Recombination between sequences in nonhomologous positions.

Crossing-over between the dispersed repeated sequences found in eukaryotic genomes would generate chromosomal rearrangements. The stability of the yeast genome suggests the existence of some constraint on the ability of these sequences to interact by recombination. We have constructed strains with two alleles of the his3 gene located on different chromosomes. Gene conversion accounts for the majority of the recombination events between these genes, but about 10% of the events are crossovers that result in a reciprocal translocation. When one of the alleles is on an autonomously replicating centromere plasmid, recombination is 5- to 10-fold more frequent than when both alleles are chromosomal, suggesting that higher-order chromosome structure may play a role in restricting interchromosomal recombination. We have also used the translocation to deduce the orientation of the his3 and rRNA genes relative to their centromeres.