Integration of Proviral DNA in Chicken Cells Infected with Schmidt-Ruppin Rous Sarcoma Virus Is Not Enhanced by DNA Repair

Abstract

The effect DNA repair might have on the integration of exogenous proviral DNA into host cell DNA was investigated by comparing the efficiency ofproviral DNA integration in normal chicken embryonic fibroblasts and in chicken embryonic fibroblasts treated with UV or 4-nitroquinoline-1-oxide. The cells were treated with UV or 4-nitroquinoline-1-oxide at various time intervals ranging from 6 h before to 24 h after infection with Schmidt-Ruppin strain A of Rous sarcoma virus. The chicken embryonic fibroblasts were subsequently cultured for 18 to 21 days to ensure maximal integration and elimination ofnonintegrated exogenous proviral DNA before DNA was extracted. Integration of proviral DNA into the cellular genome was quantitated by hybridization of denatured cellular DNA on filters with an excess of 3H-labeled 35S viral RNA. The copy number ofthe integrated proviruses in normal cells and in infected cells was also determined from the kinetics of liquid RNA-DNA hybridization in DNA excess. Both RNA excess and DNA excess methods of hybridization indicate that two to three copies of the endogenous provirus appear to be present per haploid normal chicken cell genome and that two to three copies of the provirus of Schmidt-Ruppin strain A of Rous sarcoma virus become integrated per haploid cell genome after infection. The copy number of viral genome equivalents integrated per cell treated with UV or 4-nitroquinoline-1-oxide at different time intervals before or after infection did not differ from the copy number in untreated but infected cells. This finding supports our previous report that the integration of oncornavirus proviral DNA is restricted to specific sites in the host cell DNA and suggests a specific mechanism for integration.

Prophage Mu induction as a tool to analyze mobile genetic element responses to external agents

“…Alternatively, it may have been advantageous for a cell to suppreks DNA transposition durifig the repair of 52/DuBOW AND SHINDER damage to DNA in order to prevent further, and irreparable, genomic rearrangements caused by the movement of these elements. The inhibition of DNA transposition has also been observed after treatment of cells with ultraviolet light and mitomycin C for bacteriophage Mu (Giphart-Gassler and Van de Putte, 1978), and ultraviolet light for RNA tumor viruses (Tsuruo and Baluda, 1977). X-rays induce double-stranded breaks in DNA and have been found to induce retrovirus proviruses (Varmus, 1982) and maize transposable elements (McClintock, 1951).…”