Effects of VM26 (teniposide), a specific inhibitor of type II DNA topoisomerase, on SV40 DNA replication in vivo.

Simian Virus 40 (SV40) infected cells were pulse labeled with (3H) thymidine and chased either in the absence or in the presence of the cytotoxic drug VM26 (teniposide). We investigated the structure of labeled SV40 DNA and found that VM26 had no significant effect on replicative chain elongation but strongly inhibited the conversion of late replication intermediates to mature DNA daughter molecules. The late replicative SV40 DNA intermediates which accumulate in VM26 treated cells contained essentially full length labeled DNA strands. These newly synthesized strands were not part of two catenated interlocked SV40 monomers suggesting that the block occurred prior to the final ligation reaction. Since VM26 is known to be a specific inhibitor of DNA topoisomerase II we conclude that this enzyme is dispensable for the chain elongation of replicating SV40 DNA, but that it is essential for the termination of SV40 DNA replication cycles.     

Topoisomerase I is preferentially associated with normal SV40 replicative intermediates, but is associated with both replicating and nonreplicating SV40 DNAs which are deficient in histones.

Based on the use of equilibrium centrifugation in CsCl to separate covalent complexes between topoisomerase I and DNA from protein-free DNA, it was concluded previously that the topoisomerase is preferentially associated with replicating SV40 DNA (Champoux, J. J. 1988. J. Virol. 62:3675-3683). One explanation for the failure to find the enzyme associated with nonreplicating viral DNA is that most of the completed DNA is rapidly sequestered for encapsidation and inaccessible to topoisomerase I. This explanation has been ruled out in the present work by the finding that topoisomerase I in COS-1 cells is also preferentially associated with the replicative form of an SV40 origin-containing plasmid that lacks the genes coding for the virion structural proteins and therefore cannot be encapsidated. Thus it appears that some structural feature of the replicating DNA or the replication complex specifically recruits the topoisomerase to the DNA. SV40 DNA which is produced in the presence of the protein synthesis inhibitor, puromycin, is deficient in histones and as a result lacks normal chromatin structure. Topoisomerase I was found to be associated with SV40 DNA under these conditions whether or not it was replicating. This observation is interpreted as an indication that under normal conditions, chromatin structure limits access of topoisomerase I to the nonreplicating viral DNA.     

Camptothecin, a specific inhibitor of type I DNA topoisomerase, induces DNA breakage at replication forks.

The structure of replicating simian virus 40 minichromosomes, extracted from camptothecin-treated infected cells, was investigated by biochemical and electron microscopic methods. We found that camptothecin frequently induced breaks at replication forks close to the replicative growth points. Replication branches were disrupted at about equal frequencies at the leading and the lagging strand sides of the fork. Since camptothecin is known to be a specific inhibitor of type I DNA topoisomerase, we suggest that this enzyme is acting very near the replication forks. This conclusion was supported by experiments with aphidicolin, a drug that blocks replicative fork movement, but did not prevent the camptothecin-induced breakage of replication forks. The drug teniposide, an inhibitor of type II DNA topoisomerase, had only minor effects on the structure of these replicative intermediates.     

Inhibition of DNA synthesis by aphidicolin induces supercoiling in simian virus 40 replicative intermediates.

Highly torsionally stressed replicative intermediate SV40 DNA molecules are produced when ongoing replicative DNA synthesis is inhibited by aphidicolin, a specific inhibitor of DNA polymerase alpha. The high negative superhelical density of these molecules can be partially released by intercalating drugs such as chloroquine or ethidium bromide. The torsionally stressed replicative intermediates bind to monoclonal anti-Z-DNA antibodies. Electron microscopy of anti-Z-DNA cross-linked to torsionally stressed replicative intermediates shows that the antibody specifically binds close to the replication forks. The superhelical structures are not formed when SV40 DNA replication is inhibited by both aphidicolin and novobiocin, suggesting that a topoisomerase type II-like enzyme is somehow involved in the introduction of torsional strain in replicative intermediate DNA. One interpretation of our data is that fork movement continues to some rather limited extent when SV40 DNA synthesis in replicative chromatin is blocked by aphidicolin. After deproteinization, the exposed single-stranded DNA branches reassociate to form paranemic DNA structures with left-handed helical stretches, while the reduced linking number of the parental strands induces a high negative superhelical density.     

