Heterozygosity alters Msh5 binding to meiotic chromosomes in the baker's yeast.

Meiotic crossovers are initiated from programmed DNA Double Strand Breaks (DSBs). The Msh4-Msh5 heterodimer is an evolutionarily conserved mismatch repair related protein complex that promotes meiotic crossovers by stabilizing strand invasion intermediates and joint molecule structures such as Holliday junctions. In vivo studies using homozygous strains of the baker's yeast Saccharomyces cerevisiae (SK1) show that the Msh4-Msh5 complex associates with DSB hotspots, chromosome axes, and centromeres. Many organisms have heterozygous genomes that can affect the stability of strand invasion intermediates through heteroduplex rejection of mismatch containing sequences. To examine Msh4-Msh5 function in a heterozygous context, we performed ChIP-Seq analysis in a rapidly sporulating hybrid S. cerevisiae strain (S288c-sp/YJM789, containing sporulation enhancing QTLs from SK1), using SNP information to distinguish reads from homologous chromosomes. Overall, Msh5 localization in this hybrid strain was similar to that determined in the homozygous strain (SK1). However, relative Msh5 levels were reduced in regions of high heterozygosity, suggesting that high mismatch densities reduce levels of recombination intermediates to which Msh4-Msh5 binds. Msh5 peaks were also wider in the hybrid background compared to the homozygous strain (SK1). We determined regions containing heteroduplex DNA by detecting chimeric sequence reads with SNPs from both parents. Msh5 bound DSB hotspots overlap with regions that have chimeric DNA, consistent with Msh5 binding to heteroduplex-containing recombination intermediates.