DNA crystals are a bacterium's best friend.

For many bacteria, when the going gets tough, the tough stop growing. Faced with a lack of nutrients, microbes may shut down many of their cellular activities, such as replication, and simply wait for better days.

During this so-called stationary phase, some bacteria shield their DNA from degradation by embedding the genetic material within crystals of a protein called Dps, Abraham Minsky of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues report in the July 1 NATURE.

"It's an amazing possibility. This could represent a new paradigm for how cells protect themselves in situations of starvation, dryness, or whatever," says microbiologist John R. Battista of Louisiana State University in Baton Rouge.

The story of Dps begins in 1992, when a research team led by Roberto Kolter of Harvard Medical School in Boston described the novel DNA-binding protein produced by nutrient-deprived colonies of the bacterium Escherichia coli. "In stationary phase, Dps is one of the most abundant proteins in the bacteria," notes Steven E. Finkel, a Harvard colleague of Kolter.

Kolter's team subsequently found that the protein protects the DNA from highly reactive oxygen-containing molecules called free radicals. Because many bacteria other than E. coli also make proteins resembling Dps, Kolter and his colleagues speculated that they had found a common bacterial strategy to safeguard DNA.

Left unresolved was the question of how Dps exerts its protection. In 1997, the team deduced the shape of Dps and concluded that 12 copies of the protein normally aggregate to form a hollow, spherical complex. Since the structure of Dps resembles that of an iron-binding protein called ferritin, the scientists suggested that the Dps complex sequesters iron within it, preventing the metal from helping to produce DNA-damaging free radicals.

The new work reveals another explanation for Dps' protective talents, one not incompatible with the iron-sequestering theory. Minsky and his colleagues, working with Kolter's group, found that when they added DNA to a solution of Dps, the protein spheres aggregated and quickly formed crystals.

Microscopic analysis revealed that the Dps crystals had incorporated the DNA within their structure. This biocrystallization appears to offer an effective method of preserving DNA integrity, says Minsky.

He and his colleagues next studied an E. coli strain genetically engineered to overproduce the Dps protein. Under normal growth conditions, no Dps crystals formed within the bacteria. When starved, however, the microbes created large Dps crystals like those observed in the test-tube experiments. Similar, though smaller, crystals arose in normal E. coli deprived of nutrients.

The starvation-induced crystallization reverses without damaging the microorganisms. "Once nutrients are supplied, [the crystals] disappear within a very short timescale, and the bacteria are viable and growing," says Minsky.

While the Dps spheres appear to crystallize by packing together like oranges in a crate, how bacterial DNA integrates itself into the crystals remains unclear. "We believe DNA wraps around the outside of the [Dps] protein complexes," says Finkel, noting that such a scenario would allow Dps to separate DNA from any iron within the spheres.

Since actively growing bacteria have free-floating Dps complexes, Minsky's group has begun to look for molecules within starved bacteria that trigger the unusual crystallization.

Why haven't scientists taken note of this process in their decades of research with E. coli? Perhaps because investigators tend to study growing bacteria. "The stationary phase of the bacterial life cycle has not really been appreciated until recently," notes Finkel.