Post translational modification of Exo1 in Saccharomyces cerevisiae

Meiosis is the cell division that produces haploid gametes for the purpose of sexual reproduction. During this division it is essential for homologous chromosomes to be securely paired and segregated in order for the gametes to receive a single copy of each chromosome. An important protein in this process is the exonuclease Exo1. Exo1 has two important and distinct roles during meiosis: resection of DNA at double-strand breaks (DSBs) exposing single stranded DNA suitable for strand invasion, and resolution of double-Holliday junctions (dHJs) as cross-overs. Exo1 also acts as a nuclease during DSB repair during mitosis. Previous studies have shown that Exo1 is phosphorylated in response to DNA damage in mitotically cycling cells. The role of this phosphorylation is yet to be definitively determined. This study aimed to test the hypothesis that Exo1 might also be phosphorylated in response to meiotic DSBs. It was confirmed that Exo1 was phosphorylated in response to mitotic DNA damage by designing a tagged version of Exo1 in the Saccharomyces cerevisiae background SK1, a strain commonly used for meiotic experiments. Meiotic progression and spore viability appeared to be normal in cells with exo1 mutated at the phosphorylation sites reported as active in mitosis. In meiotic studies Exo1 was found to be phosphorylated during meiosis, and this phosphorylation was different to that seen in previous mitotic studies. It was found that this phosphorylation was transient during meiosis and that it reflected the presence of Spo11-DSBs and their repair.