Deinococcus radiodurans is one of the bacterium that is resistant to ionizing radiation. To say, that they evolved this capacity by natural selection or mutation is difficult, because the background radiation on earth is very low as compared to what this organism can endure for it to have evolved in this manner. However, it has been observed that dessication also causes double stranded DNA breaks similar to that which occur after radiation exposure. Hence, it can be rationalized that Deinococcus actually developed a mechanism to combat dessication or dehydration which is now being proved useful to combat high doses of radiation.
This is not the only bacterium that shows radiation resistance . There are some other bacteria (Kineococcus radiotolerans, Rubrobacter xylanophilus) as well as archae (Thermococcus gammatolerans) that show radiation resistance. However this particular guy has received much attention. How come these apparently unrelated species show resistance to high doses of radiation? There have been 2 postulates about this. One states that radiation resistance was a widespread phenomenon in the beginning of the world and slowly has been lost by the species. The other says it is a result of convergent evolution or horizontal gene transfer.
It has been found that Deinococcus radiotolerans can survive a radiation dose of 5000 Gy without any adverse effect. What helps it withstand this high stress level? The factors responsible for this ability as well as the exact mechanisms have still not been confirmed but the possibilities have been enumerated.
The Deinococcus has a segmented genome with two chromosomes 2.64 Mb (Chromosome I) and 0.41 Mb (Chromosome II). Apart from this it also has a 0.18 Mb megaplasmid and a 0.045 Mb plasmid. It has between 4 to 10 genome copies per cell depending upon the bacterial growth phase which are stacked one over the other . The more the number of genome copies, more is the resistance to radiation stress. More the number of genome copies means there is more probability of survival of crucial genetic information. This information from the surviving copies may then be used to repair the other damaged strands. The condensed genome is also said to play a part in radiation resistance. Since, it is tightly packed together, even though the strands may be fragmented they cannot diffuse away from each other and hence repair may become somewhat easier.
It has been experimentally found that in presence of a lot of Mn (II) the bacterium can withstand radiation stress better than when it is depleted of them. However in both the instances, the impact is the same, its only that the survival is better in presence of Mn (II). It is not that the bacterium does not undergo damage but it has a very efficient and error free repair mechanism to repair the damage that has taken place. In fact, the cell growth is halted till all the damaged DNA is repaired and then only the bacterium continues with its life cycle.
Also, new loci have been found in the genome suggesting newer enzymes or at least novel mechanisms of DNA repair that need to be further looked into. However a lot of work still needs to be carried out so that the mysteries of this "strange" bacterium are revealed.
Reference:
http://www.biochem.wisc.edu/faculty/cox/lab/pdfs/38.pdf
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