type C (XauC) causes a milder disease and only infects Citrus aurantifolia. Trehalase catalyzes hydrolysis of the disaccharide trehalose, a sugar that has been reported to be related to Xcc pathogenicity. We expressed the recombinant gene product and assessed Xcc trehalase structural and kinetics data. The recombinant protein presented 42.7% of secondary structures in α-helix and 13% in β-sheets, no quaternary structure in solution, and Michaelis-Menten constant (K_M) of 0.077 mM and V_max 55.308 μMol glucose.min^-1.mg protein^-1 for trehalose. A Xcc mutant strain (XccΔtreA) was produced by gene deletion from Xcc genome. Enzymatic activity of trehalase was determined in Xcc, XauC and XccΔtreA cellular lysates, showing the highest values for XauC in in vitro infective condition and no activity for XccΔtreA. Finally, leaves of Citrus aurantifolia infected with XccΔtreA showed much more drenching and necrosis than those infected by wild type Xcc. We concluded that trehalase contributes to alleviate bacterial virulence and that inability for trehalose hydrolysis may promote higher Xcc infectivity.

Action of Multiple Cell Wall-Degrading Enzymes Is Required for Elicitation of Innate Immune Responses During Xanthomonas oryzae pv. oryzae Infection in Rice

Xanthomonas oryzae pv. oryzae secretes a number of plant cell wall-degrading enzymes (CWDEs) whose purified preparations induce defense responses in rice. These defense responses are suppressed by X. oryzae pv. oryzae using type 3 secretion system (T3SS) effectors and a type 3 secretion system mutant (T3SS(-)) of X. oryzae pv. oryzae is an inducer of rice defense responses. We assessed the role of individual CWDEs in induction of rice defense responses during infection, by mutating them in the genetic background of a T3SS(-). We mutated the genes for five different plant CWDEs secreted by X. oryzae pv. oryzae, including two cellulases (clsA and cbsA), one xylanase (xyn), one pectinase (pglA), and an esterase (lipA), singly in a T3SS(-) background. We have demonstrated that, as compared with a T3SS(-) of X. oryzae pv. oryzae, a cbsA(-)T3SS(-), a clsA(-)T3SS(-), and a xyn(-)T3SS(-) are deficient in induction of rice immune responses such as callose deposits and programmed cell death. In comparison, a lipA(-) T3SS(-) and a pglA(-)T3SS(-) is as efficient in induction of host defense responses as a T3SS(-). Overall, these results indicate that the collective action of X. oryzae pv. oryzae-secreted ClsA, CbsA, and Xyn proteins is required for induction of rice defense responses during infection.

The ATP-dependent RNA helicase HrpB plays an important role in motility and biofilm formation in Xanthomonas citri subsp. citri

BACKGROUND

RNA helicases are enzymes that catalyze the separation of double-stranded RNA (dsRNA) using the free energy of ATP binding and hydrolysis. DEAD/DEAH families participate in many different aspects of RNA metabolism, including RNA synthesis, RNA folding, RNA-RNA interactions, RNA localization and RNA degradation. Several important bacterial DEAD/DEAH-box RNA helicases have been extensively studied. In this study, we characterize the ATP-dependent RNA helicase encoded by the hrpB (XAC0293) gene using deletion and genetic complementation assays. We provide insights into the function of the hrpB gene in Xanthomonas citri subsp. citri by investigating the roles of hrpB in biofilm formation on abiotic surfaces and host leaves, cell motility, host virulence of the citrus canker bacterium and growth in planta.

RESULTS

The hrpB gene is highly conserved in the sequenced strains of Xanthomonas. Mutation of the hrpB gene (∆hrpB) resulted in a significant reduction in biofilms on abiotic surfaces and host leaves. ∆hrpB also exhibited increased cell dispersion on solid medium plates. ∆hrpB showed reduced adhesion on biotic and abiotic surfaces and delayed development in disease symptoms when sprayed on susceptible citrus leaves. Quantitative reverse transcription-PCR assays indicated that deletion of hrpB reduced the expression of four type IV pili genes. The transcriptional start site of fimA (XAC3241) was determined using rapid amplification of 5'-cDNA Ends (5'RACE). Based on the results of fimA mRNA structure predictions, the fimA 5' UTR may contain three different loops. HrpB may be involved in alterations to the structure of fimA mRNA that promote the stability of fimA RNA.

CONCLUSIONS

Our data show that hrpB is involved in adherence of Xanthomonas citri subsp. citri to different surfaces. In addition, to the best of our knowledge, this is the first time that a DEAH RNA helicase has been implicated in the regulation of type IV pili in Xanthomonas.

