Perturbations in simian virus 40 DNA synthesis by ultraviolet light

DNA damage-induced phosphorylation of a replicative DNA helicase results in inhibition of DNA replication through attenuation of helicase function

A major function of the DNA damage responses (DDRs) that act during the replicative phase of the cell cycle is to inhibit initiation and elongation of DNA replication. It has been shown that DNA replication of the polyomavirus, SV40, is inhibited and its replication fork is slowed by cellular DDR responses. The inhibition of SV40 DNA replication is associated with enhanced DDR kinase phosphorylation of SV40 Large T-antigen (LT), the viral DNA helicase. Mass spectroscopy was used to identify a novel highly conserved DDR kinase site, T518, on LT. In cell-based assays expression of a phosphomimetic form of LT at T518 (T518D) resulted in dramatically decreased levels of SV40 DNA replication, but LT-dependent transcriptional activation was unaffected. Purified WT and LT T518D were analyzed in vitro. In concordance with the cell-based data, reactions using SV40 LT-T518D, but not T518A, showed dramatic inhibition of SV40 DNA replication. A myriad of LT protein-protein interactions and LT's biochemical functions were unaffected by the LT T518D mutation; however, LT's DNA helicase activity was dramatically decreased on long, but not very short, DNA templates. These results suggest that DDR phosphorylation at T518 inhibits SV40 DNA replication by suppressing LT helicase activity.

Eukaryotic Translesion DNA Synthesis on the Leading and Lagging Strands: Unique Detours around the Same Obstacle

During S-phase, minor DNA damage may be overcome by DNA damage tolerance (DDT) pathways that bypass such obstacles, postponing repair of the offending damage to complete the cell cycle and maintain cell survival. In translesion DNA synthesis (TLS), specialized DNA polymerases replicate the damaged DNA, allowing stringent DNA synthesis by a replicative polymerase to resume beyond the offending damage. Dysregulation of this DDT pathway in human cells leads to increased mutation rates that may contribute to the onset of cancer. Furthermore, TLS affords human cancer cells the ability to counteract chemotherapeutic agents that elicit cell death by damaging DNA in actively replicating cells. Currently, it is unclear how this critical pathway unfolds, in particular, where and when TLS occurs on each template strand. Given the semidiscontinuous nature of DNA replication, it is likely that TLS on the leading and lagging strand templates is unique for each strand. Since the discovery of DDT in the late 1960s, most studies on TLS in eukaryotes have focused on DNA lesions resulting from ultraviolet (UV) radiation exposure. In this review, we revisit these and other related studies to dissect the step-by-step intricacies of this complex process, provide our current understanding of TLS on leading and lagging strand templates, and propose testable hypotheses to gain further insights.

Beyond xeroderma pigmentosum: DNA damage and repair in an ecological context. A tribute to James E. Cleaver

The ability to repair DNA is a ubiquitous characteristic of life on Earth and all organisms possess similar mechanisms for dealing with DNA damage, an indication of a very early evolutionary origin for repair processes. James E. Cleaver's career (initiated in the early 1960s) has been devoted to the study of mammalian ultraviolet radiation (UVR) photobiology, specifically the molecular genetics of xeroderma pigmentosum and other human diseases caused by defects in DNA damage recognition and repair. This work by Jim and others has influenced the study of DNA damage and repair in a variety of taxa. Today, the field of DNA repair is enhancing our understanding of not only how to treat and prevent human disease, but is providing insights on the evolutionary history of life on Earth and how natural populations are coping with UVR-induced DNA damage from anthropogenic changes in the environment such as ozone depletion.

Inhibition of initiation of simian virus 40 DNA replication during acute response of cells irradiated by ultraviolet light

