Engineered interphase chromosome loops guide intrachromosomal recombination.

How large-scale topologies regulate interphase chromosome function remains an important question in eukaryotic cell biology. Looped structures are thought to modulate transcription by pairing promoters with distant control elements and to orchestrate intrachromosomal recombination events by pairing appropriate recombination partners. To explore the effects of chromosomal topology on intrachromosomal recombination, distinct loop geometries were engineered into chromosome III of the budding yeast Saccharomyces cerevisiae. These topologies were created by employing pairs of lac operator clusters to serve as pairing sites and a modified lac repressor to perform the role of a protein cross-bridge. The influence of these engineered loops on the selection of donor loci during mating-type switching was evaluated using novel genetic and molecular methods. These experiments demonstrate that engineered interphase chromosome loops are biologically active-capable of influencing the course of intrachromosomal recombination. They also provide insight into the mechanism of mating-type switching by revealing a causal relationship between defined chromosomal topologies and the choice of donor locus.