The fate of parental nucleosomes during SV40 DNA replication.

The fate of parental nucleosomes during the replication of chromatin templates was studied using a modification of the cell-free SV40 DNA replication system. Plasmid DNA molecules containing the SV40 origin were assembled into chromatin with purified core histones and fractionated assembly factors derived from HeLa cells. When these templates were replicated in vitro, the resulting progeny retained a nucleosomal organization. To determine whether the nucleosomes associated with the progeny molecules resulted from displacement of parental histones during replication followed by reassembly, the replication reactions were performed in the presence of control templates. It was observed that the progeny genomes resulting from the replication of chromatin templates retained a nucleosomal structure, whereas the progeny of the control DNA molecules were not assembled into chromatin. Additional experiments, involving direct addition of histones to the replication reaction mixtures, confirmed that the control templates were not sequestered in some form which made them unavailable for nucleosome assembly. Thus, our data demonstrate that parental nucleosomes remain associated with the replicating molecules and are transferred to the progeny molecules without displacement into solution. We propose a simple model in which nucleosomes ahead of the fork are transferred intact to the newly synthesized daughter duplexes.     

Chromatin assembly during DNA replication in somatic cells.

Newly replicated DNA is assembled into chromatin through two principle pathways. Firstly, parental nucleosomes segregate to replicated DNA, and are transferred directly to one of the two daughter strands during replication fork passage. Secondly, chromatin assembly factors mediate de-novo assembly of nucleosomes on replicating DNA using newly synthesized and acetylated histone proteins. In somatic cells, chromatin assembly factor 1 (CAF-1) appears to be a key player in assembling new nucleosomes during DNA replication. It provides a molecular connection between newly synthesized histones and components of the DNA replication machinery during the S phase of the cell division cycle.     

Coordinated leading and lagging strand synthesis during SV40 DNA replication in vitro requires PCNA

Proliferating cell nuclear antigen (PCNA) is a cell cycle and growth regulated protein required for replication of SV40 DNA in vitro. Its function was investigated by comparison of the replication products synthesized in its presence or absence. In the completely reconstituted replication system that contains PCNA, DNA synthesis initiates at the origin and proceeds bidirectionally on both leading and lagging strands around the template DNA to yield duplex, circular daughter molecules. In contrast, in the absence of PCNA, early replicative intermediates containing short nascent strands accumulate. Replication forks continue bidirectionally from the origin, but surprisingly, only lagging strand products are synthesized. Thus two stages of DNA synthesis have been defined, with the second stage requiring PCNA for coordinated leading and lagging strand synthesis at the replication fork. We suggest that during eukaryotic chromosome replication there is a switch to a PCNA-dependent elongation stage that requires two distinct DNA polymerases.     

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.     

Purification of a cellular replication factor, RF-C, that is required for coordinated synthesis of leading and lagging strands during simian virus 40 DNA replication in vitro.

Cell extracts (S100) derived from human 293 cells were separated into five fractions by phosphocellulose chromatography and monitored for their ability to support simian virus 40 (SV40) DNA replication in vitro in the presence of purified SV40 T antigen. Three fractions, designated I, IIA, and IIC, were essential. Fraction IIC contained the known replication factors topoisomerases I and II, but in addition contained a novel replication factor called RF-C. The RF-C activity, assayed in the presence of I, IIA, and excess amounts of purified topoisomerases, was detected in both cytosol and nuclear fractions, but was more abundant in the latter fraction. RF-C was purified from the 293 cell nuclear fraction to near homogeneity by conventional column chromatography. The reconstituted reaction mix containing purified RF-C could replicate SV40 origin-containing plasmid DNA more efficiently than could the S100 extract, and the products were predominantly completely replicated, monomer molecules. Interestingly, in the absence of RF-C, early replicative intermediates accumulated and subsequent elongation was aberrant. Hybridization studies with strand-specific, single-stranded M13-SV40 DNAs showed that in the absence of RF-C, abnormal DNA synthesis occurred preferentially on the lagging strand, and leading-strand replication was inefficient. These products closely resembled those previously observed for SV40 DNA replication in vitro in the absence of proliferating-cell nuclear antigen. These results suggest that an elongation complex containing RF-C and proliferating-cell nuclear antigen is assembled after formation of the first nascent strands at the replication origin. Subsequent synthesis of leading and lagging strands at a eucaryotic DNA replication fork can be distinguished by different requirements for multiple replication components, but we suggest that even though the two polymerases function asymmetrically, they normally progress coordinately.     

