Photoactivation of aerobically and anaerobically grown diploid yeast cells under aerobe and hypoxic conditions.

It has been suggested that photoreactivation may occur by a mechanism which is similar to a photosensi­ tised splitting of pyrimidine-dimers1. Since photo­ sensitised processes depend very often on the presence or absence of oxygen, it is of interest in this context to know whether the gaseous environment has any ef­ fect on the photoreactibility of UV-irradiated cells. If the cytochrome system is involved in the photoreactiva­ tion mechanism one might, furthermore, expect a dif­ ference between aerobically and anaerobically grown cells. Diploid yeast Saccharomyces cerevisiae, strain 211, originally obtained from W. L a s k o w s k i , was used for this investigation. The cells were grown to the statio­ nary phase in Wickerham medium bubbled with air or nitrogen. They were irradiation in open P e t r i dishes under continuous stirring at room temperature. Sur­ vivors were counted as macrocolonies on Wickerham agar (2%) after 3 days incubation at 30 °C in the dark. The irradiation source was a low pressure mer­ cury lamp, Osram HNS 12. Photoreactivation was per­ formed by exposing the cell suspension in glass tubes to a Madza Al/215 bulb, operated at 10 V at a distance of about 10 cm. During this treatment, which lasted