Suppression of Intrachromosomal Gene Conversion in Mammalian Cells by Small Degrees of Sequence Divergence

Corresponding author: Alan S. Waldman, Department of Biological Sciences, University of South Carolina, 700 Sumter St., Columbia, SC 29208., awaldman{at}sc.edu (E-mail)

Communicating editor: M. LICHTEN

Pairs of closely linked defective herpes simplex virus (HSV) thymidine kinase (tk) gene sequences exhibiting various nucleotide heterologies were introduced into the genome of mouse Ltk^- cells. Recombination events were recovered by selecting for the correction of a 16-bp insertion mutation in one of the tk sequences. We had previously shown that when two tk sequences shared a region of 232 bp of homology, interruption of the homology by two single nucleotide heterologies placed 19 bp apart reduced recombination nearly 20-fold. We now report that either one of the nucleotide heterologies alone reduces recombination only about 2.5-fold, indicating that the original pair of single nucleotide heterologies acted synergistically to inhibit recombination. We tested a variety of pairs of single nucleotide heterologies and determined that they reduced recombination from 7- to 175-fold. Substrates potentially leading to G-G or C-C mispairs in presumptive heteroduplex DNA (hDNA) intermediates displayed a particularly low rate of recombination. Additional experiments suggested that increased sequence divergence causes a shortening of gene conversion tracts. Collectively, our results suggest that suppression of recombination between diverged sequences is mediated via processing of a mispaired hDNA intermediate.

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