Diploidy and repair of radiation damage in Saccharomyces mutants

Diploid wild-type cells of Saccharomyces and cells homozygous for one of the single genes r^s₁, r^s₂ of the two alleles of the r^s₃ gene (rad 2 gene) were tested for their response to ionizing irradiation in the presence and absence of oxygen. Both immediate and delayed plating (agar-holding) techniques were used.An identical oxygen enhancement ratio (OER) of 2.5 was observed in r^s₁r^s₁, r^s₂r^s₂ cells and the wild type. In r^s_3?1r^s_3?1 and r^s_3?2r^s_3?2 cells the OER was reduced. Their sensitivity to ionizing irradiation in oxic conditions was the same as that of the wild type.r^s₂r^s₂ cells showed an increase in relative sensitivity by a factor of 4 in both conditions of irradiation. The relative sensitivity of r^s₁r^s₁ cells was increased by a factor of 2.5 in hypoxic conditions and the sensitivity in hypoxia was identical to that of wild type cells in oxic conditions. In contrast to haploid r^s₁ cells the oxygen effect was fully expressed in diploid r^s₁r^s₁ cells.The results on diploid strains are discussed in terms of three independent genetically determined repair systems in Saccharomyces. The expression of these systems is influenced by ploidy.