The Coprinus cinereus adherin Rad9 functions in Mre11-dependent DNA repair, meiotic sister-chromatid cohesion, and meiotic homolog pairing

Mitotic sister-chromatid cohesion (SCC) is known to depend in part on conserved proteins called adherins, which although necessary for SCC are not themselves localized between sister chromatids. We have examined mitotic DNA-repair and meiotic chromosome behavior in the Coprinus cinereus adherin mutant rad9-1. Genetic pathway analysis established that Rac19 functions in an Mre11-dependent pathway of DNA repair. Using fluorescence in situ hybridization, we found that the rad9-1 mutant is defective in the establishment of meiotic homolog pairing at both interstitial and subtelomeric sites but in the maintenance of pairing at only interstitial loci. To determine the role of Rac19 in meiotic SCC, we hybridized nuclear spreads simultaneously with a homolog-specific probe and a probe that recognizes both members of a homologous pair. We found that Rac19 is required for wild-type levels of meiotic SCC, and that nuclei showing loss of cohesion were twice as likely also to fail at homolog pairing. To ask whether the contribution of Rac19 to homolog pairing is solely in the establishment of SCC, we examined a rad9-1;msh5-22 double mutant, in which premeiotic DNA replication is inhibited. The msh5-22 mutation partially suppressed the deleterious effects of the rad9-1 mutation on homolog pairing; however, pairing in the double mutant still was significantly lower than in the msh5-22 single mutant control. Because the role of Rad9 in homolog pairing is not obviated by the absence of a sister chromatid, we conclude that adherins have one or more early meiotic functions distinct from the establishment of cohesion.