Purification and characterization of a DNA polymerase beta from Drosophila*

Antiviral potential of plant polysaccharide nanoparticles actuating non-specific immunity

The development of high-end targeted drugs and vaccines against modern pandemic infections, such as COVID-19, can take a too long time that lets the epidemic spin up and harms society. However, the countermeasures must be applied against the infection in this period until the targeted drugs became available. In this regard, the non-specific, broad-spectrum anti-viral means could be considered as a compromise allowing overcoming the period of trial. One way to enhance the ability to resist the infection is to activate the nonspecific immunity using a suitable driving-up agent, such as plant polysaccharides, particularly our drug Panavir isolated from the potato shoots. Earlier, we have shown the noticeable anti-viral and anti-bacterial activity of Panavir. Here we demonstrate the pro-inflammation activity of Panavir, which four-to-eight times intensified the ATP and MIF secretion by HL-60 cells. This effect was mediated by the active phagocytosis of the Panavir particles by the cells. We hypothesized the physiological basis of the Panavir proinflammatory activity is mediated by the indol-containing compounds (auxins) present in Panavir and acting as a plant analog of serotonin.

Magnetic isotope and magnetic field effects on the DNA synthesis

Magnetic isotope and magnetic field effects on the rate of DNA synthesis catalysed by polymerases β with isotopic ions (24)Mg(2+), (25)Mg(2+) and (26)Mg(2+) in the catalytic sites were detected. No difference in enzymatic activity was found between polymerases β carrying (24)Mg(2+) and (26)Mg(2+) ions with spinless, non-magnetic nuclei (24)Mg and (26)Mg. However, (25)Mg(2+) ions with magnetic nucleus (25)Mg were shown to suppress enzymatic activity by two to three times with respect to the enzymatic activity of polymerases β with (24)Mg(2+) and (26)Mg(2+) ions. Such an isotopic dependence directly indicates that in the DNA synthesis magnetic mass-independent isotope effect functions. Similar effect is exhibited by polymerases β with Zn(2+) ions carrying magnetic (67)Zn and non-magnetic (64)Zn nuclei, respectively. A new, ion-radical mechanism of the DNA synthesis is suggested to explain these effects. Magnetic field dependence of the magnesium-catalysed DNA synthesis is in a perfect agreement with the proposed ion-radical mechanism. It is pointed out that the magnetic isotope and magnetic field effects may be used for medicinal purposes (trans-cranial magnetic treatment of cognitive deceases, cell proliferation, control of the cancer cells, etc).

The multi-replication protein A (RPA) system--a new perspective

Replication protein A (RPA) complex has been shown, using both in vivo and in vitro approaches, to be required for most aspects of eukaryotic DNA metabolism: replication, repair, telomere maintenance and homologous recombination. Here, we review recent data concerning the function and biological importance of the multi-RPA complex. There are distinct complexes of RPA found in the biological kingdoms, although for a long time only one type of RPA complex was believed to be present in eukaryotes. Each complex probably serves a different role. In higher plants, three distinct large and medium subunits are present, but only one species of the smallest subunit. Each of these protein subunits forms stable complexes with their respective partners. They are paralogs as complex. Humans possess two paralogs and one analog of RPA. The multi-RPA system can be regarded as universal in eukaryotes. Among eukaryotic kingdoms, paralogs, orthologs, analogs and heterologs of many DNA synthesis-related factors, including RPA, are ubiquitous. Convergent evolution seems to be ubiquitous in these processes. Using recent findings, we review the composition and biological functions of RPA complexes.

