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Ashley Charpentier Microbe Wiki 04/05/16

Deinococcus radiodurans’ Viability on Mars

Figure 1. http://i.telegraph.co.uk/multimedia/archive/03455/mars2_3455949b.jpg

Background and Historical Significance

   Deinococcus radiodurans has been considered as a model for extraterrestrial life due to its nature as an polyextremophile.  D. radiodurans is known to survive in extreme levels of drought, lack of nutrients, low temperatures, and, most notably, high levels of radiation.  It is able to withstand up to 1,000 times the amount of radiation which would prove lethal to the average human.  This bacterium can be found in both the arid and nutrient deficient environment of the Antarctic Dry Valley as well as nutrient rich environments such as animal feces[4].  Because of its tolerance, D. radiodurans is often the test subject of experiments to determine the ability of an organism to survive under simulated Martian conditions[5].

Genome Structure

Figure 2 [2]

   D. radiodurans is a spherical, non spore forming, non motile bacterium containing a red pigment from carotenoids[1].  It is a gram positive bacteria, having a very thick cell wall made up of peptidoglycan[6].  Radiation of a cell causes the greatest damage in double stranded breaks (DSB’s) of DNA and is a major cause of cell death.  Dehydration can also cause a similar type of damage within DNA[1], as seen in figure 2[2].  What makes this microbe unique is its ability to repair the strands of DNA following DSB’s caused by radiation; D. radiodurans can survive hundreds of DNA DSB’s caused by irradiation per haploid[9].  D. radiodurans does not possess a mechanism to prevent these breaks, however, but does possess a series of mechanisms to repair the broken strands at a fast rate.  A major key to the success of D. radiodurans’ ability to recover from intense doses of irradiation comes from a combination of the bacterium containing 4 to 10 extra copies of genes on all chromosomes and plasmids[6][1] and the nucleotides being contained in a tight structured ring[2].  The presence of so many extra copies of genes allows them to be more readily available for recombination.  The tight structure of the nucleotides prevents the fragments of DNA from diffusing far from the site[2].

Mechanisms and Adaptations for Radiation Resistance

Figure 3 [2]

   The first thing the cell does upon exposure to irradiation is delay all replication processes for a period of time greater than the time it takes to make the necessary repairs to the DNA strands[2].  Unlike what is seen in Escherichia Coli, the genome does not use the RecB or RecC proteins, a part of the RecBCD enzyme which processes duplex DNA ends to produce a single-stranded DNA tail for the RecA protein to bind on.  However, the RecA protein is essential for radiation repair because it helps to find overlapping fragments and splice them together[9]; a demonstration of this repair can be seen in figure 2[2].  About one and a half hours after irradiation, recombination due to RecA independent process occurs and begins repairing the fragments.  Several hours later RecA dependent repairs become more critical[11].  RecA forms filaments on the single-stranded DNA and hydrolyzes ATP and dATP (a nucleotide precursor used in cells for DNA synthesis) and promotes DNA strand reactions.  RecA promotes ATP and dATP dependent reactions with different pH profiles.  dATP is hydrolyzed at a pH of 7.5 and 8.1 but only exchanges DNA strands efficiently at a pH of 8.1.  ATP, on the other hand, supports efficient DNA strand replacement at both the pH of 7.5 and 8.1 through heterologous insertions[11].
   In a nutrient deprived environment, like what could be found on Mars, DdrA is another critical adaptation of D. radiodurans.  DdrA is critical to long term viability in Martian type conditions because it effectively buys time for conditions to improve to the point at which cell rebuilding can occur[2].  DdrA serves to preserve the fragmented ends of DSB’s from further degradation as a result of nuclease digestion. Approximately a half hour following irradiation, the DdrA levels within the cell increase by a factor of twenty to thirty and proceeds to bind with only the 3’ ends of single-stranded DNA to protect them[7].
   This microbe has another potential adaptation in the accumulation of manganese.  High concentrations of Mn(II) increase the cell’s ability to survive irradiation.  Consistent with D. radiodurans’ other adaptations, a high concentration of Mn(II) will not act to prevent the occurrence of DSB’s, but rather helps tolerate the cell damage caused by irradiation.  Mn(II) can help to protect the proteins by seeking out Reactive Oxygen Species, a large contributor to DNA damage after irradiation[3].

