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Thu, Nov 18, 1999 Reuters World Report by Maggie Fox, Health and Science Correspondent 

A pink bacterium that shrugs off the worst radiation and which has been taught to thrive on toxic waste is yielding genetic secrets that could lead not only to better waste clean-up but better treatments for cancer, researchers said on Thursday.

Teams at The Institute for Genomic Research (TIGR) in Rockville, Maryland and at the Department of Energy said they had sequenced the genome -- the entire collection of genes -- of Deinococcus radiodurans.

They say it seems to have thousands of unique genes that help it clean up the damage done to its DNA by radiation, helping it survive where any other creature would die.

Found living happily 40 years ago in a can of food that had been irradiated to kill germs, D. radiourans has intrigued scientists ever since. It can survive 1.5 million rads of gamma irradiation -- a dose 3,000 times the amount that would kill a human. It also pops back to life after being dried out and can live through high doses of ultraviolet radiation. Just last year, researchers genetically engineered it to eat up toxic chemicals such as toluene and mercury.

"This is a significant accomplishment," Secretary of Energy Bill Richardson said in a statement. "Besides the insights into the way cells work, this new research may help provide a new safe and inexpensive tool for some of the nation's most difficult cleanup challenges."

The team at TIGR, which has now sequenced the genomes of 10 important micro-organisms, used the "shotgun" method to make multiple copies of every piece of genetic material in the bacterium. Writing in the journal Science, they said they overlapped these to make a full map of its genes. They found about 3,100 genes arranged on two circular chromosomes, TIGR president Claire Fraser said.

The secret to Deinococcus's toughness seems to be a large number of genes that allow it not to prevent damage from radiation, heat and other assaults, but to repair that damage quickly and efficiently enough to allow it to survive. Radiation, heat and chemicals create breaks in the double helix of DNA that makes up the genes. This can kill a cell outright, or cause it to make mistakes as it replicates itself -- mistakes that can kill it, or cause cancer and other disease.

"A unique mechanism may contribute to D. radiourans' resistance to DNA damage," the researchers wrote in their Science report. "This organism transports damaged nucleotides out of the cell, which potentially prevents their reincorporation into the genome." In other words, it throws out the damaged pieces of genetic material.

"Other cells try to recycle building blocks of DNA," Fraser said in a telephone interview. "The last thing you want floating around in cytoplasm of the cell is a large number of nucleotides that have been damaged in some way by the radiation."

In addition, the bacterium has other unique genes that must help it clean up damage before it causes harm. "This absolutely has implications for understanding DNA damage in cancer," Fraser said. Many cancers are known to be caused by mutations in the genes responsible for fixing damaged DNA.

"Somewhere between 500 and 1,000 genes in this organism are going to be unique to Deinococcus," Fraser said. "My guess is that some are absolutely critical to the ability of Deinococcus to withstand radiation."

A year ago Michael Daly and colleagues at the University of Minnesota inserted four genes into Deinococcus that gave it the ability to break down some kinds of toxic chemicals and to convert mercury into a less dangerous form. The Energy Department hopes to make use of genetic information about Deinococcus to figure out even better ways to do this.

Comment (webmaster): This should not be confused with radiation resistance of prions. That is a property of proteins generally. DNA however is adversely affected. In this bacteria, radiation does cause lethal double-stranded breaks. However, the bacterium can repair them provided the cell is still healthy metabolically.

It seems unlikely that irradiation of meat will help much. Bacteria such as this will continue to flourish until radiation levels are such that the meat itself would be severely damaged.

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