Effect of gamma-radiation on aqueous solutions of DNA's of different base composition

Application of spectral methods for studying DNA damage induced by gamma-radiation

Spectral methods can provide a variety of possibilities to determine several types of radiation-induced DNA damage, such as nucleobase destruction and local denaturation. DNA UV absorption and CD spectra measured at room temperature undergo noticeable alteration under the action of γ-radiation. We have applied the Spirin method of total nucleobases determination, and have measured the molar extinction coefficient of DNA and DNA CD spectra for solutions with different NaCl concentrations (3mM-3.2M) and containing MgCl₂, exposed to γ-radiation with the doses of 0-10³Gy. The melting temperatures of DNA in irradiated solutions at the doses of 0-50Gy were obtained with the help of spectrophotometric melting. It was found that the amount of destructed nucleobases and radiation-induced loss of DNA helicity significantly decreases with the rise of the ionic strength of the irradiated solution. Substitution of a portion of Na⁺ ions on Mg²⁺ while keeping the total ionic strength constant (μ=5mM) does not affect the considered radiation effects. The role of the structure and composition of the DNA secondary hydration layer in the radiation-induced damages is discussed.

Transcriptomic Profile of Arabidopsis Rosette Leaves during the Reproductive Stage after Exposure to Ionizing Radiation

We attempted to obtain a transcriptomic profile of ionizing radiation-responsive genes in Arabidopsis plants using Affymetrix ATH1 whole-genome microarrays. The Arabidopsis plants were irradiated with 200 Gy gamma rays at the early reproduction stage, 33 days after sowing. Rosette leaves were harvested during the postirradiation period from 36 to 49 days after sowing and used for the microarray analysis. The most remarkable changes in the genome-wide expression were observed at 42 days after sowing (9 days after the irradiation). We identified 2165 genes as gamma-ray inducible and 1735 genes as gamma-ray repressible. These numbers of affected genes were almost two to seven times higher than those at other times. In a comparison of the control and irradiated groups, we also identified 354 differentially expressed genes as significant by applying Welch's t test and fold change analysis. The gene ontology analysis showed that radiation up-regulated defense/ stress responses but down-regulated rhythm/growth responses. Specific expression patterns of 10 genes for antioxidant enzymes, photosynthesis or chlorophyll synthesis after irradiation were also obtained using real-time quantitative PCR analysis. We discuss physiological and genetic alterations in the antioxidative defense system, photosynthesis and chlorophyll metabolism after irradiation at the reproductive stage.

A Comparative Evaluation of Radiation-Induced DNA Damage using Real-Time PCR: Influence of Base Composition

To study the radiosensitivity of DNA segments at the open reading frame (gene) level, real-time PCR was used to analyze DNA damages induced by ionizing radiation. After irradiation (1, 3 and 5 kGy) of genomic DNA purified from Salmonella typhimurium, real-time PCR based on SYBR Green fluorescence and melting temperature was performed using various primer sets targeting the rfbJ, rfaJ, rfaB, hilD, ssrB, pipB, sopD, pduQ, eutG, oadB, ccmB and ccmA genes. The ccmA and ccmB genes, which existed as two copies on the chromosome and had a high GC content ( approximately 70%), showed much lower radiosensitivities than the other genes tested, particularly at 5 kGy; this distinctive feature was seen only when the genes were located on the chromosome, regardless of copy number. Our results reinforce the concept that gene sensitivity to ionizing radiation depends on the base composition and/or the spatial localization of the gene on the chromosome.

Low doses of alpha- and gamma-radiation enhance DNA thermal stability

Isolated calf thymus DNA in buffered solutions has been exposed to 0-150 Gy of alpha- and gamma-radiation. The effects of alpha- and gamma-radiation on the thermal stability and electrophoretic mobility of the DNA molecules have been studied by UV spectroscopic 'melting' and Pulsed Field Gel Electrophoresis (PFGE), respectively. The observed thermal denaturation parameters were fitted to the energy propagation descriptive model. The experimental results for the samples exposed to relatively low (low) doses indicate an increased thermal stability and a reduced mobility over that of the controls. The expected overall degradation of the DNA molecules was confirmed for the samples exposed to high doses. Our results are in good agreement with the predictions of the energy propagation model, which now is also tested in the low dose region and for an additional type of ionising radiation (alpha-particles). Our findings are consistent with conformational changes at low doses resulting in a DNA form characterised by localised alterations, which affect the energy flow along the DNA molecule.

