Repair of DNA loops involves DNA-mismatch and nucleotide-excision repair proteins

A number of enzymes recognize and repair DNA lesions(1). The DNA-mismatch repair system corrects base-base mismatches and small loops, whereas the nucleotide-excision repair systems removes pyrimidine dimers and other helix-distorting lesions. DNA molecules with mismatches or loops can arise as a consequence of heteroduplex formation during meiotic recombination(2). In the yeast Saccharomyces cerevisiae, repair of mismatches results in gene conversion or restoration, and failure to repair the mismatch results in post-meiotic segregation (PMS) (Fig. 1). The ratio of gene-conversion to PMS events reflects the efficiency of DNA repair(3,4). By examining the PMS patterns in yeast strains heterozygous for a mutant allele with a 26-base-pair insertion, we find that the repair of 26-base loops involves Msh2 (a DNA-mismatch repair protein) and Rad1 (a protein required for nucleotide-excision repair).