A permeable cell system for studying DNA replication in synchronized HeLa cells

Subphases of DNA replication in Drosophila cells

Exponentially growing Drosophila S2 cells in suspension culture were synchronized at low- and high-resolution centrifugal elutriation, and DNA synthesis was measured by [(3)H]-thymidine incorporation throughout the S phase. At low resolution, one repair peak at the G(1)/G(0) border and two replication peaks known as early and late S subphases were observed. At high resolution, six chronologic compartments were distinguished. The distribution of these peaks indicated one repair peak at 2.05 C value, one minor replication peak at 2.43C, and four major subphases of replication corresponding to 2.64C, 2.89C, 3.32C, and 3.60C, representing 6.7%, 3.4%, 15.3%, 20.4%, 32.1%, and 22.0% of the synthetic activity, respectively. The five major peaks of cell growth with 2.32C, 2.56C, 2.85C, 3.18C, and 3.58C values consistently preceded those of replication subphases.

Multiple subphases of DNA replication in Chinese hamster ovary (CHO-K1) cells

Replicative DNA synthesis has been measured throughout the S phase in synchronized populations of Chinese hamster ovary cells. When exponentially growing, cells in suspension cultures were subjected to counterflow centrifugal elutriation and the resolution power was increased the biphasic replication profile has been resolved and multiple subphases were distinguished. These replication peaks, termed replication checkpoints, are distributed evenly throughout the S phase. The replication checkpoints have been characterized by their average C values corresponding to 2.05, 2.12, 2.2, 2.45, 2.6, 2.8, 2.95, 3.15, 3.3, 3.45, and 3.85.

Increasing transduction efficiency of recombinant murine retrovirus vectors by initiation of endogenous reverse transcription: potential utility for genetic therapies

Reverse transcription of retroviral genomic RNA in a target cell is influenced by cellular factors, including the concentration of deoxyribonucleoside triphosphates (dNTPs). In addition, recent data have demonstrated that reverse transcription can be driven within human immunodeficiency virus type 1 virions, prior to infection of a cell, by increasing extracellular concentrations of dNTPs. In attempts to increase the transduction efficiency of recombinant murine leukemia virus vectors, endogenous reverse transcription was initiated within cell-free, recombinant murine leukemia virus virions in the presence of relatively high concentrations of dNTPs. As a result, the expression of transduced genes via these retroviral vectors was increased approximately 10-fold by treatment of virions with dNTPs. Combined with our previous data, these observations suggest that virion-associated DNA synthesis can occur in diverse groups of retroviruses and positively alter retroviral infectivity. As such, these manipulations may be useful for increasing the efficiency of retrovirus-mediated gene delivery.

Introduction and metabolism of pentose and hexose phosphates in permeabilized Morris hepatoma 5123TC cells

Metabolism of arabinose 5-P, ribose 5-P and glucose 6-P in permeabilized and resealed Morris hepatoma 5123TC cells was investigated by measuring the contribution of these compounds to nucleic acid biosynthesis. The level of [14C]-arabinose (non-phosphorylated) incorporation into nucleic acids was slight, presumably due to the low activity of the transport system or the absence or low activity of a specific 'kinase' enzyme. The permeabilizing procedure involved the brief treatment of Morris hepatoma 5123TC cells with lysolecithin and resulted in a cell population which was permeable to charged compounds i.e. sugar phosphates and nucleotides, that otherwise could not cross the plasma membrane. The permeabilized (and resealed cells) retained normal cellular morphology and intactness of specific organelles as judged by the maintenance of functional properties. Following permeabilization, these cells resealed when transferred back to normal growth medium, and continued to divide and increase at the same rates as control non-permeabilized cell cultures. The permeabilized cells incorporated deoxyribonucleotides ([methyl -3H]-TTP) into DNA at a linear rate of 0.047 nmol per 10(7) cells min-1, representing 90-100 per cent of the DNA synthesis rate in vivo. The permeabilization technique, when coupled with procedures to establish cell synchrony, permitted the comparative estimate of the contributions of [14C]-labelled arabinose 5-P, ribose 5-P and glucose 6-P to RNA, DNA, amino acids, CO2, lactate and sugar mono- and bisphosphates. The percentage of [14C]-isotope incorporated into total nucleic acids by these three labelled sugar phosphates were 2.3, 4.9 and 6.3 respectively. Possible reasons for the lower incorporation of 14C from arabinose 5-P are given. The results are consistent with the proposal that arabinose 5-P, an intermediate of the L-type pentose pathway activity of 5123TC cells, was incorporated into nucleic acids by its interconversion with ribulose 5-P and ribose 5-P and thus into PRPP. This study represents the first report of sugar phosphate as opposed to free sugar metabolism by tumour cells in culture.