To study the mechanism by which ultraviolet (UV) light inhibits DNA replication, we examined the effects of UV 254 nm irradiation on the replication of simian virus 40 (SV40) DNA and SV40-based plasmid in monkey cells. The study was designed to determine the relative contributions made by inhibition of replication initiation and chain elongation to the immediate inhibition of DNA replication following UV irradiation. We used two-dimensional neutral-alkaline electrophoresis to examine the behaviour of replication intermediates unambiguously. Kinetic analysis using this technique showed that initiation of replication started to decline at 15 min post-irradiation. When the pulse label incorporated in SV40 replication intermediates before irradiation was chased for 1 h, most of the label was found in mature Form I and II molecules. This indicated that replication elongation took place on damaged template. We also used a transfection technique to show that heavily irradiated plasmids replicated efficiently in unirradiated transfected cells. By the transfection technique, we observed that UV irradiation of host cells dose-dependently inhibited replication of transfected non-irradiated plasmids, suggesting that the inhibition of DNA replication is due to a global change in cellular physiology induced by UV. This change was also apparent from poor staining of the chromatin by fluorescent-DNA-binding dyes immediately after UV irradiation of intact cells. We conclude that a significant fraction of chain elongation proceeds on damaged templates and DNA replication during the acute response of cells irradiated with UV is mainly controlled by the inhibition of replication initiation.

Inducible cellular responses to ultraviolet light irradiation and other mediators of DNA damage in mammalian cells

Both naturally occurring and carcinogen-induced tumors display not only point mutations in cellular oncogenes but also more complex changes in cellular oncogenes and other cellular genes. For this and other reasons, it seems likely that DNA damage in mammalian cells can induce alterations in gene expression that may have both short and long term consequences in the target cell. The purpose of this review is to summarize current available information on inducible responses to UV-irradiation and other mediators of DNA damage in mammalian cells, and to provide some working hypotheses. We have divided these responses into three time frames, immediate (0-12 hours), early (12-48) and late (beyond 48 hours). Immediate responses include the action of DNA repair enzymes, some of which are induced as a consequence of DNA damage, and transient inhibition of DNA synthesis. Within the past few years considerable evidence has accumulated that during this immediate period there is increased expression of certain cellular oncogenes, proteases and proteins whose functions remain to be identified. It is of interest that the expression of some of these genes is also induced by certain growth factors, tumor promoters and heat shock. Alterations in gene expression during the subsequent "early" period (12-48 hrs.) have not been studied in detail, but it is during this period that one can detect increased replication of several types of viruses in cells that harbor these viruses. We have examined in detail the induction of asynchronous polyoma DNA replication (APR) in a rat fibroblast cell line carrying integrated copies of this DNA. We have obtained evidence that UV-irradiation of these cells leads to the synthesis of a 40 kd protein, within the first 1-24 hrs after irradiation, that binds to a specific sequence TGACAACA in the regulatory region of polyoma DNA. We suggest that this protein acts together with other proteins to induce APR and that this serves as a useful model for understanding the mechanisms responsible for amplification of cellular genes, a phenomenon often seen in malignant tumors. Finally, we discuss how the events occurring during the immediate and early periods following DNA damage might lead to late effects in the target cell that are stable and contribute to the genotype and phenotype of some of the progeny of these cells that are destined to become tumor cells.

Replication of simian virus 40 DNA after UV irradiation: evidence of growing fork blockage and single-stranded gaps in daughter strands

The molecular mechanisms of in vivo inhibition of mammalian DNA replication by exposure to UV light (at 254 nm) was studied in monkey and human cells infected with simian virus 40. Analysis of viral DNA by electron microscopy and sucrose gradients confirmed that the presence of UV-induced lesions severely blocks DNA synthesis, and thus the conversion of replicative intermediates (RIs) into fully replicated form I DNA is inhibited by UV irradiation. These blocked RI molecules present several special features when visualized by electron microscopy. (i) In excision repair-proficient monkey and human cells they are composed of a double-stranded circular DNA with a double-stranded tail whose size corresponds to the average interpyrimidine dimer distance, as determined by the dimer-specific T4 endonuclease V. (ii) In excision repair-deficient human cells from patients with xeroderma pigmentosum, UV-irradiated RIs present a Cairns-like structure similar to that observed for replicating molecules obtained from unirradiated infected cells. (iii) Single-stranded gaps are visualized in the replicated portions of UV-irradiated RI molecules; such regions are detected and clearly distinguishable from double-stranded DNA when probed by a specific single-stranded DNA-binding protein such as the bacteriophage T4 gene 32 product. Consistent with the presence of gaps in UV-irradiated RI molecules, single-strand-specific S1 nuclease digestion causes a shift in their sedimentation properties when analyzed in neutral sucrose gradients compared with undamaged molecules. These results are in agreement with and reinforce the model in which UV lesions are a barrier to the replication fork movement when present in the template for the leading strand; when lesions are in the template for the lagging strand they inhibit synthesis or completion of Okazaki fragments, leaving gaps opposite the lesion. Moreover, cellular DNA repair-linked endonucleolytic activity may induce double-stranded breaks in the blocked region of the replication forks, resulting in the tailed structures observed in viral DNA molecules obtained from excision repair-proficient cell lines.

