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Li H, Li L, Wu W, Wang F, Zhou F, Lin Y. SvSTL1 in the large subunit family of ribonucleotide reductases plays a major role in chloroplast development of Setaria viridis. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2022; 111:625-641. [PMID: 35608125 DOI: 10.1111/tpj.15842] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/03/2021] [Revised: 05/04/2022] [Accepted: 05/20/2022] [Indexed: 06/15/2023]
Abstract
Ribonucleotide reductases (RNRs) are essential enzymes in DNA synthesis. However, little is known about the RNRs in plants. Here, we identified a svstl1 mutant from the self-created ethyl methanesulfonate (EMS) mutant library of Setaria viridis. The mutant leaves exhibited a bleaching phenotype at the heading stage. Paraffin section analysis showed the destruction of the C4 Kranz anatomy. Transmission electron microscopy results further demonstrated the severely disturbed development of some chloroplasts. MutMap analysis revealed that the SvSTL1 gene is the primary candidate, encoding a large subunit of RNRs. Complementation experiments confirmed that SvSTL1 is responsible for the phenotype of svstl1. There are two additional RNR large subunit homologs in S. viridis, SvSTL2 and SvSTL3. To further understand the functions of these three RNR large subunit genes, a series of mutants were generated via CRISPR/Cas9 technology. In striking contrast to the finding that all three SvSTLs interact with the RNR small subunit, the phenotype varied along with the copies of chloroplast genome among different svstl single mutants: the svstl1 mutant exhibited pronounced chloroplast development and significantly fewer copies of the chloroplast genome than the svstl2 or svstl3 single mutants. These results suggested that SvSTL1 plays a major role in the optimal function of RNRs and is essential for chloroplast development. Furthermore, through the analysis of double and triple mutants, the study provides new insights into the finely tuned coordination among SvSTLs to maintain normal chloroplast development in the emerging C4 model plant S. viridis.
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Affiliation(s)
- Huanying Li
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
| | - Lin Li
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
| | - Weichen Wu
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
| | - Fei Wang
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
| | - Fei Zhou
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
| | - Yongjun Lin
- National Key Laboratory of Crop Genetic Improvement and National Center of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan, 430070, China
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2
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Tang LY, Matsushima R, Sakamoto W. Mutations defective in ribonucleotide reductase activity interfere with pollen plastid DNA degradation mediated by DPD1 exonuclease. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2012; 70:637-49. [PMID: 22239102 DOI: 10.1111/j.1365-313x.2012.04904.x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Organellar DNAs in mitochondria and plastids are present in multiple copies and make up a substantial proportion of total cellular DNA despite their limited genetic capacity. We recently demonstrated that organellar DNA degradation occurs during pollen maturation, mediated by the Mg(2+) -dependent organelle exonuclease DPD1. To further understand organellar DNA degradation, we characterized a distinct mutant (dpd2). In contrast to the dpd1 mutant, which retains both plastid and mitochondrial DNAs, dpd2 showed specific accumulation of plastid DNAs. Multiple abnormalities in vegetative and reproductive tissues of dpd2 were also detected. DPD2 encodes the large subunit of ribonucleotide reductase, an enzyme that functions at the rate-limiting step of de novo nucleotide biosynthesis. We demonstrated that the defects in ribonucleotide reductase indirectly compromise the activity of DPD1 nuclease in plastids, thus supporting a different regulation of organellar DNA degradation in pollen. Several lines of evidence provided here reinforce our previous conclusion that the DPD1 exonuclease plays a central role in organellar DNA degradation, functioning in DNA salvage rather than maternal inheritance during pollen development.
