While most bacteria wither at a stray x-ray, others can soak up immense doses of ionizing radiation without a problem. The disparity has nothing to do with how bacteria protect their DNA, as most researchers had thought. Instead, according to a new study, what matters is how they shield their proteins. The insight challenges conventional understanding of radiation damage, hints at new ways to cleanup radioactive waste, and offers the hope of safer forms of radiation therapy.
If there's a superhero of the bacterial world, it's Deinococcus radiodurans. The bacterium's name--roughly translated as "strange berry that withstands radiation"--says it all: The bug can survive doses of radiation up to 10,000 Grays (Gy)--a level lethal to other bacteria and indeed most cells in general. (Humans check out at 10 Gy). How does D. radiodurans do it? Radiation damages the DNA of different bacterial species to the same degree, explains Michael Daly, a radiation biologist at the Uniformed Services University of the Health Sciences in Bethesda, Maryland, so the superbug must be protecting some other system critical to its survival.
Daly's previous work has suggested that the proteins of D. radiodurans' DNA repair systems work much more efficiently than those in sensitive bacteria. He suspected the difference might have something to do with the metal manganese, which is 300 times more abundant in resistant bacterial species than in sensitive ones. In the new study, Daly's team examined manganese's ability to scavenge the damaging free radicals created by radiation. The researchers discovered a manganese-based chemical complex in D. radiodurans that inactivated the free radicals that damage proteins but not those that harm chromosomal DNA. "Proteins in sensitive bacteria are much more sensitive to radiation than DNA" is, says Daly, whose team reports its findings in the April issue of PLoS Biology. "If you can protect proteins from radiation, you're in a good position to recover."
The team has partially purified the manganese complex and hopes to find a way to deliver it into a variety of cell types. Bioremediation researchers might be able to add it to microbes to better clean up toxic waste that harbors radioactive material, for example. And Daly hopes the complex could also boost the radiation-resistance of human cells, speeding recovery for patients in radiation therapy.
"The ideas presented differ radically from mainstream thinking concerning bacterial radioresistance," says John Battista, a microbiologist at Louisiana State University in Baton Rouge. Although more experiments are needed, Battista says the study indicates that "the rest of us have been searching for the answers to radioresistance by studying the wrong target." Still, David Thaler, a microbiologist at Rockefeller University in New York City, proposes that manganese might have a second role of improving certain proteins' ability to repair irradiated DNA. Studies have shown that the metal can increase the ability of the enzymes to fix broken chromosomes, he points out, and its high levels in D. radiodurans may give the bacterium a critical boost.