The mode of action of aphidicolin on DNA synthesis in isolated nuclei

Participation of deoxyribonucleic acid polymerase alpha in amplification of ribosomal deoxyribonucleic acid in Xenopus laevis

Aphidicolin, a known inhibitor of eucaryotic deoxyribonucleic acid (DNA) polymerase alpha, efficiently inhibited amplification of ribosomal DNA during oogenesis in Xenopus laevis. DNA polymerase alpha, but not DNA polymerase gamma, as isolated from ovaries, was sensitive to aphidicolin. DNA polymerase beta was not detectable in Xenopus ovary extracts. Therefore, DNA polymerase alpha plays a major role in ribosomal ribonucleic acid gene amplification.

Azidothymidine triphosphate is an inhibitor of both human immunodeficiency virus type 1 reverse transcriptase and DNA polymerase gamma

The reverse transcriptase from human immunodeficiency virus type 1 was purified from the virus to near homogeneity. The enzyme was shown to possess both RNA-dependent and DNA-dependent DNA-synthesizing activity. Activated DNA as a heteropolymeric substrate was used as efficiently as was the homopolymeric substrate poly(rA)-oligo(dT). The Michaelis-Menten constants were determined for each of the four nucleotides needed to elongate a natural template primer. Azidothymidine triphosphate, a well-known inhibitor of the enzyme, inhibited the enzyme competitively with respect to dTTP and noncompetitively with respect to the other nucleotides. Azidothymidine triphosphate acted as an efficient inhibitor of cellular DNA polymerase gamma, whereas other enzymes of eucaryotic DNA metabolism, namely, DNA polymerase alpha-primase and DNA polymerase beta, were not inhibited. This finding may explain why some acquired immunodeficiency syndrome patients suffer side effects during azidothymidine therapy.

Microtubule-associated protein-2 stimulates DNA synthesis catalyzed by the nuclear matrix

Microtubule-associated protein-2 (MAP-2) isolated from porcine brains stimulated DNA synthesis catalyzed by the nuclear matrix isolated from Physarum polycephalum in the presence of activated DNA as exogenous templates. The degree of the stimulation depended on the amount of the nuclear matrix, but not on that of the template. MAP-2 also stimulated DNA polymerase alpha activity solubilized from nuclei, but not DNA polymerase beta activity. These results suggest that MAP-2 stimulates DNA synthesis by interacting with the putative DNA replication machinery including DNA polymerase alpha bound to the matrix. Similar stimulation occurred in the nuclear matrix isolated from HeLa and rat ascites hepatoma cells, which strongly suggests that MAP-2 is involved in the control of DNA replication in eukaryotic cells.

In situ enzymology of DNA replication and ultraviolet-induced DNA repair synthesis in permeable human cells

Using permeable diploid human fibroblasts, we have studied the deoxyribonucleoside triphosphate concentration dependences of ultraviolet- (UV-) induced DNA repair synthesis and semiconservative DNA replication. In both cell types (AG1518 and IMR-90) examined, the apparent Km values for dCTP, dGTP, and dTTP for DNA replication were between 1.2 and 2.9 microM. For UV-induced DNA repair synthesis, the apparent Km values were substantially lower, ranging from 0.11 to 0.44 microM for AG1518 cells and from 0.06 to 0.24 microM for IMR-90 cells. Control experiments established that these values were not significantly influenced by nucleotide degradation during the permeable cell incubations or by the presence of residual endogenous nucleotides within the permeable cells. Recent data implicate DNA polymerase delta in UV-induced repair synthesis and suggest that DNA polymerases alpha and delta are both involved in semiconservative replication. We measured Km values for dGTP and dTTP for polymerases alpha and delta, for comparison with the values for replication and repair synthesis. Km values for polymerase alpha were 2.0 microM for dGTP and 5.0 microM for dTTP. For polymerase delta, the Km values were 2.0 microM for dGTP and 3.5 microM for dTTP. The deoxyribonucleotide Km values for DNA polymerase delta are much greater than the Km values for UV-induced repair synthesis, suggesting that when polymerase delta functions in DNA repair, its characteristics are altered substantially either by association with accessory proteins or by direct posttranslational modification. In contrast, the deoxyribonucleotide binding characteristics of the DNA replication machinery differ little from those of the isolated DNA polymerases.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of DNA polymerase alpha by aphidicolin derivatives

