An induced mitochondrial DNA polymerase from Tetrahymena

Purification and characterization of an inducible mitochondrial DNA polymerase from Tetrahymena thermophila

Treatment of the eukaryotic organism Tetrahymena with various types of DNA-damaging agents has been reported to cause a 35-fold induction of a mitochondrial DNA polymerase. We here report that the enzyme can be induced in large-scale cultures by exposure of the cells to thymine starvation and/or intercalating agents. The induced DNA polymerase has been purified to near homogeneity, with a specific activity of approx. 300,000 units/mg protein. The relative molecular mass of the active form of the enzyme is approx. 100,000, as determined by glycerol gradient sedimentation. The subunit structure has been analysed by SDS polyacrylamide gel electrophoresis of the highly purified preparation and by immunoprecipitation with a monoclonal antibody directed to the DNA polymerase. A polypeptide of Mr 47,000 has been observed to be a subunit of the enzyme. This corresponds to the size of the subunits suggested for mitochondrial DNA polymerase from chicken embryos and mouse myeloma cells.

Purification and properties of a low molecular weight DNA polymerase from Neurospora crassa

A third DNA polymerase 'C' with low molecular weight was isolated and purified 3700-fold from ground hyphae of Neurospora crassa WT 74 A, which shows similarities to beta- and gamma-polymerases from higher eukaryotes: preference for poly(rA)(dT) as a template/primer, inhibition by p-chloromercuribenzoate, resistance against N-ethylmaleimide up to 10 mmol/l, and molecular weight of about 40000. This polymerase elutes as a distinct peak from DEAE-cellulose at 0.60 mol/l KCl and has an optimum for K+ at 2-20 mmol/l, for Mn2+ at 0.8 mmol/l, for Mg2+ at 4.0 mmol/l, the pH optimum is 8.0. Its Km is 1.5 mumol/l using dTTP as substrate. The enzyme activity described here is free of endonuclease but contains detectable amounts of exonuclease.

Studies on the regulation of the branched chain amino acyl-tRNA synthetases of the fungusNeurospora crassa

The specific activities of the branched chain amino acyl-tRNA synthetases from the cytosolic and mitochondrial fractions ofN. crassa were low in dormant conidia and increased during germination, reaching a maximum 8 h after inoculation. This stage of development is characterised by high rates of many other cellular activities.The increases in activity of synthetases of both cytosol and mitochondria are inhibited by cycloheximide indicating that they are synthesized on cytoplasmic ribosomes. The mitochondrial synthetases show a stimulation of their specific activity when mitochondrial RNA and protein synthesis are inhibited by either ethidium bromide or chloramphenicol suggesting that a mitochondrial translation product regulates the synthesis of the mitochondrial synthetases.The activities of amino acyl-tRNA synthetases are dependent on energy production. When respiration is uncoupled from oxidative phosphorylation, synthetase specific activities decrease although the activities of other mitochondrial enzymes like NADH-dehydrogenase increase. This phenomenon suggests that more than one mechanism regulates the synthesis of mitochondrial proteins which are formed on cytoplasmic ribosomes.The synthesis of branched chain amino acyl-tRNA synthetases ofNeurospora is neither repressed by their cognate amino acids, nor is there inhibition by the precursors of these amino acids, as has been observed in other amino acyl-tRNA synthetases of various organism includingNeurospora.

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.

Increased DNA polymerase and ATP-dependent deoxyribonuclease activities following DNA damages in mycobacterium smegmatis

Treatment of growing cultures of Mycobacterium smegmatis with alkylating agents (methyl methanesulphonate, ethyl methanesulphonate, nitrogen mustard, or mitomycin C) or with ultraviolet light resulted in enhanced specific activities of a DNA polymerase and of an ATP-dependent deoxyribonuclease. Similar results had previously been obtained with hydroxyurea and with iron limitation. The three of these treatments which were tested (methyl methanesulphonate, mitomycin C and hydroxyurea) produced strand breaks or alkali-labile regions in the DNA of this organism. The increased enzyme activities could be prevented by simultaneous treatment with inhibitors of protein synthesis. In contrast, treatment of the cultures with intercalating agents (ethidium bromide, acridine orange, or proflavine), 5-fluorouracil, caffeine, or nalidixic acid, inhibited DNA synthesis without increasing the enzyme activities. These treatments did not produce strand breaks in the DNA of this organism. The results support the hypothesis that, in M. smegmatis, damage to DNA induces increased synthesis of enzymes associated with DNA repair.

