A nuclear system for DNA replication from synchronized HeLa cells

Hypoxanthine Reduces Radiation Damage in Vascular Endothelial Cells and Mouse Skin by Enhancing ATP Production via the Salvage Pathway

An effective method that can protect radiation-damaged tissues from apoptosis and promote tissue repair has not been reported to date. Hypoxanthine (Hx) is an intermediate metabolite in the purine degradation system that serves as a substrate for ATP synthesis via the salvage pathway. In this study, we focused on the transient decrease in intracellular ATP concentration after radiation exposure and examined the protective effect of Hx against radiation-induced tissue damage. Human umbilical vein endothelial cells were X irradiated, and the cell viability and incidence of apoptosis and DNA double-strand breaks (DSBs) were evaluated at different Hx concentrations. We found that in the presence of 2-100 μM Hx, the percentages of DSBs and apoptotic cells after 2, 6 and 10 Gy dose of radiation significantly decreased, whereas cell viability increased in a concentration-dependent manner. Moreover, the addition of Hx increased the levels of AMP, ADP, and ATP in the cells at 2 h postirradiation, suggesting that Hx was used for adenine nucleotide synthesis through the salvage pathway. Administration of a xanthine oxidoreductase inhibitor to a mouse model of radiation dermatitis resulted in increased blood Hx levels that inhibited severe dermatitis and accelerated recovery. In conclusion, the findings provide evidence that increasing the levels of Hx to replenish ATP could be an effective strategy to reduce radiation-induced tissue damage and elucidating the detailed mechanisms underlying the protective effects of Hx could help develop new protective strategies against radiation.

The involvement of ATP produced via (ADP-Ribose)n in the maintenance of DNA replication apparatus during DNA repair

The formation of ATP produced from poly(ADP-ribose) [(ADP-R)n] has been suggested to be required to repair damaged DNA. Here we investigate whether this ATP is involved in DNA replication processes during DNA repair. Poly(ADP-ribosyl)ated mid-S phase cell nuclei, which were isolated from synchronized HeLa S3 cells followed by the treatment with a DNA damaging agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), were revealed to retain DNA replication synthesizing activity during preincubation for de-poly(ADP-ribosyl)ation only in the presence of pyrophosphate (PPi) before DNA synthesis was started by adding 3 mM ATP. This DNA replication activity was not maintained in the presence of a potent and specific inhibitor of poly(ADP-ribose) glycohydrolase (PARG), Oenothein B (Oen B) during the preincubation with PPi. In the preincubation with PPi, muM orders of ATP was produced from (ADP-R)n. These results point to an important function of ATP generated from (ADP-R)n in nuclei for the maintenance of replication apparatus during DNA repair.

A densely methylated DNA island is associated with a chromosomal replication origin in the human RPS14 locus

We describe a 258-bp densely methylated DNA island (DMI) and chromosomal origin of bidirectional DNA replication within the transcribed portion of the human RPS14 intron 1. Together with the DMIs previously detected in two functional Chinese hamster replication origins [see Ref. 1, pp. 5636-5644], observations described in this report strengthen the correlation between densely methylated DNA islands and active mammalian chromosomal replication origins. Accordingly, DMIs may prove to be reliable physical markers for origins of bidirectional DNA replication in complex genomic DNAs of higher animals.

