Effects of actinomycin D and of mitomycin C on the development of Rofs sarcoma virus

Actinomycin D inhibits human immunodeficiency virus type 1 minus-strand transfer in in vitro and endogenous reverse transcriptase assays

In this report we demonstrate that human immunodeficiency virus type 1 (HIV-1) minus-strand transfer, assayed in vitro and in endogenous reactions, is greatly inhibited by actinomycin D. Previously we showed that HIV-1 nucleocapsid (NC) protein (a nucleic acid chaperone catalyzing nucleic acid rearrangements which lead to more thermodynamically stable conformations) dramatically stimulates HIV-1 minus-strand transfer by preventing TAR-dependent self-priming from minus-strand strong-stop DNA [(-) SSDNA]. Despite this potent activity, the addition of NC to in vitro reactions with actinomycin D results in only a modest increase in the 50% inhibitory concentration (IC50) for the drug. PCR analysis of HIV-1 endogenous reactions indicates that minus-strand transfer is inhibited by the drug with an IC50 similar to that observed when NC is present in the in vitro system. Taken together, these results demonstrate that NC cannot overcome the inhibitory effect of actinomycin D on minus-strand transfer. Other experiments reveal that at actinomycin D concentrations which severely curtail minus-strand transfer, neither the synthesis of (-) SSDNA nor RNase H degradation of donor RNA is affected; however, the annealing of (-) SSDNA to acceptor RNA is significantly reduced. Thus, inhibition of the annealing reaction is responsible for actinomycin D-mediated inhibition of strand transfer. Since NC (but not reverse transcriptase) is required for efficient annealing, we conclude that actinomycin D inhibits minus-strand transfer by blocking the nucleic acid chaperone activity of NC. Our findings also suggest that actinomycin D, already approved for treatment of certain tumors, might be useful in combination therapy for AIDS.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Deficiency of 60 to 70S RNA in murine leukemia virus particles assembled in cells treated with actinomycin D

Production of particles with the ultrastructural appearance of C-type virions persisted for at least 6 h in actinomycin D-treated cells infected with murine leukemia virus. This phenomenon occurred despite severe inhibition of viral RNA synthesis. Virus particles present in a 6-h harvest sedimented in sucrose gradients with the buoyant density characteristic of RNA tumor viruses (1.16 g/cm(3)) and exhibited high levels of reverse transcriptase activity in response to the exogenous template polyriboadenylic acid.oligo deoxythymidylic acid in the range of untreated controls. However, RNase-sensitive endogenous activity was only (1/5) the level found in controls. This observation correlated with a marked reduction in infectivity. Kinetic studies on the appearance of labeled RNA in banded virions revealed that within the first hour after addition of actinomycin D, particles contained 60 to 70S RNA and two low-molecular-weight RNA species corresponding to 8 and 4S RNA. After approximately 1 h of incubation with actinomycin D, 60 to 70S RNA could not be detected and 4S RNA was the predominant species. These findings suggest that murine leukemia virus particles assembled in the presence of actinomycin D are deficient in 60 to 70S viral RNA.

Mechanism of action of ribonuclease H isolated from avian myeloblastosis virus and Escherichia coli

Purified preparations of RNA-dependent DNA polymerase isolated from avain myeloblastosis virus contain RNase H activity. Labeled ribohomopolymers are degraded in the presence of their complementary deoxyribopolymer, except [(3)H]poly(U).poly(dA). The degradation products formed from [(3)H]poly(A).poly(dT) were identified as oligonucleotides containing 3'-hydroxyl and 5'-phosphate termini, while AMP was not detected. The nuclease has been characterized as a processive exonuclease that requires ends of poly(A) chains for activity. Exonucleolytic attack occurs in both 5' to 3' and 3' to 5' directions.RNase H has also been purified from E. coli. This nuclease degrades all homoribopolymers tested in the presence of their complementary deoxyribopolymers to yield oligonucleotides with 5'-phosphate and 3'-hydroxyl termini. E. coli RNase H has been characterized as an endonuclease.

