Control of DNA replication by protein synthesis at defined times during the S period in Physarum polycephalum

Time of replication of ARS elements along yeast chromosome III

The replication of putative replication origins (ARS elements) was examined for 200 kilobases of chromosome III of Saccharomyces cerevisiae. By using synchronous cultures and transfers from dense to light isotope medium, the temporal pattern of mitotic DNA replication of eight fragments that contain ARSs was determined. ARS elements near the telomeres replicated late in S phase, while internal ARS elements replicated in the first half of S phase. The results suggest that some ARS elements in the chromosome may be inactive as replication origins. The actively expressed mating type locus, MAT, replicated early in S phase, while the silent cassettes, HML and HMR, replicated late. Unexpectedly, chromosome III sequences were found to replicate late in G1 at the arrest induced by the temperature-sensitive cdc7 allele.

Time of replication of ARS elements along yeast chromosome III

The replication of putative replication origins (ARS elements) was examined for 200 kilobases of chromosome III of Saccharomyces cerevisiae. By using synchronous cultures and transfers from dense to light isotope medium, the temporal pattern of mitotic DNA replication of eight fragments that contain ARSs was determined. ARS elements near the telomeres replicated late in S phase, while internal ARS elements replicated in the first half of S phase. The results suggest that some ARS elements in the chromosome may be inactive as replication origins. The actively expressed mating type locus, MAT, replicated early in S phase, while the silent cassettes, HML and HMR, replicated late. Unexpectedly, chromosome III sequences were found to replicate late in G1 at the arrest induced by the temperature-sensitive cdc7 allele.

DNA replication fork progression rate and temporal organization of S phase in normal epidermis and in basal cell carcinoma

The double-pulse labeling technique for DNA fiber autoradiography was applied to epidermal cells from normal human skin and from human basal cell carcinoma (BCC). We aimed to measure the size and replication rate of the replication unit (RU) for both types of cell and to account, from these results, for our previous observation of a near doubling of S-phase duration in BCC, compared with normal skin. The mean RU size was 76 +/- 4 micron in BCC, not significantly different from the 68 +/- 6 micron value found in normal skin, so the mean of those two values (i.e., 72 micron), was used in further calculations. The rate of replication fork progression was 0.59 +/- 0.005 micron/min in the normal epidermis and 0.33 +/- 0.03 micron/min in BCC, corresponding to a replication time of the average RU equal to 61 min and 109 min, respectively. Thus, with an unchanged RU size in BCC, the observed 1.8-fold decrease in the rate of fork progression in the tumor can account entirely for our previous observation of a 1.8-fold increase in S-phase duration in this tumor, without requiring the assumption of any change in the temporal organization of DNA synthesis in the malignant cells. Considering S phase as an ordered process in which a major part, if not all, of the genome replicates at genetically determined times, we suggest that the clusters of replication units are, in turn, organized into temporally defined "sets". These sets are composed of all the clusters (whatever their chromosomal location) that are programmed to initiate replication during the same fraction of the S period. This hypothesis implies that DNA synthesis in a given set is triggered by some event coupled to progression of replication in the immediately preceding set. Based on a S-phase duration of 10.2 hours in normal skin and of 19.2 hours in BCC (our previous data), and assuming perfect synchrony and homogeneity of the clusters within each set and of each cluster's constitutive RUs, the minimum number of sequentially replicating sets, in both instances, can be estimated as roughly equal to 10.

Patterns of histone acetylation in Physarum polycephalum. H2A and H2B acetylation is functionally distinct from H3 and H4 acetylation

Histone acetylation has previously been correlated with both chromosome replication and transcription. We present evidence that (a) confirms both correlations in the true slime mold, Physarum polycephalum and (b) shows that quite a different pattern of acetate turnover is associated with replication compared with transcription. The pattern associated with replication involves turnover of acetate on all four core histones on species containing one or two acetates per molecule. This pattern was resolved from the transcription-associated pattern by three different procedures: (a) detailed analysis of gels of histones pulse-labelled with acetate; (b) the pattern of acetylation of histones pulse-labelled with [3H]lysine; and (c) the pattern of acetylation of soluble histones. The pattern associated with transcription is restricted to histones H3 and H4 and occurs mostly on highly acetylated species. This pattern was resolved by (a) analysis of gels of histones pulse-labelled with acetate; (b) the pattern of histone acetylation in G2 phase of the cell cycle; and (c) the pattern of histone acetylation in the presence of cycloheximide.

