Replicative senescence is a permanent cell cycle arrest in response to extensive telomere shortening. To understand the mechanisms behind a permanent arrest, we screened for factors affecting replicative senescence in budding yeast lacking telomere elongation pathways. Intriguingly, we found that DNA polymerase epsilon (Pol epsilon) acts synergistically with Exo1 nuclease to maintain replicative senescence. In contrast, the Pol epsilon-associated, checkpoint and replication protein Mrc1 facilitates escape from senescence. To understand this paradox, in which DNA-synthesizing factors cooperate with DNA-degrading factors to maintain arrest, whereas a checkpoint protein opposes arrest, we analyzed the dynamics of double and single stranded DNA (ssDNA) at chromosome ends during senescence. We found evidence for cycles of DNA resection, followed by re-synthesis. We propose that resection of the shortest telomere, activating a Rad24(Rad17)-dependent checkpoint pathway, alternates in time with an Mrc1-regulated, Pol epsilon-re-synthesis of a short, double-stranded chromosome end, which in turn activates a Rad9(53BP1)-dependent checkpoint pathway. Therefore, instead of one type of DNA damage, different types (ssDNA and a double strand break-like structure) alternate in a vicious circle, each activating a different checkpoint sensor. Every time resection and re-synthesis switches, a fresh signal initiates, thus preventing checkpoint adaptation and ensuring the permanent character of senescence.
|Evidence ID||Analyze ID||Interactor||Interactor Systematic Name||Interactor||Interactor Systematic Name||Type||Assay||Annotation||Action||Modification||Phenotype||Source||Reference||Note|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Gene Ontology Term||Gene Ontology Term ID||Qualifier||Aspect||Method||Evidence||Source||Assigned On||Annotation Extension||Reference|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Phenotype||Experiment Type||Experiment Type Category||Mutant Information||Strain Background||Chemical||Details||Reference|
|Evidence ID||Analyze ID||Regulator||Regulator Systematic Name||Target||Target Systematic Name||Experiment||Assay||Construct||Conditions||Strain Background||Reference|