Inhibitory effect of prostaglandin D2 on DNA synthesis in nuclei

Prostaglandin (PG) D2 treatment inhibited DNA synthesis in isolated nuclei of mastocytoma P-815, 2-E-6 cells. On treatment with PGD2 (10 micrograms/ml), the inhibition was distinct by 8 hrs, and complete after 18 hrs. This effect of PGD2 on DNA synthesis in nuclei was not direct or mediated by cyclic AMP, but was a cell-mediated reaction. The cytoplasmic fractions of PGD2-treated and untreated cells both had stimulatory effects and their potencies were the same except for that of the cytoplasmic fraction of 8 hr-treated cells, which was less than that of the cytoplasmic fraction of untreated cells. On treatment with PGD2, inhibition of DNA synthesis in the nuclei began after 8 hrs, and this inhibition could not be reversed, even by adding the cytoplasmic fraction from untreated cells to the assay system. A nuclear salt extract prepared by adding 0.3 M NaCl to nuclei of cells that had been treated with PGD2 for 18 hrs had a much smaller stimulatory effect on DNA synthesis of salt-treated nuclei than an extract of nuclei from untreated cells. It is suggested that inhibition of cell growth by PGD2 is not mediated by intracellular cyclic AMP, but that PGD2 induces a factor(s) that inhibits nuclear DNA synthesis.

Ataxia-telangiectasia cell extracts confer radioresistant DNA synthesis on control cells

We have investigated in greater detail the radioresistant DNA synthesis universally observed in cells from patients with ataxia-telangiectasia (A-T). The approach employed in this study was to permeabilize cells with lysolecithin after gamma-irradiation and thus facilitate the introduction of cell extract into these cells. This permeabilization can be reversed by diluting the cells in growth medium. Cells treated in this way show the characteristic inhibition (control cells) or lack of it (A-T cells) after exposure to ionizing radiation. Introduction of A-T cells extracts into control cells prevented the radiation-induced inhibition of DNA synthesis normally observed in these cells. A-T cell extracts did not change the level of radioresistant DNA synthesis in A-T cells. Control cell extracts on the other hand did not influence the pattern of inhibition of DNA synthesis in either cell type. It seems likely that the agent involved is a protein because of its heat lability and sensitivity to trypsin digestion. It has a molecular weight (MW) in the range 20-30 000 D. The development of this assay system for a factor conferring radioresistant DNA synthesis on control cells provides a means of purifying this factor, and ultimately an approach to identifying the gene responsible.

The presence of intact mitochondrial DNA in HeLa cell nuclei

Restriction analysis of DNA labelled with [32P]dCTP in an in vitro replication system with isolated nuclei from early S phase cells showed preferential labelling of restriction fragments derived from mitochondrial DNA (mtDNA) by a replication machinery distinct from that responsible for bulk nuclear DNA replication. Use of restriction nucleases with one recognition site in mtDNA gave rise to 16.5 kbp long fragments corresponding to full-length linearized mtDNA, indicating the presence of intact mtDNA in the isolated cell nuclei. Incorporation of dNTPs into mtDNA was not restricted to the S phase of the cell cycle. We were unable to increase the labelling of mtDNA by the addition of purified mitochondria or mtDNA to the nuclear replication system. These and other results presented is evidenced that the presence of mtDNA in the isolated nuclei was not due to uptake during preparation, thus indicating its presence in the cell nucleus in vivo.

Nuclear protein phosphorylation in isolated nuclei from HeLa cells. Evidence that 32P incorporation from [gamma-32P]GTP is catalyzed by nuclear kinase II

A nuclear system for studying nuclear protein phosphorylation is characterized, using as phosphate donor either low levels of [gamma-32P]GTP, low levels of [gamma-32P]ATP, or low levels of labeled ATP plus excess unlabeled GTP. Since nuclear casein kinase II is the only described nuclear protein kinase to use GTP with high affinity, low levels of GTP should specifically assay this enzyme. ATP should measure all kinases, and ATP plus unlabeled GTP should measure all kinases except nuclear casein kinase II (ATP-specific kinases). The results are consistent with these predictions. In contrast with the ATP-specific activity, endogenous phosphorylation with GTP was enhanced by 100 mM NaCl, inhibited by heparin and quercetin, stimulated by polyamines, and did not use exogenous histone as substrate. The GTP- and ATP-specific kinases phosphorylated different subsets of about 20 endogenous polypeptides each. Addition of purified casein kinase II enhanced the GTP-supported phosphorylation of the identical proteins that were phosphorylated by endogenous kinase. These results support the hypothesis that activity measured with GTP is catalyzed by nuclear casein kinase II, though other minor kinases which can use GTP are not ruled out. Preliminary observations with this system suggest that the major nuclear kinases exist in an inhibited state in nuclei, and that the effects of polyamines on nuclear casein kinase II activity are substrate specific. This nuclear system is used to determine if the C-proteins of hnRNP particles, previously shown to be substrates for nuclear casein kinase II in isolated particles, is phosphorylated by GTP in intact nuclei. The results demonstrate that the C-proteins are effectively phosphorylated by GTP, but in addition they are phosphorylated by ATP-specific kinase activity.

