Enzyme involved in DNA replication in the axolotl. I. Analysis of the forms and activities of DNA polymerase and DNA ligase during development

Expression of DNA ligases I and II during oogenesis and early development of Xenopus laevis

We have analyzed the expression of DNA ligase I protein during oogenesis and early development of Xenopus laevis. The protein is already present in stage I oocytes and then accumulates throughout oogenesis to reach a steady state level by stage VI. It remains at this level at least until tadpole stage. In stage VI oocytes DNA ligase I protein is almost exclusively localized in the germinal vesicle. We have partially purified a DNA ligase II activity from stage VI oocytes, unfertilized eggs, and stage 8 embryos. An 80-kDa polypeptide can be specifically adenylated in all three purified extracts. It is not recognized by antibodies directed against DNA ligase I and is active on oligo(dT)-poly(rA) substrate. It could therefore represent DNA ligase II protein. The presence of both DNA ligases I and II in oocytes and embryos is inconsistent with the DNA ligase model that had been previously proposed for amphibia.

Growth and DNA replication in rabbit blastocysts

DNA content and DNA polymerase activity were measured on rabbit blastocysts removed from the uterus at 24-hr intervals over the period of days 4-7 postcoitum (pc). Median DNA content increased 53 times over the 72-hr period, from 25.3 ng on day 4 to 1,360 ng on day 7. Median DNA polymerase activity (fmole of radiolabeled nucleotide incorporated in 30 min at 37 degrees C) increased 393-fold from day 4 to day 7: 32.8 to 12,900. These embryos also increased in surface area and volume by 334-fold and 6,078-fold, respectively. Litters containing individuals with high DNA content also tended to have similar individuals with high DNA polymerase activity. Therefore, DNA polymerase activity may be a useful measure of the potential for the next cell division. A large amount of variation existed between blastocysts in all parameters measured. An analysis of variance, conducted to partition variation between litters and within litters, determined that within-litter variation was actually greater than that between litters, resulting in intraclass correlation coefficients less than 0.5. There was also a positive regression of DNA content and DNA polymerase activity on surface area in 6- and 7-day-old blastocysts after eliminating variation attributable to litters. The developmental pattern of DNA polymerase activity in the rabbit may be quantitatively different from that described in the mouse. The pattern in mammals is very different from that described in several nonmammalian species.

Reinvestigation of DNA ligase I in axolotl and Pleurodeles development

We have recently shown that the exclusion process causing the replacement of DNA ligases II by DNA ligase I in amphibian eggs after fertilization does not occur in the case of Xenopus laevis [Hardy, S., Aoufouchi, S., Thiebaud, P., and Prigent, C., (1991) Nucleic Acids Res. 19, 701-705]. Since this result is in contradiction with the situation reported in axolotl and Pleurodeles we decided to reinvestigate such results in both species. Three different approaches have been used: (1) the substrate specificity of DNA ligase I; (2) the DNA ligase-AMP adduct reaction and (3) the immunological detection using antibodies raised against the X.laevis DNA ligase I. Our results clearly demonstrate that DNA ligase I activity is associated with a single polypeptide which is present in oocyte, unfertilized egg and embryo of both amphibians. Therefore, the hypothesis of a change in DNA ligase forms, resulting from an expression of the DNA ligase I gene in axolotl and Pleurodeles early development must be rejected. We also show that, in contradiction with published data, the unfertilized sea urchin egg contains a DNA ligase activity able to join blunt ended DNA molecules.

Mechanism of inhibition of DNA ligase in Ara-C treated cells

The activity of DNA ligase, the enzyme involved in ligation of DNA fragments and also in DNA repair is inhibited by Ara-C. The exposure of two human leukemic cell lines, K562 and HL-60 to 10(-5) M Ara-C for 3 h, induces a decrease of DNA ligase activity by 40% in K562 and 92% in HL-60. This decreased activity is due to an inhibition by Ara-CTP of the ligase-adenylate complex generation, the crucial step in the action of this enzyme. The activity of the semi-purified ligase as well as the formation of ligase-adenylate complex are decreased in the presence of Ara-CTP. These results demonstrate that Ara-C via its active form Ara-CTP inhibits DNA ligase activity through the inhibition of the ligase-adenylate complex. Other inhibitors of DNA synthesis, such as hydroxyurea, do not exert the same inhibitory effect. The inhibition of DNA ligase activity may be partly responsible for the cytotoxicity of Ara-C.

