Mlh1 is unique among mismatch repair proteins in its ability to promote crossing-over during meiosis.

Abstract

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.

Exo1 Roles for Repair of DNA Double-Strand Breaks and Meiotic Crossing Over inSaccharomyces cerevisiae

“…Consistent with this view, none of the eight chromosome III disomes was recombinant. In the absence of Mlh1, Mlh3, Msh4, Msh5, Zip1, Zip2, and Mer3, crossing over is reduced twofold to threefold during meiosis, leading to abnormal reductional division (Ross and Roeder, 1994;Sym and Roeder, 1994;Hollingsworth et al, 1995;Hunter and Borts, 1997;Chua and Roeder, 1998;Nakagawa and Ogawa, 1999;Wang et al, 1999). Mlh1, Mlh3, Msh4, and Msh5 are homologues of mismatch repair proteins, and both Mlh1 and Mlh3, like Exo1, are also required for mismatch correction; thus, a close mechanistic correlation between mismatch repair and crossing over is suggested.…”

“…Our genetic data show that crossing over in the exo1 msh4 double mutant is no more reduced than in msh4, arguing that Exo1 and Msh4 work in a common pathway. There is a strong interaction between EXO1 and MSH2; MSH2 encodes a homologue of the MutS protein of E. coli and plays a central part in recognition of mismatch base pairing , but it does not play a role in crossing over (Hunter and Borts, 1997). Exo1 physically interacts with Msh2, and EXO1 acts in the MSH2-dependent mismatch repair pathway.…”

“…Gene conversion has no significant role in segregation (Roeder, 1997). A mutation in any one of a group of genes (ZIP1, ZIP2, MSH4, MSH5, MLH1, MLH3, and MER3) reduces crossing over (Ross and Roeder, 1994;Sym and Roeder, 1994;Hollingsworth et al, 1995;Hunter and Borts, 1997;Chua and Roeder, 1998;Nakagawa and Ogawa, 1999;Wang et al, 1999). Among these, MSH4, MSH5, MLH1, and MLH3 are homologues of the mismatch repair proteins of E. coli, suggesting a mechanistic relationship between mismatch repair and crossing over .…”

Negative epistasis between natural variants of the Saccharomyces cerevisiae MLH1 and PMS1 genes results in a defect in mismatch repair

“…The initial spore viability of the mlh1⌬ strain was 70.6%, with 50.6% of the tetrads being incomplete, i.e., containing less than four viable spores (data not shown). Much of this initial reduction in spore viability is attributable to the meiotic crossover defect associated with the mlh1⌬ mutation (27). The wild-type and cMLH1-kPMS1 strains displayed initial spore viabilities of 94.3% and 91.1%, respectively, with 15.5% and 17.5% incomplete tetrads, respectively.…”

Regulating double-stranded DNA break repair towards crossover or non-crossover during mammalian meiosis

“…Strains carrying mutations in one of these genes have a reduction of CO whereas NCO are, in general, not affected. In addition, formation of CO in this pathway also requires Mlh1 and Mlh3 proteins, thought to act at a late step in the recombination reaction (Hunter & Borts 1997, Wang et al 1999, Alpi et al 2003, Argueso et al 2004. A unique manifestation of CO regulation is positive interference which leads to the non-random distribution of CO that are more widely spaced than expected (Jones 1984).…”