Mutational studies of human DNA polymerase alpha. Lysine 950 in the third most conserved region of alpha-like DNA polymerases is involved in binding the deoxynucleoside triphosphate

The function of a lysine residue, Lys950, of human DNA polymerase alpha located in the third most conserved region and conserved in all of the alpha-like polymerases was analyzed by site-directed mutagenesis. Lys950 was mutagenized to Arg, Ala, or Asn. The mutant enzymes were expressed in insect cells infected with recombinant baculoviruses and purified to near homogeneity. The mutant enzymes had specific activities ranging from 8 to 22% of the wild type. All three Lys950 mutants utilized Mn2+ as metal activator more effectively than the wild type enzyme and showed an increase in Km values for deoxynucleoside triphosphate but not k(cat) values in reactions with either Mg2+ or Mn2+ as the metal activator. Although mutation of the Lys950 residue caused an increase in Km values for deoxynucleoside triphosphates, mutations of Lys950 to Arg, Ala, or Asn did not alter the mutant enzymes' misinsertion efficiency in reactions with Mg2+ as a metal activator as compared with that of the wild type, suggesting that the base of the incoming deoxynucleoside triphosphate is not the structural feature interacting with the Lys950 side chain. In reaction with Mn2+ as a metal activator, all three Lys950 mutants had an improved fidelity for deoxynucleotide misinsertion compared to wild type. Inhibition studies of the three Lys950 mutant derivatives with an inhibitor, structural analogs of deoxynucleoside triphosphate, and pyrophosphate suggest that the deoxyribose sugar and beta-,gamma-phosphate groups are not the structural feature recognized by the Lys950 side chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of herpes simplex virus 1 origin-binding protein with DNA polymerase alpha

The herpes simplex virus 1 (HSV-1) genome encodes seven polypeptides that are required for its replication. These include a heterodimeric DNA polymerase, a single-strand-DNA-binding protein, a heterotrimeric helicase/primase, and a protein (UL9 protein) that binds specifically to an HSV-1 origin of replication (oris). We demonstrate here that UL9 protein interacts specifically with the 180-kDa catalytic subunit of the cellular DNA polymerase alpha-primase. This interaction can be detected by immunoprecipitation with antibodies directed against either of these proteins, by gel mobility shift of an oris-UL9 protein complex, and by stimulation of DNA polymerase activity by the UL9 protein. These findings suggest that enzymes required for cellular DNA replication also participate in HSV-1 DNA replication.

Variation in protein and RNA synthesis activity in isolated mitochondria of the developing rice (Oryza sativa L.) panicle

We have studied variation in mitochondrial protein and RNA synthesis during the development of a specialized rice (Oryza sativa L.) reproductive organ in a bacteria-free environment. Mitochondria were prepared from the maturing panicle during microsporogenesis when meiosis occurred and from etiolated seedlings at two growth stages. We found (1) that there was no discernible qualitative difference among the polypeptides synthesized by these three mitochondrial samples; (2) that the quantity of proteins synthesized by panicle mitochondria was approximately 3 times that of the seedling mitochondria, while the two seedling samples exhibited only a minor quantitative difference; (3) that panicle and seedling mitochondria samples synthesized qualitatively the same RNA but at distinctly different rates and that more RNA products were synthesized by panicle than by seedling mitochondria. These results, taken together, suggest that either the regulation of mitochondrial transcription and translation or the copy number of mitochondrial DNA per mitochondrion change discretely in the developing panicle and consequently that the level of mitochondrial gene expression increases considerably during the development of the reproductive structure in rice.