DNA and RNA polymerase activities of nuclei and hypotonic extracts of nuclei isolated from tomato golden mosaic virus infected tobacco leaves

Nuclei and hypotonically leached extracts of nuclei prepared from tomato golden mosaic virus (TGMV)-infected Nicotiana benthamiana leaves have been used in in vitro DNA and RNA polymerisation reactions. The synthesis of virus-specific DNA was resistant to aphidicolin, sensitive to N-ethylmaleimide and dideoxy TTP, and stimulated by KC1 and ATP. Variably virion (+) and complementary (-) strand DNA of both the A and B genomic components were synthesised. Virus-specific RNA was synthesised in reactions which were initiated prior to nuclei isolation and leaching. From inhibitor studies and salt requirements RNA synthesis appeared to be catalysed by a DNA-dependent RNA polymerase type II enzyme. Both components of the TGMV genome were transcribed in a bidirectional fashion with a prevalence in some experiments of transcripts derived from DNA component A.

Isolated HeLa cell nuclei synthesize meaningful DNA

DNA replicated at the beginning of S phase was labelled by incubating nuclei isolated from cells arrested at the G/S border with radioactive deoxyribonucleoside triphosphate in a reaction mixture sustaining DNA synthesis. By hybridization against ribosomal RNA bound to nitrocellulose, the fraction of the labelled DNA which was complementary to rRNA could be quantified, and the stability of the RNA-DNA hybrids could be estimated by sequential elution of DNA at increasing temperatures. The results obtained indicate that the isolated nuclei make "meaningful" DNA, as judged by the melting characteristics of the hybrids between rRNA and the in vitro replicated DNA. Hybridization of the labelled DNA against rRNA fractionated by electrophoresis and blotted onto nitrocellulose verified the presence of sequences complementary to 18 S and 28 S rRNA.

Cellular integrity is required for inhibition of initiation of cellular DNA synthesis by reovirus type 3

Synchronized HeLa cells, primed for entry into the synthesis phase by amethopterin, were prevented from initiating DNA synthesis 9 h after infection with reovirus type 3. However, nuclei isolated from synchronized cells infected with reovirus for 9 or 16 h demonstrated a restored ability to synthesize DNA. The addition of enucleated cytoplasmic extracts from infected or uninfected cells did not affect this restored capacity for synthesis. The addition of ribonucleotide triphosphates to nuclei isolated from infected cells stimulated additional DNA synthesis, suggesting that these nuclei were competent to initiate new rounds of DNA replication. Permeabilization of infected cells did not restore the ability of these cells to synthesize DNA. Nucleoids isolated from intact or permeabilized cells, infected for 9 or 16 h displayed an increased rate of sedimentation when compared with nucleoids isolated from uninfected cells. Nucleoids isolated from the nuclei of infected cells demonstrated a rate of sedimentation similar to that of nucleoids isolated from the nuclei of uninfected cells. The inhibition of initiation of cellular DNA synthesis by reovirus type 3 appears not to have been due to a permanent alteration of the replication complex, but this inhibition could be reversed by the removal of that complex from factors unique to the structural or metabolic integrity of the infected cell.