DNA-dependent DNA polymerase from yeast mitochondria. Dependence of enzyme activity on conditions of cell growth, and properties of the highly purified polymerase

The activity of DNA polymerase was determined in gradient-purified mitochondria from yeast cells grown under a variety of conditions. The specific enzyme activity was found to be dependent on the degree of aeration of the cells, and on the carbon source used for the medium. It was sensitive to glucose repression, and was enhanced about two-fold by the growth of yeast cells in the presence of ethidium bromide. Mitochondria DNA polymerase was highly purified and several properties were determined. Sucrose density gradient centrifugation, and dodecylsulfate-polyacylamide gel electrophoresis revealed the following structure: a monomer of molecular weight around 60 000 aggregated under relatively high salt concentration (0.2 M phosphate buffer) to a dimer of about 120 000 which under low salt concentration (0.2 M Tris-HCl buffer) formed higher aggregates. For optimal activity an Mg2+ ion concentration of 50 mM was found necessary, Mn ions did not promote activity at any concentration tested (0.5--50 mM). Indeed, if added to Mg2+-containing assays, Mn2+ strongly inhibited enzyme activity at low concentrations. This might be an explanation for the inducation of mitochondrial mutants in yeast cells grown in the presence of Mn2+ ions. Mitochondrial DNA polymerase activity was strongly inhibited by low concentrations of the -SH reagent p-chloromercuribenzoate, the nucleotide analogue cytosine arabinoside triphosphate also exerted an inhibitory effect. An about 50% decrease of activity was observed in the presence of 1 mM o-phenanthroline in assay mixture containing DNA at about the Km concentration. The enzyme preferred a gapped template primer, poly(dA) - (dT)10, over nicked DNA and was unable to use a polyribonucleotide template, poly(rA) - (dT)10. In the purest preparations no exonuclease activity could be detected.

A DNA polymerase from Ustilago maydis. 1. Purification and properties of the polymerase activity

A DNA polymerase from Ustilago maydis has been purified to apparent homogeneity. The native enzyme possesses a subunit structure consisting of 50000 and 55000-dalton monomers. The apparent sedimentation coefficient of the polymerase activity in the absence of salt is 8.4 S (Mr=180000-200000), that in its presence (0.6 M NaCl or 0.12 M KCl) being 6.3 S (Mr=80000-100000). Low concentrations of EDTA also converted the 8.4-S to a 6.3-S form, whereas magnesium ions catalysed the reverse association. The enzyme has an absolute requirement for both a DNA or RNA template and a DNA primer. For homopolymer templates the primer requirement was satisified by a short complementary oligodeoxynucleotide, but oligoribonucleotides were extremely inefficient primers. With the template-primer poly(dA) X (dT)12, the enzyme added an average of 50 dTMP nucleotides on to each primer molecule, whereas with poly(rA) X (dT)12, this figure was 300. The enzyme also possesses an associated deoxyribonuclease activity. No other DNA polymerase activity was detected in cell-free extracts of U. maydis.

Iolation and charcterization of a DNA-binding non-histone protein from Tetrahymena pyriformis

Three proteins (A, B and C) that bind specifically to single-stranded DNA have been isolated from the eukaryotic organism Tetrahymena pyriformis. Their molecular weights are 47 000, 41 000 and 32 000. The amino acid composition of the A protein indicates that it is a non-histone protein and sucrose gradient centrifugation shows that it binds to bacteriophage M13 DNA and to oligo (dT)100 in a cooperative manner. The exonucleolytic degradation of oligo (dT)100 is prevented when it is bound to the A protein. The effect of A protein on the exonucleolytic reaction confirms the cooperative manner of binding of A protein binding to oligo (dT)100 and shows that this process may be prevented by high ionic strength. The A protein seems to be without enzymatic activity.

Accumulation of replicative intermediates of mitochondrial DNA in Tetrahymena pyriformis grown in ethidium bromide

The effect of growth of Tetrahymena pyriformis in ethidium bromide (EthBr) on the structure and synthesis of mitochondrial DNA (mtDNA) has been investigated. During the first 5 h of growth in EthBr, mtDNA synthesis is inhibited 95% or more. After 10-15 h, this block is partially released and large numbers of replicating molecules accumulate, indicating that inhibition by EthBr primarily affects the rate of chain growth and not the initiation of new rounds of replication. The accumulated molecules sediment more rapidly than normal Tetrahymena mtDNA and do not contain enough single-strand regions to distinguish them from normal Tetrahymena mtDNA when banded in buoyant CsCl or NaI gradients. Electron microscopy shows that the predominant species in this rapidly sedimenting DNA is a linear molecule containing one symmetrical double-stranded replication loop of varying size located at its center. No degradation of mtDNA from cells grown in EthBr was detected in alkaline velocity gradients.