Rad6-Bre1 mediated histone H2Bub1 protects uncapped telomeres from exonuclease Exo1 in Saccharomyces cerevisiae

Histone H2B lysine 123 mono-ubiquitination (H2Bubl), catalyzed by Rad6 and Brel in Saccharomyces cerevisiae, modulates chromatin structure and affects diverse cellular functions. H2Bubl plays roles in telomeric silencing and telomere replication. Here, we have explored a novel role of H2Bubl in telomere protection at uncapped telomeres in yku70 Delta and cdc13-1 cells. Deletion of RAD6 or BRE1, or mutation of H2BK123R enhances the temperature sensitivity of both yku70 Delta and cdc13-1 telomere capping mutants. Consistently, BREI deletion increases accumulation of telomeric single-stranded DNA (ssDNA) in yku70 Delta and cdc13-1 cells, and EXO1 deletion improves the growth of yku70 Delta brel Delta and cdc13-1 bre1 Delta cells and decreases ssDNA accumulation. Additionally, deletion of BRE1 exacerbates the rate of entry into senescence of yku70 Delta mrel1 Delta cells with telomere defects, and increases the recombination of subtelomeric Y' element that is required for telomere maintenance and survivor generation. Furthermore, Exol contributes to the abrupt senescence of yku70 Delta mrel1 Delta bre1 Delta cells, and Rad51 is essential for Y' recombination to generate survivors. Finally, deletion of BRE1 or mutation of H2BK123R results in nucleosome instability at subtelomeric regions. Collectively, this study provides a mechanistic link between H2Bub1-mediated chromatin structure and telomere protection after telomere uncapping.