Purification of a primase activity associated with DNA polymerase alpha from HeLa cells

DNA Damage Tolerance by Eukaryotic DNA Polymerase and Primase PrimPol

PrimPol is a human deoxyribonucleic acid (DNA) polymerase that also possesses primase activity and is involved in DNA damage tolerance, the prevention of genome instability and mitochondrial DNA maintenance. In this review, we focus on recent advances in biochemical and crystallographic studies of PrimPol, as well as in identification of new protein-protein interaction partners. Furthermore, we discuss the possible functions of PrimPol in both the nucleus and the mitochondria.

Archaeal primase: bridging the gap between RNA and DNA polymerases

In the evolution of life, DNA replication is a fundamental process, by which species transfer their genetic information to their offspring. DNA polymerases, including bacterial and eukaryotic replicases, are incapable of de novo DNA synthesis. DNA primases are required for this function, which is sine qua non to DNA replication. In Escherichia coli, the DNA primase (DnaG) exists as a monomer and synthesizes a short RNA primer. In Eukarya, however, the primase activity resides within the DNA polymerase alpha-primase complex (Pol alpha-pri) on the p48 subunit, which synthesizes the short RNA segment of a hybrid RNA-DNA primer. To date, very little information is available regarding the priming of DNA replication in organisms in Archaea. Available sequenced genomes indicate that the archaeal DNA primase is a homolog of the eukaryotic p48 subunit. Here, we report investigations of a p48-like DNA primase from Pyrococcus furiosus, a hyperthermophilic euryarchaeote. P. furiosus p48-like protein (Pfup41), unlike hitherto-reported primases, does not catalyze by itself the synthesis of short RNA primers but preferentially utilizes deoxynucleotides to synthesize DNA fragments up to several kilobases in length. Pfup41 is the first DNA polymerase that does not require primers for the synthesis of long DNA strands.

Identification of domains of the HPV11 E1 protein required for DNA replication in vitro

The HPV E1 and E2 proteins along with cellular factors, are required for replication of the viral genome. In this study we show that in vitro synthesized HPV11 E1 can support DNA replication in a cell-free system and is able to cooperate with E2 to recruit the host polymerase alpha primase to the HPV origin in vitro. Deletion analysis revealed that the N-terminal 166 amino acids of E1, which encompass a nuclear localization signal and a cyclin E-binding motif, are dispensable for E1-dependent DNA replication and for recruitment of pol alpha primase to the origin in vitro. A shorter E1 protein lacking the N-terminal 190 amino acids supported cell-free DNA replication at less than 25% the efficiency of wild-type E1 and was active in the pol alpha primase recruitment assay. An even shorter E1 protein lacking a functional DNA-binding domain due to a truncation of the N-terminal 352 amino acids was inactive in both assays despite the fact that it retains the ability to associate with E2 or pol alpha primase in the absence of ori DNA. We provide additional functional evidence that E1 interacts with pol alpha primase through the p70 subunit of the complex by showing that p70 can be recruited to the HPV origin by E1 and E2 in vitro, that the domain of E1 (amino acids 353-649) that binds to pol alpha primase in vitro is the same as that needed for interaction with p70 in the yeast two-hybrid system, and that exogenously added p70 competes with the interaction between E1 and pol alpha primase and inhibits E1-dependent cell-free DNA replication. On the basis of these results and the observation that pol alpha primase competes with the interaction between E1 and E2 in solution, we propose that these three proteins assemble at the origin in a stepwise process during which E1, following its interaction with E2, must bind to DNA prior to interacting with pol alpha primase.

Response of Xenopus Cds1 in cell-free extracts to DNA templates with double-stranded ends

Although homologues of the yeast checkpoint kinases Cds1 and Chk1 have been identified in various systems, the respective roles of these kinases in the responses to damaged and/or unreplicated DNA in vertebrates have not been delineated precisely. Likewise, it is largely unknown how damaged DNA and unreplicated DNA trigger the pathways that contain these effector kinases. We report that Xenopus Cds1 (Xcds1) is phosphorylated and activated by the presence of some simple DNA molecules with double-stranded ends in cell-free Xenopus egg extracts. Xcds1 is not affected by aphidicolin, an agent that induces DNA replication blocks. In contrast, Xenopus Chk1 (Xchk1) responds to DNA replication blocks but not to the presence of double-stranded DNA ends. Immunodepletion of Xcds1 (and/or Xchk1) from egg extracts did not attenuate the cell cycle delay induced by double-stranded DNA ends. These results imply that the cell cycle delay triggered by double-stranded DNA ends either does not involve Xcds1 or uses a factor(s) that can act redundantly with Xcds1.

