A ribonucleic acid-dependent deoxyribonucleic acid polymerase was found in virions of visna virus. The enzyme product was resistant to ribonuclease and alkaline hydrolysis but susceptible to the digestion of deoxyribonuclease. Visna virus is a medium-sized, ether-sensitive virus of sheep (8). It is formed by budding of the cytoplasmic membrane of the host cells and contains ribonucleic acid (RNA) as demonstrated by radioactive labeling of purified virus (2; Lin and Thormar,Virology, in press), but its multiplication is inhibited by 5-bromodeoxyuridine and actinomycin D, indicating a requirement for synthesis and function of deoxyribonucleic acid (DNA; reference 7). These properties of visna virus are similar to those of RNA tumor viruses (8).
A single-stranded ribonucleic acid(s) has been isolated from purified virions of visna virus. It consists of two major components, namely 63S and "4S," under the conditions employed for ribonucleic acid (RNA) extraction. The 63S component can be converted to subunits by heat and dimethylsulfoxide treatments. Analyses by base composition indicate that the "4S" RNA isolated from visna virus is not a random breakdown product of the 63S component as a result of extraction, nor is it randomly derived from cellular RNA.
Measles virus has been suggested to cause subacute sclerosing panencephalitis (SSPE), a slow central nervous system disease of children. However, several questions remain about the pathogenesis of SSPE. For example, it is not known whether alteration of the measles virus genome has a role in the initiation and persistence of the disease. Several studies have compared the RNA and protein composition of wild-type (wt) and SSPE strains of measles virus in a search for markers characteristic of the latter. All the studies used SSPE strains that had reverted to the budding, virion-producing form, similar to wt. We have shown, however, that only cell-associated non-budding strains of SSPE virus cause an SSPE-like persistent infection in young ferrets. Strong cell association and cell-fusing activity were essential for the virulence of measles virus in the brains of experimental animals and possibly humans. We have, therefore, compared the protein composition of virulent SSPE strains to that of the budding, non-virulent SSPE and wt strains. We report here that the M protein was not detectable in non-budding SSPE strains D.R., Biken and IP-3, and strain D.R. contained very little H protein.
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