Studies on DNA from normal and scrapie-affected mouse brain

Does the infective agent of scrapie replicate without nucleic acid? An assessment

The dogma of a unique status for the scrapie agent falling outside the virological spectrum is critically examined in the light of the circumstances which gave rise to it, and it is concluded that such an extreme view cannot be justified. The dogma arose in the first place by a combination of inadequate methodology and the lack of comparable data from other systems. It has been sustained partly by the same factors, and partly by a general failure to understand the impact on all relevant investigations of the exceptionally tenacious binding of infective agent to host-cell (membrane) components. This has not only greatly distorted the experimental findings, but as a consequence has resulted in extensive data misinterpretation. It is concluded that there is no hard evidence for the absence of a nucleic acid core in the scrapie agent so long as it is accepted that this is very small--i.e., of the order of 30 KDa (kilodaltons)--and is embedded within a cell membrane component matrix which protects it by forming a sequestered microenvironment: further that this is by far the simplest hypothesis.

Conservation of infectivity in purified fibrillary extracts of scrapie-infected hamster brain after sequential enzymatic digestion or polyacrylamide gel electrophoresis

Infectious extracts of scrapie-infected hamster brain enriched for scrapie-associated fibrils and scrapie amyloid protein (PrP) were partially denatured and subjected to either polyacrylamide gel electrophoresis with subsequent isolation of the PrP band or sequential enzymatic digestion with deglycosidase, phospholipase, proteinase, and several different nucleases. Infectivity measurements of these various specimens revealed a convincing association between infectivity and scrapie amyloid protein, with or without its sugar chains and disulfide bonds, and did not support the hypothesis that nucleic acid is involved in replication.

Thiocyanate and hydroxyl ions inactivate the scrapie agent

To probe the macromolecular structure of the scrapie agent and explore conditions for monomerization, the stability of the agent in low concentrations of inorganic ions was determined. A reduction by a factor of 10(5) in scrapie titer was found on exposure of the agent to 1 M KSCN or 0.3 M NaOH. In addition to the inactivation by thiocyanate ions, other chaotropic ions such as guanidinium and trichloroacetate inactivate the scrapie agent. Removal of thiocyanate ions by dialysis or glass permeation chromatography prevented the reduction in scrapie agent infectivity. Addition of equimolar amounts of (NH4)2SO4, a nonchaotrope, to preparations containing 1 M KSCN also prevented the loss of scrapie infectivity. In contrast, neutralization of the alkali-treated fractions with HCl did not restore infectivity. Acidification of partially purified fractions did not cause inactivation of the agent but did result in precipitation of the infectious agent. Inactivation by relatively low concentrations of chaotropic ions is consistent with many observations, all of which suggest that the scrapie agent contains a protein component that is essential for the maintenance of infectivity. Thus, it is unlikely that the agent is composed only of a "naked" nucleic acid. Certainly, if the agent were a naked nucleic acid, its lability in alkali virtually eliminates the possibility that it is composed of a single-stranded molecule of DNA.

Viroids: structure and function

Viroids are nucleic acid species of relatively low molecular weight and unique structure that cause several important diseases of cultivated plants. Similar nucleic acid species may be responsible for certain diseases of animals and humans. Viroids are the smallest known agents of infectious disease. Unlike viral nucleic acids, viroids are not encapsidated. Despite their small size, viroids replicate autonomously in cells of susceptible plant species. Known viroids are single-stranded, covalently closed circular, as well as linear, RNA molecules with extensive regions of intramolecular complementarity; they exist in their native state as highly base-paired rods.

Relation between Carp (multiple-sclerosis associated) agent and multiple sclerosis

It is suggested that the Carp (multiple-sclerosis associated and scrapie associated) agents may be the V-R.N.A. form of slow (D.N.A.) viruses which are the aetiological agents of the two diseases.