mlh3 mutations in baker’s yeast alter meiotic recombination outcomes by increasing noncrossover events genome-wide

Mlh1-mlh3 is an endonuclease hypothesized to act in meiosis to resolve double holliday junctions into crossovers. it also plays a minor role in eukaryotic dna mismatch repair (mmr). to understand how mlh1-mlh3 functions in both meiosis and mmr,we analyzed in baker’s yeast 60 new mlh3 alleles. five alleles specifically disrupted mmr,whereas one (mlh3-32) specifically disrupted meiotic crossing over. mlh1-mlh3 representatives for each class were purified and characterized. both mlh1-mlh3-32 (mmr+,crossover-) and mlh1-mlh3-45 (mmr-,crossover+) displayed wild-type endonuclease activities in vitro. msh2-msh3,an msh complex that acts with mlh1-mlh3 in mmr,stimulated the endonuclease activity of mlh1-mlh3-32 but not mlh1-mlh3-45,suggesting that mlh1-mlh3-45 is defective in msh interactions. whole genome recombination maps were constructed for wild-type and mmr+crossover-,mmr-crossover+,endonuclease defective and null mlh3 mutants in an s288c/yjm789 hybrid background. compared to wild-type,all of the mlh3 mutants showed increases in the number of noncrossover events,consistent with recombination intermediates being resolved through alternative recombination pathways. our observations provide a structure-function map for mlh3 that reveals the importance of protein-protein interactions in regulating mlh1-mlh3’s enzymatic activity. they also illustrate how defective meiotic components can alter the fate of meiotic recombination intermediates,providing new insights for how meiotic recombination pathways are regulated. © 2017 al-sweel et al.