Distinct Functions in Regulation of Meiotic Crossovers for DNA Damage Response Clamp Loader Rad24(Rad17) and Mec1(ATR) Kinase.

The number and distribution of meiotic crossovers (COs) are highly regulated, reflecting the requirement for COs during the first round of meiotic chromosome segregation. CO control includes CO assurance and CO interference, which promote at least one CO per chromosome bivalent and evenly-spaced COs, respectively. Previous studies revealed a role for the DNA damage response (DDR) clamp and the clamp loader in CO formation by promoting interfering COs and interhomolog recombination, and also by suppressing ectopic recombination. In this study, we use classical tetrad analysis of to show that a mutant defective in , which encodes the DDR clamp loader ( in other organisms), displayed reduced CO frequencies on two shorter chromosomes ( and ), but not on a long chromosome (chromosome ). The residual COs in the mutant do not show interference. In contrast to , mutants defective in the ATR kinase homolog Mec1, including a null and a kinase-dead mutant, show slight or few defects in CO frequency. On the other hand, COs show defects in interference, similar to the mutant. Our results support a model in which the DDR clamp and clamp-loader proteins promote interfering COs by recruiting pro-CO Zip, Mer, and Msh proteins to recombination sites, while the Mec1 kinase regulates CO distribution by a distinct mechanism. Moreover, CO formation and its control are implemented in a chromosome-specific manner, which may reflect a role for chromosome size in regulation.