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-O (RAV-O) DNAs in exogenously infected turkey and chicken cells. Restriction endonuclease analyses of the DNAs of RAV-O-infected cells indicated that unique integration sites of RAV-O DNA were detectable in clones of RAV-O-infected cells but not in mass-infected cell cultures. In addition, the sites of integration of RAV-O DNA differed in each of the seven clones of RAV-O-infected cells examined. Thus, endogenous RAV-O proviruses appeared to integrate at multiple sites in cellular DNA, which were distinct from the sites of integration of endogenous RAV-O genomes. Since exogenous RAV-O proviruses are expressed at 10³- to 10⁴-fold-higher levels and are 10³- to 10⁴-fold more infectious in transfection assays than the endogenous RAV-O genome of uninfected V⁺ chicken cells, these results are consistent with the hypothesis that transcription of the endogenous RAV-O genome is regulated by flanking cellular DNA sequences. Although all RAV-O-infected cells contained infectious RAV-O DNA and produced high titers of RAV-O compared with uninfected V⁺ cells, different clones of RAV-O-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-O-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-O proviruses in different clones or with the infectivity of RAV-O produced by different clones, indicating that differences either in modification of RAV-O DNAs or in the cellular sequences flanking exogenous RAV-O DNAs were responsible for the observed differences in expression and infectivity.