Genetic analysis of baker's yeast Msh4-Msh5 reveals a threshold crossover level for meiotic viability.
During meiosis, the Msh4-Msh5 complex is thought to stabilize single-end invasion intermediates that form during early stages of recombination and subsequently bind to Holliday junctions to facilitate crossover formation. To analyze Msh4-Msh5 function, we mutagenized 57 residues in Saccharomyces cerevisiae Msh4 and Msh5 that are either conserved across all Msh4/5 family members or are specific to Msh4 and Msh5. The Msh5 subunit appeared more sensitive to mutagenesis. We identified msh4 and msh5 threshold (msh4/5-t) mutants that showed wild-type spore viability and crossover interference but displayed, compared to wild-type, up to a two-fold decrease in crossing over on large and medium sized chromosomes (XV, VII, VIII). Crossing over on a small chromosome, however, approached wild-type levels. The msh4/5-t mutants also displayed synaptonemal complex assembly defects. A triple mutant containing a msh4/5-t allele and mutations that decreased meiotic double-strand break levels (spo11-HA) and crossover interference (pch2Δ) showed synergistic defects in spore viability. Together these results indicate that the baker's yeast meiotic cell does not require the ∼90 crossovers maintained by crossover homeostasis to form viable spores. They also show that Pch2-mediated crossover interference is important to maintain meiotic viability when crossovers become limiting.
Nishant
K T
KT
Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York, United States of America.
Chen
Cheng
C
Shinohara
Miki
M
Shinohara
Akira
A
Alani
Eric
E
D003434:Crossing Over, Genetic; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D008540:Meiosis; D050296:Microbial Viability; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D013172:Spores, Fungal / Q000166:cytology / Q000235:genetics / Q000378:metabolism
D004268:DNA-Binding Proteins; C091158:MSH4 protein, S cerevisiae; C094801:MSH5 protein, S cerevisiae; D009687:Nuclear Proteins; C528119:Pch2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins