DNA polymerase alpha from the nuclear matrix of cells infected with simian virus 40.

The nuclear matrix prepared from normal, simian virus 40 (SV40)-infected, and SV40-transformed cells contained DNA polymerase activities. Approximately 12% of the total DNA polymerase activities in isolated nuclei remained with the nuclear matrix. alpha-polymerase was the major matrix DNA polymerase activity as judged by sensitivity to various inhibitors: aphidicolin, dideoxy-TTP, and N-ethylmaleimide. Approximately 2-4 fold higher DNA polymerase activity was detected in matrices obtained from lytically infected and virus-transformed cells than that found in normal cells. In lytically infected cells, 30-50% of the matrix-bound DNA polymerase activity solubilized by sonication co-sedimented with majority of the matrix T-antigen, and was co-precipitated with anti-T sera. The results suggest that alpha-polymerase and viral T-antigen may form a functional complex in the matrix.     

Inhibition of simian virus 40 DNA replication by specific modification of T-antigen with oxidized ATP

We covalently bound periodate-oxidized ATP (oATP) to purified simian virus 40 (SV40) large T-antigen and determined the effect of this modification on viral DNA replication and three other biochemical activities of T-antigen. The oATP bound specifically to T-antigen, inhibiting the ATPase activity and preventing T-antigen from activating SV40 DNA replication in vitro. In contrast, binding of oATP had no effect on the DNA-binding activity of T-antigen nor on its ability to form a complex with DNA polymerase alpha. These results provide direct biochemical evidence suggesting that the T-antigen ATPase activity is necessary for viral DNA replication.     

Induction of the replicative synthesis of DNA by SV40 virus T antigen introduced into cells using liposomes

The role of virus SV40 T-antigen in the induction of cell DNA synthesis during its incorporation into cell liposomes was studied, using monolamellar liposomes obtained by phase reversal with incorporated highly purified T-antigen. Immunofluorescence studies revealed that T-antigen effectively penetrates inside the cells and after 10 hours is accumulated in the nuclei, where its level remains unchanged for 24 hours. Injections of purified T-antigen into the renal cells of serum-starved CV1 monkeys resulted in an almost 10-fold increase in the number of DNA-synthesizing cells 18 hours after the exposure. The same effect was observed during stimulation of a 10% serum culture. Removal of T-antigen from the preparation by specific immunoadsorption eliminated this effect. Centrifugation of cells grown in the presence of bromodeoxyuridine in a CsCl gradient was used to demonstrate the replicative type of cell DNA synthesis during T-antigen induction.     

The release of growth arrest by microinjection of adenovirus E1A DNA.

The induction of DNA synthesis in growth-arrested mouse fibroblasts (NIH 3T3) was studied by microinjection of different constructs of adenovirus DNA using SV40 DNA and plasmid DNA as positive and negative controls. The E1A region of adenovirus types 2 and 12 appears to be sufficient to induce cellular DNA synthesis after growth arrest in approximately 30% of the cells and both 13S and 12S cDNA constructs mediate this effect. The presence of the E1A protein products as assayed by immunofluorescence does not strictly correlate with the induction of DNA synthesis in microinjected cells in contrast to the SV40 large T-antigen. Microinjection of truncated fragments of the Ad12 E1A region suggests, however, that the protein products of 12S and 13S may be involved in the induction process. A sequence comparison of the SV40 T-antigen and the adenovirus E1A products identified a region of significant homology providing a basis for a hypothesis concerning the evolution of T-antigen genes in DNA viruses.     

DNA binding properties of simian virus 40 T-antigens synthesized in vivo and in vitro.