Subunit protein-affinity isolation of Drosophila DNA polymerase catalytic subunit

gfLittle is known at present about the biochemical properties of very large-sized Drosophila DNA polymerases. In a previous study, we tried to purify Drosophila pol. catalytic subunit from embryos through seven column chromatographies and study its biochemical properties. However, we failed to characterize it precisely because an insufficient amount of the enzyme was generated. In this report, we describe direct purification from Drosophila embryos to near homogeneity using Drosophila DNA polymerase second subunit (Drosophila pol. 2) protein-conjugated affinity column chromatography and characterization of the enzyme in detail. To our knowledge this is the first demonstration of native DNA polymerase purification with activity using a subunit protein-affinity column. We observed new characteristics of Drosophila pol. catalytic subunit as follows: Drosophila pol. catalytic subunit synthesized DNA processively in the presence of both Mn(2+) and Mg(2+) ions, but Mn(2+) inhibited the 3'-5' proofreading activity, thereby decreasing the fidelity of DNA replication by 50%.

Purification of Drosophila DNA polymerase zeta by REV1 protein-affinity chromatography

Studies on the biochemical properties of very-large-size eukaryotic DNA polymerases have been limited by the difficulty in obtaining sufficient purified forms of each enzyme. Our aim was to determine and elucidate the biochemical properties of one such polymerase, pol zeta (DNA polymerase zeta) from Drosophila melanogaster (Dmpol zeta). Using an REV1 (UV-revertible gene 1) protein-affinity column, we have isolated the enzyme directly from Drosophila embryos. Completely purified Dmpol zeta was found to have a molecular mass of approx. 240 kDa, and to be sensitive to aphidicolin and resistant to ddTTP (2',3'-dideoxythymidine-5-triphosphate) and N-ethylmaleimide. The enzyme has a preference for poly(dA)/oligo(dT)(10:1) as a template primer and has high processivity for DNA synthesis. Moreover, Dmpol zeta showed significantly higher fidelity compared with Rattus norvegicus DNA polymerase, an error-prone DNA polymerase, in an M13 forward mutation assay. The activities of bypassing pyrimidine dimers and (6-4) photoproducts and extending from mismatched primer-template termini in (6-4) photoproduct by Dmpol zeta were not detected. Drosophila REV7 interacted with Dmpol zeta in vitro, but did not influence the DNA synthesis activity of Dmpol zeta. The present study is the first report about characterization of purified pol zeta from multicellular organisms, and the second concerning the characterization of yeast pol zeta.

Molecular cloning and expression during development of the Drosophila gene for the catalytic subunit of DNA polymerase epsilon

We have cloned the genomic DNA and cDNA of Drosophila DNA polymerase epsilon (pol-epsilon) catalytic subunit (GenBank No. AB035512). The gene is separated into four exons by three short introns, and the open reading frame consists of 6660 base pairs (bp) capable of encoding a polypeptide of 2220 amino acid residues. The calculated molecular mass is 255018, similar to that of mammalian and yeast homologues. The deduced amino acid sequence of the pol-epsilon catalytic subunit shares approximately 41% identity with human and mouse homologues as well as significant homology those of C. elegans, S. cerevisiae and S. pombe. Similar to the pol-epsilon catalytic subunits from other species, the pol-epsilon catalytic subunit contains domains for DNA polymerization and 3'-5' exonuclease in the N-terminal region, and two potential zinc-finger domains in the C-terminal regions. Interestingly, a 38 amino acid sequence in the C-terminal region from amino acid positions 1823 to 1861 is similar to the site for Mycoplasma ATP binding and/or ATPase domain (GenBank No. P47365). Northern hybridization analysis indicated that the gene is expressed at the highest levels in unfertilized eggs, followed by zero to 4h embryos and adult females, and then embryos at other embryonic stages, instar larva stages and adult males. Low levels of the mRNA were also detected at the pupa stage. This pattern of expression is similar to those of DNA replication-related enzymes such as DNA polymerase alpha and delta except for the high level of expression in adult males.

