Transformation of rodent cells with avian Rous sarcoma virus (RSV) opened new ways to studying virus integration and expression in nonpermissive cells. We were interested in (i) the molecular changes accompanying fusion of RSV-transformed mammalian cells with avian cells leading to virus rescue and (ii) enhancement of this process by retroviral gene products. The RSV-transformed hamster RSCh cell line was characterized as producing only a marginal amount of env mRNA, no envelope glycoprotein, and a small amount of unprocessed Gag protein. Egress of viral unspliced genomic RNA from the nucleus was hampered, and its stability decreased. Cell fusion of the chicken DF-1 cell line with RSCh cells led to production of env mRNA, envelope glycoprotein, and processed Gag and virus-like particle formation. Proteosynthesis inhibition in DF-1 cells suppressed steps leading to virus rescue. Furthermore, new aberrantly spliced env mRNA species were found in the RSCh cells. Finally, we demonstrated that virus rescue efficiency can be significantly increased by complementation with the env gene and the highly expressed gag gene and can be increased the most by a helper virus infection. In summary, Env and Gag synthesis is increased after RSV-transformed hamster cell fusion with chicken fibroblasts, and both proteins provided in trans enhance RSV rescue. We conclude that the chicken fibroblast yields some factor(s) needed for RSV replication, particularly Env and Gag synthesis, in nonpermissive rodent cells.
Importance: One of the important issues in retrovirus heterotransmission is related to cellular factors that prevent virus replication. Rous sarcoma virus (RSV), a member of the avian sarcoma and leukosis family of retroviruses, is able to infect and transform mammalian cells; however, such transformed cells do not produce infectious virus particles. Using the well-defined model of RSV-transformed rodent cells, we established that the lack of virus replication is due to the absence of chicken factor(s), which can be supplemented by cell fusion. Cell fusion with permissive chicken cells led to an increase in RNA splicing and nuclear export of specific viral mRNAs, as well as synthesis of respective viral proteins and production of virus-like particles. RSV rescue by cell fusion can be potentiated by in trans expression of viral genes in chicken cells. We conclude that rodent cells lack some chicken factor(s) required for proper viral RNA processing and viral protein synthesis.