Relationship between replication of simian virus 40 DNA and specific events of the host cell cycle

Inhibition of Autographa californica nuclear polyhedrosis virus replication in high-density Trichoplusia ni cell cultures

Replication of the Autographa californica nuclear polyhedrosis virus was studied in Trichoplusia ni (TN-368) tissue culture cells under cell density conditions which approximated a confluent monolayer (5.7 x 10(5) cells/cm(2) growth surface). These high-density cultures were not refractory to infection but the synthesis of both the occluded and nonoccluded forms of the virus were inhibited greater than 98% when compared to inoculation of low cell density cultures (1.4 x 10(5) cells/cm(2)). The inhibition phenomenon was not due to depletion of a medium component or a diffusable cell-associated factor. The inhibition was reversible and required cell-to-cell contact. Viral-induced protein synthesis studies using 35S-labeled methionine incorporation indicated an 18-hr delay in the synthesis of one of the four early proteins under high cell density conditions. DNA:DNA dot hybridization and 3H-labeled thymidine incorporation studies indicated that cell-to-cell contact reversibly inhibited both cellular and viral DNA synthesis.

Phosphorylation of simian virus 40 T antigen on Thr 124 selectively promotes double-hexamer formation on subfragments of the viral core origin

Cell cycle-dependent phosphorylation of simian virus 40 (SV40) large tumor antigen (T-ag) on threonine 124 is essential for the initiation of viral DNA replication. A T-ag molecule containing a Thr-->Ala substitution at this position (T124A) was previously shown to bind to the SV40 core origin but to be defective in DNA unwinding and initiation of DNA replication. However, exactly what step in the initiation process is defective as a result of the T124A mutation has not been established. Therefore, to better understand the control of SV40 replication, we have reinvestigated the assembly of T124A molecules on the SV40 origin. Herein it is demonstrated that hexamer formation is unaffected by the phosphorylation state of Thr 124. In contrast, T124A molecules are defective in double-hexamer assembly on subfragments of the core origin containing single assembly units. We also report that T124A molecules are inhibitors of T-ag double hexamer formation. These and related studies indicate that phosphorylation of T-ag on Thr 124 is a necessary step for completing the assembly of functional double hexamers on the SV40 origin. The implications of these studies for the cell cycle control of SV40 DNA replication are discussed.

Cyclin-dependent kinase regulation of the replication functions of polyomavirus large T antigen

The amino-terminal portion of polyomavirus (Py) large T antigen (T Ag) contains two phosphorylation sites, at T187 and T278, which are potential substrates for cyclin-dependent kinases (CDKs). Our experiments were designed to test whether either or both of these sites are involved in the origin DNA (ori DNA) replication function of Py T Ag. Mutations were generated in Py T Ag whereby either or both threonines were replaced with alanine, generating T187A, T278A, and double-mutants (DM [T187A T278A]) mutant T Ags. We found that the Py ori DNA replication functions of T278A and DM, but not T187A, mutant T Ags were abolished both in vivo and in vitro. Consistent with this finding, it was shown that the ori DNA binding and unwinding activities of mutant T278A Py T Ag were greatly impaired. Moreover, whereas wild-type Py T Ag is an efficient substrate for phosphorylation by cyclin A-CDK2 and cyclin B-cdc2 complexes, it is phosphorylated poorly by a cyclin E-CDK2 complex. In contrast to mutant T187A, which behaved similarly to the wild-type protein, T278A was only weakly phosphorylated by cyclin B-cdc2. These data thus suggest that T278 is an important site on Py T Ag for phosphorylation by CDKs and that loss of this site leads to its various defects in mediating ori DNA replication. S- and G2-phase-specific CDKs, but not a G1-specific CDK, can phosphorylate wild-type T Ag, which suggests yet another reason why DNA tumor viruses require actively cycling host cells.

Regulation of prothymosin alpha during the cell cycle

A number of studies have indicated that the small nuclear acidic protein prothymosin alpha is associated with cellular-proliferation events. For example, c-myc causes immediate transcriptional activation of prothymosin alpha, and prothymosin alpha antisense oligonucleotides inhibit myeloma cell division. To investigate the regulation of prothymosin alpha, we examined its mRNA and protein levels during the cell cycle of mononuclear cells and fibroblastic cells. We isolated immunoreactive material from cellular extracts and immunolocalized the protein to the nucleus during the cell cycle. We reported here that the material present in the cells is prothymosin alpha rather than the amino-terminal peptide thymosin alpha 1. [3H]Thymidine-incorporation studies associate maximum accumulation of mRNA and protein with the S/G2 phase of the cell cycle. This induction of prothymosin alpha mRNA seems to resemble cyclin B expression and is more pronounced in fibroblasts. Moreover, transient-transfection experiments indicate that transcription factor E2F is a strong positive regulator of the prothymosin alpha gene. Our results are consistent with the hypothesis that prothymosin alpha is involved in proliferation checkpoints of the cell cycle.

cdc2 phosphorylation of threonine 124 activates the origin-unwinding functions of simian virus 40 T antigen

