Carbonyldiphosphonate, a selective inhibitor of mammalian DNA polymerase delta

The RNA triphosphatase domain of L protein of Rinderpest virus exhibits pyrophosphatase and tripolyphosphatase activities

L protein of the Rinderpest virus, an archetypal paramyxovirus possesses RNA-dependent RNA polymerase activity which transcribes the genome into mRNAs as well as replicates the RNA genome. The protein also possesses RNA triphosphatase (RTPase), guanylyltransferase (GTase) and methyltransferase enzyme activities responsible for capping the mRNAs in a conventional pathway similar to that of the host pathway. Subsequent to the earlier characterization of the GTase activity of L protein and identification of the RTPase domain of the L protein, we report here, additional enzymatic activities associated with the RTPase domain. We have characterized the pyrophosphatase and tripolyphosphatase activities of the L-RTPase domain which are metal-dependent and proceed much faster than the RTPase activity. Interestingly, the mutant proteins E1645A and E1647A abrogated the pyrophosphatase and tripolyphosphatase significantly, indicating a strong overlap of the active sites of these activities with that of RTPase. We discuss the likely role of GTase-associated L protein pyrophosphatase in the polymerase function. We also discuss a possible biological role for the tripolyphosphatase activity hitherto considered insignificant for the viruses possessing such activity.

Exploring the role of the α-carboxyphosphonate moiety in the HIV-RT activity of α-carboxy nucleoside phosphonates

As α-carboxy nucleoside phosphonates (α-CNPs) have demonstrated a novel mode of action of HIV-1 reverse transcriptase inhibition, structurally related derivatives were synthesized, namely the malonate 2, the unsaturated and saturated bisphosphonates 3 and 4, respectively and the amide 5. These compounds were evaluated for inhibition of HIV-1 reverse transcriptase in cell-free assays. The importance of the α-carboxy phosphonoacetic acid moiety for achieving reverse transcriptase inhibition, without the need for prior phosphorylation, was confirmed. The malonate derivative 2 was less active by two orders of magnitude than the original α-CNPs, while displaying the same pattern of kinetic behavior; interestingly the activity resides in the “L”-enantiomer of 2, as seen with the earlier series of α-CNPs. A crystal structure with an RT/DNA complex at 2.95 Å resolution revealed the binding of the “L”-enantiomer of 2, at the polymerase active site with a weaker metal ion chelation environment compared to 1a (T-α-CNP) which may explain the lower inhibitory activity of 2.

Determination of growth inhibitory action point of interferon gamma on WISH cells in cell cycle progression and the window of responsiveness of the cells to the interferon

We had earlier shown that human foetal epithelial cells (WISH), growth-inhibited by interferon gamma (IFNgamma), were reversibly detained at a point prior to DNA synthesis. In the present study, we determined the window of action of IFNgamma in the G1 phase duration and the exact point of detention of WISH cells in cell cycle progression with respect to the known points of detention by the inhibitors of DNA replication initiation (aphidicolin and carbonyl diphosphonate) and of activation of replication protein A (6-dimethylaminopurine), of which RPA activation being the earlier event compared to DNA replication initiation in cell cycle progression. WISH cells, which were released from IFNgamma-induced arrest, permeabilised and exposed independently to these inhibitors show that IFNgamma detains WISH cells prior to initiation of DNA synthesis. Further, exposure of IFNalpha-synchronized (at G0/G1) or mimosine-synchronized (at G1/S) WISH cells to IFNgamma, which was added at different time points post-release from the synchronizing agent, showed that the cells were promptly responsive to the growth inhibitory action of IFNgamma only during the first 11h in G1 phase. Taken together, these results suggest that IFNgamma inhibits growth of WISH cells by detaining them at a point prior to initiation of DNA synthesis and that the IFN acts within the first 11h in G1 phase of the cell cycle.

