Shi Miaomiao, Wan Fen, Mao Yinting, Gao Haichun
Institute of Microbiology and College of Life Sciences, Zhejiang University, Hangzhou, Zhejiang, China.
Key Laboratory for Agro-Microbial Research and Utilization, Zhejiang Province, Hangzhou, Zhejiang, China.
J Bacteriol. 2015 Jul;197(13):2179-2189. doi: 10.1128/JB.00154-15. Epub 2015 Apr 20.
Oxidative stresses triggered by reactive oxygen species (ROS) that damage various cellular components are unavoidable for virtually all living organisms. In defense, microorganisms have evolved sophisticated mechanisms to sense, respond to, and battle against ROS. Shewanella oneidensis, an important research model for applied and environmental microbes, employs OxyR to mediate the response to H2O2 by derepressing the production of the major H2O2 scavenger KatB as a major means toward these goals. Surprisingly, despite enhanced H2O2 degradation, the oxyR mutant carries a viability deficiency phenotype (plating defect), which can be suppressed by the addition of exogenous iron species. Experiments showed that the defect was not due to iron starvation. Rather, multiple lines of evidence suggested that H2O2 generated abiotically in lysogeny broth (LB) is responsible for the defect by quickly killing mutant cells. We then showed that the iron species suppressed the plating defect by two distinct mechanisms, either as an H2O2 scavenger without involving living cells or as an environmental cue to stimulate an OxyR-independent response to help cells cope with oxidative stress. Based on the suppression of the plating defect by overproduction of H2O2 scavengers in vivo, we propose that cellular components that are vulnerable to H2O2 and responsible for the defect may reside outside the cytoplasm.
In bacteria, OxyR is the major regulator controlling the cellular response to H2O2. The loss of OxyR results in reduced viability in many species, but the underlying mechanism is unknown. We showed in S. oneidensis that this defect was due to H2O2 generated abiotically in LB. We then showed that this defect could be corrected by the addition of Fe(2+) or catalase to the LB or increased intracellular production of catalase. Further analyses revealed that Fe(2+) was able not only to decompose H2O2 directly but also to stimulate the activity of OxyR-independent H2O2-scavenging enzymes. Our data indicate that iron species play a previously underappreciated role in protecting cells from H2O2 in environments.
活性氧（ROS）引发的氧化应激会损害各种细胞成分，这对几乎所有生物体来说都是不可避免的。作为防御机制，微生物已经进化出复杂的机制来感知、响应并对抗ROS。嗜铁钩端螺旋菌（Shewanella oneidensis）是应用微生物和环境微生物的重要研究模型，它利用OxyR通过解除对主要H2O2清除剂KatB产生的抑制来介导对H2O2的响应，以此作为实现这些目标的主要手段。令人惊讶的是，尽管H2O2降解增强，但oxyR突变体具有生存力缺陷表型（平板接种缺陷），添加外源铁物种可以抑制这种表型。实验表明，该缺陷并非由于铁饥饿。相反，多条证据表明，溶原肉汤（LB）中非生物产生的H2O2通过快速杀死突变细胞导致了该缺陷。然后我们表明，铁物种通过两种不同机制抑制平板接种缺陷，要么作为不涉及活细胞的H2O2清除剂，要么作为一种环境信号来刺激不依赖OxyR的响应，以帮助细胞应对氧化应激。基于体内过量产生H2O2清除剂对平板接种缺陷的抑制作用，我们提出，易受H2O2影响并导致该缺陷的细胞成分可能存在于细胞质之外。
在细菌中，OxyR是控制细胞对H2O2响应的主要调节因子。OxyR的缺失导致许多物种的生存力降低，但其潜在机制尚不清楚。我们在嗜铁钩端螺旋菌中表明，这种缺陷是由于LB中非生物产生的H2O2所致。然后我们表明，向LB中添加Fe(2+)或过氧化氢酶或增加细胞内过氧化氢酶的产生可以纠正这种缺陷。进一步分析表明，Fe(2+)不仅能够直接分解H2O2，还能够刺激不依赖OxyR的H2O2清除酶的活性。我们的数据表明，铁物种在保护细胞免受环境中H2O2影响方面发挥了以前未被充分认识的作用。