Intranuclear localization of histone acetylation in Physarum polycephalum and the structure of functionally active chromatin

Based on studies of histone acetylation in vivo in Physarum polycephalum, we present the following hypotheses: (1) Transcription-specific histone acetylation on histones H3 and H4 is a localized process at the nuclear matrix; (2) Histone acetylation in the S phase, which is specific for newly synthesized histones, occurs in an intranuclear nonlocalized process. These hypotheses can explain: (1) the histone specificity of histone acetylation that is dependent on the functional state of the chromatin; (2) the apparent absence of turnover of histone acetylation in the bulk of the chromatin despite a definite low level of steady-state acetylation of all four core histones in bulk chromatin; (3) the pattern of butyrate-induced hyperacetylation observed for active and inactive chromatin.

Mitotic cell cycle control in Physarum. Unprecedented insights via flow-cytometry

High resolution flow-cytometric studies of isolated macroplasmodial nuclei of the myxomycete. Physarum polycephalum provide definite evidence for the persistence of natural synchrony at mitosis throughout the entire cell cycle, i.e. completely synchronous DNA replication and traverse of G2. Even if nuclei within a given macroplasmodium belong to two distinct genome size classes (mixoploidy), they cycle and traverse mitosis in strict synchrony. This cannot be explained by current models of regulation of division based solely upon nuclear size and/or nuclear/cytoplasm ratios. Constitutional DNA content variation was apparent among all tested strains, and loss of late-replicating, presumably AT-rich DNA accounts for this variation. A constant duration of the S phase is maintained, irrespective of DNA content, via differential slowdown of replication rates during the 2nd and 3rd hours of replication. A frequently described extension of nuclear replication into G2 could not be substantiated. Interference with DNA and protein synthesis provides the first evidence for a brief "G1 phase" equivalent of 3-4 min duration in asynchronous microplasmodial cultures, and temporally assigns a protein synthesis-dependent "transition point" for completion of mitosis and initiation of DNA synthesis at 5 min prior to actual division nuclei which have passed this point at the time of addition of cycloheximide replicate 5% of their DNA before they become arrested. These findings provide strong experimental support for the transition point concept of cell cycle control, and additionally are commensurate with some form of the replicon-set hypothesis in Physarum.

Patterns of histone acetylation in the cell cycle of Physarum polycephalum

Labeling of histones in the naturally synchronous cell cycle of Physarum polycephalum with short pulses of tritiated acetate in vivo clearly showed three distinct patterns of histone acetate turnover. In G2 phase, turnover of acetate was observed only in histones H3 and H4, predominantly on the multiple acetylated forms. No acetate turnover was found in histones H2A and H2B compared with histones H3 and H4. In S phase, intense labeling was seen in all four core histones, in histones H3 and H4 predominantly in the low acetylated forms. In addition, cotranslational acetylation of the amino-terminal serines of histones H4 and H1 was observed during S phase. During mitosis, from condensation at prophase to decondensation after telophase, acetate turnover is almost zero. This suggests that within the mitotically condensed chromosomes all potential histone acetylation sites are masked. In G2 phase, when transcription is occurring, only histones H3 and H4 are available for acetate turnover, but in S phase, when both transcription and replication occur, all four histones are available for acetate turnover.

In the higher plant Pisum sativum maturation of nascent DNA is blocked by cycloheximide, but only after 4-8 replicons are joined

Velocity sedimentation in alkaline sucrose gradients, single cell autoradiography and cytophotometry were used to determine if protein synthesis is required for the maturation of nascent replicons to chromosomal-sized molecules in cultured pea-root cells. The results obtained showed that cycloheximide at 5 and 10 microgram/ml, added either before or during labeling with tritiated thymidine, blocked maturation of nascent DNA at an intermediate size of 72-140 X 10(6) daltons single-stranded DNA. To reach this size, nascent replicons - which are 18 X 10(6) daltons single-stranded DNA each - were replicated and groups of 4-8 replicons were joined even though protein synthesis was reduced to 15% of the control. Further maturation of the nascent molecules to chromosomal size, however, was prevented and this resulted in the accumulation of nascent molecules in the 72-140 X 10(6) daltons range. The experiments also showed that the joining of nascent replicons is not an absolute function of late S or G2 phase of the cell cycle, since cells treated with cycloheximide and blocked in mid-S phase had nascent DNA of a size corresponding to 4-8 joined replicons. Finally, the results support the hypothesis that at least one step in the process of nascent DNA maturation may require replication, during late-S phase, of DNA segments that are interspersed within replicon-clusters that replicate early in the S phase.