Active site mapping of the catalytic mouse primase subunit by alanine scanning mutagenesis

In the eukaryotic cell, DNA synthesis is initiated by DNA primase associated with DNA polymerase alpha. The eukaryotic primase is composed of two subunits, p49 and p58, where the p49 subunit contains the catalytic active site. Mutagenesis of the cDNA for the p49 subunit was initiated to demonstrate a functional correlation of conserved residues among the eukaryotic primases and DNA polymerases. Fourteen invariant charged residues in the smaller catalytic mouse primase subunit, p49, were changed to alanine. These mutant proteins were expressed, purified, and enzymatically characterized for primer synthesis. Analyses of the mutant proteins indicate that residues 104-111 are most critical for primer synthesis and form part of the active site. Alanine substitution in residues Glu105, Asp109, and Asp111 produced protein with no detectable activity in direct primase assays, indicating that these residues may form part of a conserved carboxylic triad also observed in the active sites of DNA polymerases and reverse transcriptases. All other mutant proteins showed a dramatic decrease in catalysis, while mutation of two residues, Arg162 and Arg163, caused an increase in Km(NTP). Analysis of these mutant proteins in specific assays designed to separately investigate dinucleotide formation (initiation) and elongation of primer indicates that these two activities utilize the same active site within the p49 subunit. Finally, mutations in three active site codons produced protein with reduced affinity with the p58 subunit, suggesting that p58 may interact directly with active site residues.

DNA replication in vitro by recombinant DNA-polymerase-alpha-primase

DNA-polymerase-alpha--primase complex contains four subunits, p180, p68, p58, and p48, and comprises a minimum of two enzymic functions. We have cloned cDNAs encoding subunits of DNA-polymerase-alpha--primase from human and mouse. Sequence comparisons showed high amino acid conservation among the mammalian proteins. We have over-expressed the single polypeptides and co-expressed various subunit complexes using baculovirus vectors, purified the proteins and investigated their biochemical properties. The purified mouse p48 subunit (Mp48) alone had primase activity. Purification of co-expressed Mp48 and Mp58 subunits yielded stable DNA primase of high specific activity. Co-expression of all four subunits yielded large quantities of tetrameric DNA-polymerase-alpha--primase. The p180, p58 and p48 polypeptides were also co-expressed and immunoaffinity purified as a trimeric enzyme complex. The tetrameric and trimeric DNA-polymerase-alpha--primase complexes showed both DNA primase and DNA polymerase activities. The tetrameric recombinant DNA-polymerase-alpha--primase synthesized double-stranded M13 DNA and replicated polyoma viral DNA in vitro efficiently.

On the association of DNA primase activity with the nuclear matrix in HeLa S3 cells

We have reinvestigated the association of DNA primase activity with the nuclear matrix prepared from exponentially growing HeLa S3 cells. We have found that 25-30 per cent of the nuclear primase activity resists extraction with 2 M NaCl and digestion with Dnase I. Unlike previous investigations, done with the same cell line, the results showed that nuclear matrix-bound DNA primase activity represented less than 10 per cent of the total cell activity. Association of high levels of primase activity with the nuclear matrix was strictly dependent on a 37 degrees C incubation of isolated nuclei prior to subfractionation. Evidence was obtained that the method used for preparing nuclei can have a dramatic effect on the amount of primase activity which is recovered both in the postnuclear supernatant and in isolated nuclei, thus seriously affecting the interpretation of the results about the quantity of DNA primase activity bound to the nuclear matrix.

HIV-1 reverse transcriptase inhibitor from Phyllanthus niruri

An aqueous extract of Phyllanthus niruri (Euphorbiaceae) inhibited human immunodeficiency virus type-1 reverse transcriptase (HIV-1-RT). The inhibitor against HIV-1-RT in this plant was purified by combination of three column chromatographies, Sephadex LH-20, cellulose, and reverse-phase high-performance liquid chromatography. The inhibitor was then identified by nuclear magnetic resonance (NMR) spectra as repandusinic acid A monosodium salt (RA) which was originally isolated from Mallotus repandus. The 50% inhibitory doses (ID50) of RA on HIV-1-RT and DNA polymerase alpha (from HeLa cells) were 0.05 microM and 0.6 microM, respectively, representing approximately a 10-fold more sensitivity of HIV-1-RT compared with DNA polymerase alpha. RA was shown to be a competitive inhibitor with respect to the template-primer while it was a noncompetitive inhibitor with respect to the substrate. RA as low as 10.1 microM inhibited HIV-1-induced cytopathogenicity in MT-4 cells. In addition, 4.5 microM of RA inhibited HIV-1-induced giant cell formation of SUP-T1 approximately 50%. RA (2.5 microM) inhibited up to 90% of HIV-1 specific p24 antigen production in a Clone H9 cell system.