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MESH Headings
- Arabidopsis/genetics
- Arabidopsis/metabolism
- Arabidopsis Proteins/genetics
- Arabidopsis Proteins/metabolism
- DNA, Mitochondrial/genetics
- DNA, Mitochondrial/metabolism
- DNA, Plant/genetics
- DNA, Plant/metabolism
- Exoribonucleases/genetics
- Exoribonucleases/metabolism
- Gene Expression Regulation, Enzymologic
- Gene Expression Regulation, Plant
- Genetic Complementation Test
- Luminescent Proteins/genetics
- Luminescent Proteins/metabolism
- Microscopy, Electron, Scanning
- Microscopy, Fluorescence
- Mutation
- Phenotype
- Plants, Genetically Modified
- Plastids/genetics
- Pollen/genetics
- Pollen/ultrastructure
- Reverse Transcriptase Polymerase Chain Reaction
- Ribonucleotide Reductases/genetics
- Ribonucleotide Reductases/metabolism
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Affiliation(s)
- Lay Yin Tang
- Institute of Plant Science and Resources, Okayama University, Kurashiki, Okayama, Japan
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3
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Gustavson MD, Molinaro AM, Tedeschi G, Camp RL, Rimm DL. AQUA analysis of thymidylate synthase reveals localization to be a key prognostic biomarker in 2 large cohorts of colorectal carcinoma. Arch Pathol Lab Med 2008; 132:1746-52. [PMID: 18976010 DOI: 10.5858/132.11.1746] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/06/2008] [Indexed: 11/06/2022]
Abstract
CONTEXT Increased thymidylate synthase expression is a marker for decreased survival in colorectal cancer. OBJECTIVE Thymidylate synthase localizes to both the nucleus and cytoplasm, but how the relationship of these expression levels affects colon cancer outcome has yet to be determined. DESIGN Using AQUA, we assessed prognosis of thymidylate synthase expression as a function of subcellular localization in 2 retrospective cohorts of colorectal carcinoma. We used the first cohort (n = 599) as a training set, subsequently validating optimal expression cut points in the second cohort (n = 447). RESULTS A significant association between decreased 5-year disease-specific survival and increased nuclear expression (16% decreased survival [72% to 56%] for the top 60% of nuclear-expressing tumors [P < .001]) and cytoplasmic expression (12% decreased survival [70% to 58%] for the top 54% of cytoplasmic-expressing tumors [P = .02]) was observed for the training set. A higher nuclear to cytoplasmic ratio also correlated significantly with decreased survival (15% decreased survival [66% to 51%] for the top 25% of tumors [P < .001]). Applying these findings to the validation set, as a function of time to recurrence, only the ratio (P = .03 [expression ratio]; P = .18 [nuclear]; P = .71 [cytoplasmic]) showed a significant association with decreased time to recurrence. Additionally, the expression ratio significantly added to the prognostic value given by the primary tumor pathologic classification and nodal status. CONCLUSIONS These data suggest the relationship of nuclear to cytoplasmic thymidylate synthase expression, given as a ratio of continuous AQUA scores, to be a strong predictor of colon cancer survival.
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Affiliation(s)
- Mark D Gustavson
- Department of Pathology, Yale University, New Haven, CT 06511, USA.
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Ribonucleotide reduction is a cytosolic process in mammalian cells independently of DNA damage. Proc Natl Acad Sci U S A 2008; 105:17801-6. [PMID: 18997010 DOI: 10.1073/pnas.0808198105] [Citation(s) in RCA: 85] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Ribonucleotide reductase provides deoxynucleotides for nuclear and mitochondrial (mt) DNA replication and repair. The mammalian enzyme consists of a catalytic (R1) and a radical-generating (R2 or p53R2) subunit. During S-phase, a R1/R2 complex is the major provider of deoxynucleotides. p53R2 is induced by p53 after DNA damage and was proposed to supply deoxynucleotides for DNA repair after translocating from the cytosol to the cell nucleus. Similarly R1 and R2 were claimed to move to the nucleus during S-phase to provide deoxynucleotides for DNA replication. These models suggest translocation of ribonucleotide reductase subunits as a regulatory mechanism. In quiescent cells that are devoid of R2, R1/p53R2 synthesizes deoxynucleotides also in the absence of DNA damage. Mutations in human p53R2 cause severe mitochondrial DNA depletion demonstrating a vital function for p53R2 different from DNA repair and cast doubt on a nuclear localization of the protein. Here we use three independent methods to localize R1, R2, and p53R2 in fibroblasts during cell proliferation and after DNA damage: Western blotting after separation of cytosol and nuclei; immunofluorescence in intact cells; and transfection with proteins carrying fluorescent tags. We thoroughly validate each method, especially the specificity of antibodies. We find in all cases that ribonucleotide reductase resides in the cytosol suggesting that the deoxynucleotides produced by the enzyme diffuse into the nucleus or are transported into mitochondria and supporting a primary function of p53R2 for mitochondrial DNA replication.