The effect of recombinant human erythropoietin in treating the anemia of prematurity

Anemia of prematurity (AOP) has been conventionally treated with erythrocyte transfusions. Recent investigations have reported the use of recombinant human erythropoietin (rHuEPO) as an alternative for treating AOP. The potential of rHuEPO in increasing erythropoiesis implies its clinical usefulness. The effect of rHuEPO on reticulocyte count as well as other parameters of blood cells was examined in 14 premature babies with AOP. The average birth body weight and gestational age of these premature babies were 1533.71 +/- 61.66 g (Mean +/- SEM) and 31.36 +/- 0.49 weeks respectively. They received the first dose of rHuEPO at age 26.14 +/- 2.03 days with a hemoglobin level by 9.40 +/- 0.27 g/dL and hematocrit level of 28.20 +/- 0.81%. They were given rHuEPO 200 U/kg subcutaneously every other day for 10 doses, and iron 3 mg/kg and vitamin E 25 IU/kg per os every day. Average erythropoietin level of the patients on entry into this study was low (7.66 +/- 1.10 mu/mL). After treatment with rHuEPO for 20 days, the corrected reticulocyte count increased from 0.64 +/- 0.10% to 1.68 +/- 0.42% on Day 5 (P < 0.05), 1.96 +/- 0.41% on Day 12 (P < 0.05), 1.77 +/- 0.43% on Day 20 (P < 0.05), and hematocrit increased from 28.2 +/- 0.81% to 29.58 +/- 1.02% (p < 0.05) on Day 20. Bone marrow aspirates on Day 10 for 9 infants revealed moderate to high cellularity, mostly with erythroblasts (47.89 +/- 1.78%); the M/E ratio was low (0.57 +/- 0.05). The granulocyte series and megakaryocyte could be well visualised.(ABSTRACT TRUNCATED AT 250 WORDS)

Down-regulation of genes encoding DNA replication proteins during cell cycle exit

We have studied the expression of genes encoding DNA replication proteins during different cell growth events. Gene expression of human DNA polymerase alpha-DNA primase, a principal chromosomal replication enzyme complex, is up-regulated during the entrance of a cell from quiescence into the mitotic cell cycle. In contrast, expression of these genes is greatly reduced in fibroblasts rendered temporarily quiescent by contact inhibition or serum starvation. In actively cycling cells, DNA polymerase alpha-DNA primase genes are expressed at all stages of the cell cycle. To investigate how their gene expression is regulated in cells permanently exiting the cell cycle during terminal differentiation, we used a novel method to obtain a pure population of such cells. In this report, we describe the down-regulation of gene expression of DNA polymerase alpha during both HL-60 (human myeloid) and MEL (mouse erythroleukemia) cell differentiation. Gene expression of the two subunits of DNA primase, p49 and p58, is also down-regulated at the mRNA level in differentiated MEL cells. In differentiated HL-60 cells, the decline of DNA polymerase alpha gene expression occurs at both the transcript and protein levels. Down-regulation of DNA polymerase alpha at the steady state transcript level is caused, at least in part, by a decreased rate of transcription initiation without transcription elongation block.

Active oxygen species are involved in the induction of micronuclei by arsenite in XRS-5 cells

The human carcinogen, arsenic, is genotoxic to mammalian cells in vitro. The mechanism is largely unknown, although the involvement of free radicals has been suggested. Since the X-ray sensitive Chinese hamster ovary cell line, XRS-5, is also sensitive to several free-radical generating agents, including H2O2, we have used this cell line to test whether the genotoxic effect of arsenite is mediated via the generation of active-oxygen species. The results indicate that the XRS-5 cells are more sensitive to arsenite in terms of cell-killing and micronucleus induction compared to the parental CHO-K1 cells. The level of arsenic uptake and release, the levels of elementary components for arsenic detoxification, glutathione and glutathione S-transferase activities in these two cell lines are very similar. The XRS-5 cells, however, were shown to have 6-fold lower catalase activity in comparison to CHO-K1 cells. Moreover, catalase could effectively reduce the frequency of arsenite-induced micronuclei. These results indicate that the low catalase activity may be an important reason why XRS-5 cells are more sensitive to the toxic effects of arsenite, and arsenite probably induces micronuclei via the overproduction of H2O2. The XRS-5 cells had a higher background level of micronuclei, and were also more sensitive to gamma-rays in terms of induction of micronuclei. Catalase, however, did not reduce the background level or the frequency of gamma-ray-induced micronuclei. Therefore, the lower catalase activity seems to bear little relation to the high background level of micronuclei and the hypersensitivity to gamma-rays in micronucleus induction in XRS-5 cells.

