The nature of the scrapie agent.

There now seems little doubt that the infective agent of scrapie cannot be accommodated within current concepts of virology/molecular biology. It is proposed: that the basic infective entity is a nucleic acid fragment (oligonucleotide) of some 40 bp coupled with specific (but host encoded) protein totalling approximately 10(5) daltons, a significant proportion of which is in the form of proteolipid; that the nucleic acid fragment reprograms the host cell on the chemically switched microprocessor network analogy already proposed; that the nucleic acid fragment has no initiation sequence for replication: it is therefore non-infective; that the nucleic acid fragment can replicate when associated with the specific protein component because the resulting complex is able to displace mobile genetic element flanking sequences (similar to the yeast delta sequence). The function of the protein is to provide a scaffolding which allows the nucleic acid fragment to be assimilated into the replication cycle of the mobile genetic element as a whole.