Inactivation of Ku-mediated end joining suppresses mec1Δ lethality by depleting the ribonucleotide reductase inhibitor Sml1 through a pathway controlled by Tel1 kinase and the Mre11 complex

RAD53 and MEC1 are essential Saccharomyces cerevisiae genes required for the DNA replication and DNA damage checkpoint responses. Their lethality can be suppressed by increasing the intracellular pool of deoxynucleotide triphosphates. We report that deletion of YKU70 or YKU80 suppresses mec1Δ, but not rad53Δ, lethality. We show that suppression of mec1Δ lethality is not due to Ku^--associated telomeric defects but rather results from the inability of Ku^- cells to efficiently repair DNA double strand breaks by nonhomologous end joining. Consistent with these results, mec1Δ lethality is also suppressed by lif1Δ, which like yku70Δ and yku80Δ, prevents nonhomologous end joining. The viability of yku70Δ mec1Δ and yku80Δ mec1Δ cells depends on the ATM-related Tell kinase, the Mre11-Rad50-Xrs2 complex, and the DNA damage checkpoint protein Rad9. We further report that this Mec1-independent pathway converges with the Rad53/Dun1-regulated checkpoint kinase cascade and leads to the degradation of the ribonucleotide reductase inhibitor Sml1.