KatG, the Bifunctional Catalase of Xanthomonas citri subsp. citri, Responds to Hydrogen Peroxide and Contributes to Epiphytic Survival on Citrus Leaves

Xanthomonas citri subsp. citri (Xcc) is the bacterium responsible for citrus canker. This bacterium is exposed to reactive oxygen species (ROS) at different points during its life cycle, including those normally produced by aerobic respiration or upon exposition to ultraviolet (UV) radiation. Moreover, ROS are key components of the host immune response. Among enzymatic ROS-detoxifying mechanisms, catalases eliminate H₂O₂, avoiding the potential damage caused by this specie. Xcc genome includes four catalase genes. In this work, we studied the physiological role of KatG, the only bifunctional catalase of Xcc, through the construction and characterization of a modified strain (XcckatG), carrying an insertional mutation in the katG gene. First, we evaluated the involvement of KatG in the bacterial adaptive response to H₂O₂. XcckatG cultures exhibited lower catalase activity than those of the wild-type strain, and this activity was not induced upon treatment with sub-lethal doses of H₂O₂. Moreover, the KatG-deficient mutant exhibited decreased tolerance to H₂O₂ toxicity compared to wild-type cells and accumulated high intracellular levels of peroxides upon exposure to sub-lethal concentrations of H₂O₂. To further study the role of KatG in Xcc physiology, we evaluated bacterial survival upon exposure to UV-A or UV-B radiation. In both conditions, XcckatG showed a high mortality in comparison to Xcc wild-type. Finally, we studied the development of bacterial biofilms. While structured biofilms were observed for the Xcc wild-type, the development of these structures was impaired for XcckatG. Based on these results, we demonstrated that KatG is responsible for Xcc adaptive response to H₂O₂ and a key component of the bacterial response to oxidative stress. Moreover, this enzyme plays an important role during Xcc epiphytic survival, being essential for biofilm formation and UV resistance.

Solid-substrate bioprocessing of cow dung for the production of carboxymethyl cellulase by Bacillus halodurans IND18

The production of carboxymethyl cellulase (CMCase) by Bacillus halodurans IND18 under solid substrate fermentation (SSF) using cow dung was optimized through two level full factorial design and second order response surface methodology (RSM). The central composite design (CCD) was employed to optimize the vital fermentation parameters, such as pH of the substrate, concentration of nitrogen source (peptone) and ion (sodium dihydrogen phosphate) sources in medium for achieving higher enzyme production. The optimum medium composition was found to be 1.46% (w/w) peptone, 0.095% (w/w) sodium dihydrogen phosphate and pH 8.0. The model prediction of 4210IU/g enzyme activity at optimum conditions was verified experimentally as 4140IU/g. The enzyme was active over a broad temperature range (40-60±1°C) and pH (7.0-9.0) with maximal activity at 60±1°C and pH 8.0. This study demonstrated the potential of cow dung as novel substrate for CMCase production.

The dual nature of trehalose in citrus canker disease: a virulence factor for Xanthomonas citri subsp. citri and a trigger for plant defence responses

Xanthomonas citri subsp. citri (Xcc) is a bacterial pathogen that causes citrus canker in susceptible Citrus spp. The Xcc genome contains genes encoding enzymes from three separate pathways of trehalose biosynthesis. Expression of genes encoding trehalose-6-phosphate synthase (otsA) and trehalose phosphatase (otsB) was highly induced during canker development, suggesting that the two-step pathway of trehalose biosynthesis via trehalose-6-phosphate has a function in pathogenesis. This pathway was eliminated from the bacterium by deletion of the otsA gene. The resulting XccΔotsA mutant produced less trehalose than the wild-type strain, was less resistant to salt and oxidative stresses, and was less able to colonize plant tissues. Gene expression and proteomic analyses of infected leaves showed that infection with XccΔotsA triggered only weak defence responses in the plant compared with infection with Xcc, and had less impact on the host plant's metabolism than the wild-type strain. These results suggested that trehalose of bacterial origin, synthesized via the otsA-otsB pathway, in Xcc, plays a role in modifying the host plant's metabolism to its own advantage but is also perceived by the plant as a sign of pathogen attack. Thus, trehalose biosynthesis has both positive and negative consequences for Xcc. On the one hand, it enables this bacterial pathogen to survive in the inhospitable environment of the leaf surface before infection and exploit the host plant's resources after infection, but on the other hand, it is a tell-tale sign of the pathogen's presence that triggers the plant to defend itself against infection.

