Winged-helix transcriptional factors play important roles in the control of gene expression in many organisms. In the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens, the winged-helix protein BigR, a member of the ArsR/SmtB family of metal sensors, regulates transcription of the bigR operon involved in bacterial biofilm growth. Previous studies showed that BigR represses transcription of its own operon through the occupation of the RNA polymerase-binding site; however, the signals that modulate its activity and the biological function of its operon are still poorly understood. Here we show that although BigR is a homodimer similar to metal sensors, it functions as a novel redox switch that derepresses transcription upon oxidation. Crystal structures of reduced and oxidized BigR reveal that formation of a disulfide bridge involving two critical cysteines induces conformational changes in the dimer that remarkably alter the topography of the winged-helix DNA-binding interface, precluding DNA binding. This structural mechanism of DNA association-dissociation is novel among winged-helix factors. Moreover, we demonstrate that the bigR operon is required for hydrogen sulfide detoxification through the action of a sulfur dioxygenase (Blh) and sulfite exporter. As hydrogen sulfide strongly inhibits cytochrome c oxidase, it must be eliminated to allow aerobic growth under low oxygen tension, an environmental condition found in bacterial biofilms, xylem vessels, and root tissues. Accordingly, we show that the bigR operon is critical to sustain bacterial growth under hypoxia. These results suggest that BigR integrates the transcriptional regulation of a sulfur oxidation pathway to an oxidative signal through a thiol-based redox switch.Winged-helix proteins belong to a larger ensemble of helixturn-helix (HTH) 4 factors employed by living organisms to sense and respond to diverse environmental cues (1). For example, in bacterial cells, winged-helix transcriptional factors function as metal sensors, which control metal tolerance (2). In the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens, the winged-helix repressor protein BigR (biofilm growth-associated repressor) controls the expression of the bigR operon through the recognition of the Ϫ10 region in the operator site, thus blocking transcription of the operon genes (3). bigR operons are evolutionarily conserved in some plantassociated bacteria and human opportunistic pathogens and encode BigR itself, membrane transporters, and Blh, a DUF442-␤-lactamase domain protein related to the mitochondrial sulfur dioxygenase ETHE1 involved in sulfide detoxification in mammals (3, 4). Previous studies have shown that the bigR operon is actively transcribed in Xylella and Agrobacterium biofilms and that mutants lacking BigR can attach more tightly to glass and root surfaces than normal bacteria. Although these results strongly indicated that the bigR operon plays an important role in bacterial biofilm formation or cell adhesion, the biological function ...

The citrus (Citrus sinensis) cyclophilin CsCyp is a target of the Xanthomonas citri transcription activator-like effector PthA, required to elicit cankers on citrus. CsCyp binds the citrus thioredoxin CsTdx and the carboxyl-terminal domain of RNA polymerase II and is a divergent cyclophilin that carries the additional loop KSGKPLH, invariable cysteine (Cys) residues Cys-40 and Cys-168, and the conserved glutamate (Glu) Glu-83. Despite the suggested roles in ATP and metal binding, the functions of these unique structural elements remain unknown. Here, we show that the conserved Cys residues form a disulfide bond that inactivates the enzyme, whereas Glu-83, which belongs to the catalytic loop and is also critical for enzyme activity, is anchored to the divergent loop to maintain the active site open. In addition, we demonstrate that Cys-40 and Cys-168 are required for the interaction with CsTdx and that CsCyp binds the citrus carboxyl-terminal domain of RNA polymerase II YSPSAP repeat. Our data support a model where formation of the Cys-40-Cys-168 disulfide bond induces a conformational change that disrupts the interaction of the divergent and catalytic loops, via Glu-83, causing the active site to close. This suggests a new type of allosteric regulation in divergent cyclophilins, involving disulfide bond formation and a loop-displacement mechanism.