Adenovirus deoxyribonucleic acid replication. Characterization of the enzyme activities of a soluble replication system

Quantitative estimation of tumor metastasis by measurement of DNA polymerse activity

Metastatic tumor burden in the lung of C57BL/6 or BDF1 mice was quantitated by measuring DNA polymerase alpha activity in the lung of tumor-bearing animals. DNA polymerase activity in the lung increased time-dependently following the inoculation of Lewis lung carcinoma (s.c.) or B16 melanoma variant B16-B2 (i.v.). In the Lewis lung carcinoma system, the number of metastatic modules and the weight of lung also increased time-dependently. Results from the B16 melanoma showed that the increase in lung nodules occurred 10 to 20 days after i.v. inoculation of tumor cells. DNA polymerase activity increased significantly during this period. Because the lung nodules were very small there was no obvious concomitant increase in lung weight. Since no significant infiltration of host cells was observed in the lung in response to metastatic foci, the rise in DNA polymerase activity should be due to tumor cells and not to infiltrating host cells. When the metastasis of Lewis lung carcinoma was inhibited by adriamycin and cyclophosphamide, decrease in DNA polymerase activity in the lung occurred. These results indicate that the degree of tumor metastasis can be quantitated by measuring DNA polymerase activity.

Adenovirus DNA replication in vivo: properties of short DNA molecules extracted from infected cells

Adenovirus type 5 (Ad5) DNA replicating in intact HeLa cells was pulse-labeled with [3H]thymidine extracted by the procedure of Hirt and analyzed on neutral sucrose gradients. In addition to viral replicative forms (over 33 kb), slowly sedimenting species of DNA (0.05-3 kb) was observed. Hybridization analysis showed that this DNA contained about 30% Ad5 DNA sequences. Analysis of the sensitivity of this DNA to the 5' OH-specific spleen exonuclease after alkali or RNAase treatment revealed that about 80% of these molecules contained ribonucleotides. DNA in this fraction was labeled at the 5' end with 32P and hybridized together with control 3H-labeled Ad5 DNA to Hpa I restriction fragments immobilized on nitrocellulose paper. Many of these small molecules were located near the termini. Alkali treatment prior to hybridization decreased the 32P/3H ratio throughout the genome. This suggests that some of these Ad5 molecules possess ribonucleotides and therefore may be intermediates in discontinuous DNA replication. Longer molecules (over 0.5 kb) were found sedimenting with viral replicative forms and mature DNA. The Ad5 molecules in this fraction showed no evidence of alkali-labile termini.

Resistance of adenoviral DNA replication to aphidicolin is dependent on the 72-kilodalton DNA-binding protein

Aphidicolin is a highly specific inhibitor of DNA polymerase alpha and has been most useful for assessing the role of this enzyme in various replication processes (J. A. Huberman, Cell 23:647-648, 1981). Both nuclear DNA replication and simian virus 40 DNA replication are highly sensitive to this drug (Krokan et al., Biochemistry 18:4431-4443, 1979), whereas mitochondrial DNA synthesis is completely insensitive (Zimmerman et al., J. Biol. Chem. 255:11847-11852, 1980). Adenovirus DNA replication is sensitive to aphidicolin, but only at much higher concentrations. These patterns of sensitivity are seen both in vivo and in vitro (Krokan et al., Biochemistry 18:4431-4443, 1979). A temperature-sensitive mutant of adenovirus type 5 known as H5ts125 is able to complete but not initiate new rounds of replication at nonpermissive temperatures (P. C. van der Vliet and J. S. Sussenbach, Virology 67:415-426, 1975). When cells infected with H5ts125 were shifted from permissive (33 degrees C) to nonpermissive (41 degrees C) conditions, the residual DNA synthesis (elongation) showed a striking increase in sensitivity to aphidicolin. The temperature-sensitive mutation of H5ts125 is in the gene for the 72-kilodalton single-stranded DNA-binding protein. This demonstrated that the increased resistance to aphidicolin shown by adenovirus DNA replication was dependent on that protein. It also supports an elongation role for both DNA polymerase alpha and the 72-kilodalton single-stranded DNA-binding protein in adenovirus DNA replication. Further support for an elongation role of DNA polymerase alpha came from experiments with permissive temperature conditions and inhibiting levels of aphidicolin in which it was shown that newly initiated strands failed to elongate to completion.

