Deinococcus Radiodurans - Ionizing Radiation Resistance Mechanisms

Ionizing Radiation Resistance Mechanisms

Deinococcus accomplishes its resistance to radiation by having multiple copies of its genome and rapid DNA repair mechanisms. It usually repairs breaks in its chromosomes within 12–24 hours through a 2-step process. First, D. radiodurans reconnects some chromosome fragments through a process called single-stranded annealing. In the second step, a protein mends double-strand breaks through homologous recombination. This process does not introduce any more mutations than a normal round of replication would.

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.

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, he performed an experiment in which he demonstrated that mutant strains of D. radiodurans which 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. In addition to DNA repair, D. radiodurans use LEA proteins (Late Embryogenesis Abundant proteins) expression to protect against desiccation.

Scanning electron microscopy analysis has shown that DNA in D. radiodurans is organized into tightly packed toroids, which may facilitate DNA repair.

A team of Croatian and French researchers led by Miroslav Radman have bombarded D. radiodurans to study the mechanism of DNA repair. At least two copies of the genome, with random DNA breaks, can form DNA fragments through annealing. Partially overlapping fragments are then used for synthesis of homologous regions through a moving D-loop that can continue extension until they find complementary partner strands. In the final step, there is crossover by means of RecA-dependent homologous recombination.

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 proposed the idea that "protein, rather than DNA, is the principal target of the biological action of in sensitive bacteria, and extreme resistance in Mn-accumulating bacteria is based on protein protection".

A team of Russian and American scientists proposed that the radioresistance of D. radiodurans had a Martian origin. 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 an extraterrestrial origin.

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.