17-Acetylaphidicolin was 10-fold weaker and two derivatives lacking hydroxyl groups at the 16 and 17 positions were 100-fold weaker than aphidicolin as inhibitors of DNA polymerase alpha from HeLa and Chinese hamster ovary cells. 17,18-Diacetyl, 3,17,18-triacetyl and 3-epi derivatives of aphidicolin were inactive. Active compounds were, like aphidicolin, competitive with dCTP and did not inhibit aphidicolin-resistant DNA polymerases.

Cell cycles and in vitro transdifferentiation and regeneration of isolated, striated muscle of jellyfish

Isolated, mononucleated, cross-striated muscle cells of a medusa can transdifferentiate in vitro to various new cell types and even form a complex regenerate. The transdifferentiation events follow a strict pattern. The first new cell type resembles smooth muscle and is formed without a preceding DNA replication. This cell type behaves like a stem cell and by quantal cell cycles produces all other new cell types. Some preparations develop an inner and an outer layer separated by a basal lamella. Formation of these layers does not depend on DNA replication. When layers do not form, each division results in nerve cells and smooth muscle cells. If separation into layers occurs, then a regenerate will be formed, and in the course of only two cell cycles all necessary cell types to form a functional regenerate will differentiate.

Abrogation of the effects of aphidicolin on NIH3T3 and V79 cells by caffeine

In this communication I show that caffeine (1,3,7-trimethylxanthine) stimulates [3H]thymidine incorporation in aphidicolin-treated V79 and NIH3T3 cells. Flow microfluorometric analysis showed that caffeine, partially or fully, abrogates the cell cycle progression block produced by aphidicolin. Increased cell growth is also observed in cultures treated with both aphidicolin and caffeine compared to cultures treated with aphidicolin only. Microscopic examination of V79 cultures treated with aphidicolin for 8 h showed a marked reduction in the frequency of round mitotic cells, as is expected from a drug which inhibits progression through the cell cycle by inhibiting DNA replication; this effect of aphidicolin was also reduced by caffeine. Biochemical analysis showed that caffeine did not directly interfere with the inhibition of DNA polymerase-alpha by aphidicolin. Analysis of dNTP pools indicated that caffeine increased the level of dCTP in V79 cells. In aphidicolin-treated V79 cells, the increase in the dCTP level due to exogenous cytidine was almost completely blocked; caffeine also substantially overcame this effect of aphidicolin. These results indicate that caffeine produces its effects on aphidicolin-treated cells by altering the dCTP metabolism.

In vitro transdifferentiation of striated muscle to smooth muscle cells of a medusa

Mononucleated striated muscle cells can be isolated from anthomedusae and cultivated in artificial seawater. In the cultivated muscle the differentiated state is maintained and DNA synthesis is not observed. The isolated striated muscle can be activated by collagenase treatment to transdifferentiate into various new cell types. Between the second and third day following collagenase treatment DNA synthesis is initiated, and mitosis and de novo flagellum formation occur in the isolated muscle. Under these circumstances all isolated striated muscle fragments produce both smooth muscle cells and y-cells (Schmid and Alder, 1984). In experiments, in which either transcription (actinomycin D) or translation (cycloheximide) is inhibited, the activated striated muscle cells do not transdifferentiate but maintain their differentiated state. Inhibition of DNA replication (aphidicolin), however, results in uniform transdifferentiation of striated muscle to smooth muscle cells in the absence of y-cell types (Schmid and Alder, 1984). The fluorescence stain NBD-phallacidin is used to monitor the characteristic change of F-actin pattern of these isolates.

