Effect of inhibiting the cellular synthesis of RNA, DNA and protein on DNA replicative activity of isolated S-phase nuclei

Distinct mechanistic responses to replication fork stalling induced by either nucleotide or protein deprivation

Incidents that slow or stall replication fork progression, collectively known as replication stress, represent a major source of spontaneous genomic instability. Here, we determine the requirement for global protein biosynthesis on DNA replication and associated downstream signaling. We study this response side by side with dNTP deprivation; one of the most commonly used means to investigate replication arrest and replicative stress. Our in vitro interrogations reveal that inhibition of translation by cycloheximide (CHX) rapidly impairs replication fork progression without decoupling helicase and polymerase activities or inducing DNA damage. In line with this, protein deprivation stress does not activate checkpoint signaling. In contrast to the direct link between insufficient dNTP pools and genome instability, our findings suggest that replication forks remain stable during short-term protein deficiency. We find that replication forks initially endure fluctuations in protein supply in order to efficiently resume DNA synthesis upon reversal of the induced protein deprivation stress. These results reveal distinct cellular responses to replication arrest induced by deprivation of either nucleotides or proteins.

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.

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.

Independent regulation of the nuclear and mitochondrial DNA synthesis induced by either Simian virus 40 or serum in mouse fibroblast 3T3 cells

Resting cultures of 3T3 cells (an established line of mouse fibroblasts) were released from density inhibition by either infection with Simian virus 40 or addition of serum. The increased rate of thymidine incorporation into DNA, induced by these two agents, was measured in the presence and in the absence of three inhibitory conditions (cycloheximide or dibutyryladenosine 3':5'-monophosphate added to the medium, or lack of anchorage). The inhibition was found to be quite similar in cultures stimulated by virus or serum; under the same conditions, however, the incorporation into mitochondrial DNA was much less inhibited than that into nuclear DNA. The experiments also suggest that new protein synthesis may not be necessary, for either virus or serum, to start the inductive mechanism.

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 synthesis in isolated HeLa cell nuclei. Optimalization of the system and characterization of the product

DNA replication in isolated nuclei from synchronized HeLa cells has been studied in an effort to optimalize the system and characterize the product. The synthesis was highly dependent on the four deoxyribonucleoside triphosphates, ATP, and Mg2+. Optimum pH was about 7.8. The system was further stimulated by monovalent ions with NH4Cl and Tris-HCl (each 65 mM) being the most effective. The four ribonucleoside triphosphates and glycerol gave a slight but very reproducible and additive stimulation. Low concentrations of spermine and spermidine (0.2-1.5 X 10(-4) M) were also slightly stimulatory (10-15%) whereas higher concentrations were inhibitory. The reaction product was DNase sensitive, and banded at 1.699 g/ml in neutral CsCl together with bulk HeLa nuclear DNA. When studied by neutral CsCl and alkaline Cs2SO4 gradients, the incorporation of [3H]TTP was mainly (more than 85%) due to further elongation of strands initiated in vivo as evidenced by BrdUrd labeling.

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.

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

Treatment of HeLa cells with a hypotonic buffer solution makes them permeable to nucleotides. Cells which are in S-phase at the time of treatment continue to synthesize DNA when supplied with the four deoxyriboside triphosphates, ATP, Mg2+, and the proper ionic environment. DNA replication extends from sites which were active in the cells prior to treatment. The product is confined to the nucleus and is sensitive to deoxyribonuclease. Under optimum conditions, up to 5% of the HeLa genome can be replicated from exogenous nucleotides. In synchronized cultures the level of DNA replicase activity, as measured in permeable cells at different points in the cell cycle, correlates with the rate of [14C] thymidine incorporation measured in the living, untreated cells.

Hydroxyurea

Hydroxyurea (HU) is an anti-leukaemia and anti-tumour drug which has also found limited application in the treatment of dermatological disorders. It is a potent inhibitor of DNA synthesis in many organisms from viruses to man and in cell culture systems. To a lesser degree it can also inhibit RNA and protein synthesis. It is anti-mitotic and cytotoxic depending on the concentration used, the duration of exposure, and the sensitivity of the organism. In most cells HU is active mainly in the S-phase of the cycle and because of the easy reversibility of its action it has been used as a synchronising agent in cell cultures with some success. There is conflicting evidence about the ability of HU to act as an inhibitor of the natural DNA repair mechanisms in cells exposed to radiation or alkylating agents. HU has, however, been shown to induce chromosome damage in a number of organisms including man. It is clearly teratogenic in mammals and can cross the placental barrier at least in rats and the golden hamster, but it is believed to have only a limited mutagenic activity. Studies involving a direct comparison of the action of HU with other compounds suggest that the = C--NHOH moiety is responsible for its biological activity.

Vitamin B12 and the macromolecular composition of Euglena. 3. Effect of cycloheximide on the recovery from B12-induced unbalanced growth

When cycloheximide is added to (B12)-deficient cultures before or after replenishment of the cells with B(12), reversion of these cells is inhibited. This inhibition is not caused by interference of the inhibitor in the uptake of B(12) as measured by division kinetics. Cycloheximide does not inhibit the initial increase in the rate of DNA synthesis caused by B(12) replenishment, but within 30-45 min the rate decreases and DNA synthesis ceases. Cycloheximide added to replenished deficient cells after completion of DNA duplication inhibits cell division. The total cellular protein and RNA in replenished cells treated with cycloheximide does not change. B(12) added to deficient cells does not stimulate the incorporation of [(14)C]leucine into protein during resumption and completion of DNA duplication. However, there is a large increase in [(14)C]leucine incorporation into the protein of these cells soon after completion of DNA duplication and before resumption of cell division. The addition of cycloheximide to B(12)-replenished or to nonreplenished deficient cells rapidly inhibits the incorporation. We suggest that the addition of B(12) accelerates the rate of DNA synthesis in the deficient cells and that possibly no new protein synthesis is required except for mitosis. However, protein synthesis is needed for continuous DNA synthesis.