SummaryTwo peroxiredoxins, Prx1 and Prx2, were previously identified in Vibrio vulnificus. Besides OxyR1, a homologue of Escherichia coli OxyR (EcOxyR), OxyR2 that shares low homology with EcOxyR was first identified in V. vulnificus. OxyR2 activated prx2 during aerobic growth, while OxyR1 activated prx1 only when exposed to exogenous H2O2. OxyR2 was oxidized to form a reversible C206 to C215 disulphide bond by sensing low levels of H2O2, which were insufficient to oxidize OxyR1, and only the oxidized OxyR2 activated prx2. OxyR25CA, in which all cysteine residues except for C206 and C215 were replaced with alanines, and its mutants, OxyR25CA-C206S and OxyR25CA-C215S, were constructed. OxyR25CA and OxyR25CA-C215S directly bound to a specific binding sequence centred at −56.5 from the prx2 transcription start site, albeit with different binding affinities. The binding sequence consisted of four ATCGnt elements spaced by a helical turn and aligned in the twofold dyad symmetry, suggesting that OxyR2 binds DNA as a tetramer. OxyR25CA-C206S also directly bound to DNA comprising more extended sequences, indicating that oxidized and reduced OxyR2 adopt different conformational states, leading to altered DNA contacts. The oxyR2 mutation reduced cytotoxicity and growth during infection, indicating that OxyR2 is essential for the pathogenesis of V. vulnificus.

Background: Peroxiredoxins are ubiquitous antioxidant enzymes reducing toxic peroxides. Results: Two 2-Cys peroxiredoxins of Vibrio vulnificus are different in expression patterns, sensitivities to overoxidation, and kinetic properties. Conclusion: The two peroxiredoxins are optimized for detoxifying different ranges of H 2 O 2 . Significance: This study is the first report on the coexistence of the two peroxiredoxins along with their distinct roles in a single bacterium.

Pathogenic Vibrio species cause diseases in diverse marine animals reared in aquaculture. Since their pathogenesis, persistence, and survival in marine environments are regulated by quorum sensing (QS), QS interference has attracted attention as a means to control these bacteria in aquatic settings. A few QS inhibitors of Vibrio species have been reported, but detailed molecular mechanisms are lacking. Here, we identified a novel, potent, and selective Vibrio QS inhibitor, named QStatin [1-(5-bromothiophene-2-sulfonyl)-1H-pyrazole], which affects Vibrio harveyi LuxR homologues, the well-conserved master transcriptional regulators for QS in Vibrio species. Crystallographic and biochemical analyses showed that QStatin binds tightly to a putative ligand-binding pocket in SmcR, the LuxR homologue in V. vulnificus, and changes the flexibility of the protein, thereby altering its transcription regulatory activity. Transcriptome analysis revealed that QStatin results in SmcR dysfunction, affecting the expression of SmcR regulon required for virulence, motility/chemotaxis, and biofilm dynamics. Notably, QStatin attenuated representative QS-regulated phenotypes in various Vibrio species, including virulence against the brine shrimp (Artemia franciscana). Together, these results provide molecular insights into the mechanism of action of an effective, sustainable QS inhibitor that is less susceptible to resistance than other antimicrobial agents and useful in controlling the virulence of Vibrio species in aquacultures.

SAAving vitamin A–mediated immunity The vitamin A metabolite retinol is critical for B and T cell development and homing to the gut. Intestinal myeloid cells such as dendritic cells and macrophages take up retinol and process it into retinoic acid (RA), which in turn initiates RA-dependent gene expression programs in lymphocytes. Bang et al . identified LDL receptor-related protein 1 (LRP1) as a myeloid cell surface receptor for retinol. LRP1 binds retinol chaperoned by serum amyloid A (SAA) proteins, and SAA–retinol complexes are then endocytosed and metabolized by myeloid cells. Mice lacking either Saa or myeloid-specific Lrp1 exhibited profound impairments in vitamin A–mediated immunity. B and T cell trafficking to the intestine, immunoglobulin A production by B cells, and protection from enteric Salmonella Typhimurium infection were all diminished when either of these crucial players was missing. —STS

Vitamin A is a dietary component that is essential for the development of intestinal immunity. Vitamin A is absorbed and converted to its bioactive derivatives retinol and retinoic acid by the intestinal epithelium, yet little is known about how epithelial cells regulate vitamin A-dependent intestinal immunity. Here we show that epithelial cell expression of the transcription factor retinoic acid receptor β (RARβ) is essential for vitamin A-dependent intestinal immunity. Epithelial RARβ activated vitamin A-dependent expression of serum amyloid A (SAA) proteins by binding directly to Saa promoters. In accordance with the known role of SAAs in regulating Th17 cell effector function, epithelial RARβ promoted IL-17 production by intestinal Th17 cells. More broadly, epithelial RARβ was required for the development of key vitamin A-dependent adaptive immune responses, including CD4+ T-cell homing to the intestine and the development of IgA-producing intestinal B cells. Our findings provide insight into how the intestinal epithelium senses dietary vitamin A status to regulate adaptive immunity, and highlight the role of epithelial cells in regulating intestinal immunity in response to diet.