Mre11 exonuclease activity removes the chain-terminating nucleoside analog gemcitabine from the nascent strand during DNA replication.

The Mre11 nuclease is involved in early responses to DNA damage, often mediated by its role in DNA end processing. MRE11 mutations and aberrant expression are associated with carcinogenesis and cancer treatment outcomes. While, in recent years, progress has been made in understanding the role of Mre11 nuclease activities in DNA double-strand break repair, their role during replication has remained elusive. The nucleoside analog gemcitabine, widely used in cancer therapy, acts as a replication chain terminator; for a cell to survive treatment, gemcitabine needs to be removed from replicating DNA. Activities responsible for this removal have, so far, not been identified. We show that Mre11 3' to 5' exonuclease activity removes gemcitabine from nascent DNA during replication. This contributes to replication progression and gemcitabine resistance. We thus uncovered a replication-supporting role for Mre11 exonuclease activity, which is distinct from its previously reported detrimental role in uncontrolled resection in recombination-deficient cells.

The msh2 gene of Schizosaccharomyces pombe is involved in mismatch repair, mating-type switching, and meiotic chromosome organization.

We have identified in the fission yeast Schizosaccharomyces pombe a MutS homolog that shows highest homology to the Msh2 subgroup. msh2 disruption gives rise to increased mitotic mutation rates and increased levels of postmeiotic segregation of genetic markers. In bandshift assays performed with msh2Delta cell extracts, a general mismatch-binding activity is absent. By complementation assays, we showed that S. pombe msh2 is allelic with the previously identified swi8 and mut3 genes, which are involved in mating-type switching. The swi8-137 mutant has a mutation in the msh2 gene which causes a truncated Msh2 peptide lacking a putative DNA-binding domain. Cytological analysis revealed that during meiotic prophase of msh2-defective cells, chromosomal structures were frequently formed; such structures are rarely found in the wild type. Our data show that besides having a function in mismatch repair, S. pombe msh2 is required for correct termination of copy synthesis during mating-type switching as well as for proper organization of chromosomes during meiosis.