Paparazzi of the DNA World

An x-ray snapshot of crystallized DNA polymerase, an enzyme that copies our genetic blueprint, has revealed a remarkable ability to function while in crystal form, according to a report in the current issue of Nature. By making a fast series of images, the researchers say, they may be able to create a movie that captures DNA replication in action.

DNA comes mainly as paired strands of nucleotides. When a cell divides, the strands peel apart, and DNA polymerase builds matching strands one nucleotide at a time. Although biologists use DNA polymerases every day in the lab to copy DNA, they have only a sketchy understanding of how the enzyme works.

To try to glimpse DNA polymerase doing its job, Lorena Beese and her colleagues at Duke University first mixed a DNA strand of 13 nucleotides with a matching strand lacking five nucleotides, then added DNA polymerase from a kind of hot-spring bacteria. After crystallizing the DNA and enzyme together, the researchers bathed the crystal in a solution of nucleotides. A timed x-ray exposure clearly showed that the enzyme had wrapped around the DNA strands and had added a nucleotide to the partial strand. Beese says that in previous attempts to crystallize active DNA polymerase, the enzyme was too constrained to do its job. In the latest work, she says, her group was lucky that "the crystal permitted enough space to extend the chain without running into another molecule."

Beese next hopes to capture all the perambulations of the enzyme as it adds nucleotides while moving down a DNA strand. One way to do this would be to slow the enzyme by chilling it to near zero degrees. Alternatively, she says, the team could make the images using synchrotron radiation, superfast x-ray bursts that function something like a strobe light.

Other researchers aren't lining up just yet for tickets to a slow-motion movie of DNA replication. "It won't be easy," says Thomas Steitz, a biochemist at Yale University. But then again, he says, "you should never say 'never' in science."

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