Lucidenic acid O and lactone, new terpene inhibitors of eukaryotic DNA polymerases from a basidiomycete, Ganoderma lucidum

Terpenoids, 1, 2 and 3, which selectively inhibit eukaryotic DNA polymerase activities, were isolated from the fruiting body of a basidiomycete, Ganoderma lucidum, and their structures were determined by spectroscopic analyses. New terpenes, lucidenic acid O (1) and lucidenic lactone (2), prevented not only the activities of calf DNA polymerase alpha and rat DNA polymerase beta, but also these of human immunodeficiency virus type 1 reverse transcriptase. Cerevisterol (3), which was reported to be a cytotoxic steroid, inhibited only the activity of DNA polymerase alpha. Although these compounds did not influence the activities of prokaryotic DNA polymerases and other DNA metabolic enzymes such as T7 RNA polymerase and deoxyribonuclease I.

A new DNA polymerase species from Drosophila melanogaster: a probable mus308 gene product

Harris et al. [P.V. Harris, O.M. Mazina, E.A. Leonhardt, R.B. Case, J.B. Boyd, K.C. Burtis, Molecular cloning of Drosophila mus308, a gene involved in DNA cross-link repair with homology to prokaryotic DNA polymerase I genes, Mol. Cell. Biol., 16 (1996) 5764-5771.] reported the molecular cloning of Drosophila mus308 gene, and its nucleotide and protein sequences similar to DNA polymerase I. In the present study, we attempted to find and isolate the gene product by purifying a DNA polymerase fraction not present in mus308 flies. A new DNA polymerase with properties different from those of any known polymerase species was identified and partially purified from the wild-type fly embryos through ten column chromatographies. The enzyme was resistant to aphidicolin, but sensitive to ddTTP and NEM. Human proliferating cell nuclear antigen (PCNA) and Drosophila replication protein A (RP-A) did not affect the polymerase activity. It preferred poly(dA)/oligo(dT) as a template-primer. The molecular mass was about 230 kDa with a broad peak region of 200 to 300 kDa in HiPrep16/30 Sephacryl S-300 gel filtration. These properties a different from those of all reported Drosophila polymerase classes such as alpha, beta, gamma, delta, epsilon and zeta and closely resemble those of the gene product expected from the nucleotide sequence. The new polymerase species appears to have ATPase and 3'-5' exonuclease activities as shown by the chromatographies.

4-Hydroxy-17-methylincisterol, an inhibitor of DNA polymerase-alpha activity and the growth of human cancer cells in vitro

An ergosterol derivative, 4-hydroxy-17-methylincisterol (HMI), was found to be an inhibitor of mammalian DNA polymerases in vitro. HMI inhibited the activity of calf thymus DNA polymerase alpha (pol. alpha). Among the polymerases tested, pol. alpha was the most sensitive to inhibition by HMI, and the inhibition was concentration dependent. The inhibitory effect of HMI on pol. alpha was almost the same as that shown by aphidicolin, a well-known potent pol. alpha inhibitor. HMI had relatively less effect on rat DNA pol. beta, human immunodeficiency virus type 1 reverse transcriptase (HIV-RT), and calf thymus terminal deoxynucleotidyl transferase (TdT) in vitro, and did not influence the activities of prokaryotic DNA polymerases such as Klenow Fragment of DNA polymerase I, or the DNA-metabolic enzyme DNase I. HMI was found to be able to prevent the growth of human cancer cell lines originating from patients with leukemia or various solid tumors; its IC50 values ranged from 7.5 to 12 microM. We also synthesized other ergosterol derivatives and tested them, and found that two compounds, 17-methylincisterol and 4-acetyl-17-methylincisterol, have similar inhibitory effects.