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.

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.

Preirradiation of host cells does not alter blockage of simian virus 40 replication forks by pyrimidine dimers

Do damage-inducible responses in mammalian cells alter the interaction of lesions with replication forks? We have previously demonstrated that preirradiation of the host cell mitigates UV inhibition of SV40 DNA replication; this mitigation can be detected within the first 30 min after the test irradiation. Here we test the hypotheses that this mitigation involves either (1) rapid dimer removal, (2) rapid synthesis of daughter strands past lesions (trans-dimer synthesis), or (3) continued progression of the replication fork beyond a dimer. Cells preirradiated with UV were infected with undamaged SV40, and the effects of UV upon viral DNA synthesis were measured within the first hour after a subsequent test irradiation. In preirradiated cells, as well as in non-preirradiated cells, pyrimidine dimers block elongation of daughter strands; daughter strands grow only to a size equal to the interdimer distance along the parental strands. There is, within this first hour after UV, no evidence for trans-dimer synthesis, nor for more rapid dimer removal either in the bulk of the parental DNA or in molecules in the replication pool. Progression of the replication forks was analyzed by electron microscopy of replicating SV40 molecules. Dimers block replication-fork progression in preirradiated cells to the same extent as in non-preirradiated cells. These experiments argue strongly against the hypotheses that preirradiation of host cells results in either the rapid removal of dimers, trans-dimer synthesis, or continued replication-fork progression beyond dimers.

Preirradiation of host (monkey) cells mitigates the effects of UV upon simian virus 40 DNA replication

We are examining the effects of preirradiation of host (monkey) cells upon the replication of UV-damaged SV40. Control cells and cells preirradiated with low fluences (5 or 10 J/m2) of UV were infected with undamaged SV40, and the immediate effects of a subsequent irradiation were determined. UV inhibited total SV 40 DNA synthesis (incorporation of thymidine into viral DNA) in both preirradiated and control cells, but the extent of inhibition was less in the preirradiated cells. A test fluence of 60 J/m2 to SV40 replicating in preirradiated cells reduced synthesis only as much as a test fluence of 25 J/m2 in control cells. The fraction of recently replicated SV40 molecules that re-entered the replication pool and subsequently completed one round of replication in the first 2 h after UV was also decreased less in the preirradiated cells. Thus preirradiation of the host cell mitigates the immediate inhibitory effects of a subsequent UV exposure upon SV40 replication.

Pyrimidine dimers block simian virus 40 replication forks

UV light produces lesions, predominantly pyrimidine dimers, which inhibit DNA replication in mammalian cells. The mechanism of inhibition is controversial: is synthesis of a daughter strand halted at a lesion while the replication fork moves on and reinitiates downstream, or is fork progression itself blocked for some time at the site of a lesion? We directly addressed this question by using electron microscopy to examine the distances of replication forks from the origin in unirradiated and UV-irradiated simian virus 40 chromosomes. If UV lesions block replication fork progression, the forks should be asymmetrically located in a large fraction of the irradiated molecules; if replication forks move rapidly past lesions, the forks should be symmetrically located. A large fraction of the simian virus 40 replication forks in irradiated molecules were asymmetrically located, demonstrating that UV lesions present at the frequency of pyrimidine dimers block replication forks. As a mechanism for this fork blockage, we propose that polymerization of the leading strand makes a significant contribution to the energetics of fork movement, so any lesion in the template for the leading strand which blocks polymerization should also block fork movement.

Pyrimidine dimers block simian virus 40 replication forks

UV light produces lesions, predominantly pyrimidine dimers, which inhibit DNA replication in mammalian cells. The mechanism of inhibition is controversial: is synthesis of a daughter strand halted at a lesion while the replication fork moves on and reinitiates downstream, or is fork progression itself blocked for some time at the site of a lesion? We directly addressed this question by using electron microscopy to examine the distances of replication forks from the origin in unirradiated and UV-irradiated simian virus 40 chromosomes. If UV lesions block replication fork progression, the forks should be asymmetrically located in a large fraction of the irradiated molecules; if replication forks move rapidly past lesions, the forks should be symmetrically located. A large fraction of the simian virus 40 replication forks in irradiated molecules were asymmetrically located, demonstrating that UV lesions present at the frequency of pyrimidine dimers block replication forks. As a mechanism for this fork blockage, we propose that polymerization of the leading strand makes a significant contribution to the energetics of fork movement, so any lesion in the template for the leading strand which blocks polymerization should also block fork movement.