In vitro DNA replication in association with the nuclear matrix of permeable mammalian cells

DNA replication has been studied in in vitro cultured bovine liver cells permeabilized in 0.02% Triton X-100. The Km for TTP was 20 microM. The initial incorporation rate at 10 microM TTP concentration was about 12% of the in vivo synthesis and declined very strongly within 1 h. A similar decline of the incorporation rate was found at 0.12 microM TTP concentration. DNAase I digestion of DNA-matrix complexes obtained from isolated nuclei in 2 M NaCl revealed that newly replicated DNA was preferentially bound to the nuclear matrix. A similar digestion with S1 nuclease caused a selective release of short duplexes of Okazaki fragments with the complementary parental strand. The results show that in vivo replication continues in permeabilized cells in an almost unchanged way, except for a gradual decline of its rate which is mainly due to inactivation of one or more essential components.

Permeable embryonic cells of sea urchins as a model for studying nucleus-cytoplasm interactions

Sea urchin embryonic cells were made permeable by treating them with glycerol solution for the purpose of exchanging cytoplasmic components. When proteoglycans prepared from embryos of advanced stages of development permeated into these glycerinated cells, these substances accumulated rapidly in the nucleus, being bound with chromatin.

DNA chain growth as a function of age in intact and permeabilized WI-38 and MRC-5 cells

The rate of incorporation of deoxythymidine triphosphate (dTTP) into acid-precipitable material of permeabilized MRC-5 and WI-38 cells as well as the rate of DNA chain growth in both intact and permeabilized cells was examined as a function of cell age. Although both the total rate of dTTP incorporation and the percentage of labeled cells decreased as cultures aged, we could detect no decrease in the rate of DNA chain growth from passages 29 to 53 for MRC-5 and from passages 34 to 50 for WI-38 cells. Since the older passages were in phase III growth and since, in our hands, the WI-38 cells used for this study senesced at passage 51, we conclude that a decrease in the rate of DNA chain growth is not related to in vitro aging.

A system of DNA replication in HeLa nuclei treated with inhibitors of protein synthesis

An in vitro DNA synthesizing system consisting os isolated nuclei from HeLa cells which had been treated with inhibitors of protein synthesis was investigated. Treatment with both 30 microgram/ml cycloheximide and 10 microgram/ml puromycin of S-phase cells reduced the rate of DNA synthesis immediately; however, the overall DNA synthesis continued for up to 4 h with a diminished rate and then ceased. In the nuclei which were isolated from the cells which had been incubated with these drugs for 6 h, little incorporation of [3H]TTP into acid-insoluble materials was observed. Addition of cytosol prepared from cells actively synthesizing DNA induced the incorporation of [3H]TTP in these nuclei, while little induction was observed by the addition of cytosol prepared from drug-treated cells in spite of the fact that the latter cytosol stimulated DNA synthesis in isolated nuclei from non-treated cells. The induced DNA synthesis was shown to require Mg2+, all four deoxyribonucleoside triphosphates and ATP, and to proceed discontinuously. The activity inducing DNA synthesis in drug-treated nuclei fluctuated with the phases in a cell cycle and it was not ascribed solely to DNA polymerase alpha nor to DNA ligase.