Dispersive segregation of nucleosomes during replication of simian virus 40 chromosomes

The distribution of preformed ("old") histone octamers between the two arms of DNA replication forks was analyzed in simian virus 40(SV40)-infected cells following treatment with cycloheximide to prevent nucleosome assembly from nascent histones. Viral chromatin synthesized in the presence of cycloheximide was shown to be deficient in nucleosomes. Replicating SV40 DNA (wild-type 800 and capsid assembly mutant, tsB11) was radiolabeled in either intact cells or nuclear extracts supplemented with cytosol. Nascent nucleosomal monomers were then released by extensive digestion of isolated nuclei, nuclear extracts or isolated viral chromosomes with micrococcal nuclease. The labeled nucleosomal DNA was purified and found to hybridize to both strands of SV40 DNA restriction fragments taken from each side of the origin of DNA replication, whereas Okazaki fragments hybridized only to the strand representing the retrograde DNA template. In addition, isolated, replicating SV40 chromosomes were digested with two strand-specific exonucleases that excised nascent DNA from either the forward or the retrograde side of replication forks. Pretreatment of cells with cycloheximide did not result in an excess of prenucleosomal DNA on either side of replication forks, but did increase the amount of internucleosomal DNA. These data are consistent with a dispersive model for nucleosome segregation in which "old" histone octamers are distributed to both arms of DNA replication forks.     

Disruption of the nucleosomes at the replication fork.

The fate of parental nucleosomes during chromatin replication was studied in vitro using in vitro assembled chromatin containing the whole SV40 genome as well as salt-treated and native SV40 minichromosomes. In vitro assembled minichromosomes were able to replicate efficiently in vitro, when the DNA was preincubated with T-antigen, a cytosolic S100 extract and three deoxynucleoside triphosphates prior to chromatin assembly, indicating that the origin has to be free of nucleosomes for replication initiation. The chromatin structure of the newly synthesized daughter strands in replicating molecules was analysed by psoralen cross-linking of the DNA and by micrococcal nuclease digestion. A 5- and 10-fold excess of protein-free competitor DNA present during minichromosome replication traps the segregating histones. In opposition to published data this suggests that the parental histones remain only loosely or not attached to the DNA in the region of the replication fork. Replication in the putative absence of free histones shows that a subnucleosomal particle is randomly assembled on the daughter strands. The data are compatible with the formation of a H3/H4 tetramer complex under these conditions, supporting the notion that under physiological conditions nucleosome core assembly on the newly synthesized daughter strands occurs by the binding of H2A/H2B dimers to a H3/H4 tetramer complex.     

Structure of replicating simian virus 40 minichromosomes. The replication fork, core histone segregation and terminal structures

