EXO1 and MSH4 differentially affect crossing-over and segregation

Abstract

The 5'-3' exonuclease Exo1p from Saccharomyces cerevisiae is required for wild-type levels of meiotic crossing-over and normal meiotic chromosome segregation as is the meiosis-specific MutS homologue, Msh4p. Mutations in both genes reduce crossing-over by approximately two-fold, but deltamsh4 strains have significantly lower viability and a higher frequency of meiosis I non-disjunction. Epistasis analysis indicates a complex interaction between the two genes. Although crossing-over was not detectably lower in the double mutant, viability was significantly worse than either single mutant. Such a result suggests that the two genes are affecting meiotic viability by distinct mechanisms. We propose that deltaexo1 affects chromosome segregation by reducing crossing-over, while deltamsh4 affects both the frequency and distribution of crossovers. Mutation in EXO1 reduces gene conversion frequencies significantly at some but not all loci, suggesting that other enzymes are also involved in DNA resection. We propose that Exo1p plays an early role in establishing some recombination intermediates by generating single-stranded tails. The role of Msh4p is suggested to be in determining whether some recombination intermediates are resolved as crossover events and in generating crossover interference. The synergistic effect of deltaexo1deltamsh4 on spore viability suggests that the two genes have partially compensatory roles in a process affecting meiotic success.

A Mutation in the Putative MLH3 Endonuclease Domain Confers a Defect in Both Mismatch Repair and Meiosis inSaccharomyces cerevisiae

“…Khazanehdari and Borts 2000). Consistent with this, a high percentage of two-spore viable sister tetrads was seen in mlh3D (86%, n ¼ 194 two-spore viable tetrads) and mlh3-D523N (88%, n ¼ 198).…”

Systematic Mutagenesis of the Saccharomyces cerevisiae MLH1 Gene Reveals Distinct Roles for Mlh1p in Meiotic Crossing Over and in Vegetative and Meiotic Mismatch Repair

“…These observations suggest that Mlh1p may activate mismatch repair and crossing-over factors through a common mechanism that is not well understood. A potential candidate for activation by Mlh1p is Exo1p, a factor which also promotes meiotic crossing over and physically interacts with Mlh1p (34,35,65,66). Additional genetic analyses will be required to determine whether Mlh1p and Exo1p function in a common meiotic pathway.…”

Local chromosome context is a major determinant of crossover pathway biochemistry during budding yeast meiosis

“…Most of the remaining events are repaired by a meiosis-specific CO pathway, in which an ensemble of meiotic proteins, called the ZMM proteins, stabilize early recombination intermediates and promote their maturation into double Holliday junction joint molecules (Allers and Lichten, 2001a;Bö rner et al, 2004;Lynn et al, 2007;Schwacha and Kleckner, 1994). These ZMM-stabilized joint molecules (JMs) are subsequently resolved as COs (Sourirajan and Lichten, 2008) through the action of the MutLg complex, which contains the Mlh1, Mlh3, and Exo1 proteins (Argueso et al, 2004;Khazanehdari and Borts, 2000;Wang et al, 1999;Zakharyevich et al, 2010Zakharyevich et al, , 2012. MutLg does not appear to make significant contributions to mitotic COs (Ira et al, 2003).…”