Deinococcus radiodurans is capable of withstanding an acute dose of 5,000
grays (Gy), or 500,000 rad, of
ionizing radiation with almost no loss of viability, and an acute dose of 12,000 grays with 10% survivability. A dose of 5,000 Gy is estimated to introduce several dozens double-strand breaks (DSBs) into the organism's DNA: given the estimated rate of 0.005 DSB/Gy/Mbp, the approximately 3.2 Mbp bacterial genome should have received 80 DSBs if it was haploid. For comparison, a chest X-ray or Apollo mission involves about 1 mGy, 5 Gy can kill a human, 200–800 Gy will kill
E. coli, and more than 4,000 Gy will kill the radiation-resistant
tardigrade.
Mechanisms of ionizing-radiation resistance DNA structure Deinococcus accomplishes its resistance to radiation by having multiple copies of its
genome.
Scanning electron microscopy analysis has shown that DNA in
D. radiodurans is organized into tightly packed
toroids, which may facilitate DNA repair.
DNA repair Deinococcus radiodurans has a unique quality in which it can repair both
single- and
double-stranded DNA. When damage is apparent to the cell, it brings the damaged DNA into a compartmental ring-like structure where the DNA is repaired, and then is able to fuse the nucleoids from the outside of the compartment with the damaged DNA.
Deinococcus usually repairs breaks in its chromosomes within 12–24 hours by a 2-step process. • First,
D. radiodurans reconnects some chromosome fragments by a process called
single-stranded annealing. This is facilitated having multiple copies of the genome, and as few as two copies can perform annealing. Partially overlapping fragments are then used for synthesis of
homologous regions through a moving
D-loop that can continue extension until the fragments find
complementary partner strands.
Deinococcus radiodurans is capable of genetic transformation, a process by which DNA derived from one cell can be taken up by another cell and integrated into the recipient genome by homologous recombination. This may help if the DNA in a single cell is insufficient for repair into a complete chromosome. Natural genetic transformation under stressful conditions in
D. radiodurans is associated with
repair of DNA damage. When DNA damages (e.g. pyrimidine dimers) are introduced into donor DNA by UV irradiation, the recipient cells efficiently repair the damages in the transforming DNA, as they do in cellular DNA, when the cells themselves are irradiated.
Additional protective mechanisms Michael Daly has suggested the bacterium uses
manganese complexes as
antioxidants to protect itself against radiation damage. In 2007 his team showed that high intracellular levels of manganese(II) in
D. radiodurans protect proteins from being oxidized by radiation, and they proposed the idea that "protein, rather than DNA, is the principal target of the biological action of [ionizing radiation] in sensitive bacteria, and extreme resistance in Mn-accumulating bacteria is based on protein protection". In 2016, Massimiliano Peana
et al. reported a spectroscopic study through NMR, EPR, and ESI-MS techniques on the Mn(II) interaction with two peptides, DP1 (DEHGTAVMLK) and DP2 (THMVLAKGED), whose amino acid composition was selected to include the majority of the most prevalent amino acids present in a Deinococcus radiodurans bacterium cell-free extract that contains components capable of conferring extreme resistance to ionizing radiation. In 2018, M. Peana and C. Chasapis reported by a combined approach of bioinformatic strategies based on structural data and annotation, the Mn(II)-binding proteins encoded by the genome of DR and proposed a model for Manganese interaction with DR proteome network involved in ROS response and defense. In 2009,
nitric oxide was reported to play an important role in the bacteria's recovery from radiation exposure: the gas is required for division and proliferation after DNA damage has been repaired. A gene was described that increases nitric oxide production after UV radiation, and in the absence of this gene, the bacteria were still able to repair DNA damage, but would not grow. A few more mechanisms (LEA and SDBC) are described in the following section.
Evolution of ionizing-radiation resistance A persistent question regarding
D. radiodurans is how such a high degree of radioresistance could evolve. Natural
background radiation levels are very low—in most places, on the order of 0.4 mGy per year, and the highest known background radiation, near
Ramsar, Iran, is only 260 mGy per year. With naturally occurring background radiation levels so low, organisms evolving mechanisms specifically to ward off the effects of high radiation are unlikely. In the distant geological past, higher background radiation existed both due to more
primordial radionuclides not yet having decayed and due to effects of things like the
natural nuclear fission reactors at Oklo, Gabon, which were active some 1.7
billion years ago. However, even if adaptations to such conditions
did evolve during that time,
genetic drift would almost certainly have eliminated them if they provided no (other) evolutionary benefit. A team of Russian and American scientists proposed that the radioresistance of
D. radiodurans had a
Martian origin. They suggested that evolution of the microorganism could have taken place on the Martian surface until it was delivered to Earth on a
meteorite. However, apart from its resistance to radiation,
Deinococcus is genetically and biochemically very similar to other terrestrial life forms, arguing against a unique extraterrestrial origin. Valerie Mattimore of
Louisiana State University has suggested the radioresistance of
D. radiodurans is simply a side effect of a mechanism for dealing with prolonged cellular
desiccation (dryness). To support this hypothesis, she performed an experiment in which she demonstrated that mutant strains of
D. radiodurans that are highly susceptible to damage from
ionizing radiation are also highly susceptible to damage from prolonged desiccation, while the wild-type strain is resistant to both. It was also shown that desiccation induces double-stranded DNA breaks with patterns similar to extreme ionizing radiation. In addition to DNA repair,
D. radiodurans use LEA proteins (
Late Embryogenesis Abundant proteins) expression to protect against desiccation. In this context, also the robust cell envelope of
D. radiodurans through its main protein complex, the S-layer Deinoxanthin Binding Complex (SDBC), strongly contributes to both physiological functions and its extreme radioresistance. In fact, this protein complex acts as a shield against electromagnetic stress, as in the case of ionizing radiation exposure, but also stabilizes the cell envelope against possible consequent high temperatures and desiccation. ==Applications==