Considerable evidence argues that prions are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrP) designated PrPSc. Many experimental approaches have failed to find evidence for a scrapie-specific nucleic acid, yet the identification of distinct scrapie "strains" seriously challenges this conclusion. Distinct scrapie "strains" are characterized by specific incubation times when passaged through a particular host. Two new avenues of investigation into the structure and biology of prions may help resolve the conundrum of how the prion particle can carry the information required for "strains", yet no scrapie-specific nucleic acid has been identified. First, we recently found that incubation time phenotypes of Syrian hamster (SHa) scrapie "strains" change with passage in transgenic (SHaPrP) mice as well as in Armenian (AHa) and Chinese (CHa) hamsters. These findings give rise to an experimental approach that might begin to unravel the molecular basis of scrapie "strains" or isolates. The impact of single and multiple amino acid exchanges in hamster PrP molecules will be investigated with respect to: 1) scrapie incubation times for different "strains", 2) synthesis of prions with different properties, 3) spontaneous development of prion diseases, 4) species barriers, and 5) neuropathologic changes. From the results of these studies, we should be able to determine which amino acid residues in PrP modify the properties of specific scrapie "strains". Second, we plan to exploit an exciting recent observation that transgenic (Tg) mice expressing the Gerstmann-Straussler-Scheinker syndrome (GSS) mutant mouse (Mo) PrP transgene spontaneously develop CNS degeneration. This discovery has created a promising and novel strategy for determining whether or not infectious prions can be generated de novo in the absence of any inoculum. We shall extend these dramatic results by varying the amino acid side chains at codons 102, 117, and 200 all of which are sites of point mutations in GSS and familial Creutzfeldt-Jakob disease (CJD). The proposed studies should identify amino acid substitutions that cause spontaneous CNS degeneration and possibly those that produce infectious prions de novo. If our investigations demonstrate convincingly that infectious prions are generated de novo in uninoculated Tg mice expressing mutant PrP transgenes, then these findings will argue persuasively that prions are devoid of foreign nucleic acid in accord with the results of many earlier studies. The knowledge gained from the proposed studies may allow us to predict the properties of prions carrying specific PrP sequences and to produce them de novo from mutant PrP transgenes.