Phosphorylation of simian virus 40 (SV40) T antigen on threonine 124 activates viral DNA replication in vivo and in vitro. We have manipulated the modification of T-antigen residue 124 both genetically and biochemically and have investigated individual replication functions of T antigen under conditions suitable for in vitro DNA replication. We find that the hexamer assembly, helicase, DNA polymerase alpha-binding, and transcriptional-autoregulation functions are independent of phosphorylation of threonine 124. In contrast, neither T antigen with an alanine mutation of threonine 124 made in human cells nor unphosphorylated T antigen made in Escherichia coli binds the SV40 replication origin as stably as phosphorylated wild-type T antigen does. Furthermore, modification of threonine 124 is essential for complete unwinding of the SV40 replication origin. We conclude that phosphorylation of threonine 124 enhances specific interactions of T antigen with SV40 origin DNA. Our findings do not exclude the possibility that phosphorylation of threonine 124 may affect additional undefined steps in DNA replication. We also show that DNase footprinting and KMnO4 modification assays are not as stringent as immunoprecipitation and origin-dependent strand displacement assays for detecting defects in the origin-binding and -unwinding functions of T antigen. Differences in the assays may explain discrepancies in previous reports on the role of T-antigen phosphorylation in DNA binding.

Cell cycle control of polyomavirus-induced transformation

The cell cycle dependence of polyomavirus transformation was analyzed in infections of nonpermissive Fischer rat (FR3T3) cells released from G0. A 5- to 100-fold (average, ca. 20-fold) difference in relative frequency of transformation was found for cells infected in the early G1 phase of the cell cycle compared with cells infected in G2. Differences in the relative level of early viral gene expression in those two cell populations were equivalent to those obtained for transformation frequencies. The difference in transformation potential was accounted for only in part by a cell cycle control of viral adsorption (2- to 15-fold effect). Furthermore, in cells infected in the early G1 phase, viral gene expression was induced as a big synchronous burst of large transcripts of variable sizes, delayed till the G1 phase of the cell cycle after that in which infection took place. Thus, the results demonstrate that the abortive infection cycle of G0-released FR3T3 cells is cell cycle regulated at least at two steps: adsorption and another early step, nuclear transport, decapsidation, up to or including the transcription of the viral early genes. The cell cycle regulation of these steps results in a similar regulation of the abortive and stable transformation processes, although it is more pronounced for the latter. A model implicating c-fos and c-jun is proposed.

Control of simian virus 40 DNA replication by the HeLa cell nuclear kinase casein kinase I

The initiation of simian virus 40 (SV40) DNA replication is regulated by the phosphorylation state of the viral initiator protein, large T antigen. We describe the purification from HeLa cell nuclei of a 35-kDa serine/threonine protein kinase that phosphorylates T antigen at sites that are phosphorylated in vivo and thereby inhibits its ability to initiate SV40 DNA replication. The inhibition of both origin unwinding and DNA replication by the kinase is reversed by protein phosphatase 2A. As determined by molecular weight, substrate specificity, autophosphorylation, immunoreactivity, and limited sequence analysis, this kinase appears to be identical to casein kinase I, a ubiquitous serine/threonine protein kinase that is closely related to a yeast kinase involved in DNA metabolism. The HeLa cell phosphorylation cycle that controls the initiation of SV40 DNA replication may also play a role in cellular DNA metabolism.

Control of simian virus 40 DNA replication by the HeLa cell nuclear kinase casein kinase I

The initiation of simian virus 40 (SV40) DNA replication is regulated by the phosphorylation state of the viral initiator protein, large T antigen. We describe the purification from HeLa cell nuclei of a 35-kDa serine/threonine protein kinase that phosphorylates T antigen at sites that are phosphorylated in vivo and thereby inhibits its ability to initiate SV40 DNA replication. The inhibition of both origin unwinding and DNA replication by the kinase is reversed by protein phosphatase 2A. As determined by molecular weight, substrate specificity, autophosphorylation, immunoreactivity, and limited sequence analysis, this kinase appears to be identical to casein kinase I, a ubiquitous serine/threonine protein kinase that is closely related to a yeast kinase involved in DNA metabolism. The HeLa cell phosphorylation cycle that controls the initiation of SV40 DNA replication may also play a role in cellular DNA metabolism.

Quantitation of simian virus 40 T-antigen correlated with the cell cycle of permissive and non-permissive cells

These studies examined cell cycle progression and quantitative changes in T-antigen following infection by SV40. Single cells were assayed by multiparameter flow cytometric analysis (FCM) for DNA content and T-antigen expression. Conditions were used which permitted permissive, semi-permissive, and non-permissive cells to be monitored through two rounds of DNA synthesis induced by SV40. The permissive cells included the monkey kidney cell lines; CV-1, Vero and BSC-1 and the COS-1 and COS-7 which are CV-1 cells transformed with an origin defective SV40. The non-permissive cell strains included mouse embryo fibroblasts, Chinese hamster fibroblasts, and IMR-90, a human diploid fibroblast. Cell types differed in the maximal amount of T-antigen expressed per cell. Additionally, all cell types expressed a limited quantity of T-antigen for each cell cycle phase and the quantity increased in each successive phase. The level in each phase was increased only two-fold when 100 times more virus was used. Thus, for an infected population the quantity of T-antigen was dependent on cell cycle distribution. High levels of T-antigen were not required for permissive infection; however, permissive cells were distinguished from non-permissive cells by the G2 levels. Permissive G2 cells had more than double the T-antigen content expressed in G1, while nonpermissive G2 cells had less than a two-fold increase over G1 levels. The appearance of cells with tetraploid DNA content and the failure to undergo mitosis correlated to the higher T-antigen levels in the G2 of the permissive cells. Two other strains of SV40, 776, and VA45 exhibit similar values for T-antigen expression and movement into tetraploid DNA content. This study establishes the levels of T-antigen correlated to the cell cycle and cell type.