DNA replication in Physarum polycephalum: electron microscopic analysis of patterns of DNA replication in the presence of cycloheximide

DNA from synchronously replicating nuclei of Physarum polycephalum was studied electron microscopically after 15, 30, 60, and 90 or 120 min of replication in the presence or absence of the protein synthesis inhibitor cycloheximide. The replication-loop size-distribution showed that replication fork progression is severely retarded in the presence of cycloheximide. Analysis of replication-loop frequency showed a similar pattern in control and cyclo-heximide-treated samples, with an increase from 15 to 30 and 60 min. This suggests, surprisingly, that initiations of new replicons either may not be inhibited by cycloheximide or, alternatively, that all initiations have already taken place at the very start of S-phase. The latter conclusion is favored in the light of previous results in our laboratory, discussed here.

Isolation of Physarum DNA segments that support autonomous replication in yeast

Hybrid plasmids containint the bacterial resistance-transfer factor pBR322 and the yeast leu2+ gene have been used to isolate DNA fragments of Physarum that are capable of initiating DNA replication in a yeast host. Five of forty hybrid plasmids containint Physarum sequences transform leu2- yeast to Leu+ at high frequency. The resulting Leu+ transformants are characterized by phenotypic instability. Supercoiled plasmid molecules containing pBR322 sequences can be detected in the transformed yeast, indicating that the transforming DNA replicates autonomously. Plasmid DNA isolated from Leu+ yeast can transform leuB bacteria. The hybrid plasmid recovered from the Leu+ bacterial transformants is identical to the original plasmid, indicating structural integrity is maintained during passage through the yeast host. These hybrid plasmids containing Physarum sequences have the same characteristics as those containing autonomously replicating yeast chromosomal sequences. As the temporal sequence of DNA replication is particularly accessible to study in Physarum plasmodia, the functional significance of these segments should be amenable to study.

Cycloheximide resistance of Physarum polycephalum

In the presence of cycloheximide, wild-type plasmodia of Physarum polycephalum exhibit an immediate decrease in deoxyribonucleic acid synthesis, a reduction in the incorporation of [3H]thymidine into thymidine triphosphate, and an increase in the level of thymidine triphosphate, as well as a decrease in protein synthesis. In this study, we have utilized a cycloheximide-resistant (Cycr) amoebic strain selected from a population of cells mutagenized with nitrosoguanidine. Segregation data indicate that the resistance is due to a single mutation. We have used this Cycr mutant to construct Cycr plasmodial strains. Ribosomes isolated from such Cycr plasmodia showed resistance to cycloheximide in vitro, in contrast to ribosomes isolated from wild-type plasmodia. The Cycr plasmodia showed none of the cycloheximide-induced biochemical effects. Plasmodia heterozygous for the resistance marker were sensitive to cycloheximide with regard to growth but showed an intermediate response in the biochemical parameters. Heterokaryons formed by fusion of various proportions of the sensitive and resistant plasmodia showed a resistance with regard to both growth and biochemical parameters which was directly related to the fraction of Cycr plasmodia present in the heterokaryons. The data are consistent with the hypothesis that the effects of cycloheximide on deoxyribonucleic acid synthesis and nucleoside metabolism are secondary to the effect of the drug on protein synthesis in this organism.

Isolation of a stimulatory factor for nuclear DNA replication

Aqueous extracts of isolated nuclei and intact plasmodia of Physarum contain a heat-stable stimulator of nuclear DNA replication. The stimulatory factor is present throughout the mitotic cycle, and its activity is unaffected by prior exposure of plasmodia to cycloheximide. The stimulatory substance has been partially purified by heat treatment, precipitation with ethanol, chromatography on DEAE cellulose, and gel filtration. The purified material contains both carbohydrate and protein, and exhibits a molecular weight of about 30 000. The active substance increases the rate and overall extent of DNA replication in S-phase nuclei, but does not trigger the initiation of DNA synthesis in nuclei isolated from G2-phase plasmodia. The stimulatory material contains little or no deoxyribonuclease or DNA polymerase activity, and it does not affect DNA polymerase activity assayed using a purified DNA template.

