In bacteria, OxyR is a peroxide sensor and transcription regulator, which can sense the presence of reactive oxygen species and induce antioxidant system. When the cells are exposed to H2O2, OxyR protein is activated via the formation of a disulfide bond between the two conserved cysteine residues (C199 and C208). In Deinococcus radiodurans, a previously unreported special characteristic of DrOxyR (DR0615) is found with only one conserved cysteine. dr0615 gene mutant is hypersensitive to H2O2, but only a little to ionizing radiation. Site-directed mutagenesis and subsequent in vivo functional analyses revealed that the conserved cysteine (C210) is necessary for sensing H2O2, but its mutation did not alter the binding characteristics of OxyR on DNA. Under oxidant stress, DrOxyR is oxidized to sulfenic acid form, which can be reduced by reducing reagents. In addition, quantitative real-time PCR and global transcription profile results showed that OxyR is not only a transcriptional activator (e.g., katE, drb0125), but also a transcriptional repressor (e.g., dps, mntH). Because OxyR regulates Mn and Fe ion transporter genes, Mn/Fe ion ratio is changed in dr0615 mutant, suggesting that the genes involved in Mn/Fe ion homeostasis, and the genes involved in antioxidant mechanism are highly cooperative under extremely oxidant stress. In conclusion, these findings expand the OxyR family, which could be divided into two classes: typical 2-Cys OxyR and 1-Cys OxyR.

In order to reveal the mechanisms of the extreme radioresistance and DNA repair in Deinococcus radiodurans, we examined proteome changes in a wild-type strain following gamma-irradiation using two-dimensional polyacrylamide gel electrophoresis and Silver-staining. The expression levels of 26 protein spots showed significant changes under radiation stress. Of these spots, 21 were identified with peptide mass fingerprinting using matrix-assisted laser desorption/ionization-time of flight mass spectrometry after tryptic in-gel digestion. These proteins exhibited various cellular functions, including (i) translation; (ii) transcription; (iii) signal transduction; (iv) post-translational modification, protein turnover, chaperones; (v) carbohydrate transport and metabolism; (vi) energy production and conversion; (vii) nucleotide transport and metabolism; (viii) inorganic ion transport and metabolism; (ix) DNA replication, recombination and repair; and (x) yet unknown. Most of the proteins have not previously been reported to be relevant to radioresistance.

Here we present direct evidence for the vital role of RecO in Deinococcus radiodurans's radioresistance. A recO null mutant was constructed using a deletion replacement method. The mutant exhibited a growth defect and extreme sensitivity to irradiation with gamma rays and UV light. These results suggest that DNA repair in this organism occurs mainly via the RecF pathway.

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