Replicative DNA synthesis and unscheduled DNA synthesis in permeable sarcoma cells studied by nuclease digestion

About 20% of DNA replicated in vitro in permeable mouse ascites sarcoma cells showed higher sensitivity to staphylococcal nuclease than the sensitivity of bulk DNA, and the remaining part showed the same nuclease sensitivity as that of parental chromatin DNA. The sensitivity of DNA replicated in permeable cells was higher than that of DNA newly replicated in vivo in intact cells, and close to that of DNA newly replicated in vivo in the presence of cycloheximide. Bleomycin-induced unscheduled DNA synthesis in permeable cells was highly sensitive to the nuclease. The results suggest that DNA replicated in vitro and parental nuclear protein form immature nucleosomes, probably in the same way as in vivo chromatin replication in the presence of protein synthesis inhibitors. It also appears that bleomycin-induced, unscheduled DNA synthesis occurs largely in the internucleosomal region.

An assay system for factors involved in mammalian DNA replication

An assay for cellular factors stimulating DNA synthesis by partially lysed CHO cells is presented. The assay is based on the observation that in highly lysed cells, DNA synthesis, as determined by [3H]dTTP incorporation, was only 2-5% of that in gently lysed cells, and that this low level of DNA synthesis could be increased by a factor of approx. 50 by the addition of CHO cell extract (i.e. supernatant of a cell homogenate subjected to high-speed centrifugation). Highly lysed cells were obtained by treatment with 0.1% Brij-58 and 240 mM KCl, while for the preparation of gently lysed cells, 0.01% Brij-58 and 80 mM KCl were used. Incorporation of [3H]dTTP reflected DNA synthesis qualitatively similar to that in intact cells. It was semiconservative, and no repair synthesis was detected unless cells were irradiated with ultraviolet light prior to parital lysis. DNA molecules of 4 S were synthesized and converted to DNA of more than 25 S via 6-12-S intermediates. DNA synthesis was restricted to nuclei from cells in S phase, and cell extract did not induce DNA synthesis in nuclei from cells in G1 phase. Stimulation of DNA synthesis by cell extract was concentration-dependent. Cell extract activity was recovered to more than 50% after (NH4)2SO4 precipitation. Heat-inactivation experiments suggested that cell extract contained at least tow factors timulating DNA replication. This system may, therefore, be used for the purification and characterization of factors participating in DNA replication of mammalian cells.

Dependence of mammalian DNA replication on DNA supercoiling. I. Effects of ethidium bromide on DNA synthesis in permeable Chinese hamster ovary cells

Chinese hamster ovary cells labelled with [14C]thymidine were made permeable, incubated with various concentrations of the intercalating dye ethidium bromide, and centrifuged through neutral sucrose gradients. The gradient profiles of these cells were qualitatively similar to those obtained by centrifuging DNA from untreated, lysed permeable cells through gradients containing ethidium bromide. The sedimentation distance of DNA had a biphasic dependence on the concentration of ethidium bromide, suggesting that the dye altered the amount of DNA supercoiling in situ. The effect of ethidium bromide intercalation on incorporation of [3H]dTMP into acid-precipitable material in an in vitro DNA synthesis mixture was measured. The incorporation of [3H]dTMP was unaffected by less than 1 microgram/ml of ethidium bromide, enhanced up to two-fold by 1--10 microgram/ml, and inhibited by concentrations greater than 10 micrograms/ml. Alkaline sucrose gradient analysis revealed a higher percentage of small DNA fragments (6--20 S) in the cells treated with 2 micrograms/ml ethidium bromide than in control cells. These fragments attained parental size within the same time as the fragments in control cells. In cells treated with 2 micrograms/ml ethidium bromide, a significant fraction of newly synthesized DNA resulted from new starts, whereas in untreated cells practically none of the newly synthesized DNA resulted from new starts. These results suggest that relaxation of DNA supercoiled structures ahead of the replication fork generates spurious initiations of DNA synthesis and that in intact cells the rate of chain elongation is limited by supercoiled regions ahead of the growing point.