Carbonylbisphosphonate and (diazomethylene)bisphosphonate analogues of AZT 5(')-diphosphate

A novel nucleotide analogue is described, in which the alpha,beta-phosphoric anhydride oxygen of a nucleoside 5(')-diphosphate is replaced by a carbonyl group: the carbonylbisphosphonate analogue 5 of 2('),3(')-dideoxy-3(')-azidothymidine 5(')-diphosphate (AZT 5(')-diphosphate). 5 was synthesized from tetramethyl (diazomethylene)bisphosphonate 1 via the trimethyl ester 4 of the corresponding AZT 5(')-(diazomethylene)bisphosphonate 6, which is also a new type of nucleotide analogue. The ultimate product 5 was isolated by reverse-phase HPLC, and characterized by 31P, 13C, and 1H NMR; and by high-resolution mass spectrometry. The ketone group of 5 is a visible chromophore (yellow) and reversibly forms a colorless hydrate. The ketone hydrate 'pK' is about 4.2 when excess of magnesium ion is present. The potential of such analogues as novel inhibitors of enzymes mediating nucleotide-dependent biochemical processes is discussed.

A DNA polymerase epsilon inhibitor activates the ribo and deoxyribo modes of primase expression and induces a unique phenomenon of primer accumulation

Carbonyldiphosphonate (COMDP), a selective inhibitor of DNA polymerase (pol) epsilon, strongly stimulates expression of the ribo and deoxyribo modes of primase (Pr) activities of the Pr-DNA pol alpha enzyme complex associated with special cytoplasmic nucleoprotein complexes of chicken leukemic myeloblasts [J. Ríman and A. Sulová, Acta Virol. 41 (1997) 181-214]. Besides stimulation, COMDP uncouples the Pr activities from those of DNA pol alpha, inducing in this way a unique phenomenon of accumulation of primers of basic length. In the presence of dNTPs, the COMDP effect is counteracted by excess of mimosine. The mutually exclusive effects of these agents are discussed.

Preferential inhibition of DNA polymerases alpha, delta, and epsilon from Novikoff hepatoma cells by inhibitors of cell proliferation

DNA polymerases alpha, delta and epsilon from normal regenerating rat liver and Novikoff hepatoma cells were purified about 300-fold, characterized, and checked for sensitivity towards drugs known to inhibit cell proliferation. Characterization included (a) identification of associated proteins, (b) measurement of physiochemical constants (including sedimentation coefficients, diffusion coefficients, calculation of relative molecular masses), (c) quantification of catalytic activities using specific DNA primer templates (Km values) and specific inhibitors (Ki values), and (d) discrimination between DNA polymerases from normal cells and those from malignant cells using inhibitors of cell proliferation. (a) DNA primase associated with DNA polymerase alpha, and 3'-5' exonuclease accompanying DNA polymerases delta and epsilon had similar activities. (b) Comparison of physicochemical and catalytic properties of DNA polymerases from both sources revealed similarities but also some important differences. Sedimentation and diffusion coefficients of DNA polymerases alpha and epsilon from malignant cells differed significantly. (c) The DNA-binding domain of DNA polymerases alpha and epsilon from hepatoma cells was altered since Km values, determined with several specific DNA primer-templates, were higher. Furthermore, dNTP-binding sites of DNA polymerases from malignant cells, when probed with specific inhibitors (aphidicolin, butylphenyl-dGTP, carbonyldiphosphonate, and dideoxy-TTP) showed significantly lower Ki values, indicating lower affinity to deoxyribonucleoside 5'-triphosphates. (d) Sixteen drugs representative of various modes of interaction with DNA and protein were chosen. Dose/response experiments were performed and the concentration at which the polymerizing activity was reduced to 50% was calculated (K50 values). Preferential inhibition of DNA polymerases alpha, delta, and epsilon from Novikoff hepatoma cells was found for: the intercalating drugs doxorubicin, daunorubicin, amsacrine, mitoxantrone, quinacrine and ethidium bromide, the minor-groove binders distamycin and netropsin, the ATPase-blocking agents novobiocin and coumamycin, and the topoisomerase I inhibitors camptothecin and topotecan. When the sensitivity of polymerases delta and epsilon was measured using poly(dA.dT) as a primer-template, the preferential inhibition of the enzymes from malignant cells was even more pronounced. Drugs known to trap the DNA-topoisomerase-II complex, etoposide, nalidixic acid, teniposide, and merbarone did not affect DNA polymerases irrespective of the source. Since the majority of the inhibitors used, particularly intercalators and minor-groove binders, act by modification of the primer-template, inhibition of DNA synthesis must have occurred through weakening of non-covalent bonds between DNA and catalytic polypeptides. Consequently, preferential inhibition of DNA polymerases from malignant cells seems to be indicative of abnormally diminished binding of the enzymes to their primer-templates. This effect may be caused by conformational alterations in polymerases from malignant cells which affect the DNA binding domains. Similarly, changes in physicochemical and kinetic constants are indicative of alterations of dNTP-binding domains.

