Dependence of intrachromosomal recombination in mammalian cells on uninterrupted homology.

Recombination between a 360-base-pair (bp) segment of a wild-type thymidine kinase gene (tk) from each of three different strains (F, MP, and 101) of herpes simplex virus type one and a complete herpes simplex virus type 1 (strain F) tk gene containing an 8-bp insertion mutation was studied. The pairs of tk sequences resided as closely linked repeats within the genome of mouse LTK- cells. The frequency of recombination between sequences exhibiting 232 bp of uninterrupted homology and containing no mismatches other than the insertion mutation was comparable to the frequency of recombination between two sequences exhibiting four additional nucleotide mismatches distributed in such a way to preserve the 232-bp stretch of contiguous homology. In contrast, the placement of only two single-nucleotide mismatches (in addition to the insertion mutation) in such a manner to reduce the longest uninterrupted homology to 134 bp resulted in a 20-fold reduction in recombination. We conclude that the rate of intrachromosomal recombination in mammalian cells is determined by the amount of uninterrupted homology available and not by the total number of mismatches within a given interval of DNA. Furthermore, efficient recombination appears to require between 134 and 232 bp of uninterrupted homology; single-nucleotide heterologies are most likely sufficient to disrupt the minimal efficient recombination target. We also observed that if recombination was allowed to initiate within sequences exhibiting perfect homology, the event could propagate through and terminate within adjacent sequences exhibiting 19% base pair mismatch. We interpret this to mean that heterology exerts most of its impact on early rather than late steps of intrachromosomal recombination in mammalian cells.