The structure of replicating simian virus 40 (SV40) minichromosomes was studied by DNA crosslinking with trimethyl-psoralen. The procedure was used both in vitro with extracted SV40 minichromosomes as well as in vivo with SV40-infected cells. Both procedures gave essentially the same results. Mature SV40 minichromosomes are estimated to contain about 27 nucleosomes (error +/- 2), except for those molecules with a nucleosome-free gap, which are interpreted to contain 25 nucleosomes (error +/- 2). In replicative intermediates, nucleosomes are present in the unreplicated parental stem with the replication fork possibly penetrating into the nucleosomal DNA before the histone octamer is removed. Nucleosomes reassociate on the newly replicated DNA branches at distances from the branch point of 225 ( +/- 145) nucleotides on the leading strand and of 285( +/- 120) nucleotides on the lagging strand. In the presence of cycloheximide, daughter duplexes contained unequal numbers of nucleosomes, supporting dispersive and random segregation of parental nucleosomes. These were arranged in clusters with normal nucleosome spacing. We detected a novel type of interlocked dimer comprising two fully replicated molecules connected by a single-stranded DNA bridge. We cannot decide whether these dimers represent hemicatenanes or whether the two circles are joined by a Holliday-type structure. The joining site maps within the replication terminus. We propose that these dimers represent molecules engaged in strand segregation.     

Topoisomerase II plays an essential role as a swivelase in the late stage of SV40 chromosome replication in vitro.

The effects of topoisomerases I and II on the replication of SV40 DNA were examined using an in vitro replication system of purified proteins that constitutes the monopolymerase system. In the presence of the two topoisomerases, two distinct nascent DNAs were formed. One product arising from the replication of the leading template strand was approximately half the size of the template DNA, whereas the other product derived from the lagging template strand consisted of short DNAs. These products were synthesized from both SV40 naked DNA and SV40 chromosomes. For the replication of SV40 naked DNA, either topoisomerase I or II maintained replication fork movement and supported complete leading strand synthesis. When SV40 chromosomes were replicated with the same proteins, reactions containing only topoisomerase I produced shorter leading strands. However, mature size DNA products accumulated in reactions supplemented with topoisomerase II, as well as in reactions containing only topoisomerase II. In the presence of crude extracts of HeLa cells, VP-16, a specific inhibitor of topoisomerase II, blocked elongation of the nascent DNA during the replication of SV40 chromosomes. These results indicate that topoisomerase II plays a crucial role as a swivelase in the late stage of SV40 chromosome replication in vitro.     

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.     

Transfer of nucleosomes from parental to replicated chromatin.

Simian virus 40 (SV40) minichromosomes were used as the substrate for in vitro replication. Protein-free SV40 DNA or plasmids, carrying the SV40 origin of replication, served as controls. Replicated minichromosomal DNA possessed constrained negative superhelicity indicative of the presence of nucleosomes. The topological state of replicated minichromosomal DNA was precisely determined by two-dimensional gel electrophoresis. We show that most or all nucleosomes, present on the replicated minichromosomal DNA, were derived from the parental minichromosome substrate. The mode and the rate of nucleosome transfer from parental to minichromosomal daughter DNA were not influenced by high concentrations of competing replicating and nonreplicating protein-free DNA, indicating that nucleosomes remain associated with DNA during the replication process. The data also show that parental nucleosomes were segregated to the replicated daughter DNA strands in a dispersive manner.     

Cleavage of model replication forks by fission yeast Mus81-Eme1 and budding yeast Mus81-Mms4

The blockage of replication forks can result in the disassembly of the replicative apparatus and reversal of the fork to form a DNA junction that must be processed in order for replication to restart and sister chromatids to segregate at mitosis. Fission yeast Mus81-Eme1 and budding yeast Mus81-Mms4 are endonucleases that have been implicated in the processing of aberrant DNA junctions formed at stalled replication forks. Here we have investigated the activity of purified Mus81-Eme1 and Mus81-Mms4 on substrates that resemble DNA junctions that are expected to form when a replication fork reverses. Both enzymes cleave Holliday junctions and substrates that resemble normal replication forks poorly or not at all. However, forks where the equivalents of either both the leading and lagging strands or just the lagging strand are juxtaposed at the junction point, or where either the leading or lagging strand has been unwound to produce a fork with a single-stranded tail, are cleaved well. Cleavage sites map predominantly between 3 and 6 bp 5' of the junction point. For most substrates the leading strand template is cleaved. The sole exception is a fork with a 5' single-stranded tail, which is cleaved in the lagging strand template.     