Prevalence of cytomegalovirus antibodies in Norwegian kidney-transplant recipients and their living donors. A comparative study of two different methods

376 sera from Norwegian kidney-transplant recipients and their living, related donors, were tested for the presence of cytomegalovirus antibodies (CMV-ab) by the complement fixation test (CF), and for IgG CMV-ab by the enzyme-linked immuno-sorbent assay (ELISA). Whenever the results of the two methods differed, the sera were also tested by the indirect fluorescence antibody test (FAT). 325 of 376 sera showed corresponding results, giving a concordance of 86 per cent between CF and ELISA, ELISA being the more sensitive of the two, with a sensitivity in the same range as FAT, and high specificity with only 0.3 per cent false positive results. The prevalence of CMV-ab in Norwegian kidney-transplant recipients and their living, related donors, differed very little. The overall CMV-ab seropositivity was 76 per cent. The prevalence increased from 50 per cent at the age of 20 years, to 80 per cent at 30 years and 90 to 100 per cent from 60 years of age.

The role of RNA primer in discontinuous DNA replication in isolated nuclei from HeLa cells

RNA-primed discontinuous DNA synthesis was studied in an in vitro system consisting of washed nuclei from synchronized S-phase HeLa cells. A new technique proved useful for the purification of short nascent fragments of DNA (Okazaki fragments). Mercurated dCTP was substituted for dCTP in the DNA synthesis reaction. Short nascent pieces (4-6 S) of mercurated DNA were found to bind preferentially to sulfhydryl-agarose, and could be eluted with mercaptoethanol. The isolated fragments were assayed for the presence of covalently linked RNA by the spleen exonuclease method described by Kurosawa et al. (Kurosawa, Y., Ogawa T., Hirose, S., Okazaki, T. and Okazaki, R. (1975) J. Mol. Biol. 96, 653-664). Following a 30 s incubation with [3H]TTP in the absence of added ribonucleotides, approximately 20% of the nascent strands synthesized in washed nuclear preparations had RNA attached. These RNA primers either preexisted in the nuclei or were formed from endogenous ribonucleotides. The 5' ends of the primers appeared to be largely in a phosphorylated state. In the absence of added ribonucleotides, these RNA-DNA linkages disappeared with 2 min, whereas if ribonucleotides were added, the number of RNA primers increased to 40% and remained at this level for greater than 2 min. To obtain maximal levels of RNA primer, the addition of all three of the ribonucleotides, rCTP, rGTP, rUTP (0.1mM), as well as high levels of rATP (5mM) was required. Addition of ribonucleotides also markedly enhanced the amount of nascent DNA fragments synthesized. However, in the absence of added ribonucleotides, after RNA primers had disappeared, nascent DNA fragments were still initiated at a significant rate. These results suggest that RNA primers play an important role in the initiation of Okazaki fragments but that synthesis can also be initiated by alternative mechanisms. An important role for ATP in RNA primer synthesis is suggested.

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.

Role of cytosol proteins in DNA chain growth and chromatin replication in Friend erythroleukemia cell nuclei

The influence of cytosol proteins on the replication of DNA and chromatin in isolated nuclei from Friend erythroleukemia cells has been investigated. The overall process has been clearly shown to proceed stepwise. In the absence of cytosol proteins DNA chain growth tends to stop after the addition of approximately 200 nucleotides to the ends of growing chains. In the presence of cytosol proteins these sections grow to approximately 250 nucleotides, and participate in the stepwise extension of the replication process through adjacent nucleosomal sections of the template. Immediately following pulse labeling, the newly replicated DNA resides in a chromatin form which appears to be relatively resistant to digestion by micrococcal nuclease. During a chase interval, the association of the pulse-labeled DNA with nuclear proteins matures to a form which yields lengths of DNA upon digestion with micrococcal nuclease that correspond to mono-, di-, tri- and polynucleosomal units of chromatin. In the absence of cytosol proteins the nuclease resistant state of the labeled DNA tends to predominate and persist. The data support the view that DNA replication in a chromosomal setting proceeds stepwise over successive nucleosomal sections of template made accessible by the interaction of the cytosol proteins at or near the replication fork.