A mushroom fruiting body-inducing substance inhibits activities of replicative DNA polymerases

We found and isolated two natural products in the extract from a basidiomycete, Ganoderma lucidum, as eukaryotic DNA polymerase inhibitors. The compounds were identified as cerebrosides, (4E,8E)-N-D-2'-hydroxypalmitoyl- 1-O-beta-D-glucopyranosyl-9-methyl-4,8-sphingadienine and (4E,8E)-N-D-2'-hydroxystearoyl-1-O-beta-D-glucopyranos yl-9-methyl- 4,8-sphingadienine and were found to be identical to the mushroom fruiting body-inducing substances (FIS) reported. These cerebrosides selectively inhibited the activities of replicative DNA polymerases, especially the alpha-type, from phylogenetically broad eukaryotic species, whereas they hardly influenced the activities of DNA polymerase beta, prokaryotic DNA polymerases, terminal deoxynucleotidyl transferase, HIV reverse transcriptase, RNA polymerase, deoxyribonuclease I, and ATPase. The inhibition of another replicative polymerase, the delta-type, was moderate. The inhibitions of the replicative polymerases were dose-dependent, and the IC50 for animal or mushroom DNA polymerase alpha was achieved at approximately 12 micrograms/ml (16.2 microM) and for animal DNA polymerase delta at 57 micrograms/ml (77.2 microM). FIS is possibly a DNA polymerase inhibitor specific to the replicative enzyme group, and the fruiting body formation may be required for the suppression of the DNA replication or the vegetative growth of the mycelium.

Studies on inhibitors of mammalian DNA polymerase alpha and beta: sulfolipids from a pteridophyte, Athyrium niponicum

Three sulfolipid compounds, 1, 2, and 3, have been isolated from a higher plant, a pteridophyte, Athyrium niponicum, as potent inhibitors of the activities of calf DNA polymerase alpha and rat DNA polymerase beta. The inhibition by the sulfolipids was concentration dependent, and almost complete inhibition of DNA polymerase alpha and DNA polymerase beta was achieved at 6 and 8 microg/mL, respectively. The compounds did not influence the activities of calf thymus terminal deoxynucleotidyl transferase, prokaryotic DNA polymerases such as the Klenow fragment of DNA polymerase I, T4 DNA polymerase and Taq polymerase, the DNA metabolic enzyme DNase I, and even a DNA polymerase from a higher plant, cauliflower. Similarly, the compounds did not inhibit the activity of the human immunodeficiency virus type 1 reverse transcriptase. The kinetic studies of the compounds showed that DNA polymerase alpha was inhibited non-competitively with respect to the DNA template and substrate, whereas DNA polymerase beta was inhibited competitively with both the DNA template and substrate. The binding to DNA polymerase beta could be stopped with non-ionic detergent, but the binding to DNA polymerase alpha could not.

Fomitellic acids, triterpenoid inhibitors of eukaryotic DNA polymerases from a basidiomycete, Fomitella fraxinea

Four new triterpenoid compounds, 1-4, were isolated from the mycelium of a basidiomycete, Fomitella fraxinea, and their structures determined by spectroscopic analyses. Compounds 1-5 inhibited calf DNA polymerase alpha and rat DNA polymerase beta, with respective minimum inhibitory concentration (MIC) ranges of 35-75 and 90-130 microM.

DNA polymerase epsilon from Drosophila melanogaster

We identified a DNA polymerase species in Drosophila melanogaster embryos, and purified it. This polymerase shared some common properties with DNA polymerase epsilon from mammals and yeast as follows; it has a preference for poly(dA)/oligo(dT) as a template/primer, it is highly processive in DNA synthesis, it co-fractionates with 3'-5' exonuclease activity, it is sensitive to aphidicolin and is resistance to ddTTP. The polymerase activity was inhibited in the immuno-precipitation assay with anti-pol-epsilon antibodies, which were produced against a polypeptide coded on the cDNA of a putative Drosophila pol-epsilon we isolated previously. Using these antibodies, Western blot analysis revealed that this polymerase is a 250kDa polypeptide, which is the same size as observed in mammals and yeast. These results indicate that Drosophila produces the epsilon-class of DNA polymerase, and like mammals or yeast, possesses the 5 typical classes of DNA polymerases (alpha to epsilon) in its embryos.

