Across the tree of life, radiation resistance is governed by antioxidant Mn ²⁺, gauged by paramagnetic resonance

Significance

Decades of functional genomic efforts have failed to predict the ability of cells to survive ionizing radiation (IR). Evidence is mounting that small high-symmetry antioxidant complexes of manganous ions with metabolites (H-Mn ²⁺) are responsible for cellular IR resistance, and that H-Mn ²⁺ protects the proteome, not the genome, from IR-induced reactive oxygen species. We show that the amount of H-Mn ²⁺ in nonirradiated living cells is readily gauged by electron paramagnetic resonance (EPR) spectroscopy and highly diagnostic of their DNA repair efficiency and survival after gamma-radiation exposure. This spectroscopic measure of cellular H-Mn ²⁺ content is the strongest known biological indicator of cellular IR resistance between and within organisms across the three domains of the tree of life, with potential applications including optimization of radiotherapy.

Abstract

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 ²⁺) 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 ²⁺ 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 ²⁺ present as H-complexes (H-Mn ²⁺), 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 ²⁺ complexes, not antioxidant enzymes (e.g., Mn superoxide dismutase), govern IR survival. As the pool of intracellular metabolites needed to form H-Mn ²⁺ 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.