Simian virus 40 large T- and small t-antigens have been shown previously to share immunological determinants and common sequences and to have roles in virus-induced cell transformation. However, only large T-antigen is a DNA binding protein. Under all conditions tested, small t-antigen did not interact with DNA. Large T-antigen synthesized in infected cells bound to both native calf thymus and simian virus 40 DNAs. As its binding efficiency was less than 100%, it is likely that there are different forms of T-antigen which vary in their affinity for DNA. Large T-antigen synthesized in cell-free protein-synthesizing systems primed by simian virus 40 mRNA also bound to DNA-cellulose, whereas small t-antigen similarly synthesized in vitro did not. An 82,000-molecular-weight T-antigen polypeptide synthesized in cell-free protein-synthesizing systems primed by simian virus 40 complementary RNA transcribed in vitro from simian virus 40 DNA by Escherichia coli RNA polymerase bound efficiently to simian virus 40 DNA. As this product did not share sequences with the small t-antigen, it can be concluded that the amino-terminal portion of the T-antigen is not required for some of its specific DNA binding properties.     

A possible mechanism by which SV40 T-antigen stimulates rRNA synthesis

Binding of SV40 T-antigen to RNA polymerase I could account for the observation that T-antigen stimulates rRNA synthesis and that nucleoli do not stain for T-antigen. Two tests were performed to detect binding: a) RNA polymerase I was isolated and assayed in the presence of T-antigen; polymerase activity was enhanced. b) RNA polymerase I and T-antigen were mixed and then T-antigen complement fixation assays performed, complement fixation was not inhibited.     

Recent progress in studies of polyomavirus tumour antigens

The polyomavirus tumour (T) antigens were originally identified by their reactivity with antisera from tumour-bearing animals. The primary structure of the three T-antigens has been established by combining the information from the nucleotide sequencing of DNA, RNA analysis, and peptide mapping. The functions of the T-antigens in productive infection and cellular transformation have largely been analysed by using virus mutants. The large T-antigen binds specifically to polyomavirus DNA. This binding is probably linked to the activity of the protein in the control of viral DNA and RNA synthesis. In addition, the large T-antigen has the ability to confer an unlimited growth potential to cells in culture. The middle T-antigen is a primary inducer of cellular transformation. The part of this protein that is located in the plasma membrane, is associated with a tyrosine kinase activity. The small T-antigen, finally, has not yet been studied extensively. However, small T-antigen has to be expressed to allow a complete productive infection cycle in mouse cells.     

Induction of DNA polymerase beta during proliferation of mitogen-stimulated human lymphocytes.

On induction of proliferation of human peripheral blood mononuclear cells by phytohemagglutinin treatment, DNA polymerase beta activity increases markedly before and during DNA replication. The increase of enzymatic activity seems to be well correlated with the increase of DNA polymerase beta mRNA, which is induced by enhanced expression of the DNA polymerase beta gene. These data suggest that DNA polymerase beta is involved in DNA repair, which is linked to replicative DNA synthesis, or directly in replicative DNA synthesis in normal proliferating cells.     

Simian virus 40 large T-antigen function is required for induction of tetraploid DNA content during lytic infection.

Infection of quiescent CV-1 cells with simian virus 40 mutant tsA30 at 37 degrees C resulted in the induction of two rounds of cellular DNA synthesis in T-antigen-positive cells, as previously described for wild-type simian virus 40. Following infection with tsA30 at 40.5 degrees C, T-antigen-positive cells were induced into S phase and reached a diploid G2 DNA content; however, a second S phase was not initiated. The failure of tsA30-infected CV-1 cells to enter tetraploid S phase at 40.5 degrees C identifies a T-antigen function, distinct from T-antigen functions responsible for stimulation of cell DNA synthesis, which is required for initiation of a second round of DNA synthesis without mitosis.     

Regulation of gene amplification and expression in cells that constitutively express a temperature sensitive SV40 T-antigen.

