In addition, expression of oxyR is positively regulated by cAMP-activated Crp protein during exponential growth and negatively regulated by RpoS when cells enter stationary phase [38] Based on microarray assays, a new function as regulator of S-nitrosylation (a nitrosative stress regulon) under anaerobic conditions has been determined for OxyR [4] and it is distinct from the OxyR regulon activated by oxidative stress [4] OxyR consists of 305 amino acid residues and is a member of the LysR family of bacterial transcription factors [3, 7] It is a tetramer in solution [] OxyR consists of two domains, the N-terminal domain, carrying a helix-turn-helix motif, and the C-terminal regulatory domain. A flexible linker connects both domains. The C-terminal domain contains the determinants for oligomerization and two redox-active cysteines, Cys199 and Cys208 [] OxyR functions as a reversible cellular redox switch. In the presence of 100 nM H₂O₂, activation occurs by the formation of an intramolecular disulfide bond between Cys199 and Cys208 [14] It was determined that concentrations of >2 μM extracellular H₂O₂ trigger the intracellular OxyR stress response, and >5 μM begins to impair cell growth in minimal medium [] Consistent with this, in minimal medium dose-dependent growth retardation occurred in the wild type and in a Glucose Minimal Media Optimized Strain (GMOS), but no significant impact of H₂O₂ on the growth profile of iron-evolved strains was noted, as overexpression of OxyR-regulated ROS-detoxifying genes rendered the strains tolerant to peroxide []. In contrast with activation, inactivation of OxyR involves reduction of the disulfide bond by glutaredoxin 1 and glutathione [] Oxidation is significantly faster than reduction, allowing for transient activation in a reducing environment [] OxyR is also activated under nitrosative stress; this reaction involves S-nitrosylation of OxyR [] The crystal structures of the regulatory domain of OxyR in both its reduced and oxidized forms have been solved. Comparison of both structures reveals that oxidation is accompanied by a significant structural change in the regulatory domain [] OxyR also senses sulfane sulfur (H₂S_n), which is toxic under both anaerobic and aerobic conditions at high levels. OxyR is required to deal with H₂O₂ only under aerobic conditions, whereas it responds to H₂S_n under both aerobic and anaerobic conditions [39] Sulfane sulfur modifies OxyR at Cys¹⁹⁹ to form a protein persulfide that in turn activates the expression of thioredoxin, glutaredoxin, and catalase. The Cys residues C199 and C208 are required for such induction, and OxyR Cys¹⁹⁹ persulfidation is likely the mechanism of sensing H₂S_n [39] In systematic studies of oligomerization, it was shown that some members of the LysR family, like OxyR, interact with other members of the family to form heterodimers, but the physiological significance of this is unknown []. OxyR can bind as a pair of dimers bending to the DNA [] OxyR recognizes a motif comprised of ATAGnt elements spaced at 10-bp intervals [12] The binding to DNA differs significantly for the oxidized and reduced forms [12] Reduced OxyR binds DNA at two pairs of major grooves separated by one helical turn, while oxidized OxyR occupies four consecutive major grooves. When bound to DNA, oxidized OxyR stimulates transcription by interacting with the C-terminal domain of the α-subunit of RNA polymerase [] OxyR homologs are widely present in bacteria; 4772 homologs of OxyR were identified and distributed in 4494 bacterial genomes [39] A review of the function of OxyR in protecting against nitrosative stress under anaerobic conditions revealed parallels between regulation of S-nitrosylation and hypoxic signaling by nitric oxide (NO) in mammalian cells, and in mitochondria in particular [] Review: [].