Colon replacement from esophagus. Clinical experience from 240 cases

The clinical experience in the colon replacement of the esophagus in 240 cases is reported. The overall operative morbidity was 17.5%; the incidence of the anastomotic leaks was 10.4%; and the mortality was 2.80%. The points in surgical technique were: 1) utilizing the left colic artery as the supporting vessel if possible, based on the anatomy of the colon vessel; 2) using the colon segment in an isoperistaltic position, which has been proved much physiological; 3) single-layer anastomosis which is simple and reliable, with minimal inflammation and quick healing; and 4) choosing the channel of the colon transplant according to the disease condition, the age, and the function of the heart and the lung of the patients.

Enzymatic characterization of the individual mammalian primase subunits reveals a biphasic mechanism for initiation of DNA replication

The enzymatic mechanism of primase was investigated using Escherichia coli and baculoviral overexpressed mouse primase subunits, p49 and p58. Neither of the singly purified primase subunits displayed primase activity alone, but the p49 subunit was able to extend a riboprimer, indicating that this subunit contains an RNA polymerase activity. The p58 subunit cooperated with the p49 subunit in binding the initiating purine to form the initial dinucleotide. After initiation, the p49 subunit alone was sufficient to extend the growing primer, but both the rate of p49 primer extension and its stability were influenced by the p58 subunit. The Km(ATP) in primer synthesis on poly(dT) of the p49-p58 heterodimeric primase complex was 10-fold higher than the Km(ATP) of the single p49 subunit in a ribo(A) primer extension assay. In addition, labeled ATP cross-linked to both of the individually purified subunits but with a striking difference in affinities; cross-linking was 11-fold more efficient to the p49 subunit. The interaction of the two primase subunits with polymerase alpha was also investigated. Immunoprecipitation experiments indicate that only the p58 subunit directly contacts the p180 subunit of DNA polymerase alpha. Competition experiments in the coupled primase-polymerase assay with a catalytically inactive mutant of DNA polymerase alpha and the Klenow fragment suggest that the DNA polymerase alpha-primase complex does not dissociate from the primer during the transition from RNA to DNA synthesis.

Mutational studies of human DNA polymerase alpha. Serine 867 in the second most conserved region among alpha-like DNA polymerases is involved in primer binding and mispair primer extension

The second most conserved region of alpha-like DNA polymerases, region II, spans a block of 40 amino acid residues centered at the core sequence -DFNSLYPSII-. In the previous paper, we described mutational studies of 3 amino acid residues in region II which includes 2 amino acid residues in the core sequence. We showed that residues Asp860 and Tyr865 in the core sequence are involved in substrate deoxynucleotide triphosphate (dNTP) binding. We further showed that the phenyl moiety of the Tyr865 side chain interacts with the incoming dNTP and is responsible for the misinsertion fidelity of the enzyme. In this report, we investigated the function of 2 serine residues, Ser863 and Ser867, in this core sequence. Mutation of these 2 Ser residues to either Ala or Thr yielded mutant enzymes with similar Km for dNTPs, kcat, processivity, and misinsertion fidelity of DNA synthesis as the wild type enzyme. However, mutation of Ser867 to Ala demonstrated a 30-fold increase in Km for primer-template and a 5-fold higher KD for binding primer-template. DNA footprinting experiments of primer with the dideoxynucleotide terminus indicated that the structural feature of the primer recognized by Ser867 is the 3'-OH terminus. Single-stranded DNA inhibition data suggest that removal of the hydroxyl side chain of Ser867 affects the polymerase's interaction with primer and not with template. Mutation of Ser867 to Ala also decreases the mutant enzyme's Km for dNTP to extend a mispaired primer and thus enhances its capacity to extend a mispaired primer terminus. These data support the conclusion that the hydroxyl side chain of Ser867 of human DNA polymerase alpha is involved in primer interaction during DNA synthesis and plays an essential role in mispair extension fidelity of DNA synthesis.