Synthesis of Superhelical Simian Virus 40 Deoxyribonucleic Acid in Cell Lysates*.

In vivo-labeled SV40 replicating DNA molecules can be converted into covalently closed superhelical SV40 DNA (SV40(I) using a lysate of sv40-infected monkey cells containing intact nuclei. Replication in vitro occurred at one-third the in vivo rate for 30 min at 30 degrees. After 1 hour of incubation, about 54% of the replicating molecules had been converted to SV40(I), 5% to nicked, circular molecules (SV40(II), 5% to covalently closed dimers; the remainder failed to complete replication although 75% of the prelabeled daughter strands had been elongated to one-genome length. Density labeling in vitro showed that all replicating molecules had participated during DNA synthesis in vitro. Velocity and equilibrium sedimentation analysis of pulse-chased and labeled DNA using radioactive and density labels suggested that SV40 DNA synthesis in vitro was a continuation of normal ongoing DNA synthesis. Initiation of new rounds of SV40 DNA replication was not detectable.     

Characterization of Simian virus 40 DNA component II during viral DNA replication

Replicating molecules of Simian virus 40 DNA labeled during a short pulse with [³H]thymidine have been fractionated by ultracentrifugation methods and the open circular form (DNA component II) has been characterized. The pulse-labeled DNA component II is a relatively small constituent (1 to 3%) of the pool of replicating molecules. Examination of the circular (18 S) and linear (16 S) strands of DNA component II by alkaline sedimentation and by degradation using exonuclease III of Escherichia coli reveals that the newly synthesized DNA is principally in the linear strand. Cleavage of pulse-labeled DNA component II by an fi⁺, R-factor restriction endonuclease from E. coli demonstrates that the interruption in the pulse-labeled strand is specifically located at the termination point for replication.During a chase period of 20 minutes the amount of DNA component II increases to about 6 to 8% of the total labeled viral DNA. The kinetics of formation of superhelical, DNA component I and disappearance of replicative intermediates are linear during the chase period. After several hours of continuous labeling of replicating viral DNA, the DNA component II pool consists mainly of molecules labeled in both strands with the interruption non-specifically located in either strand. These molecules probably arise by the random introduction of single-strand breaks in newly synthesized DNA component I. During short periods of continuous labeling with [³H]thymidine, the ratio of DNA components I to II increases as a function of the pulse duration. These results support a model for 8V 40 DNA replication in which the open circular form is a precursor of the superhelical form.     

Molecular analysis of enhanced replication of UV-damaged simian virus 40 DNA in UV-treated mammalian cells.

Irradiation of simian virus 40 (SV40)-infected cells with low fluences of UV light (20 to 60 J/m2, inducing one to three pyrimidine dimers per SV40 genome) causes a dramatic inhibition of viral DNA replication. However, treatment of cells with UV radiation (20 J/m2) before infection with SV40 virus enhances the replication of UV-damaged viral DNA. To investigate the mechanism of this enhancement of replication, we analyzed the kinetics of synthesis and interconversion of viral replicative intermediates synthesized after UV irradiation of SV40-infected cells that had been pretreated with UV radiation. This enhancement did not appear to be due to an expansion of the size of the pool of replicative intermediates after irradiation of pretreated infected cells; the kinetics of incorporation of labeled thymidine into replicative intermediates were very similar after irradiation of infected control and pretreated cells. The major products of replication of SV40 DNA after UV irradiation at the low UV fluences used here were form II molecules with single-stranded gaps (relaxed circular intermediates). There did not appear to be a change in the proportion of these molecules synthesized when cells were pretreated with UV radiation. Thus, it is unlikely that a substantial amount of DNA synthesis occurs past pyrimidine dimers without leaving gaps. This conclusion is supported by the observation that the proportion of newly synthesized SV40 form I molecules that contain pyrimidine dimers was not increased in pretreated cells. Pulse-chase experiments suggested that there is a more efficient conversion of replicative intermediates into form I molecules in pretreated cells. This could be due to more efficient gap filling in relaxed circular intermediate molecules or to the release of blocked replication forks. Alternatively, the enhanced replication observed here may be due to an increase in the excision repair capacity of the pretreated cells.     