Temperature-sensitive mutants of Physarum polycephalum: viability, growth, and nuclear replication

Using a selfing strain of Physarum polycephalum that forms haploid plasmodia, we have isolated temperature-sensitive growth mutants in two ways. The negative selectant, netropsin, was used to enrich for temperature-sensitive mutants among a population of mutagenized amoebae, and, separately, a nonselective screening method was used to isolate plasmodial temperature-sensitive mutants among clonal plasmodia derived from mutagenized amoebae. Complementation in heterokaryons was used to sort the mutants into nine functional groups. When transferred to the restrictive temperature, two mutants immediately lysed, whereas the remainder slowed or stopped growing. Of the two lytic mutants, one affected both amoebae and plasmodia, and the other affected plasmodia alone. The growth-defective mutants were examined for protein and deoxyribonucleic acid synthesis and for aberrations in mitotic behavior. One mutant may be defective in both protein and deoxyribonucleic acid synthesis, and another only in deoxyribonucleic acid synthesis. The latter shows a striking reduction in the frequency of postmitotic reconstruction nuclei at the restrictive temperature. We believe that this mutant, MA67, is affected in a step in the nuclear replication cycle occurring late in G2. Execution of this step is necessary for both mitosis and chromosome replication.

Fluctuations in proline and other free amino acids during the mitotic cycle of the myxomycete Physarum polycephalum

1. During synchronous growth of the acellular slime mould Physarum polycephalum the free amino pool had two maxima, one of 650 units [nmol/plasmodium dry weight (mg)] at metaphase and the other of 780 units in mid G2 with minima of 550 units before and after mitosis. 2. Proline formed 20--25% of the total pool with aspartic acid, glutamic acid, threonine, valine, leucine, lysine and arginine making up 55% of the pool. 3. The fluctuation of proline during the mitotic cycle was quite different from that of the other amino acids and was transiently very low during telophase.

Size distribution and maturation of newly replicated DNA through the S and G2 phases of Physarum polycephalum

The size distribution of newly made DNA and the dynamics of size maturation of progeny DNA molecules were studied in the synchronous S and G2 phases of Physarum polycephalum. Pulse labeling of DNA and analysis of the products on alkaline sucrose gradients showed that synthesis of primary replication units (which will also be referred to as "Okazaki" fragments) occurred throughout the S period. Pulse and pulse-chase experiments revealed a distinct pattern of size maturation. An apparently linear increase in molecular weight of progeny DNA molecules during the first hour of the S phase occurred at a rate of approximately 4-5 X 10(5) daltons per min at 26 degrees C, corresponding to the joining of 6-8 Okazaki fragments. The resulting 35-45S (1.1-2.2 X 10(7) daltons) DNA molecules may correspond to the Physarum "replicon." The further size increases of the newly made DNA appear to occur in steps, possibly reflecting a clustering of isochronous replicons along the chromatide. These observations are discussed with regard to mechanisms of DNA replication and size maturation.

Sister chromatid exchange in lymphocytes from patients with malignant lymphoma

Sister chromatid exchange (SCE) frequencies were studied in differentially stained lymphocytes from 47 patients with malignant lymphoma. Thirteen patients were untreated when studied. The mean SCE frequency [+/- standard error (SE)] for these patients was 12.7 +/- 0.9 per mitosis. The mean score for 40 controls was 6.1 +/- 0.3. SCE mean scores were significantly higher in the untreated patients than in the controls (P less than 0.001). Seven patients were treated with radiotherapy alone. They demonstrated a mean SCE frequency (8.8 +/- 0.8) significantly lower (P less than 0.01) than that found in untreated patients. Eleven patients received cyclophosphamide within 4 weeks prior to study. They demonstrated a mean SCE frequency (14.3 +/- 1.3) significantly higher (P less than 0.05) than that found in patients who had received regimens that did not contain cyclophosphamide in the prior 4 weeks (11.1 +/- 1.3) or who had been off drugs for at least 8 weeks (10.1 +/- 0.8). Our data suggest that untreated patients with malignant lymphoma have elevated SCE frequencies, which may be further increased by certain chemotherapeutic agents.

