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Friday, September 29, 2006

Scientific American: Cheating DNA Death: How an Extremophile Repairs Shattered Chromosomes

Fifty years ago, scientists uncovered a microbe capable of withstanding radiation in canned meat that had been bombarded with gamma rays. Named Deinococcus radiodurans--or "strange berry that withstands radiation" [pictured at right]--the microorganism can survive doses of radiation up to 500 times that which would kill a human. These doses shatter D. radiodurans's DNA--just as they would in a human--but the microbe can repair its broken DNA and spring back to life within hours, depending on the dose. Researchers in France have finally determined how the most durable of extremophiles manages this trick. "We have discovered the mechanism by which a clinically dead cell resurrects back to life," explains Miroslav Radman of INSERM, France's public biomedical research institution. "This extreme radiation resistance is but a by-product of its selection for resistance to desiccation."

Both desiccation and radiation break D. radiodurans's chromosomes into short DNA fragments. Radman and his colleagues blasted the microbe with one megarad of gamma radiation, enough to sterilize food but well below D. radiodurans's resistance threshold. Nevertheless, its chromosomes broke down into short strands of DNA. For the next hour and a half, the cells appeared dead, but by the end of three hours the chromosomes of D. radiodurans were reassembled and fully functioning.

Close observation of this miracle revealed that DNA synthesis was at work, in which each fragment serves as a template and extends itself by removing damaged ends and overlapping with a fragment that matches part of its sequence of nucleotides--all with the help of an enzyme known as PolA. Ultimately this results in single long strands of repaired DNA, as much as 30 times longer than the longest repetitive sequences of D. radiodurans's DNA.

But such single long strands of DNA do little to resurrect the microorganism until the second stage of the newly discovered process kicks in: the simple pairing discovered by Watson and Crick decades ago--adenine (A) bonds with thymine (T), and cytosine (C) bonds with guanine (G). By inserting a special version of the nucleotide thymine that only binds to single strands of DNA--known as 5-bromodeoxyuridine--the researchers could observe as the single strands bonded with complementary strands to form complete chromosomes. "Once the chromosome is functional, the synthesis of all cellular components starts, and the cellular life is back," Radman says.

The process--dubbed extended synthesis-dependent strand annealing--solves the mystery of how D. radiodurans survives radiation and repairs the damage it causes, according to the paper presenting the result published online in Nature on September 28. It also shows that the plucky microbug actually synthesizes DNA faster during such recovery than during its own normal replication. But it does not solve the mystery of how common enzymes, such as PolA, work so much better in D. radiodurans than in other microorganisms that radiation kills for good, notes Michael Daly of the Uniformed Services University of the Health Sciences.

Regardless, scientists are now closer to understanding the remarkable strength of this "strange berry" and perhaps putting it to work. "Because Deinococcus can survive death, I like to dream that it can teach us how to resurrect dead neurons," Radman says. "Plus, I would send [it] to seed life on sterile planets--directed panspermia." A big role may await this tiny extremophile.
Scientific American: Cheating DNA Death: How an Extremophile Repairs Shattered Chromosomes


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