Intermediate in SV40 DNA Chain Growth

PREVIOUS studies of the DNA replication of simian virus 40 (SV40), an oncogenic member of the papoyavirus group, have been concerned with separation and characterization of replicative intermediates^1–4. Circular replicating intermediates have been identified for SV40^1–3, as well as for the similar replication system of polyoma viral DNA5,6. The replicative intermediates of SV40 DNA have been observed by electron microscopy to contain two forks, three branches and no free ends^1–3 as is the case for the circular replicating molecules of polyoma, bacteriophage λ⁷, Escherichia coli⁸ and colicin E1 in mini-cells^9,10. An important property of replicative intermediates of SV40 DNA that has also been observed in replicating molecules of colicin E1¹⁰ is that most molecules contain a superhelical region in the unreplicated portion of the molecule¹.     

Replicative intermediates in UV-irradiated simian virus 40

We have used Simian virus 40 (SV40) as a probe to study the replication of UV-damaged DNA in mammalian cells. Viral DNA replication in infected monkey kidney cells was synchronized by incubating a mutant of SV40 (tsA58) temperature-sensitive for the initiation of DNA synthesis at the restrictive temperature and then adding aphidicolin to temporarily inhibit DNA synthesis at the permissive temperature while permitting pre-replicative events to occur. After removal of the drug, the infected cells were irradiated at 100 J/m2 (254 nm) to produce 6-7 pyrimidine dimers per SV40 genome, and returned to the restrictive temperature to prevent reinitiation of replication from the SV40 origin. Replicative intermediates (RI) were labeled with [3H]thymidine, and isolated by centrifugation in CsCl/ethidium bromide gradients followed by BND-cellulose chromatography. The size distribution of daughter DNA strands in RI isolated shortly after irradiation was skewed towards lengths less than the interdimer spacing in parental DNA; this bias persisted for at least 1 h after irradiation, but disappeared within 3 h, by which time the size of the newly-synthesized DNA exceeded the interdimer distance. No significant excision of dimers from parental strands in either replicative intermediates or Form I (closed circular) DNA molecules was detected. These data are consistent with the hypothesis that replication forks are temporarily blocked by dimers encountered on the leading strand side of the fork, but that daughter strand continuity opposite dimers is eventually established. Evidence was obtained for the generation at late times after irradiation, of Form I molecules in which the daughter DNA strands contain dimers. Thus DNA strand exchange as well as trans-dimer synthesis may be involved in the generation of supercoiled Form I DNA from UV-damaged SV40 replicative intermediates.     

Filamentous Bacterial Viruses V. Asymmetric Replication of fd Duplex Deoxyribonucleic Acid

Short pulses (30 sec at 32 C) of (3)H-thymidine were found primarily in the viral strands of replicating fd deoxyribonucleic acid (DNA), even at a time when most DNA being synthesized was duplex DNA. Much of the labeled viral strand DNA was longer than unit length, but some was shorter than unit length. Most of the corresponding complementary-strand DNA was recovered in closed supercoiled duplex molecules, even for short pulses; the remainder of the complementary-strand DNA was found in replicative intermediates in pieces shorter than unit length. Some of the viral strands in open replicating DNA lacked a corresponding complementary strand.     

Immunoprecipitation of SV40 replicating minichromosomes complexed with bacteriophage T4 gene 32 protein.

Simian Virus 40 (SV40) DNA replication is a useful model to study eukaryotic cell DNA replication because it encodes only one replication protein and its genome has a nucleoprotein structure ('minichromosome') indistinguishable from cellular chromatin. Late after infection SV40 replicating DNA molecules represent about 5% of total viral minichromosomes. Since gene 32 protein (P32) from bacteriophage T4 interacts with single-stranded DNA and SV40 replication complexes are expected to contain single-stranded regions at the replication forks, we asked whether P32 might be used to isolate replicating SV40 minichromosomes. When nuclear extracts from SV40 infected cells were treated sequentially with P32 and anti-P32 antibodies, pulse-labeled minichromosomes were selectively immunoprecipitated. Agarose gel electrophoresis analysis confirmed that immunoprecipitated material corresponded to SV40 replicative intermediates. Protein analysis of the pelleted material revealed several proteins of viral and cellular origin. Among them, T antigen and histones were found to be complexed with at least other three proteins from cellular origin, to the replicative complexes. Additionally, anti-P32 antibodies were able to detect three cellular proteins of approximately 70, 32 and 13 kDa in western blots. These proteins could correspond to those found as part of an eukaryotic multisubunit single-stranded DNA binding protein. The use of P32 and anti-P32 antibodies thus allows the separation of replicating from mature SV40 minichromosomes and can constitute a novel method to enrich and to study replicative active chromatin.     