Gap filling and not replication fork progression is the rate-limiting step in the replication of UV-damaged simian virus 40 DNA

We have analyzed the structural characteristics of simian virus 40 replicative intermediate DNA produced after UV irradiation and the kinetics of conversion of this intermediate DNA into form I DNA. Replicative intermediate DNA isolated at 30 or 60 min after UV irradiation consists primarily of two species of molecules that sediment in neutral sucrose gradients as either Cairns theta structures or relaxed monomeric circles. Replication forks on the Cairns intermediate DNA are symmetrically located with respect to the origin of replication, ruling out the possibility of asymmetric pauses or blocks to replication fork progression at damage sites. The relaxed circles contain at least one randomly located discontinuity in the daughter strand. Pulse-chase experiments demonstrated that a UV fluence-dependent fraction of the Cairns intermediate DNA progresses through the relaxed circular intermediate before being converted to completed form I molecules. Disappearance of Cairns intermediate DNA occurs at the same rate in irradiated and unirradiated cells, whereas completion of the relaxed circular intermediate DNA occurs at a slow rate, relatively independent of UV fluence. These data support a model for replication of UV-damaged DNA in which replication rapidly continues past damage sites via a gap formation event.

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.

Gap filling and not replication fork progression is the rate-limiting step in the replication of UV-damaged simian virus 40 DNA

We have analyzed the structural characteristics of simian virus 40 replicative intermediate DNA produced after UV irradiation and the kinetics of conversion of this intermediate DNA into form I DNA. Replicative intermediate DNA isolated at 30 or 60 min after UV irradiation consists primarily of two species of molecules that sediment in neutral sucrose gradients as either Cairns theta structures or relaxed monomeric circles. Replication forks on the Cairns intermediate DNA are symmetrically located with respect to the origin of replication, ruling out the possibility of asymmetric pauses or blocks to replication fork progression at damage sites. The relaxed circles contain at least one randomly located discontinuity in the daughter strand. Pulse-chase experiments demonstrated that a UV fluence-dependent fraction of the Cairns intermediate DNA progresses through the relaxed circular intermediate before being converted to completed form I molecules. Disappearance of Cairns intermediate DNA occurs at the same rate in irradiated and unirradiated cells, whereas completion of the relaxed circular intermediate DNA occurs at a slow rate, relatively independent of UV fluence. These data support a model for replication of UV-damaged DNA in which replication rapidly continues past damage sites via a gap formation event.

Accommodation of pyrimidine dimers during replication of UV-damaged simian virus 40 DNA

UV irradiation of simian virus 40-infected cells at fluences between 20 and 60 J/m2, which yield one to three pyrimidine dimers per simian virus 40 genome, leads to a fluence-dependent progressive decrease in simian virus 40 DNA replication as assayed by incorporation of [3H]deoxyribosylthymine into viral DNA. We used a variety of biochemical and biophysical techniques to show that this decrease is due to a block in the progression of replicative-intermediate molecules to completed form I molecules, with a concomitant decrease in the entry of molecules into the replicating pool. Despite this UV-induced inhibition of replication, some pyrimidine dimer-containing molecules become fully replicated after UV irradiation. The fraction of completed molecules containing dimers goes up with time such that by 3 h after a UV fluence of 40 J/m2, more than 50% of completed molecules contain pyrimidine dimers. We postulate that the cellular replication machinery can accommodate limited amounts of UV-induced damage and that the progressive decrease in simian virus 40 DNA synthesis after UV irradiation is due to the accumulation in the replication pool of blocked molecules containing levels of damage greater than that which can be tolerated.

