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Abstract. Nucleic acid hybridization studies were made between 71S-AMV-RNA and DNA from leukemic myeloblasts and from normal chicken cells. There was homology between the viral RNA and chicken cell DNA and'to a greater extent between viral RNA and leukemic cell DNA. Leukemic cell DNA hybridized approximately twice as much viral RNA as did normal chicken DNA. Thermal melting studies showed that the viral RNA bound to normal and leukemic cell DNA consists of long polynucleotides (Tm = 870 and 920C, respectively, in 2 X saline citrate). This suggests that the leukemic cells contain a DNA template of the viral RNA.

Freshly explanted leukemic myeloblasts produce avian myeloblastosis virus (AMV) at a constant rate without any obvious cytopathic effect; therefore, subviral components are continually synthesized at a steady rate. The incorporation of various radioactive precursors into virions was monitored by determination of radioactivity in purified virus after density equilibrium sedimentation in preformed sucrose gradients. The kinetics of incorporation of 3H-uridine have shown that there is an average time interval of 3 to 4 hr (half-life) between the time viral ribonucleic acid (RNA) is synthesized and the time it is released as a mature virus particle; this represents the average time interval spent by AMV-RNA in an intracellular pool. Studies with '4C-phenylalanine have revealed that some protein synthesis takes place at or near the cell surface immediately prior to maturation and release of virus. "4C-glucosamine also appears to be incorporated into the outer viral envelope shortly before maturation. On the other hand, there is an average lag of about 16 to 20 hr before '4C-ethanolamine incorporated into intracellular lipoprotein appears in free virions; this probably reflects the kinetics of replacement of cellular surface membrane. Actinomycin D inhibits AMV-RNA within 30 min but permits the maturation of AMV to continue for at least 2 hr. AMV released in the presence of actinomycin D contains AMV-RNA synthesized before the addition of the drug.

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 virusspecific 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 890C in 2x 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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