SEARCH FOR A PUTATIVE SCRAPIE GENOME IN PURIFIED PRION FRACTIONS REVEALS A PAUCITY OF NUCLEIC-ACIDS

Scrapie can be transmitted by novel infectious pathogens termed prions. No evidence for a scrapie-specific nucleic acid has been detected to date. To investigate amounts, types and sizes of nucleic acid molecules associated with prions in purified preparations, aliquots were deproteinized, and the nucleic acids analysed by PAGE and silver staining. Digestion with nucleases and exposure to Zn2+ prior to analysis substantially diminished the content of nucleic acids, but did not alter the prion titre indicating that those nucleic acids which were removed are not essential for infectivity. Since a single species of scrapie-specific nucleic acid could not be identified, we explored the unprecedented possibility of scrapie-specific nucleic acids of variable length which are biologically active. If such molecules of variable length exist then they might be hidden within the background smear on silver-strained gels after PAGE. A new procedure designated return refocusing gel electrophoresis (RRGE) was developed to identify heterogeneous nucleic acids in purified prion fractions. The content of variable length nucleic acids was reduced by a factor of 10 by exhaustive Bal 31 exonuclease digestion after dispersion of purified prions into detergent-lipid-protein complexes. For example, a typical sample after Bal 31 digestion contained approximately 4 ng of nucleic acid of variable length and 10(8.7) ID50 units of scrapie prion infectivity. Consideration of different models for a hypothetical scrapie-specific nucleic acid suggests that such a molecule would have to be: (i) quite small (< 100 nucleotides), (ii) possess a particle-to-infectivity ratio near unity or (iii) heterogeneous in size. Although our results do not eliminate the possibility that prions possess a scrapie-specific nucleic acid of variable length, they narrow considerably the spectrum of features specifying such a candidate molecule.