Purification and Properties of Deoxyribonucleic Acid Polymerase from Rat Liver Mitochondria

Translesion DNA polymerases in eukaryotes: what makes them tick?

Life as we know it, simply would not exist without DNA replication. All living organisms utilize a complex machinery to duplicate their genomes and the central role in this machinery belongs to replicative DNA polymerases, enzymes that are specifically designed to copy DNA. "Hassle-free" DNA duplication exists only in an ideal world, while in real life, it is constantly threatened by a myriad of diverse challenges. Among the most pressing obstacles that replicative polymerases often cannot overcome by themselves are lesions that distort the structure of DNA. Despite elaborate systems that cells utilize to cleanse their genomes of damaged DNA, repair is often incomplete. The persistence of DNA lesions obstructing the cellular replicases can have deleterious consequences. One of the mechanisms allowing cells to complete replication is "Translesion DNA Synthesis (TLS)". TLS is intrinsically error-prone, but apparently, the potential downside of increased mutagenesis is a healthier outcome for the cell than incomplete replication. Although most of the currently identified eukaryotic DNA polymerases have been implicated in TLS, the best characterized are those belonging to the "Y-family" of DNA polymerases (pols η, ι, κ and Rev1), which are thought to play major roles in the TLS of persisting DNA lesions in coordination with the B-family polymerase, pol ζ. In this review, we summarize the unique features of these DNA polymerases by mainly focusing on their biochemical and structural characteristics, as well as potential protein-protein interactions with other critical factors affecting TLS regulation.

A mechanistic view of human mitochondrial DNA polymerase gamma: providing insight into drug toxicity and mitochondrial disease

Mitochondrial DNA polymerase gamma (Pol gamma) is the sole polymerase responsible for replication of the mitochondrial genome. The study of human Pol gamma is of key importance to clinically relevant issues such as nucleoside analog toxicity and mitochondrial disorders such as progressive external ophthalmoplegia. The development of a recombinant form of the human Pol gamma holoenzyme provided an essential tool in understanding the mechanism of these clinically relevant phenomena using kinetic methodologies. This review will provide a brief history on the discovery and characterization of human mitochondrial DNA polymerase gamma, focusing on kinetic analyses of the polymerase and mechanistic data illustrating structure-function relationships to explain drug toxicity and mitochondrial disease.

The eureka enzyme: the discovery of DNA polymerase

The identification and partial purification by Arthur Kornberg and his colleagues in 1956 of an enzyme - DNA polymerase I of Escherichia coli - that catalysed the stable incorporation of deoxyribonucleotides into DNA in vitro came as a surprise. At the time, most scientists in the field believed that DNA synthesis was too complicated to be accurately reflected outside the living cell.

DNA topoisomerase I from calf thymus mitochondria is associated with a DNA binding, inner membrane protein

During purification of the type I DNA topoisomerase from calf thymus mitochondria, two polypeptides, p78 and p63, cofractionate with the enzymatic activity (Lazarus et al., (1987) Biochemistry 26, 6195-6203). The two polypeptides are released from a mitochondrial inner membrane preparation by nonionic detergent lysis and both adsorb strongly to a single-stranded DNA agarose column. We have attempted to characterize the relationship between these two polypeptides and have found the following: (i) the mitochondrial topoisomerase is active in free (monomer) and associated (heterodimer) form; (ii) the catalytic activity resides solely in p78, as adjudged by both the covalent linkage of the enzyme to substrate DNA and the ability of the enzyme to relax supercoils; (iii) at low ionic strength the enzyme is active in monomer form with p78 alone being sufficient for activity; (iv) in high salt, the high molecular weight species is a 140-kDa heterodimer composed of one p78 and one p63; and (v) the two polypeptides are not structurally related as digestion with V8 protease results in distinct proteolytic fragment patterns. These results suggest that p63 may have an important role in the metabolism of the mitochondrial topoisomerase.

Effect of activators and inhibitors on the activity of mitochondrial DNA polymerase

At a concentration of 0.5 to 3 mmol/l, ATP stimulates the activity of mitochondrial DNA polymerase of Neurospora crassa under the optimum reaction conditions; at higher concentrations, an inhibitory effect is observed. 4-Chloromercuribenzoate (1 mmol/L), a thiol inhibitor, decreases the enzyme activity two-fold, while N-ethylmalcimide (2 mmol/L) has no effect. Ethidium bromide (up to 10 mumol/L) and heparin (up to 0.4 micrograms/mL) reduce the activity by 60%. ddTTP does not affect the DNA polymerase reaction. The best in vitro template is the activated calf-thymus DNA.

