High homology is not required at the site of strand invasion during recombinational double-strand break repair in mammalian chromosomes.

We investigated the impact of sequence divergence on DNA double-strand break (DSB) repair occurring via recombination in cultured thymidine kinase deficient mouse fibroblasts. We stably transfected cells with a DNA construct harboring a herpes thymidine kinase (tk) gene (the "recipient") rendered nonfunctional by insertion of an oligonucleotide containing the recognition site for endonuclease I-SceI. The construct also contained a closely linked truncated "donor" tk sequence. The donor could potentially restore function to the recipient gene via recombination provoked by induction of a DSB at the I-SceI site in the recipient. Repair events were recoverable by selection for tk-positive clones. The donor contained 33 mismatches relative to the recipient. The mismatches were clustered, forming a localized segment of DNA sequence displaying about 20% divergence relative to the recipient, and the mismatched segment was surrounded by regions of high homology. When the donor was aligned with the recipient, the DSB site in the recipient aligned opposite the mismatched segment, allowing us to potentially capture recombinational repair events initiating between diverged sequences. Previous work demonstrated that mammalian cells effectively avoid recombination between 20% diverged sequences. In the current study we asked whether flanking regions of high homology would enable genetic exchange between highly diverged sequences or, instead, would rejection of exchange between diverged sequences remain unchanged. We found that by surrounding mismatches with high homology, suppression of recombination between diverged sequences was overcome. Strikingly, we recovered a high frequency of gene conversion tracts positioned entirely within the mismatched sequences. We infer that such events were enabled by homologous pairing interactions between sequences surrounding the site of strand invasion. Our results suggest a search for high homology prior to recombination that is not mediated by an invading DNA terminus.