Early changes in the synthesis of proteins with affinity for single-stranded DNA during the onset of transformation in NRK cells

Early alterations in the synthesis of proteins which bind to single-stranded DNA have been examined following the onset of transformation in NRK cells transformed by a heat-sensitive mutant (ts339) of Rous sarcoma virus. Transformation was initiated by shifting quiescent cultures from nonpermissive to permissive temperatures. Cultures were prelabelled with [3H]leucine for several generations at the non-permissive temperature, and with [35S]methionine at times after shift to the permissive temperature. Cytosol extracts were passed through sequential columns of double-stranded and single-stranded DNA bound to cellulose. Within the first hour of transformation there was an increase in the synthetic rate of proteins binding tightly to single-stranded DNA, but not to double-stranded DNA. More loosely bound protein fractions showed no such early synthetic increase. Electrophoresis of the fraction eluted from single stranded DNA-cellulose with 2 M NaCl demonstrated the presence of a major protein of 93 000 daltons, which comprised more than 0.1% of the cytosol protein. The synthesis of the 93 000 dalton protein increased continuously over the first 4 h interval after the onset of transformation. The synthetic rate of a 35 000 dalton protein, a major DNA-binding polypeptide found in mammalian cells, began to increase after a 1-h lag, following the onset of transformation. The protein fraction containing the 93 000 dalton protein had considerable unwinding activity, depressing the melting temperature of poly(dA-dT) by 39 degrees C. The protein fraction containing the bulk of the 35 000 dalton protein did not have unwinding activity. Transformation-induced DNA synthesis was measured in cells made permeable to deoxyribonucleoside triphosphates at times after shift to the permissive temperature. It was determined that synthesis of DNA began within the first 1--2 h after the onset of transformation. We conclude that the early transformation-associated synthesis of SS93 and perhaps other proteins binding to single-stranded DNA may be related to early transformation-associated changes preparatory to DNA replication and subsequent growth.

Stability of DNA replicating activity in permeabilized mouse cells during preincubation

When permeabilized cells, treated with detergent and made permeable to the nucleoside triphosphates, were preincubated briefly without nucleoside triphosphates, the activity of DNA replication was lost rapidly. This loss of DNA replicating activity was prevented when the mixture of nucleoside triphosphates (5 mM ATP and 0.1 mM each of dATP, dGTP, dCTP and TTP, the same concentrations contained in reaction mixture) was added to the permeabilized cells during the incubation. Each of deoxyribonuclesode triphosphates or ribonucleoside triphosphates, when added at 5 mM, was effective to varying degrees, but ATP was the most effective. These results suggests that there exists a process or factor(s) that requires ATP for DNA replication in mammalian cells, and that its decay during the preincubation could be prevented by ATP.

Changes of mitochondrial DNA polymerase-gamma activity in synchronized mouse cell cultures

Following synchronization by a double hydroxyurea block, mouse cell cultures exhibited a period of accelerated precursor incorporation into mitochondrial DNA during the late nuclear S phase. Peak activity of mitochondrial DNA polymerase-gamma occurred concurrent to the interval of accelerated organelle DNA synthesis. Mixing experiments suggested that the variations in mitochondrial DNA polymerase activity during the cell cycle were not due to free inhibitors in the enzyme preparations examined.

Synthesis of DNA and poly(adenosine diphosphate ribose) in normal and chronic lymphocytic leukemia lymphocytes

Peripheral blood lymphocytes were isolated from 9 patients with chronic lymphocytic leukemia (CLL) and 12 normal control donors. The cells were assayed for synthesis of DNA and poly-(adenosine diphosphate ribose) (poly[ADPR]) immediately after isolation and on successive days following their treatment with phytohemagglutinin (PHA). Two different techniques were used to measure DNA synthesis. In the standard technique, DNA synthesis was measured by incubating intact cells with [(3)H]deoxythymidine. In the new technique, the lymphocytes were first rendered permeable to nucleotides, then DNA synthesis was measured by incubating them with [(3)H]deoxythymidine triphosphate in the presence of deoxyATP, deoxyGTP, deoxyCTP, ATP, and Mg(++). Both assays showed the anticipated rise in DNA synthesis after PHA stimulation of normal cells. PHA-stimulated lymphocytes from patients with CLL demonstrated low levels of DNA synthesis in both assay systems. The initial levels of poly(ADPR) synthesis were greater in CLL lymphocytes than in normal cells. Studies with a T-cell-depleted population of normal cells showed the same activity for poly(ADPR) synthesis that was demonstrated by the original population of normal cells. PHA stimulation produced an increase in poly(ADPR) synthesis in both the normal and CLL cells. The increase in poly(ADPR) synthesis in normal cells was coincident with the increase in DNA synthesis. The increase in poly(ADPR) synthesis in the CLL cells was dissociated from the delayed and diminished increase in DNA synthesis. Thus, CLL cells have higher than normal initial levels of poly(ADPR) synthesis. Poly(ADPR) synthesis is dissociated from DNA synthesis in CLL cells whereas it varies directly with DNA synthesis in normal lymphocytes.