Purification and characterization of a benzene hydroxylase from rat liver mitochondria

An enzyme has been purified to electrophoretic homogeneity from rat liver mitoplasts which metabolizes benzene to phenol. The enzyme has a Mr of 52,000 and requires NADPH, adrendoxin, and adrenodoxin reductase for activity. Benzene hydroxylase activity could be inhibited by carbon monoxide and SKF-525A, and by specific inhibitors of microsomal benzene metabolism. The purified enzyme also oxidized phenol to catechol. The data suggest that a cytochrome P-450 of mitochondrial origin is involved in benzene metabolism, and provide another example of a role for the mitochondrion in xenobiotic activation.

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.

Studies on mitochondrial type I topoisomerase and on its function

We have reported previously that rat liver mitochondria contain a topoisomerase and have shown it to be distinct from the nuclear enzyme by its sensitivity to Berenil and ethidium bromide. We report here some additional characterization. The enzyme differs further from its nuclear counterpart in its failure to bind to ssDNA cellulose and its chromatographic behavior on Sephadex; the latter procedure yields an Mr of 44 000 for the mitochondrial and 70 000 for the nuclear enzyme. The topoisomerase is strongly associated with mitochondrial membranes; only 10% of the activity could be extracted. The pH optimum of the enzyme falls between 6.0 and 8.5, with an NaCl optimum of 0.13 M in 0.1 M Tris (pH 8.3). Dithiothreitol is required, while N-ethylmaleimide is inhibitory. Tosylphenylalanine chloromethyl ketone, a serine proteinase inhibitor, abolishes activity; another, phenylmethanesulfonyl fluoride, has no effect. Berenil, a non-intercalating drug, and four of its analogues all inhibit with up to 100-fold differences in potency. No dependence on ATP, Mg2+, or both together could be shown. Neither novobiocin nor oxolinic acid shows any inhibitory effect. Nicked circles are generated in the presence of DMSO. These three observations are consistent with the topoisomerase being of the Type I class. Positively supercoiled pBR322 DNA, whose 6-8 positive turns were generated by altering solution conditions, is relaxed by the enzyme, indicating a lack of requirement for a negatively supercoiled substrate. We have also examined a partially purified preparation of the corresponding mitochondrial enzyme from mouse L cells. This enzyme is largely similar in properties to the rat liver enzyme. In isolated mitochondria, Berenil causes biphasic alterations in [3H]dATP incorporation into DNA, 10(-4) mM stimulating 2-fold, while higher concentrations inhibit. [3H]UTP incorporation into mitochondrial RNA also follows this pattern.

Covalent binding of benzene and its metabolites to DNA in rabbit bone marrow mitochondria in vitro

Rabbit bone marrow mitochondria, stripped of their outer membrane (mitoplasts), have been shown to carry out the NADPH-dependent bioactivation of radiolabelled benzene in vitro to metabolites capable of covalently binding to mtDNA, thereby inhibiting transcription. The metabolites of benzene produced in bone marrow cells by the microsomal cytochrome P-450 are thought to be phenol, catechol, hydroquinone and p-benzoquinone (Andrews et al., Life Sci., 25 (1979) 567; Irons et al., Chem.-Biol. Interact., 30 (1980) 241). Incubation of mitoplasts from rabbit bone marrow cells in vitro with varying concentrations of the putative microsomal metabolites showed a concentration-dependent inhibition of RNA synthesis. The 50% inhibitory molar concentration (IC50) for each metabolite was determined to be: 1,2,4- benzenetriol , 6.3 X 10(-7); p-benzo-quinone, 2 X 10(-6); phenol, 2.5 X 10(-5); hydroquinone, 5 X 10(-5); catechol, 2 X 10(-3); benzene, 1.6 X 10(-2). DNA, isolated from rabbit bone marrow cell or rat liver mitoplasts prelabelled in DNA with [3H]dGTP and exposed to [14C]benzene in vitro, was enzymatically hydrolyzed to nucleosides which were chromatographed on a Sephadex LH-20 column to separate free nucleosides from nucleoside-adducts. The elution profiles indicated that rat liver mtDNA contained six guanine nucleoside-adducts and rabbit bone marrow cell mtDNA contained seven guanine nucleoside-adducts. Incubation of bone marrow mitoplasts in vitro in the presence of benzene and the hydroxyl radical scavenger, mannitol, resulted in the inhibition of formation of four of the guanosine-adducts. When [3H]dATP was substituted as the prelabelled precursor nucleotide, the LH-20 column profile indicated that two adenine nucleoside-adducts were also formed from benzene in vitro. Furthermore, a comparison of the Sephadex LH-20 column profiles of purine adducts derived from [14C]benzene- and [3H]dGMP-labelled mtDNA with profiles generated by individually incubating each of the putative unlabelled metabolites with bone marrow mitoplasts in vitro has indicated that p-benzoquinone, phenol, hydroquinone and 1,2,4- benzenetriol form adducts with guanine. One of the two adenosine-adducts may arise from hydroquinone; the compound forming the other adduct is unknown at the present time. Exposure of mitoplasts to catechol in vitro resulted in the formation of a guanine nucleoside-adduct that was present in rat liver mtDNA but absent from the DNA isolated from rabbit bone marrow cell mitoplasts exposed to [14C]benzene in vitro. This suggests that catechol is probably not a major metabolite of benzene formed in bone marrow cell mitochondria.

