A role for the MutL homologue <i>MLH2</i> in controlling heteroduplex formation and in regulating between two different crossover pathways in budding yeast

Abstract

Background and aims: Mismatch repair proteins play important roles during meiotic recombination in the budding yeast Saccharomyces cerevisiae and most eukaryotic organisms studied to date. To study the functions of the mismatch repair protein Mlh2p in meiosis, we constructed mlh2Δ strains and measured rates of crossing over, gene conversion, post-meiotic segregation and spore viability. We also analysed mlh1Δ, mlh3Δ, msh4Δ, msh5Δ, exo1Δ and mus81Δ mutant strains singularly and in various combinations. Results: Loss of MLH2 resulted in a small but significant decrease in spore viability and a significant increase in gene conversion frequencies but had no apparent effect on crossing over. Deletion of MLH2 in mlh3Δ, msh4Δ or msh5Δ strains resulted in significant proportion of the “lost” crossovers found in single deletion strains being regained in some genetic intervals. We and others propose that there are at least two pathways to generate crossovers in yeast (Ross-Macdonald and Roeder, 1994; Zalevsky et al., 1999; Khazanehdari and Borts, 2000; Novak et al., 2001; de los Santos et al., 2003). Most recombination intermediates are processed by the “major”, Msh4-dependent pathway, which requires the activity of Mlh1p/Mlh3p/Msh4p/Msh5p as well as a number of other proteins. The minor pathway(s) utilizes Mms4p/Mus81p. We suggest that the absence of Mlh2p allows some crossovers from the MSH4 pathway to traverse the MUS81-dependent pathway.

Distinct Regulation of Mlh1p Heterodimers in Meiosis and Mitosis in Saccharomyces cerevisiae

“…crossing over that were significantly lower than the wild type in all intervals studied (P , 0.00511, G-test of homogeneity; Table 2) as observed previously (Wang et al 1999;Abdullah et al 2004). To determine whether MLH3 functions in the same pathway as MLH1 to promote crossing over as expected, a double mlh1D mlh3D mutant was compared to the homozygous mlh3D single mutant.…”

“…At least two distinct pathways contribute to the production of crossover events in Saccharomyces cerevisiae. The major pathway is dependent on Msh4p/Msh5p and the mismatch repair proteins Mlh1p and Mlh3p (Ross-MacDonald and Roeder 1994;Hollingsworth et al 1995;Hunter and Borts 1997;Wang et al 1999;Abdullah et al 2004) and the second pathway is dependent on Mus81p/Mms4p endonuclease (de los Santos et al 2001(de los Santos et al , 2003.…”

A Two-Pathway Analysis of Meiotic Crossing Over and Gene Conversion in Saccharomyces cerevisiae

“…Deletions of Mlh1 cause a reduction in crossing over, generally to a lesser degree than do deletions of Msh4 or Msh5 (Wang et al 1999;Abdullah et al 2004;Argueso et al 2004). Deletions of Mlh1 are hypostatic to deletions of Msh4 (Wang et al 1999;Argueso et al 2004), clearly indicating that Mlh1 promotes crossing over in the disjunction pathway.…”

“…Confirmation of this expectation is befogged by claims in the literature regarding the effect, if any, of Mlh1 on crossover interference. Abdullah et al (2004) wrote, '' . .…”

“…Shortcomings in the analyses by Abdullah et al (2004): Abdullah et al (2004) presented a three-factor test for interference on chromosome III. The data revealed interference in wild type and fit well with the null hypothesis of no interference in the mlh1 deletion mutant.…”