Fatty acids selectively inhibit eukaryotic DNA polymerase activities in vitro

The in vitro relationship between eukaryotic DNA polymerases and fatty acids was investigated. Some fatty acids strongly inhibited the activities of DNA polymerase alpha and/or beta in vitro. The kinetics of inhibition by linoleic acid showed that DNA polymerase alpha was non-competitively inhibited with respect to the DNA template and substrate (dTTP), while DNA polymerase beta was inhibited competitively with both DNA and substrate.

Potential role of DNA polymerase beta in gene therapy against cancer: a case for colorectal cancer

Genetic instability characterized by the accumulation of mutations of tumor suppressor genes and oncogenes appears to be associated with carcinogenesis in colorectal and other cancers. Mutations of DNA polymerase beta (pol beta) and related chromosomal alterations appear to be consistent with the causal role of a "mutator phenotype' in carcinogenesis. However, homozygous knockout pol beta mutations appear to interfere with embryogenesis. Increased pol beta activity (i.e. relative to pol alpha activity) has been associated with cell cycle arrest. The related aphidicolin-resistant DNA replication has been observed primarily in differentiating cells, including the mammalian blastocyst, adrenal cortex, thyroid, anterior pituitary, and the mechanism of endoreduplication (amitotic over-replication of DNA) can be traced to lower eukaryotes. This increased activity in relation to terminal commitment is inconsistent with a simple "DNA repair' view of pol beta. It is therefore proposed that pol beta may play a more fundamental role in cellular differentiation through involvement in a putative subgenomic DNA replication-based model of terminal gene expression. Thus genetic instability, loss of differentiation, and carcinogenesis may result from aberration(s) or "derailment' of such replication-based mechanism of terminal gene expression. It is suggested to examine the relationship of DNA pol beta to genomic instability and carcinogenesis using genetic analyses and antisense technology with possible applications for gene therapy against colorectal cancer.

Purification and characterization of a beta-like DNA polymerase from Trypanosoma cruzi

A DNA polymerase was purified to near homogeneity from Trypanosoma cruzi epimastigotes. This preparation had a major polypeptide of 50 kDa and a minor band of 45 kDa. SDS-PAGE studies and a novel colorimetric activity gel technique demonstrated that the 50-kDa polypeptide chain is the catalytic subunit of this T. cruzi DNA polymerase. Western blot analysis of different purification stage fractions strongly suggests that this 50-kDa protein is the intact catalytic subunit and does not correspond to a degradation product from a larger one. This T. cruzi DNA polymerase is insensitive to aphidicolin, butylphenyldeoxyguanosine triphosphate, berenil, ethidium bromide and N-ethylmaleimide, but is markedly inhibited by the dideoxythymidine triphosphate analogue. Studies with different DNA templates showed that the DNA polymerase prefers activated DNA as substrate and that it cannot elongate oligoriboadenylate primers. The data presented in this paper are consistent with the hypothesis that this enzyme corresponds to a beta-like DNA polymerase present in the parasitic protozoon T. cruzi.

Effects of DNA polymerase beta gene over-expressed in transgenic Drosophila on DNA repair and recombination

DNA polymerase beta (pol beta) cDNA of rat fused to an enhancer-promoter region plus a poly(A) signal sequence of actin 5C gene of Drosophila (abbreviated pol beta) was transferred to the Drosophila genome. Three of four constructed transgenic strains possessing transgene pol beta on different chromosomes were studied. Levels of the pol beta transcript and those of the polymerization activity of pol beta were markedly elevated in cultured cells transfected with pol beta-bearing vectors as well as in embryos of the transgenic strains. The popular idea that DNA polymerase beta participates in DNA repair was not supported by the observation that a pair of a normal and a pol beta strain, and the other pair of a mei-9 mei-41 (DNA-repair deficient double mutations) strain and a pol beta mei-9 mei-41 strain, showed no difference in survival within each pair after treatment with ultraviolet light, methylmethane sulfonate and mitomycin C. The other idea that DNA polymerase beta participates in recombination was supported by the findings that spontaneous frequency of recombination, either meiotic or mitotic, is significantly higher in a transgenic pol beta strain than in a non-transgenic strain. The enhanced recombination frequency in the pol beta strain may, however, reflect an indirect effect of over-produced pol beta proteins on chromosomal stability. Whatever the direct effect of rat pol beta is, the transgenic pol beta flies will be useful for study of the physiological role of pol beta and the mechanism of recombination.

