Nuclear fallout wouldn't faze Deinococcus radiodurans. This hardy microbe can weather blasts of radiation 1500 times stronger than doses that would wipe out any other organism. How the bug does this is a mystery. Now, researchers suggest that damage control is helped by the microbe's tendency to pack extra copies of its genome into tidy doughnuts of DNA.
Slam an ordinary creature with a high dose of radiation and the results are most unpleasant: tissue swells, strands of DNA rupture, and chromosomes shatter. All organisms have proteins that check for and repair broken DNA, but there's only so much wreckage they can repair. In contrast, D. radiodurans' repair kit is unusually gifted. This may be partly due to the bacterium's physical structure, researchers suspect. The bug has not one, but four copies of its genome in separate compartments. And studies have shown that DNA packed in a doughnut-shaped torus is easier to repair, because the torus holds the broken strands in place until they can be spliced back together.
To investigate this life-saving stitchwork, Abraham Minsky and colleagues at the Weizmann Institute of Science in Rehovot, Israel, and elsewhere exposed thousands of bugs to high doses of radiation. Then they allowed the microbes to recover, quick-freezing them at different stages to catch the repair mechanisms in action. After a radiation insult, the DNA initially holds its doughnut shape in each of the four compartments while repairs begin, the team reports in the 10 January issue of Science. In the second phase of repair, the DNA doughnuts uncoil. Each copy sends portions of itself through holes between compartment barriers to mingle with tendrils from other genome copies. Because redundant copies of the genome make it likely that not all parts will be damaged in exactly the same place, the mingling allows repairs to be made using unbroken sections in one copy as a template to repair the damaged ones, the team suggests.
The evidence doesn't convince microbiologist John Battista at Louisiana State University in Baton Rouge. The connection between DNA doughnuts and the ability to survive radiation would be more convincing, he says, if the doughnuts turn up in six other species of closely related bacteria that are just as radiation resistant. If any of D. radiodurans' hardy relatives lack the rings, he says, "there is no story here."