The roles of Mek1 and cohesins in suppressing meiotic intersister double-strand break repair

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Callender, Tracy Liane
The Graduate School, Stony Brook University: Stony Brook, NY.
Reciprocal exchange between homologs, in combination with sister chromatid cohesion, promotes proper segregation of homologs at Meiosis I. Missegregation of chromosomes during meiosis causes abnormal chromosome numbers in gametes. Errors in meiotic chromosome segregation can lead to infertility and, in humans, are responsible for genetic disorders like Trisomy 21. Meiotic recombination is initiated by double-strand breaks (DSBs). Repair of these breaks is biased to occur by invasion of homologs, not sister chromatids. This bias is mediated both by a meiosis-specific recombinase, Dmc1 and by the suppression of Rad51/Rad54 mediated recombination between sister chromatids. In dmc1 mutants, DSBs fail to get repaired, triggering a recombination checkpoint resulting in a prophase arrest. Mek1, a meiosis-specific kinase activated by DSBs, is required to prevent DSB repair using sister chromatids. To confirm that inactivation of Mek1 promotes intersister repair, two-dimensional gel analysis was used to look at intersister and interhomolog joint molecules (JMs). Inactivation of a conditional allele of Mek1 in a dmc1 background resulted exclusively in intersister JMs. To determine whether MEK1 suppression of intersister repair requires the presence of homologs, DSB repair was analyzed in haploid strains containing mek1 and dmc1. The finding that dmc1 haploids have unrepaired breaks that are repaired in the absence of MEK1 demonstrates that the mechanism by which MEK1 suppresses intersister repair is specific to sister chromatids. Interestingly, repair in wild-type haploids is either delayed or absent. Comparing DSB repair at different hotspots in disomic haploid and haploid strains, I found that DSBs are repaired only on the disomic chromosomes, indicating that repair on different chromosomes occurs independently. REC8 encodes a meiosis-specific subunit of cohesin. DSB repair in rec8 diploids is impaired during meiosis. DSB repair was analyzed in diploid and haploid strains containing rec8 and rec8 mek1. DSBs were efficiently repaired when MEK1 is absent, indicating that REC8 is specifically required for interhomolog repair.