Mutational studies of human DNA polymerase alpha. Identification of residues critical for deoxynucleotide binding and misinsertion fidelity of DNA synthesis

Conserved site-directed mutations were introduced into the second most conserved amino acid region, region II, of the human DNA polymerase alpha catalytic subunit. These mutants were expressed in the baculovirus system and purified to near homogeneity. The mutants had polymerase activity ranging from 4 to 60% compared with the wild type polymerase alpha. Steady-state kinetic analysis of mutants G841A, D860A, D860S, D860N, Y865S, and Y865F demonstrated no significant difference in their Km values for primer-template compared with that of the wild type enzyme. In contrast, mutants D860A, Y865S, and Y865F showed a 5-10-fold increase in the Km for deoxynucleotide triphosphate (dNTP) compared with the wild type enzyme. DNA synthetic fidelity studies of these mutants showed that mutant Y865S but not Y865F had a greater than 10-fold higher misinsertion efficiency than the wild type enzyme in Mg(2+)-catalyzed reactions. However, with Mn2+ as the metal activator, Y865S and Y865F demonstrated a 2- and 9-fold higher misinsertion efficiency, respectively. These results indicate that Asp860 and Tyr865 in region II of human DNA polymerase alpha are involved in incoming dNTP substrate binding. Using three deoxynucleotide structural analogs as probes, we show that the nucleotide base is the structural requirement for dNTP binding with Tyr865. Furthermore, abolishing the hydrophobic phenyl ring side chain of Tyr865 by replacing tyrosine with serine rendered the enzyme resistant to aphidicolin. Results of these studies strongly suggest that the phenyl ring of Tyr865 directly interacts with the nucleotide base moiety of the dNTP and plays a critical role in the misinsertion fidelity of DNA synthesis. Although mutation of Gly841 to Ala did not affect the binding of primer-template, it had a significant decrease in kcat, an increase in Km for dNTP, a striking decrease of processivity, and also resistance to aphidicolin. Thus, mutation of this residue, Gly841, which is highly conserved among the alpha-like DNA polymerases, appears to affect both catalysis and substrate deoxynucleotide binding. This suggests that Gly841 is essential for the maintenance of the overall structure of the polymerase alpha catalytic site.

In vivo species specificity of DNA polymerase alpha

The DNA polymerase alpha enzymes from human, and budding (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are homologous proteins involved in initiation and replication of chromosomal DNA. Sequence comparison of human DNA polymerase alpha with that of S. cerevisiae and S. pombe shows overall levels of amino acid sequence identity of 32% and 34%, respectively. We report here that, despite the sequence conservation among these three enzymes, functionally active human DNA polymerase alpha fails to rescue several different conditional lethal alleles of the budding yeast POL1 gene at nonpermissive temperature. Furthermore, human DNA polymerase alpha cannot complement a null allele of budding yeast POL1 either in germinating spores or in vegetatively growing cells. In fission yeast, functionally active human DNA polymerase alpha is also unable to complement the disrupted pol alpha::ura4+ allele in germinating spores. Thus, in vivo, DNA polymerase alpha has stringent species specificity for initiation and replication of chromosomal DNA.

Fission yeast with DNA polymerase delta temperature-sensitive alleles exhibits cell division cycle phenotype

