New light on bearded DNA protection.

New light on bacterial DNA protection

Some people get going. Others go shopping. But with bacteria, says Scott C. Mohr, "when the going gets tough, the tough sporulate."

The Boston University chemist is referring to the process by which some bacteria encase themselves in tough protein coatings and go into states of near-suspended animation to weather periods of harsh environmental conditions. Bacterial spores can sit in bone-dry soil for 60 years or more, then spring to life when conditions again become conducive to growth. "You have to cook the bejesus out of these things to kill them," Mohr says.

Researchers have puzzled over how sporulated bacteria protect their precious genetic cargo from heat, desiccation and doses of ultraviolet light lethal to their sporeless siblings. Mohr and Peter Setlow of the University of Connecticut Health Center in Farmington, working with two Boston University colleagues, now appear to have solved the riddle of how these hardy microbes protect their genes from ultraviolet radiation. Their report appears in the January PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.88, No.1).

Previous work had shown that bacteria in the early stages of sporulation produce large quantities of proteins called small acid-soluble spore proteins (SASPs), which seem to help protect DNA from the ravages of ultraviolet light. In cells lacking SASPs, ultraviolet light has a deadly effect on DNA sections that have two thymine molecules next to each other. Ultraviolet rays permanently transmogrify these adjoining thymines into gnarled, double-looped structures that interfere with DNA replication and repair.

In the new work, the mechanism by which SASPs prevent this fatal reaction comes to light. The team's laboratory experiments show that SASPs bind to DNA, unwinding it ever so slightly. This changes the geometry of the DNA's thymine pairs, leaving them relatively nonreactive to ultraviolet light.

By studying SASPs, scientists may learn more about how the earliest cells survived the high ultraviolet levels present before the emergence of Earth's radiation-filtering stratospheric ozone layer, Mohr says. Moreover, he says, ongoing studies of the mechanics behind SASPs' arm-twisting of DNA may someday aid researchers seeking biological tools to alter the structure and function of DNA sequences in bacteria and higher cells.