Molecular Mechanisms for Cellular Responses to Ionizing Radiation and Oxidative Stresses

大肠杆菌具有一个调节子，该调节子是由soxRS调节子介导的超氧化物生成（氧化还原循环）剂诱导的氧化应激反应。soxRS调节子的诱导发生在转录激活的两个阶段。由超氧化物应激直接激活的SoxR蛋白刺激soxS基因的转录，这反过来激活soxRS调节子中的某些基因。SoxR是一种34 kDa的同源二聚体，在多肽链中含有一个氧化还原活性[2Fe-2S]簇。关于这种激活过程的分子机制知之甚少。在厌氧条件下，SoxR蛋白也被SH氧化剂二酰胺激活，这表明SoxR是通过其氧化还原状态激活的。我们发现，SoxS的诱导在缺乏谷氧还蛋白和硫氧还蛋白系统的基因的突变体中增强。另一方面，还原型谷氧还蛋白的过表达降低了甲萘醌对SoxS的诱导水平。大肠杆菌（E.coli）细胞具有特异性的抗H_2O_2防御系统，转录激活因子OxyR介导了这种防御系统。本实验旨在阐明硫氧还蛋白、谷氧还蛋白及其还原酶系统在OxyR的氧化还原中的作用。利用OxyR特异性抗体，我们发现大肠杆菌中硫氧还蛋白和谷氧还蛋白的氧化还原状态影响OxyR的氧化还原。OxyR的氧化还原状态与kat G：：lacZ基因的表达水平和动力学密切相关。大肠杆菌trxAgrxA突变株在未经H_2O^2处理的情况下，β-半乳糖苷酶的活性仍然很高。突变体细胞含有显著量的激活形式的OxyR。

Escherichia coli possesses a regulon which is induced in response to oxidative stress by superoxide-generating (redox-cycling) agents mediated by the soxRS regulator. Induction of the soxRS regulon occurs in two stages of transcriptional activation. SoxR protein that is directly activated by superoxide stress stimulates the transcription of the soxS gene, which in turn activates certain genes in the soxRS regulon. SoxR is a 34-kDa homodimer containing one redox-active [2Fe-2S] cluster in the polypeptide chain. Little is known about the molecular mechanisms of such activation process. The SoxR protein is also activated by diamide, a SH-oxidizing agent, under anaerobic conditions, indicating that the SoxR is activated via its redox state. We found that the induction of SoxS is enhanced in the mutants lacking the genes of the glutaredoxin and thioredoxin systems. On the other hand, the over-expression of reduced form of glutaredoxin decreased the level of SoxS induction by menadione. E.coli cells possess a specific defense system against H_2O_2 mediated by the transcriptional activator OxyR.The inactive form of OxyR protein present in unstressed cell is oxidized upon exposure to peroxide stress and converted into its active conformation as a transcriptional activator. Reversible intramolecular disulfide bond formation between Cys-199 and Cys-208 regulates the activity of OxyR.The present experiments were done to elucidate the role of thioredoxin, glutaredoxin and their reductase systems in oxidation and reduction of OxyR.By using the OxyR-specific antibody, we found that the oxidation-reduction of OxyR was affected by the redox states of thioredoxin and glutaredoxin in E.coli. The redox state of OxyR well correlated to the level and kinetics of the expression of kat G : : lacZ gene. The high level of β-galactosidase activity occurred in E.coli trxAgrxA mutant without treatment with H_2O^2. The mutant cells contained significant amount of activated form of OxyR.