DNA polymerases alpha and delta are essential enzymes believed to play critical roles in initiation and replication of chromosome DNA. In this study, we show that the genes for Schizosaccharomyces pombe (S.pombe) DNA polymerase alpha and delta (pol alpha+ and pol delta+) are essential for cell viability. Disruption of either the pol alpha+ or pol delta+ gene results in distinct terminal phenotypes. The S.pombe pol delta+ gene is able to complement the thermosensitive cdc2-2 allele of Saccharomyces cerevisiae (S.cerevisiae) at the restrictive temperature. By random mutagenesis in vitro, we generated three pol delta conditional lethal alleles. We replaced the wild type chromosomal copy of pol delta+ gene with the mutagenized sequence and characterized the thermosensitive alleles in vivo. All three thermosensitive mutants exhibit a typical cell division cycle (cdc) terminal phenotype similar to that of the disrupted pol delta+ gene. Flow cytometric analysis showed that at the nonpermissive temperature all three mutants were arrested in S phase of the cell cycle. The three S.pombe conditional pol delta alleles were recovered and sequenced. The mutations causing the thermosensitive phenotype are missense mutations. The altered amino acid residues are uniquely conserved among the known polymerase delta sequences.

Selective action of 4'-azidothymidine triphosphate on reverse transcriptase of human immunodeficiency virus type 1 and human DNA polymerases alpha and beta

4'-Azidothymidine (ADRT) is a novel nucleoside analogue that exhibits potent inhibitory activity against the replication of human immunodeficiency virus (HIV) in lymphocytes. The mechanisms by which ADRT inhibits HIV reverse transcriptase (HIV-RT) as ADRT 5'-triphosphate (ADRT-TP), the active intracellular metabolite of ADRT, and as the ADRT-MP molecule incorporated into DNA were examined and compared to their effects on human DNA polymerases alpha and beta. Inhibition of HIV-RT by ADRT-TP is competitive against TTP and is more potent against RNA to DNA synthesis (Ki = 0.009 microM versus Km = 3.3 microM for TTP) than it is against DNA to DNA synthesis (Ki = 0.95 microM versus Km = 16.3 microM for TTP). ADRT-TP is also a more potent inhibitor for primer elongation on RNA template than on DNA template. ADRT-TP is a poor inhibitor of human DNA polymerases alpha (Ki = 62.5 microM) and beta (Ki = 150 microM) (Chen et al., 1992). The consequences of ADRT incorporation into DNA are strikingly different for the HIV-RT and for human DNA polymerases alpha and beta. DNA polymerases alpha and beta incorporate a single ADRT-MP molecule into nascent DNA at a very slow rate and continue to elongate. They are unable to incorporate a second consecutive ADRT-MP. However, HIV-RT is able to efficiently incorporate two consecutive ADRT molecules. Incorporation of two consecutive ADRT-MP molecules by HIV-RT prevents further DNA chain elongation. Incorporation of two ADRT-MP molecules separated by one deoxyribonucleoside monophosphate (dAMP, dCMP, or dGMP) also abolishes DNA chain elongation by HIV-RT.

Amino-dextran-deferoxamine: a potential polymeric heterobifunctional agent for high-level 111In-labeling of anti-melanoma monoclonal antibody TP41.2

Amino-dextran-10 (ADX-10) was partially oxidized to polyaldehyde-ADX which was then reacted with deferoxamine (DFO) to form a Schiff's base and converted into a secondary amine, ADX-DFO (I) with ten moles of DFO per mole of ADX. ADX-DFO was chelated with Indium or 111In to yield ten moles of In or 111In per mole of ADX-DFO. A selective maleimide derivatization of (I) with sulfosuccinimidyl-4-(p-maleimidophenyl) butyrate yielded (II), which contained 3 moles of maleimide groups per mole of (II). The sulfhydryl-amidinium derivatization of the monoclonal antibody (MoAb) TP41.2 with 2-IT produced (III). Compounds (II) and (III) were combined to form the thioether space-arm linkage of (IV), which was subsequently radiolabeled with 111In to yield (V). MoAb-DFO-111In, (VI), was also prepared for a control study. Direct cell binding revealed the immunoreactivity of (V) to be 79.7% and that of (VI) to be 60.3%. The in vitro stability of (V) at 4, 24, and 48 hours resulted in 1.7%, 7.0% and 16.0% hydrolysis respectively, as compared with 2.1%, 8.7% and 18.5% hydrolysis of the control (VI), at the same time intervals. In a biodistribution study in non-tumor rats at 4, 24, and 48 hours post-injection, the liver concentration at 48 hours was 2.97% (ID/g) for (V) and 4.84% (ID/g) for (VI). This novel technique for radiolabeling antibodies allows for a high level of radiometallic labeling, preservation of immunoreactivity, and reduction of uptake by the liver.

