THE NOVEL DNA-DAMAGE CHECKPOINT PROTEIN DDC1P IS PHOSPHORYLATED PERIODICALLY DURING THE CELL-CYCLE AND IN RESPONSE TO DNA-DAMAGE IN BUDDINGYEAST

The DDC1 gene was identified, together with MEC3 and other checkpoint genes, during a screening for mutations causing synthetic lethality wh en combined with a conditional allele altering DNA primase. Deletion o f DDC1 causes sensitivity to UV radiation, methyl methanesulfonate (MM S) and hydroxyurea (HU), ddc1 Delta mutants are defective in delaying G(1)-S and G(2)-M transition and in slowing down the rate of DNA synth esis when DNA is damaged during G(1), G(2) or S phase, respectively. T herefore, DDC1 is involved in all the known DNA damage checkpoints. Co nversely, Ddc1p is not required for delaying entry into mitosis when D NA synthesis is inhibited. ddc1 and mec3 mutants belong to the same ep istasis group, and DDC1 overexpression can partially suppress, MMS and HU sensitivity of mec3 Delta strains, as well as their checkpoint def ects. Moreover, Ddc1p is phosphorylated periodically during a normal c ell cycle and becomes hyperphosphorylated in response to DNA damage, B oth phosphorylation events are at least partially dependent on a funct ional MEC3 gene.