Multiple mechanisms control chromosome integrity after replication fork uncoupling and restart at irreparable UV lesions

DNA replication forks pause in front of lesions on the template, eventually leading to cytotoxic chromosomal rearrangements. The in vivo structure of damaged eukaryotic replication intermediates has been so far elusive. Combining electron microscopy (EM) and two-dimensional (2D) gel electrophoresis, we found that UV-irradiated S. cerevisiae cells uncouple leading and lagging strand replication at irreparable UV lesions, thus generating long ssDNA regions on one side of the fork. Furthermore, small ssDNA gaps accumulate along replicated duplexes, likely resulting from repriming events downstream of the lesions on both leading and lagging strands. Translesion synthesis and homologous recombination counteract gap accumulation, without affecting fork progression. The DNA damage checkpoint contributes to gap repair and maintains a replication-competent fork structure. We propose that the coordinated action of checkpoint, recombination, and translesion synthesis-mediated processes at the fork and behind the fork preserves the integrity of replicating chromosomes by allowing efficient replication restart and filling the resulting ssDNA gaps.     

Emetine allows identification of origins of mammalian DNA replication by imbalanced DNA synthesis, not through conservative nucleosome segregation.

In the presence of emetine, an inhibitor of protein synthesis, nascent DNA on forward arms of replication forks in hamster cell lines containing either single or amplified copies of the DHFR gene region was enriched 5- to 7-fold over nascent DNA on retrograde arms. This forward arm bias was observed on both sides of the specific origin of bidirectional DNA replication located 17 kb downstream of the hamster DHFR gene (OBR-1), consistent with at least 85% of replication forks within this region emanating from OBR-1. However, the replication fork asymmetry induced by emetine does not result from conservative nucleosome segregation, as previously believed, but from preferentially inhibiting Okazaki fragment synthesis on retrograde arms of forks to produce 'imbalanced DNA synthesis'. Three lines of evidence support this conclusion. First, the bias existed in long nascent DNA strands prior to nuclease digestion of non-nucleosomal DNA. Second, the fraction of RNA-primed Okazaki fragments was rapidly diminished. Third, electron microscopic analysis of SV40 DNA replicating in the presence of emetine revealed forks with single-stranded DNA on one arm, and nucleosomes randomly distributed to both arms. Thus, as with cycloheximide, nucleosome segregation in the presence of emetine was distributive.     

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.     

Eukaryotic genome instability in light of asymmetric DNA replication

The eukaryotic nuclear genome is replicated asymmetrically, with the leading strand replicated continuously and the lagging strand replicated as discontinuous Okazaki fragments that are subsequently joined. Both strands are replicated with high fidelity, but the processes used to achieve high fidelity are likely to differ. Here we review recent studies of similarities and differences in the fidelity with which the three major eukaryotic replicases, DNA polymerases α, δ, and ?, replicate the leading and lagging strands with high nucleotide selectivity and efficient proofreading. We then relate the asymmetric fidelity at the replication fork to the efficiency of DNA mismatch repair, ribonucleotide excision repair and topoisomerase 1 activity.     

Multiple replication factors augment DNA synthesis by the two eukaryotic DNA polymerases, alpha and delta.

DNA synthesis by two eukaryotic DNA polymerases, alpha and delta, was studied using a single-strand M13 DNA template primed at a unique site. In the presence of low amounts of either DNA polymerase alpha or delta, DNA synthesis was limited and short DNA strands of approximately 100 bases were produced. Addition of replication factors RF-A, PCNA and RF-C, which were previously shown to be required for SV40 DNA replication in vitro, differentially stimulated the activity of both DNA polymerases. RF-A and RF-C independently stimulated DNA polymerase alpha activity 4- to 6-fold, yielding relatively short DNA strands (less than 1 kb) and PCNA had no effect. In contrast, polymerase delta activity was stimulated co-operatively by PCNA, RF-A and RF-C approximately 25- to 30-fold, yielding relatively long DNA strands (up to 4 kb). Neither RF-C nor RF-A appear to correspond to known polymerase stimulatory factors. RF-A was previously shown to be required for initiation of DNA replication at the SV40 origin. Results presented here suggest that it also functions during elongation. The differential effects of these three replication factors on DNA polymerases alpha and delta is consistent with the model that the polymerases function at the replication fork on the lagging and leading strand templates respectively. We further suggest that co-ordinated synthesis of these strands requires dynamic protein-protein interactions between these replication factors and the two DNA polymerases.     

