Scientists are reporting that, for the first time, they have made an artificial prion, or misfolded protein, that can, by itself, produce a deadly infectious disease in mice and may help explain the roots of mad cow disease.

The findings, being reported today in the journal Science, are strong evidence for the “protein-only hypothesis,” the controversial idea that a protein, acting alone without the help of DNA or RNA, a cousin of DNA, can cause certain kinds of infectious diseases.

The concept was introduced in 1982 by Dr. Stanley Prusiner, a neurology professor at the University of California, San Francisco, who led the new study. The hypothesis is still unsettling to many scientists who have been taught that only bacteria and viruses containing genetic information can spread infectious diseases.

Over the years, the idea that a misfolded prion protein, because of its shape alone, could trigger an infectious disease has been gaining acceptance, but it has never been conclusively proven. Proteins, which are strings of amino acids, fold into distinct shapes to carry out various functions in the body. It is not completely known what makes proteins fold incorrectly. But the theory is that when a misfolded prion comes in contact with other prions, they, too, become misfolded and a disease is spread.

In this study, Dr. Prusiner said, researchers essentially created a man-made prion and injected it into mouse brains and produced disease. Then, they took tissue from the diseased mice and injected it into other mice, which also got the same disease.

Dr. Prusiner said in a telephone interview that he was “flabbergasted” that it took 22 years to prove the hypothesis, but that his laboratory was able to overcome earlier technical difficulties with a new set of experiments. “We have compelling evidence,” he said. “We’ve done it all.”

Dr. Christopher Dobson, a professor of chemical and structural biology at Cambridge University and an expert on protein folding, said: “This is the key experiment everyone was waiting for. While there is never absolute proof in science, this experiment puts the protein-only theory of transmission beyond reasonable doubt.”

Still, some said the evidence was not sufficient.

Dr. Bruce Chesebro, chief of the Laboratory of Persistent Viral Diseases at the federal Rocky Mountain Laboratories, said the experiment showed that the prion produced something in the mice. But he said it was still unclear whether the prion was causing the infection or just exposing an underlying infectious process.

Dr. Laura Manuelidis, a neuropathologist at the Yale University Medical School, one of Dr. Prusiner’s most vocal critics, said the prion strain that turned up in the experiment looked like a mouse prion frequently used in Dr. Prusiner’s laboratory. She said that meant that something else might have caused the infection.

“Basically I think the data look like contamination,” Dr. Manuelidis said, possibly stemming from “inadequately washed instruments.”

Dr. Prusiner, who won the 1997 Nobel Prize in Physiology or Medicine for his prion research, said some of his critics would never be satisfied. “They’ll say we need to do 10 more years of experiments,” he said. “It’s just silly.”

Dr. Prusiner predicted that the newly gleaned information would lead to more effective ways to diagnose and treat a family of deadly diseases called transmissible spongiform encephalopathies, or T.S.E.’s, believed to be caused by aberrant proteins.

The stakes are enormous. Last week, a British citizen who died from other causes was found to have been infected by a human form of T.S.E. from a routine blood transfusion. The human form, called variant Creutzfeldt-Jakob disease, is contracted from eating cattle infected with mad cow disease. At least two people who died from the variant disease gave blood before falling ill, which means many more Britons could be infected. The disease can take years to manifest itself in humans.

American agriculture officials are testing thousands of cattle in an effort to determine if mad cow disease is a problem in this country, but the tests are notoriously imperfect. A deeper understanding of protein diseases should lead to tests that can diagnose the disease even in cattle that show no symptoms.

The biology of protein diseases is new and often difficult to grasp, Dr. Prusiner said. He said that many proteins caused disease when they adopted an abnormal shape and formed toxic fibrils that created havoc in the brain or body. Alzheimer’s disease, Parkinson’s disease, Type 2 diabetes and at least two dozen other human disorders may be caused by misshapen proteins, he said. Those diseases, however, do not involve prions, which are the only misfolded proteins known to be infectious.

The hallmarks of a T.S.E. are spongy holes and inflammation in the brain. Some of the diseases also feature clumps of fibrils called amyloids.

When Dr. Prusiner introduced the protein-only hypothesis, he was greeted with skepticism. Laboratories around the world have tried many times to inject synthetic prions into mice, but the experiments never worked, because the mice never got sick, Dr. Prusiner said. But this time, he said, the mice did get sick from the synthetic prion. The protein fragment was synthetic to avoid the possibility that something in a live cell might have contributed to the disease.

In this experiment, Dr. Prusiner injected a protein fragment that he reasoned could be the core source of the infection.

“We waited,” he said. “And waited. At 300 days, none of the animals got sick. We thought the experiment had not worked.”

But over the next 200 days, every animal developed spongiform degeneration, Dr. Prusiner said. A brain extract from a sick animal was injected into normal mice with different prion structures. They, too, got sick.

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