FIG. 1.

trans activation of leukemogenic retrovirus infection of nonpermissive hamster cells by an sFr-RBD-receptor complex. The hamster-derived cell line CHTGmCAT1 was challenged with retroviral vectors containing E. coli lacZ and bearing the envelope glycoprotein from FeLV-A, FeLV-B, FeLV-T, or the murine amphotropic MCF247 or xenotropic NZB virus. A virus expressing an altered xenotropic envelope glycoprotein lacking an RBD (ΔRBD) was also studied. Infection was measured as a function of the concentration of the purified sFr-RBD added to the culture medium and scored by enumerating the foci of cells expressing β-galactosidase 2 days after exposure to virus. Virus infection titers, shown in infectious units per milliliter of virus-containing medium (i.u. per milliliter), were determined by endpoint dilution.