Electrophoretic separation of nucleic acids: evaluation by video and photographic densitometry

Detection of pyrimidine dimers and monoadducts induced by 7-methylpyrido(3,4-c) psoralen and UVA in Chinese hamster V79 cells by enzymatic cleavage and high-pressure liquid chromatography

The photochemotherapeutically active psoralen derivative 7-methylpyrido(3,4-c) psoralen (MePyPs) has been recently shown to be able to photoinduce monoadducts of the C4-cycloaddition type as well as pyrimidine dimers in DNA in vitro. In the present study, we report on the induction of these two types of photolesions in mammalian cells in culture. The MePyPs photocycloadducts were quantified in V79 Chinese hamster cells after treatment with MePyPs plus UVA following enzymatic hydrolysis of the DNA by DNase I, S1 nuclease and acidic phosphatase treatments. Concomitantly induced pyrimidine dimers were determined by two methods, high-pressure liquid chromatography and alkaline gel electrophoresis after dimer-specific endonucleolytic cleavage. The results show that, in Chinese hamster cells treated with MePyPs plus UVA, the yield of pyrimidine dimers is approximately 5-10% that of MePyPs-DNA photocycloadducts. Because psoralen monoadditions to DNA alone are generally not considered as being very phototoxic, a synergistic interaction of monoadditions with pyrimidine dimers may be expected to occur in order to explain the high photobiological effectiveness of this psoralen derivative.

Repair of DNA double-strand breaks induced in Saccharomyces cerevisiae using different gamma-ray dose-rates: a pulsed-field gel electrophoresis analysis

We investigated the effects of gamma-ray exposures at high dose-rate (HDR, 23.2 Gy/min) and low dose-rate (LDR, 0.47 Gy/min) on survival and the induction of DNA double-strand breaks (dsb) in a diploid wild-type (D7) and the repair-deficient mutant strain rad52/rad52 of Saccharomyces cerevisiae. Analysis by pulsed-field gel electrophoresis (PFGE) using a contour homogeneous electric field apparatus revealed that, at HDR, in the range 0-400 Gy, dsb are induced as a linear function of gamma-ray dose. Liquid holding recovery in non-nutrient medium (LHR) for 48 h of wild-type cells treated at HDR, significantly increased survival and reduced the yield of dsb. Such changes did not occur in rad52/rad52 cells defective in the repair of dsb. Thus, in gamma-irradiated wild-type cells, an efficient repair of dsb is taking place during LHR. Treatments of wild-type cells at LDR resulted in higher survival and an approximately two-fold lower yield of dsb than at HDR. Such a dose-rate effect was absent in rad52/rad52 cells suggesting that, in wild-type cells during LDR exposures, significant amounts of dsb can be repaired. This repair could be very much accentuated by 48-h LHR of wild-type cells treated at LDR. The relationship observed between gamma-ray survival and dsb repair clearly indicates that increases in survival of wild-type cells, during LDR as compared with HDR exposures and after LHR, are strongly related to the repair of dsb.

DNA double-strand break induction in yeast by X-rays and alpha-particles measured by pulsed-field gel electrophoresis

Pulsed-field gel electrophoresis was used to separate the chromosomes of the diploid yeast Saccharomyces cerevisiae 211*B after irradiation with X-rays and alpha-particles. After electrophoresis, gels were stained with ethidium bromide, placed on a UV-transilluminator and photographed with a digitizing camera connected to a personal computer. The pictures obtained were processed with the help of specially developed software which allows for the correction of the camera's shading effect and background fluorescence. Linearity between DNA amount and fluorescence was demonstrated. Fluorescence intensity for the band with the lowest electrophoretic mobility was found to decrease exponentially with dose. Based on the known size of the native DNA molecules, double-strand break yields could be calculated. These were found to be (8.2 +/- 0.4) and (14.8 +/- 0.5)10(-12) (g/mol)-1 Gy-1 for 80 keV X-rays and 3.5 MeV 241Am alpha-particles respectively which gives a relative biological effectiveness of 1.8 +/- 0.1.