Xylan utilization regulon in Xanthomonas citri pv. citri Strain 306: gene expression and utilization of oligoxylosides

Xanthomonas citri pv. citri strain 306 (Xcc306), a causative agent of citrus canker, produces endoxylanases that catalyze the depolymerization of cell wall-associated xylans. In the sequenced genomes of all plant-pathogenic xanthomonads, genes encoding xylanolytic enzymes are clustered in three adjacent operons. In Xcc306, these consecutive operons contain genes encoding the glycoside hydrolase family 10 (GH10) endoxylanases Xyn10A and Xyn10C, the agu67 gene, encoding a GH67 α-glucuronidase (Agu67), the xyn43E gene, encoding a putative GH43 α-l-arabinofuranosidase, and the xyn43F gene, encoding a putative β-xylosidase. Recombinant Xyn10A and Xyn10C convert polymeric 4-O-methylglucuronoxylan (MeGXn) to oligoxylosides methylglucuronoxylotriose (MeGX3), xylotriose (X3), and xylobiose (X2). Xcc306 completely utilizes MeGXn predigested with Xyn10A or Xyn10C but shows little utilization of MeGXn. Xcc306 with a deletion in the gene encoding α-glucuronidase (Xcc306 Δagu67) will not utilize MeGX3 for growth, demonstrating the role of Agu67 in the complete utilization of GH10-digested MeGXn. Preferential growth on oligoxylosides compared to growth on polymeric MeGXn indicates that GH10 xylanases, either secreted by Xcc306 in planta or produced by the plant host, generate oligoxylosides that are processed by Xyn10 xylanases and Agu67 residing in the periplasm. Coordinate induction by oligoxylosides of xyn10, agu67, cirA, the tonB receptor, and other genes within these three operons indicates that they constitute a regulon that is responsive to the oligoxylosides generated by the action of Xcc306 GH10 xylanases on MeGXn. The combined expression of genes in this regulon may allow scavenging of oligoxylosides derived from cell wall deconstruction, thereby contributing to the tissue colonization and/or survival of Xcc306 and, ultimately, to plant disease.

Molecular mechanisms associated with xylan degradation by Xanthomonas plant pathogens

Xanthomonas pathogens attack a variety of economically relevant plants, and their xylan CUT system (carbohydrate utilization with TonB-dependent outer membrane transporter system) contains two major xylanase-related genes, xynA and xynB, which influence biofilm formation and virulence by molecular mechanisms that are still elusive. Herein, we demonstrated that XynA is a rare reducing end xylose-releasing exo-oligoxylanase and not an endo-β-1,4-xylanase as predicted. Structural analysis revealed that an insertion in the β7-α7 loop induces dimerization and promotes a physical barrier at the +2 subsite conferring this unique mode of action within the GH10 family. A single mutation that impaired dimerization became XynA active against xylan, and high endolytic activity was achieved when this loop was tailored to match a canonical sequence of endo-β-1,4-xylanases, supporting our mechanistic model. On the other hand, the divergent XynB proved to be a classical endo-β-1,4-xylanase, despite the low sequence similarity to characterized GH10 xylanases. Interestingly, this enzyme contains a calcium ion bound nearby to the glycone-binding region, which is required for catalytic activity and structural stability. These results shed light on the molecular basis for xylan degradation by Xanthomonas and suggest how these enzymes synergistically assist infection and pathogenesis. Our findings indicate that XynB contributes to breach the plant cell wall barrier, providing nutrients and facilitating the translocation of effector molecules, whereas the exo-oligoxylanase XynA possibly participates in the suppression of oligosaccharide-induced immune responses.

Characterisation of a novel endo-xyloglucanase (XcXGHA) from Xanthomonas that accommodates a xylosyl-substituted glucose at subsite -1

A xyloglucan-specific endo-1,4β-glucanase (XcXGHA) from Xanthomonas citri pv. mangiferaeindicae has been cloned, expressed in Escherichia coli, purified and characterised. The XcXGHA enzyme belongs to CAZy family GH74 and has catalytic site residues conserved with other xyloglucanases in this family. At its optimal reaction conditions, pH 7.0 and 40 °C, the enzyme has a k cat/K M value of 2.2 × 10(7) min(-1) M(-1) on a tamarind seed xyloglucan substrate. XcXGHA is relatively stable within a broad pH range (pH 4-9) and up to 50 °C (t 1/2, 50 °C of 74 min). XcXGHA is proven to be xyloglucan-specific, and a glycan microarray study verifies that XcXGHA catalyses cleavage of xyloglucan extracted from both monocot and dicot plant species. The enzyme catalyses hydrolysis of tamarind xyloglucan in a unique way by cleaving XXXG into XX and XG (X is xylosyl-substituted glucose; G is unsubstituted glucose), is able to degrade more complex xyloglucans and notably is able to cleave near more substituted xyloglucan motifs such as L [i.e. α-L-Fucp-(1 → 2)-β-D-Galp-(1 → 2)-α-D-Xylp-(1 → 6)-β-D-Glcp]. LC-MS/MS analysis of product profiles of tamarind xyloglucan which had been catalytically degraded by XcXGHA revealed that XcXGHA has specificity for X in subsite -1. The 3D model suggests that XcXGHA consists of two seven-bladed β-propeller domains with the catalytic center formed by the interface of these two domains, which is conserved in xyloglucanases in the GH74 family. However, the XcXGHA has two amino acids (D264 and R472) that differ from the conserved residues of other GH74 xyloglucanases. These two amino acids were predicted to be located on the opposite side of the active site pocket, facing each other and forming a closing surface above the active site pocket. These two amino acids may contribute to the unique substrate specificity of the XcXGHA enzyme.