Mechanism of activation of simian virus 40 DNA replication by protein phosphatase 2A

The catalytic subunit of protein phosphatase 2A (PP2Ac) stimulates the initiation of replication of simian virus 40 DNA in vitro by dephosphorylating T antigen at specific phosphoserine residues (K. H. Scheidtmann, D. M. Virshup, and T. J. Kelly, J. Virol. 65:2098-2101, 1991). To better define the biochemical mechanism responsible for this stimulation, we investigated the effect of PP2Ac on the interaction of T antigen with wild-type and mutant origins of replication. Analysis of the binding of T antigen to the wild-type origin as a function of protein concentration revealed that binding occurs in two relatively discrete steps: the assembly of a T-antigen hexamer on one half-site of the origin, followed by the assembly of the second hexamer on the other half-site. The major effect of PP2Ac was to stimulate binding of the second hexamer, so that the binding reaction became much more cooperative. This observation suggests that dephosphorylation of T antigen by PP2Ac primarily affects interactions between the two hexamers bound to the origin. Pretreatment with PP2Ac increased the ability of the bound T antigen to unwind the origin of replication but had no effect on the intrinsic helicase activity of the protein. Thus, dephosphorylation of PP2Ac appears to increase the efficiency of the initial opening of the origin by T antigen. An insertion mutation at the dyad axis in the simian virus 40 origin, which altered the structural relationship of the two halves of the origin, abolished the effect of the phosphatase on the cooperativity of binding and completely prevented origin unwinding. These findings suggest that the ability of T antigen to open the viral origin of DNA replication is critically dependent on the appropriate functional interactions between T-antigen hexamers and that these interactions are regulated by the phosphorylation state of the viral initiator protein.

Mechanism of activation of simian virus 40 DNA replication by protein phosphatase 2A

The catalytic subunit of protein phosphatase 2A (PP2Ac) stimulates the initiation of replication of simian virus 40 DNA in vitro by dephosphorylating T antigen at specific phosphoserine residues (K. H. Scheidtmann, D. M. Virshup, and T. J. Kelly, J. Virol. 65:2098-2101, 1991). To better define the biochemical mechanism responsible for this stimulation, we investigated the effect of PP2Ac on the interaction of T antigen with wild-type and mutant origins of replication. Analysis of the binding of T antigen to the wild-type origin as a function of protein concentration revealed that binding occurs in two relatively discrete steps: the assembly of a T-antigen hexamer on one half-site of the origin, followed by the assembly of the second hexamer on the other half-site. The major effect of PP2Ac was to stimulate binding of the second hexamer, so that the binding reaction became much more cooperative. This observation suggests that dephosphorylation of T antigen by PP2Ac primarily affects interactions between the two hexamers bound to the origin. Pretreatment with PP2Ac increased the ability of the bound T antigen to unwind the origin of replication but had no effect on the intrinsic helicase activity of the protein. Thus, dephosphorylation of PP2Ac appears to increase the efficiency of the initial opening of the origin by T antigen. An insertion mutation at the dyad axis in the simian virus 40 origin, which altered the structural relationship of the two halves of the origin, abolished the effect of the phosphatase on the cooperativity of binding and completely prevented origin unwinding. These findings suggest that the ability of T antigen to open the viral origin of DNA replication is critically dependent on the appropriate functional interactions between T-antigen hexamers and that these interactions are regulated by the phosphorylation state of the viral initiator protein.

Simian virus 40 minichromosomes as targets for retroviral integration in vivo

We present a method for studying multiple retroviral integration events into a small DNA target in vivo. Episomal simian virus 40 (SV40) genomes established by infection of CV-1 cells served as integration targets during subsequent infection with murine leukemia virus (MLV). Using a PCR-based assay for the abundance and distribution of integration events, nonrandom integration of MLV DNA into SV40 DNA is detectable as early as 4 hr and reaches a maximum level by 8 hr after MLV infection. The level of integration but not the distribution of integration sites is sensitive to the stage in the SV40 life cycle at which MLV infection is performed. Using a temperature-sensitive tumor (T) antigen mutant SV40 strain, we observed that active replication of the target DNA is not required for efficient integration in vivo. The distribution of integration sites in vivo is closely approximately by in vitro reactions with isolated SV40 minichromosomes as integration targets. However, the degree of bias between the most and least favored sites is greater in vivo than in vitro.

Time-course development of differentiated hepatocarcinoma and lung metastasis in transgenic mice

A precise targeting of the SV40 T early region expression in the liver of transgenic mice was obtained using 700 bp of the antithrombin III regulatory sequences to control oncogene expression. In the strain expressing the highest level of large T antigen (Tag), the incidence of hepatocarcinoma was 100%. The evolution was reproducible and characterized by a marked cytolysis occurring as early as 4 weeks, when no morphological and histological modifications were visible, a preneoplastic state marked by a progression from hyperplasia to proliferative nodules composed of highly differentiated cells exhibiting a high Tag expression, which elicited tumor formation in nude mice and could proliferate in vitro, and hepatocellular carcinoma associated, in 10% of the cases, with lung metastasis. These transgenic mice constituted a useful model for therapeutic assays and fundamental studies on carcinogenesis.