Combined effects of aphidicolin and retinoic acid on proliferation and differentiation of human leukaemic (HL-60) cells

The relationships between replicative DNA synthesis and retinoic acid (RA)-induced differentiation of human promyelocytic leukaemic (HL-60) cells are evaluated with the use of Aphidicolin, a specific and reversible inhibitor of DNA polymerase alpha (alpha). Addition of a sublethal concentration of Aphidicolin (0.4 microM) in culture for 3 days suppresses DNA synthesis to a similar level of the resting stage (day 8) in control cultures. DNA synthesis is reactivated to the level observed in the growing stage of control cultures once Aphidicolin is removed after 3 days in culture. The level of DNA synthesis at the early stage of RA-induction (day 3) is suppressed by only 17% when compared to control cultures. The inhibitory effect of Aphidicolin on DNA synthesis in both control cultures and RA-induced cell cultures is similar. However, no reactivation of DNA synthesis is observed after removal of Aphidicolin on day 3 from RA-induced cell cultures. Flow cytometric analysis of DNA content on day 3 reveals that cells accumulate in G1 and early S phases of the cell cycle after exposure to Aphidicolin with or without RA. Of interest is the fact that, while Aphidicolin alone did not induce cells to differentiate, neither did it interfere with RA-induced cell differentiation (the rate of RA-induced cell differentiation in the presence of Aphidicolin is similar to that of RA-treated cultures in the absence of Aphidicolin). These results suggest that the combined use of Aphidicolin and RA may inhibit leukaemic cell proliferation more effectively without causing severe cytotoxicity and without interfering with RA-induced cell differentiation.

Inhibition of replicative DNA synthesis in nuclei of Strongylocentrotus intermedium urchin embryo by 2',3'-dideoxy-3'-aminonucleoside 5'-triphosphates

2',3'-Dideoxy-3'-aminonucleoside 5'-triphosphates have been shown to be inhibitors of replicative DNA synthesis in isolated nuclei of sea urchin embryo. These compounds inhibit the Okazaki fragment synthesis. The effect of 2',3'-dideoxy-3'-aminothymidine 5'-triphosphate and arabinothymidine 5'-triphosphate is reversible when adding the corresponding substrate for DNA synthesis, 2'-deoxythymidine 5'-triphosphate.

Transfection of the DNA polymerase-alpha gene

DNA polymerase-alpha is the major replicative DNA polymerase in animal cells. The gene coding for a mutant DNA polymerase-alpha was transferred from one cell to another by transfection of DNA from mutant cells. The DNA was isolated from a mutant hamster cell line resistant to aphidicolin, a specific inhibitor of DNA polymerase-alpha, and transferred into an aphidicolin-sensitive cell line. The resulting transfectants exhibited increased survival in the presence of aphidicolin and contained an aphidicolin-resistant DNA polymerase-alpha.

DNA polymerase in nuclei isolated from herpes simplex virus type-2-infected cells. Characterization of the reaction product and inhibition by substrate analogs

Nuclei isolated from herpes simplex virus (HSV) type 2-infected KB cells were examined for their capacity to serve as an in situ source of herpes DNA polymerase. In contrast to purified enzymes with added template, approx. 80% of the DNA synthesized in isolated nuclei was viral. The average size of DNA fragments labeled in vitro was 3.2 X 10(6) Da. Based on an increase in DNA density when nuclei were incubated in the presence of BrdUTP rather than dTTP, 16% of the nucleotides were added during the in vitro reaction. Sucrose gradient analysis of DNA polymerase activity in extracts of isolated nuclei demonstrated the nearly exclusive presence of herpes DNA polymerase. Km concentrations for the four dNTPs were from 0.14 to 0.55 microM. DNA synthesis was inhibited competitively by the 5'-triphosphates of ara-A and ara-C (Ki = 0.03 and 0.22 microM, respectively) but not by the 5'-triphosphate of dideoxythymidine. aATP also served as a substrate (Km = 0.014 microM) for the reaction. We conclude that nuclei from HSV-infected cells have significant advantages for the detailed study of inhibitors of herpesvirus replication.