DNA in uninfected and virus-infected cells complementary to avian tumor virus RNA

The 70S RNA component of several avian tumor viruses was hybridized with DNA extracted from avian tumor virus-infected and uninfected chicken and Japanese quail cells. Tritium-labeled 70S RNAs from Rous sarcoma virus (RSV), Rous associated virus-1 (RAV-1), RAV-60, and Schmidt-Ruppin-RSV (SR-RSV) hybridize from 3 to 10 times more with DNA from uninfected chicken cells than with DNA from Escherichia coli, calfthymus, or baby hamster kidney cells. After infection of chicken cells with RSV(RAV-1), SR-RSV, or RAV-2, the amount of 70S avian tumor virus [(3)H]RNA hybridized increases by 1.6 times. The specificity of the hybridization reaction was shown by the specific competition of 70S SR-RSV [(3)H]RNA with 70S RNA from RSV(RAV-1), and not with RNA from Sendai virus or chicken cells. There was no difference in the hybridization of 70S RNA from RSV (RAV-1), RAV-1, or RAV-60 with DNA either from chicken cells that contain RAV-60 in a nonreplicating form or from chicken cells that do not appear to contain RAV-60. These results indicate that both types of uninfected chicken cells contain DNA that is complementary to RNA from several avian tumor viruses and that the amount of complementary DNA increases in such cells after infection with an avian tumor virus. The RNAs of genetically different avian tumor viruses appear to have indistinguishable base sequences by this technique.

Evidence for a DNA replicative genome for RNA-containing tumor viruses

The requirement for DNA synthesis in the reproduction of RNA tumor viruses was examined using 5-bromodeoxyuridine (BrdU). Addition of BrdU for the first 12 hr after exposure of cells to Rous sarcoma virus resulted in a large decrease in infectious progeny. However, incorporation of [(3)H]uridine into the viral RNA of virions was unaffected. Also, the numbers of virions visualized by electron microscopy were similar in BrdU-treated and untreated samples. Viral group-specific antigen was synthesized after BrdU treatment, and BrdU had little effect on transformation of cells by Rous sarcoma virus. The results support the hypothesis that the replicative genome of RNA-containing tumor viruses contains DNA.

DNA complementary to viral RNA in leukemic cells induced by avian myeloblastosis virus

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 (T(m) = 87 degrees and 92 degrees C, respectively, in 2x saline citrate). This suggests that the leukemic cells contain a DNA template of the viral RNA.

Incorporation of precursors into ribonucleic acid, protein, glycoprotein, and lipoprotein of avian myeloblastosis virions

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 (3)H-uridine have shown that there is an average time interval of 3 to 4 hr (half-life) between the time viral ribo-nucleic 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 (14)C-phenylalanine have revealed that some protein synthesis takes place at or near the cell surface immediately prior to maturation and release of virus. (14)C-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 (14)C-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.

On the role of DNA synthesis in avian tumor virus infection

Preinfection of chick embryo fibroblasts with an avian tumor virus does not eliminate the requirement of early DNA synthesis for other avian tumor viruses superinfecting the same cells several days after the primary inoculum. The function served by the early DNA in avian tumor virus infection either fails to persist into late infection or is genetically specific for each viral type.

Deoxyribonucleic acid-dependent ribonucleic acid polymerase activity in cells infected with influenza virus

Deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA) polymerase activity was assayed on nuclear preparations of chick embryo fibroblast cells at various times after infection with an influenza A virus (fowl plague virus) and was compared with the activity of uninfected cells. Polymerase activity was increased by about 60% by 2 hr after infection, and this increase coincided with an increase in RNA synthesis in infected cells, as determined by pulse-labeling with uridine. No difference could be detected between the polymerases of infected and uninfected cells as to their requirements for DNA primer, divalent cations, and nucleoside triphosphates, and they were equally sensitive to addition of actinomycin D to the reaction mixture. It is possible that host cell DNA-dependent RNA polymerase is involved in the replication of influenza virus RNA.