Synaptonemal Complex Proteins of Budding Yeast Define Reciprocal Roles in MutSγ-Mediated Crossover Formation

During meiosis, crossover recombination creates attachments between homologous chromosomes that are essential for a precise reduction in chromosome ploidy. Many of the events that ultimately process DNA repair intermediates into crossovers during meiosis occur within the context of homologous chromosomes that are tightly aligned via a conserved structure called the synaptonemal complex (SC), but the functional relationship between SC and crossover recombination remains obscure. There exists a widespread correlation across organisms between the presence of SC proteins and successful crossing over, indicating that the SC or its building block components are procrossover factors. For example, budding yeast mutants missing the SC transverse filament component, Zip1, and mutant cells missing the Zip4 protein, which is required for the elaboration of SC, fail to form MutS gamma-mediated crossovers. Here we report the reciprocal phenotype-an increase in MutS gamma-mediated crossovers during meiosis-in budding yeast mutants devoid of the SC central element components Ecm11 or Gmc2, and in mutants expressing a version of Zip1 missing most of its N terminus. This novel phenotypic class of SC-deficient mutants demonstrates unequivocally that the tripartite SC structure is dispensable for MutS gamma-mediated crossover recombination in budding yeast. The excess crossovers observed in SC central element-deficient mutants are Msh4, Zip1, and Zip4 dependent, clearly indicating the existence of two classes of SC proteins-a class with procrossover function(s) that are also necessary for SC assembly and a class that is not required for crossover formation but essential for SC assembly. The latter class directly or indirectly limits MutSg-mediated crossovers along meiotic chromosomes. Our findings illustrate how reciprocal roles in crossover recombination can be simultaneously linked to the SC structure.