The effect of bacterial DNA gyrase inhibitors on DNA synthesis in mammalian mitochondria

Using isolated rat liver mitochondria, which have previously been shown to carry out true replicative DNA synthesis, we have obtained results which are in accord with the presence and functioning of a DNA gyrase in this organelle. The effects of the Escherichia coli DNA gyrase inhibitors, novobiocin, coumermycin, nalidixic acid and oxolinic acid, upon mtDNA replication suggest the involvement of the putative mitochondrial enzyme in various aspects of this process. First, the preferential inhibition of [3H]dATP incorporation into highly supercoiled DNA together with the appearance of labeled, relaxed DNA are consistent with the involvement of a gyrase in the process of generating negative supercoils in mature mtDNA. Second, the overall depression of incorporation of labeled dATP into mtDNA, including the reduction of radioactivity incorporated into replicative intermediates, suggests a 'swivelase' role for the putative gyrase, and this hypothesis is further supported by results obtained on sucrose gradient centrifugation of heat-denatured, D-loop mtDNA. Here, the synthesis of the completed clean circles is inhibited while 9 S initiator strand synthesis is not, suggesting that chain elongation is blocked by the gyrase inhibitors.

A phylogenetic study on vertebrate mitochondrial DNA polymerase

We have started a phylogenetic survey for the mitochondrial DNA polymerase and present in this study the results obtained for all the different classes for the vertebrates. The operating conditions include the purification of mitochondria, the analysis of the DNA polymerase activity in the extract and the determination of the sedimentation coefficient on sucrose gradients. The utilization of digitonin for removing the external membrane of the organelle and contaminating proteins has been generalized since this detergent shows no effect on the activities of either DNA polymerases alpha or gamma. The results obtained for the mitochondria of different classes of vertebrates show that the activity responding to the specific assay of DNA polymerase gamma tended invariably to increase during purification while that of DNA polymerase alpha tended to decrease. Furthermore in almost all the cases the gamma-polymerase represented the only DNA polymerase activity found in the mitochondria after digitonin treatment. The analysis of the sedimentation patterns of the mitochondrial DNA polymerase strongly suggests the presence of a single type of DNA polymerase showing the typical properties of the gamma-polymerase. It is concluded that the vertebrate mitochondria contain a well-defined and unique form of DNA polymerase which corresponds to the DNA polymerase gamma.

The role of DNA polymerases alpha, beta and gamma in nuclear DNA synthesis

The effects of the inhibitors 2'3' dideoxythymidine triphosphate (ddTTP) and 1-beta-D-arabinofuranosyl cytosine triphosphate (araCTP) on DNA synthesis in isolated S-phase HeLa S3 nuclei have been examined. These effects are compared with the effects of the same inhibitors in partially purified preparations of DNA polymerases alpha and beta. The effect of ddTTP on partially purified DNA polymerase gamma was also tested. DNA polymerases beta and gamma were very sensitive to ddTTP whereas DNA polymerase alpha and DNA synthesis in isolated nuclei were quite resistant. The synthesis and subsequent ligation of primary DNA pieces ('Okazaki fragments') were not affected by the presence of this inhibitor. DNA synthesis in isolated nuclei and DNA polymerase alpha activity were very sensitive to araCTP whereas DNA polymerase beta was almost totally resistant to the inhibitor. The results indicate a major role for DNA polymerase alpha in DNA replication.

Distinctive properties of mammalian DNA polymerases

DNA polymerase-alpha and -beta can be distinguished from one another by the differential effects of N-ethylmaleimide, KCl, ara-CTP and temperature, as well as on the basis of sedimentation. The sensitivity of DNA polymerase-beta to elevated temperatures as compared to DNA polymerase-alpha provides a new means of distinguishing between these two enzymes even in crude extracts and a possible probe for determining their function. DNA polymerase-alpha and -beta share several properties in common, including the ability to readily incorporate dUTP in place of dTTP. The Km for dUTP varies from 10 to 30 micron with different preparations of DNA polymerase-alpha and -beta. Thus, in mammalian cells, dUMP could be incorporated into DNA, and if excised by an endonuclease, would lead to discontinuities. Initial analyses of fidelity in direct comparative studies indicate that beta-class DNA polymerases are highly accurate in base selection when copying poly[d(A-T)]. Less than one molecule of dGMP is incorporated for every 12 000-45 000 molecules of dAMP and dTMP polymerized. DNA polymerase-alpha is somewhat less accurate, making one mistake for every 4000-10 000 correct nucleotides incorporated. Since both polymerases lack an exonucleolytic activity, this accuracy must be the result of selectivity for the complementary nucleotide by the polymerase.

Changes of mitochondrial DNA polymerase-gamma activity in synchronized mouse cell cultures

Following synchronization by a double hydroxyurea block, mouse cell cultures exhibited a period of accelerated precursor incorporation into mitochondrial DNA during the late nuclear S phase. Peak activity of mitochondrial DNA polymerase-gamma occurred concurrent to the interval of accelerated organelle DNA synthesis. Mixing experiments suggested that the variations in mitochondrial DNA polymerase activity during the cell cycle were not due to free inhibitors in the enzyme preparations examined.

Template specificity of rat mitochondrial DNA polymerase

Mitochondrial DNA polymerase was purified 2300-fold over isolated mitochondria from rat liver. Template-primer specificities of this enzyme were investigated. Activated DNA was satisfactorily used as an active template-primer, but both native and denatured DNAs showed a slight activity. Synthetic polynucleotide, poly(dA) - oligo(dT)10 was found to have a high efficiency under the same condition for activated DNA. When the closed-circular, nicked and gapped Co1E1 DNAs were employed as a template-primer, the enzyme could only utilize the gapped DNA, indicating that the displacement synthesis was not catalyzed by the enzyme itself. The enzyme also copied poly(A) - oligo(dT)10 in high efficiency at pH 7.5 in the presence of MnCl2. Such RNA-directed DNA polymerase activity of the enzyme was further characterized. Cofractionated endouclease activity was completely separated from the enzyme by glycerol gradient centrifugation.

Identity of DNA polymerase gamma from synaptosomal mitochondria and rat-brain nuclei

DNA polymerase gamma and mitochondrial DNA polymerase were isolated from brain nuclei and synaptosomes respectively. The presence of a single DNA polymerase in synaptosomal mitochondria was established by chromatography on DEAE-cellulose, phosphocellulose and DNA-cellulose, as well as by sedimentation analysis and isoelectric focusing. A great similarity between the purified nuclear DNA polymerase gamma and the mitochondrial enzyme was found by the following criteria: chromatographic behaviour in three column systems; essentially complete inhibition by N-ethyl-maleimide (2 mM); optimal requirements of Mn2+ (0.1 mM), Mg2+ (5 mM) and pH (8.0); template preferences, poly(A) - (dT)20-25 larger than activated DNA larger than poly(dA) - (dT)12-18; lack of activity on single-stranded polynucleotides and (dT)12-primed mRNA; molecular weight (180000), sedimentation (9.2 S) and isoelectric point (pI 5.4). We therefore conclude that brain nuclear DNA polymerase gamma and synaptosomal mitochondrial DNA polymerase are closely related and may even be identical.

