Melting experiment concerning the topological structure of closed circular double-stranded DNA

DNA nicking by ultraviolet radiation is enhanced in the presence of iron and of oxygen

Double-stranded covalently closed circular supercoiled DNA (ccc DNA) from plasmid pUK 9 was irradiated in vitro at defined wavelengths in the UV region (290, 313 and 365 nm). The nicking was monitored by electrophoresis on agarose gels, ethidium staining and densitometric quantitation of supercoiled and relaxed moieties. At the explored wavelengths, the dose required for introducing one nick per million phosphodiester bonds diminishes with increased concentration of added ferric iron, whereas the effect of cupric iron is practically negligible. Adding metal chelators or bubbling argon prior to the irradiation results in a dramatic increase in the dose required for introducing one nick per million phosphodiester bonds. Taken together, these results seem to indicate that iron and oxygen play a role as cofactors in the UV-induced nicking of ccc DNA in vitro.

Enhancement of oxidative DNA degradation by histidine: the role of stereochemical parameters

The nicking of supercoiled DNA by H2O2 and ferrous iron has been studied in a variety of environmental conditions. The replicative form of phage fd DNA (fd RF DNA) was used for investigating the phenomenon. The rate of nicking was measured in 10 mM NaCl. The addition of 1 mM Tris-HCl buffer (pH 7.5) slowed down the rate of nicking, the addition of 0.1 mM histidine enhanced it. The simultaneous presence of 1 mM Tris-HCl buffer and of 0.1 mM histidine further enhanced the rate of nicking of fd RF DNA. Increasing the concentration of NaCl dramatically reduced the rate of the reaction. The degradation of fd RF DNA was determined as a function of the concentration of histidine (0-5 mM): the rate increases with concentration, reaches a maximum and then decreases. In the presence of histidine, increasing the concentration of Tris leads to a similar phenomenon. In the absence of histidine, Tris always quenches the degradation of DNA. Electron spin resonance measurements failed to detect an enhancement of the signal characteristic for the hydroxyl radical when histidine was added to the solution containing hydrogen peroxide and ferrous iron. When the nicking of DNA is achieved via the process of auto-oxidation of ferrous iron (i.e., in the absence of added H2O2), histidine only reduces the rate of reaction in a dose-dependent manner, in the explored range of concentrations. In the presence of H2O2 and ferrous iron, histidine enhances the rate of nicking of double-stranded DNA in its supercoiled as well as in its relaxed state, but fails to modify the rate of nicking of fd DNA when it is in its vegetative, single-stranded form.

Histidine modulates the clastogenic effect of oxidative stress

The kinetics of the formation of single-strand breaks induced by H2O2 in DNA is more rapid in Eagle's Modified Minimal Essential Medium (MEM) than in phosphate-buffered saline (PBS). Among the components of MEM, we found that histidine increases the rate of degradation of DNA by H2O2 in PBS dose-dependently. In hamster lung fibroblasts histidine increases the cytotoxicity of H2O2, as well as the number of sister-chromatid exchanges and the frequency of micronuclei induced by hydrogen peroxide.

Effect of UV irradiation at defined wavelengths on the tertiary structure of double-stranded covalently closed circular DNA

Double-stranded, covalently closed, supercoiled circular DNA from phage fd (replicative form) was irradiated with increasing doses of UV light at 254 nm, 290 nm, 313 nm and 365 nm, and subjected to electrophoresis on agarose slab gels. Increasing the doses of UV light at 254 and 290 nm promotes a smooth reduction in the electrophoretic mobility of the sample, as would be expected if the major effect of light at these two wavelengths were to induce the formation of photoproducts leading to the unwinding of the double strand. At high doses, UV light at 290 nm introduces single-strand breaks (1.2 kJ m-2 per nick per million phosphodiester bonds). UV light at 313 nm promotes an abrupt change in the electrophoretic mobility, as would be expected if the effect of this wavelength were to induce single-strand breaks, leading to the transformation of the supercoiled molecules in their relaxed form (23 kJ m-2 in order to introduce one nick per million phosphodiester bonds). UV light at 365 nm also promotes single-strand breaks in DNA (140 kJ m-2 per nick per million phosphodiester bonds).