DNA double strand break (DSB) repair is essential, not only for the maintenance of genome stability and therefore animal survival, but also for completion of meiosis. Exonucleases, such as Exo1, play key roles in DNA repair. Exo1 was first isolated in fission yeast extracts that possessed 5'' to 3'' exonuclease activity. Since then Exo1 has been implicated in a multitude of eukaryotic DNA metabolic pathways, such as telomere maintenance and DSB repair1. Moreover, Exo1 is implicated in mismatch repair (MMR); a process crucial for removing misincorporated nucleotides during DNA replication. MMR deficiency results in instability of simple repeat sequences, a phenomenon referred to as microsatellite instability (MSI). Several MMR factors also affect homologous recombination (HR) and DNA damage signaling2. Here, we introduce the C. elegans homologue of Exo1 and investigate how it contributes to genome stability. The C. elegans F45G2.3 gene (hereafter referred to as
exo-1) encodes a protein that is highly similar to yeast and mammalian Exo1. We show that
exo-1 deletion mutants have elevated MSI, suggesting a conserved role in MMR. In addition, we found that
exo-1 mutants are hypersensitive to g-irradiation, in a manner that is epistatic with the well-conserved HR factor,
brc-1. This suggests a role for
exo-1 in HR, which is the principle DSB repair pathway active in the worm germline. In contrast, deletion mutants of
mlh-1, which is a non-redundant factor in MMR, are not sensitive to g-irradiation, which implies that
exo-1 has an MMR-independent role in DSB repair in the germline. During meiosis, programmed DSBs are introduced by the topoisomerase
spo-11 and repair of these breaks is essential for completion of meiosis. Under normal conditions,
exo-1 mutants are able to complete meiosis, indicating
exo-1 is either not involved in the repair of
spo-11 induced DSBs or that it acts redundantly with other factors. In order to identify factors that may be working redundantly with
exo-1, we performed a genome-wide synthetic lethal RNAi screen. This screen identified several genes required for development of
exo-1 deficient animals and their contribution to DSB repair is being investigated. Additionally, we are searching for redundant factors via a candidate gene approach, focusing on the worm''s RecQ helicases. This was prompted by recent studies showing that Sgs1, the sole RecQ helicase in yeast, acts in parallel with Exo1 in processing DSBs in mitotic cells3. 1) Tran PT et al. EXO1: a multi-tasking eukaryotic nuclease, DNA Repair (2004) 2) Jiricny J. The multifaceted mismatch-repair system, Nat. Rev. Mol. Cell Biol. (2006) 3) Klein HL. DNA endgames, Nature (2008).