Hyperactivation of the yeast DNA damage checkpoint by TEL1 and DDC2 overexpression

The evolutionarily conserved yeast Mec1 and Tel1 protein kinases, as well a s the Mec1-interacting protein Ddc2, are involved in the DNA damage checkpo int response. We show that regulation of Tell and Ddc2-Mec1 activities is i mportant to modulate both activation and termination of checkpoint-mediated cell cycle arrest. In fact, overproduction of either Tell or Ddc2 causes a prolonged cell cycle arrest and cell death in response to DNA damage, impa iring the ability of cells to recover from checkpoint activation. This cell cycle arrest is independent of Mec1 in UV-irradiated Tel1-overproducing ce lls, while it is strictly Mec1 dependent in similarly treated DDC2-overexpr essing cells. The Rad53 checkpoint kinase is instead required in both cases for cell cycle arrest, which correlates with its enhanced and persistent p hosphorylation, suggesting that unscheduled Rad53 phosphorylation might pre vent cells from re-entering the cell cycle after checkpoint activation. In addition, Tell overproduction results in transient nuclear division arrest and concomitant Rad53 phosphorylation in the absence of exogenous DNA damag e independently of Mec1 and Ddc1.