Genetic evidence that aphidicolin inhibits in vivo DNA synthesis in Chinese hamster ovary cells

Using a genetic approach, Chinese hamster ovary (CHO) cells sensitive (aphS) and resistant (aphR) to aphidicolin were grown in the presence or absence of various DNA polymerase inhibitors, and the newly synthesized DNA isolated from [32P]dNMP-labelled, detergent-permeabilized cells, was characterized after fractionation by gel electrophoresis. The particular aphR mutant CHO cell line used was one selected for resistance to aphidicolin and found to possess an altered DNA polymerase of the alpha-family. The synthesis of a 24 kb replication intermediate was inhibited in wild-type CHO cells grown in the presence of aphidicolin, whereas the synthesis of this replication intermediate was not inhibited by this drug in the mutant CHO cells or in the aphidicolin-resistant somatic cell hybrid progeny constructed by fusion of wild-type and mutant cell lines. Arabinofuranosylcytosine (ara-C), like aphidicolin, inhibited the synthesis of this 24 kb DNA replication intermediate in the wild-type CHO cells but not in the aphR mutant cells. However, carbonyldiphosphonate (COMDP) inhibited the synthesis of the 24 kb replication intermediate in both wild-type and mutant cells. N2-(p-n-Butylphenyl)-2' deoxyguanisine-5'-triphosphate (BuPdGTP) was found to inhibit the formation of Okazaki fragments equally well in the wild-type and mutant cell lines and thus led to inhibition of synthesis of DNA intermediates in both cases. It appears that aphidicolin and ara-C both affect a common target on the DNA polymerase, which is different from that affected by COMDP in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA polymerases alpha, delta, and epsilon of Novikoff hepatoma cells differ from those of normal rat liver in physicochemical and catalytic properties

To investigate whether DNA replication in malignant cells deviates from that of normal cells we compared DNA polymerases alpha, delta, and epsilon from normal rat liver to the enzymes from fast-growing (malignant) Novikoff hepatoma cells. DNA polymerases were purified 300-fold by three chromatographic steps. Characterization included measurement of physicochemical constants (including sedimentation coefficients, diffusion coefficients, calculation of relative molecular masses), quantitation of catalytic activities using specific DNA primer templates (Km values) and inhibitors (Ki values), and identification of polypeptides which are strongly associated with DNA polymerases. Comparison of physicochemical and catalytic properties of DNA polymerases from both sources revealed similarities but also some important differences. DNA primase associated with DNA polymerase alpha, and 3'-5' exonuclease accompanying DNA polymerases delta and epsilon had similar activities. In contrast, the DNA-binding domain of DNA polymerases alpha and epsilon from hepatoma cells was altered since Km values, determined with the specific primer templates gapped calf thymus DNA and poly(dA.dT), were higher. Furthermore, sedimentation and diffusion coefficients, Stokes' radii, and frictional coefficient ratios of DNA polymerases alpha and epsilon from malignant cells significantly deviated. In addition, when the dNTP-binding sites were probed with specific inhibitors (aphidicolin, butylphenyl-dGTP, carbonyldiphosphonate, and dideoxy-TTP), significantly lower Ki values were obtained for the polymerases from Novikoff cells indicating lower affinity of the dNTP binding site to deoxyribonucleoside 5'-triphosphates. Altered catalytic and molecular properties are possibly a consequence of malignant transformation. It is to be expected that similar changes occur in DNA polymerases of other tumors. In particular, diminished affinity to primer templates and weakened nucleotide binding leads to lowered specificity of nucleotide selection in the base-pairing process and is therefore likely to cause an enhanced mutation rate during malignant progression.

Some phosphonic acid analogs as inhibitors of pyrophosphate-dependent phosphofructokinase, a novel target in Toxoplasma gondii