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Bissoon-Haqqani S, Moyana T, Jonker D, Maroun JA, Birnboim HC. Nuclear expression of thymidylate synthase in colorectal cancer cell lines and clinical samples. J Histochem Cytochem 2005; 54:19-29. [PMID: 15956025 DOI: 10.1369/jhc.5a6642.2005] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Thymidylate synthase (TS) [TYMS; OMIM reference number (188,350)] is normally considered to be a cytoplasmic enzyme. However, a few reports have suggested it may also be present in the nucleus. To explore this in more detail, we used a highly specific polyclonal antibody to TS and a combination of techniques, including immunocytochemistry, confocal microscopy, cell fractionation, and Western blotting. We developed cell line HeLa-55, a HeLa derivative that grossly overexpresses TS. Although the vast majority of TS was in the cytoplasm, some TS also was seen in the nucleus. TS in parental HeLa cells and in normal human fibroblasts was seen exclusively in the cytoplasm. HeLa-55 cells exposed to 5-fluorodeoxyuridine were fractionated and examined by Western blotting. Interestingly, both free TS and the ternary complex of TS were seen in the cytoplasmic fraction but only free TS was detected in the nuclear fraction. Amongst different cell lines examined, HCT-15 and normal fibroblasts showed no nuclear TS, HCC-2998 and SW-620 showed a small amount of nuclear TS, and HT-29, RKO, and HCT-116 showed a strong nuclear TS signal. Nuclear staining was clearly evident in some clinical colorectal specimens, both normal and malignant. This staining was definitively shown to be TS by competition with recombinant TS protein. A putative leucine-rich nuclear export sequence was identified but its function could not be confirmed. We conclude that small amounts of TS protein is present in the nucleus of some cell types but further work is needed to determine the significance of this observation.
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Gribaudo G, Riera L, Lembo D, De Andrea M, Gariglio M, Rudge TL, Johnson LF, Landolfo S. Murine cytomegalovirus stimulates cellular thymidylate synthase gene expression in quiescent cells and requires the enzyme for replication. J Virol 2000; 74:4979-87. [PMID: 10799571 PMCID: PMC110849 DOI: 10.1128/jvi.74.11.4979-4987.2000] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Herpesviruses accomplish DNA replication either by expressing their own deoxyribonucleotide biosynthetic genes or by stimulating the expression of the corresponding cellular genes. Cytomegalovirus (CMV) has adopted the latter strategy to allow efficient replication in quiescent cells. In the present report, we show that murine CMV (MCMV) infection of quiescent fibroblasts induces both mRNA and protein corresponding to the cellular thymidylate synthase (TS) gene, which encodes the enzyme that catalyzes the de novo synthesis of thymidylic acid. The increase in TS gene expression was due to an increase in gene transcription, since the activity of a reporter gene driven by the mouse TS promoter was induced following MCMV infection. Mutagenesis of the potential E2F-responsive element immediately upstream from the TS essential promoter region abolished the virus-mediated stimulation of the TS promoter, suggesting that the transactivating activity of MCMV infection was E2F dependent. Cotransfection experiments revealed that expression of the viral immediate-early 1 protein was sufficient to mediate the increase in TS promoter activity. Finally, MCMV replication and viral DNA synthesis were found to be inhibited by ZD1694, a quinazoline-based folate analog that inhibits TS activity. These results demonstrate that upregulation of cellular TS expression is required for efficient MCMV replication in quiescent cells.
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Affiliation(s)
- G Gribaudo
- Department of Public Health and Microbiology, University of Turin, Turin, Italy.
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7
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Cinquina CC, Grogan E, Sun R, Lin SF, Beardsley GP, Miller G. Dihydrofolate reductase from Kaposi's sarcoma-associated herpesvirus. Virology 2000; 268:201-17. [PMID: 10683342 DOI: 10.1006/viro.1999.0165] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Kaposi's sarcoma-associated herpesvirus (KSHV) is the first human virus known to encode dihydrofolate reductase (DHFR), an enzyme required for nucleotide and methionine biosynthesis. We have studied the purified KSHV-DHFR enzyme in vitro and analyzed its expression in cultured B-cell lines derived from primary effusion lymphoma (PEL), an AIDS-associated malignancy. The amino acid sequence of KSHV-DHFR is most similar to human DHFR (hDHFR), but the viral enzyme contains an additional 23 amino acids at the carboxyl-terminus. The viral DHFR, overexpressed and purified from E. coli, was catalytically active in vitro. The K(m) of KSHV-DHFR for dihydrofolate (FH(2)) was 2.4 microM, which is significantly higher than the K(m) of recombinant hDHFR (rhDHFR) for FH(2) (390 nM). K(m) values for NADPH were similar for the two enzymes, about 1 microM. KSHV-DHFR was inhibited by folate antagonists such as methotrexate (K(i): 200 pM), aminopterin (K(i): 610 pM), pyrimethamine (K(i): 29 nM), trimethoprim (K(i): 2.3 microM), and piritrexim (K(i): 3.9 nM). In all cases, K(i) values for these folate antagonists were higher for KSHV-DHFR than for rhDHFR. The viral enzyme was expressed at levels two- to tenfold higher than hDHFR in PEL cell lines as an early lytic cycle gene. KSHV-DHFR mRNA and protein appeared from 6 to 24 h after chemical induction of the KSHV lytic cycle. Epitope-tagged KSHV-DHFR and rhDHFR both localized to the nucleus of transfected cells, while other KSHV nucleotide metabolism genes localized to the cytoplasm. DHFR activity was not essential for viral replication in cultured PEL cells. Since hDHFR was not detectable in peripheral blood mononuclear cells (PBMCs), KSHV-DHFR may function to provide increased DHFR activity in vivo in infected cells that have little or none of their own enzyme.
