"When this gene (irrE) is not functional you have a very sensitive organism," says Ashlee Earl, a lead researcher on the study. "At a dose of radiation that would not normally kill D. radiodurans, there is less than .001 percent survival without irrE."
With a name that literally means "strange berry that can withstand radiation," D. radiodurans is the world's most radiation resistant organism, able to survive up to 1.5 million rads, over 1000 times more radiation than practically any other organism.For reference, 1000 rads is enough to kill the average human. For decades scientists have been trying to figure out what makes it so hard to kill. Understanding how this bacterium survives such extreme conditions could help researchers with everything from cleaning up nuclear waste sites to preventing cancer.
To better define the mechanisms used by the bacterium to resist radiation, Earl and her team, which included researchers from The Institute for Genomic Research (TIGR), studied a mutant strain of D. radiodurans that could not survive high radiation levels. This strain carried mutations on two genes, one of which was irrE. When the irrE gene was restored, the bacterium was once again resistant to radiation, a finding that suggested the gene was responsible, at least in some part, for resistance.
Further study of irrE suggested that the gene is not directly responsible for DNA repair following radiation exposure but instead it controls other genes that are known in other organisms to be associated with DNA repair. It's role is also not specific to radiation. The gene is responsible in some part for D. radiodurans resistance to other environmental damage, including ultraviolet light.
The gene is probably not solely responsible for the bacterium's remarkable restorative powers, says Earl. "We are not exactly sure how it functions in relation to other regulatory genes. It could serve as anything from a damage sensor to something that directly controls gene expression. All we really know is the bacterium cannot survive radiation or other types of damage without it."
Earl and her colleagues are currently working on identifying which cellular functions irrE controls. "In defining which genes irrE controls, we can better focus our attention to a small subset of genes that are most important to D. radiodurans response to radiation damage," she says.
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The American Society for Microbiology (ASM) is the largest single life science society, composed of over 42,000 scientists, teachers, physicians, and health professionals. Its mission is to promote research and training in the microbiological sciences and to assist communication between scientists, policymakers, and the public to improve health, economic well being, and the environment.
The Institute for Genomic Research (TIGR) is a not-for-profit research institute based in Rockville, Maryland. TIGR, which sequenced the first complete genome of a free-living organism in 1995, has been at the forefront of the genomic revolution since the institute was founded in 1992. TIGR conducts research involving the structural, functional, and comparative analysis of genomes and gene products in viruses, bacteria, archaea, and eukaryotes.