Evidence for a change in expression of DNA ligase genes in the Pleurodeles waltlii germ line during gonadogenesis

The expression of DNA ligase genes was studied using the nuclear transplantation approach in the germ line of Pleurodeles waltlii (P. waltlii) just before and during gonadogenesis. Germ cell (GC) nuclei were isolated from larvae of P. waltlii and transplanted into unfertilized Ambystoma mexicanum eggs. DNA ligase activity in these eggs was then analyzed after sucrose gradient fractionation. The activity of DNA ligase I (heavy form, 7.5 S) of P. waltlii was present when the transplanted GC nuclei were isolated before the first histological appearance of gonadogenesis. At the beginning of genital ridge formation and thereafter, DNA ligase I activity was replaced by that of DNA ligase II (light form, 7 S). Expression of form I was found to be sensitive to inhibitors of translation and transcription, while that of form II was not. Therefore, the change in DNA ligase activity of the transferred nuclei of P. waltlii germ cells was assumed to be the consequence of a change in gene activity, namely, the repression of the gene encoding DNA ligase I. This change in the gene-regulated state could be linked to protein modifications of the chromatin. These results indicate that, at the beginning of gonadogenesis, germ cells receive information leading to a new state of differentiation.

Changes in the catalytic properties of DNA ligases during early sea urchin development

Two distinct DNA ligases are expressed during early sea urchin embryogenesis. A light form (50 kDa) is found in unfertilized eggs (oocyte form) and a heavier enzyme (110 kDa) is observed at the two-cell stage (embryonic form). The chronology of the change reveals that the embryonic form is detected 90 min after fertilization. After the two proteins were purified, their catalytic properties were studied using different substrates. The oocyte ligase acts only on deoxypolymers while the embryonic form also ligates heteropolymers. The two enzymes were found to undergo both nick and cohesive-end ligation. With different kinds of restriction sites it was observed that the embryonic enzyme could also ligate blunt-ended DNA. These catalytic properties account for sealing of exogenous DNA and concatenation following DNA injection into eggs. The role of the oocyte form of the enzyme is unclear; one speculation is a role in repair of DNA breaks which might accumulate during long-term sperm and oocyte storage in the gonad.

Activity of DNA and RNA polymerases in resurfacing rabbit corneal epithelium

Activity of RNA polymerases I, II and III (distinguished using alpha-amanitin) and activity of DNA polymerases alpha and beta (distinguished using N-ethylmaleimide) were assayed for varying intervals and at varying substrate (UTP or dTTP) concentrations in the purified nuclear fraction from corneal epithelium of carbamylcholine-treated and control eyes of rabbits with resurfacing acid burn defects. Incorporation was linear with time for all enzymes up to 30 min. In 10 min assays at varying substrate concentrations, all polymerases from carbamylcholine-treated eyes had significantly elevated Vmax compared to corresponding control enzymes. The drug also increased apparent affinity of RNA polymerase II for UTP and apparent affinity of DNA polymerases alpha and beta for dTTP. Results are discussed in relation to potential mechanisms by which effects of carbamylcholine on polymerase activity may be mediated.

Expression of DNA ligase genes by ram spermatid nuclei and RNA in amphibian eggs

During animal development and gametogenesis two DNA ligases are found and successively expressed. In this study the two DNA ligases present in the axolotl egg and the two ligases present during ram sperm cell maturation were distinguished by biochemical and immunological methods. The expression of the genes for the heavy and light ram DNA ligases has been studied using transplantation of spermatid and sperm nuclei in axolotl eggs. We found that ram DNA ligases were expressed in axolotl egg cytoplasm. The exclusion phenomenon between the heavy and light form of DNA ligase is species-specific and involves a cytoplasmic mediator. In the transplanted ram germ cell nuclei the heavy ram DNA ligase expression was found to be sensitive to inhibitors of transcription while the light one was not. When mRNA was used, no exclusion process was observed and both the heavy and light enzyme expression were sensitive to cycloheximide and not to aamanitin. These results are discussed in terms of the possible stability of the gene-regulated state following nuclear transfer.

Cyclic nucleotides in muscarinic regulation of DNA and RNA polymerase activity in cultured corneal epithelial cells of the rabbit

DNA and RNA polymerase activities in the purified nuclear fraction from cultured rabbit corneal epithelial cells were assayed over a range of substrate (labeled dTTP or UTP) concentrations using calf thymus DNA as template. Effects of carbamylcholine on polymerase activities were evaluated over a range of drug concentrations including those saturating muscarinic receptors. Carbamylcholine significantly (p less than 0.001) enhanced activity of both polymerases, both in nuclei incubated with the drug during assay and in nuclei from carbamylcholine-treated cells. Drug effects were blocked by atropine. Regression analysis of Hill plots for variation of polymerase activity with carbamylcholine concentration indicated half-maximal activity of both polymerases at approximately 1 microM carbamylcholine. Mechanisms by which carbamylcholine may alter polymerase activities are discussed in relation to effects of the drug on nuclear enzymes of cyclic nucleotide metabolism and on cyclic nucleotide-dependent protein phosphorylation.