Role of DNA polymerase gamma in adenovirus DNA replication. Mechanism of inhibition by 2',3'-dideoxynucleoside 5'-triphosphates

In contrast to cellular or SV40 DNA replication, adenovirus type 5 (Ad5) or type 2 (Ad2) DNA synthesis in isolated nuclei is strongly inhibited by low concentrations of 2',3'-dideoxythymidine 5'-triphosphate (ddTTP). On the basis of differential sensitivity of cellular DNA polymerases, a role of DNA polymerase gamma in adenovirus DNA replication has been proposed. We have investigated the mechanism of inhibition of adenovirus DNA synthesis, using [alpha-32P]ddTTP and other dNTP analogues. Both ddATP and ddGTP were as inhibitory as ddTTP, while ddCTP had an even stronger effect on adenovirus DNA replication. DNA polymerase alpha was resistant to all four ddNTP's, while DNA polymerase gamma was very sensitive. The inhibition by ddTTP in isolated infected nuclei was slowly reversible. [alpha-32P]ddTTP was incorporated into Ad5 DNA as a chain-terminating nucleotide, and the analogue could be used as a substrate by DNA polymerase gamma. Under similar conditions, incorporation in cellular DNA or using DNA polymerase alpha was not observed. The nucleoside analogues ddA and ddC suppressed adenovirus. DNA replication in intact cells and reduced plaque formation. These results provide further evidence for a function of DNA polymerase gamma in adenovirus DNA synthesis.

DNA synthesis in chromatin preparations from human fibroblasts infected by cytomegalovirus

Chromatin prepared from (14C)-thymidine pulse labelled cytomegalovirus-infected human fibroblasts 72 hours postinfection exhibited under appropriate conditions endogenous activity of (3H)-thymidine triphosphate incorporation which was relatively salt-resistant and phosphonoacetic acid-sensitive. Isopycnic centrifugation of the doubly labelled DNA in CsCl revealed that cell-free incoporation occurred into viral as well as into host cell DNA. Density labelling experiments with bromodeoxyuridine triphosphate suggested the incoporation into viral DNA to be due to replicative DNA synthesis. Chromatin from infected cells contained, in addition to cellular, viral DNA polymerase activity.

Purification and properties of a DNA ligase from a soluble DNA replication complex

A DNA ligase has been purified from a subnuclear soluble replication complex isolated from adenovirus type 2-infected human KB cells. DNA ligase activity could not be demonstrated using an exogenous template until the complex was dissociated, suggesting that the ligase activity may be a component of the complex. The purified enzyme was free of endonuclease, exonuclease, 5'-nucleotidase, and phosphatase activities, and had a molecular weight of 105 000, as estimated by sedimentation in a glycerol gradient. The ligase requires ATP and a divalent cation for activity. The optimum of the reaction is at pH 7.8 in 50--100 mM Tris-HCl buffer and 10--20 mM MgCl2. Monovalent salts greatly stimulate ligase activity and the optimum was found at 150 mM. The reaction is very sensitive to high temperature; maximum activity was observed at 25--30 degrees C. ATP is the sole required cofactor and NAD, dATP and GTP could not replace the requirement for ATP. The Km for ATP is 60 microM. The Km for DNA is 250 microgram/ml or 1.6 nmol of terminal phosphate/ml and thus the enzyme shows relatively weak affinity for exogenous DNA. The maximum conversion of 32P into a phosphatase-resistant form is approximately 1.3% of the total, whereas T4 ligase, under the same conditions, can convert more than 25% of phosphate into a resistant form.

Levels of cellular DNA polymerases in synchronized bovine paravovirus-infected cells

Infection of synchronized bovine fetal spleen cells with bovine parvovirus results in changes in the levels and patterns of DNA polymerases alpha and gamma during the cell cycle. The pattern of DNA polymerase alpha activity closely paralled viral DNA synthesis and the production of progeny virus, and levels, of this enzyme were threefold greater than in mock-infected cells during the period of maximal viral DNA synthesis. DNA polymerase gamma activity remained slightly elevated during viral DNA replication. Levels and patterns of DNA polymerase beta were similar in mock- and virus-infected cells.