StoS, a hybrid histidine kinase sensor of Xanthomonas oryzae pv. oryzae, is activated by sensing low O₂ concentration and is involved in stress tolerance and virulence

Bacteria have two-component signal transduction systems (TCSTS), which are important devices for receiving various environmental signals. A TCSTS generally consists of a sensor histidine kinase (HK) and a response regulator (RR) that contains a receiver domain. There are also hybrid-type HK (HyHK) that comprise a HK with a receiver domain within one molecule. In this study, we show that the deletion mutant of a HyHK XOO_0635 (StoS) of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight of rice, had decreased stress tolerance to high osmolarity, sodium, and H₂O₂. Growth of the StoS mutant was delayed, and viability was lower than the wild type in medium and in rice leaves. We found that StoS regulates the expression of various genes including XOO_3715, XOO_0131, and stoS itself. A domain search revealed a PAS domain with a heme pocket in StoS, implying that the HyHK functions as an O₂ sensor. When the bacteria were incubated in low oxygen, the StoS-dependent expression of XOO_0131 and XOO_3715 became higher. Therefore, StoS is activated by sensing a low O2 concentration in its environs and is involved in gene expression for adapting to various stressful conditions.

Genome mining reveals the genus Xanthomonas to be a promising reservoir for new bioactive non-ribosomally synthesized peptides

BACKGROUND

Various bacteria can use non-ribosomal peptide synthesis (NRPS) to produce peptides or other small molecules. Conserved features within the NRPS machinery allow the type, and sometimes even the structure, of the synthesized polypeptide to be predicted. Thus, bacterial genome mining via in silico analyses of NRPS genes offers an attractive opportunity to uncover new bioactive non-ribosomally synthesized peptides. Xanthomonas is a large genus of Gram-negative bacteria that cause disease in hundreds of plant species. To date, the only known small molecule synthesized by NRPS in this genus is albicidin produced by Xanthomonas albilineans. This study aims to estimate the biosynthetic potential of Xanthomonas spp. by in silico analyses of NRPS genes with unknown function recently identified in the sequenced genomes of X. albilineans and related species of Xanthomonas.

RESULTS

We performed in silico analyses of NRPS genes present in all published genome sequences of Xanthomonas spp., as well as in unpublished draft genome sequences of Xanthomonas oryzae pv. oryzae strain BAI3 and Xanthomonas spp. strain XaS3. These two latter strains, together with X. albilineans strain GPE PC73 and X. oryzae pv. oryzae strains X8-1A and X11-5A, possess novel NRPS gene clusters and share related NRPS-associated genes such as those required for the biosynthesis of non-proteinogenic amino acids or the secretion of peptides. In silico prediction of peptide structures according to NRPS architecture suggests eight different peptides, each specific to its producing strain. Interestingly, these eight peptides cannot be assigned to any known gene cluster or related to known compounds from natural product databases. PCR screening of a collection of 94 plant pathogenic bacteria indicates that these novel NRPS gene clusters are specific to the genus Xanthomonas and are also present in Xanthomonas translucens and X. oryzae pv. oryzicola. Further genome mining revealed other novel NRPS genes specific to X. oryzae pv. oryzicola or Xanthomonas sacchari.

CONCLUSIONS

This study revealed the significant potential of the genus Xanthomonas to produce new non-ribosomally synthesized peptides. Interestingly, this biosynthetic potential seems to be specific to strains of Xanthomonas associated with monocotyledonous plants, suggesting a putative involvement of non-ribosomally synthesized peptides in plant-bacteria interactions.