Fidelity studies of the human DNA polymerase alpha. The most conserved region among alpha-like DNA polymerases is responsible for metal-induced infidelity in DNA synthesis

Mutational studies in the highly conserved region I domain of the human DNA polymerase alpha enzyme demonstrated a change in metal cation-specific catalysis. Here, we extend the investigation to include the fidelity of DNA synthesis by these mutants, studying misinsertion, mispair extension, and the nucleotide analog utilization. The fidelity of region I mutants and wild type human DNA polymerase alpha enzyme were analyzed with either Mg2+ or Mn2+ as the metal activator. Despite the known mutagenic effect of Mn2+ in causing polymerases to misinsert nucleotides and to utilize dideoxynucleotides, we have found that two region I mutants, D1002N and T1003S, which utilize Mn2+ in catalysis more effectively than Mg2+, actually have a 70- and 40-fold higher misinsertion fidelity, respectively, in Mn(2+)-catalyzed reactions than that of the wild type enzyme. The enhanced misinsertion fidelity of these two mutants in Mn(2+)-catalyzed reactions is due to Km discrimination of the incorrect nucleotide where the D1002N and T1003S had a 850- and 62-fold higher Km for insertion of incorrect than correct nucleotide, respectively. In Mg(2+)-catalyzed reactions, all of the region I mutants exhibited similar misinsertion efficiencies as the wild type polymerase. Study of mispair extension showed that in Mn(2+)-catalyzed ractions, the wild type polymerase alpha enzyme readily extended mispair termini. In contrast, the two region I mutants, D1002N and T1003S, were unable to extend the mispaired termini in either Mg(2+)- or Mn(2+)-catalyzed reactions. These results suggest that the side chains of region I amino acids play an essential role in the Mn(2+)-induced infidelity during DNA synthesis by human DNA polymerase alpha. The effects of the metal activator on the utilization of two nucleotide analogs, 3'-azido-3'-deoxythymidine triphosphate and ddCTP, by the region I mutants were also investigated.

Mutational analysis of the human DNA polymerase alpha. The most conserved region in alpha-like DNA polymerases is involved in metal-specific catalysis

Five site-directed mutations were introduced at the most conserved amino acids in region I (YGDTDS) of the human DNA polymerase alpha catalytic subunit. Mutant proteins were expressed in the baculovirus system by an improved method and purified by a rapid one-step purification in high yield and high specific activity. The Asp1004 to Asn mutation produced a protein with no detectable polymerase activity while other mutations gave activities from 1 to 20% of the wild type polymerase activity. Steady state kinetic analysis of the active mutants indicates that none of the mutations caused a change in Km(dNTP) or KD(DNA), but all active mutants showed a decrease in kcat. Thus, the effect of these conserved mutations is manifest in altered rates of catalysis. Two mutations, Asp1002 to Asn and Thr1003 to Ser, caused the enzyme to utilize Mn2+ more effectively in catalysis than Mg2+, suggesting that these amino acids are involved in metal binding. Rates of catalysis by the D1002N and T1003S mutants, as well as Y1000F mutant were improved 80-, 30-, and 70-fold, respectively, on homopolymer templates when Mn2+ replaced Mg2+ as the activator metal. The results from these mutational studies suggest that this highly conserved region binds the metal which is essential for catalysis. The Asp1002 may participate directly in chelating the metal. Results from the T1003S mutant suggest that the beta-methyl group of the threonine side chain might be locked in a hydrophobic pocket preventing free rotation around the C alpha-C beta bond, thus positioning the Thr1003 hydroxyl group to form a crucial bond with the metal ion. In addition, D1002N and T1003S displayed a 20-fold resistance to aphidicolin compared to the wild type polymerase alpha, and all of the active mutants displayed altered sensitivity to butylphenyl-dGTP. Models of the involvement of region I in catalysis and aphidicolin interaction are proposed. The mutational studies presented in this report will serve as a prototype for the functional role of region I in catalysis for all alpha-like DNA polymerases.

