Neighbor-Joining unrooted phylogenetic tree was built using the PHYLIP program on the basis of 16S rRNA sequences. Five Thermus species (T. thermophilus, T. flavus, T. aquaticus, T. igniterrae, T. filiformis) are included as an outgroup. The Deinococcus/Thermus group is deeply branched in bacterial phylogenetic trees with putative relationships with the actinobacteria/cyano-bacteria branch. The tree reconstructions can be found elsewhere (see for example, Wolf et al., BMC Evol. Biol. 2001 Oct 20;1(1):8)
Most radiation resistant bacteria that have been reported are spore-forming pathogens and are not remarkably radiation resistant when growing vegetatively [Thornley et al., 1963; Van Gerwen et al., 1999]. Bacteria belonging to the family Deinococcaceae are distinctly unusual [Minton, 1994; Minton, 1996; Battista et al., 1999]. Not only are they the most radiation resistant organisms discovered, but also they are vegetative, easily cultured, and nonpathogenic. Despite ubiquitous distribution and ancient derivation, only about twenty species of the family Deinococcaceae have been described [Minton, 1996; Ferreira et al., 1997; Makarova et al., 2007]. Deinococcus radiodurans (strain R1) was the first of the deinobacteria to be discovered and was isolated in Oregon in 1956 [Anderson et al., 1956] from X-ray sterilized canned meat that had spoiled. Culture yielded a red-pigmented, non-sporulating, Gram-positive coccus that was extremely resistant to ionizing-radiation, ultra violet light (UV), hydrogen peroxide and numerous other agents that damage DNA [Moseley et al., 1983; Minton, 1994; Wang et al., 1995; Lange et al., 1998], as well as highly resistant to desiccation [Sanders et al., 1979; Mattimore et al., 1996]. It is an aerobic, tetrad-forming soil bacterium that is most famous for its extreme resistance to ionizing radiation; it not only can survive acute exposures to gamma radiation that exceed 15,000 Gy without lethality or induced mutation [Daly et al., 1994], but it also displays luxuriant growth in the presence of chronic irradiation (60 Gy/hour) [Lange et al., 1997] without any effect on its growth rate. For comparison, an acute exposure of just 5-10 Gy is lethal to the average human [Thornley et al., 1963]. Adding to the growing resource of genetic technologies available for D. radiodurans is the recent whole-scale sequencing and annotation of its genome [White et al., 1999; Makarova et al., 2001]. The D. radiodurans strain R1 genome consists of two chromosomes (DR_Main: 2.65 Mbp and DR412: 412 kbp ), one megaplasmid (DR177: 177 kbp), and one plasmid (46 kbp) [White et al., 1999], encoding 3,193 predicted genes. This combination of factors has positioned D. radiodurans as a promising candidate for the study of mechanisms of DNA damage and repair, as well as its exploitation for practical purposes such as bioremediation of radioactive waste sites.
Shortly after the isolation of D. radiodurans R1 in 1956, a second strain of D. radiodurans (Sark) was discovered as an air contaminant in a hospital in Ontario [Murray et al., 1958]. Since then, six closely related radiation-resistant species have been identified: Deinococcus radiopugnans from haddock tissue [Davis et al., 1963], Deinococcus radiophilus from Bombay duck [Davis et al., 1963], Deinococcus proteolyticus from the feces of Lama glama [Kobatake et al., 1973], the rod-shaped Deinococcus grandis from elephant feces [Oyaizu et al., 1987], and the two slightly thermophilic species Deinococcus geothermalis and Deinococcus murrayi from hot springs in Portugal and Italy, respectively [Ferreira et al., 1997]. These species together form a distinct eubacterial phylogenetic lineage, believed to be most closely related to the Thermus genus, and it has been proposed that Deinococcus and Thermus form a eubacterial phylum [Woese et al., 1987; Weisberg et al., 1989; Woese et al., 1992; Narumi et al., 1996; Rainey et al., 1997]. To date, the natural distribution of the deinobacteria has not been explored by systematic investigation. Isolations have been world-wide, but diverse and patchy in distribution. In addition to those noted above, sites of isolation include damp soil near a lake in England [Masters et al., 1991], weathered granite from the Antarctic Dry Valleys [Counsell et al., 1986], irradiated medical instruments, air purification systems, a shielding pool for a 60Co radiation source in Denmark [Kristensen et al., 1981; Christensen et al., 1981], and numerous isolations by this laboratory from desert environments. It is possible that their extreme proficiency at DNA repair is related to the selective advantage in environments prone to damage arising during long periods of desiccation [Mattimore et al., 1996].
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