Effect of DNA conformation on the hydroxyl radical-induced formation of 8,5'-cyclopurine 2'-deoxyribonucleoside residues in DNA

Reactions of hydroxyl radicals with DNA form a variety of base and sugar products and 8,5'-cyclopurine 2'-deoxyribonucleoside residues in DNA. Here we report the effect of DNA conformation on the yields of 8,5'-cyclopurine 2'-deoxynucleosides and the ratios of their (5'R)- and (5'S)-diastereomers. Calf thymus DNA in native (double-stranded DNA) or heat-denatured form (single-stranded DNA) was exposed to hydroxyl radicals generated by ionizing radiation in nitrous oxide-saturated phosphate buffer. Doses ranging from 10 to 40 Gy were used to ensure low levels of damage to DNA and thus to preserve its secondary structure in experiments with double-stranded DNA (ds-DNA). After irradiation, DNA was hydrolysed enzymatically to 2'-deoxyribonucleosides. The hydrolysates were dried, trimethylsilylated, and analyzed by capillary gas chromatography-mass spectrometry with selected-ion monitoring. An internal standard was used for quantitative measurements and added to DNA samples prior to enzymatic hydrolysis. The yields of 8,5'-cyclo-2'-deoxyadenosine and 8,5'-cyclo-2'-deoxyguanosine in single-stranded DNA (ss-DNA) were higher than those in ds-DNA. The (5'R)-diastereomers of both compounds were found to predominate over their (5'S)-diastereomers in ss-DNA. In contrast, the yields of the (5'S)-diastereomers in ds-DNA were slightly higher than those of the (5'R)-diastereomers. The G values of 8,5'-cyclo-2'-deoxyadenosine in ss-DNA and ds-DNA were 0.042 and 0.025, respectively. Those of 8,5'-cyclo-2'-deoxyguanosine in ss-DNA and ds-DNA were 0.038 and 0.017, respectively.

Unpaired bases in phage DNA after gamma-irradiation in-situ and in-vitro

Phage Lambda DNA, gamma-irradiated in-situ and in-vitro, has been analyzed for unpaired bases by melting, reannealing, and cleavage with Sl nuclease which is specific for single-stranded DNA. DNA, irradiated in-situ, i.e., in the phage particle, contained sites being sensitive to Sl nuclease. These single-stranded lesions were passed over and conserved during reannealing, whereas adjacent DNA regions reannealed specifically. Complementary base-pairing was restored after Sl nuclease treatment. Comparison of the Tm-data before and after Sl nuclease treatment indicated that the single-stranded regions were removed by the enzyme. In contrast, DNA irradiated in-vitro, i.e., gamma-irradiated in aqueous solution, failed to match complementarily and was not sensitive to Sl nuclease. Thus it appears that lesions leading to unpaired bases were randomly distributed in DNA irradiated in-vitro, but occurred in clusters after irradiation in-situ. Most probably these clusters contain damaged bases which in turn caused localized disruption of the hydrogen bonds between complementary base pairs.

Dependence of gamma-irradiated Y-peak and melting of DNA on concentration and ionic environment

Y-peak is found to be a function of ionic strength and concentrations of DNA. The Y-peak reveals close dynamic interaction between DNA and solvent system. Electronic transitions responsible for Y-peak are not the same transitions that are responsible for X-peak. Y-peak's electronic transitions are indicative of charge transfer complex formation between DNA and solvent system. gamma-Irradiation induces hyperchromicity due to strand separation at lower doses. A-T base pairs are first to undergo coiled state as shown by delta Tm spread. Strand chopping and saturation of double bonds of the exposed bases by free radicals (H degrees and OH degrees) give rise to hypochromic regions at X-peak. Rise in ionic strength and the concentration of DNA has protective effect against gamma-damage. Y-peak is found to be a function of solvent, whereas, X-peak is independent of solvent nature.

An electrochemical analysis of changes in properties of DNA caused by ionizing and ultraviolet radiations

Electrooxidation and electroreduction of gamma- and u.v.-irradiated DNA were studied by means of differential pulse voltammetry at the graphite electrode and differential pulse polarography at the dropping mercury electrode. Two separated voltammetric oxidation peaks G and A were used for monitoring conformational changes in guanine . cytosoine (GC) and adenine . thyminde (AT) pairs respectively in irradiated double-stranded (ds) DNA. Pulse-polarographic reduction peak III was used for detection of denatured DNA in the irradiated samples of ds DNA. It was found that the heights of peaks G and A of ds DNA did not change with the radiation dose after relatively low doses of gamma- and u.v.-radiations (up to ca. 40 krads and 1 X 10(4) Jm-2, respectively, when no single-stranded (ss) DNA was detected in the irradiated DNA samples. After higher doses of radiation the occurrence of ss DNA or ss segments in the irradiated samples of ds DNA was accompanied by an increase of peaks G and A; however, peak A grew more rapidly with the increasing dose than peak G. It was concluded that the results obtained support the assumption, according to which regions of ds DNA rich in AT pairs are more susceptible to denaturation caused by gamma- and u.v.-radiations.