Quinolone antibiotics inhibit eucaryotic DNA polymerase alpha and beta, terminal deoxynucleotidyl transferase but not DNA ligase

DNA polymerases alpha and beta, Terminal deoxynucleotidyl Transferase and DNA ligases from chicken thymus were purified to homogeneity. Quinolone antibiotics (nalidixic acid, oxolinic acid and pefloxacin ) known to inhibit DNA replication were tested for their effects on these enzymes. DNA ligase activity was not affected by the three drugs. DNA polymerases alpha and beta were inhibited by competitive mechanisms. Surprisingly, Terminal deoxynucleotidyl Transferase was strongly inhibited by the three compounds and more efficiently by nalidixic acid. The significance of these results is discussed in terms of the possible involvement of the enzymes in the respective DNA replication and repair processes.

Changes in DNA polymerase alpha, beta, gamma activities during early development of the teleost fish Misgurnus fossilis (loach)

DNA polymerase alpha, beta, gamma activities were determined after fractionation of loach (Misgurnus fossilis) cell extracts in glycerol gradients. The extracts of mature eggs, liver and testes cells yielded similar specific activities of DNA polymerase gamma (9-15 units/g protein) but differed markedly in the specific activities of DNA polymerases alpha and beta. A high activity of DNA polymerase alpha was revealed in the egg extract (2.0 X 10(3) units/g protein), while no DNA polymerase alpha activity was detected in liver cells. The specific beta-polymerase activity in the extract of mature eggs was extremely low (3.6 units/g protein), about two orders of magnitude lower than the enzyme activity in the extracts of liver or testes cells. The specific activity of DNA polymerases alpha and gamma changes insignificantly during oocyte maturation and embryogenesis up to the stage of hatching (50 h at 21.5 degrees C). As the mass of oocytes and embryos remains virtually unchanged at the stages studied, no substantial changes were observed in the alpha and gamma-polymerase activities per egg or embryo. The specific beta-polymerase activity in embryo cells increases about threefold by the completion of epiboly (20 h).

Control of DNA ligase molecular forms in nucleocytoplasmic combinations of axolotl and Pleurodeles

A light form of DNA ligase (EC 6.5.1.2), the only form present in oocytes of the axolotl (Ambystoma mexicanum), has been shown to be replaced by a heavy form of the enzyme when the egg enters cleavage. This early biochemical event has been assumed to rely on direct nuclear input. Sucrose gradient analysis permits discrimination between enzymes from axolotl and the sharp-ribbed salamander (Pleurodeles waltlii) for both heavy and light enzymatic forms of DNA ligase. Genetic activity of blastula nuclei transplanted in activated cytoplasm has been tested by determination of the enzymatic forms and specific types of DNA ligases when the implanted egg enters cleavage. A blastula nucleus of Pleurodeles in axolotl cytoplasm determines a heavy ligase of the Pleurodeles type. Conversely, a haploid androgenetic nucleus of Pleurodeles in axolotl cytoplasm controls a light ligase of the Pleurodeles type. Reciprocal experiments give homologous results. To our knowledge, this is the earliest nucleus-dependent synthesis revealed in development for any system. The heavy ligase of one species may coexist with the light form of the other species but not with the light form of its own specific type. Inhibition of the production of the heavy form for one genome results in the expression of the light form. We conclude that genetic control of DNA ligase in very early development involves structural genes for heavy and light forms of enzyme, with an exclusion process operating an alternative expression of corresponding genes. This exclusion relationship between nonallelic genes is species specific.

Isolation of the messenger RNA for 8S DNA ligase in early developing axolotl egg and its cell free translation

A new DNA ligase activity is expressed when the Axolotl eggs enter cleavage. The messenger RNA can be labelled by [3H] uridine thereby indicating its de novo synthesis. This new genetic expression is occurring just before cleavage and is the earliest found during Amphibian development. The newly synthesized [3H] mRNA can be translated in vitro in the rabbit reticulocyte lysate system. The resulting product is a 160 K protein specifically immunoprecipitated with the antiserum directed against 8S DNA ligase. This in vitro translated polypeptide exhibits 8S DNA ligase activity specific of activated or fertilized eggs but does not display 6S DNA ligase activity of non activated eggs.

DNA ligase in Axolotl egg: a model for study of gene activity control

Replacement of the light form of DNA ligase (6 S) by the heavy form (8 S) in activated egg of Axolotl has been studied as a model for change in genetic activity exerted by the female pronucleus. Nuclear transplantation shows that a blastula nucleus is able to govern the replacement of the light ligase by the heavy one. The result is not the same if the grafted nucleus is taken from an androgenetic embryo, devoid of the heavy enzyme. Therefore the change in the properties of the female pronucleus appears stable and autoreproducible. Gamma irradiations delivered at different times after activation establish that the replacement of the ligase forms depends on an intact nucleus up to 3 hr 30 min after activation, and thereafter is achieved independently of any nuclear damage. Inhibitors of DNA replication impede the change of enzymatic form in reversible process, suggesting new chromatin synthesis as prerequisite for expression of the new genetic activity. The quantitative level of DNA ligase activity does not show any dose effect when one or many nuclei are present in the same cytoplasm. However, a change in nucleotide concentration results in a change in DNA ligase activity, indicating cytoplasmic control of enzymatic regulation.