Xanthomonas oryzae pv. oryzae type III effector XopN targets OsVOZ2 and a putative thiamine synthase as a virulence factor in rice

Xanthomonasoryzae pv. oryzae (Xoo) is spread systemically through the xylem tissue and causes bacterial blight in rice. We evaluated the roles of Xanthomonas outer proteins (Xop) in the Xoo strain KXO85 in a Japonica-type rice cultivar, Dongjin. Five xop gene knockout mutants (xopQ_KXO85, xopX_KXO85, xopP1_KXO85, xopP2_KXO85, and xopN_KXO85) were generated by EZ-Tn5 mutagenesis, and their virulence was assessed in 3-month-old rice leaves. Among these mutants, the xopN_KXO85 mutant appeared to be less virulent than the wild-type KXO85; however, the difference was not statistically significant. In contrast, the xopN_KXO85 mutant exhibited significantly less virulence in flag leaves after flowering than the wild-type KXO85. These observations indicate that the roles of Xop in Xoo virulence are dependent on leaf stage. We chose the xopN gene for further characterization because the xopN_KXO85 mutant showed the greatest influence on virulence. We confirmed that XopN_KXO85 is translocated into rice cells, and its gene expression is positively regulated by HrpX. Two rice proteins, OsVOZ2 and a putative thiamine synthase (OsXNP), were identified as targets of XopN_KXO85 by yeast two-hybrid screening. Interactions between XopN_KXO85 and OsVOZ2 and OsXNP were further confirmed in planta by bimolecular fluorescence complementation and in vivo pull-down assays. To investigate the roles of OsVOZ2 in interactions between rice and Xoo, we evaluated the virulence of the wild-type KXO85 and xopN_KXO85 mutant in the OsVOZ2 mutant line PFG_3A-07565 of Dongjin. The wild-type KXO85 and xopN_KXO85 mutant were significantly less virulent in the mutant rice line. These results indicate that XopN_KXO85 and OsVOZ2 play important roles both individually and together for Xoo virulence in rice.

[Synthesis of cefatrizine by recombinant alpha-amino acid ester hydrolase]

To explore the enzymatic route of cefatrizine synthesis, alpha-amino acid ester hydrolase (AEH) gene was cloned from the whole genome of Xanthomonas rubrillineans, and expressed heterologously in Escherichia coli BL21 (DE3). The effects of temperature, pH and substrates' molar ratio upon the transformation yield of cefatrizine by purified recombinant AEH were investigated. The monomer of AEH was determined as 70 kDa by SDS-PAGE. The optimal pH and temperature reaction were (6.0 +/- 0.1) and 36 degrees C for cefatrizine synthesis. The transformation yield was 64.3% under 36 degrees C, pH (6.0 +/- 0.1), when the concentrations of two substrates were about 30 mmol/L (7-ATTC) and 120 mmol/L (HPGM x HCl), respectively, and the enzyme consumption was 22 U/mL. The results pave the way for optimization of the industrial enzymatic synthesis of cefatrizine.

[Purification and characterization of alpha-amino acid ester hydrolase from Xanthomonas rubrillineans]

OBJECTIVE

The intracellular alpha-amino acid ester hydrolase (AEH) from Xanthomonas rubrillineans was purified and characterized.

METHODS

AEH was extracted by butyl acetate, and then purified to electrophoretic homogeneity by calcium phosphate gel precipitation, ammonium sulfate fraction precipitation, anion exchange with DEAE Sephadex A-50, cation exchange with CM cellulose 52, and Sephadex G 200 column chromatography.

RESULTS

The subunit molecular mass of AEH was 70 kDa by SDS-PAGE. The optimal reaction pH for cefaclor synthesis was 6.8, and optimal temperature was 42 degrees C. The enzyme was stable between pH 5.0 and 8.0, and at 35 degrees C. The enzyme activity was enhanced by Mn2+ and Ca2+, however was strongly inhabited by Cu2+, Fe2+ and high concentration of acetone. The kinetic parameters that the enzyme hydrolyzed D-Phenylglycine methyl ester, D-Hydroxyphenylglycine methyl ester and cefaclor were determined, and the values of k(cat)/K(m) were 123.7 +/-3.7, 2.9 +/- 0.6 and 101.3 +/- 2.1 mmol(-1) x s(-1) x L respectively. The k(cat)/K(m) values indicated that the enzyme hydrolyzed D-Phenylglycine methyl ester more efficiently than other substrates. The mechanism of enzymatic reaction with bi-substrates by AEH is Ping-Pong kinetics, and the k(cat) value that the enzyme catalyzed the synthesis of cefaclor is 547.3 +/- 38.2 s(-1).

CONCLUSION

The studies of AEH from Xanthomonas rubrillineans were rare, and our research may provide an important basis for industrial application of AEH for beta-lactam antibiotics synthesis.

