Deinococcus radiodurans

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iDeinococcus radiodurans

D. radiodurans

Scientific classification

Kingdom:	Bacteria
Phylum:	Deinococcus-Thermus
Order:	Deinococcales
Genus:	Deinococcus
Species:	D. radiodurans

Binomial name

Deinococcus radiodurans
Brooks & Murray, 1981

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.

The term Deinobacter has been replaced by Deinococcus based on evaluation of ribosomal RNA sequences.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.

1 History
2 Radioactivity resistance mechanisms
3 Applications
4 References
5 See also
6 External links

[edit] 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.

[edit] Radioactivity resistance mechanisms

Deinococcus accomplishes its resistance to radiation by having multiple copies of its genome and rapid DNA repair mechanisms. As a consequence of its hardiness it has been nicknamed "Conan the Bacterium" (after Conan the Barbarian); its official name literally means "strange berry that withstands radiation".

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. 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 <ref>Mattimore V, Battista JR (1996). "Radioresistance of Deinococcus radiodurans: functions necessary to survive ionizing radiation are also necessary to survive prolonged desiccation". JOURNAL OF BACTERIOLOGY 178 (3): 633–637. PMID 8550493.</ref>. In addition to DNA repair, D. radiodurans use LEA (Late Embryogenesis Abundant) protein<ref>Goyal K, Walton LJ, Tunnacliffe A (2005). "LEA proteins prevent protein aggregation due to water stress". BIOCHEMICAL JOURNAL 388 (Part 1): 151–157. PMID 15631617.</ref> expression to protect against desiccation<ref>Battista JR, Park MJ, McLemore AE (2001). "Inactivation of two homologues of proteins presumed to be involved in the desiccation tolerance of plants sensitizes Deinococcus radiodurans R1 to desiccation". CRYOBIOLOGY 43 (2): 133–139. PMID 11846468.</ref>.

Michael Daly of the Uniformed Services University of the Health Sciences suggests that the bacterium uses manganese to protect itself against radiation damage<ref>Pearson, Helen (30 September 2004). Secret of radiation-proof bugs proposed. Internal antioxidants may shield cells from radiation damage. news@nature.com. Retrieved on 2006-06-19.</ref>.

Scanning electron microscopy analysis has shown that DNA in D. radiodurans is organized into tightly packed toroids, which may facilitate DNA repair<ref>Levin-Zaidman S, Englander J, Shimoni E, Sharma AK, Minton KW, Minsky A (2003). "Ringlike structure of the Deinococcus radiodurans genome: a key to radioresistance?". SCIENCE 299 (5604): 254–256. PMID 12522252.</ref>.

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<ref name="MRadman">Zahradka K, Slade D, Bailone A, Sommer S, Averbeck D, Petranovic M, Lindner AB, Radman M (2006). "Reassembly of shattered chromosomes in Deinococcus radiodurans". NATURE 443 (7111): 569-573. PMID 17006450.</ref>.

[edit] 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<ref>Brim H, McFarlan SC, Fredrickson JK, Minton KW, Zhai M, Wackett LP, Daly MJ (2000). "Engineering Deinococcus radiodurans for metal remediation in radioactive mixed waste environments". NATURE BIOTECHNOLOGY 18 (1): 85–90. PMID 10625398.</ref>. 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<ref name="MRadman">Zahradka K, Slade D, Bailone A, Sommer S, Averbeck D, Petranovic M, Lindner AB, Radman M (2006). "Reassembly of shattered chromosomes in Deinococcus radiodurans". NATURE 443 (7111): 569-573. PMID 17006450.</ref>.