Biological activity of cloned retroviral DNA in microinjected cells

Avian retroviral DNA molecules that had been cloned from infected cells by using recombinant DNA techniques were microinjected into either uninfected chicken embryo fibroblasts (CEF) or CEF transformed by the envelope glycoprotein-deficient Bryan strain of Rous sarcoma virus [RSV(--)cells]. Retroviral DNA injected into RSV(--) cells directed transcription of envelope mRNA, which was then able to complement the RSV(--) env deficiency and promote the production of infectious transforming virus. The retroviral DNA also directed the production of fully infectious virus after injection into uninfected cells or RSV(--) cells. Virus production began within 3--4 hr after microinjections. When 100 DNA molecules per cell were injected, almost all injected cells produced infectious virus. As the number of injected molecules per cell was decreased, a corresponding decrease was observed in the number of cells that produced infectious virus. DNA injected into the cytoplasm was 1/50th to 1/10th as effective in virus production as DNA molecules injected into the nucleus. DNA molecules containing one or two tandem copies of the viral long terminal repeat were equally effective in virus production.