The Preference for Error-Free or Error-Prone Postreplication Repair in Saccharomyces cerevisiae Exposed to Low-Dose Methyl Methanesulfonate Is Cell Cycle Dependent

Cells employ error-free or error-prone postreplication repair (PRR) processes to tolerate DNA damage. Here, we present a ge-nome-wide screen for sensitivity to 0.001%methyl methanesulfonate (MMS). This relatively low dose is of particular interest because wild-type cells exhibit no discernible phenotypes in response to treatment, yet PRRmutants are unique among repair mutants in their exquisite sensitivity to 0.001%MMS; thus, low-dose MMS treatment provides a distinctive opportunity to study postreplication repair processes. We show that upon exposure to low-dose MMS, a PRR-defective rad18mutant stalls into a lengthy G2 arrest associated with the accumulation of single-stranded DNA (ssDNA) gaps. Consistent with previous results fol-lowing UV-induced damage, reactivation of Rad18, even after prolonged G2 arrest, restores viability and genome integrity. We further show that PRR pathway preference in 0.001%MMS depends on timing and context; cells preferentially employ the error-free pathway in S phase and do not requireMEC1-dependent checkpoint activation for survival. However, when PRR is re-stricted to the G2 phase, cells utilize REV3-dependent translesion synthesis, which requires aMEC1-dependent delay and results in significant hypermutability. The DNA damage response (DDR) employs a signal transduc-tion network to delay cell cycle progression and promoteDNA repair (1). While it is well known that DNA damage checkpoints are critical for maintaining genome integrity, how the cell bal-ances between checkpoint arrest and cell proliferation in the set