Control of DNA replication: Effect of spermine on DNA polymerase activity in nuclei isolated from Physarum polycephalum

A DNA polymerase with unusual properties from the slime mold Physarum polycephalum

Two forms of a DNA polymerase have been purified from microplasmodia of Physarum polycephalum by poly(ethyleneimine) precipitation and chromatography on DEAE-Sephacel, phosphocellulose, heparin Sepharose, hydroxyapatite, DNA-agarose, blue-Sepharose. They were separated from DNA polymerase alpha on phosphocellulose and from each other on heparin-Sepharose. Form HS1 enzyme was 30-40% pure and form HS2 enzyme 60% with regard to protein contents of the preparations. Form HS2 enzyme was generated from form HS1 enzyme on prolonged standing of enzyme preparations. The DNA polymerases were obtained as complexes of a 60-kDa protein associated with either a 135-kDa (HS1) or a 110-kDa (HS2) DNA-polymerizing polypeptide in a 1:1 molar stoichiometry. The biochemical function of the 60-kDa protein remained unknown. The complexes tended to dissociate during gradient centrifugation and during partition chromatography as well as during polyacrylamide gradient gel electrophoresis under nondenaturing conditions at high dilutions of samples. Both forms existed in plasmodia extracts, their proportions depending on several factors including those which promoted proteolysis. The DNA polymerases resembled eucaryotic DNA polymerase beta by several criteria and were functionally indistinguishable from each other. It is suggested that lower eucaryotes contain repair DNA polymerases, which are similar to those of eubacteria on a molecular mass basis.

The effect of polyamines of DNA synthesis in vitro

Nuclei and DNA-dependent polymerases alpha and beta were isolated from exponentially growing baby hamster kidney 21/C13 cells and were used to study the effects of polyamines on DNA synthesis in vitro. The greatest effect was observed with spermine, which inhibited both nuclear DNA synthesis and the activity of partially purified DNA polymerase alpha. At 2.5 mM spermine, the maximum concentration used, we observed 58 and 68% inhibition of DNA synthesis by isolated nuclei and polymerase alpha, respectively. In contrast, spermidine caused a small increase in nuclear DNA synthesis at low concentrations (0.5 mM) and inhibition at higher concentrations (2.5 mM); it had no significant effect on the partially purified polymerase alpha. Neither polyamine had any appreciable effect on polymerase beta activity. The results are consistent with the concept that DNA polymerase alpha catalyses the observed DNA synthesis in isolated nuclei.

Stimulation of Bacillus subtilis transformation by spermidine

Addition of spermidine in millimolar concentrations to Bacillus subtilis cells curing competence development increases transformability. The spermidine must be added at least 30 min before DNA for maximum stimulation. An incubation period of about 30 minutes is also required for the maximum uptake of labeled spermidine. The amount of DNA initially attached and the rate of DNA uptake are increased to the same extent as transformation. The rate of protein synthesis is also equivalently increased. These observations are consistent with an increase in the number of competent cells in the cell population; this increase is mediated by a spermidine-stimulated protein synthesis.

DNA synthesis in HeLa cells and isolated nuclei after treatment with an inhibitor of spermidine synthesis, methyl glyoxal bis(guanylhydrazone)

Addition of methyl glyoxal bis(guanylhydrazone) to HeLa S3 suspension cultures resulted in increased putrescine levels and decreased spermidine and spermine levels preceding a drop in incorporation of [3H]thymidine, [3H]uridine and [14C]leucine into macromolecules. When putrescine, spermidine, spermine or cadaverine was added simultaneously with methyl glyoxal bis(guanylhydrazone), the drug had no detectable effect on the synthesis of macromolecules. In nuclei isolated from cells treated with methyl glyoxal bis(guanylhydrazone) the reduction in the rate of DNA synthesis was equal to the reduction of [3H]thymidine incorporation in the corresponding whole cells. The capability of the nuclei to synthesize DNA could not be restored by adding spermidine or spermine to the system in vitro. The rate of DNA chain elongation was only reduced slightly by methyl glyoxal bis(guanylhydrazone) indicating that decreased levels of spermidine and spermine lead to a decrease in the number of replication units active in DNA synthesis within each cell.

