Researchers at the University of Pennsylvania School of Medicine have shown the form of a crucial smallpox virus and the manner with which it attaches// itself to the DNA .The enzyme, which is named, as a topoisomerase has become a very important drug target for devising novel methods to combat small pox.

"This enzyme is one of the most closely studied DNA-modifying enzymes in biology," says Frederic D. Bushman, PhD, Professor of Microbiology, one of the senior authors. "The structure of the DNA complex has been long-awaited." DNA-modifying enzymes bind to specific sequences in the genetic code to aid in the many steps of DNA replication.

The smallpox virus is one of the most easily transmissible infectious diseases known to humans, resulting in up to 30 percent mortality. The efficiency with which it spreads, combined with the deadly nature of the disease, has raised fears that smallpox could be revived for use in bioterrorism. Knowing the exact three-dimensional structure of smallpox virus proteins could help researchers design antiviral agents, but few structures of whole viral proteins exist.

Poxviruses are large viruses that contain two strands of DNA and replicate themselves entirely in the cytoplasm of infected cells. Poxviruses do not take over the genetic machinery inside the nucleus of the host cell, as many viruses do. Because of this strategy, poxviruses encode many of the enzymes they need to replicate their own genes, and hence reproduce. One of these enzymes is a topoisomerase, which is used by the virus to relieve the excessive twisting of DNA strands that normally occurs during DNA replication and transcription of the viral genes. Upon initial infection, the poxviruses come already equipped with some proteins, including topoisomerases, to kick-start replication.

The structure was determined in a collaborative effort between the Bushman lab and the lab of the other senior author Gregory D. Van Duyne,
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