Metal Balance Helps Explain Survival of Microbial "Superhero" by Marcia Stone
Bacteria belonging to the family Deinococcaceae are some of the most radiation-resistant organisms yet discovered. Deinococcus (Micrococcus) radiodurans strain R1 (ATCC BAA-816) was first reported in 1956 by A. W. Anderson and coworkers of the Oregon Agricultural Experimental Station, Corvalis, Oregon. This obligate aerobic bacterium typically grows in rich medium as clusters of two cells (diplococci) in the early stages of growth, and as clusters of four cells (tetracocci) in the late stages of growth, is non-pathogenic, and best known for its ability to survive extremely high doses of acute ionizing radiation (10,000 Gy) without cell-killing. For comparison, 5 Gy is lethal to the average human, and 2,000 Gy can sterilize a culture of Escherichia coli. D. radiodurans is capable of growth under chronic radiation (60 Gy/hour) and resistant to other DNA damaging conditions including exposure to desiccation, ultraviolet (UV) light, and hydrogen peroxide. The genes and cellular pathways underlying the survival strategies of D. radiodurans are under investigation, and its resistance characteristics are being exploited in the development of bioremediation processes for cleanup of highly radioactive US Department of Energy waste sites, and in the development of radioprotectors.
Death By Protein Damage
The modern founding concept of radiation biology that deals with X-rays and γ-rays is that ionizing radiation is dangerous because of its damaging effects on DNA. Mounting experimental evidence does not fit into this theoretical framework, instead supporting that radiation resistance is governed by protein damage. Recent studies from several independent labs implicate protein damage as the major probable cause of death in irradiated cells. Whereas DNA lesion-yields in cells exposed to a given dose of radiation appear to be fixed, protein-lesion yields are variable and closely related to survival. There are profound practical implications to this new view of radiation toxicity. Basically, if you want to survive radiation, protect your proteins! D. radiodurans has shown us how to protect proteins from radiation and other sources of reactive oxygen species (ROS). For the latest review see Michael J Daly (2012) Death by protein damage in irradiated cells.
One original goal of radiobiology was to explain why cells are so sensitive to ionizing radiation (IR). Early studies in bacteria incriminated DNA as the principal radiosensitive target, an assertion that remains central to modern radiation toxicity models. More recently, the emphasis has shifted to understanding why bacteria such as Deinococcus radiodurans are extremely resistant to IR (1), by focusing on DNA repair systems expressed during recovery from high doses of IR (2). Unfortunately, as key features of DNA-centric hypotheses of extreme resistance have grown weaker (3), the study of alternative cellular targets has lagged far behind, mostly because of their relative biological complexity. Recent studies have shown that extreme levels of bacterial IR resistance correlate with high intracellular Mn(II) concentrations (4), and resistant and sensitive bacteria are equally susceptible to IR-induced DNA damage (~0.005 DSB/Gy/haploid genome). Our recent work has established a mechanistic link between the orthophosphate complex of Mn2+ and protection of proteins from radiation damage (5a, 5b, 5c). In contrast to resistant bacteria, naturally sensitive bacteria are highly susceptible to IR-induced protein oxidation. We have proposed that sensitive bacteria sustain lethal levels of protein damage at radiation doses that elicit relatively little DNA damage, and that extreme resistance in bacteria is dependent on protein protection (6).
A breakthrough application developed from Death by Protein Damage has been the preparation of ionizing radiation-sterilized whole-bacterial cell and whole-virus vaccines. Our approach to protecting proteins at supra-lethal doses (25-40 kGy) has been successfully tested, where the epitopes of cells and viruses treated with reconstituted Deinococcus Mn-peptide complexes (Cell Host & Microbe, 12(1):117-124, 2012) survive doses of gamma-radiation which obliterate their genomes. This approach has produced mouse-vaccines which are absolutely non-infective yet highly immunogenic and protective. The rapidity of vaccine development that is achieved by killing whole isolated pathogens makes this a powerful approach against bioterror threats and emerging infections caused by poorly characterized new or rapidly mutating agents, such as pandemic influenza and HIV.
Image overlay of transmission electron microscopy, light microscopy, and x-ray fluorescence microprobe analyses of Deinococcus radiodurans. Depth-average abundance of Mn (blue, green, pink) and Fe (red) are shown within a single D. radiodurans diplococcus.
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