Gaps and forks in DNA replication: Rediscovering old models

Most current models for replication past damaged lesions envisage that translesion synthesis occurs at the replication fork. However older models suggested that gaps were left opposite lesions to allow the replication fork to proceed, and these gaps were subsequently sealed behind the replication fork. Two recent articles lend support to the idea that bypass of the damage occurs behind the fork. In the first paper, electron micrographs of DNA replicated in UV-irradiated yeast cells show regions of single-stranded DNA both at the replication forks and behind the fork, the latter being consistent with the presence of gaps in the daughter-strands opposite lesions. The second paper describes an in vitro DNA replication system reconstituted from purified bacterial proteins. Repriming of synthesis downstream from a blocked fork occurred not only on the lagging strand as expected, but also on the leading strand, demonstrating that contrary to widely accepted beliefs, leading strand synthesis does not need to be continuous.     

A novel assay for examining the molecular reactions at the eukaryotic replication fork: activities of replication protein A required during elongation.

Studies to elucidate the reactions that occur at the eukaryotic replication fork have been limited by the model systems available. We have established a method for isolating and characterizing Simian Virus 40 (SV40) replication complexes. SV40 rolling circle complexes are isolated using paramagnetic beads and then incubated under replication conditions to obtain continued elongation. In rolling circle replication, the normal mechanism for termination of SV40 replication does not occur and the elongation phase of replication is prolonged. Thus, using this assay system, elongation phase reactions can be examined in the absence of initiation or termination. We show that the protein requirements for elongation of SV40 rolling circles are equivalent to complete SV40 replication reactions. The DNA produced by SV40 rolling circles is double-stranded, unmethylated and with a much longer length than the template DNA. These properties are similar to those of physiological replication forks. We show that proteins associated with the isolated rolling circles, including SV40 T antigen, DNA polymerase alpha, replication protein A (RPA) and RF-C, are necessary for continued DNA synthesis. PCNA is also required but is not associated with the isolated complexes. We present evidence suggesting that synthesis of the leading and lagging strands are co-ordinated in SV40 rolling circle replication. We have used this system to show that both RPA-protein and RPA-DNA interactions are important for RPA's function in elongation.     

Fork-Remodeling Helicase Rad5 Preferentially Reverses Replication Forks with Gaps in the Leading Strand

DNA damage bypass pathways promote the replication of damaged DNA when replication forks stall at sites of DNA damage. Template switching is a DNA damage bypass pathway in which fork-reversal helicases convert stalled replication forks into four-way DNA junctions called chicken foot intermediates, which are subsequently extended by replicative DNA polymerases. In yeast, fork-reversal is carried out by the Rad5 helicase using an unknown mechanism. To better understand the mechanism of Rad5 and its specificity for different fork DNA substrates, we used a FRET-based assay to observe fork reversal in real time. We examined the ability of Rad5 to bind and catalyze the reversal of various fork DNA substrates in the presence of short gaps in the leading or lagging strand as well as in the presence or absence of RPA and RNA primers in the lagging strand. We found that Rad5 preferentially reverses fork DNA substrates with short gaps (10 to 30 nt.) in the leading strand. Thus, Rad5 preferentially reverses fork DNA substrates that form chicken foot intermediates with 5′ overhangs that can be extended by replicative DNA polymerases during the subsequent steps of template switching.