Assay of ribonucleotide reduction in nucleotide-permeable hamster cells

Ribonucleotide reduction was measured in Chinese hamster ovary cells made permeable to nucleotides by treatment with the detergent Tween-80. When compared to the respective ribonucleotide reductase activity in partially purified cell extracts, CDP and GDP reductase activities in permeabilized cells responded in a similar fashion to dithiothreitol, pH, MgCl2, FeCl3, substrate concentration and the presence of positive or negative allosteric effectors. At low protein concentrations both CDP and GDP reduction with whole cells increased linearly with cell number and was greater than the activity in corresponding cell extracts. Permeabilized cells were used to measure the level of CDP and GDP reductase in a hamster cell line resistant to the cytotoxic effects of hydroxyurea. The hydroxyurea-resistant cell line contained four to ten times more CDP and GDP reductase activity compared to parental or revertant cell lines. The permeabilized cell assay was also used to measure CDP and GDP reductase activities in Chinese hamster ovary cells synchronized by isoleucine starvation. CDP reductase activity was low in G1 arrested cells but increased 10-fold by 16 hours after the readdition of isoleucine to the growth medium. GDP reductase, which is present at much higher levels, is similarly induced after isoleucine addition, but only by 2-fold. The maximum activity of both CDP and GDP reductase occurred from 14 to 16 hours after isoleucine addition, which corresponded to the period of maximum DNA synthesis.

Differential effects of heparin on replicative DNA synthesis and unscheduled DNA synthesis

The effect of heparin on DNA synthesis was compared between replicative DNA synthesis and unscheduled DNA synthesis. Replicative DNA synthesis in permeable cells or nuclei prepared from rapidly growing mouse ascites sarcoma cells was inhibited by heparin. Unscheduled DNA synthesis in nuclei isolated from normal rat liver or from mouse ascites sarcoma cells in stationary phase was stimulated by heparin at low concentrations but inhibited by high heparin concentrations. DNA polymerase activity assayed with activated calf thymus DNA and DNA polymerase alpha purified partially from mouse ascites sarcoma cells was inhibited with either calf thymus histones or heparin. DNA synthesis inhibited with histones was partially reactivated by heparin. Replicative DNA synthesis in permeable cells was inhibited by adding histones to the assay mixture, and the inhibited DNA synthesis was partially reactivated by low concentrations of heparin. These results indicated that the replicated sites (or replication machinery) in permeable cells or nuclei were largely unrestricted by histones and that heparin inhibition of replicative DNA synthesis was due to the direct inhibitory interaction of heparin with some essential component(s), such as DNA polymerase, of replication machinery.

Nature of DNA synthesized in the presence of ATP in isolated nuclei

Optimal DNA synthesis in isolated Ehrlich ascites tumor cell nuclei depends upon the presence of ATP. The increased synthesis in the presence of ATP is shown to be due primarily to increased synthesis per active site rather than initiation of synthesis at different sites. The size of the short pieces observed in the presence of ATP is larger than the corresponding pieces observed in the absence of ATP.

