The Evolutionarily Conserved Repetitive Sequence d(TG·AC)n Promotes Reciprocal Exchange and Generates Unusual Recombinant Tetrads During Yeast Meiosis

Abstract

We have studied the genetic behavior of the alternating copolymer d(TG·AC)_n inserted into a defined position in the genome of the yeast Saccharomyces cerevisiae. When d(TG·AC)_n sequences were present at the HIS3 locus on homologous chromosomes, diploid cells undergoing meiosis generated an excess of tetrads containing reciprocally recombined products with crossover points close to the repetitive DNA insert. Most of these tetrads exhibited gene conversion of a d(TG·AC)_n insert. However, the insertion of d(TG·AC)_n sequences had no effect on the frequency of gene conversion of closely linked marker genes. Surprisingly, when d(TG·AC)_n sequences were present on only one homolog at the HIS3 locus, one-half of the tetrads exhibiting nonparental segregation for marker genes that flanked the repetitive DNA insert were very unusual and appeared to have arisen by multiple recombination events in the vicinity of the d(TG·AC)_n insert. Similar multiply recombinant tetrads were seen in crosses in which d(TG·AC)_n sequences were present on both homologs. Combined, the data strongly suggest that d(TG·AC)_n sequences significantly enhance reciprocal meiotic recombination and may be important in causing multiple recombination events to occur within a relatively small region of the yeast chromosome. Molecular evidence is presented that clearly documents the postmeiotic segregation of an 80-base stretch of d(TG·AC)_n.