Restriction enzyme cleavage of ultraviolet-damaged simian virus 40 and pBR322 DNA

Cleavage of specific DNA sequences by the restriction enzymes EcoRI, HindIII and TaqI was prevented when the DNA was irradiated with ultraviolet light. Most of the effects were attributed to cyclobutane pyrimidine dimers in the recognition sequences; the effectiveness of irradiation was directly proportional to the number of potential dimer sites in the DNA. Combining EcoRI with dimer-specific endonuclease digestion revealed that pyrimidine dimers blocked cleavage within one base-pair on the strand opposite to the dimer but did not block cleavage three to four base-pairs away on the same strand. These are the probable limits for the range of influence of pyrimidine dimers along the DNA, at least for this enzyme. The effect of irradiation on cleavage by TaqI seemed far greater than expected for the cyclobutane dimer yield, possibly because of effects from photoproducts flanking the tetranucleotide recognition sequence and the effect of non-cyclobutane (6-4)pyrimidine photoproducts involving adjacent T and C bases.

Accommodation of pyrimidine dimers during replication of UV-damaged simian virus 40 DNA

UV irradiation of simian virus 40-infected cells at fluences between 20 and 60 J/m2, which yield one to three pyrimidine dimers per simian virus 40 genome, leads to a fluence-dependent progressive decrease in simian virus 40 DNA replication as assayed by incorporation of [3H]deoxyribosylthymine into viral DNA. We used a variety of biochemical and biophysical techniques to show that this decrease is due to a block in the progression of replicative-intermediate molecules to completed form I molecules, with a concomitant decrease in the entry of molecules into the replicating pool. Despite this UV-induced inhibition of replication, some pyrimidine dimer-containing molecules become fully replicated after UV irradiation. The fraction of completed molecules containing dimers goes up with time such that by 3 h after a UV fluence of 40 J/m2, more than 50% of completed molecules contain pyrimidine dimers. We postulate that the cellular replication machinery can accommodate limited amounts of UV-induced damage and that the progressive decrease in simian virus 40 DNA synthesis after UV irradiation is due to the accumulation in the replication pool of blocked molecules containing levels of damage greater than that which can be tolerated.

Inhibition of simian virus 40 DNA replication by ultraviolet light

The effects of ultraviolet light (uv) upon SV40 DNA synthesis in monkey cells were examined to determine whether replication forks were halted upon encountering lesions in the DNA, or alternatively whether lesions were rapidly bypassed. Ultraviolet light inhibits elongation of nascent DNA strands; the extent of incorporation of [3H]deoxythymidine ( [3H]dT) into DNA decreases with increasing uv fluence. Inhibition begins within minutes of irradiation, and becomes more pronounced with increasing time after irradiation. The synthesis of form I (covalently closed) molecules is inhibited even more severely than is total incorporation: post-uv incorporation is predominantly into replication intermediates. In contrast to previous reports, we find that replication intermediates labeled after uv resemble those in unirradiated cells, and contain covalently closed parental strands. DNA strands made after uv are approximately the size of parental DNA which has been cleaved at pyrimidine dimers by a uv endonuclease, indicating that they do not extend past dimers. The hypothesis that replication forks are halted upon encountering pyrimidine dimers in the template strand is consistent with these data.

U.V. enhanced reactivation of U.V.-and gamma-irradiated adenovirus in normal human fibroblasts

An enhanced reactivation (UVER) of U.V.-irradiated as well as of gamma-irradiated human adenovirus type 2 (Ad 2) was detected following infection of normal human fibroblasts which had been pre-irradiated with U.V. light. U.V.-irradiated or non-irradiated fibroblasts were infected with either non-irradiated or irradiated Ad 2, and at 48 hours after infection cells were examined for the presence of viral structural antigens (Vag) using immunofluorescent staining. Results obtained using 5 different normal fibroblast strains showed that irradiation of host monolayers with 10J/m2 immediately prior to infection gave a U.V. enhanced reactivation (UVER) factor +/- standard error equal to 3 . 1 +/- 1 . 2 for virus U.V.-irradiated with 1 . 2 x 10(3) J/m2, and 2 . 1 +/- 0 . 5 for virus gamma-irradiated with 2 x 10(4) Gy. For a fixed survival of about 5 . 9 x 10(-2) for irradiated virus, the efficiency of UVER for gamma-irradiated virus was about 0 . 18, slightly less than the value of about 0 . 24 obtained for U.V.-irradiated virus. The results of time course experiments indicated that while U.V.-irradiation of normal host monolayers prior to infection gave rise to an increased rate of Vag formation for infection by unirradiated Ad 2, U.V.-irradiation of the cells increased the proportion of cells able to repair U.V.-damaged virus as well as allowing an earlier onset and/or increased rate of synthesis of Vag from a U.V.-damaged template. Similar experiments involving gamma-ray enhanced reactivation (gamma-RER) of irradiated Ad 2 indicated that gamma-RER and UVER may operate, in part at least, by different mechanisms in normal human cells.

