The ribonucleotide reductase inhibitor Sml1 is a new target of the Mec1/Rad53 kinase cascade during growth and in response to DNA damage

The evolutionarily conserved protein kinases Mec1 and Rad53 are required fo r checkpoint response and growth. Here we show that their role in growth is to remove the ribonucleotide reductase inhibitor Sm11 to ensure DNA replic ation. Sm11 protein levels fluctuate during the cell cycle, being lowest du ring S phase. The disappearance of Sm11 protein in S phase is due to post-t ranscriptional regulation and is associated with protein phosphorylation, B oth phosphorylation and diminution of Sm11 require MEC1 and RAD53, Moreover , failure to remove Sm11 in mec1 and rad53 mutants results in incomplete DN A replication, defective mitochondrial DNA propagation, decreased dNTP leve ls and cell death. Interestingly, similar regulation of Sm11 also occurs af ter DNA damage. In this case, the regulation requires MEC1 and RAD53, as we ll as other checkpoint genes. Therefore, Sm11 is a new target of the DNA da mage checkpoint and its removal is a conserved function of Mec1 and Rad53 d uring growth and after damage.