EFFICIENT REPAIR OF ALL TYPES OF SINGLE-BASE MISMATCHES IN RECOMBINATION INTERMEDIATES IN CHINESE-HAMSTER OVARY CELLS - COMPETITION BETWEENLONG-PATCH AND G-T GLYCOSYLASE-MEDIATED REPAIR OF G-T MISMATCHES

Repair of all 12 single-base mismatches in recombination intermediates was investigated in Chinese hamster ovary cells. Extrachromosomal rec ombination was stimulated by double-strand breaks in regions of shared homology. Recombination was predicted to occur via single-strand anne aling, yielding heteroduplex DNA (hDNA) with a single mismatch. Nicks were expected on opposite strands flanking hDNA, equidistant from the mismatch. Unlike studies of covalently closed artificial hDNA substrat es, all mismatches were efficiently repaired, consistent with a nick-d riven repair process. The average age repair efficiency for all mispai rs was 92% with no significant differences among mispairs. There was s ignificant strand-independent repair of G-T --> GC, with a slightly gr eater bias in a CpG context. Repair of C-A was also biased (toward C-G ), but no A-C --> G-C bias was found, a possible sequence context effe ct. No other mismatches showed evidence of biased repair, but among he tero-mismatches, the trend was toward retention of C or G vs. A or T. Repair of both T-T and G-T mismatches was much less efficient in misma tch repair-deficient cells (similar to 25%), and the residual G-T repa ir was completely biased toward G-C. Our data indicate that single-bas e mismatches in recombination inter-mediates are substrates for at lea st two competing repair systems.