Department of Chemistry, Northwestern University, Evanston, IL 60208.
Department of Pathology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814.
Proc Natl Acad Sci U S A. 2017 Oct 31;114(44):E9253-E9260. doi: 10.1073/pnas.1713608114. Epub 2017 Oct 17.
Despite concerted functional genomic efforts to understand the complex phenotype of ionizing radiation (IR) resistance, a genome sequence cannot predict whether a cell is IR-resistant or not. Instead, we report that absorption-display electron paramagnetic resonance (EPR) spectroscopy of nonirradiated cells is highly diagnostic of IR survival and repair efficiency of DNA double-strand breaks (DSBs) caused by exposure to gamma radiation across archaea, bacteria, and eukaryotes, including fungi and human cells. IR-resistant cells, which are efficient at DSB repair, contain a high cellular content of manganous ions (Mn) in high-symmetry (H) antioxidant complexes with small metabolites (e.g., orthophosphate, peptides), which exhibit narrow EPR signals (small zero-field splitting). In contrast, Mn ions in IR-sensitive cells, which are inefficient at DSB repair, exist largely as low-symmetry (L) complexes with substantially broadened spectra seen with enzymes and strongly chelating ligands. The fraction of cellular Mn present as H-complexes (H-Mn), as measured by EPR of live, nonirradiated Mn-replete cells, is now the strongest known gauge of biological IR resistance between and within organisms representing all three domains of life: Antioxidant H-Mn complexes, not antioxidant enzymes (e.g., Mn superoxide dismutase), govern IR survival. As the pool of intracellular metabolites needed to form H-Mn complexes depends on the nutritional status of the cell, we conclude that IR resistance is predominantly a metabolic phenomenon. In a cross-kingdom analysis, the vast differences in taxonomic classification, genome size, and radioresistance between cell types studied here support that IR resistance is not controlled by the repertoire of DNA repair and antioxidant enzymes.
尽管人们为了了解电离辐射（IR）抗性的复杂表型而进行了协同的功能基因组研究，但基因组序列并不能预测细胞是否具有 IR 抗性。相反，我们报告说，非照射细胞的吸收-显示电子顺磁共振（EPR）光谱高度诊断了暴露于伽马辐射时，从古菌、细菌和真核生物（包括真菌和人类细胞）的 DNA 双链断裂（DSB）的 IR 生存和修复效率。IR 抗性细胞在 DSB 修复方面效率很高，其细胞内含有高浓度的锰离子（Mn），这些 Mn 以高对称性（H）抗氧化复合物的形式存在，并与小代谢物（如正磷酸盐、肽）结合，表现出狭窄的 EPR 信号（小零场分裂）。相比之下，Mn 离子在 IR 敏感细胞中，这些细胞在 DSB 修复方面效率较低，主要以低对称性（L）复合物的形式存在，其光谱明显变宽，与酶和强螯合剂结合。通过对活的、非照射的 Mn 充足细胞的 EPR 测量，细胞内 Mn 以 H-复合物（H-Mn）形式存在的分数是目前已知的在代表生命三个领域的生物之间和内部最强的生物 IR 抗性的量度指标：抗氧化 H-Mn 复合物，而不是抗氧化酶（如 Mn 超氧化物歧化酶），控制着 IR 生存。由于形成 H-Mn 复合物所需的细胞内代谢物库取决于细胞的营养状态，我们得出结论，IR 抗性主要是一种代谢现象。在跨域分析中，这里研究的细胞类型在分类学分类、基因组大小和辐射抗性方面的巨大差异表明，IR 抗性不受 DNA 修复和抗氧化酶的 repertoire 控制。
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