Initiation of simian virus 40 DNA replication requires the interaction of a specific domain of human DNA polymerase alpha with large T antigen

Initiation of cell-free simian virus 40 (SV40) DNA replication requires the interaction of DNA polymerase alpha/primase with a preinitiation complex containing the viral T antigen and cellular proteins, replication protein A, and topoisomerase I or II. To further understand the molecular mechanisms of the transition from preinitiation to initiation, the intermolecular interaction between human DNA polymerase alpha and T antigen was investigated. We have demonstrated that the human DNA polymerase alpha catalytic polypeptide is able to associate with SV40 large T antigen directly under physiological conditions. A physical association between these two proteins was detected by coimmunoprecipitation with monoclonal antibodies from insect cells coinfected with recombinant baculoviruses. A domain of human polymerase alpha physically interacting with T antigen was identified within the amino-terminal region from residues 195 to 313. This domain of human polymerase alpha was able to form a nonproductive complex with T antigen, causing inhibition of the SV40 DNA replication in vitro. Kinetics of the inhibition indicated that this polymerase domain can inhibit viral replication only during the preinitiation stage. Extra molecules of T antigen could partially overcome the inhibition only prior to initiation complex formation. The data support the conclusion that initiation of SV40 DNA replication requires the physical interaction of T antigen in the preinitiation complex with the amino-terminal domain of human polymerase alpha from amino acid residues 195 to 313.

Cell cycle expression of two replicative DNA polymerases alpha and delta from Schizosaccharomyces pombe

We have investigated the expression of two Schizosaccharomyces pombe replicative DNA polymerases alpha and delta during the cell cycle. The pol alpha+ and pol delta+ genes encoding DNA polymerases alpha and delta were isolated from S. pombe. Both pol alpha+ and pol delta+ genes are single copy genes in haploid cells and are essential for cell viability. In contrast to Saccharomyces cerevisiae homologs, the steady-state transcripts of both S. pombe pol alpha+ and pol delta+ genes were present throughout the cell cycle. Sequence analysis of the pol alpha+ and pol delta+ genes did not reveal the Mlu I motifs in their upstream sequences that are involved in cell cycle-dependent transcription of S. cerevisiae DNA synthesis genes as well as the S. pombe cdc22+ gene at the G1/S boundary. However, five near-match Mlu I motifs were found in the upstream region of the pol alpha+ gene. S. pombe DNA polymerases alpha and delta proteins were also expressed constantly throughout the cell cycle. In addition, the enzymatic activity of the S. pombe DNA polymerase alpha measured by in vitro assay was detected at all stages of the cell cycle. Thus, these S. pombe replicative DNA polymerases, like that of S. pombe cdc17+ gene, are expressed throughout the cell cycle at the transcriptional and protein level. These results indicate that S. pombe has at least two regulatory modes for the expression of genes involved in DNA replication and DNA precursor synthesis.

Protein affinity chromatography reveals cell cycle dependent association of cellular factors with human DNA polymerase alpha

