Infectivity of visna virus DNA

Deoxyribonucleic acid isolated from cells infected with the RNA containing slow virus visna confers on uninfected cells the capacity to synthesize new virus. The cytopathic agent isolated in these experiments has the biological and biochemical properties of visna virus, and the active material is DNA by several criteria, the most important of which is the loss of infectivity with prior treatment of the DNA with deoxyribonuclease. The efficiency of infection with DNA is markedly enhanced by presenting DNA to the cell as a calcium DNA phosphate precipitate. Because of the lytic nature of visna virus infection, the infectivity of visna DNA can be assayed quantitatively by plaque formation. Our major findings with this infectivity assay include the following: (1) Duplex DNA is infectious while denatured DNA is not. (2) At least one complete copy of proviral DNA is integrated in the host genome. (3) The minimal infective size of duplex DNA corresponds to a transcript of one subunit of viral RNA. (4) However, the concentration dependence of infectivity is two hit over the size range of 10-30 million molecular weight. These findings suggest that the unique genetic information of the viral genome is distributed over more than one subunit and that the proviral DNA transcripts are unlinked in the cell. The usefulness of the DNA infectivity assay and the implications of the visna provirus for persistent viral infections are discussed.