Luo Li, Qi Ming-Sheng, Yao Shi-Yi, Cheng Hai-Ping, Zhu Jia-Bi, Yu Guan-Qiao
National Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China.
Acta Biochim Biophys Sin (Shanghai). 2005 Jun;37(6):421-8. doi: 10.1111/j.1745-7270.2005.00055.x.
The process of symbiotic nitrogen fixation results in the generation of reactive oxygen species such as the superoxide anion (O2-) and hydrogen peroxide (H2O2). The response of rhizobia to these toxic oxygen species is an important factor in nodulation and nitrogen fixation. In Sinorhizobium meliloti, one oxyR homologue and three catalase genes, katA, katB, and katC were detected by sequence analysis. This oxyR gene is located next to and divergently from katA on the chromosome. To investigate the possible roles of oxyR in regulating the expression of catalases at the transcriptional level in S. meliloti, an insertion mutant of this gene was constructed. The mutant was more sensitive and less adaptive to H2O2 than the wild type strain, and total catalase/peroxidase activity was reduced approximately fourfold with the OxyR mutation relative to controls. The activities of KatA and KatB and the expression of katA::lacZ and katB::lacZ promoter fusions were increased in the mutant strain compared with the parental strain grown in the absence of H2O2, indicating that katA and katB are repressed by OxyR. However, when exposed to H2O2, katA expression was also increased in both S. meliloti and Escherichia coli. When exposed to H2O2, OxyR is converted from a reduced to an oxidized form in E. coli. We concluded that the reduced form of OxyR functions as a repressor of katA and katB expression. Thus, in the presence of H2O2, reduced OxyR is converted to the oxidized form of OxyR that then results in increased katA expression. We further showed that oxyR expression is autoregulated via negative feedback.
共生固氮过程会产生活性氧物种，如超氧阴离子（O2-）和过氧化氢（H2O2）。根瘤菌对这些有毒氧物种的反应是结瘤和固氮的一个重要因素。通过序列分析，在苜蓿中华根瘤菌中检测到一个oxyR同源物和三个过氧化氢酶基因katA、katB和katC。该oxyR基因位于染色体上katA的旁边，且转录方向相反。为了研究oxyR在苜蓿中华根瘤菌转录水平上调控过氧化氢酶表达的可能作用，构建了该基因的插入突变体。与野生型菌株相比，该突变体对H2O2更敏感且适应性更差，相对于对照，OxyR突变导致总过氧化氢酶/过氧化物酶活性降低了约四倍。与在无H2O2条件下生长的亲本菌株相比，突变体菌株中KatA和KatB的活性以及katA::lacZ和katB::lacZ启动子融合体的表达增加，表明katA和katB受到OxyR的抑制。然而，当暴露于H2O2时，苜蓿中华根瘤菌和大肠杆菌中的katA表达也会增加。在大肠杆菌中，当暴露于H2O2时，OxyR会从还原形式转变为氧化形式。我们得出结论，OxyR的还原形式作为katA和katB表达的阻遏物发挥作用。因此，在H2O2存在的情况下，还原型OxyR会转变为氧化型OxyR，进而导致katA表达增加。我们进一步表明，oxyR表达通过负反馈进行自我调节。