DNA polymerase alpha/primase (Pol alpha) is the key replication enzyme in eukaryotic cells. This enzyme synthesizes and elongates short RNA primers at an unwound origin of replication. Pol alpha was used as an affinity ligand to identify cellular replication factors interacting with it. Protein complexes between Pol alpha and cellular factors were analyzed by co-immunoprecipitations with monoclonal antibodies directed against Pol alpha and by protein affinity chromatography of cell extracts derived from pure G1- and S-phase cell populations on Pol alpha affinity columns. Co-immunoprecipitations resulted in the identification of a polypeptide with a molecular weight of 46 kDa. For Pol alpha affinity chromatography, the ligand was purified from insect cells infected with a recombinant baculovirus encoding the catalytic subunit (p180) of Pol alpha (Copeland and Wang, 1991). With 5 x 10(8) infected Sf9 cells, a rapid one step purification protocol was used which yielded in five hours 0.6 mg pure enzyme with a specific activity of 140,000 units/mg. The G1- and S-phase cell populations were generated by block, release and counterflow centrifugal elutriation of exponentially growing human MANCA cells. Starting with 2 x 10(9) non synchronous cells, 5 x 10(8) G1-phase cells were isolated. Chromatography of cell extracts derived from G1- or S-phase cells on Pol alpha affinity columns resulted in identifying several polypeptides in the range of 40-70 kDa. Some of these polypeptides are more abundant in eluates derived from S-phase extracts than from G1-phase extracts.

Photoreactive 111In-cyclodextrin inclusion complex: a new heterobifunctional reagent for antibody labeling

The compound of interest, N-5-azido-2-nitrobenzoylaminomethyl-111In-acetylacetone-alpha-cycl odextrin (CD) (V) was synthesized by the selective tosylation of alpha-CD to form 6-tosyl-6-deoxy-CD, which was then reacted with NaN3 to form 6-azido-6-deoxy-CD (II). This was followed by catalytic hydrogenation to yield III. Compound III and 111In-acetylacetone were mixed to form an inclusion complex, which was then reacted with N-5-azido-2-nitrobenzoyloxysuccinimide to yield compound V. Anti-melanoma MAbTP41.2 was added to compound V, followed by immediate photoreactivation labeling by u.v. light at 320 nm. The final product VI was purified from a Sephadex G-50 column. 111In-DTPA-MAbTP41.2 was also prepared as a control. Immunoreactivity via the cell-binding assay of VI was 87%, compared with 57% by the BADTPA method. Biodistribution in non-tumor rats yielded a liver concentration in %ID/g of 3.5, 1.7 and 1.0 for compound VI, compared to the 5.5, 5.2 and 3.1 for the BADTPA compound, at 4, 24 and 48 h post-injection, respectively.

Imaging of cardiac transplantation rejection in primates using two new antimyosin agents

Indium-111-labeled monoclonal antimyosin Fab has been used to image myocardial infarction, myocarditis and cardiac transplant rejection with localization in myocytes that have suffered irreversible loss of cell membrane integrity. Technical factors potentially limiting clinical usefulness of 111In antimyosin include dosimetry (72 hr half-life of 111In), slow blood clearance of antibody proteins delaying optimal imaging to 24 to 48 hr postinjection and nontarget organ uptake. Therefore, two new antimyosin imaging agents experimentally shown to potentially improve dosimetry, shorten time from injection to imaging or decrease nonspecific cell binding were evaluated in a primate cardiac transplant model. The two agents evaluated were polylysine 111In-antimyosin (0.023 mg Fab modified with a 3.3 kd polymer of polylysine and labeled with 111In) and 99mTc-antimyosin (0.5 mg Fab' antimyosin labeled using the RP-1 ligand technique). A total of eight baboons were studied: three with heterotopic (cervical) xenographs, three with orthotopic allographs and two control animals. Each animal was injected first with 12-23 mCi of 99mTc-RP-1 antimyosin and 5-16 hr after completion of imaging, was injected with 0.72-1.88 mCi of 111In-polylysine antimyosin (PIs) and reimaged 12-48 hr later. The imaging results were compared to the histology of the animals. Biexponential curves were fit to the blood sample data and rate constants were determined and expressed as T1/2 values. There were no significant differences between the two agents in either the early fast components or the late slow components. On planar imaging, there was blood-pool activity at 10-12 hr postinjection of both agents, but by 16-24 hr postinjection, blood pool was negligible on the 111In-PIs scans. Both agents were concentrated in the rejected cardiac tissue. The slow blood-pool clearance combined with the 6 hr half-life of 99mTc-RP-1 AMA make this agent less promising for detection of diffuse myocardial uptake than 111In Fab modified with polylysine.