Simian cells have been transformed with SV40 origin-defective recombinant plasmids containing the tsA209 T-antigen gene. These plasmids contain deletions of either 5 or 52 nucleotides that include the BglI site at the SV40 ori, are defective for replication in COS-1 cells but retain a functional SV40 early promoter. Two cell lines transformed with these plasmids, U4 and S7, and their respective clonal derivatives E5 and F11, contain the tsA209 T-antigen gene integrated into the cell DNA and express T-antigen as detected by immunoprecipitation and immunofluorescence. These cells behave as ts-COS cells, since they complement in a temperature dependent manner the replication of an SV40 derived recombinant plasmid. When transfected with recombinant plasmids containing the chloramphenicol acetyl transferase (CAT) gene cloned into SV40 replicons, ts-COS cells were able to regulate the induction of the CAT activity by temperature. The ratios of CAT activity observed at permissive versus restrictive temperature were in the range of 20-400. Thus, these ts-COS cells are useful systems for the regulated expression of cloned genes in simian cells.     

Mutational analysis of simian virus 40 T-antigen primosome activities in viral DNA replication

The recruitment of DNA polymerase alpha-primase (pol-prim) is a crucial step in the establishment of a functional replication complex in eukaryotic cells, but the mechanism of pol-prim loading and the composition of the eukaryotic primosome are poorly understood. In the model system for simian virus 40 (SV40) DNA replication in vitro, synthesis of RNA primers at the origin of replication requires only the viral tumor (T) antigen, replication protein A (RPA), pol-prim, and topoisomerase I. On RPA-coated single-stranded DNA (ssDNA), T antigen alone mediates priming by pol-prim, constituting a relatively simple primosome. T-antigen activities proposed to participate in its primosome function include DNA helicase and protein-protein interactions with RPA and pol-prim. To test the role of these activities of T antigen in mediating priming by pol-prim, three replication-defective T antigens with mutations in the ATPase or helicase domain have been characterized. All three mutant proteins interacted physically and functionally with RPA and pol-prim and bound ssDNA, and two of them displayed some helicase activity. However, only one of these, 5030, mediated primer synthesis and elongation by pol-prim on RPA-coated ssDNA. The results suggest that a novel activity, present in 5030 T antigen and absent in the other two mutants, is required for T-antigen primosome function.     

New Deoxyribonucleic Acid Polymerase Induced by Bacillus subtilis Bacteriophage PBS2

The deoxyribonucleic acid (DNA) of Bacillus subtilis phage PBS2 has been confirmed to contain uracil instead of thymine. PBS2 phage infection of wild-type cells or DNA polymerase-deficient cells results in an increase in the specific activity of DNA polymerase. This induction of DNA polymerase activity is prevented by actinomycin D and chloramphenicol. In contrast to the major B. subtilis DNA polymerase, which prefers deoxythymidine triphosphate (dTTP) to deoxyuridine triphosphate (dUTP), the DNA polymerase in crude extracts of PBS2-infected cells is equally active whether dTTP or dUTP is employed. This phage-induced polymerase may be responsible for the synthesis of uracil-containing DNA during PBS2 phage infection.     

T-antigen interactions with chromatin and p53 during the cell cycle in extracts from xenopus eggs.

The role of SV40 large tumor T-antigen in replication of viral DNA is well established, but it is still unclear how T-antigen triggers cellular replication and cell transformation in non-permissive cells. Here, we used Xenopus egg extracts which reproduce most nuclear events linked to the cell cycle in vitro to analyze its interaction with genomic chromatin during the cell cycle. We show that T-antigen associates with chromatin before the nuclear membrane formation, and further demonstrate that the nuclear membrane is not necessary for its import into the nucleus. We show that the interaction of T-antigen with the endogenous chromatin does not occur at replication foci nor at RPA pre-replication centers. Immunoprecipitations as well as sucrose gradient experiments, indicate that the endogenous pool of p53 interacts with T-antigen. In addition, a transient association of both proteins with the nuclear matrix is observed during the ongoing DNA synthesis. These data are discussed in view of the T-antigen and p53 activity during the cell cycle.     

Polyomavirus-plasmid recombinants capable of replicating have an enhanced transforming potential.

