Abstract
When telomerase is absent and/or telomeres become critically short, cells undergo a progressive decline in viability termed senescence. The telomere checkpoint model predicts that cells will respond to a damaged or critically short telomere by transiently arresting and activating repair of the telomere. We examined the senescence of telomerase-deficient Saccharomyces cerevisiae at the cellular level to ask if the loss of telomerase activity triggers a checkpoint response. As telomerase-deficient mutants were serially subcultured, cells exhibited a progressive decline in average growth rate and an increase in the number of cells delayed in the G2/M stage of the cell cycle. MEC3, MEC1, and DDC2, genes important for the DNA damage checkpoint response, were required for the cell cycle delay in telomerase-deficient cells. In contrast, TEL1, RAD9, and RAD53, genes also required for the DNA damage checkpoint response, were not required for the G2/M delay in telomerase-deficient cells. We propose that the telomere checkpoint is distinct from the DNA damage checkpoint and requires a specific set of gene products to delay the cell cycle and presumably to activate telomerase and/or other telomere repair activities.