Bacterial redox sensors

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Abstract
Bacteria have sensitive and specific sensors — involving redox-active cofactors such as haem, flavins, pyridine nucleotides and iron–sulphur clusters, or redox-sensitive amino-acid side chains such as cysteine thiols — that monitor redox signals including oxygen, cytoplasmic redox state or the production of reactive oxygen species. Redox sensors control the processes that function to maintain redox homeostasis, usually at the level of transcription. This review explains the systems that have evolved to allow bacteria to sense and respond to different redox signals. Thiol-based sensor function is reviewed. Typically, these sensors use cysteine modification to sense redox alterations. Examples include OxyR in Escherichia coli, the σR-RsrA system in Streptomyces coelicolor, CrtJ and the RegB–RegA in Rhodobacter sphaeroides, and OhrR from Bacillus subtilis. The importance of Fe–S proteins in redox sensing is illustrated by the functions of several redox sensors from E. coli that use oxidation of Fe–S clusters to monitor the redox status of cell compartments and the environment to produce appropriate transcriptional responses — these include SoxR, Fnr, aconitase and IcsR. Haem also functions to sample redox states in bacteria — examples include FixL in Sinorhizobium meliloti and Dos in E. coli, both of which function as oxygen sensors that coordinate the haem cofactor to a PAS (PER–ARNT–SIM) fold. By contrast, in B. subtilis the haem sensor is coordinated to a globin fold in the HemAT protein, which controls flagellar rotation in response to oxygen tension in chemotaxis. Other redox sensors use the coenzymes FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide), which perceive redox states owing to their electron-carrying capacity. Those redox sensors reviewed here include the nitrogen-fixation regulation genes in Klebsiella pneumoniae, S. meliloti and Azotobacter vinelandii, and Aer, which mediates regulation of aerotaxis in E. coli. Similarly, NAD cofactors can shuttle electrons and are used by regulators described in this review. Examples of such regulators include the Rex repressor in S. coelicolor, which monitors NAD status and respiratory activity according to oxygen availability and seems to be conserved among Gram-positive bacteria, and CbbR from the chemoautotroph Xanthobacter flavus, which samples whether there is enough reducing power available for carbon fixation. Finally, the authors address the role of quinones in redox sensing. These cofactors might oxidize cysteines directly in the ArcB–ArcA system of E. coli that regulates gene expression under microaerobic and anaerobic growth conditions.