Strand Breaks In Plasmid DNA Research Articles

Strand breaks in plasmid DNA, natural and brominated, by low-energy X-rays.

Single- and double-strand breaks were measured in Col E1 plasmid DNA, natural and partially brominated, irradiated with monoenergetic X-rays (from a synchrotron radiation source) on both sides of the K-absorption edge for Br. The fraction of the undamaged supercoiled form decreased exponentially with the photon dose; its yield in the brominated DNA did not exhibit any energy-dependence. This result is consistent with the calculated relative contributions of photoelectric interactions with individual component atoms which show that an electron flux originating in light atoms outweighs that from bromine. However, X-rays of energy above the Br K-edge appear slightly more efficient in producing double-strand breaks. This result, as well as those reported in the literature on the Auger enhancement of different biological endpoints in cells containing brominated DNA, seem to suggest that the positive charge and submolecular effects associated with the photo-absorption in bromine play some role in damaging processes, besides the initial distribution of deposited electron energy.

Free radical formation and DNA strand breakage during metabolism of diaziquone by NAD(P)H quinone-acceptor oxidoreductase (DT-diaphorase) and NADPH cytochrome c reductase

One-electron reduction of diaziquone (AZQ) by purified rat liver NADPH cytochrome c reductase was associated with formation of AZQ semiquinone, superoxide anions, hydrogen peroxide, and hydroxyl radicals as indicated by ESR spin-trapping studies. Reactive oxygen formation correlated with AZQ-dependent production of single and double PM2 plasmid DNA strand breaks mediated by this system as detected by gel electrophoresis. Direct two-electron reduction of AZQ by purified rat liver NAD(P)H (quinone acceptor) oxidoreductase (QAO) was also associated with formation of AZQ semiquinone, superoxide anions, hydrogen peroxide, and hydroxyl radicals as detected by ESR spin trapping. Furthermore, PM2 plasmid DNA strand breaks were detected in the presence of this system. Plasmid DNA strand breakage was inhibited by dicumarol (49 ± 5%), catalase (57 ± 2.3%), SOD (42.2 ± 3.6%) and ethanol (41.1 ± 3.9%) showing QAO and reactive oxygen formation was involved in the PM2 plasmid DNA strand breaks observed. These results show that both one- and two-electron enzymatic reduction of AZQ give rise to formation of reactive oxygen species and DNA strand breaks. Autoxidation of the AZQ semiquinone and hydroquinone in the presence of molecular oxygen appears to be responsible for these processes. QAO appears to be involved in the metabolic activation of AZQ to free radical species. The cellular levels and distribution of this enzyme may play an important role in the response of tumor and normal cells to this antitumor agent.

Sapurimycin, new antitumor antibiotic produced by Streptomyces. Producing organism, fermentation, isolation and biological properties.

In screening actinomycetes for antitumor compounds, Streptomyces sp. DO-116 was found to produce a new antitumor antibiotic sapurimycin. It is structurally related to, but distinct from, kapurimycins. The antibiotic was produced in a fermentation medium supplemented with high porous polymer resin which adsorbs antibiotic in the culture and results in an increase of titer. Active material was separated from the polymer resin by a solvent extraction procedure and isolated by repeated solvent extraction, adsorption chromatography and HPLC. Sapurimycin was active against bacteria, particularly Gram-positive organisms. It exhibited antitumor activity against leukemia P388 and sarcoma 180 in mice. Sapurimycin caused single strand breaks in supercoiled plasmid DNA in vitro. These results are discussed in comparison with data for kapurimycins.

Induction of DNA strand breaks by RSU-1069, a nitroimidazole-aziridine radiosensitizer: Role of binding of both unreduced and radiation-reduced forms to DNA, in vitro