Pyrophosphate-dependent phosphofructokinase (PPi-PFK) was identified previously in Toxoplasma gondii as the only kinase that phosphorylates fructose-6-P to fructose-1,6-bisP. Since such an enzyme is not present in mammals, it was considered to be a good target for prospective selective inhibitors of the parasite. We have examined the effects of several phosphonic acid derivatives, analogs of pyrophosphate, on PPi-PFK activity, as well as on the replication of T. gondii in human foreskin fibroblast (HFF) cells. The most active compound in inhibiting PPi-PFK was tetrasodium carbonyldiphosphonate. Several bisphosphonates and related arylhydrazones showed inhibition of the enzyme, but with higher IC50 values. Although several phosphonoacetic acid derivatives also inhibited PPi-PFK, as a group they were less potent than the bisphosphonate derivatives. Comparison among the structures of various inhibitors and their effects against PPi-PFK indicates that a carbonyl (C=O) or amino (C=N) group between two phosphoryl moieties is associated with more potent enzyme inhibiton. Tetrasodium carbonyldiphosphonate did not show a significant effect against replication of T. gondii cells, probably because, as a charged molecule, it could not cross the cell membrane to reach the intracellular parasite. Tetraisopropyl carbonyldiphosphonate 2,4-dinitrophenylhydrazone showed some selective inhibitory effect against replication of the parasite in the HFF cells and protected the mammalian cells from damage by T. gondii. The results indicate that carbonyldiphosphonic acid is a good prototype compound that is amenable to chemical manipulation, which, in turn, may optimize selective inhibition of T. gondii PPi-PFK and increase accessibility to the intracellular parasite.

Effects of antiviral nucleoside analogs on human DNA polymerases and mitochondrial DNA synthesis

Inhibition constants were determined for 16 nucleoside analog triphosphates against human DNA polymerases alpha, beta, gamma, and epsilon, and 7 nucleoside analogs were examined as inhibitors of mitochondrial DNA synthesis in human Molt-4 cells in culture. The results demonstrate no clear quantitative or qualitative correlation between inhibition of DNA polymerases, particularly mitochondrial DNA polymerase gamma, and the inhibition of mitochondrial DNA synthesis in Molt-4 cell culture. Furthermore, the data indicate that inhibition of isolated DNA polymerases may not be predictive of in vitro or in vivo toxicity. Finally, it is not clear whether inhibition of mitochondrial DNA synthesis will be an accurate predictor of the potential in vivo toxicity of antiviral nucleoside analogs.

Differential inhibition of eukaryotic DNA polymerases by halenaquinol sulfate, a p-hydroquinone sulfate obtained from a marine sponge

Halenaquinol sulfate, a p-hydroquinone sulfate obtained from a marine sponge, inhibited the activity of eukaryotic DNA polymerases in varying degrees; the Ki values for DNA polymerases, alpha, beta, delta and epsilon were 1.3, 80, 17.5 and 2.0 microM, respectively, whereas it was less effective against E. coli DNA polymerase I. The inhibition occurred competitively with each of dATP and dTTP, but non-competitively with dCTP, dGTP and the template DNA. Thus, halenaquinol sulfate is demonstrated to be a potential inhibitor of DNA polymerases alpha and epsilon, and be a useful tool for analyzing the dNTP binding sites of DNA polymerases.

Inhibitor analysis of calf thymus DNA polymerases alpha, delta and epsilon

Quantitative effects of inhibitors of the replicative DNA polymerases (pol) alpha, delta and epsilon from calf thymus are reported under similar assay conditions. Carbonyldiphosphonate was a competitive inhibitor of pols delta and epsilon, with 4- to 6-fold selectivity compared to pol alpha. Aphidicolin inhibited pols alpha and delta with 6- to 10-fold selectivity compared to pol epsilon. The 'butylphenyl' nucleotides, BuPdGTP and BuAdATP, inhibited pol alpha with at least 1000-fold selectivity compared to pols delta and epsilon. The use of these inhibitors under similar assay conditions permits the discrimination of the three enzymes.

Analysis of inhibitors of bacteriophage T4 DNA polymerase

Bacteriophage T4 DNA polymerase was inhibited by butylphenyl nucleotides, aphidicolin and pyrophosphate analogs, but with lower sensitivities than other members of the B family DNA polymerases. The nucleotides N2-(p-n-butylphenyl)dGTP (BuPdGTP) and 2-(p-n-butylanilino)dATP (BuAdATP) inhibited T4 DNA polymerase with competitive Ki values of 0.82 and 0.54 microM with respect to dGTP and dATP, respectively. The same compounds were more potent inhibitors in truncated assays lacking the competitor dNTP, displaying apparent Ki values of 0.001 and 0.0016 microM, respectively. BuPdGTP was a substrate for T4 DNA polymerase, and the resulting 3'-BuPdG-primer:template was bound strongly by the enzyme. Each of the non-substrate derivatives, BuPdGDP and BuPdGMPCH2PP, inhibited T4 DNA polymerase with similar potencies in both the truncated and variable competitor assays. These results indicate that BuPdGTP inhibits T4 DNA polymerase by distinct mechanisms depending upon the assay conditions. Reversible competitive inhibition predominates in the presence of dGTP, and incorporation in the absence of dGTP leads to potent inhibition by the modified primer:template. The implications of these findings for the use of these inhibitors in the study of B family DNA polymerases is discussed.