Nucleoprotein complexes released from lymphoma nuclei that contain the abl oncogene and RNA and DNA polymerase and RNA primase activities

We report on the discovery and isolation of DNA- and RNA-containing macromolecular nuclear complexes whose purified major DNA possessed electrophoretic mobilities of approximately 90 and approximately 25 kbp. The deoxyribonucleoprotein-ribonucleoprotein complexes contain RNA and DNA polymerase and primase activities and were isolated from nuclei of murine RAW117 large-cell lymphoma cells by restriction digestion with Msp-I, gentle extraction with solutions containing MgCl2, but without chelating agents, and low ionic strength gel electrophoresis. Two-dimensional (isoelectric focusing/M(r)) gel electrophoresis and silver staining of the proteins of the complexes after treatment with DNase I indicated the presence of approximately 30 protein components. In vitro DNA and RNA polymerase/primase assays showed that the DNP/RNP complexes had very high enzyme specific activities. Using the DNP/RNP complexes a discrete DNA polymerase alpha product of approximately 85 kbp was synthesized that was not synthesized in the presence of the DNA polymerase alpha inhibitor aphidicolin. RNA polymerase assays in the presence of excess alpha-amanitin indicated that the complexes possessed significant RNA polymerase I activity. Preparing the complexes at various times after the release of cells from a double thymidine block showed the complexes as well as the complex-associated enzyme activities to be cell-cycle dependent. The DNA and RNA polymerase-related activities were highest in late S phase, 7 and 9 h, respectively, after release from the double thymidine block. The complexes synthesized a specific in vitro DNA polymerase product using endogenous substrate and nucleotide precursors.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Plasmids bearing mammalian DNA-replication origin-enriched (ors) fragments initiate semiconservative replication in a cell-free system

Four plasmids containing monkey (CV-1) origin-enriched sequences (ors), which we have previously shown to replicate autonomously in CV-1, COS-7 and HeLa cells (Frappier and Zannis-Hadjopoulos (1987) Proc. Natl. Acad. Sci. USA 84, 6668-6672), were found to replicate in an in vitro replication system using HeLa cell extracts. De novo site-specific initiation of replication on plasmids required the presence of an ors sequence, soluble low-salt cytosolic extract, poly(ethylene glycol), a solution containing the four standard deoxyribonucleoside triphosphates and an ATP regenerating system. The major reaction products migrated as relaxed circular and linear plasmid DNAs, both in the presence and absence of high-salt nuclear extracts. Inclusion of high-salt nuclear extract was required to obtain closed circular supercoiled molecules. Replicative intermediates migrating slower than form II and topoisomers migrating between forms II and I were also included among the replication products. Replication of the ors plasmids was not inhibited by ddTTP, an inhibitor of DNA polymerase beta and gamma, and was sensitive to aphidicolin indicating that DNA polymerase alpha and/or delta was responsible for DNA synthesis. Origin mapping experiments showed that early in the in vitro replication reaction, incorporation of nucleotides occurs preferentially at ors-containing fragments, indicating ors specific initiation of replication. In contrast, the limited incorporation of nucleotides into pBR322, was not site specific. The observed synthesis was semiconservative and appeared to be bidirectional.

DNA replication of human polyomavirus JC is stimulated by NF-I in vivo

Using a combination of Bal31 deletion mutagenesis and site-directed mutagenesis, we analyzed the sequence requirements for the DNA replication of the human neurotropic polyomavirus JC. In addition to defining the late side boundary of the viral core origin we demonstrated that the viral enhancer stimulates replication in vivo. Three regions within the viral enhancer increased the rate of replication, with sequences directly adjacent to the late side of the core origin exhibiting the strongest effect. These sequences interact with various cellular proteins, among them NF-I. Point mutations within the NF-I site abolished the stimulation of DNA replication concomitant with a strong reduction in NF-I binding. By contrast, point mutations which did not interfere with NF-I binding did not influence the rate of replication in vivo. Stimulation of JCV DNA replication could only be observed in vivo, but not in vitro, indicating a role of NF-I in determining chromatin structure.

