Meiotic Chromosome Synapsis-Promoting Proteins Antagonize the Anti-Crossover Activity of Sgs1

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Abstract
Sgs1, the budding yeast homolog of the mammalian BLM helicase, has been implicated in preventing excess recombination during both vegetative growth and meiosis. Most meiotic crossover (CO) recombination requires full function of a set of yeast proteins (Zip1, Zip2, Zip3, Zip4/Spo22, Mer3, Msh4, and Msh5, termed the SIC or ZMM proteins) that are also required for homologous chromosome synapsis. We report here genetic and molecular assays showing that sgs1 single mutants display relatively modest increases in CO recombination (less than 1.6-fold relative to wild-type). In contrast, a much greater CO increase is seen when an sgs1 mutation is introduced into the CO- and synapsis-deficient zip1, zip2, zip3, mer3, or msh4 mutants (2- to 8-fold increase). Furthermore, close juxtaposition of the axes of homologous chromosomes is restored. CO restoration in the mutants is not accompanied by significant changes in noncrossover (NCO) recombinant frequencies. These findings show that Sgs1 has potent meiotic anti-CO activity, which is normally antagonized by SIC/ZMM proteins. Our data reinforce previous proposals for an early separation of meiotic processes that form CO and NCO recombinants. Most eukaryotic cells are diploid (two copies of each chromosome per cell), but gametes (in animals, sperm and eggs) are haploid (one chromosome copy). Gametes are produced from diploid cells during meiosis. The two copies of each chromosome are brought together in end-to-end alignment (synapsis), and then are connected by crossover recombination, which involves the joining of DNA from one chromosome copy to DNA of the other. Crossovers are critical for chromosome separation in the diploid-to-haploid transition, and also promote genetic diversity by shuffling parental genotypes. In contrast, during mitotic cell growth, crossovers create genome rearrangements and loss of heterozygosity, which are associated with cancer and other diseases. A DNA-unwinding enzyme, called BLM in mammals and Sgs1 in budding yeast, prevents mitotic crossover recombination by taking apart intermediates that would otherwise give rise to crossovers. This paper shows that yeast proteins that promote meiotic chromosome synapsis also protect recombination intermediates from Sgs1. If any of these proteins are absent, Sgs1 prevents both crossover formation and synapsis. These findings show how modulating the activity of a single critical enzyme can either prevent or promote crossover recombination, which threatens genome stability in mitosis but is essential for genome transmission in meiosis.