Bacteriophage T4 and human type I DNA ligases relax DNA under joining conditions

Both bacteriophage T4 and human type I DNA ligases in the presence of a mixture of ATP, AMP and PPi altered the topological properties of a supercoiled substrate by a step-wise reaction eventually leading to a population of fully relaxed, covalently closed products. In the presence of only AMP and PPi DNA products containing nicks with 3'OH/5'P termini accumulated in the presence of bacteriophage T4 DNA ligase, suggesting reversal of the entire joining reaction, but not in the presence of human DNA ligase I. Both DNA ligases became deoxyadenylylated in the presence of dATP, but the joining reaction did not proceed to completion. However, with both enzymes the full relaxing reaction took place in the presence of dAMP alone and in the presence of a mixture of dATP, dAMP and PPi. In no case could the joining reaction be reversed by dAMP and PPi. Related experiments with modified derivatives of deoxyribonucleoside 5'-triphosphates and PPi gave results in accord with these observations. The AMP dependent DNA relaxation catalysed by DNA ligases was insensitive to the presence of exonuclease III. These results indicate that controlled relaxation of the substrate by both DNA ligases occurs as a separate reaction rather than by simple reversal of the joining reaction. These findings support the hypothesis that in vivo the DNA topoisomerising ligases relax their substrate at the replication fork both during and separately from ligation of a pre-existing nick.

Isolation and partial characterization of a high-molecular-weight DNA polymerase from Leishmania mexicana

This paper describes for the first time the isolation and characterization of a high-molecular-weight predominant DNA polymerase from the genus Leishmania, which are parasitic flagellated protozoa. Like mammalian DNA polymerase alpha, the leishmanial DNA polymerase, designated DNA polymerase A, is of high-molecular-weight, is sensitive to N-ethylmaleimide and is inhibited by high ionic strength. Unlike mammalian DNA polymerase alpha, but similar to the predominant DNA polymerase isolated from the related lower eukaryotic organisms, Trypanosoma cruzi and Crithidia fasciculata, the leishmanial DNA polymerase A is resistant to inhibition by aphidicolin, a potent inhibitor of DNA replication in mammalian cells and of DNA polymerase alpha. The DNA polymerase A was purified 28,000-fold and properties such as pH optimum, salt sensitivity, template requirements and response to DNA polymerase inhibitors were determined. A low-molecular-weight DNA polymerase was detected during the isolation procedures, but was separated from the polymerase A activity. Differences in responses to specific antisera and specific mammalian DNA polymerase alpha inhibitors suggest that the leishmanial high-molecular-weight A enzyme is sufficiently different to suggest this enzyme as a chemotherapeutic target.

Eukaryotic DNA replication

The past decade has witnessed an exciting evolution in our understanding of eukaryotic DNA replication at the molecular level. Progress has been particularly rapid within the last few years due to the convergence of research on a variety of cell types, from yeast to human, encompassing disciplines ranging from clinical immunology to the molecular biology of viruses. New eukaryotic DNA replicases and accessory proteins have been purified and characterized, and some have been cloned and sequenced. In vitro systems for the replication of viral DNA have been developed, allowing the identification and purification of several mammalian replication proteins. In this review we focus on DNA polymerases alpha and delta and the polymerase accessory proteins, their physical and functional properties, as well as their roles in eukaryotic DNA replication.

