Direct selection for 12 mutations (blm) conferring hypersensitivities to lethal effects of bleomycins in Saccharomyces cerevisiae resulted in mutants exhibiting cross-hypersensitivity to ionizing radiation and hydrogen peroxide. Remaining mutations did not confer cross-hypersensitivity to radiation. All blm mutations were recessive, except codominant blm3-1, and were assigned to seven complementation groups.In order to improve our understanding of radiomimetic damage to cells and how cells respond to and reverse such damage, we isolated a large number of mutants (blm [17]) of Saccharomyces cerevisiae on the basis of their hypersensitivities to lethal effects of the bleomycin family of lowmolecular-weight chemical congeners (Mr approximately 1,500 to 1,600). Objectives of the current study were to determine for 12 blm mutants the inheritance patterns of the mutant genes, genetic complementation among the mutants, hypersensitivities conferred by the blm mutations to ionizing and UV radiation and hydrogen peroxide, and temperature sensitivities. A common property of these agents is the generation of hydroxyl free radicals, which mediate DNA breaks (for examples, see references 1-3, 6-8, 10-15, 23, 24, 27-31). Meiotic segregants for this study were constructed by conventional techniques of crossing, sporulation, and dissection (22, 26). Culturing, media preparation, ionizing and UV irradiation, and quantitative survival assays were carried out as previously described (17)(18)(19)(20)22).The range of sensitivities conferred by the blm mutations and the importance of each defective gene on survival are illustrated in Fig. 1. Survival of the parental strain (Fig. 1A) decreased approximately linearly from 100 to 16% + 0.38% in a dose range where survival of each mutant strain (Fig. 1B through L MATa, ade240, cycl45 [17,21]) and mutant strains. Cells were plated and grown on nonsynthetic complete medium (1% Bacto-Yeast extract, 2% Bacto-Peptone, 2% dextrose, and 0.08 mg of adenine sulfate per ml, solidified with 1 to 1.5% Noble agar or Bacto-Agar [YPAD]) containing 0 to 14 ,ug of bleomycin per ml. Survival at 23 and 37°C is compared for mutants exhibiting temperature-sensitive expression of bleomycin sensitivity, except for X1-13, whose survival at 37°C at high test concentrations was too low to be determined accurately. For all other mutants, survival at 23°C is illustrated. The X2-27, X3-27, X4-9, and X7-20 mutants exhibited slower, poor, or
