The RAD6/BRE1 histone modification pathway in saccharomyces confers radiation resistance through a RAD51-dependent process that is independent of RAD18

We examine ionizing radiation (IR) sensitivity and epistasis relationships of several Saccharomyces mutants affecting post-translational modifications of histones H2B and H3. Mutants bre1 Delta, Ige1 Delta, and rtf1 Delta, defective in histone H2B lysine 123 ubiquitination, show IR sensitivity equivalent to that of the dot1 Delta mutant that we reported on earlier, consistent with published findings that Dot1p requires H2B K123 ubiquitination to fully methylate histone H3 K79. This implicates progressive K79 methylation rather than mono-methylation in IR resistance. The set2 Delta mutant, defective in H3 K36 methylation, shows mild IR sensitivity whereas mutants that abolish H3 K4 methylation resemble wild type. The dot1 Delta, bre1 Delta, and lge1 Delta mutants show epistasis for IR sensitivity. The paf1 Delta mutant, also reportedly defective in H2B K123 ubiquitination, confers no sensitivity. The rad6 Delta, rad51null, rad50 Delta, and rad9 Delta mutations are epistatic to bre1 Delta and dot1 Delta, but rad18 Delta and rad5 Delta show additivity with bre1 Delta, dot1 Delta, and each other. The bre1 Delta rad18 Delta double mutant resembles rad6 Delta in sensitivity; thus the role of Rad6p in ubiquitinating H2B accounts for its extra sensitivity compared to rad18 Delta. We conclude that IR resistance conferred by BRE1 and DOT1 is mediated through homologous recombinational repair, not postreplication repair, and confirm findings of a G(1) checkpoint role for the RAD6/BRE1/DOT1 pathway.