Homogeneity and complexity of avian oncornavirus proviral DNA determined by molecular hybridization

Measurement of proviral genes in uninfected and avian myeloblastosis virus-infected cells by hybridization with 3H-labeled complementary DNA probe excess

Viral RNA (vRNA) from avian myeloblastosis virus or DNA from virus-infected and uninfected cells was hybridized with [3H]DNA complementary to viral RNA ([3H]cDNA) under conditions of [3H]cDNA excess. When [3H]cDNA was used to drive the hybridization reaction with vRNA, a rate constant of 33.2 liters/mol-s was obtained. The same rate constant was obtained when vRNA excess was used as the driver. The specific activities of the [3H]DNA probe, estimated from kinetic measurements of the hybridization reaction and from the amount of [3H]cDNA in hybrid form at equilibrium, were 9.1 and 8.6 cpm/pg, respectively. DNA isolated from uninfected cells contained five or six copies of proviral DNA per cell genome. DNA isolated from erythrocytes infected with avian myeloblastosis virus had an additional five or six viral genes added to the cell genome, and the virus-infected target cell (myeloblasts) contained about 15 additional copies of proviral DNA per cell. The use of excess [3H]cDNA probe is an easy and accurate method to quantify the frequency of proviral DNA sequences in cell DNA and to measure a small amount (40 to 200 pg) of vRNA. Probe excess hybridization offers a number of advantages over other procedures and these are discussed.

Integration of proviral DNA in chicken cells infected with Schmidt-Ruppin Rous sarcoma virus is not enhanced by DNA repair

The effect DNA repair might have on the integration of exogenous proviral DNA into host cell DNA was investigated by comparing the efficiency of proviral 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 of nonintegrated 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 (3)H-labeled 35S viral RNA. The copy number of the 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.