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Viruses: At the Threshold of Life |
Chapter: 4
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A Solution–55 Years Later After microbiologists established the existence of viruses at the turn of the century, a search began for a virus that could cause cancer. To many investigators, the search seemed foolhardy because cancer did not appear to be an infectious disease. Nevertheless, one virus did emerge as an apparent cause of a type of cancer.
In 1911, an American physician, Francis Peyton Rous, was studying chickens that had a tumor of the connective tissues called a sarcoma. Rous decided to test the tumor for virus content, and he mashed up a section of tissue and passed it through a bacterial filter. To his astonishment, the clear filtrate caused tumors in healthy chickens. Rous did not refer to the infecting material as a virus, but others gradually did, and for many decades thereafter, the "Rous sarcoma virus" remained as a clear-cut example of a cancer-causing virus. The virus soon became an important tool of cancer researchers. In 1966, Rous was awarded the Nobel Prize in Physiology or Medicine, 55 years after his discovery.
However, the mystery of the Rous sarcoma virus deepened, as knowledge about the transformation of normal cells to cancer cells emerged. Virologists believed that cancer viruses released DNA into the cell cytoplasm, whereupon the DNA inserted itself into the cell’s chromosome. But the Rous sarcoma virus was composed of RNA, and RNA had never been found inserted in a chromosome. Moreover, the genetic code for transformation was presumably supplied by DNA, not by RNA.
The mystery was solved by Howard Temin and David Baltimore. In 1970, these investigators announced the discovery of a viral enzyme named reverse transcriptase. This enzyme uses RNA as a template and synthesizes a molecule of DNA complementary to the RNA. The new DNA could then insert into the chromosome and transform the cell. The finding of reverse transcriptase completed the story of Rous’ discovery and won for Temin and Baltimore the 1975 Nobel Prize in Physiology or Medicine.
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