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Viral genes code for viral proteins.

I 8.36 The typical genome of a retrovirus contains gag, pol, and env genes.

8.37 HIV principally attacks T lymphocytes.

Electron micrograph showing a T cell infected with HIV. (Courtesy of Dr. Hans Gelderblom.)

hereditary information in the form of DNA or RNA, and so how are prions able to reproduce without nucleic acid?

Prions were first recognized as unusual infectious agents that cause scrapie, a disease of sheep that destroys the brain. In 1982, Prusiner purified the scrapie pathogen and reported that it consisted entirely of protein. Prusiner and his colleagues eventually showed that the prion protein (PrP) is derived from a normal protein that is encoded by a gene found throughout eukaryotes, including yeast. Normal PrP (PrPC) is folded into a helical shape, but the protein can also fold into a flattened p sheet that causes scrapie (PrPSc) ( FIGURE 8.38). When PrPSc is present, it interacts with and causes PrPC to fold into the disease-causing form of the protein; infection with PrPSc converts an individual's normal PrP protein into abnormal PrP that forms prions. Accumulation of the PrPSc in the brain appears to be responsible for the neurological degeneration associated with diseases caused by prions. This explanation for prion diseases, called the "protein only" hypothesis, is not universally accepted; some scientists still believe that these diseases are caused by an as-of-yet unisolated virus.

Prions cause scrapie, bovine spongiform encephalopa-thy (BSE, or "mad cow" disease), and kuru, an exotic disorder spread among New Guinea aborigines by ritualistic cannibalism. They also play a role in some inherited human neurodegenerative disorders, including Creutzfeldt-Jakob disease and Gerstmann-Scheinker disease. In these inherited diseases, the PrP gene is mutated and produces a type of PrP that is more susceptible to folding into PrPSc. Nearly all those who carry such a mutated gene eventually produce prions and get the disease.

Some cases of human prion diseases have been traced to injections of growth hormone, which until recently was obtained from the brains of human cadavers infected with prions. In England, an epidemic of mad cow disease erupted in the late 1980s, the origin of which was traced to cattle feed containing the remains of sheep infected with scrapie.

www.whfreeman.com/pierce For more information on prions, prion-caused diseases, and Stanley Prusiner's account of his hunt for the secret of prions.

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