July 10, 2000 Nearly fifty years after a landmark paper proposed the existence of what later came to be called jumping genes, scientists are getting their first clear snapshot of one caught in midair.
In the July 7 issue of the journal Science, a UW-Madison team describes the 3-dimensional, atomic structure of an enzyme that allows a transposable genetic element in a bacterium to "jump" from one part of DNA to another.
The structure of this protein-DNA complex -- featured on the journal's cover -- gives researchers a new framework for understanding how transposable elements operate, according to the paper's lead authors Ivan Rayment and Bill Reznikoff. The finding also may accelerate the search for new drugs to inhibit AIDS.
"Transposable elements have the potential to remodel genomes and to facilitate the movement of genetic information, such as antibiotic resistance," says Reznikoff, a molecular geneticist.
The transposition of DNA is central to genetics and evolution. Transposable elements are an important source of the mutations on which natural selection operates. Scientists estimate that transposable elements make up as much as 30 percent of the human genome, for example.
In Science, the Wisconsin team describes the 3-dimensional structure of the Escherischia coli Tn5 transposase bound to the Tn5 transposable element. "Our discovery is an important step in understanding the structural basis for transposition," says Rayment, a crystallographer and molecular biologist.
The team's findings have implications for AIDS researchers because the human immunodeficiency virus-1 (HIV-1) uses a process similar to DNA transposition to insert itself into human DNA.
"Just as enzymes called transposases make transposition possible, enzymes called integrases catalyze similar events in retroviruses, including HIV-1," Rayment says. "Researchers have now studied the catalytic core of five different transposases and integrases, and they show remarkable similarity. Therefore, a clear image of one of them provides greater understanding of all similar ones."
To control AIDS, researchers in the pharmaceutical industry are screening compounds that can inhibit HIV-1 integrase, according to Rayment and Reznikoff. Because HIV-1 integrase and Tn5 transposase have similar structures, the Wisconsin scientists believe they now have a model system that can help scientists identify or design compounds effective in controlling HIV-1.
The paper's co-authors include Douglas Davies and Igor Goryshin. Davies worked with Rayment to develop the DNA-enzyme crystals and analyze them using X-ray crystallography. Goryshin, a molecular biologist, worked with Reznikoff in developing, isolating and purifying the transposase. The research team -- all with the Department of Biochemistry in the College of Agricultural and Life Sciences -- worked together to solve the structure of the complex.
In 1951, geneticist Barbara McClintock proposed "controlling elements" to explain genetic patterns she observed in corn. Many geneticists were slow to appreciate the importance McClintock's discovery, for which she received a Nobel Prize in 1983. However, researchers have since made remarkable progress in understanding the molecular nature transposable elements.
Past studies of the structure of the enzymes that trigger transposition have focused on the core region that cuts the element from DNA, Rayment says. Researchers have not known what the entire enzyme looks like or how it binds to and interacts with DNA. Capturing the 3-dimensional structure of the protein-DNA complex, allowed the UW-Madison team to present a much clearer view of how the enzyme and DNA interact at the molecular level.
Prior to transposition, one copy of Tn5 transposase binds to a specific region at one end of the transposon and a second copy binds to an identical region at the opposite end. Neither enzyme can cut DNA at the site to which it binds. When events produce a loop in the Tn5 transposable element the two enzymes at the ends come together. The Wisconsin research shows how the architecture of the resulting protein-DNA complex positions each enzyme so it can then cleave the opposite end of the transposable element DNA from its initial binding site. The Tn5-enzyme complex can then move freely before it inserts itself into a new location.
The research was supported by: state funding to the College of Agricultural and Life Sciences, and grants from the National Institute of General Medical Sciences; National Institute of Arthritis and Musculoskeletal and Skin Diseases; the U.S. Department of Energy; and a Vilas Associates Award from the UW-Madison.
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