The frequency of transformation of rodent fibroblasts by polyomavirus is enhanced by a viral gene product, large T-antigen. However, this effect of large T-antigen cannot be demonstrated with pBR322-cloned viral DNA. Recently, it was discovered that pBR322 contains cis-acting sequences inhibitory to DNA replication in mammalian cells. Because polyomavirus large T-antigen is required for viral DNA replication, we examined the possibility that our inability to demonstrate a requirement for large T-antigen in transformation with pBR322-cloned viral DNA was due to the failure of the chimeric DNA to replicate in the transfected cells. To this end we constructed polyomavirus recombinant molecules with a plasmid (pML-2) that lacks these "poison" sequences and measured their capacity to transform cells. Here we report that recombinant plasmids capable of replicating in the transfected cells transform these cells at frequencies approximately sixfold greater than their replication-defective counterparts.     

Simian virus 40 cRNA is processed into functional mRNA in microinjected monkey cells.

Monkey cells, microinjected with simian virus 40 (SV40) in vitro synthesized cRNA produce full-size tumor (T)-antigen. This was verified by analyzing immunoprecipitates of microinjected cells by polyacrylamide gel electrophoresis. Early SV40 DNA contains an intron within the large T-antigen coding sequences. Therefore, cRNA copied in vitro from the early DNA strand requires removal of the intron in order to become a functional mRNA. Polyadenylation of the cRNA in vitro by Escherichia coli poly(A)-polymerase increased the biological activity of the RNA. Detection of T-antigen by gel electrophoresis required as little as 50 poly(A)-cRNA injected cells. Splicing of the microinjected cRNA appears to be a nuclear process. Cells enucleated by cytochalasin B prior to injection do not synthesize large T-antigen. However, small t-antigen, a protein with a continuous sequence, is synthesized in these cells. Finally, it is shown that the process of splicing is not required for the transport of mRNA from the nucleus into the cytoplasm. Authentic T-antigen mRNA, isolated from virus infected cells, induced T-antigen synthesis with similar efficiency after either nuclear or cytoplasmic injection.     

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen.

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.     

DNA polymerases in polyoma virus-infected mouse kidney cells.

Infection of arrested mouse kidney cells by polyoma virus results in the induction of the cellular 6-8S DNA polymerase activity. Levels of this enzyme increase two- to threefold in the cytoplasm but seven- to tenfold in nuclei and nuclear extract, suggesting an accumulation of the enzyme in the nucleus. Experiments using the inhibitor of DNA synthesis, fluordeoxyuridine, indicate that this accumulation is linked to active DNA synthesis. The activity and cellular distribution of the small 3.4S DNA polymerase remains unchanged.     

Cellular and Epstein-Barr virus specific DNA polymerases in virus-producing Burkitt's lymphoma cell lines.

We have determined the levels of cellular DNA polymerases and Epstein-Barr virus specific DNA polymerase in three Burkitt's lymphoma cell lines producing varying amounts of EBV, one of which was induced by 12-0-tetra-decanoylphorbol-13-acetate (TPA). There was a proportional increase in the level of EBV-DNA polymerase with an increase in the percent of virus-producing cells. However, there was a reciprocal relationship between the levels of EBV-DNA polymerase and DNA polymerase alpha i.e., in cell line containing the highest level of EBV-DNA polymerase, activity of DNA polymerase alpha, but not of DNA polymerase beta, was reduced to an insignificantly low level. TPA does not have any direct effect on activities of either EBV-DNA polymerase or DNA polymerase alpha. EBV-DNA polymerases isolated from cells grown with or without TPA are indistinguishable in their properties such as elution position on phosphocellulose column, molecular weight, mono and divalent cation requirements, pH optimum, and other requirements for optimum activity. Addition of crude extracts of cells grown in presence of TPA to the purified DNA polymerase alpha did not inhibit its activity indicating that the observed loss was not due to any specific inhibitor present in TPA treated cells. Raji, a nonproducer cell line, did not contain EBV-DNA polymerase. There was no induction of EBV-DNA polymerase when Raji cells were grown in presence of TPA. The phenomenon of reduction in the levels of DNA polymerase alpha in cells induced to produce EBV may represent a mechanism by which the host DNA replication is shut off following virus infection.