title: Characterization of palindromic loop mismatch repair tracts in mammalian cells.
abstract: Single- and multi-base (loop) mismatches can arise in DNA by replication errors, during recombination, and by chemical modification of DNA. Single-base and loop mismatches of several nucleotides are efficiently repaired in mammalian cells by a nick-directed, MSH2-dependent mechanism. Larger loop mismatches (> or =12 bases) are repaired by an MSH2-independent mechanism. Prior studies have shown that 12- and 14-base palindromic loops are repaired with bias toward loop retention, and that repair bias is eliminated when five single-base mismatches flank the loop mismatch. Here we show that one single-base mismatch near a 12-base palindromic loop is sufficient to eliminate loop repair bias in wild-type, but not MSH2-defective mammalian cells. We also show that palindromic loop and single-base mismatches separated by 12 bases are repaired independently at least 10% of the time in wild-type cells, and at least 30% of the time in MSH2-defective cells. Palindromic loop and single-base mismatches separated by two bases were never repaired independently. These and other data indicate that loop repair tracts are variable in length. All tracts extend at least 2 bases, some extend <12 bases, and others >12 bases, on one side of the loop. These properties distinguish palindromic loop mismatch repair from the three known excision repair pathways: base excision repair which has one to six base tracts, nucleotide excision repair which has approximately 30 base tracts, and MSH2-dependent mismatch repair, which has tracts that extend for several hundred bases.
authors.affiliation: Department of Molecular Genetics and Microbiology, University of New Mexico School of Medicine, Albuquerque, NM 87131, USA.
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Miller
authors.forename: Colin A
authors.initials: CA
authors.lastname: Bill
authors.forename: Jac A
authors.initials: JA
authors.lastname: Nickoloff
mesh_terms: D000818:Animals; D020137:Base Pair Mismatch; D016466:CHO Cells; D006224:Cricetinae; D004260:DNA Repair