Effect of cycloheximide on maturation of replicative intermediates into high-molecular-weight DNA in Tetrahymena

Replicative intermediates of discrete size (approximately 41 S) are observed in the eukaryotic organism Tetrahymena pyriformis, when the organism is grown under defined physiological conditions. The intermediates (believed to represent replicons) are synthesized and accumulated over longer periods of time (less than 90 min), if the cells are treated with low concentrations of cycloheximide. Under these conditions the rate of total DNA synthesis is only slightly inhibited (less than 15%), while maturation of intermediate DNA into high-molecular-weight DNA is completely blocked (greater than 98%). Cycloheximide appears to inhibit the maturation process more specifically than other protein synthesis inhibitors. Studies of the accumulated intermediates on alkaline buoyant density gradients demonstrate that initiation of new putative replicons occurs during treatment with cycloheximide.

Preferential binding of S-phase proteins to temporally-characteristic units of replication in Physarum polycephalum

Synchronous plasmodia of Physarum polycephalum were pulse-labeled with 3H-thymidine in early or late portions of the S-phase, and the binding capacity of the replicated DNA for isochronous S-phase plasmodial proteins assessed by nitrocellulose filter binding assay. Replication units replicating during the first one-third of the S-phase preferentially bind cytosol proteins present in plasmodia engaged in early S DNA replication, while late S replicating DNA exhibits a corresponding preferential binding of plasmodial proteins present only in late S plasmodia. Temporally-characteristic nascent replication units were isolated by Hydroxylapatite column chromatography and were found to contain binding sites for isochronous proteins.

DNA replication in Physarum polycephalum: characterization of DNA replication products made in vivo in the presence of cycloheximide in strains sensitive and resistant to cycloheximide

Synchronous plasmodia of cycloheximide-sensitive and cycloheximide-resistant strains of Physarum polycephalum were labelled with 3[H]-deoxyadenosine in pulse and pulse-chase experiments in presence and absence of cycloheximide. The replication products were studied with alkaline sucrose gradient sedimentation analysis. We show that the action of cycloheximide on DNA replication in Physarum is mediated through the ribosome, since the ribosomally located resistance also makes the plasmodial DNA replication refractile to the action of cycloheximide. Cycloheximide caused inhibition of three stages in DNA replication in the wild type: first, the formation of primary replication units ("Okazaki" size fragments), secondly, the ligation of primary units into secondary ("Replicon" size) units and thirdly, the ligation of secondary units into mature DNA.

Accumulation of replicative DNA intermediates in Tetrahymena after excision-repairable damage to DNA

Inhibition of DNA synthesis in the eukaryotic organism Tetrahymena pyriformis by thymine starvation results in the formation and accumulation of a distinct class of DNA during the first hour after re-initiation of the synthesis. This DNA, when synthesized in the presence of 5-bromodeoxyuridine (BrdUrd), bands as a distinct peak at an intermediate density of (see article) in a neutral CsCl buoyant density gradient. Following short pulses of BrdUrd, the DNA of intermediate density accounts for up to 90% of the newly synthesized DNA and binding assays on nitrocellulose filters show the presence of single-stranded regions. Alkaline buoyant density and alkaline velocity gradients clearly demonstrate the presence of non-covalently linked newly synthesized fragments with an average length shorter than that of the parental strands. Pulse experiments show that the DNA of intermediate density is progressively converted to fully hybrid (light - heavy) DNA molecules. It is therefore suggested that in response to damage caused by thymine starvation, a replicative DNA intermediate accumulates at the growing point. A similar phenomenon has also been observed after irridation with ultraviolet light. The effect of the protein synthesis inhibitor cycloheximide on the formation on the intermediate and the fully hybrid material is also described.

DNA replication in homogenates of Physarum polycephalum

Homogenates of Physarum polycephalum incorporate [3H]dATP into nuclear DNA at an initial rate of approximately 15% of the in vivo rate. To attain this level of synthesis, cultures, are homogenized in a medium containing Mg++, EGTA, glucose and spermine. Incorporation is strongly stimulated by the addition of ATP and all four deoxyribonucleoside triphosphates to homogenates prior to incubation. Various inorganic cations other than Mg++ either do not affect synthesis or are inhibitory. Incorporation is inhibited by a nonionic detergent, Triton X-100. DNA synthesis in this cell-free nuclear system is similar in several respects to that which occurs in vivo: (1) The rate of DNA synthesis in the intact organism at a given time in the mitotic cycle is reflected by the level of synthesis in homogenates prepared from cultures at that time of the cycle; (2) DNA strands labeled in vitro exhibit alkaline sucrose density gradient sedimentation properties similar to those of daughter-strand DNA pulse-labeled in vivo; and (3) Homogenates of cultures which were pre-treated with cycloheximide incorporate [3H]dATP at about 60% of the level observed in homogenates of untreated controls.

