Summary• Type II secretion (T2S) systems of many plant-pathogenic bacteria often secrete cell wall-degrading enzymes into the plant apoplast.• Here, we show that the Xps-T2S system from the plant pathogen Xanthomonas campestris pv vesicatoria (Xcv) promotes disease and contributes to the translocation of effector proteins that are delivered into the plant cell by the type III secretion (T3S) system.• The Xcs-T2S system instead lacks an obvious virulence function. However, individual xcs genes can partially complement mutants in homologous xps genes, indicating that they encode functional components of T2S systems. Enzyme activity assays showed that the Xps system contributes to secretion of proteases and xylanases. We identified the virulence-associated xylanase XynC as a substrate of the Xps system. However, homologs of known T2S substrates from other Xanthomonas spp. are not secreted by the T2S systems from Xcv. Thus, T2S systems from Xanthomonas spp. appear to differ significantly in their substrate specificities.• Transcript analyses revealed that expression of xps genes in Xcv is activated by HrpG and HrpX, key regulators of the T3S system. By contrast, expression of xynC and extracellular protease and xylanase activities are repressed by HrpG and HrpX, suggesting that components and substrates of the Xps system are differentially regulated.

Sialylation of glycoproteins and glycolipids is catalyzed by sialyltransferases in the Golgi of mammalian cells, whereby sialic acid residues are added at the nonreducing ends of oligosaccharides. Because sialylated glycans play critical roles in a number of human physio‐pathological processes, the past two decades have witnessed the development of modified sialic acid derivatives for a better understanding of sialic acid biology and for the development of new therapeutic targets. However, nothing is known about how individual mammalian sialyltransferases tolerate and behave towards these unnatural CMP‐sialic acid donors. In this study, we devised several approaches to investigate the donor specificity of the human β‐d‐galactoside sialyltransferases ST6Gal I and ST3Gal I by using two CMP‐sialic acids: CMP‐Neu5Ac, and CMP‐Neu5N‐(4pentynoyl)neuraminic acid (CMP‐SiaNAl), an unnatural CMP‐sialic acid donor with an extended and functionalized N‐acyl moiety.

Dickeya dadantii is a pectinolytic phytopathogen enterobacterium that causes soft rot disease on a wide range of plant species. The virulence of D. dadantii involves several factors, including the osmoregulated periplasmic glucans (OPGs) that are general constituents of the envelope of proteobacteria. In addition to the loss of virulence, opg-negative mutants display a pleiotropic phenotype, including decreased motility and increased exopolysaccharide synthesis. A nitrosoguanidine-induced mutagenesis was performed on the opgG strain, and restoration of motility was used as a screen. The phenotype of the opg mutant echoes that of the Rcs system: high level activation of the RcsCD-RcsB phosphorelay is needed to activate exopolysaccharide synthesis and to repress motility, while low level activation is required for virulence in enterobacteria. Here, we show that mutations in the RcsCDB phosphorelay system restored virulence and motility in a D. dadantii opg-negative strain, indicating a relationship between the Rcs phosphorelay and OPGs.Osmoregulated periplasmic glucans (OPGs) are general periplasmic constituents of the envelope of most proteobacteria. Their common features are that glucose is the sole constituent sugar, and their abundance in the periplasm increases as the osmolarity of the medium decreases. In Enterobacteriaceae and related bacteria, the glucose backbone synthesis is catalyzed by both products of the opgGH operon (5). Studies of several bacterial pathogens, including Dickeya dadantii, showed the importance of OPGs for virulence (4,5,18,25,26).Dickeya dadantii is a member of the pectinolytic erwiniae causing soft rot disease in a wide range of plant species (33). The virulence of D. dadantii is associated with the synthesis and the secretion of a set of plant cell wall-degrading enzymes (pectinases, cellulases, and proteases) causing maceration of the plant tissues (22). D. dadantii synthesize OPGs containing 5 to 12 glucose units joined by ␤,1-2 linkages and branched by ␤,1-6 linkages that are substituted with succinyl and acetyl residues (11). The opgG or opgH mutants unable to synthesize OPGs show a pleiotropic phenotype. They are nonvirulent on chicory leaves and potato tubers, and synthesis and secretion of pectate-lyases, cellulases, and proteases are reduced (32). Motility is severely reduced, while exopolysaccharide secretion is increased (mucoid phenotype) (32). Data suggest that the opg mutants are impaired in perception of the environment, which prevents D. dadantii from recognizing host cells, suggesting a possible dysfunction of phosphorelay signaling pathways, major systems required for environmental perception in bacteria (6). In these systems, upon stimuli, a kinase/phosphatase sensor autophosphorylates and transfers the phosphate group to a cytoplasmic regulator which modulates expression of target genes.Here, we show that mutations in the rcsC and rcsB genes, encoding, respectively, the sensor and the cognate regulator of the RcsCD-RcsB phosphorelay, suppress several phenotypes o...