Chick-embryo DNA polymerase gamma. Identity of gamma-polymerases purified from nuclei and mitochondria

The level of DNA polymerase gamma as compared to DNA polymerases alpha and beta has been determined in chick embryo by means of specific tests: the amount of gamma-polymerase in the 12-day-old chick embryo reaches about 15% of the total polymerase activity. This enzyme is mainly localized in nuclei and mitochondria, where it represents the prevailing if not the unique DNA polymerase activity. The mitochondrial DNA polymerase gamma is likely to be associated with the internal membrane or the matrix of this organelle since it is not removed by digitonin treatment. The gamma-polymerases have been purified from chick embryo nuclei and mitochondria 500-700 times by means of DEAE-cellulose, phosphocellulose and hydroxyapatite chromatographies. The purified mitochondrial DNA polymerase gamma is closely related to the homologous enzyme purified from the nuclei of the same cells. So far, they cannot be distinguished on the basis of their sedimentation, catalytical properties and response to inhibitors or denaturating agents. The purified gamma enzymes are distinct from the chick embryo DNA polymerases alpha and beta and are not inhibited by antibodies prepared against the latter enzymes. The nuclear and mitochondrial gamma-polymerases do not respond to the oncogenic RNA virus DNA polymerase assay with natural mRNAs.

DNA synthetic capabilities of differentiating sperm cells

Spermatogenic cells separated by velocity sedimentation were analysed by a micro-procedure for differentiation-associated changes in DNA synthetic capabilities. DNA-dependent DNA polymerase activity is maximal in premeiotic and meiotic cells, sequentially declines in progressively more differentiated spermiogenic cells to a minimum value in testicular spermatozoa which is 1/14 of the maximum. No further decrease of activity is observed during the subsequent process of sperm cell maturation and, at the end-differentiated state, the potential of sperm cells for DNA synthesis is demonstrated by the presence of substantial activities of thymidine and thymidylate kinases as well as DNA polymerase activity, as determined by in vitro assay, are polymerase. Although levels of DNA polymerase activity, as determined by in vitro assay, are negatively correlated with the state of differentiation, the findings support the hypothesis that, in this cell system, DNA synthetic enzymes may not be limiting factors in the control of DNA synthesis.

Mitochondrial DNA polymerase from rat liver

A DNA-dependent DNA polymerase from rat liver mitochondria was partially purified and characterized. Mitochondrial DNA polymerase has been found to be quite different from other DNA-dependent DNA polymerases alpha and beta present in the rat liver in the following points: elution patterns in a DEAE-cellulose column chromatography, sedimentation coefficients determined by the glycerol gradient centrifugation in the presence of high salt, and sensitivities to N-ethylmaleimide, ethidium bromide and KCl.

Chloramphenicol-induced loss of mitochondrial DNA polymerase activity in HeLa cells

HeLa cells exposed to chloramphenicol for approximately one cell generation were found to contain a mitochondria-associated DNA polymerase with a significantly lower specific activity than that of control cells. This observation was not due to the presence of inhibitors in mitochondrial DNA polymerase preparations of chloramphenicol-treated cell cultures. In addition, there was no accumulation of a typical mitochondrial DNA polymerase in the post-mitochondrial supernatant of drug-treated cells.

Synthesis of DNA in isolated nuclei from differentiated mammalin epidermal cells

Epidermal spinous and granular cells from the newborn rat neither replicate their nuclear DNA nor proliferate in vitro under conditions which support both processes in basal cells. However, as shown by autoradiography, (3H)thymidine and (14C)bromodeoxyuridine do label nuclei removed from spinous cells but not from granular cells. CsCl density gradient centrifugation of DNA obtained from early differentiated nuclei which had been exposed in vitro to (14C)bromodeoxyuridine, indicated that a considerable level of the tracer was present in the nucleic acid and suggested that replication of the genome had occurred. Therefore, spinous cells appear to retain the capability of reproducing nuclear DNA. Since differentiated cells appear to have the "diploid" level of DNA, these observations point to the replication of DNA as a possible locus of the mitotic inhibition which is coincident with epidermal differentiation.

DNA polymerases of rat liver. Partial characterization and effect of various inhibitors

