Integration of Rous sarcoma virus DNA into chicken embryo fibroblasts: no preferred proviral acceptor site in the DNA of clones of singly infected transformed chicken cells.

We analyzed retroviral integration into a host genome by using avian sarcoma virus infection of natural target cells under conditions where secondary integration via virus spread was inhibited. This was accomplished by using the noninfectious pol- env- alpha variant of the Bryan high-titer strain of Rous sarcoma virus. A total of 12 independent Bryan high-titer Rous sarcoma virus-transformed chicken embryo fibroblast clones were obtained and mapped by using restriction endonucleases. Provirus-cell junction fragments were identified with appropriate hybridization probes. We found that expression of the viral genes could occur after proviral integration at many sites on the chicken genome and that there was no apparent preference for specific integration sites.