Our previous studies have shown that during the lytic infection of sheep cells in culture, visna virus produces an unintegrated viral DNA which is a linear double-stranded molecule with a molecular weight of approximately 6 × 106 daltons. We have now studied this DNA using the method of Southern to locate the cleavage sites of a number of restriction endonucleases on the unintegrated visna viral DNA. Using these sites, we have constructed a physical map of the linear viral DNA. In addition, our analysis has identified two species of closed circular viral DNA in the Hirt supernatant fraction of visna virus-infected cells.
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