Pulsed-field gel electrophoresis of chromosomal DNA of Saccharomyces pastorianus after exposure to x-rays (30-50 keV) and neutrons (14 MeV)

Chromosomes of budding yeast Saccharomyces pastorianus were used to determi ne the extent of DNA double-strand breaks (DSBs) induced by x-rays (30-50 k eV) and 14 MeV neutrons. The yeast chromosomes were separated by pulsed-fie ld gel electrophoresis (PFGE) and the proportion of unbroken molecules corr esponding to the largest chromosome no. IV (1500 kbp) was used to calculate the DSB frequency assuming a random distribution of hits. To determine the protective contribution of the cell environment, chromosomes embedded in a garose plugs as well as intact yeast cells, were irradiated under condition s completely inhibiting DNA repair. Following irradiation, the intact cells were also embedded in agarose plugs and the chromosomes isolated to perfor m PFGE. All radiation experiments resulted in a linear dose-effect curve fo r DSBs. For both radiation qualities, the yield of DSBs for exposed isolate d chromosomes exceeded that for intact yeast cells by a factor of 13. The r elative biological effectiveness (RBE) of 13 MeV neutrons in the induction of DNA DSBs was about 2.5. This figure was found to be identical for the in vivo and in vitro exposure of yeast chromosomes (neutrons 36.7 and 2.8, x- rays 14.5 and 1.1x10(-8) DSBxBp(-1) Gy(-1) for isolated DNA and intact cell s, respectively).