Eukaryotic DNA primase. Abortive synthesis of oligoadenylates

Calf thymus DNA polymerase alpha-primase, human placenta DNA polymerase alpha-primase and human placenta DNA primase synthesized oligoriboadenylates of a preferred length of 2-10 nucleotides and multimeric oligoribonucleotides of a modal length of about 10 monomers on a poly(dT) template. The dimer and trimer were the prevalent products of the polymerization reaction. However, only the oligonucleotides from heptamers to decamers were elongated efficiently by DNA polymerase alpha.

Reverse transcriptase in archaebacteria. Purification and characterization of a primase-reverse-transcriptase complex from Halobacterium halobium

A primase-reverse-transcriptase of Halobacterium halobium was purified by column chromatography on DEAE-cellulose, hydroxyapatite and carboxymethyl-cellulose, followed by sedimentation on a glycerol gradient. The enzyme is a multifunctional enzyme containing reverse transcriptase. DNA polymerase and RNase H activities and does not require a performed primer to initiate DNA synthesis. Using a single-stranded DNA as template, this enzyme synthesizes oligonucleotides (8-12 bases) that can be used a primer by Escherichia coli DNA nucleotidyltransferase I (DNA polymerase I, Klenow fragment). Two polypeptides of 67 and 57 kDa were found after 14750-fold purification of the enzyme.

Metabolism and action of purine nucleoside analogs

Recent investigations have identified many new purine nucleoside analogs that act as antimetabolites. This article focuses on the metabolism and mechanisms of action of tiazofurin, 3-deazaguanosine, neplanocin A, arabinosyladenine in combination with inhibitors of adenosine deaminase, arabinosyl-2-fluoroadenine, and 2-chloro-2'-deoxyadenosine, drugs that are either currently being evaluated in clinical trials or are close to that stage. The diverse metabolic requirements for activation, unique mechanisms of action, and differential biological activities of these compounds are characterized and evaluated for prospective therapeutic application.

A novel primase-free form of murine DNA polymerase alpha induced by infection with minute virus of mice

Two species of DNA polymerase alpha free of primase activity were identified in extracts of Ehrlich mouse cells that had been infected with minute virus of mice. Primase-free forms of DNA polymerase alpha eluted with 150 and 180 mM NaCl during ion-exchange chromatography on DEAE-cellulose columns, exhibited sedimentation coefficients of 11 S and 8.2 S, respectively, and were inhibited by aphidicolin, N2-(p-n-butylphenyl)-9-(2-deoxy-beta-D-ribofuranosyl)guanine 5'-triphosphate, and 2-(p-n-butylanilino)-9-(2-deoxy-beta-D-ribofuranosyl)adenine 5'-triphosphate. The ratio of primase-free DNA polymerase alpha to the DNA polymerase alpha-primase complex increased from 1.5 to greater than 100 during the course of infection, and free primase was produced during the MVM replicative cycle.

Structural study of immunoaffinity-purified DNA polymerase alpha-DNA primase complex from calf thymus

The DNA polymerase alpha-DNA primase complex was purified over 17,000-fold to near homogeneity from calf thymus using an immunoaffinity column. Sodium dodecyl sulfate gel electrophoresis revealed three polypeptides with molecular weights of 140, 50 and 47 kDa, in a ratio of 1:2:0.25. The complex showed a sedimentation coefficient of 9.7 S, a Stokes radius of 56 A and a native molecular weight of 250-260 kDa. Taken together, the data suggest that the calf thymus dNA polymerase alpha-DNA primase complex is essentially a heterotrimer of large (140 kDa) and small (50 kDa) subunits in a ratio of 1:2, with a globular conformation. Electron-microscopic studies of the complex revealed a spherical particle of 120 A in diameter, in agreement with the physiochemical results. The binding of the complex to DNA was also demonstrated.