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Affiliation(s)
- C C Cinquina
- Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut, 06520, USA
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8
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Samsonoff WA, Reston J, McKee M, O'Connor B, Galivan J, Maley G, Maley F. Intracellular location of thymidylate synthase and its state of phosphorylation. J Biol Chem 1997; 272:13281-5. [PMID: 9148948 DOI: 10.1074/jbc.272.20.13281] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Thymidylate synthase (TS), an enzyme that is essential for DNA synthesis, was found to be associated mainly with the nucleolar region of H35 rat hepatoma cells, as determined both by immunogold electron microscopy and by autoradiography. In the latter case, the location of TS was established through the use of [6-3H]5-fluorodeoxyuridine, which forms a tight ternary complex of TS with 5-fluorodeoxyuridylate (FdUMP) and 5, 10-methylenetetrahydrofolylpolyglutamate within the cell. However, with H35 cells containing 50-100-fold greater amounts of TS than unmodified H35 cells, the enzyme, although still in the nucleus, was located primarily in the cytoplasm as shown by autoradiography and immunohistochemistry. In addition, TS was also present in mitochondrial extracts of both cell lines, as determined by enzyme activity measurements and by ternary complex formation with [32P]FdUMP and 5,10-methylenetetrahydrofolate. Another unique observation is that the enzyme appears to be a phosphoprotein, similar to that found for other proteins associated with cell division and signal transduction. The significance of these findings relative to the role of TS in cell division remains to be determined, but suggest that this enzyme's contribution to the cell cycle may be more complex than believed previously.
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Affiliation(s)
- W A Samsonoff
- Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, New York 12201-0509, USA
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9
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Thymidylate synthase is localized to the nuclear periphery in the yeast Saccharomyces cerevisiae. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)37199-5] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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10
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Xu Y, Plunkett W. Regulation of thymidine kinase and thymidylate synthase in intact human lymphoblast CCRF-CEM cells. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)41537-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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11
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Mathews CK. Enzyme organization in DNA precursor biosynthesis. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1993; 44:167-203. [PMID: 8434123 DOI: 10.1016/s0079-6603(08)60220-2] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Affiliation(s)
- C K Mathews
- Department of Biochemistry and Biophysics, Oregon State University, Corvallis 97331
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12
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Reddy GP. Compartmentation of deoxypyrimidine nucleotides for nuclear DNA replication in S phase mammalian cells. J Mol Recognit 1989; 2:75-83. [PMID: 2636898 DOI: 10.1002/jmr.300020205] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
DNA synthesis in S phase Chinese hamster embryo fibroblast cells in the presence of exogenous 3H-dUrd shows incorporation of the labeled precursor with very little dilution by the large unlabeled intracellular precursor pools. Full mixing would predict a specific activity 10-fold less than that measured. This coupled with the finding that 80% of the radioactivity derived from the exogenous 3H-dUrd appears in the karyoplasts implies a compartmentation where 3H-dUMP and 3H-dTTP derived from exogenous 3H-dUrd do not mix freely with endogenous cytoplasmic pools.