Phosphorylation of large tumour antigen by cdc2 stimulates SV40 DNA replication

Simian virus 40 large tumour antigen (T) is a replication origin binding protein required for viral DNA synthesis. Unphosphorylated T antigen is deficient in promoting DNA replication in vitro but can be activated by phosphorylation at residue threonine 124 by the cdc2 protein kinase. This observation demonstrates that T is regulated by phosphorylation and provides a model for cdc2 function in the control of DNA replication.

Analysis of simian virus 40 infection of CV-1 cells by quantitative two-color fluorescence with flow cytometry

Quantitative two-color fluorescent analysis of Simian virus (SV40) infection of permissive CV-1 cells was investigated. Analysis included by quantitation of cellular DNA, the early viral tumor (T) antigen with a monoclonal antibody, and late viral (V) antigens with a polyclonal antibody. T antigen was detected in all phases of the cell cycle at 6 and 12 h, after SV40 infection of growth arrested cells. At later time intervals, the percentage of T-antigen-positive cells increased with the induction of the cells into successive rounds of DNA synthesis and an increase in tetraploid-polyploid cells. The amount of T antigen per cell increased as the cells entered the successive stages of the cell cycle (G0/G1----G2 + M----tetraploid S and G2 + M). The V antigen from adsorbed virus was detected immediately after infection. Synthesis of V antigen began in late S and G2 + M phases of the cell cycle. This quantitative analysis allows a definitive determination of antigen per cell in a population correlated with the cell cycle and may be useful in correlating viral and cellular events with transformation.

The intranuclear location of simian virus 40 polypeptides VP2 and VP3 depends on a specific amino acid sequence

A cDNA fragment coding for poliovirus capsid polypeptide VP1 was inserted into a simian virus 40 (SV40) genome in the place of the SV40 VP1 gene and fused in phase to the 3' end of the VP2-VP3 genes. Simian cells were infected with the resulting hybrid virus in the presence of an early SV40 mutant used as a helper. Indirect immunofluorescence analysis of the infected cells using anti-poliovirus VP1 immune serum revealed that the SV40/poliovirus fusion protein was located inside the cell nucleus. Deletions of various lengths were generated in the SV40 VP2-VP3 portion of the hybrid gene using BAL31 nuclease. The resulting virus genomes expressed spliced fusion proteins whose intracellular location was either intranuclear or intracytoplasmic, depending on the presence or absence of VP2 amino acid residues 317 to 323 (Pro-Asn-Lys-Lys-Lys-Arg-Lys). This was confirmed by site-directed mutagenesis of the Lys residue at position 320. Modification of Lys-320 into either Thr or Asn abolished the nuclear accumulation of the fusion protein. It is concluded that at least part of the sequence of VP2 amino acids 317 to 323 allows VP2 and VP3 to remain stably located inside the cell nucleus. The proteins are most probably transported from the cell cytoplasm to the cell nucleus by interaction, with VP1 acting as a carrier.

Identification of temperature-sensitive DNA- mutants of Chinese hamster cells affected in cellular and viral DNA synthesis

We described a strategy which facilitates the identification of cell mutants which are restricted in DNA synthesis in a temperature-dependent manner. A collection of over 200 cell mutants temperature-sensitive for growth was isolated in established Chinese hamster cell lines (CHO and V79) by a variety of selective and nonselective techniques. Approximately 10% of these mutants were identified as ts DNA- based on differential inhibition of macromolecular synthesis at the restrictive temperature (39 degrees C) as assessed by incorporation of [3H]thymidine and [35S]methionine. Nine such mutants, selected for further study, demonstrated rapid shutoff of DNA replication at 39 degrees C. Infections with two classes of DNA viruses extensively dependent on host-cell functions for their replication were used to distinguish defects in DNA synthesis itself from those predominantly affecting other aspects of DNA replication. All cell mutants supported human adenovirus type 2 (Ad2) and mouse polyomavirus DNA synthesis at the permissive temperature. Five of the nine mutants (JB3-B, JB3-O, JB7-K, JB8-D, and JB11-J) restricted polyomavirus DNA replication upon transfection with viral sequences at 33 degrees C and subsequent shift to 39 degrees C either before or after the onset of viral DNA synthesis. Only one of these mutants (JB3-B) also restricted Ad2 DNA synthesis after virion infection under comparable conditions. No mutant was both restrictive for Ad2 and permissive for polyomavirus DNA synthesis at 39 degrees C. The differential effect of these cell mutants on viral DNA synthesis is expected to assist subsequent definition of the biochemical defect responsible.

Identification of temperature-sensitive DNA- mutants of Chinese hamster cells affected in cellular and viral DNA synthesis

We described a strategy which facilitates the identification of cell mutants which are restricted in DNA synthesis in a temperature-dependent manner. A collection of over 200 cell mutants temperature-sensitive for growth was isolated in established Chinese hamster cell lines (CHO and V79) by a variety of selective and nonselective techniques. Approximately 10% of these mutants were identified as ts DNA- based on differential inhibition of macromolecular synthesis at the restrictive temperature (39 degrees C) as assessed by incorporation of [3H]thymidine and [35S]methionine. Nine such mutants, selected for further study, demonstrated rapid shutoff of DNA replication at 39 degrees C. Infections with two classes of DNA viruses extensively dependent on host-cell functions for their replication were used to distinguish defects in DNA synthesis itself from those predominantly affecting other aspects of DNA replication. All cell mutants supported human adenovirus type 2 (Ad2) and mouse polyomavirus DNA synthesis at the permissive temperature. Five of the nine mutants (JB3-B, JB3-O, JB7-K, JB8-D, and JB11-J) restricted polyomavirus DNA replication upon transfection with viral sequences at 33 degrees C and subsequent shift to 39 degrees C either before or after the onset of viral DNA synthesis. Only one of these mutants (JB3-B) also restricted Ad2 DNA synthesis after virion infection under comparable conditions. No mutant was both restrictive for Ad2 and permissive for polyomavirus DNA synthesis at 39 degrees C. The differential effect of these cell mutants on viral DNA synthesis is expected to assist subsequent definition of the biochemical defect responsible.