Avirulence proteins AvrBs7 from Xanthomonas gardneri and AvrBs1.1 from Xanthomonas euvesicatoria contribute to a novel gene-for-gene interaction in pepper

A novel hypersensitive resistance (HR) in Capsicum baccatum var. pendulum against the bacterial spot of pepper pathogen, Xanthomonas gardneri, was introgressed into C. annuum cv. Early Calwonder (ECW) to create the near-isogenic line designated as ECW-70R. A corresponding avirulence gene avrBs7, in X. gardneri elicited a strong HR in ECW-70R. A homolog of avrBs7, avrBs1.1, was found in X. euvesicatoria 85-10, which showed delayed HR on ECW-70R leaves. Genetic analysis confirmed the presence of a single dominant resistance gene, Bs7, corresponding to the two avr genes. Both AvrBs7 and AvrBs1.1 share a consensus protein tyrosine phosphatase (PTP) active site domain and can dephosphorylate para-nitrophenyl phosphate. Mutation of Cys(265) to Ser in the PTP domain and subsequent loss of enzymatic activity and HR activity indicated the importance of the PTP domain in the recognition of the Avr protein by the Bs7 gene transcripts. Superpositioning of AvrBs7 and AvrBs1.1 homology models indicated variation in the geometry of the loops adjacent to the active sites. These predicted structural differences might be responsible for the differences in HR timing due to differential activation of the resistance gene. Mutating the PTP domain of AvrBs1.1 to match that of AvrBs7 failed to activate HR on ECW-70R, indicating the possibility of differential substrate specificities between AvrBs1.1 and AvrBs7.

Crystal structure of malonyl CoA-Acyl carrier protein transacylase from Xanthomanous oryzae pv. oryzae and its proposed binding with ACP

Xanthomonas oryzae pv. oryzae (Xoo) is a plant bacterial pathogen that causes bacterial blight (BB) disease, resulting in serious production losses of rice. The crystal structure of malonyl CoA-acyl carrier protein transacylase (XoMCAT), encoded by the gene fabD (Xoo0880) from Xoo, was determined at 2.3 Å resolution in complex with N-cyclohexyl-2-aminoethansulfonic acid. Malonyl CoA-acyl carrier protein transacylase transfers malonyl group from malonyl CoA to acyl carrier protein (ACP). The transacylation step is essential in fatty acid synthesis. Based on the rationale, XoMCAT has been considered as a target for antibacterial agents against BB. Protein-protein interaction between XoMCAT and ACP was also extensively investigated using computational docking, and the proposed model revealed that ACP bound to the cleft between two XoMCAT subdomains.

Post-synthetic modification of plant cell walls by expression of microbial hydrolases in the apoplast

The systematic creation of defined cell wall modifications in the model plant Arabidopsis thaliana by expression of microbial hydrolases with known specific activities is a promising approach to examine the impacts of cell wall composition and structure on both plant fitness and cell wall recalcitrance. Moreover, this approach allows the direct evaluation in living plants of hydrolase specificity, which can differ from in vitro specificity. To express genes encoding microbial hydrolases in A. thaliana, and target the hydrolases to the apoplast compartment, we constructed an expression cassette composed of the Cauliflower Mosaic Virus 35S RNA promoter, the A. thaliana β-expansin signal peptide, and the fluorescent marker protein YFP. Using this construct we successfully introduced into Colombia-0 plants three Aspergillus nidulans hydrolases, β-xylosidase/α-arabinosidase, feruloyl esterase, acetylxylan esterase, and a Xanthomonas oryzae putative a-L: -arabinofuranosidase. Fusion with YFP permitted quick and easy screening of transformants, detection of apoplastic localization, and protein size confirmation. Compared to wild-type Col-0, all transgenic lines showed a significant increase in the corresponding hydrolytic activity in the apoplast and changes in cell wall composition. Examination of hydrolytic activity in the transgenic plants also showed, for the first time, that the X. oryzae gene indeed encoded an enzyme with α-L: -arabinofuranosidase activity. None of the transgenic plants showed a visible phenotype; however, the induced compositional changes increased the degradability of biomass from plants expressing feruloyl esterase and β-xylosidase/α-arabinosidase. Our results demonstrate the viability of creating a set of transgenic A. thaliana plants with modified cell walls to use as a toolset for investigation of how cell wall composition contributes to recalcitrance and affects plant fitness.

Determination of the crystal structure and active residues of FabV, the enoyl-ACP reductase from Xanthomonas oryzae

Background

Enoyl-ACP reductase (ENR) catalyses the last reduction reaction in the fatty acid elongation cycle in bacteria and is a good antimicrobial target candidate. FabV is the most recently discovered class of ENR, but we lack information about the atomic structure and the key residues involved in reductase activity except for the known conserved tyrosine and lysine residues in the Y-X₈-K active site motif.

