Deinococcus radiodurans

Deinococcus radiodurans ("strange berry that withstands radiation", formerly called Micrococcus radiodurans) is an extremophilic bacterium, and is the most radioresistant organism known. While a dose of 10 Gy is sufficient to kill a human, and a dose of 60 Gy is sufficient to kill all cells in a culture of E. coli, D. radiodurans is capable of withstanding an instantaneous dose of up to 5,000 Gy with no loss of viability, and an instantaneous dose of up to 15,000 Gy with 37% viability. It can survive heat, cold, dehydration, vacuum, and acid, and because of its resistance to more than one extreme condition, D. radiodurans is known as a polyextremophile. It has also been listed as the world's toughest bacterium in "The Guiness Book Of World Records" because of its extraordinary resistance to several extreme conditions. It has been classified as a Gram-positive bacterium. The term Deinobacter has been replaced by Deinococcus based on evaluation of ribosomal RNA sequences. Several other species within the genus have been described, and they are related to heat-resistant bacteria such as Thermus; the group is accordingly known as Deinococcus-Thermus.

History
D. radiodurans was discovered in 1956 by A.W. Anderson at the Oregon Agricultural Experiment Station in Corvallis, Oregon. Experiments were being performed to determine if canned food could be sterilized using high doses of gamma radiation. A tin of meat was exposed to a dose of radiation that was thought to kill all known forms of life, but the meat subsequently spoiled. D. radiodurans was isolated from the meat.

Radioactivity 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-strand annealing. In the second step, a protein mends double-strand breaks through homologous recombination. As a consequence of its hardiness it has been nicknamed "Conan the Bacterium" (after Conan the Barbarian).

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 Guarapari, Brazil is only 175 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 and John R. Battista of Louisiana State University have suggested that the radioresistance of D. radiodurans is simply a side-effect of a mechanism for dealing with prolonged cellular desiccation (dryness). To support this hypothesis, they performed an experiment in which they 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 (Late Embryogenesis Abundant) protein expression to protect against desiccation.

Michael Daly of the Uniformed Services University of the Health Sciences suggests that the bacterium uses manganese to protect itself against radiation damage.

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 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 .

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. . It has been hypothesised that such objects might have attained sufficient kinetic energy to achieve escape velocity as a result of a major meteorite impact on the Martian surface (this being part of a hypothesised process known as panspermia). The heating effects of that impact and of entry into the Earth's atmosphere would have had to have been insufficient to sterilise such an object. However, apart from its resistance to radiation, Deinococcus is genetically and biochemically quite similar to other terrestrial life forms, arguing against an extraterrestrial origin.

Applications
Using genetic engineering Deinococcus has been used for bioremediation to consume and digest solvents and heavy metals, even in a highly radioactive site. The bacterial mercuric reductase gene has been cloned from Escherichia coli into Deinococcus to detoxify the ionic mercury frequently found in radioactive waste generated from nuclear weapons manufacture. Those researchers developed a strain of Deinococcus that could detoxify both mercury and toluene in mixed radioactive wastes.

Some have speculated that mechanisms of DNA repair used by D. radiodurans could be incorporated into the genome of higher species as a means of rejuvenation.

Some scientists have suggested that D. radiodurans could be genetically manipulated to produce various medicines.

The Craig Venter Institute has used a system derived from the rapid DNA repair mechanisms of D. radiodurans to assemble synthetic genes into chromosomes to make synthetic organisms.

Trivia

 * The computer game Anarchy Online features an item called Deinococcus radiodurans which is used to make a "stim" (potion) that reduces damage from radioactive sources.