Restriction fragment primed phi X174 single-stranded DNA as template for DNA polymerase alpha and beta. Detection and partial purification of a polymerase alpha stimulating factor

Template-primers constructed of phiX174 single-stranded viral DNA hybridized to a restriction fragment of phiX174 RF DNA can be used for extensive polymerization by DNA polymerase alpha. Polymerization is dependent upon a restriction fragment containing a 3'OH. The products of the reaction have been identified by agarose gel electrophoresis. Polymerization of 150--400 nucleotides can be obtained in 1h depending upon the restriction fragment used as primer. Synthesis may be limited by barriers in the primary or secondary structure of the template. A factor which stimulates the rate of alpha polymerase activity on these templates was partially purified. This factor does not stimulate alpha polymerase on activated DNA. The stimulating factor sediments at 5.5 S in glycerol gradients containing 0.4M potassium phosphate and has an apparent molecular weight of 70 000 on Sephadex G-100.

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.

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.

Cellular polyamine depletion reduces DNA synthesis in isolated lymphocyte nuclei

The accumulation of putrescine, spermidine and spermine in activated bovine lymphocytes was blocked by the combined action of two inhibitors of polyamine biosynthesis, methylglyoxal bis(guanylhydrazone) (MGBG) and alpha-methylornithine. Lymphocytes were cultured under three conditions: (1) alpha-methylornithine alone, (2) MGBG alone, or (3) alpha-methylornithine plus MGBG. DNA synthesis in nuclei isolated from these cells was reduced from control rates by approx. 10, 55 and 75%, respectively. In each case, the degree of inhibition was similar to that observed with the intact cells. Stimulation of nuclear DNA synthesis with the postnuclear supernatant fraction was not affected by polyamine depletion of the cells. Several experiments indicate that the reduced rate of in vitro DNA synthesis was caused by the lack of polyamines and not by alternate effects of the drugs. No inhibition was observed (1) when spermidine was added to inhibited cultures 12 h before harvest and nuclear isolation, (2) when the drugs were added after polyamines had accumulated, and (3) when the drugs were added directly to the in vitro assay. In addition, the degree of inhibition of in vitro DNA synthesis correlated with the degree of polyamine deficiency. These in vitro studies confirm the results obtained with whole cells and support the hypothesis that DNA synthesis is one cellular site of action of the naturally occurring polyamines.

In vitro HeLa cell DNA synthesis. II. Partial characterization of soluble factors stimulating nuclear dna synthesis

When HeLa cells are lysed in hypotonic buffer and the lysate is fractionated by centrifugation into particulate and soluble components, the particulate component (nuclei) is deficient compared to unfractionated lysate in the extent to which it can carry out in vitro DNA synthesis. In addition, rapidly labeled short DNA chains (Okazaki pieces) accumulate in purified nuclei, and are chased into higher molecular weight DNA to a lesser degree than in unfractinated lysate. When purified nuclei were reconstituted with soluble component, the capacity of the nuclei for in vitro DNA synthesis was fully restored, as was the capacity of the nuclei for conversion of Okazaki pieces to higher molecular weight DNA. This suggests that the soluble component contains a factor or factors necessary for normal DNA replication. The major incorporation-stimulating activity was partially characterized and partially purified from the soluble component. It is heat labile, non-dialyzable, partially recoverable in the supernatant after pH 5 precipitation, found mainly in a 55--85% saturated (NH4)2SO4 fraction, and is included on Sephadex G-100. After passage through Sephadex G-100, the activity displays increased instability to storage at either 4 degrees C or --70 degrees C. Part of the activity does not bind to phosphocellulose at pH 7.2 and low salt; no additional activity can be recovered in a 0.5 M KC1 wash of the phosphocellulose column. The Okazaki-piece-joining activity was found, along with the bulk of the incorporation-stimulating activity, in the 55--85% (NH4)2SO4 fraction. These findings provide some of the groundwork for future attempts to completely purify and characterize those activities in the soluble component of cell lysates which are involved in DNA replication.