A meiotic DNA polymerase from Coprinus cinereus: further purification and characterization

A meiotic DNA polymerase that is present at a high level of activity in meiotic cells of a basidiomycete, Coprinus cinereus, was purified to near homogeneity using synthetic RNA homopolymer [poly(C)] cellulose column chromatography. This report presents the first extensive purification and characterization of any eukaryotic DNA polymerase having a role in meiosis. This enzyme is a single polypeptide with a molecular mass of 65,000. Activity in this enzyme requires magnesium ions and occurs at an optimal pH of 7.5. It is strongly inhibited by dideoxythymidine triphosphate but is relatively insensitive to aphidicolin and N-ethylmaleimide and can use poly(C)/oligo(dG)12-18 as a template-primer. Polymerase activity can be found only in cells at meiotic prophase, even though the enzyme has been identified in somatic cells in an inactive state using immunoblot analysis. Its distinctive distribution makes possible a genetic and biochemical analysis of functional role of a meiotic DNA polymerase in meiotic recombination, repair and synthesis.

Potentiation of RNA polymerase II transcription by Gal4-VP16 during but not after DNA replication and chromatin assembly

Purified, reconstituted chromatin templates containing regular, physiological nucleosome spacing were transcribed in vitro by RNA polymerase II along with the Gal4-VP16 activator. When Gal4-VP16 was prebound to DNA before reconstitution of either H1-deficient or H1-containing chromatin, the resulting templates were transcribed with a similar efficiency. Under such conditions, we observed long-range (1000 bp) activation of transcription in vitro with H1-containing chromatin, but not naked DNA templates. When Gal4-VP16 was added to preassembled chromatin, the H1-deficient chromatin was transcriptionally active, whereas the H1-containing chromatin, which possessed properties similar to native chromatin, was transcriptionally inert. We then mimicked DNA replication and chromatin assembly at a replication fork and found that Gal4-VP16 could potentiate transcription during, but not after, replication and assembly of histone H1-containing chromatin. These experiments provide biochemical data that support a DNA replication-dependent mechanism for reconfiguration of chromatin structure and activation of transcription by Gal4-VP16.

DNA polymerase-beta from the pupal ovaries of Bombyx mori

The silk glands of Bombyx mori, a highly replicative tissue contains high levels of DNA polymerases alpha, delta and epsilon but not DNA polymerase-beta. However, we detected the latter activity in the gonadal tissues, viz. the pupal ovaries and testes of B. mori. The enzyme has been purified to homogeneity from the pupal ovaries by a series of column chromatographic and affinity purification steps. The enzyme satisfied the criteria to be designated as DNA polymerase-beta based on its small size, requirement for high concentration of monovalent cations for catalytic activity, sensitivity to ddTTP and insensitivity to aphidicolin. It is a monomeric polypeptide of M(r) 40 kDa, and the Km for dNTPs ranges between 8-20 microM. DNA polymerase-beta is biochemically and immunologically distinct from DNA polymerase-alpha from the silk glands of B. mori. The enzyme showed a preference for gapped DNA, and could not elongate ultraviolet irradiated template beyond the pyrimidine dimers. The absence of any associated primase and exonuclease activities from this enzyme, and its conspicuous absence in the highly replicative tissue, imply that it is unlikely to participate in the DNA endoreplication process.