DNA polymerase and simian virus 40 infection of resting monkey cells: induction of aphidicolin resistant alpha-polymerase

Tightly confluent monkey cell lines BSC-1 and CV-1 held in stale medium for several days exhibited an extremely low level of thymidine incorporation into cellular DNA. Yet, these cells contained a level of alpha-polymerase equal to about 15% of the level in rapidly dividing cells, and they still were capable of supporting replication of SV40 DNA. SV40 infection and culture in stale medium resulted in a four-fold induction of alpha-polymerase in CV-1 cells, whereas no change in alpha-polymerase level was observed in BSC-1 cells. Characterization of alpha-polymerase partially purified from infected CV-1 cells revealed that 80-90% of the enzyme activity was aphidicolin resistant. SV40 DNA replication in resting CV-1 cells, however, was aphidicolin sensitive. SV40 infection of resting CV-1 cells may induce an aphidicolin-resistant enzyme or lead to a modified alpha-polymerase species.

Cellular factor stimulating DNA synthesis in nuclei isolated from sea urchin embryos

DNA synthesis in isolated nuclei of morula-stage embryos of sea urchin was studied. Embryonic extracts of cleaving embryos (but not unfertilized eggs) stimulated DNA synthesis in the in vitro system. A stimulatory factor was identified which eluted at 0.52 M KCl during chromatography on DEAE-cellulose column. This factor was inactivated by heat treatment and trypsin digestion, and was resolved into three active peaks by gel filtration (Strokes radii of 6.3, 4.6, and 4.1 nm, respectively).

Specific inhibitors of eukaryotic DNA synthesis and DNA polymerase alpha, 3-deoxyaphidicolin and aphidicolin-17-monoacetate

Of several phytotoxins isolated from culture filtrates of Phoma betae Frank PS-13, an incitant of leaf spot disease of sugar beet, three have been identified as aphidicolin, 3-deoxyaphidicolin and aphidicolin-17-monoacetate. Aphidicolin is a selective inhibitor of eukaryotic DNA polymerase alpha (Ikegami et al. (1978) Nature 275, 458-460). Consequently, we studied the action mechanism of 3-deoxyaphidicolin and aphidicolin-17-monoacetate. These aphidicolin analogues markedly inhibited the in vivo DNA synthesis of sea urchin embryos and HeLa cells but not RNA and protein syntheses. Only DNA polymerase alpha, not DNA polymerase beta and gamma, was inhibited by these drugs. The mode of action of these analogues on DNA polymerase alpha from the sea urchin was competitive inhibition with respect to dCTP with Ki values of 0.44 micrograms/ml for deoxyaphidicolin and 0.89 micrograms/ml for aphidicolin monoacetate, respectively. None of the other three dNTPs competed with these drugs. A similar inhibitory mode was observed using the enzyme from HeLa cells and toad oocytes. These drugs at a concentration of 2 micrograms/ml caused a delay in the cleavage of fertilized eggs of the sea urchin and decomposition before blastulation, indicating the possibility of achromosomal cleavage because of the absence of DNA synthesis. Based on the above, it is concluded that these analogues can be used as other inhibitors of eukaryotic DNA synthesis and DNA polymerase alpha.

Effects of aphidicolin on cell proliferation, repair of potentially lethal damage and repair of DNA strand breaks in Ehrlich ascites tumour cells exposed to X-rays

The effects of aphidicolin, a specific inhibitor of DNA alpha-polymerase, have been studied on various cellular end-points and on DNA strand break repair. In the concentration range 0.02-2 micrograms/ml DNA synthesis was strongly inhibited resulting in a concomittant loss of cell proliferation ability; RNA and protein synthesis were unaffected in this range. At these concentrations PLD repair in X-irradiated plateau-phase cells was unaffected even after 7 hours treatment with aphidicolin; however, at higher concentrations (greater than 2 micrograms/ml) PLD repair was inhibited. We show that in the low concentration range (less than 2 micrograms/ml) PLD repair can be seen in exponentially growing cells and from experiments with synchronized cells we establish that the PLD repair observed can be attributed to the S-phase population, the survival of G1-cells not being affected by aphidicolin. The 'promotion' of PLD repair in exponentially growing cells was in excess of that observed for the same cells in balanced salt solution in which PLD repair is usually observed. At high concentrations (greater than 2 micrograms/ml) of aphidicolin, both X-irradiated and control S-cells were killed increasingly as concentration increased. The repair of DNA strand breaks (single and double) was unaffected in the low concentration range, but a strong inhibition was observed at high concentration. It is concluded from these results that alpha-polymerase, which is strongly inhibited at low concentrations of aphidicolin as evidenced by the inhibition of DNA synthesis, plays no major part in the repair of DNA strand breaks or in the repair of PLD.