High molecular weight DNA intermediates synthesized by permeabilized L cells

Mouse L cells rendered permeable to deoxynucleoside triphosphates synthesize DNA as an extension of replication forks that were active in the intact cells. The permeabilization process does not affect the size of the bulk cell DNA. Intermediate molecular weight DNA, synthesized in the intact cells, is neither degraded to small molecular weight DNA nor processed into bulk cell DNA following the permeabilization process. DNA synthesized by the permeable cells demonstrates a heterogenous distribution in alkaline sucrose gradients, with peaks at 26 S and 71 S. Pulse-chase experiments demonstrate that these two classes of DNA intermediates do not have a precursor product relation. They appear to be synthesized independently and at the same rate. The results are compatible with continuous synthesis of DNA along one template strand and discontinuous synthesis along the opposite strand. DNA synthesis in isolated L cell nuclei was compared to the process in permeabilized cells with the results demonstrating that the rate of DNA synthesis is slower in the nuclei than it is in the permeable cells. Alkaline sucrose gradient studies demonstrate that the DNA synthesized by isolated nuclei is smaller than the DNA synthesized by the permeable cells.

Study of DNA synthesis in chromatin isolated from HeLa cells

When incubated in vitro, HeLa cell chromatin can synthesize DNA at rate comparable to that observed with isolated nuclei. The in vitro DNA synthetic activity of chromatin reflects DNA synthesis in intact cells since chromatin from cells in S phase are several times more active thatn preparations derived from mitotic cells. The requirements for the synthesis of DNA by chromatin preparations are also similar to those of isolated nuclei and the size of the DNA pieces made in both systems is roughly comparable. The chromatin system offers several advantages not available with isolated nuclei. Chromatin will synthesize DNA for a much longer time than isolated nuclei so that larger amounts of DNA can be synthesized in vitro In addition, although chromatin has its own endogenous ability to synthesize DNA, it is markedly stimulated by the presence of exogenously added HeLa cell DNA polymerase alpha, beta, and gamma, and, thus, may provide a new template system for the study of DNA synthesis.

Synthesis of a compound soluble in organic solvents from deoxycytidine triphosphate in permeabilized normal human lymphocytes

Normal human lymphocytes may be rendered permeable to deoxynucleoside triphosphates. When [3H]dCTP is furnished to permeabilized lymphocytes tow compounds are formed: DNA and a compound soluble in organic solvents. [3H]dCTP incorporation is higher in stimulated lymphocytes than in unstimulated cells. Some characteristics of the system are reported.

DNA synthesis in detergent-treated mouse ascites sarcoma cells

Mouse ascites sarcoma cells (SR-C3H/He cells) were made permeable to nucleoside triphosphates by treatment with nonionic detergents in a nearly isotonic condition. The permeable cells synthesized DNA in the presence of the four deoxyribonucleoside triphosphates, ATP, Mg2+, and the proper ionic environment. The optimum detergent concentration for DNA synthesis was 0.015--0.020% with Triton X-100, 0.020% with Nonidet P-40, and about 0.0025% with Brij 58. Higher concentrations of detergents were rather inhibitory to DNA synthesis. DNA synthesis in Triton-permeabilized cells was thought to be replicative, and the activity in the optimum conditions was much higher than that measured in hypotonic permeable cells or in isolated nuclei. These studies show the potential usefulness of detergent treatment for examining DNA replication in mammalian cells in vitro.

Semiconservative DNA replication in vitro. I. Properties of two systems derived from mouse P-815 cells by permeabilization or lysis with Brij-58

To study DNA replication in vitro, mouse P-815 cells were either permeabilized by hypotonic treatment or gently lysed with the detergent Brij-58. In the presence of KCl, EGTA, creatine phosphokinase, creatine phosphate, sucrose, dithiothreitol, CTP, GTP, UTP, and HEPES at pH 7.8, both in vitro systems required similar concentrations of all four deoxyribonucleoside triphosphates, ATP, Mg2+, and dextran. Incorporation of [3H] dTTP was due to semiconservative DNA replication and was restricted to S-phase nuclei. No repair replication was detectable. In the first 20 min, the rate of DNA replication in vitro was 30--40% of the in vivo rate, and after 60 min, about 3% of the genome were replicated. Preexisting bulk DNA was not fragmented during permeabilization or lysis with Brij-58 and during replication in vitro. Size distributions of growing strands in vitro were similar to those found in vivo. Neither cycloheximide (100 mug/ml) nor hydroxyurea (3 mM) inhibited DNA replication in vitro.

