The study of DNA repair is not new. For years scientists have recognized the importance of this process in maintaining the integrity of our genes. For example, when DNA strands that have become separated are not rejoined, the resulting damage can lead to the uncontrolled cell growth that characterizes cancer. In recent years, researchers at other institutions began elucidating the biology of DNA ligases -- the enzymes that rejoin, or "ligate," the separated ends of two DNA strands.
Less is known, however, about the structure and mechanism of action of RNA ligases. Dr. Shuman has been a leader in exploring the enzymes that modify RNA, and he has collaborated for several years with Dr. Lima in analyzing their structures and mechanisms. While working in the Shuman and Lima labs, Mr. Nandakumar had been studying how the Rnl2 enzyme works. In an effort to better visualize what this ligase looks like in action, he performed x-ray crystallography studies to generate images of the enzyme at work. (X-ray crystallography is a technique in which the three-dimensional structure of molecules is determined by means of diffraction patterns that are produced when a beam of x-rays is directed at crystals of a molecule.)
What the scientists produced were detailed three-dimensional images demonstrating how Rnl2 prompts the release of phosphate groups from ATP (an energy source for the repair reaction) and how Rnl2 then recognizes a "nick" in a broken RNA strand and seals the broken ends back together.
It is the first time that anyone has shown how a ligase of any type is involved in all three steps of such a critical process. "We found that this process itself is very subtle and that the geometric changes that occur are actually quite subtle too," said Dr. Lima. "We didn't expect it to be so simple and yet so elegant."
