Integration, expression, and infectivity of exogenously acquired proviruses of Rous-associated virus-O

We investigated the integration sites, infectivities, and expression of Rous-associated virus-0 (Rav-0) DNAs in exogenously infected turkey and chicken cells. Restriction endonuclease analyses of the DNAs of RAV-0-infected cells indicated that unique integration sites of RAV-0 DNA were detectable in clones of RAV-0-infected cells but not in mass-infected cell cultures. In addition, the sites of integration of RAV-0 DNA differed in each of the seven clones of RAV-0-infected cells examined. Thus, endogenous RAV-0 proviruses appeared to integrate at multiple sites in cellular DNA, which were distinct from the sites of integration of endogenous RAV-0 genomes. Since exogenous RAV-0 proviruses are expressed at 10(3)- to 10(4)-fold-higher levels and are 10(3)- to 10(4)-fold more infectious in transfection assays than the endogenous RAV-0 genome of uninfected V+ chicken cells, these results are consistent with the hypothesis that transcription of the endogenous RAV-0 genome is regulated by flanking cellular DNA sequences. Although all RAV-0-infected cells contained infectious RAV-0 DNA and produced high titers of RAV-0 compared with uninfected V+ cells, different clones of RAV-0 infected chicken cells differed by as much as 30-fold in their levels of virus production. The infectivity of the DNA of each clone of RAV-0-infected cells correlated with the amount of virus produced by that clone. However, these differences did not appear to be correlated either with the number of exogenous RAV-0 proviruses in different clones or with the infectivity of RAV-0 produced by different clones, indicating that differences either in modification of RAV-0 DNAs or in the cellular sequences flanking exogenous RAV-0 DNAs were responsible for the observed differences in expression and infectivity.