FIG. 5.

Survival curves of Mn-accumulating organisms exposed to γ-radiation. The numbers in parentheses indicate the approximate number of survivable DSBs inflicted per haploid genome at the dose that kills 90% of the organisms (22). Note that the ability of Mn-accumulating organisms to mend DSBs after exposure to γ-rays is much greater than in most bacteria which do not accumulate significant Mn (25). The high efficiency of repair enzymes in Mn-accumulating organisms under oxidative stress is attributed to the formation of Mn antioxidants that scavenge ROS and prevent the inactivation of enzymes. Mn-accumulating organisms: Bacterium—Deinococcus radiodurans (ATCC BAA-816) grown under Mn-depleted or Mn-replete conditions (25); Diploid yeast—S. cerevisiae (JW1777) (9); Nematode worm—Caenorhabditis elegans (39). Many explanations for the cause of the long shoulders of survival curves of Mn-accumulating organisms have been proposed. The most favored hypotheses begin with the premise that the yield of DSBs is linear with dose (0.004 DSB/Gy/Mbp) and that the non-linearity of the survival curves is caused by dose-dependent changes in the efficiency/accuracy of enzymatic DNA repair, which in many cell types appears to be dependent on the accumulation of Mn antioxidants (22). Note that the x-axis is a logarithmic scale. Radiation source: ⁶⁰Co γ-radiation. DSBs, DNA double-strand breaks; ROS, reactive oxygen species.