Two-handed protein may protect DNA.

Scientists have found a new clue to the remarkable capability of an unusual bacterial species, Deinococcus radiodurans, to survive large doses of radiation and extended periods without water.

This bacterium can withstand a dose of radiation thousands of times more intense than what would prove lethal to a person. Although the radiation scrambles the microbe's DNA, D. radiodurans has an unmatched talent for repairing the genetic damage. This skill apparently evolved as a way to protect the bacterium from DNA mutations produced during long-term dehydration (SN: 12/12/98, p. 376).

Microbiologists have struggled to understand how D. radiodurans so easily stitches its DNA back together. One factor may be molecules called single-stranded DNA-binding (SSB) proteins, which have been implicated in the copying and repairing of genes. "In all organisms, [these] proteins are critical for genome stability, says James L. Keck of the University of Wisconsin Medical School in Madison.

D. radiodurans produces many more SSB proteins than do radiation-sensitive bacteria, such as Escherichia coli, according to Keck. Prompted by that oddity, Keck's team took a close look at the shape of the D. radiodurans' SSB protein.

Typically, four SSB protein molecules combine into a complex, each part having one DNA-binding region. In the June 8 Proceedings of the National Academy of Sciences, however, Keck and his colleagues report that a single D. radiodurans' SSB molecule contains two DNA-binding regions, each with a slightly different shape. Moreover, the baeterium's SSB complexes have only two components.

The differences are a "tantalizing hint" that the bacterium has developed unusual ways to deal with its damaged DNA, says Keek.