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

Abstract

We examine ionizing radiation (IR) sensitivity and epistasis relationships of several Saccharomyces mutants affecting post-translational modifications of histones H2B and H3. Mutants bre1Δ, lge1Δ, and rtf1Δ, defective in histone H2B lysine 123 ubiquitination, show IR sensitivity equivalent to that of the dot1Δ 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 set2Δ mutant, defective in H3 K36 methylation, shows mild IR sensitivity whereas mutants that abolish H3 K4 methylation resemble wild type. The dot1Δ, bre1Δ, and lge1Δ mutants show epistasis for IR sensitivity. The paf1Δ mutant, also reportedly defective in H2B K123 ubiquitination, confers no sensitivity. The rad6Δ, rad51null, rad50Δ, and rad9Δ mutations are epistatic to bre1Δ and dot1Δ, but rad18Δ and rad5Δ show additivity with bre1Δ, dot1Δ, and each other. The bre1Δ rad18Δ double mutant resembles rad6Δ in sensitivity; thus the role of Rad6p in ubiquitinating H2B accounts for its extra sensitivity compared to rad18Δ. We conclude that IR resistance conferred by BRE1 and DOT1 is mediated through homologous recombinational repair, not postreplication repair, and confirm findings of a G1 checkpoint role for the RAD6/BRE1/DOT1 pathway. Copyright © 2006 by the Genetics Society of America.