Methodology/Principal Findings

Here we report the crystal structure of FabV from Xanthomonas oryzae (xoFabV). The crystal structure of this enzyme has been solved to 1.6 Å resolution in space group P2₁2₁2₁. The model of xoFabV consists of one monomer in the asymmetric unit which is composed of 13 α-helices and 11 β-strands, representing a canonical Rossmann fold architecture. Structural comparison presents that the locations of the conserved tyrosine (Y236) and lysine (K245) residues in the Y-X₈-K active site motif of xoFabV and the Y-X₆-K motif of ecFabI are notably similar. However, the conformations of Y236 in xoFabV and Y156 in ecFabI are distinct. Structure-based site-directed mutagenesis and enzymatic activity assays reveal that in addition to the conserved Y236 and K245 in the Y-X₈-K motif, Y53, D111 and Y226 are key residues implicated in the reductase activity, and F113 and T276 are also important for enzyme function. Moreover, a proposed active lysine located immediately after the Y-X₈-K motif in FabV from Burkholderia mallei (bmFabV) is altered to an inactive V246 in xoFabV.

Conclusions/Significance

We determine the first crystal structure of the FabV enzyme and identify several residues important for its enzymatic activity. These findings lay a solid foundation for the development of specific antibacterial inhibitors of the pathogenic bacteria, such as Vibrio cholerae, Burkholderia species and Xanthomonas species.

The rsmA-like gene rsmA(Xoo) of Xanthomonas oryzae pv. oryzae regulates bacterial virulence and production of diffusible signal factor

The plant-pathogenic prokaryote Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight, one of the most destructive diseases of rice. A nonpolar mutant of the rsmA-like gene rsmA(Xoo) of the Xoo Chinese strain 13751 was constructed by homologous integration with a suicide plasmid. Virulence tests on a host plant, namely the hybrid rice cultivar Teyou 63, showed that the mutant had lost its virulence almost completely, whereas tests on a nonhost, namely castor-oil plant (Ricinus communis), showed that the mutant had also lost the ability to induce a hypersensitive response in the nonhost. In addition, the rsmA(Xoo) mutant produced significantly smaller amounts of the diffusible signal factor, extracellular endoglucanase, amylase and extracellular polysaccharide, but showed significantly higher glycogen accumulation, bacterial aggregation and cell adhesion. The expression of most hrp genes, genes encoding AvrBs3/PthA family members, rpfB, xrvA, glgA, eglXoB and XOO0175 (encoding an α-amylase) was down-regulated in the rsmA(Xoo) mutant. All phenotypes and expression levels of the tested genes in the rsmA(Xoo) mutant were restored to their levels in the wild-type by the presence of rsmA(Xoo) in trans. These results indicate that rsmA(Xoo) is essential for the virulence of Xoo.

Molecular cloning and characterization of a beta-L-Arabinobiosidase in Bifidobacterium longum that belongs to a novel glycoside hydrolase family

Extensin is a glycoprotein that is rich in hydroxyprolines linked to β-L-arabinofuranosides. In this study, we cloned a hypBA2 gene that encodes a novel β-L-arabinobiosidase from Bifidobacterium longum JCM 1217. This enzyme does not have any sequence similarity with other glycoside hydrolase families but has 38-98% identity to hypothetical proteins in Bifidobacterium and Xanthomonas strains. The recombinant enzyme liberated L-arabinofuranose (Araf)-β1,2-Araf disaccharide from carrot extensin, potato lectin, and Araf-β1,2-Araf-β1,2-Araf-β-Hyp (Ara(3)-Hyp) but not Araf-α1,3-Araf-β1,2-Araf-β1,2-Araf-β-Hyp (Ara(4)-Hyp) or Araf-β1,2-Araf-β-Hyp (Ara(2)-Hyp), which indicated that it was specific for unmodified Ara(3)-Hyp substrate. The enzyme also transglycosylated 1-alkanols with retention of the anomeric configuration. This is the first report of an enzyme that hydrolyzes Hyp-linked β-L-arabinofuranosides, which defines a new family of glycoside hydrolases, glycoside hydrolase family 121.

A single amino acid substitution in the SdhB protein of succinate dehydrogenase determines resistance to amicarthiazol in Xanthomonas oryzae pv. oryzae

BACKGROUND

Xanthomonas oryzae pv. Oryzae Ishiyama, a causal agent of rice bacterial leaf blight, was found to be sensitive in vitro to the systemic fungicide amicarthiazol (2-amino-4-methylthiazole -5-carboxanilide), which is a potent inhibitor of succinate dehydrogenase (SDH, EC 1.3.99.1). This paper aimed to determine the molecular resistance mechanism of X. oryzae pv. oryzae to amicarthiazol.