Nuclease modification in Chinese hamster cells hypersensitive to DNA cross-linking agents--a model for Fanconi anemia

Fanconi anemia is a human inherited disease that is characterized by cellular hypersensitivity to DNA cross-linking agents. A number of potential experimental models for that disorder have been developed by selecting mutants that are hypersensitive to bifunctional mutagens. The six mutants of that class in Drosophila, all of which map to the mus308 locus, express an alteration in a mitochondrial nuclease. A recent extension of that observation to cell lines from complementation group A of Fanconi anemia has established a new cellular phenotype for that disorder. In the current study an analogous enzyme has been analyzed in eight recently isolated Chinese hamster cell lines that are hypersensitive to cross-linking agents. Among these lines. V-H4 and V-B7 are shown to exhibit an enzyme modification analogous to that observed in the mutant Drosophila and human cells. These results validate the nuclease assay as an indicator of the Fanconi defect and further establish the V-H4 cell line as a valuable cellular model for analysis of the Fanconi A defect.

Relationship of eukaryotic DNA replication to committed gene expression: general theory for gene control

The historic arguments for the participation of eukaryotic DNA replication in the control of gene expression are reconsidered along with more recent evidence. An earlier view in which gene commitment was achieved with stable chromatin structures which required DNA replication to reset expression potential (D. D. Brown, Cell 37:359-365, 1984) is further considered. The participation of nonspecific stable repressor of gene activity (histones and other chromatin proteins), as previously proposed, is reexamined. The possible function of positive trans-acting factors is now further developed by considering evidence from DNA virus models. It is proposed that these positive factors act to control the initiation of replicon-specific DNA synthesis in the S phase (early or late replication timing). Stable chromatin assembles during replication into potentially active (early S) or inactive (late S) states with prevailing trans-acting factors (early) or repressing factors (late) and may asymmetrically commit daughter templates. This suggests logical schemes for programming differentiation based on replicons and trans-acting initiators. This proposal requires that DNA replication precede major changes in gene commitment. Prior evidence against a role for DNA replication during terminal differentiation is reexamined along with other results from terminal differentiation of lower eukaryotes. This leads to a proposal that DNA replication may yet underlie terminal gene commitment, but that for it to do so there must exist two distinct modes of replication control. In one mode (mitotic replication) replicon initiation is tightly linked to the cell cycle, whereas the other mode (terminal replication) initiation is not cell cycle restricted, is replicon specific, and can lead to a terminally differentiated state. Aberrant control of mitotic and terminal modes of DNA replication may underlie the transformed state. Implications of a replicon basis for chromatin structure-function and the evolution of metazoan organisms are considered.

Alteration of a nuclease in Fanconi anemia

Fanconi anemia is a cancer-prone disease characterized by progressive loss of blood cells, skeletal defects and stunted growth. Studies of a nuclease acting on double-stranded DNA have revealed an enzyme alteration in cells derived from Fanconi patients. A particulate fraction isolated from cultured human lymphoblasts and fibroblasts was solubilized with detergent and subjected to isoelectric focusing. Nuclease activity observed in four normal cell lines bands in a pH gradient with a pI of 6.3. Four cell lines belonging to complementation group A exhibit an increase in the pI of that nuclease to 6.8. These observations provide a new diagnostic for this disorder. Analysis of this enzyme in tetraploid cultures derived from fusion of normal and Fanconi cells suggest that the normal phenotype is dominant. That observation supports the hypothesis that the Fanconi A gene is required for modification of the nuclease pI. Definition of the molecular basis of this enzyme alteration should provide insight into the primary genetic lesion in this disorder.

