Investigating the role of Sgs1 in the suppression of meiotic homeologous recombination in Saccharomyces cerevisiae

Homeologous recombination is the genetic exchange that occurs between diverged DNA sequences. This type of recombination can be detrimental to the cell, as it could lead to deletions, duplications and even chromosome loss. Therefore, it must be suppressed in order to maintain the integrity of the genome. The mismatch repair (MMR) complex, along with the 3’-to-5’ helicase Sgs1, has been implicated in this early role in meiosis. We propose a model in which the MMR complex scans the genome searching for single end invasion (SEI) events occurring between diverged sequences. On finding such events, the MMR complex binds to them and impedes their progression. The MMR complex then recruits the Sgs1 helicase which unwinds the heteroduplex DNA, allowing the invading strand to continue its homology search. To test this hypothesis, we investigated whether the interaction between Sgs1 and one member of this MMR complex, Mlh1, affected the ability of Sgs1 to suppress homeologous recombination in the baker’s yeast Saccharomyces cerevisiae. In addition, we investigated which domains of Sgs1 were required for this suppression. The data presented here show that the interaction between Sgs1 and Mlh1 may be important in the suppression of homeologous recombination at the SEI stage. In addition to this, we present data suggesting that the interaction between Sgs1 and the type IA topoisomerase Top3 may also be important in the resolution of recombination intermediates formed between diverged sequences. We suggest that there may still be additional factors that are utilised by the cell in order to maintain the barrier to inter-species recombination. Sgs1 has also been suggested to function at a later stage of meiosis, in the decatenation of Holliday junctions. This process was proposed to be carried out in concert with Top3. We present data that conflict with this hypothesis. We propose that interactions between Sgs1 and the type II topoisomerase Top2 are required in the decatenation of pre-existing replication errors prior to the onset of meiosis. The data implicate Sgs1 in the pre-meiotic replication checkpoint to aid in the repair of errors caused during DNA replication prior to meiosis. We also hypothesise an additional role of Sgs1 in the activation of this pre-meiotic replication checkpoint for the process of sporulation. This investigation therefore emphasises the importance of Sgs1 in the early stages of meiosis.