Phosphorylation of mammalian DNA topoisomerase I and activation by protein kinase C

The influence of mammalian DNA topoisomerase I phosphorylation on enzyme activity has been investigated. Dephosphorylation by calf intestine alkaline phosphatase abolished the DNA relaxing activity of DNA topoisomerase I and the sensitivity of the enzyme to its specific inhibitor, camptothecin. DNA topoisomerase I could be reactivated by incubation with purified protein kinase C. DNA topoisomerase I was then able to relax supercoiled DNA processively, like the native enzyme, and to cleave 32P-end-labeled SV40 DNA fragments at the same sequences as the native enzyme in the presence of camptothecin. These results show that active DNA topoisomerase I is a phosphoprotein and suggest a possible regulatory role of protein kinase on topoisomerase I activity and on its sensitivity to camptothecin.     

Structure of Replicating Simian Virus 40 Deoxyribonucleic Acid Molecules

Properties of replicating simian virus 40 (SV40) deoxyribonucleic acid (DNA) have been examined by sedimentation analysis and by direct observation during a lytic cycle of infection of African green monkey kidney cells. Two types of replicating DNA molecules were observed in the electron microscope. One was an open structure containing two branch points, three branches, and no free ends whose length measurements were consistent with those expected for replicating SV40 DNA molecules. A second species had the same features as the open structure, but in addition it contained a superhelix in the unreplicated portion of the molecule. Eighty to ninety per cent of the replicative intermediates (RI) were in this latter configuration, and length measurements of these molecules also were consistent with replicating SV40 DNA. Replicating DNA molecules with this configuration have not been described previously. RI, when examined in ethidium bromide-cesium chloride (EB-CsCl) isopycnic gradients, banded in a heterogeneous manner. A fraction of the RI banded at the same density as circular SV40 DNA containing one or more single-strand nicks (component II). The remaining radioactive RI banded at densities higher than that of component II, and material was present at all densities between that of supercoiled double-stranded DNA (component I) and component II. When RI that banded at different densities in EB-CsCl were examined in alkaline gradients, cosedimentation of parental DNA and newly replicated DNA did not occur. All newly replicated DNA sedimented more slowly than did intact single-stranded SV40 DNA, a finding that is inconsistent with the rolling circle model of DNA replication. An inverse correlation exists between the extent of replication of the SV40 DNA and the banding density in EB-CsCl. Under alkaline conditions, the parental DNA strands that were contained in the RI sedimented as covalently closed structures. The sedimentation rates in alkali of the covalently closed parental DNA decreased as replication progressed. Based on these observations, some possible models for replication of SV40 DNA are proposed.     

Effects of VM26, a specific inhibitor of type II DNA topoisomerase, on SV40 chromatin replication in vitro.

We have examined the influence of VM26 (teniposide), a specific inhibitor of mammalian type II DNA topoisomerase, on the replication of SV40 minichromosomes in vitro. The replication system we used consists of replicative intermediate SV40 chromatin as substrate which is converted to mature SV40 chromatin in the presence of ATP, deoxynucleotides and a protein extract from uninfected cells. The addition of 100 microM VM26 to this system reduces DNA synthesis to 70 to 80 percent of the control and leads to an accumulation of 'late replicative intermediates'. The VM26 induced block of replication was not released by the addition of large quantities of type I DNA topoisomerase. We conclude, that type II DNA topoisomerase is essential for the final replication steps leading from late Cairns structures of replicative intermediates to monomeric minichromosomes. It appears that type I DNA topoisomerase can function as a swivelase during most of the replicative elongation phase, but must later be replaced by type II DNA topoisomerase.     