Three distinct DNA-dependent DNA polymerase activities have been partially purified from normal rat liver. Soluble activities are separable into two distinct fractions (P1 and P2) by phosphocellulose chromatography. A low-molecular-weight DNA polymerase was isolated from purified nuclei. The enzymes were characterized according to chromatographic and sedimentation behavior, enzymological properties, and response to various inhibitors. The results indicate that fraction P1 corresponds to the high-molecular-weight enzyme and suggest that polymerase P2 may be derived from partial dissociation of the high-molecular-weight enzyme. The molecular weight of polymerase P1 was estimated to be about 250 000 by Sephadex column chromatography. Both fraction P2 and nuclear DNA polymerase appeared to be low-molecular-weight enzymes. However, the molecular size of these activities was apparently different. The estimated molecular weights of nuclear and P2 enzyme are about 40 000 and 25 000, respectively. As with the nuclear enzyme, polymerase P2 (but not P1) appeared to be free of detectable exonuclease activity. All of these polymerases showed a marked preference for initiated polydeoxyribonucleotide templates. The rat liver polymerases differed in their ability to use poly[d(A-T)-A1 primer-template, as is shown by the ratios of their activity with this synthetic polymer to that with activated DNA: 0.5, 2.75, and 1.34 for P1, P2, and nuclear polymerase, respectively. Denatured DNA was a poor template for both enzymes P1 and P2, but it was inert as template for the nuclear enzyme. Although each of these polymerases required all four deoxynucleoside triphosphates for maximal activity, they catalyzed a high rate of synthesis in the absence of one or more deoxynucleoside triphosphates. Such a 'limited' synthesis was much more extensive for polymerase P2 and nuclear enzyme than for P1 was the most sensitive of the three to sulphydryl reagents, ehtidium bromide, heparin, and single-stranded DNA. The responses of P2 and nuclear enzymes to various inhibitors were very similar. However, these two enzymes respond differently to heat and high ionic strength.

A DNA-directed DNA polymerase from murine liver mitochondria

A DNA-directed DNA polymerase has been isolated from murine liver mitochondria. The mitochondrial DNA polymerase is distinguishable from other DNA polymerases found in the nucleus and cytosol of murine cells by several enzymatic and physical properties. It is stimulated 5--6-fold by 0.15 M KCl, does not require a sulfhydryl reducing agent for activity, and is inhibited by ethidium bromide or ATP. The enzyme has a sedimentation coefficient of 8.8 S in the presence of up to 0.5 M KCl, a molecular weight of 150--170000, and utilizes natural templates in the following order of preference: activated DNA (100%), single stranded DNA (24%), and native DNA (5%).

Differentiation and characterization of the cytoplasmic and nuclear deoxyribonucleic acid polymerases from baby hamster kidney cells

Distinct DNA polymerase activities have been found in the cytoplasmic and nuclear fractions of a baby hamster kidney cell line. They were separated by chromatography on DEAE-cellulose and partially purified by ammonium sulfate fractionation, DNA - cellulose and linear sucrose gradients. The cytoplasmic DNA polymerase exhibited an S-coefficient of 6.95 S in 0.15 M NaCl and its activity was highly sensitive to inhibition by N-ethylmaleimide and elevated temperatures, regardless of the presence of DNA template or other cofactors. It was stimulated by monovalent salts in the order of NH4 Cl greater than KCl greater than NaCl greater than CsCl greater than LiCl (inhibitory). The DNA polymerase extracted from nuclei sedimented with an S-value of 3.47 S, was resistant to inactivation by N-ethylmaleimide, and maximally stimulated by NaCl, while also being inhibited by LiCl. For optimal activity, both DNA polymerase activities required a divalent cation, with MgCl2 being more effective than MnCl2. Although the optimal pH values for the two enzyme activities differed slightly, glycine - NaOH buffer induced an alkaline shift of 1.5 pH units in the optimum of both enzymes. This was accompanied by an increase in the effectiveness of MnCl2 relative to MgCl2 for the cytoplasmic DNA polymerase.

Enzymes of deoxythymidine triphosphate biosynthesis in Neurospora crassa mitochondria

Intact mitochondria of Neurospora crassa incorporate deoxythymidine 5'-monophosphate (dTMP) into deoxyribonucleic acid but not the label from (methyl-3H) deoxythymidine. Mitochondrial homogenates contain deoxythymidylate kinase (EC 2.7.4.9), deoxycytidylate aminohydrolase (dCMP deaminase) (EC 3.5.4.12), and thymidylate synthetase (EC 2.1.1b), but not thymidine kinase (EC 2.7.1.21) activity. dTMP kinase is loosely bound to the mitochondrial membrane and is solubilized by 0.4 M KCl in mitochondrial homogenates, the dCMP aminohydrolase deaminase) is bound to the inner membrane and is not solubilized by 0.4 M KCl. dTMP synthetase activity is found in the 2,000 times g particulate fractions by homogenization of mitochondria in 0.4 M KCl. The dCMP deaminase activity found in the particulate fraction of the inner membrane is efficiently regulated by the products of the pathway: deoxycytidine 5'-triphosphate activates whereas deoxythymidine 5'-triphosphate inhibits, as found for the soluble enzyme from other sources. These data indicate that mitochondria of N. crassa contain specific enzymes for the biosynthesis of deoxythymidine triphosphate.