Fig. 3. A dominant allele of rad4⁺ suppresses the DNA damage and HU-sensitive phenotypes of checkpoint mutants. (A). Survival rates are shown for wild-type and rad3-56 cells as well as rad3-56 cells carrying either pREP41-HA (p182) or pREP41-HA-rad4⁺ (p247) following exposure to 10 mM HU, 20 µg/ml bleomycin sulphate and UV irradiation (0-150 J/m²). The presence of multi-copy HA-rad4⁺ suppresses the sensitivity of rad3-56 cells to all three agents, restoring viability to wild-type levels. (B-D) Equivalent experiments for rad26 (B), rad17-w (C) and hus1-4 (C). Wild-type and the respective mutant strains carrying either pREP41-HA (p182) or pREP41-HA-rad4⁺ (p247) were assayed for survival following exposure to 20 µg/ml bleomycin sulphate, 10 mM HU or UV radiation (0-150 J/m²). Comparison of B and C with D shows that the presence of the HA-Rad4-expressing plasmid suppresses sensitivity to all three agents in rad26 and rad17-w but not hus1-4 cells when compared with control cells carrying the vector alone.

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