Expression of Endogenous Retroviral Genes in Leukemic Guinea Pig Cells

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The expression of guinea pig retrovirus (5-bromodeoxyuridine[BUdR]-induced GPV) was studied in guinea pig L2C leukemic lymphoblasts by use of molecular hybridization of viral complementary DNA (cDNA) to cellular RNA. It was found that L2C leukemic lymphoblasts, leukemic spleen, and BUdR-induced virus-producing cells contain virus-specific RNA: 0.05% (800 to 960 copies per cell), 0.02% (360 copies per cell), and 0.3% (5,120 copies per cell), respectively. Adult normal liver and spleen, on the other hand, contain less than 0.2 copy of viral RNA per cell. Both BUdR-induced cells and L2C leukemic lymphoblasts contained 14S, 22S, 35S, and 70S RNA species of total and cytoplasmic virus-specific RNA as determined by sucrose velocity gradient analysis and hybridization of sucrose gradient fractions to cDNA. Virus-specific mRNA was identified in both BUdR-induced cells and L2C leukemic lymphoblasts by the criterion that it cosedimented with purified polyribosomes in a sucrose gradient and that it changed to a lower sedimentation value if polyribosomes were disaggregated with EDTA prior to centrifugation. Virus-specific mRNA obtained from either the polyribosome region of purified polyribosomes or the released messenger region of EDTA-disaggregated purified polyribosomes consisted of 14S, 20S, and 35S species in both BUdR-induced cells and L2C leukemic lymphoblasts. Hybridization of cDNA to the RNA of L2C leukemic lymphoblasts and BUdR-induced cells was essentially complete. Additionally, leukemic lymphoblast RNA could displace 95% of the hybridization of BUdR-induced GPV 70S RNA to guinea pig DNA. The midpoints of thermal denaturation of hybrids formed between GPV cDNA and the RNA of either L2C leukemic lymphoblasts or the 70S RNA of BUdR-induced GPV were both 89°C in 2× concentrated 0.15 M NaCl plus 0.015 M sodium citrate. These results show that BUdR-induced GPV genes are essentially completely expressed in L2C leukemic lymphoblasts and that virus-specific mRNA is present, although fewer copies of RNA are present in L2C leukemic lymphoblasts than in BUdR-induced cells.

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