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Affiliation(s)
- G P Reddy
- Department of Obstetrics and Gynaecology, University of Virginia, Charlottesville 22908
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13
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Andersson M, Lewan L. Intracellular compartmentation of diadenosine tetraphosphate (Ap4A) and dTTP in rat liver. THE INTERNATIONAL JOURNAL OF BIOCHEMISTRY 1989; 21:593-8. [PMID: 2477288 DOI: 10.1016/0020-711x(89)90377-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
1. The intracellular compartmentation of diadenosine tetraphosphate (Ap4A) and of dTTP was studied in rat liver cells using non-aqueous glycerol for the isolation of cell nuclei. 2. This method allows a stepwise removal of cytoplasm from the nuclei. 3. The decrease in Ap4A or dTTP during the process was compared to the simultaneous decrease in RNA, which was taken to represent the cytoplasm. 4. In regenerating liver excised 24 hr after partial hepatectomy, Ap4A was almost equally distributed between the nucleus and cytoplasm. 5. In livers from unoperated control rats, the nuclear concentration of Ap4A was slightly elevated compared to that of whole cells. dTTP was only investigated in regenerating liver. 6. Significantly higher concentrations were found in the nuclear fractions. 7. The purest nuclei contained about 26% of whole cell levels of dTTP, while their RNA values had decreased to 7% of the whole cell RNA. 8. Considering that the liver cell nucleus comprises about 7% of the entire cell mass, a nuclear dTTP concentration of 26% indicates significantly higher dTTP levels in the nuclear compartment than in the cytoplasm of regenerating rat liver cells.
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14
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Andersson M, Lewan L, Stenram U. Compartmentation of purine and pyrimidine nucleotides in animal cells. THE INTERNATIONAL JOURNAL OF BIOCHEMISTRY 1988; 20:1039-50. [PMID: 3073978 DOI: 10.1016/0020-711x(88)90248-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Affiliation(s)
- M Andersson
- Department of Pathology, University Hospital, Lund, Sweden
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15
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Harvey G, Pearson CK. Search for multienzyme complexes of DNA precursor pathways in uninfected mammalian cells and in cells infected with herpes simplex virus type I. J Cell Physiol 1988; 134:25-36. [PMID: 2447104 DOI: 10.1002/jcp.1041340104] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Confirmatory evidence for the existence of a multienzyme complex of DNA precursor pathways in mammalian cells was obtained. Using neutral sucrose gradient centrifugation of cell lysates we found that at least five enzymes involved in DNA precursor metabolism in uninfected. S-phase BHK-cell fibroblasts cosediment at a common rate, indicative of a multienzyme complex. The enzymes include DNA polymerase thymidine kinase, ribonucleotide reductase, dihydrofolate reductase, and NDP-kinase. This complex was partially, but not completely, disrupted when lysates from GO-phase cells were centrifuged. Using lysates from cells infected with herpes simplex virus (HSV) type I some of the virus-induced ribonucleotide reductase and a minor proportion of the HSV-thymidine kinase cosedimented rapidly. The virus-induced DNA polymerase sedimented independently near the middle of the gradient, in contrast to the behaviour of the host polymerase. The enzyme associations observed were disrupted by NaCl or by inclusion of ethylenediamine tetraacetic acid during the cell lysis procedure, instead of the usual EGTA. These results indicate the importance of ionic forces in maintaining the enzyme complexes. The bulk of the DNA and the RNA present in the lysates did not sediment at the same rate as the complexes, showing that the enzymes were not simply adhering nonspecifically to these polyanions. Newly synthesised radiolabeled DNA (15 min pulse with [3H]thymidine) was not preferentially associated with the enzymes, but some functional DNA was evident in the enzyme complex fraction from the uninfected S-phase cells. DNA polymerase activity in this fraction did not require, nor was it stimulated by, exogenous "activated" DNA. Added DNA primer-template was required, however, for maximal activity of the polymerase in gradient fractions derived from GO-phase cells and from HSV-infected cells. No evidence for channeling of ribonucleotide precursors into DNA of permeabilized cells (uninfected or HSV-infected) was detected. Most rCDP was incorporated into RNA. In the uninfected, S-phase cells about 10 pmol/10(6) cells/90 min of rCDP residues was incorporated into DNA compared with 120 pmol/10(6) cells/90 min when radiolabeled dCTP was used. Nonradioactive dCTP present in equimolar concentration in the incubation with labeled rCDP did not, however, diminish the incorporation of label from the ribonucleotide. In permeabilized HSV-infected cells incorporation of radiolabel from rCDP into DNA was barely detectable.
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Affiliation(s)
- G Harvey
- Department of Biochemistry, University of Aberdeen, Marischal College, Scotland
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