Identification and characterization of a DNA primase activity present in herpes simplex virus type 1-infected HeLa cells

A novel DNA primase activity has been identified in HeLa cells infected with herpes simplex virus type 1 (HSV-1). Such an activity has not been detected in mock-infected cells. The primase activity coeluted with a portion of HSV-1 DNA polymerase from single-stranded DNA agarose columns loaded with high-salt extracts derived from infected cells. This DNA primase activity could be distinguished from host HeLa cell DNA primase by several criteria. First, the pH optimum of the HSV primase was relatively broad and peaked at 8.2 to 8.7 pH units. In contrast, the pH optimum of the HeLa DNA primase was very sharp and fell between pH 7.9 and 8.2. Second, freshly isolated HSV DNA primase was less salt sensitive than the HeLa primase and was eluted from single-stranded DNA agarose at higher salt concentrations than the host primase. Third, antibodies raised against individual peptides of the calf thymus DNA polymerase:primase complex cross-reacted with the HeLa primase but did not react with the HSV DNA primase. Fourth, freshly prepared HSV DNA primase appeared to be associated with the HSV polymerase, but after storage at 4 degrees C for several weeks, the DNA primase separated from the viral DNA polymerase. Separation or decoupling could also be achieved by gel filtration of the HSV polymerase:primase. This free DNA primase had an apparent molecular size of approximately 40 kilodaltons, whereas free HeLa DNA primase had an apparent molecular size of approximately 110 kilodaltons. On the basis of these data, we believe that the novel DNA primase activity in HSV-infected cells may be virus coded and that this enzyme represents a new and important function involved in the replication of HSV DNA.

Binding of the DNA polymerase alpha-DNA primase complex to the nuclear matrix in HeLa cells

It is well-known that there are multiple forms of DNA polymerase alpha. In order to determine which form(s) is (are) tightly bound, the activities were dissociated from DNA-poor nuclear matrices, with octyl beta-D-glucoside. Sucrose gradient sedimentation analysis revealed three bands with s values of 7.5, 10.5, and 13. The 7.5S form was free of DNA primase and represented only 10% of the total DNA polymerase alpha bound to the nuclear matrix. The 13S and the 10.5S forms each contained DNA primase activity. The 10.5S form comprised 85% of the DNA polymerase alpha activity and 95% of the DNA primase activity, dissociated from the nuclear matrix. Neither temperature of nuclease digestion nor various salt treatments of nuclei had significant effects on the proportions of DNA polymerase alpha and DNA primase activities bound to, or subsequently dissociated from, nuclear matrices. In a comparison of primase activity bound to the nuclear matrix, dissociated from the nuclear matrix, and in the soluble fraction, it was found that the bound activity had a lower ATP dependence, had less KCl inhibition, and was less sensitive to heat, compared to the dissociated and soluble activities. No differences in Mg2+ or pH dependence were noted. The amounts of DNA polymerase alpha and DNA primase activities bound to the nuclear matrix varied over the cell cycle of synchronized cells. Over the S phase, there were two peaks of matrix-bound DNA primase and two peaks of subsequently dissociated DNA polymerase alpha-DNA primase complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Eucaryotic primase

Eucaryotic primase, an enzyme that initiates de novo DNA replication, is tightly associated with polymerase alpha or yeast DNA polymerase I. It is probably a heterodimer of 5.6 +/- 0.1 S. The enzyme synthesizes oligoribonucleotides of about eight residues which are always initiated with a purine. In vitro the polymerase-primase complex initiates synthesis and pauses at preferred sites on natural single-stranded templates. The relative concentrations of ATP and GTP present in the reaction medium modulate the frequency of site recognition. Primase is strongly ATP-dependent in the presence of single-stranded DNA and of poly(dT). It also synthesizes oligo(rG) in the presence of poly(dC) very efficiently.

DNA synthesis in vitro with an endoplasmic-reticulum-DNA-polymerase complex from unfertilized sea urchin eggs