Restriction enzyme cleavage of ultraviolet-damaged DNA

SV40 and pBR322 DNAs damaged by ultraviolet light were cleaved abnormally by several restriction enzymes because of damage to pyrimidines in the recognition sequences. The use of a tandemly duplicated plasmid provided a particularly sensitive target molecule for detecting pyrimidine dimers and other possible photoproducts. The relative efficiency with which cleavage was blocked (HindIII greater than TaqI greater than EcoRI greater than BamI greater than SalI much greater than Hha I, Hae III) corresponds approximately to the relative frequency of pyrimidine dimer formation in the recognition sequences, but at a slightly higher frequency in potential sites for the non-cyclobutane T-C product. The pyrimidine dimers appear to have a range of influence that extends 1 to 3 basepairs along the DNA molecule. These effects provide clues to the way DNA damage from mutagens and carcinogens can interfere with specific enzyme-DNA interactions.

Ultraviolet irradiation inhibits encapsidation of simian virus 40 chromatin

During normal maturation and majority of pulse-labeled simian virus 40 DNA progresses from chromatin to previrions and virions within 5 h. UV light inhibits this progression. In heavily irradiated cultures (108 J m-2) most of the simian virus 40 DNA synthesized immediately before irradiation remains as chromatin for at least 5 h. This inhibition of maturation seems to be a result of the inhibition of protein synthesis. The data suggest that the pool of proteins required for maturation is sufficient to convert one-third of the simian virus 40 DNA molecules labeled in a 10-min pulse (at 33 h postinfection) from chromatin to previrions and virions and is exhausted within 1 h.

Inhibition of DNA replication by hydroxyurea and caffeine in an ultraviolet-irradiated human fibroblast cell line

DNA replication in human fibroblasts with normal excision repair was investigated after ultraviolet irradiation and incubation with caffeine or hydroxyurea. The DNA synthesized soon after irradiation had a reduced size, but that synthesized later was near normal size. When caffeine was present before labeling, it reduced the size of DNA synthesized but when added after labeling it was without effect. When irradiated cells were allowed to grow, labeled DNA increased in size steadily for 60 min to a maximum that was below control and dose-dependent. Further growth resulted in a transition of some label to parental DNA sized, but a large fraction remained permanently blocked at smaller sizes, producing bimodal distributions of DNA. The steady increase in size was inhibited by hydroxyurea. Removing cells from hydroxyurea resulted in increases similar to or slightly slower than those observed immediately after labeling, and this protocol did not permit cells to acquire any induced or enhanced capacity to replicate damaged DNA.

Ultraviolet radiation inhibits replicon initiation in S phase human cells

DNA replication was examined in ultraviolet-irradiated human fibroblasts and HeLa cells. The principal effect of exposures to low radiation fluences (less than 1.3 J/m2) was a reduced synthesis of molecules about one-half replicon in size resulting from an inhibition of replicon initiation. As the fluence of radiation was increased, inhibitory effects on strand elongation and joining masked the effect on replicon initiations.

Removal of T4 endonuclease V-sensitive sites from SV40 DNA after exposure to ultraviolet light

The induction and removal of sites sensitive to T4 endonuclease V from ultraviolet-irradiated Simian Virus 40 (SV40) Form I (supercoiled) DNA during a lytic infection of monkey CV-1 cells was investigated by agarose tube gel electrophoresis. Endonuclease-sensitive sites were induced at a rate of 0.049 sites/SV40 genome per J/m2, or 1.4 sites/1 . 10(8) daltons of DNA per J/m2. This value is similar to the yield of endonuclease-sensitive sites and pyrimidine dimers in uninfected host CV-1 cell DNA. Removal of endonuclease-sensitive sites was dose dependent and non-linear for at least 24 h after irradiation. These results suggest that SV40 DNA is subject to the excision repair mechanisms of the host cell, and that the excision of pyrimidine dimers may be one of the biochemical events underlying host cell reactivation.