Guided by the published recipe of the polio virus's genetic structure, molecular biologists at the State University of New York at Stony Brook linked small pieces of easy-to-obtain DNA to make a virus template. A routine chemical process then converted the template into the polio virus itself, the biologists report in a paper released Thursday by the journal Science.

Tests of the synthetic virus in mice produced symptoms similar to those caused by the natural polio virus, once feared for its ability to induce paralysis and sometimes death but now nearly eradicated.

It's the first time scientists have built a synthetic copy of a virus based on the genetic blueprint, or genome, of the natural form, said Eckard Wimmer, one of the paper's authors.

"Nobody has ever put together an entire genome and made virus out of it," he said in a telephone interview.

Information on virus genomes is easy to access from papers published in scientific journals and online databases. "You open the Internet and lift out a sequence and go to work and make a virus without ever having seen the virus in your laboratory," Dr. Wimmer said.

Dr. Wimmer and collaborators Jeronimo Cello and Aniko Paul undertook the project just for the sake of proving that genetic information can be used to re-create a virus. But their success also demonstrates the potential for terrorists to create from scratch a variety of deadly viruses not otherwise easy to obtain, Dr. Wimmer said.

"This work is very important to put society on alert," he said. "This is an inherent danger in biochemistry and scientific research. Society has to deal with it. ... It won't go away if we close our eyes."

Other scientists said the new report was not surprising.

"Certainly this has been something that's been possible in principle for some time," said Steven Block, a biophysicist and bioterrorism expert at Stanford University. "So certainly it's not a breakthrough in a major sense. ... But this may raise public consciousness about the issue."

Scientists have speculated that terrorists might try to create deadlier viruses, such as smallpox, from published genetic recipes.

"To chemically synthesize smallpox would be a major undertaking, but doable," said Stephen Johnston, a molecular biologist at the University of Texas Southwestern Medical Center at Dallas.

Making smallpox virus would be more difficult than making polio virus, he said. The polio virus is relatively small, composed primarily of about 7,500 chemical units called nucleotides. Smallpox is a more complicated virus and is much bigger, made from nearly 190,000 nucleotides.

"It is much more difficult to do currently, but with progress in science and technology, it will be possible in 10 to 15 years to make smallpox," Dr. Wimmer said.

Apart from spotlighting a terrorist danger, reconstructing the polio virus could lead to benefits for medicine, such as rebuilding other viruses in a weakened form to help devise new vaccines.

To produce the polio virus, Dr. Wimmer and colleagues assembled short stretches of DNA (averaging 69 nucleotides in length) into the proper sequence to match the virus's genome, the genetic blueprint for making new copies of the virus. (The DNA segments are routinely obtainable by mail order.) The polio virus itself is made of RNA, a related molecule that also can store and transmit genetic information. So the researchers mixed the DNA template with chemicals that converted it into the corresponding viral RNA. Mixed into a soup of cell parts, the RNA orchestrated production of polio virus particles.

The synthetic virus was purposely produced with slight variations from the original blueprint so that the researchers could distinguish it from the natural form. Though much weaker than the natural virus, the synthetic version still produced polio symptoms in mice that had been genetically engineered to be susceptible to infection.

Making viruses other than polio would require modifications to the methods reported by the Stony Brook scientists, Dr. Wimmer said. But Dr. Block of Stanford said he doubted that routine virus synthesis would become a major weapon in terrorist arsenals.

"Someone bent on producing biological weapons wouldn't go this route," he said. "There are much easier ways to do it."

Of greater concern, Dr. Block said, would be efforts to combine different viruses into a chemical chimera with the most dangerous properties of both.

"I'd be more worried about people patching together chimeric viruses," Dr. Block said. "Imagine something as contagious as measles but as lethal as Ebola." The polio virus synthesis, he said, is not relevant to the creation of such chimeras.

The polio synthesis does suggest, however, that future vaccination policies ought to reflect the danger of virus reconstruction from genetic data, said Dr. Wimmer. "It could be a problem that has to be taken into consideration," he said.

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