Conversion of dNTP to dNMP dependent on DNA synthesis in isolated Yoshida sarcoma nuclei

Nuclei isolated from Yoshida sarcoma cells had activity for conversion of dGTP dependent on DNA synthesis. The ratio of nucleotide generation/generation + incorporation was 0.4 +/0- 0.1, indicating that approx. 40% of the incorporated dGMP was excised. Two lines of evidence indicated the dependence of this activity on DNA synthesis. (1) The activity was observed only in the presence of ATP, which is essential for nuclear DNA synthesis. (2) Inhibitors of DNA synthesis, such as N-ethylmaleimide, aphidicolin, spermine and KCl, also inhibited ATP- or DNA synthesis-dependent dGMP generation. Although nuclei contain nucleoside triphosphatase (N-nucleotidase), this enzyme was not involved appreciably in DNA synthesis-dependent dGMP generation. The reason for this was explained by the following findings. (a) Inhibitors did not decrease dGMP production in the complete absence of DNA synthesis. (b) Inhibitors did not inactivate N-nucleotidase to the same degree as they inhibited DNA synthesis-dependent dGMP generation. (c) Addition of ATP reduced dGMP hydrolysis catalyzed by N-nucleotidase. (d) GDP has no appreciable effect on DNA synthesis-dependent dGMP generation, but had a diluting effect on dGMP production catalyzed by N-nucleotidase. These results show that the pathway of dGMP generation in isolated nuclei was switched on addition of ATP from a N-nucleotidase-catalyzed one to a DNA polymerase-exonuclease-catalyzed one.

Characterization of the effect of aphidicolin on adenovirus DNA replication: evidence in support of a protein primer model of initiation

Adenovirus DNA replication is inhibited by aphidicolin but the inhibition clearly has different parameters than the inhibition of purified DNA polymerase alpha. In adenovirus infected Hela cells, 10 micrograms/ml of aphidicolin reduced viral DNA synthesis by 80%. Cellular DNA synthesis was inhibited by 97% at 0.1 microgram/ml. 10 micrograms/ml of drug had no effect on virus yield or late protein synthesis though higher concentrations of drug (50 micrograms/ml) caused an abrupt cessation of late protein synthesis and 100 micrograms/ml reduced virus yield by 3 logs. Concentrations of the drug from 0.5 microgram/ml to 10 micrograms/ml were found to dramatically slow the rate of DNA chain elongation in vitro but not stop it completely, so that over a long period of time net incorporation was reduced only slightly compared to the control. 50 micrograms/ml or 100 micrograms/ml of drug completely inhibited incorporation in vitro. Initiation of viral DNA replication - covalent attachment of dCMP to the preterminal protein - occurs in vitro. This reaction was found to be insensitive to inhibition by aphidicolin. We thus conclude that aphidicolin exerts its effect on adenovirus DNA chain elongation, but not on the primary initiation event of protein priming.

Aphidicolin inhibits DNA synthesis by DNA polymerase alpha and isolated nuclei by a similar mechanism

Aphidicolin is a selective inhibitor of DNA polymerase alpha. In contrast to earlier reports, the drug was found to inhibit DNA synthesis catalyzed by DNA polymerase alpha and isolated HeLa cell nuclei by a similar mechanism. For both systems aphidicolin primarily competed with dCTP incorporation. However, the apparent Vmax for dCTP incorporation was reduced by 50-60% at relatively low concentrations of aphidicolin, thus the mechanism of inhibition is complex. Furthermore, a 2-5 fold increase in apparent Km for dTTP was observed in the presence of aphidicolin, but the apparent Km values for dATP and dGTP were essentially unaltered. This, together with additional evidence, suggested that the mechanism of action of aphidicolin involves a strong competition with dCMP incorporation, a weaker competition with dTMP incorporation and very little, if any, competition with dGMP and dAMP incorporation.

