Courtesy of A. Vasilenko and M. Daly, Uniformed Services University of the Health Sciences, Bethesda, Maryland, USA.

Deinococcus radiodurans (DEIRA) — literally 'strange berry that withstands radiation'— first came to the attention of scientists when it refused to die in food sterilization tests. It was noted that this bacterium is highly resistant to many extreme conditions, including insults from genotoxic chemicals, high levels of ionizing and ultraviolet radiation, and dehydration. DEIRA is so resilient to DNA damage that it can survive a dose of ionizing radiation 3,000 times higher than it would take to kill a human. Heat, dehydration and radiation normally kill cells by causing double-stranded breaks in their DNA — one of the most difficult kinds of DNA damage to repair. It's not that DEIRA can shield its genome from the radiation; but remarkably it can, within hours, stitch back together the hundreds of fragments of broken DNA that result — an outstanding feat, even by prokaryotic standards. Not surprisingly, discovering the secret behind DEIRA's efficient DNA repair talents has been the focus of many studies. Now, a report by Karlin and Mrázek suggests that DEIRA gets its diehard qualities by producing a protected cellular environment in which normal repair can occur undisturbed.

Genome repair in DEIRA was initially proposed to involve a two-step process: first, broken DNA fragments would be reconnected by annealing free single strands; second, the damaged site would be replaced by homologous recombination. The RecA protein was proposed to speed up recombination by taking advantage of DEIRA's multiple (4–10) genome complements, which, by stacking up like doughnuts on top of each other, lend their homologous segments to the repair machinery. However, the above processes and the presence of additional genome complements are not unique to DEIRA — they are typical of other prokaryotes as well.

For more clues to this mystery, Karlin and Mrázek turned to the DEIRA genome itself — using the codon preference of individual genes to predict their expression levels. Genes that deviate in codon usage from the average gene but that are similar in codon usage to ribosomal proteins (which are expressed at high levels), are also predicted to be highly expressed (PHX). Karlin and Mrázek propose that the ultraviolet resistance capacity of DEIRA is due to the abundance and versatility of PHX genes, which fall into four classes. These include genes that degrade and export damaged nucleic acids and proteins, molecular chaperones, detoxification enzymes and proteases. The abundance of PHX genes in each of these classes (particularly the first), compared to other prokaryotes, could be the intrinsic property that maintains the survival and stability of the cell when it is exposed to radiation or desiccation. Indeed, the expression levels of standard repair proteins in DEIRA (or Escherichia coli) are not in the PHX range (except one), but most chaperone, degradation and protease proteins are.

It remains to be seen whether DEIRA has novel proteins among its PHX genes, or whether it simply uses the standard repair machinery in a new and more efficient way. Meanwhile, NASA have their eyes on DEIRA for other reasons: extremophiles such as DEIRA represent a distinct life form that could give clues to the earliest inhabitants of Earth and Mars. And if that doesn't work out, there's always a use here on earth for a bug tough enough to clean up heavy metals and radioactive waste without risking its life.