Salt shield: intracellular salts provide cellular protection against ionizing radiation in the halophilic archaeon, Halobacterium salinarum NRC-1

Summary The halophilic archaeon Halobacterium salinarum NRC-1 was used as a model system to investigate cellular damage induced by exposure to high doses of ionizing radiation (IR). Oxidative damages are the main lesions from IR and result from free radicals produc- tion via radiolysis of water. This is the first study to quantify DNA base modification in a prokaryote, revealing a direct relationship between yield of DNA lesions and IR dose. Most importantly, our data dem- onstrate the significance of DNA radiation damage other than strand breaks on cell survival. We also report the first in vivo evidence of reactive oxygen species scavenging by intracellular halides in H. sali- narum NRC-1, resulting in increased protection against nucleotide modification and carbonylation of protein residues. Bromide ions, which are highly reac- tive with hydroxyl radicals, provided the greatest pro- tection to cellular macromolecules. Modified DNA bases were repaired in 2 h post irradiation, indicating effective DNA repair systems. In addition, measure- ments of H. salinarum NRC-1 cell interior revealed a high Mn/Fe ratio similar to that of Deinococcus radio- durans and other radiation-resistant microorganisms, which has been shown to provide a measure of protection for proteins against oxidative damage. The work presented here supports previous studies showing that radiation resistance is the product of mechanisms for cellular protection and detoxification, as well as for the repair of oxidative damage to cellular macromolecules. The finding that not only Mn/Fe but also the presence of halides can decrease the oxida- tive damage to DNA and proteins emphasizes the significance of the intracellular milieu in determining microbial radiation resistance.