Brown-Harvard team solves mobile DNA's surgical sleight-of-hand

PROVIDENCE, R.I. -- In a clever bit of biology called site-specific recombination, DNA can travel inside an organism, or into another organism, and perform a sort of grafting surgery that allows it to insert its chromosome into a chromosome of the target cell. The process is important because mobile DNA can carry genes that cause drug resistance or transmit viruses that cause disease or tumors that result in certain leukemias or other cancers.

But what do the surgeons look like that conduct these intricate snip-and-solder procedures?

While scientists have studied site-specific recombination for about 30 years, much remains a mystery. In the current issue of Nature, a Brown University and Harvard Medical School team reveals the crystal structure of λ-integrase (λ-INT), the protein responsible for this DNA doctoring in the lambda virus, which infects and lives off Escherichia coli (E. coli) bacteria.

The findings are important because lambda is a model virus used to understand mobile DNA, according to Arthur Landy, professor of medical science in the Department of Molecular Biology, Cell Biology and Biochemistry at Brown. "Once you see the parts and how they fit together, you have a much better understanding of how the process works," Landy said. "So this work represents a major leap."

When the lambda virus inserts its DNA into E. coli, λ-INT is the major player in the process, literally snipping the E. coli DNA strands before insertion and then sealing them up afterward. The research also suggests how three "helper" proteins – think of them as nurses – intricately fold the DNA strands during this surgery.

Marta Radman-Livaja, a postdoctoral research associate at Brown, conducted the biological and biochemical aspects of the experiments. The Harvard team – Tapan Biswas, Hideki Aihara, David Filman and Tom Ellenberger – used X-ray crystallography, a technique that combines strong X-rays and sophisticated computer software, to create a 3-D digital image of λ-INT caught in the act of carrying out site-specific recombination.

In complementary experiments carried out at Brown and published recently in the Proceedings of the National Academy of Sciences, Radman-Livaja tagged bits of lambda DNA with dyes that allowed her to draw up a map of this viral DNA during insertion.

"It was gratifying to see that two totally different experimental approaches give the same picture," Radman-Livaja said. "It means we have a very solid foundation for the next step forward in understanding this fascinating process."

The National Institutes of Health, the Jeane B. Kempner Fund and the Human Frontier Science Program funded the work.

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