[2- 14C]-RSU-1069 [1-(2-nitro-1-imidazolyl)-3-(1-aziridino)-2-propanol], either as a parent (unreduced) or following radiation reduction, binds to calf thymus DNA in vivo. Radiation-reduced RSU-1069 binds to a greater extent and more rapidly than the parent compound. RSU-1137, a nonaziridino analogue of RSU-1069, binds following radiation reduction. Radiation-reduced misonidazole (1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol) exhibits binding ratios a thousand-fold less than those of reduced RSU-1069. There is no evidence for binding of parent misonidazole. Both parent and reduced RSU-1069 cause single strand breaks (ssbs) in pSV2 gpt plasmid DNA with thereduced compound causing a greater number of breaks. Parent and reduced RSU-1137 and misonidazole do not cause ssbs. It is inferred that the aziridine moiety present in both parent and reducedRSU-1069 is required for ssb production. RSU-1069 reacts with inorganic phosphate probably via nucleophilic ring-opening of the aziridine fragment. Incubation of plasmid DNA with reduced RSU-1069 in the presence of either phosphate or deoxyribose-5-phosphate at concentrations greater than 0.35 mol dm −3 prevents strand breakage, whereas 1.2 mol dm −3 deoxyribose does not protect against strand breakage formation. From these findings it is proposed that the observed binding to DNA occurs via the aziridine and the reduced nitro group of RSU-1069 and that these two have different target sites. Binding to DNA via the reduced nitro group may serve to increase aziridine attack due to localization at or near its target.

The use of plasmid DNA to probe DNA repair functions in the yeast Saccharomyces cerevisiae.

The survival of plasmid YRp12 treated in vitro with ultraviolet- or gamma-radiation, or with restriction endonucleases, has been used to investigate in vivo RAD gene activity in Saccharomyces cerevisiae. Yields of pyrimidine dimers or single and double strand breaks in plasmid DNA were assayed by physical methods. The biological effects of these damages were assayed by transformation of wild-type cells and rad mutants from each of the major groups of radiosensitive mutants. After UV-irradiation plasmid survival depended qualitatively on the same host functions that are needed for cellular survival. After gamma-irradiation no such correspondence was found. Apart from a RAD52-dependent stimulation of transformation efficiency at low doses, other host repair functions had little effect. Stimulation of transformation corresponded with the production of double- but not single-strand breaks in plasmid sequences homologous with the yeast genome and may be linked with a transient increase in mitotic stability. More generally these data also show that transformation events using the LiCl protocol may entail the uptake of a very low number of plasmid molecules per cell over a 10-fold range of DNA concentrations.

DNA strand scission by the carbon radical derived from 2-phenyl-ethylhydrazine metabolism.

Oxyhemoglobin catalyzed oxidation of the tranquilizing drug 2- phenylethylhydrazine induces single strand breaks (nicks) in the supercoiled pBR322 plasmid DNA. Spin-trapping studies have established a clear correlation between 2-phenylethyl radical yield and the DNA strand scission activity observed during 2- phenylethylhydrazine oxidation. The same correlation is obtained in the presence of active oxygen species scavengers or when the carbon radical is generated under anaerobic conditions by ferricyanide oxidation of the drug. In addition to DNA damage, the 2- phenylethylhydrazine turnover by oxyhemoglobin promotes destruction of the hemoprotein catalyst to as yet unidentified products. These results may be relevant for the expression of the mutagenic and carcinogenic properties of hydrazine derivatives.

R factor elimination by inhibitors of thymidylate synthetase (fluorodeoxyuridine and showdomycin) and the occurrence of single strand breaks in plasmid DNA.

Fluorodeoxyuridine is bactericidal and actively eliminates R factor 1818 from Escherichia coli; both effects are prevented by the addition of thymidine. R factors 7268 and TEM are not eliminated by fluorodeoxyuridine treatment. Showdomycin is bactericidal but does not eliminate R-1818. Thymidine only partially protects showdomycin-treated cultures. Fluorouracil eliminates R-1818 at one third the rate of fluorodeoxyuridine from bacteria that are wild-type with respect to thymidine phosphorylase, but is much less effective than fluorodeoxyuridine in curing R-1818 from a thymidine phosphorylase-deficient mutant. The amount of R-1818 DNA present in the closed circular form decreases from 1% of chromosomal DNA in control cultures to 0.35% after three hours treatment with fluorodeoxyuridine; under similar conditions the amount of R-7268 DNA remains virtually constant. Showdomycin treatment apparently increases the proprotion of closed circular R-1818 DNA. The results are discussed in terms of pyrimidine metabolism and it is suggested that R-1818 elimination by fluorodeoxyuridine treatment results from the induction, by thymineless conditions, of an R factor-mediated endonuclease.