Purification and characterization of DNA polymerases from Plasmodium berghei

DNA polymerases from the malaria parasite Plasmodium berghei were purified more than 50-fold. Several distinct enzymatic activities were isolated that could be distinguished by the use of various specific DNA polymerase inhibitors. In particular, subdivision into an aphidicolin-sensitive and an aphidicolin-resistant group was possible. Further analysis allowed a better comparison with host DNA polymerases and indicated that one aphidicolin-sensitive DNA polymerase resembled DNA polymerase alpha displaying processive DNA synthesis and using RNA primers, whereas another aphidicolin-sensitive DNA polymerase was distributive and only used DNA primers. Marked differences from the host enzymes do exist, however, such as insensitivity to BuPdGTP. Another P. berghei DNA polymerase was isolated that showed characteristics of a DNA polymerase beta-like enzyme, but which differed from host DNA polymerase beta in its insensitivity to dideoxynucleotides.

A mitochondrial nuclease is modified in Drosophila mutants (mus308) that are hypersensitive to DNA crosslinking agents

The mus308 mutants of Drosophila have previously been demonstrated to be defective in an enzyme that is designated Nuclease 3 (Boyd et al. 1990b). In this study that enzyme is shown to be present in mitochondria of both wild-type flies and embryos. Since the mus308 mutants are hypersensitive to DNA crosslinking agents. Nuclease 3 is potentially required for resistance of the mitochondrial genome to such agents. In support of this hypothesis, electron microscopic studies of mus308 mutant flies that had been exposed to nitrogen mustard revealed an increased frequency of mitochondrial abnormalities. Further investigation of the defect at the enzymological level revealed that the mutants possess a new nuclease activity that is apparently a modified form of the wild-type protein. In the earlier study, enzyme extracts from mus308 mutants were found to lack an enzyme with a pI of approximately 6.2. More precisely defined assay conditions in this study revealed the appearance of a new nuclease activity with a higher pI in extracts from mutants. This observation, together with the finding that only the normal enzyme form is present in heterozygous individuals, supports the hypothesis that the mus308 locus is not the structural gene for the enzyme. Rather, the mus308 gene product is necessary for Nuclease 3 to assume the lower pI. Nuclease 3 has been partially purified and characterized from wild-type embryos. Its activity is stimulated by Mg++ and ATP. Optimum activity is found at a pH of 5.5 and a NaCl concentration of 50-100 mM. Nuclease 3 exhibits a temperature optimum of 42 degrees C and is insensitive to N-ethylmaleimide.(ABSTRACT TRUNCATED AT 250 WORDS)

Levels and size complexity of DNA polymerase beta mRNA in rat regenerating liver and other organs

A cDNA probe encoding DNA polymerase beta (beta-pol) was used to study the level and size complexity of beta-pol mRNA in regenerating rat liver and other rat tissues. An almost 2-fold increase in beta-pol mRNA was observed 18-24 h after partial hepatectomy. In most adult rat tissues (liver, heart, kidney, stomach, spleen, thymus, lung and brain) the abundance of beta-pol mRNA was low. In contrast, young brain and testes exhibited beta-pol mRNA levels 5- and 15-times higher, respectively. The observed changes in the level of beta-pol mRNA in regenerating rat liver and in developing brain are correlated with reported changes in DNA polymerase beta activity. Four different (4.0, 2.5, 2.2, 1.4 kb) transcripts hybridizing to beta-pol probe were found in all tissues examined. The 4.0 kb transcript was dominant for young and adult brain, whereas the 1.4 kb transcript was dominant for testes. The significance of these transcripts is discussed.

The influence of a protein synthesis inhibitor on sister-chromatid exchange in the plant Vicia faba

Cycloheximide strongly antagonizes the induction of sister-chromatid exchange by mitomycin C in Vicia faba root tips. This behavior is analogous to that previously observed in mammalian cells (Sono and Sakaguchi, 1981) and suggests that newly synthesized protein is also required for recombination between sister DNA molecules in plants. Conversely hydroxyurea is shown to increase the frequency of both spontaneous and induced sister-chromatid exchange. Based on these results, possible mechanisms underlying sister-chromatid exchange formation in plants are discussed with special emphasis on the absence of DNA polymerase beta in somatic tissues.