Viability Under Martian Conditions

   The environmental limits of mars include a vacuum of low pressure, an anoxic atmosphere with diurnal cycles in both temperature and relative humidity, energy-rich Ultra-Violet (UV) radiation and average temperatures of -60°C in the summer at mid latitudes to average temperatures of -126°C in the winter at the polar regions[4].  Compared to Earth, Mars has significantly higher UV radiation levels due to a thin atmosphere lacking an ozone layer to filter some of this radiation out and receives no relief from magnetic shielding[4].  
   These conditions pose great challenges for any potential of life to thrive on Mars.  The D. radiodurans cells would survive the low temperatures and lack of water and nutrients with little trouble[10]; however, these conditions are not favorable for cell growth[12].  It is extreme levels of UV radiation pose the greatest threat to D. radiodurans’ ability to remain viable under Martian conditions, despite having the presence of carotenoid pigments within its cell wall, which serve to remove free radicals and protect from UV radiation.  D. Radiodurans has been shown to have higher irradiation survival rates at temperatures of -72°C, giving the bacterium an advantage given Martian conditions[4].  However, the extremophile adaptations of the bacterium alone are not sufficient to protect against the UV radiation.  The most promise for maintaining viability relies on obtaining cover from Martian soil or sediment[4][10] to shield itself from the UV radiation.  Microbial life similar to D. radiodurans would most likely exist in subsurface habitats capable of providing the necessary UV radiation protection[6].  According to the radiation model provided by Dartnell et al. 2010, galactic cosmic rays (GCR) could cause a radiation level of 0.065Gy per year beneath 30 centimeters of dry sediment.  At this depth, D. radiodurans could theoretically survive for 1.2 million years before experiencing a high enough radiation dose to lose their viability[4].

� References

1. Blasius, M., Habscher, U., & Sommer, S. (2008). Deinococcus radiodurans : What Belongs to the Survival Kit? Critical Reviews in Biochemistry and Molecular Biology, 43(3), 221-238. 2. Cox, M. M., & Battista, J. R. (2005). Deinococcus radiodurans — the consummate survivor. Nature Reviews Microbiology Nat Rev Micro, 3(11), 882-892. 3. Daly, M. J. et al. Accumulation of Mn(II) in Deinococcus radiodurans facilitates γ-radiation resistance. Science 306, 1025–1028 (2004). 4. Dartnell, L. R., Hunter, S. J., Lovell, K. V., Coates, A. J., & Ward, J. M. (2010). Low-Temperature Ionizing Radiation Resistance of Deinococcus radiodurans and Antarctic Dry Valley Bacteria. Astrobiology, 10(7), 717-732. 5. Lewis R. Dartnell, Stephanie J. Hunter, Keith V. Lovell, Andrew J. Coates, and John M. Ward. Astrobiology. October 2010, 10(7): 717-732. doi:10.1089/ast.2009.0439. 6. Gomez, F., Mateo-Marti, E., Prieto-Ballesteros, O., Martin-Gago, J., & Amils, R. (2010). Protection of chemolithoautotrophic bacteria exposed to simulated Mars environmental conditions. Icarus, 209(2), 482-487. 7. Harris, D. R. et al. Preserving genome integrity: the DdrA protein of Deinococcus radiodurans R1. PLoS Biol. 2, 1629–1639 (2004). 8. Kim, J., Sharma, A. K., Abbott, S. N., Wood, E. A., Dwyer, D. W., Jambura, A., . . . Cox, M. M. (2002). RecA Protein from the Extremely Radioresistant Bacterium Deinococcus radiodurans: Expression, Purification, and Characterization. Journal of Bacteriology, 184(6), 1649-1660. 9. Kim, J., & Cox, M. M. (2002). The RecA proteins of Deinococcus radiodurans and Escherichia coli promote DNA strand exchange via inverse pathways. Proceedings of the National Academy of Sciences, 99(12), 7917-7921. 10. De la Vega, U. (2007). Simulation of the environmental climate conditions on martian surface and its effect on Deinococcus radiodurans. Advances in Space Research, 40(11). Retrieved March 25, 2016. 11. Zahradka, K., Slade, D., Bailone, A., Sommer, S., Averbeck, D., Petranovic, M., . . . Radman, M. (2006). Reassembly of shattered chromosomes in deinococcus radiodurans. Nature, 443(7111), 569-73. doi:http://dx.doi.org.prxy4.ursus.maine.edu/10.1038/nature05160 12. Diaz, B., & Schulze-Makuch, D. (2006). Microbial Survival Rates of Escherichia coli and Deinococcus radiodurans Under Low Temperature, Low Pressure, and UV-Irradiation Conditions, and Their Relevance to Possible Martian Life. Astrobiology, 6(2), 332-347. doi:10.1089/ast.2006.6.332