MMS1 and the repair of replication-dependent DNA damage in saccharomyces cerevisiae
In an effort to characterize DNA alkylation repair, a series of yeast mutants were isolated that are sensitive to killing by the monofunctional DNA alkylating agent methyl methanesulfonate (MMS) but not by UV or X radiation (Prakash and Prakash, 1977, Genetics 86: 33-55). Our laboratory cloned and sequenced one of the corresponding genes, 'MMS1'. The 'mms1'[Delta] mutant is sensitive to MMS, as well as to a variety of other DNA alkylating agents. Contrary to what was reported for the 'mms1-1' mutant, 'mms1'[Delta] cells do display a minor sensitivity to killing by UV and ionizing radiation. The ' mms1'[Delta] mutant displays a 'RAD9'-dependent slow growth phenotype, as well as an abnormal cellular morphology shared by recombination-defective mutants. 'mms1'[Delta] mutants show an elevated level of spontaneous DNA damage and genomic instability, as indicated by hyperrecombinant and mutator phenotypes, and an elevated basal level expression of DNA damage-inducible genes. Epistasis analysis showed that 'MMS1' is not a member of the base excision, nucleotide excision, or postreplication repair pathways. Rather, 'rad52'[Delta], defective for recombination, is epistatic to 'mms1'[Delta] for MMS-sensitivity. This was surprising, since all known 'RAD52' group mutants are extremely ã-sensitive, whereas 'mms1'[Delta] cells are not. It is hypothesized that MMS can induce replication-dependent double strand breaks, while the double strand breaks induced by ã radiation are considered to be replication-independent. It was therefore proposed that Mms1 is required for the repair of replication-dependent DNA strand breaks. In support of this, 'mms1'[Delta] cells were shown to be sensitive to killing by treatments known to induce replication-dependent DNA strand breaks, such as camptothecin, hydroxyurea, and incubation of a ' cdc2-2' strain at the restrictive temperature. Not only is ' rad52'[Delta] epistatic to 'mms1'[Delta] for MMS-sensitivity, but this relationship holds true for camptothecin sensitivity as well. Together these results implicate Mms1 in the Rad52-dependent repair of replication-dependent DNA strand breaks, and provide evidence for a postreplication repair-independent mechanism for the repair of DNA damage incurred during S phase of the cell cycle.