| The DNA damage checkpoint responds to both endogenous cell stresses such as replication fork collapse and DNA lesions resulting from exogenous sources such as ionizing-irradiation or exposure to mutagenic chemicals. In response to these stresses, the checkpoint normally arrests or slows down cell division so that repair of DNA damage can occur. Following accurate repair, the cell cycle resumes progression. In this work, I used the budding yeast S. cerevisiae as a model system to examine the mechanisms by which checkpoint activation is alleviated following repair of DNA damage at a defined double-strand DNA break. I also investigated how cells are able to keep the checkpoint system from triggering cell cycle arrest in response to the ends of linear chromosomes, which, in their most basic form, also resemble a double-strand break. I report the identification and characterization of two protein complexes---Pph3-Psy2-Psy4 and KEOPS---that regulate re-entry into the cell cycle following activation of the DNA damage checkpoint, and the integrity of telomeric function, respectively. Pph3-Psy2-Psy4 functions to dephosphorylate histone gamma-H2AX and its mutation results in a hyper-activation of key checkpoint kinases and a prolonged cell-cycle arrest. In contrast, the KEOPS complex functions to promote telomere elongation and single-stranded DNA accumulation following telomere dysfunction. I conclude that cellular viability and genome integrity require the limitation or attenuation of the checkpoint response in both space and time. |
