A Role for MutL Homologue MLH2 in controlling heteroduplex formation and in regulating between two different crossover pathways in budding yeast.

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 mlh2 strains and measured rates of crossing over, gene conversion, post-meiotic segregation and spore viability. We also analysed mlh1, mlh3, msh4, msh5, exo1 and mus81 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 mlh3, msh4 or msh5 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.