Deletion of the gag region from FBR murine osteosarcoma virus does not affect its enhanced transforming activity.

Both FBJ murine osteosarcoma virus (FBJ-MSV) and FBR-MSV induce transformation in tissue culture and osteogenic sarcomas in mice. In tissue culture, however, FBR-MSV induces larger foci with a shorter latency than those induced by FBJ-MSV. Transformation is dependent on expression of the fos oncogene in FBJ-MSV and of a gag-fos fusion protein in FBR-MSV. We have determined that the gag sequences can be deleted from FBR-MSV without affecting the high transforming activity of this virus in comparison to FBJ-MSV. The resultant virus, designated FBJ/R-MSV, has a protein coding region that is half the size of that of FBR-MSV and about one-third smaller than that of FBJ-MSV. Thus, FBJ/R-MSV will provide a useful tool for studying the transforming activity of the fos oncogene.