MMS1 and the repair of replication-dependent DNA damage in saccharomyces cerevisiae
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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 mms1Δ 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, mms1Δ cells do display a minor sensitivity to killing by UV and ionizing radiation. The mms1Δ mutant displays a RAD9-dependent slow growth phenotype, as well as an abnormal cellular morphology shared by recombination-defective mutants. mms1Δ 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, rad52Δ, defective for recombination, is epistatic to mms1Δ for MMS-sensitivity. This was surprising, since all known RAD52 group mutants are extremely γ-sensitive, whereas mms1Δ cells are not. It is hypothesized that MMS can induce replication-dependent double strand breaks, while the double strand breaks induced by γ 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, mms1Δ 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 rad52Δ epistatic to mms1Δ 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.