Virus genome integration as a function of the host cell genome replicative cycle

Mouse 3T3 cells were grown and synchronized in monolayer with the double thymidine block. Their infection with SV40 took place continuously during the cellular cycle. However, integration of viral DNA into host cell DNA occurred preferentially during the S phase. Phase G1 appeared to be necessary for virus-cell DNA recombination in S phase. Phase G2 did not alter the stability of the integrated viral genome.

Changes in the levels of viral and cellular gene-transcripts in the cell cycle of SV40 transformed mouse cells

We have analyzed the regulation of transcription of integrated SV40 DNA and of five cellular genes during the cell cycle of two lines of SV40 transformed mouse 3T3 cells. These cells (ts SV3T3) are temperature sensitive for the expression of the transformed phenotype and at the nonpermissive temperature (39 degrees C) become arrested in G1 at low serum concentrations. SV40 specific RNAs are not detected either in the nuclear or in the cytoplasmic poly(A+)RNA of quiescent cells, suggesting control at the level of transcription. After serum stimulation, however, viral transcription increases and reaches its maximum during S-phase. The expression of a group of selected housekeeping genes has received parallel analysis to determine whether other cellular genes, beside the integrated SV40, are shut off in G1 arrested cells or are expressed in restricted periods of the cell cycle. We have found that, while the mRNAs for collagen, adenosinphosphoribosiltransferase (APRT) and the mouse major histocompatibility complex (H2) are present throughout the cell cycle, the genes coding for the multifunctional protein CAD and dehydrofolate reductase are cell-cycle regulated.

Inhibition of SV40 DNA synthesis by vesicular stomatitis virus in doubly infected monkey kidney cells

Vesicular stomatitis virus (VSV) inhibited SV40 DNA synthesis in doubly infected synchronized Vero cells. Gel-electrophoretic profiles demonstrated that SV40 DNA monomers accumulated in all stages of supercoiling, regardless of whether cells were superinfected with VSV early or late in the S phase. These gel profiles were indistinguishable from ones obtained from SV40-infected, cycloheximide-treated cells in the absence of VSV infection. Radiolabel in the partial supercoils could be chased into supercoils, but only by restoring protein synthesis. The relative rates of SV40 DNA chain elongation were determined in VSV-superinfected and nonsuperinfected cells. The gradients of 3H incorporation as a function of distance from the origin of replication in pulse-labeled form I DNA were unaffected by VSV. It is concluded that VSV inhibition of SV40 DNA synthesis is an indirect result of inhibition of host cell protein synthesis and it is suggested that incompletely supercoiled SV40 chromatin is not a good template for DNA synthesis.

Cold-sensitive growth of simian virus 40 in semipermissive variants of CV1 cells

Two cell clones were isolated from the simian line CV1, permissive for simian virus 40 (SV40), by selection at low temperature with the tsA239 mutant of SV40. These clones exhibited cold-sensitive semipermissivity to both SV40 virions and SV40 DNA. On the basis of virus yields, their resistance to viral DNA was increased approximately 15 times over that of CV1 cells when the incubation temperature was lowered from 38.5 to 33.5 degrees C. A further 30- to 40-fold resistance increase was exhibited at both temperatures upon infection with SV40 virions. Partial characterization of these clones indicated that the cold sensitivity affected an early function in viral growth, between viral uncoating and the appearance of T-antigen positivity, with a burst-size decrease in all cells at the restricted temperature. This conditional defect appeared to be superimposed upon a temperature-independent uncoating defect, presumably carried in a CV1 subpopulation from which the two clones were ultimately selected.

Prereplicative events involving simian virus 40 DNA in permissive cells

Simian virus 40 DNA molecules were found to be unable to replicate for 9 h after infection, even in cells that were already replicating the DNA of preinfecting simian virus 40; after 9 h, the ability of the DNA to replicate began to rise sharply. The kinetics of activation indicated that each DNA molecule undergoes a series of slow consecutive reactions, not involving T-antigen, before it can replicate. These pre-replicative molecular transformations probably involve configurational changes; their nature and their relation to the initiation of viral DNA synthesis is discussed. Observation of the replicative behavior of one viral DNA in the presence of another was made possible by the use of two different mutants with distinguishable DNAs: a viable deletion mutant containing DNA insensitive to TAqI restriction enzyme was used to provide viral functions required for replication, and a tsA mutant with TaqI-sensitive DNA was introduced at various times as a probe to determine the ability of the DNA to replicate under different conditions.