RESULTS

UV-induced resistant mutants of X. oryzae pv. oryzae to amicarthiazol were isolated. The activity of SDH in wild-type X. oryzae pv. oryzae was strongly inhibited by amicarthiazol, while that in resistant mutants was insensitive, although their SDH activity was decreased compared with the wild-type sensitive strain without amicarthiazol. A mutation of Histidine(229) (CAC) to Tyrosine(229) (TAC) was identified in sdhB, which encoded the iron-sulfur protein subunit of SDH. The sdhB from the mutant was ligated into a cosmid, pUFR034, to generate pUFR034RAX, which conferred resistance to amicarthiazol when transformed into the wild-type sensitive strain.

CONCLUSION

A mutation of His(229) (CAC) to Tyr(229) (TAC) in SdhB was responsible for determining amicarthiazol resistance. .

Spectroscopic properties of rubber oxygenase RoxA from Xanthomonas sp., a new type of dihaem dioxygenase

Natural rubber [poly-(cis-1,4-isoprene)] is cleaved to 12-oxo-4,8-dimethyltrideca-4,8-diene-1-al (ODTD) by rubber oxygenase A (RoxA) isolated from Xanthomonas sp. RoxA has two c-type haem centres that show two distinct alpha-bands at 549 and 553 nm in the dithionite-reduced state. A well-resolved midpoint potential (E(0)') of -65 mV was determined for one haem by spectrophotometric titrations in the absence of dioxygen with dithionite and ferricyanide as reductant and oxidant, respectively. The midpoint potential of the second haem was not resolvable (E(0)' about -130 to -160 mV). One of the two haems was reduced by NADH (549 nm alpha-band), similar to bacterial dihaem peroxidases. Evidence for an electron transfer between the two haems was provided by slow reduction of the second haem (553 nm alpha-band) upon incubation of the partially reduced enzyme at room temperature. Addition of imidazole or related compounds to RoxA led to UV/vis spectral features similar to those observed for partially reduced RoxA. Notably, reduction of RoxA with dithionite or NADH, or binding of compounds such as imidazole, resulted in a reversible inactivation of the enzyme, unlike dihaem peroxidases. In line with this result, RoxA did not show any peroxidase activity. EPR spectra of RoxA as isolated showed two low-spin Fe(III) haem centres, with apparent g-values of 3.39, 3.09, 2.23, 1.92 and 1.50. A weak signal in the g=6 region resulting from a high-spin Fe(III) haem was also observed with a preparation-dependent intensity that disappeared in the presence of imidazole. Attempts to provide spectroscopic evidence for binding of the natural substrate (polyisoprene latex) to RoxA failed. However, experimental data are presented that RoxA is able to subtract redox equivalents from its substrate or from model compounds. In conclusion, RoxA is a novel type of dihaem dioxygenase with features clearly different from classical cytochrome c peroxidases.

Requirement of the galU gene for polysaccharide production by and pathogenicity and growth In Planta of Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri is the causal agent of citrus canker, which has a significant impact on citrus production. In this study, we characterized the galU gene of X. citri subsp. citri. Two galU mutants (F6 and D12) were identified in an X. citri subsp. citri EZ-Tn5 Tnp transposon library. Rescue cloning, sequence analysis, and Southern blot analysis indicated that both of these mutants had a single copy of the EZ-Tn5 transposon inserted in galU in the chromosome. Further study showed that galU was required for biosynthesis of extracellular polysaccharides (EPS; xanthan gum) and capsular polysaccharide (CPS) and biofilm formation. Mutation of galU resulted in a loss of pathogenicity for grapefruit. The loss of pathogenicity of a galU mutant resulted from its inability to grow in planta rather than from the effect on virulence genes. Quantitative reverse transcription-PCR assays indicated that mutation of galU did not impair the expression of key virulence genes, such as pthA of X. citri subsp. citri. Although D12 had a growth rate similar to that of the wild-type strain in nutrient broth, no D12 population became established in the intercellular spaces of citrus leaves. Coinoculation of a galU mutant with the wild-type strain did not promote growth of the galU mutant in planta. Defects in EPS and CPS production, pathogenicity, and growth in planta of the galU mutant were complemented to the wild-type level using plasmid pCGU2.1 containing an intact galU gene. These data indicate that the galU gene contributes to X. citri subsp. citri growth in intercellular spaces and is involved in EPS and CPS synthesis and biofilm formation.