Histone inhibition of DNA synthesis in eukaryotic cells permeable to macromolecules

Mouse L Cells, grown in suspension culture can be rendered permeable to exogenous deoxynucleoside triphosphates by a cold shock in a near isotonic buffer system. These cells use the deoxynucleotides to synthesize DNA in a semiconservative fashion. The addition of 0.05% Triton X-100 to this system increases the permeability of the cells so that exogenously supplied macromolecules gain access to the DNA. When DNAase and phosphodiesterase are added to the detergent-permeabilized cells, the cell DNA is rapidly degraded, demonstrating that the enzymes reach the DNA within the first 2 min of the incubation period. Addition of whole calf thymus histone or histone fractions to the detergent-permeabilized cells inhibits DNA synthesis. The lysine-rich histone, F is a more effective inhibitor than the arginine-rich histone, F3. The other histone fractions including the slightly lysine-rich fractions, F2a and F2b, are intermediate between F1 and F3 as inhibitors of DNA- synthesis. Kinetic analysis demonstrates that the added histones increase apparent Km and reduce V of DNA synthesis in the permeabilized cells. These studies suggest the possibility that histones alter the association of the DNA replication complex and the DNA template in a manner that reduces the rate of DNA synthesis.

Dissociation and reconstitution of the DNA replicase system of HeLa cell nuclei

The DNA replication system of S-phase HeLa nuclei has been dissociated by cautious extraction at 0 degrees C with 0.25 M NaCl. Replicase activity has been reestablished by recombination of the fractions and reduction of the salt concentration. The reconstituted system, like the starting nuclei, depended on ATP, 4dNTP, MgCl2, the proper ionic strength and the soluble cytoplasmic protein fraction. The activity of the nuclear extract showed a cell cycle dependency and was elevated in the nuclei of cells at the G1 leads to S boundary. In the presence of Mg2+ the major activity of the nuclear extract precipitated during dialysis to reduce the salt concentration; this precipitate exhibited DNA polymerase alpha activity. Chromatography of the active extracts over phosphocellulose separated the replicase supporting factors into three fractions. The major activity eluted in the fraction containing the DNA polymerase alpha activity; the other two active fractions were devoid of polymerase activity. The fraction containing DNA polymerase alpha from the nuclear extracts supported DNA replicase activity in salt-extracted nuclei whereas an equivalent level of DNA polymerase alpha from the cytoplasm was not effective. The data suggest that the DNA polymerase alpha of the salt extracts of S-phase nuclei is either different than the cytoplasmic enzyme or is associated with some essential replicase-supporting factor.

DNA replication in Chinese hamster ovary cells made permeable to nucleotides by tween-80 treatment

We have developed a method for permeabilizing CHO cells to nucleotides under conditions which allow most cells to remain viable. Permeabilized cells can carry out ATP-dependent, semiconservative synthesis of DNA. The data are consistent with the continuation of DNA synthesis in those cells in S phase at the time of treatment, possibly limited to completion of replicon synthesis without new initiations.

DNA synthesis in permeabilized mouse L cells

Mouse L cells are rendered permeable to nucleoside triphosphates by a cold shock with a near isotonic buffer. These cells retain their morphologic integrity and use exogenously supplied nucleotides and deoxynucleotides to synthesize RNA and DNA. The newly synthesized DNA is nuclear and is the product of semiconservative replication. Incorporation of deoxynucleotides into DNA by thymidine kinase-deficient cells were used to conform rigorously that the exogenously supplied deoxynucleotides were incorporated into DNA without intermediate processing through nucleosides. DNA synthesis requires the presence of Na+, ATP, all 4 deoxynucleotides, and Mg2+. The reaction is inhibited by N-ethylmaleimide, p-hydroxymercuribenzoate and actinomycin D. Hydroxy-urea and arabinosylcytosine do not inhibit the reaction whereas cytosine arabinoside triphosphate shows competitive inhibition with the deoxynucleotides. These findings indicate that the permeable cell system can be used for in situ evaluations of the replicative DNA polymerase using the endogenous DNA template.

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.