In Saccharomyces cerevisiae the MSH4-MSH5, MLH1-MLH3, and MUS81-MMS4 complexes act to promote crossing over during meiosis. MSH4-MSH5, but not MUS81-MMS4, promotes crossovers that display interference. A role for MLH1-MLH3 in crossover control is less clear partly because mlh1⌬ mutants retain crossover interference yet display a decrease in crossing over that is only slightly less severe than that seen in msh4⌬ and msh5⌬ mutants. We analyzed the effects of msh5⌬, mlh1⌬, and mms4⌬ single, double, and triple mutants on meiotic crossing over at four consecutive genetic intervals on chromosome XV using newly developed computer software. mlh1⌬ mms4⌬ double mutants displayed the largest decrease in crossing over (13-to 15-fold) of all mutant combinations, yet these strains displayed relatively high spore viability (42%). In contrast, msh5⌬ mms4⌬ and msh5⌬ mms4⌬ mlh1⌬ mutants displayed smaller decreases in crossing over (4-to 6-fold); however, spore viability (18-19%) was lower in these strains than in mlh1⌬ mms4⌬ strains. These data suggest that meiotic crossing over can occur in yeast through three distinct crossover pathways. In one pathway, MUS81-MMS4 promotes interference-independent crossing over; in a second pathway, both MSH4-MSH5 and MLH1-MLH3 promote interference-dependent crossovers. A third pathway, which appears to be repressed by MSH4-MSH5, yields deleterious crossovers. I N most eukaryotic organisms the correct segregation in both reciprocal exchanges, termed crossovers (CO), and of chromosomes at the first meiotic division requires nonreciprocal exchanges, termed noncrossovers (NCO). reciprocal exchange between homologs. The physicalThe classical double-strand break repair (DSBR) model manifestations of these crossover events, chiasmata, proproposes that these events result from alternative resoluvide the contacts between homologous chromosomes tions of a common Holliday junction intermediate (rethat are necessary for segregation ( Jones 1987). This viewed in Pâques and Haber 1999). Recent studies, cohesion or "chiasma binder" function ensures the genhowever, have suggested that COs and NCOs are proeration of a bipolar spindle in which tension is genercessed via separate pathways. In support of this idea, ated at the kinetochores (Maguire 1974

In budding yeast, the MLH1-PMS1 heterodimer is the major MutL homolog complex that acts to repair mismatches arising during DNA replication. Using a highly sensitive mutator assay, we observed that Saccharomyces cerevisiae strains bearing the S288c-strain-derived MLH1 gene and the SK1-strain-derived PMS1 gene displayed elevated mutation rates that conferred a long-term fitness cost. Dissection of this negative epistatic interaction using S288c-SK1 chimeras revealed that a single amino acid polymorphism in each gene accounts for this mismatch repair defect. Were these strains to cross in natural populations, segregation of alleles would generate a mutator phenotype that, although potentially transiently adaptive, would ultimately be selected against because of the accumulation of deleterious mutations. Such fitness ''incompatibilities'' could potentially contribute to reproductive isolation among geographically dispersed yeast. This same segregational mutator phenotype suggests a mechanism to explain some cases of a human cancer susceptibility syndrome known as hereditary nonpolyposis colorectal cancer, as well as some sporadic cancers.colorectal cancer ͉ incompatibility T he highly conserved mismatch repair (MMR) system contributes to genome stability by repairing errors that occur during DNA replication (1). In Escherichia coli, MMR is initiated by the binding of MutS protein to DNA mismatches. MutL interacts with the MutS-mismatch complex and activates downstream repair factors. Multiple MutS homologs (MSH) and MutL homologs (MLH) have evolved in eukaryotes that form heterodimers with specialized functions in DNA repair and recombination (2, 3). In Saccharomyces cerevisiae, MSH2-MSH3 and MSH2-MSH6 function in mismatch recognition, and MLH1-PMS1 is the primary MLH heterodimer in postreplicative MMR. Mutations in MSH and MLH genes that act in MMR elevate mutation rate, as measured in reversion and forward mutation assays, and reduce spore viability of diploid cells due to the accumulation of recessive lethal mutations (4-6). In addition, MMR proteins act to prevent recombination between divergent DNA sequences. This activity has been shown to prevent chromosomal rearrangements (7,8) and to enforce reproductive barriers between species (9, 10).Previously we created 60 alleles of the S. cerevisiae MLH1 gene from the S288c strain (cMLH1) in which clusters of charged residues were simultaneously changed to Ala (11). These alleles were tested for defects in MMR in the S288c (12) and SK1 (13) strains. More than one-third of the mutation set conferred a more severe MMR defect in SK1 strains than in S288c strains. Two mutations, cmlh1-29 and cmlh1-56, conferred wild-type-like phenotypes in S288c but null-like phenotypes in SK1. Introduction of the S288c PMS1 gene into the SK1 strain suppressed the mutator phenotype of these mutants, suggesting that the MMR phenotype was due to incompatibility, or negative epistasis, between MLH components (11).The influences of epistatic interactions on a wide variety of traits and proc...

Mismatch repair (MMR) is initiated by MutS family proteins (MSH) that recognize DNA mismatches and recruit downstream repair factors. We used a single-molecule DNA-unzipping assay to probe interactions between S. cerevisiae MSH2-MSH6 and a variety of DNA mismatch substrates. This work revealed a high-specificity binding state of MSH proteins for mismatch DNA that was not observed in bulk assays and allowed us to measure the affinity of MSH2-MSH6 for mismatch DNA as well as its footprint on DNA surrounding the mismatch site. Unzipping analysis with mismatch substrates containing an end blocked by lac repressor allowed us to identify MSH proteins present on DNA between the mismatch and the block, presumably in an ATP-dependent sliding clamp mode. These studies provide a high-resolution approach to study MSH interactions with DNA mismatches and supply evidence to support and refute different models proposed for initiation steps in MMR.