Studies on differentiation without cleavage in Chaetopterus: effects of inhibition of DNA synthesis with aphidicolin

The effects of aphidicolin - a powerful inhibitor of DNA polymerase alpha and of DNA replication - on normal development and on differentiation without cleavage of Chaetopterus eggs have been studied with cytological, cytochemical, and biochemical methods. The experiments show that the initial period of pseudocleavage can take place in the absence of nuclear DNA synthesis, but further development (segregation, hatching, ciliation) requires DNA synthesis. However ciliated unicellular larvae can be obtained under conditions where the DNA content of the embryos in only 40% of the controls. In fertilized eggs, aphidicolin immediately stops cleavage. The significance of these results is discussed.

Participation of deoxyribonucleic acid polymerase alpha in amplification of ribosomal deoxyribonucleic acid in Xenopus laevis

Aphidicolin, a known inhibitor of eucaryotic deoxyribonucleic acid (DNA) polymerase alpha, efficiently inhibited amplification of ribosomal DNA during oogenesis in Xenopus laevis. DNA polymerase alpha, but not DNA polymerase gamma, as isolated from ovaries, was sensitive to aphidicolin. DNA polymerase beta was not detectable in Xenopus ovary extracts. Therefore, DNA polymerase alpha plays a major role in ribosomal ribonucleic acid gene amplification.

Isolation and characterization of a UV-sensitive hypermutable aphidicolin-resistant Chinese hamster cell line

Aphidicolin is a specific inhibitor of DNA polymerase alpha and blocks DNA synthesis in vivo. The inhibition of purified alpha-polymerase has been shown to be competitive with dCTP but not with the other three deoxynucleoside triphosphates (dNTPs). In order to study the various roles that the alpha-polymerase might play in DNA replication and/or repair, we have attempted to isolate Chinese hamster V79 cells that are resistant to aphidicolin. Four resistant mutants were isolated from BrdU--black light- and UV-mutagenized cells. None of the mutants isolated contains an alpha-polymerase that is resistant, in crude extract measurements, to aphidicolin. Three mutants isolated, however, were found to be resistant to araC. Two mutants tested were found to be sensitive to cytidine and have elevated levels of dCTP or all 4 dNTPs. These results indicate that they are nucleotide pool mutants instead of alpha-polymerase mutants. One mutant, aphr-4, is characterized by the following: (1) high level of dCTP; (2) thymidine (or CdR, UdR) auxotrophic; (3) sensitive to thymidine (and AdR, GdR); (4) slow-growing; (5) cytidine sensitive; (6) UV sensitive and hypermutable at the ouabain-resistant locus; and (7) a ninefold increase in frequency of chromatid gaps and breaks when cells are exposed to BrdU-containing medium. Revertants of aphr-4 which are partially aphidicolin-resistant and retain the first three characteristics listed above, but not the others, have been isolated. The appearance of this type of revertant indicates that either aphr-4 or its "revertant" is a double mutant.

Selection of aphidicolin-resistant CHO cells with altered levels of ribonucleotide reductase

Chinese hamster ovary cells were initially selected for resistance to aphidicolin at 0.3 microgram/ml. Serial cultivation with aphidicolin at concentrations up to 5.0 micrograms/ml yielded a series of mutants with increasing resistance. The most resistant mutant isolated was 44 times more resistant to aphidicolin than the parental CHO. The alpha-polymerases, assayed in the cytoplasmic extracts of the mutants, did not increase in specific activity or differ from the parental CHO in their sensitivity to aphidicolin. When cultured in the presence of deoxythymidine, deoxyadenosine, and 1-beta-D-arabinofuranosyl cytosine (araC) the mutants showed considerably more resistance to these inhibitors than did the parental CHO. The intracellular pools of all four deoxynucleoside triphosphates (dNTPs) in the mutants increased with increasing resistance to aphidicolin. The elevated dNTP pools in the mutant most resistant to aphidicolin appear to be the result of a 4- to 8-fold increase in the level of ribonucleotide reductase (2'-deoxyribonucleoside diphosphate:oxidized thioredoxin 2'-oxidoreductase, EC 1.17.4.1).

Studies on the inhibition of highly purified calf thymus 8S and 7.3S DNA polymerase alpha by aphidicolin

On activated DNA aphidicolin competitively inhibits the incorporation of dCMP by both calf thymus DNA polymerase alpha A2 and C enzymes and inhibits the incorporation of the other three deoxynucleoside monophosphates apparently non-competitively. However, aphidicolin does not inhibit the incorporation of dAMP into poly(dT) . oligo(A)10 nor does it inhibit the incorporation of dGMP into poly(dC) . oligo(dG)10, but, it does competitively inhibit the incorporation of dTMP into poly(dA) . oligo(dT)10.

Alteration of ribonucleotide reductase in aphidicolin-resistant mutants of mouse FM3A cells with associated resistance to arabinosyladenine and arabinosylcytosine

Aphidicolin-resistant mutants of mouse FM3A cells were isolated and characterized. Most of the mutants were of a type showing associated resistance to arabinosyladenine, arabinosylcytosine, deoxyadenosine, and excess thymidine. This phenotype could also be observed in a variant line selected by resistance to a low level of arabinosylcytosine. In cell-cell hybrids, aphidicolin resistance as well as this cross-resistance behaved a codominant traits. The mutants had an increased dATP pool and decreased ability to incorporate labeled deoxycytidine into macromolecules. Genetic and biochemical evidence suggested that the mutation conferring the pleiotropic phenotype resulted from a change in ribonucleotide reductase activity such that the enzyme was desensitized to the allosteric negative effector dATP. This alteration of the enzyme could account for the marked change in deoxynucleotide pools and for the aphidicolin resistance of the mutants.

Differential effects of aphidicolin on replicative DNA synthesis and unscheduled DNA synthesis in permeable mouse sarcoma cells

Aphidicolin clearly discriminated replicative DNA synthesis from unscheduled DNA synthesis. Aphidicolin inhibited replicative DNA synthesis in permeable mouse ascites sarcoma cells. The mode of inhibition of aphidicolin was a mixed type with respect to deoxycytidine triphosphate but was non-competitive with respect to the other three deoxynucleoside triphosphates. Aphidicolin did not affect the activity of unscheduled DNA synthesis in either bleomycin-treated permeable sarcoma cells or isolated rat liver nuclei. Considering the difference in sensitivity of DNA polymerase alpha and beta to aphidicolin, and other related information reported previously, the results are compatible with the idea that DNA polymerase alpha is involved in replicative DNA synthesis and DNA polymerase beta in unscheduled DNA synthesis in the present systems.

Mechanism of inhibition of herpes simplex virus and vaccinia virus DNA polymerases by aphidicolin, a highly specific inhibitor of DNA replication in eucaryotes

The inhibition in vitro of herpes simplex virus 1 and vaccinia virus DNA polymerases by aphidicolin is primarily noncompetitive with dGTP, dATP, dTTP, DNA, and Mg2+ and competitive with dCTP in analogy with the mode of inhibition of cellular alpha-polymerase. The degree of inhibition of viral or cellular growth in vivo can be quantitatively predicted by the degree of inhibition of the isolated replicative DNA polymerases at the same concentration of aphidicolin in suitable conditions (limiting dCTP concentration). Thus, the only in vivo target for aphidicolin is probably the replicative DNA polymerase, and aphidicolin is a highly specific inhibitor of replicative nuclear DNA synthesis in eucaryotes. This, coupled with the lack of mutagenic effect, represents a valuable property for an anticancer drug. The specificity of inhibition (contrary to the aspecific effect on almost all DNA polymerases by a true competitive inhibitor, such as 1-beta-D-arabinofuranosylcytidine 5'-triphosphate) and the structure of the drug, which does not resemble that of the triphosphates, suggest that aphidicolin must recognize a site common only to the replicative DNA polymerases of eucaryotes and different from the binding site for deoxyribonucleic triphosphates and DNA, which should be similar in reparative and procaryote-type DNA polymerase; the aphidicolin binding site is probably very near to, or even overlaping with, the binding site for dCTP so that the drug mimics a competitive effect with this nucleotide.