DNA replication in mammalian cells. Altered patterns of initiation during inhibition of protein synthesis

The effects of inhibition of protein synthesis by the antibiotics cycloheximide and puromycin on the initiation of DNA replication in mouse L cells were studied. Cellular DNA was pulse labeled with [3H]thymidine of high, then of low specific activity and prepared for fiber autoradiography. Autoradiograms containing multiple (up to four) replication units were analyzed. In control cells, the proportion of replication units that initiated during a 10-min, high specific activity pulse was approximately equal to the proportion initiating immediately before the pulse. The addition of cycloheximide or puromycin at the start of the pulse inhibited the frequency of initiation in that there was a decrease by up to one-third of units initiating during the pulse relative to controls. Replication direction was also altered. Addition of the antibiotics 2 h before the pulse reduced the proportion of bidirectional units observed from 0.98 to 0.70. Antibiotic treatment for 2 h also decreased initiation synchrony in that the proportion of multiunit autoradiograms on which neighboring units showed similar replication patterns (indicating temporally coordinated initiation) was reduced by one-half. These observations indicate that inhibition of protein synthesis alters the normal pattern of DNA initiation.

Premature replication of late S period DNA regions in early S nuclei transferred to late S cytoplasm by fusion in Physarum polycephalum

Fusion of a late S period plasmodium of Physarum polycephalum to an early S period plasmodium causes premature replication of late S replicating regions in the nuclei of the early S plasmodium. The extent of ahead-of-schedule replication of late S replicating regions in early S period nuclei increases to a plateau of 16-20% for fusions with 40-70 min of phase difference, then declines for larger phase differences. The stimulatory factors for late S replicative units are present only in late S plasmodia and appear to act only on late S regions. Once replicated, early S replicating regions are not stimulated to replicate again by fusion to a plasmodium entering the S period. Our data do not discriminate between anti-termination of replication by factors of stop sites on long replicons, and a sequential initiation of replication on new, possibly non-adjacent regions, but does provide evidence that the stimulatory factors are distinct from one another and specific for certain target replicative units.

Temporal order in yeast chromosome replication

Previous work with bacteria has shown that a gene is maximally sensitive to mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine (NG) at the time it is being replicated. NG was used to test for temporal order in the replication of the genome of the unicellular eucaryote, Saccaromyces cerevisiae. Yeast cells growing exponentially were more sensitive to mutagenesis by NG than cells in which DNA synthesis had been inhibited. Further, in a synchronized population of cells, individual genetic markers exhibited maximum sensitivity to muta-genesis at distinct limited intervals within the DNA synthesis period. The peaks of sensitivity are interpreted as reflecting the times of replication of different genes. Since markers for five genes on four different chromosomes showed discrete periods of maximum sensitivity, it is likely that temporal ordering of replication exists for most genes in the yeast genome. These results imply that sites for initiation of DNA replication occur at fairly specific regions along yeast chromosomal DNA moleucles, and are activated at predetermined times in the DNA synthesis period.

[Modern research on the cell cycle and mitosis in the myxomycete Physarum polycephalum]

The plasmodial stage of Physarum polycephalum contains up to 10(9) nuclei which undergo a naturally synchronous mitosis every 8 h. Nuclear processes such as DNA and RNA synthesis as well as many cytoplasmic processes such as histone synthesis are also synchronous. Physarum polycephalum is therefore widely used in studies of cell-cycle events. This article describes experiments that may help to explain two fundamental biological processes: (1) the mechanism that triggers mitosis, (2) the structural basis of mitotic movement.

A requirement for RNA, protein and DNA synthesis in the establishment of DNA replicase activity in synchronized HeLa cells

DNA replicase activity of synchronized cultures of HeLa cells was assayed by a permeable cell technique during normal S-phase and under conditions of restricted RNA, protein, or DNA synthesis. Inhibition studies with puromycin, cycloheximide, actinomycin D, and 2-mercapto-1-(beta-4-pyridethyl)benzimidazole revealed that the establishment as well as the maintenance of DNA replicase activity in S-phase cells was dependent on the continued synthesis of both RNA and protein. Measurements during limitation of DNA replication by hydroxyurea, cytosine arabinoside, or restricted availability of thymidine indicate that a low level of DNA synthesis is required to activate or assemble some subunits of DNA replicase. Evidence for the existence of short-lived RNA and protein factors essential for DNA replicase activity is discussed.