Distribution of Replicating Simian Virus 40 DNA in Intact Cells and Its Maturation in Isolated Nuclei

The maturation of replicating simian virus 40 (SV40) chromosomes into superhelical viral DNA monomers [SV40(I) DNA] was analyzed in both intact cells and isolated nuclei to investigate further the role of soluble cytosol factors in subcellular systems. Replicating intermediates [SV40(RI) DNA] were purified to avoid contamination by molecules broken at their replication forks, and the distribution of SV40(RI) DNA as a function of its extent of replication was analyzed by gel electrophoresis and electron microscopy. With virus-infected CV-1 cells, SV40(RI) DNA accumulated only when replication was 85 to 95% completed. These molecules [SV40(RI^*) DNA] were two to three times more prevalent than an equivalent sample of early replicating DNA, consistent with a rate-limiting step in the separation of sibling chromosomes. Nuclei isolated from infected cells permitted normal maturation of SV40(RI) DNA into SV40(I) DNA when the preparation was supplemented with cytosol. However, in the absence of cytosol, the extent of DNA synthesis was diminished three- to fivefold (regardless of the addition of ribonucleotide triphosphates), with little change in the rate of synthesis during the first minute; also, the joining of Okazaki fragments to long nascent DNA was inhibited, and SV40(I) DNA was not formed. The fraction of short-nascent DNA chains that may have resulted from dUTP incorporation was insignificant in nuclei with or without cytosol. Pulse-chase experiments revealed that joining, but not initiation, of Okazaki fragments required cytosol. Cessation of DNA synthesis in nuclei without cytosol could be explained by an increased probability for cleavage of replication forks. These broken molecules masqueraded during gel electrophoresis of replicating DNA as a peak of 80% completed SV40(RI) DNA. Failure to convert SV40(RI^*) DNA into SV40(I) DNA under these conditions could be explained by the requirement for cytosol to complete the gap-filling step in Okazaki fragment metabolism: circular monomers with their nascent DNA strands interrupted in the termination region [SV40(II^*) DNA] accumulated with unjoined Okazaki fragments. Thus, separation of sibling chromosomes still occurred, but gaps remained in the terminal portions of their daughter DNA strands. These and other data support a central role for SV40(RI^*) and SV40(II^*) DNAs in the completion of viral DNA replication.     

Studies on the structure of replicative intermediates in bacteriophage M13 single stranded DNA synthesis.

Pulse-labeled replicative intermediates in M 13 single stranded DNA synthesis can be separated by dye-buoyant density centrifugation into two major fractions: Supercoiled molecules (RI I) containing viral strands of more than one genome length, and "relaxed" molecules (RI II) with labeled DNA chains shorter than unit length. It is postulated that RI II molecules might be formed in vivo by site-specific nicking of RF I molecules.     

Patterns of strongly protein-associated simian virus 40 DNA replication intermediates resulting from exposures to specific topoisomerase poisons

Exposure of infected CV-1 cells to specific type I and type II topoisomerase poisons caused strong protein association with distinct subsets of simian virus 40 (SV40) DNA replication intermediates. On the basis of the known specificity and mechanisms of action of these drugs, the proteins involved are assumed to be the respective topoisomerases. Camptothecin, a topoisomerase I poison, caused strong protein association with form II (relaxed circular) and form III (linear) viral genomes and replication intermediates having broken DNA replication forks but not with form I (superhelical) viral DNA or normal late replication intermediates which were present. In contrast, type II topoisomerase poisons caused completely replicated forms and late viral replication forms to be tightly bound to protein--some to a greater extent than others. Different type II topoisomerase inhibitors caused distinctive patterns of protein association with the replication intermediates present. Both intercalating and nonintercalating type II topoisomerase poisons caused a small amount of form I (superhelical) SV40 DNA to be protein-associated in vivo. The protein complex with form I viral DNA was entirely drug-dependent and strong, but apparently noncovalent. The protein associated with form I DNA may represent a drug-stabilized "topological complex" between type II topoisomerase and SV40 DNA.     

Biosynthetic studies of the Y base in yeast phenylalanine tRNA. Incorporation of guanine

Replicating polyoma virus DNA, pulse-labeled with ³H-thymidine, was isolated from infected mouse embryo cells by velocity sedimentation in neutral sucrose and purified by benzoylated-naphthoylated DEAE-cellulose chromatography. Nascent strands, prepared by heat denaturation of purified replicative intermediate, banded at a slightly higher buoyant density in neutral cesium sulfate gradients than single strands derived from superhelical viral DNA. Treatment of nascent strands with a mixture of ribonucleases 1A and T1 shifted their buoyant density to that of single strands derived from superhelical viral DNA. These results indicate that an oligoribonucleotide component is covalently associated with replicating polyoma DNA strands.