Topoisomerase II (topo II) catalyzes the decatenation of interlinked DNA molecules and is essential for chromosome segregation. To test the hypothesis that the noncatalytic C-terminal domain of topo II is necessary for mediating interactions with other proteins required for chromosome segregation, we used a two-hybrid cloning strategy to identify proteins that interact with S. cerevisiae topo II in vivo. One protein identified (Sgs1p) is structurally related to E. coli RecQ protein and contains helicase signature motifs. Strains lacking Sgs1p exhibit elevated levels of chromosome misseggregation during both mitotic and meiotic division. We propose a model to account for the interaction of a topoisomerase and a helicase in the faithful segregation of newly replicated eukaryotic chromosomes.

The mismatch repair system is the major barrier to genetic recombination during interspecific sexual conjugation in prokaryotes. The existence of this anti‐recombination activity has implications for theories of evolution and the isolation of species. To determine if this phenomenon occurs in eukaryotes, the effect of a deficiency of mismatch repair on the meiotic sterility of an interspecific hybrid of Saccharomyces cerevisiae and the closely related species Saccharomyces paradoxus was examined. The results demonstrate that the rare viable spores from these hybrids have high frequencies of aneuploidy and low frequencies of genetic exchange. Hybrids lacking mismatch repair genes PMS1 or MSH2 display increased meiotic recombination, decreased chromosome non‐disjunction and improved spore viability. These observations are consistent with the proposal that the mismatch repair system is an element of the genetic barrier between eukaryotic species. We suggest that an anti‐recombination activity during meiosis contributes towards the establishment of post‐zygotic species barriers.

In eukaryotes, homologs of the bacterial MutS and MutL proteins function in DNA mismatch repair and recombination pathways. The mutL homolog MLItl is required for nuclear mismatch repair. Previously, cytological analysis of MLHl.deficient mice has implied a role for Mlhl in crossing-over during meiosis. Here we demonstrate that Saccharomyces cerevisiae diploids containing a deletion of MLH1 have reduced crossing-over in addition to a deficiency in the repair of mismatched DNA during meiosis. Absence of either of the meiosis-specific mutS homologs Msh4 or Msh5 results in a similar reduction in crossing-over. Analysis of an mlhl rash4 double mutant suggests that both genes act in the same pathway to promote crossing-over. All genetic markers analyzed in mlhl mutants display elevated frequencies of non-Mendelian segregation. Most of these events are postmeiotic segregations that represent unrepaired heteroduplex. These data suggest that either restorational repair is frequent or heteroduplex tracts are shorter in wild-type cells. Comparison of mlhl segregation data with that of pmsl, rash2, rash3, and rash6 mutants show that the ability to promote crossing-over is unique to MLH1. Taken together these observations indicate that both crossing-over and gene conversion require MutS and MutL functions and that Mlhl represents an overlap between these two pathways. Models of Mlhl function are discussed.[Key Words: MutL homologs; MutS homologs; mismatch repair; meiosis; crossing-over; gene conversion.]Received January 16, 1997; revised version accepted May 6, 1997.In eukaryotes, proteins homologous to the MutS and MutL components of the bacterial long-patch mismatch repair system have evolved to perform both DNA repair and recombination functions. In Saccharomyces cerevisiae the MutS homologs Msh2, Msh3, and Msh6 and the MutL homologs Pmsl and Mlhl are involved in the repair of mismatched DNA formed during replication and recombination (Williamson et al. 1985;Kramer et al.