SV40-infected muntjac cells: cell cycle kinetics, cell ploidy and T antigen concentration

Muntjac cells in which the SV40 virus neither readily causes transformation nor replicates were used to study the effect of SV40 infection on cell ploidy and the influence of ploidy on the concentration of T antigen, which is required for the initiation of viral DNA synthesis. Both the DNA content, as measured by the flow microfluorometry of propidium iodide-DNA fluorescence, and the average number of chromosomes per cell indicated that infection with SV40 did not alter the ploidy of the host cell. SV40 infection had no effect on the ploidy distribution of muntjac cells. After immunofluorescence staining with anti-T serum and fluorescein-labeled anti-gamma G, infected and uninfected cultures were compared. In uninfected cells incubated with a 1:20 dilution of anti-T serum no fluorescence could be observed by fluorescence microscopy, but when examined by flow microfluorometry, fluorescence was detected after staining with as little as 1000-fold diluted antiserum. Determination of the amount of T antigen and DNA content in the same cell by simultaneous measurement of fluorescein isothiocyanate-conjugate and propidium iodide fluorescence, indicated that the cellular concentration of T antigen did not vary with the ploidy of the genome or the number of nuclei per cell. These results suggest that gene dosage is not a factor which determines the permissive environment for SV40 replication.

Simian virus 40-host cell interaction during lytic infection

Both exponentially growing and serum-arrested subcloned CV-1 cell cultures were infected with simian virus 40 (SV40). By 24 h after infection 96% of the nuclei of these permissive cells contained SV40 T-antigen. Analysis of the average DNA content per cell at various times after infection indicated that by 24 h most of the cells contained amounts of DNA similar to those normally found in G(2) cells. Analysis of cell cycle distributions indicated that a G(2) DNA complement was maintained by over 90% of the cells in the infected populations 24 to 48 h postinfection. Cells continued to synthesize SV40 DNA during the first 50 h after infection, and cytopathic effect was first observed 60 h after inoculation. After infection the number of mitotic cells that could be recovered by selective detachment decreased precipitously and was drastically reduced by 24 h. A study of the kinetics of decline in the number of mitotic cells suggests that this decline is related to an event during the cell cycle at or near the G(1)-S-phase border upon which commencement of SV40 DNA replication apparently depends. It was concluded that after SV40 infection, stationary cells are induced to cycle, and cycling cells complete one round of cellular DNA synthesis but do not divide. Although the infected cells continue to synthesize viral DNA, they do not appear able to reinitiate cellular DNA replication units. These results imply that the abundance of T-antigen (produced independently of cell cycle phase) in the presence of the enzymes required for continued DNA synthesis is not sufficient for reinitiation of cellular DNA synthesis.

Cell cycle-dependent multiplication of avian adenoviruses in chicken embryo fibroblasts

Propagation of CELO virus employing confluent monolayers of chicken enbryo fibroblasts (CEF) yielded virus titers one to two logs lower than those from confluent chicken kidney (CK) cells. An enhancement of virus production in CEF as measured by plaque formation was obtainedby infectng cultures in the growing non confluent state. Measurements of 3H-thymidine incorporation revealed a positive correlation between the DNA synthesis of CEF cultures at the time of inoculation and the amount of progeny virus, whereas in the CK-CELO-system no such relation was observed. Requirement of replicative fibroblasts for CELO multiplication was also demonstrated by comparison of virus replication in synchronized stationary and serum stimulated CEF cells. In stationary CEF cells arrested in the G1 phase of the cell replication cycle by serum deprivation and infected withe CELO virs, no cytopathic effect could be observed, and only very low amounts of virus were produced. But 24 hours after release of these cells for growth by serum stimulation a logarithmic rate of virus multiplication and a complete CPE occurred. Infection of synchronized CEF cultures at different stages of the cell cycle revealed that CELO multiplication was correlated with the S phase of the infected cell. In synchronized CELO infected CEF cultures viral DNA synthesis started 12 to 14 hours after growth stimulation when cells were near the end of the S phase. In contrast, no viral DNA synthesis could be measured in growth arrested CELO infected CEF cells, when cellular DNA synthesis was low. Therefore not only production of infectious virus but also viral DNA synthesis is correlated with events during the S phase of the infected CEF cell.

DNA polymerase activities in growing cells infected with simian virus 40

Growing CV1 cells were infected with simian virus 40 (SV40), and the levels of DNA polymerases-alpha, -beta, and -gamma were analyzed in the cytoplasm, nuclear Triton wash, and nucleus. In the cytoplasmic fraction, the amount of alpha-, beta-, or gamma-polymerase remained unaltered after SV40 infection. The activity of DNA polymerase-alpha increased five- to sixfold in the nuclear Triton wash and threefold in the nuclei and then remained enhanced only inside the nuclei. That of DNA polymerases-beta and gamma increased mostly in the nuclei after infection. These results suggest that DNA polymerase-alpha could be the major enzyme involved in SV40 DNA replication.

In vitro synthesis of simian virus 40 DNA. I. Synthesis by a soluble extract from infected CV1 cells

Simian Virus 40 (SV40) DNA replication was studied in vitro using cell free extracts prepared from SV40 infected CV1 cells. The cells were fractionated into a soluble cytoplasmic fraction and nuclei. The nuclei were lysed with high salt and used to prepare a soluble nuclear fraction. Both fractions displayed DNA polymerase activity as measured with activated calf thymus DNA. However, only the cytoplasmic fraction was active when SV40 DNA comonent I molecules were used as template. Under these conditions, the cytoplasmic extract was shown to catalyse the SV40 DNA dependent, in vitro incorporation of the four deoxyribonucleotides into DNA molecules which had, at both neutral and alkaline pH, the same sedimentation behavior as authentic SV40 DNA component I and component II molecules. Optimal Mg++ concentration was 5-8 mM. Incorporation of label into DNA component I molecules showed an initial lag of about 15 min., after which it was linear with time for up to 5 hrs at 32 degrees. Incorporation into DNA component II molecules proceeded without obvious lag and reached a plateau after approximately 2 hrs of incubation. It is concluded that the cytoplasmic extract supports the in vitro synthesis of SV40 DNA and that DNA component II molecules appear to be a precursor to DNA component I molecules in the reaction. Labeling of viral DNA molecules was highly dependent on ATP and on an ATP generating system. In the absence of ATP and of the energy generating system, incorporation occurred but both template and newly synthesized DNA molecules were extensively degraded.

Thymidine kinase, DNA synthesis and cancer

A resume has been presented of some recent investigations which show that DNA synthesis can be initiated in many types of quiescent animal cells by external stimuli, by introducing a quiescent nucleus into the cytoplasm of a proliferating cell, or by a virus infection. The components of the DNA replication apparatus are described. It is shown that deoxyribonucleoside triphosphate pools increase substantially in animal cells at the time DNA synthesis is initiated due to the enhanced activities of enzymes functioning in nucleotide synthesis. Especially striking is the increase of thymidine kinase activity, indicating that this enzyme may be a useful marker of the shift from the quiescent to the replicative state. The thymidine kinase isozymes of vertebrate cells have been characterized. Thymidine kinase F, which is found principally in the cytosol, is the isozyme that increases when G1 (Go) phase cells are stimulated or infected with oncogenic viruses. Chick cytosol thymidine kinase F can also be reactivated by introducing differentiated chick erythrocyte nuclei into the cytoplasm of enzyme-deficient LM (TK-) mouse cells. Furthermore, herpesviruses code for distinctive, virus-specific thymidine kinase isozymes, so that another way to transform thymidine kinase-deficient LM TK-) cells to kinase-positive cells is by infecting them with UV-irradiated herpes simplex viruses. The experiments on the activation of DNA synthesis and thymidine kinase F activity have been discussed in the context of the proliferative activity in vivo and the immortalization in culture of neoplastic cells. These experiments suggest that genes determining cell cycle proteins are readily accessible to transcription and translation in essentially all nucleated cells. The tendency of transformed cells to become multinucleated after cytochaliasin B treatment also suggests that one important difference between malignant cells and most normal cells may be the ability of malignant cells to 'stockpile' the proteins (and/or their messenger RNAs) of the DNA replicative apparatus and to maintain the 'stockpiles' in progeny cells.

Systematic variables affecting simian virus 40-induced T-antigen expression and transformation in human cells

Simian virus 40 (SV40) infection of human skin fibroblast and human tumor cells resulted in the expression of T-antigen and transformed foci. By examining various conditions of input virus multiplicity and initial cell density, the systematic variation of T-antigen determination was minimized. The most uniform results were obtained at multiplicities of about 275 plaque-forming units/cell. Within limits (5 X 10(4) to 2 X 10(5) cells/dish), initial cell density had little effect on T-antigen expression. Volume of virus inoculum was critical for some cell lines, but not for others. Cell passage level had no general effect on T-antigen expression, although specific cell lines demonstrated increased or decreased levels of T-antigen expression with serial passage for no apparent reason. T-antigen expression correlated with virus-induced cell transformation (focus formation) at two different multiplicities. In addition, T-antigen assays at 3 days gave consistently more reproducible results than transformation assays at 21 days in seven cell lines tested at two multiplicities of infection. These results defined input multiplicity as the major source of systematic variability and will permit development of a more reproducible tool in the evaluation of individuals at high risk of cancer.

Replication of human cytomegalovirus DNA: lack of dependence on cell DNA synthesis

Human cytomegalovirus (CMV) DNA synthesis was studied in 5-fluorouracil (FU)-treated and untreated human embryonic lung cells, which differ greatly with respect to the number of cells in the culture synthesizing cellular DNA. CMV DNA synthesis proceeded at the same rate in FU-treated and in untreated cells. CMV infection also reversed the inhibitory effects of FU and activated cellular DNA synthesis in some of the cells in the FU-treated culture. Autoradiographic studies showed that more than 20% of the cells in the infected FU-treated culture synthesized viral DNA when less than 1% had synthesized cellular DNA, indicating that the synthesis of viral macromolecules proceeds in cells that do not synthesize cellular DNA from the time of infection, and that viral DNA synthesis proceeds independently of the host cell DNA synthesis. Combined autoradiographic and immunofluorescence studies of both the FU-treated and untreated infected cells showed that, whereas 20% of the cells in the cultures synthesize viral DNA and viral antigens, only about 3 to 6% of those cells that synthesize cellular DNA also synthesize viral antigen. Thus, productive infection was delayed or inhibited in those cells that were stimulated by CMV infection to synthesize cellular DNA.

