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Microbe’s donut-shaped genome packs a punch
By Adam Marcus

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Talk about a tough microbe. Deinococcus radiodurans earned a place in the Guinness Book of World Records as "the world's toughest bacterium" for its ability to survive enormous doses of radiation—a thousand times more than a person could. Although radiation will damage its DNA, the microbe can put the shattered pieces back together.

A color enhanced micrograph of the red-pigmented D. radiodurans cell, highlighting the ring-like morphology of the cell genome.

This remarkable ability to mend DNA is a puzzle. Scientists have not been able to explain its origins or identify genes in D. radiodurans that confer superiority in repairing DNA.

Now, a study in today's issue of Science suggests that D. radiodurans survives because its genome is arranged in densely packed rings, called toroids. These donut-like shapes keep shards of DNA in close proximity after a dose of shattering radiation, allowing the microbe to make repairs without having to hunt around for hundreds of loose fragments.

"When they are packed in this very dense toroidal form, even when they are broken into 150 to 200 pieces, these pieces stay densely packed," says Abraham Minsky, a chemist at Israel's Weizmann Institute of Science in Rehovot and leader of the research. "The fragments do not diffuse, and the right sequence is maintained."

Minsky and his colleagues in the United States scanned the bacteria with an electron microscope to observe their chromosomes. Donut-shaped DNA packaging is rare, known to occur mainly in D. radiodurans and a few inert bacterial spores and sperm—whose job is to protect and serve up packets of genetic material.

Yet if the arrangement is so potent, why isn't it more common among microbes? Perhaps because gobs of energy are expended on maintaining the rings.

The study has drawn mixed reactions.

David Schwartz, a geneticist at the University of Wisconsin-Madison, who has studied D. radiodurans, called the genome configuration "the only possible explanation" for its immunity from radiation. "It's totally simple, totally cool," says Schwartz.

But John R. Battista, a Louisiana State University microbiologist who also works with D. radiodurans, wasn't convinced that genome shape explains the resistance. "I personally don't feel this paper has made the connection between this shape and radio-resistance," says Battista. "It's interesting and merits consideration, but I don't think they've put the nail in the coffin," he adds.

Transmission electron micrographs of D. radiodurans cells. The dark areas are ribosomes. View full

The group did not look at other species of deinococcus to see if they also had toroid chromosomes, says Battista. Nor did they examine other radioactive-resistant microbes, including a distant relative of D. radiodurans called rubrobacter. If these bacteria lack the donut-shaped DNA, something else must explain their DNA repair ability.

Indeed, Battista and his colleagues believe D. radiodurans's toughness stems from proteins it generates, not the configuration of its DNA. But so far, he added, such a molecule has not yet been reported in the literature.

Jonathan Eisen, an evolutionary biologist at The Institute for Genomic Research in Rockville, Maryland, also has questions about the paper's conclusions. He would like to see a side-by-side comparison with other radiation-resistant bacteria, as well as comparisons to species harmed by radiation.

"We need to know whether or not…radiation-sensitive bacteria have the same type of structures," Eisen says.

Minsky's team believes that while chemistry may help some organisms resist radiation damage, D. radiodurans's defenses are a product of its architecture. "Being exposed to [radiation] resulting in chromosomes being shattered to 200 fragments per chromosome is a situation that cannot be handled solely by enzymes," he says.

Some researchers hope to use D. radiodurans to clean up toxic waste sites. One potential strategy is to modify the bacterium to carry genes from other microbes that neutralize toxins, like mercury and the organic chemical toluene.

See related GNN article
»The World's Toughest Bacterium

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Levin-Zaidman, S. et al., Ringlike structure of the Deinococcus radiodurans genome: A key to radioresistance? Science 299, 254-256 (January 10, 2003).

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