Effect of ATP analogs of DNA synthesis in isolated nuclei

Optimal synthesis of DNA in Ehrlich ascites cell nuclei is shown to be dependent upon the presence of both ATP and ADP. ATP can be replaced only by dATP. An ATP regenerating system is less effective than ATP alone or ATP in combination with ADP. ATP does not stimulate DNA synthesis primarily by maintenance of deoxyribonucleotide triphosphate levels. When the inhibition of DNA synthesis by high ATP levels is taken into account, the ATP analogs adenosine 5'-(alpha,beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)-triphosphate, and adenosine 5'-(beta, gamma-imino)triphosphate can neither substitute for ATP nor inhibit the ATP stimulation of DNA synthesis. Adenosine 5'-(3-thio)triphosphate, however, is a competitive inhibitor of DNA synthesis.

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.

The nature of DNA synthesized in nuclei from mouse L1210 cells

A DNA-synthesizing system in vitro from mouse L1210 cell nuclei is described. A high proportion of the DNA synthesized by the L1210 nuclei in vitro appeared to be covalently linked complementary DNA. DNA synthesis by L1210 nuclei was not inhibited by the anti-leukaemia compound 4'-9-acridinylamino)-methansulphon-m-anisidine, although this drug binds to isolated DNA and is active against L1210 leukaemia.

The relationship between DNA strand-scission and DNA synthesis inhibition in HeLa cells treated with neocarzinostatin

Neocarzinostatin inhibits DNA synthesis in HeLa S3 cells and induces the rapid limited breakage of cellular DNA. The fragmentation of cellular DNA appears to precede the inhibition of DNA synthesis. Cells treated with drug at 37 degrees C for 10 min and then washed free of drug show similar levels of inhibition of DNA synthesis or cell growth, or of strand-scission of DNA as when cells were not washed. If cells are preincubated with neocarzinostatin at 0 degrees C before washing, the subsequent incubation of 37 degrees C results in no inhibition of DNA synthesis or cell growth, or cutting of DNA. Isolated nuclei or cell lysates derived from neocarzinostatin-treated HeLa S3 cells are inhibited in DNA synthesis but this can be overcome in cell lysates by adding activated DNA. A cytoplasmic fraction from drug-treated cells can stimulate DNA synthesis by nuclei isolated from untreated cells, whereas nuclei from drug-treated cells are not stimulated by the cytoplasmic fraction from untreated cells. By contrast, neocarzinostatin does not inhibit DNA synthesis when incubated with isolated nuclei, but it can be shown that under these conditions the DNA is already degraded and is not further fragmented by the drug. These data suggest that the drug's ability to induce breakage of cellular DNA in HeLa S3 cells is an essential aspect of its inhibition of DNA replication and may be responsible for the cytotoxic and growth-inhibiting actions of neocarzinostatin.

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 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.

Dissociation of the DNA replicase system of bovine lymphocyte nuclei

Exposure of S-phase nuclei or subnuclear preparations from phytohemagglutinin-stimulated bovine lymphocytes to 0.02 M ATP caused an immediate and almost total loss of their ability to replicate DNA in vitro. Other ribonucleoside and deoxyribonucleoside triphosphates caused a similar inhibition of DNA replication. Levels of ATP which inhibit replication cause the release of DNA polymerases alpha and beta and small pieces of DNA from these nuclei. This release occurs both at 4 and 37 degrees C. The data support the conclusion that high levels of ATP or other nucleoside triphosphates inhibit DNA replication in nuclei by dissolution of the DNA replication complex. The limited success in reconstitution of the DNA replicase complexes is discussed.

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.