An endoplasmic-reticulum-DNA-polymerase complex was prepared from unfertilized sea urchin eggs and its DNA-synthesizing activity was examined using single-stranded DNA of bacteriophage fd as a template. The complex catalyzed the ribonucleotide-dependent DNA synthesis which required dNTPs, NTPs, Mg2+ and single-stranded DNA. The DNA synthesis was sensitive to aphidicolin and N-ethylmaleimide but was resistant to 2',3'-dideoxyribosylthymine 5'-triphosphate (ddTTP) and alpha-amanitin, suggesting the involvement of DNA polymerase alpha. In parallel with the DNA synthesis, a small amount of RNA was synthesized in the presence of 100 micrograms/ml alpha-amanitin. The Km value of ribonucleotides for the RNA synthesis coincided with that for the DNA synthesis, suggesting a correlation between the DNA and RNA syntheses. Labelling of the products with [gamma-32P]ATP followed by DNA digestion with pancreatic DNase I revealed the attachment of an oligoribonucleotide (7-11 bases in length) at the 5' ends of the DNA products. These observations suggest that in DNA synthesis, primer RNA synthesis occurs first, followed by DNA chain elongation. During 1-90-min incubation, the amount of the DNA synthesized increased but the length was not significantly increased. Over 80% of the number of synthesized DNA molecules comprised a single population of short DNA fragments (60-200 bases, on average 120 bases in length) and the number of fragments increased, depending on the incubation time. However, DNA fragments of various sizes (about 100-6000 bases) were synthesized with DNA polymerase alpha solubilized from the endoplasmic-reticulum-DNA-polymerase complex. All this evidence suggests that in vitro, the complex preferentially synthesizes a particular size of short DNA fragments. The significance of the fragments is discussed.

DNA primase of human mitochondria is associated with structural RNA that is essential for enzymatic activity

DNA primase isolated from human mitochondria sediments in glycerol density gradients at 30S and 70S. These unusually high sedimentation coefficients are a result of association of the primase activity with RNA. Treatment of primase with nuclease not only affects its sedimentation behavior, but also inactivates the primase activity. The major RNA species that cofractionates with primase activity is shown by direct sequence analysis to be cytosolic 5.8S ribosomal RNA (rRNA). Specific degradation of endogenous 5.8S rRNA using ribonuclease H and oligonucleotides complementary to 5.8S rRNA results in reduction of primase activity. Other small RNAs may play a structural role in the formation of an active DNA primase complex.

DNA polymerase alpha associated primase from rat liver: physiological variations

A primase activity associated to DNA polymerase alpha from rat liver is described. Both activities were absent in normal adult rat liver but were concomitantly induced after partial hepatectomy. As previously shown for polymerase alpha and DNA topoisomerase II, primase activity reached a maximum value 40-43 h after the partial removal of the liver. Primase activity was shown to catalyze dNMP incorporation on unprimed single-stranded DNA template (M13 DNA) in the presence of rNTP. The activity was not detectable on poly(dA) or poly(dG) but was efficient on poly(dT) or poly(dC). However, the reliability of the primase assay in the presence of poly(dC) was dependent upon the degree of purification of the enzyme. The ribo primers were about 10 nucleotides long, and the reaction was completely independent of alpha-amanitin, a strong inhibitor of RNA polymerases II and III. Primase and polymerase were found tightly associated. A cosedimentation on a 5-20% sucrose gradient was always obtained, independent of the ionic strength. There was also a close coincidence between alpha-polymerase and primase activities during phosphocellulose, hydroxylapatite, and single-stranded DNA Ultrogel chromatography. It has been previously demonstrated by us and others that primase and alpha-polymerase are on separated polypeptides. The association of two activities in the replication complex and the conditions allowing their separation are discussed.

Size difference in catalytic polypeptides of two active forms of mouse DNA polymerase alpha and separation of the primase subunit from one form, DNA replicase

There are two active forms of DNA polymerase alpha in mouse cells. One form (DNA replicase) is a DNA polymerase associated with primase activity and the other form (7.3 S polymerase) has no primase activity (Yaugar, T., Kozu, T. and Seno, T. (1982) J. Biol. Chem. 257, 11121-11127). The primase activity was dissociated from partially purified DNA replicase by hydroxyapatite column chromatography in buffer containing dimethyl sulfoxide and ethylene glycol. Nearly homogeneous primase, consisting of a 58 kDa polypeptide was obtained by glycerol gradient sedimentation and DEAE-cellulose column chromatography. Experiments on the effect of proteinase treatment and measurement of the molecular weight of the catalytic polypeptide of DNA replicase after its dissociation from the primase polypeptide indicated that the primase is not part of the DNA polymerase molecule, but an independent protein associated with DNA polymerase alpha, and that the latter is a 115 kDa catalytic polypeptide. The other form of DNA polymerase alpha, 7.3 S polymerase, consists of a 72 kDa catalytic polypeptide. Thus, the two forms of mouse DNA polymerase alpha have partially, if not completely, different catalytic polypeptide structures, suggesting that the 7.3 S polymerase is not simply formed from DNA replicase by dissociation of the primase subunit.