Temperature-sensitive mutants of Balb/3T3 cells: description of a mutant affected in cellular and polyoma virus DNA synthesis

A temperature-sensitive Dna- mutant (ts-2) of the mouse cell Balb/3T3 is characterized. Studies with synchronized cells indicate that the defect is in DNA synthesis itself, rather than in progress toward its initiation. ts-2 supports polyoma DNA synthesis after infection at 33degreesC but not at 38degreesC. Viral DNA synthesis begun at 33degreesC is inhibited upon shift to 38degreesC. A procedure is proposed by which viral DNA synthesis can be used to distinguish different classes of cell Dna- mutants.

Detection of virus-specific DNA sequences by quantitative in situ hybridization during infection with SV40

In situ hybridization has been used as a quantitative method to study the replication of SV40 during lytic infection. Various parameters have been defined such as fixation and denaturation of cytological preparations, concentration of complementary (3H)-cRNA and volume of buffer used for the hybridization reaction. When all these parameters are carefully controlled, a reproducibility of +/-10% can be obtained for the quantitative study of the system SV40-monkey kidney cells (CV-1). In this system, it is evident that the synthesis of viral DNA is not synchronized in confluent cells. Moreover, a proportion of 20 to 30% of cells are not labelled whatever the multiplicity of infection. In SV40-transformed cells, it was not possible to detect significantly the integrated genome due either to the small size of the genome or to the fact that the number of genome-equivalents integrated at the same place is too low to yield a radioactivitity superior to background level for long periods of exposure.

DNA infectivity and the induction of host DNA synthesis with temperature-sensitive mutants of simian virus 40

Host DNA synthesis is induced when CV-1 (monkey kidney) cell cultures are infected at 40 C with wild-type virions or with temperature-sensitive Simian virus 40 mutants of the "early" complementation group A. Host DNA synthesis is not induced when cultures are infected with mutants of the late complementation group D. The simplest explanation for these observations, that induction depends not upon the expression of some early gene function but rather on the presence of an active D protein in the infecting virion, has been examined. Indirect experiments suggest that this explanation is not correct. Moreover, the induction of host DNA synthesis is impaired when cultures are infected with mutants of the A group at 42.5 C rather than 40 C, suggesting that the A function may be responsible for host induction. The inability of D virions to induce host DNA synthesis may reflect their inability to "uncoat" at 40C.

Replication process of the parvovirus H-1. III. Factors affecting H-1 RF DNA synthesis

Replication of the single-stranded DNA parvovirus H-1 involves the synthesis of a double-stranded DNA replicative form (RF). In this study, the metabolism of RF DNA was examined in parasynchronous hamster embryo cells. The initiation of RF DNA replication was found to occur late in S phase, as was the synthesis of the DNA upon which subsequent viral hemagglutinin synthesis is dependent. Evidence is presented which indicates that initiation of RF replication requires proteins synthesized in late S phase, but that concomittant protein synthesis is not required for the continuation of RF replication. The data also suggest a requirement for viral protein(s) for progeny strand synthesis. Incorporation of 5-bromo-2'-deoxyuridine (BUdR) into viral DNA resulted in an "all-or-none" inhibition of viral hemagglutinin and viral antigen synthesis. BUdR inactivation of viral protein function was used to explore the time of synthesis of viral DNA serving as template for viral RNA synthesis and the effect of viral protein on RF replication and progeny strand synthesis. Results of this study suggest that parental RF DNA is synthesized shortly after infection, and that viral mRNA is transcribed from only a few copies of the viral genome in each cell. They also support the conclusion that viral protein is inhibitory to RF DNA replication. Density labeling of RF DNA with BUdR, allowing separation of viral strand DNA (V) from viral complementary strand (C), provided additional data in support of the above findings.

Inhibition of herpes simplex virus type 2 replication by thymidine

Replication of herpes simplex virus type 2 (HSV-2) was impeded in KB cells which were blocked in their capacity to synthesize DNA by 2 mM thymidine (TdR). The degree of inhibition was dependent upon the concentration of TdR. In marked contrast, HSV-1 is able to replicate under these conditions. The failure of HSV-2 to replicate is probably due to the inhibition of viral DNA synthesis; there was a marked reduction in the rate of DNA synthesis as well as the total amount of HSV-2 DNA made in the presence of 2 mM TdR. We postulated that the effect of TdR on viral replication occurs at the level of ribonucleotide reductase in a manner similar to KB cells. However, unlike KB cells, an altered ribonucleotide reductase activity, highly resistant to thymidine triphosphate inhibition, was found in extracts of HSV-2-infected KB cells. This activity was present in HSV-2-infected cells incubated in the presence or absence of TdR. Ribonucleotide reductase activity in extracts of HSV-1-infected KB cells showed a similar resistance to thymidine triphosphate inhibition. These results suggest that the effect of TdR on HSV-2 replication occurs at a stage of DNA synthesis other than reduction of cytidine nucleotides to deoxycytidine nucleotides.

Viral DNA-RNA hybrids in cells infected with simian virus: the simian virus 40 transcriptional intermediates

Brief labeling of cells infected with simian virus 40 with tritiated uridine resulted in the incorporation of part of the label into material that was soluble in 1 M NaCl and sensitive to KOH, with a buoyant density close to that of DNA. The properties of this material suggest that it represents single-stranded nascent molecules of messenger RNA of simian virus 40 hydrogen-bonded over a small portion of their length to viral DNA templates. The name "simian virus 40 transcriptional intermediate molecules" is suggested for these naturally occurring DNA.RNA hybrid molecules. The DNA in the hybrid seems to be in the form of replicative intermediate molecules.