Induction of DNA polymerase activities in the regenerating rat liver

The levels of DNA polymerase alpha, DNA polymerase delta, and its accessory protein, proliferating cell nuclear antigen (PCNA) were examined in the regenerating rat liver. The levels of DNA polymerase alpha and delta activities in regenerating liver extracts were determined by the use of the DNA polymerase alpha specific inhibitor, BuAdATP [2-(p-n-butylanilino)-9-(2-deoxy-beta-D-ribofuranosyl) adenine 5'-triphosphate], and monoclonal antibodies. These reagents showed that the total DNA polymerase activities increased ca. 4-fold during regeneration and that the fraction of DNA polymerase delta activity at the peak was 40% of the total DNA polymerase activity. Immunoblots and inhibition studies using specific antibodies showed that DNA polymerase delta and epsilon and PCNA were concomitantly induced after partial hepatectomy. The levels of both DNA polymerase delta and epsilon and PCNA reached their maxima at 24-36 h post hepatectomy, i.e., at the same time that in vivo DNA synthesis reached its peak. Partial purification and characterization of DNA polymerases delta and epsilon from the regenerating rat liver were also performed. These observations suggest that the variation of DNA polymerase delta and epsilon and PCNA during liver regeneration is closely related to DNA synthesis and is consistent with their involvement in DNA replication.

Two DNA polymerases may be required for synthesis of the lagging DNA strand of simian virus 40

Agents discriminating between DNA polymerase alpha and DNA polymerases of class delta (polymerase delta or epsilon) were used to characterize steps in the synthesis of the lagging DNA strand of simian virus 40 during DNA replication in isolated nuclei. The synthesis of lagging-strand intermediates below 40 nucleotides, termed DNA primers (T. Nethanel, S. Reisfeld, G. Dinter-Gottlieb, and G. Kaufmann, J. Virol. 62:2867-2873, 1988), was selectively inhibited by butylphenyl dGTP or by neutralizing DNA polymerase alpha monoclonal antibodies. The synthesis of longer lagging chains of up to 250 nucleotides (Okazaki pieces) was affected to a lesser extent, possibly indirectly, by these agents. Aphidicolin, which inhibits both alpha- and delta-class enzymes, elicited the opposite pattern: DNA primers accumulated in its presence and were not converted into Okazaki pieces. These and previous data suggest that DNA polymerase alpha primase synthesizes DNA primers, whereas another DNA polymerase, presumably DNA polymerase delta or epsilon, mediates the conversion of DNA primers into Okazaki pieces.

DNA polymerases alpha, delta, and epsilon: three distinct enzymes from HeLa cells

DNA polymerases alpha, delta, and epsilon have been purified and characterized from the same HeLa cell extract in order to determine their relationship by comparing them from the same cell type. The catalytic properties and the primary structures of the large subunits of the DNA polymerases as compared by partial peptide mapping with N-chlorosuccinimide are different. Likewise, the small subunit of DNA polymerase epsilon appears to be distinct from the large subunit of the same polymerase and from the smaller subunits of DNA polymerase alpha. HeLa DNA polymerase delta is processive only when HeLa proliferating cell nuclear antigen is present, whereas DNA polymerase epsilon is quite processive in its absence. Inhibitor and activator spectra of DNA polymerases alpha, delta, and epsilon also distinguish the three enzymes. These results and immunologic comparisons published elsewhere support the premise that HeLa DNA polymerases alpha, delta, and epsilon are distinct enzymes that have common properties with yeast DNA polymerases I, III, and II, respectively.

Does butylphenyl-deoxyguanosine triphosphate differentially inhibit DNA polymerase alpha and delta activities in permeabilized HeLa cells?

In eukaryotic cells, two enzymes, DNA polymerases alpha and delta, are thought to play major roles in DNA synthesis. I have used butylphenyl dGTP (BuPdGTP), a potent inhibitor of purified DNA polymerase alpha, to assess the relative activities of these enzymes in two permeabilized cell systems. In both instances BuPdGTP eliminated all of the activity which was sensitive to aphidicolin. However, no conditions were found where BuPdGTP preferentially inhibited the synthesis of Okazaki fragments--the presumed products of DNA polymerase alpha activity. This implies that DNA polymerase activities on the two sides of the replication fork are unable to operate independently, being just two elements of the integrated replication machinery that undertakes DNA synthesis in permeabilized cells.

The roles of DNA polymerases alpha and delta in DNA replication

The identities and precise roles of the DNA polymerase(s) involved in mammalian cell DNA replication are uncertain. Circumstantial evidence suggests that DNA polymerase alpha and at least one form of DNA polymerase delta, that which is stimulated by Proliferating Cell Nuclear Antigen, catalyze mammalian cell replicative DNA synthesis. Further, the in vitro properties of polymerases alpha and delta suggest a model for their coordinate action at the replication fork. The present paper summarizes the current status of DNA polymerases alpha and delta in DNA replication, and describes newly available approaches to the study of those enzymes.