EDITORIAL
Journal of Virology (JVI) marks its 50th year of publishing in 2017. To highlight particularly noteworthy JVI articles over the years, 2017 issues are featuring Classic Spotlights selected from the archives by the editors. These Classic Spotlights are appearing chronologically, and in this issue, we have selected articles from 1978 and 1979.
Transformation of BALB/c-3T3 Cells by tsA Mutants of Simian Virus 40: Temperature Sensitivity of the Transformed Phenotype and Retransformation by Wild-Type Virus
Brockman (W. W. Brockman, J Virol 25:860–870, 1978, http://jvi.asm.org/content/25/3/860.abstract ) investigated the function of the A gene of simian virus 40 (SV40) in transformation of BALB/c-3T3 cells by infecting cells with wild-type (wt) SV40 and with six tsA mutants whose mutation sites map at different positions in the early region of the SV40 genome. Cells were infected at the permissive temperature, and cloned transformants were characterized for temperature sensitivity of the transformed phenotype. The majority of the tsA transformants, but none of the wt transformants, were temperature sensitive for maintenance of transformation. Furthermore, temperature-sensitive tsA transformants that were superinfected with wt SV40 were resistant to temperature inhibition of the transformed state. Since temperature-sensitive tsA transformants become temperature resistant on retransformation by wt virus, the temperature-sensitive defect lies in the product of a viral and not a cellular gene. These results imply that the protein encoded by the SV40 A gene is required to maintain the transformed phenotype in BALB/c-3T3 cells. The tsA gene encodes T antigen, and this study demonstrated that SV40 T antigen is required for transformation maintenance.
Virus-Specific DNA in the Cytoplasm of Avian Sarcoma Virus-Infected Cells Is a Precursor to Covalently Closed Circular Viral DNA in the Nucleus
Three principal forms of viral DNA have been identified in cells infected with avian sarcoma virus: (i) a linear duplex molecule synthesized in the cytoplasm, (ii) a covalently closed circular molecule found in the nucleus, and (iii) proviral DNA covalently linked to high-molecular-weight cell DNA. To define precursor product relationships among these forms of viral DNA, Shank and Varmus (P. R. Shank and H. E. Varmus, J Virol 25:104–114, 1978, http://jvi.asm.org/content/25/1/104.abstract ) investigated the relationship between cytoplasmic linear DNA and nuclear form I DNA. Viral DNA synthesized in the presence of 5-bromodeoxyuridine (BrdU) has a high density due to the substitution of BrdU for thymidine in both strands, and so BrdU was used to density label the precursor form in the cytoplasm during the first 4 h of infection by avian sarcoma virus. Further incorporation of BrdU into viral DNA was blocked by adding thymidine, and the conversion of BUdR-substituted DNA into form I molecules in the nucleus was monitored. After a chase with thymidine, a portion of the density-labeled viral DNA was transported to the nucleus and converted to a covalently closed circular form, demonstrating that that linear viral DNA, synthesized in the cytoplasm, is the precursor to closed circular DNA observed in the nucleus.
Nucleosomal Structure of Epstein-Barr Virus DNA in Transformed Cell Lines
Shaw et al. (J. E. Shaw, L. F. Levinger, and C. W. Carter, Jr., J Virol 29:657–665, 1979, http://jvi.asm.org/content/29/2/657.abstract ) examined Epstein-Barr virus (EBV) DNA organization in human B-lymphoblastoid cell lines originally derived from African Burkitt's lymphomas. DNA forms in three cell lines, virus-producing, non-virus-producing, and superinfected cells, were analyzed by micrococcal nuclease digestion. EBV viral DNA is a linear molecule. Virus-producing lines, such as P3HR-1, constantly produce infectious virus; however, only a small percentage of the cells are in lytic virus production at any time. The Raji cell line contains EBV DNA in a latent state and does not produce virus. EBV DNA in Raji cells replicates with host DNA during S-phase as covalently closed circular molecules. Raji cells can be superinfected with EBV from P3HR-1 cells, resulting in viral DNA synthesis, and viral DNA accumulates in the nucleus of infected cells. Virus particles formed during superinfection can transform human umbilical cord lymphocytes into continuous cell lines. Digests of virus-producing (P3HR-1), non-virus-producing (Raji), and superinfected Raji cell nuclei were fractionated on agarose gels, transferred to nitrocellulose, and hybridized to 32P-labeled EBV DNA. Viral DNA from Raji cells produced a series of bands that were integral multiples of a unit size, which was the same as the repeat length of host DNA. P3HR-1 viral DNA and viral DNA from superinfected Raji cells produced a smear of viral DNA. These studies showed that EBV episomal DNA of Raji cells is folded into nucleosomes, whereas most of the viral DNA of P3HR-1 and superinfected Raji cells, which is replicating in the lytic phase, is not.
Protease Required for Processing Picornaviral Coat Protein Resides in the Viral Replicase Gene
The coat protein of encephalomyocarditis (EMC) virions contains four polypeptide chains (δ, β, γ, α), which make up the repeating subunits of the protein shell. These proteins are produced during EMC virus infection by proteolytic cleavage of a large precursor protein. Palmenberg et al. (A. C. Palmenberg, M. A. Pallansch, and R. R. Rueckert, J Virol 32:770–778, 1979, http://jvi.asm.org/content/32/3/770.abstract ) partially purified EMC protease synthesized in cell extracts from rabbit reticulocytes and showed that the activity responsible for cleaving coat precursor protein cosediments with a previously unmapped virus-coded protein. Tryptic analysis showed that the protein is derived from protein D, a virus-coded component of the EMC RNA polymerase. Thus, the protease that processes EMC coat protein is within the replicase gene.
- Copyright © 2017 American Society for Microbiology.
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- EDITORIAL
- Transformation of BALB/c-3T3 Cells by tsA Mutants of Simian Virus 40: Temperature Sensitivity of the Transformed Phenotype and Retransformation by Wild-Type Virus
- Virus-Specific DNA in the Cytoplasm of Avian Sarcoma Virus-Infected Cells Is a Precursor to Covalently Closed Circular Viral DNA in the Nucleus
- Nucleosomal Structure of Epstein-Barr Virus DNA in Transformed Cell Lines
- Protease Required for Processing Picornaviral Coat Protein Resides in the Viral Replicase Gene
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