Further characterization of a DNA polymerase activity in mouse sperm nuclei

The presence of a nuclear DNA polymerase in mouse sperm from adult testes has been confirmed and the properties of this enzyme further investigated. This activity was shown to be greatly enhanced by treating the spermatozoa with methanol or ethanol before incubation in the reaction medium or by their addition in small amounts to this medium. It was protected against degradation by nuclear proteases by adding soybean trypsin inhibitor and was stimulated by ATP. It was found to be Mg2+ dependent (optimum concentration: 7.5 mM), DNA dependent, and all four deoxynucleoside triphosphates were needed for optimal reaction. The radioactive acid-precipitable product of polymerization was not eliminated by organic solvents, nor by pronase, ribonuclease or by nuclease S1; however, it was converted to a large extent to acid-soluble products by pancreatic deoxyribonuclease. Since it was only partially solubilized by Triton X-100, it therefore did not appear to be preferentially associated with the nuclear membranes. The activity recovered after incubation depended also on the pH (optimum at pH 8.3) and did not work well in a medium for DNA polymerase alpha. The temperature for maximum incorporation of nucleotides was found to be 32 degrees C and, under our conditions, the reaction was linear for 30 min. The DNA polymerase activity was inhibited by low and high concentrations of KCl. It was not lowered by N-ethylmaleimide or p-hydroxymercuribenzoate; urea slightly stimulated the reaction and this stimulation was reversed by subsequent treatment with N-ethylmaleimide. Actinomycin D (40 mug/ml), ethidium bromide (25--50 muM), netropsin (5--50 mug/ml), and spermidine (0.5--2.5 mM) lowered the polymerization of DNA precursors. The nuclear enzyme could shift from the endogenous template to activated exogenous calf thymus DNA, the resulting nuclear radioactivity being reduced. The endogenous DNP template ability was not increased by deoxyribonuclease activation according to the method of Aposhian and Kornberg (J. Biol. Chem. (1962) 237, 519--525) suggesting that the amount of DNA polymerase associated with chromatin was probably limiting the reaction. The DNA polymerase activity detected in mouse sperm nuclei has numerous properties of low molecular weight DNA polymerases (DNA polymerase beta) reported in several eukaryotic organisms.

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 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.

Ornithine decarboxylase in cardiac hypertrophy in the rat

Ornithine decarboxylase, a possible rate-limiting enzyme in the synthesis of polyamines, was assayed in hearts of normal rats, sham-operated rats, and rats subjected to aortic constriction. In the hearts of rats with constricted aortas, significantly increased enzyme activity compared with that in the hearts of sham-operated rats was observed 2, 4, 6, and 8 hours and 3, 5, and 10 days after operation, and two peaks in activity occurred--one at 4 hours and the other at 5-10 days. Increased ornithine decarboxylase activity was one of the earliest changes associated with cardiac hypertrophy. The changes in enzyme activity correlated well with the subsequent hypertrophy in the hearts of rats with aortic constriction and with the regression in the hearts of sham-operated rats, suggesting a role for polyamines in the regulation of cardiac growth. The early increase in ornithine decarboxylase activity in hearts of rats with aortic constriction was inhibited by actinomycin D, 8-azaguanine, and cycloheximide, indicating that RNA and protein synthesis are involved in the process. Actinomycin D given 30 minutes after operation or even at the time of aortic constriction failed to inhibit the increase in the enzyme activity, suggesting that the transcription required for the increase occurs early after operation. Cycloheximide given 1 hour before the rats were killed markedly decreased the enzyme activity, and the estimated half-life of cardiac ornithine decarboxylase was comparable to that of the reported short-lived liver ornithine decarboxylase. The study suggests that the synthesis of ornithine decarboxylase is influenced at the stage of transcription by mechanical stress on the myocardium and that polyamines might have a regulatory role in cardiac hypertrophy and regression.

Estrogenic induction of ornithine decarboxylase in vivo and in vitro

Injection of estrogens (17beta-estradiol or diethylstilbestrol) into immature chicks results in a marked (30- to 50-fold) increase in the ornithine decarboxylase activity of oviductal homogenates within a 4-hour period. Similar stimulations were obtained when estrogen was injected into hypophysectomized or castrated rats and the uterus was examined for decarboxylase activity. An elevation of decarboxylase activity was obtained in vitro when oviducts from immature chicks were incubated in the presence of estrogen. These data indicate a direct action of estrogen on oviduct tissue to promote a rapid increase in the activity of a specific enzyme and represent the first example of a completely in vitro enzyme response to estrogen.