Contribution of a photoreactivable component to the oxygen effect observed in certain rad mutants of Saccharomyces cerevisiae after exposure to high energy electrons.

It is shown that a fraction of damage induced by high energy electrons (25 MeV) in certain rad mutants of the yeast Saccharomyces cerevisiae can be photoreactivated. The photoreactivable damage contributes to the lethal effect of this type of irradiation and modifies the oxygen effect. Using photoreactivating light or nigrosin, the amount of photoreactivable damage is reduced and the oxygen enhancement ratio (OER) for yeast mutants increases approximately to the OER found in wild-type cells.

The formation of photoreactivable damage by direct excitation of DNA in X-irradiated E. coli cells.

The role of the direct excitation process in the formation of photoreactivable damage (pyrimidine dimers) in E. coli WP2 hcr-exr- cells has been studied. The pyrimidine dimers were detected by photoreactivation following anoxic irradiation by X-rays (220 kVp). The dose modifying factor (DMF) is 1.28 +/- 0.09. A biophysical model is used for a theoretical examination of the importance of the direct excitation process in the formation of photoreactivable damage and the experimental data are consistent with this model.

The oxygen effect in E. coli WP2: the role of the photoreactivatable component in modification of the oxygen effect in E. coli WP2 uvrA6.

The role of photo-products induced by secondary U.V. which accompanies high-energy gamma-radiations has been studied in bacteria in the presence and absence of oxygen. The photodamage makes an essential contribution to the lethal effect of ionizing radiation in U.V.-sensitive strains in anoxia and thus can influence the value of the oxygen effect. When the secondary U.V.-contribution was minimized (irradiation by X-rays or gamma-rays in the presence of 1 per cent nigrosine) the oxygen enhancement ratio was shown to increase and approximate to that for E. coli WP2 cells.