Plasmid-mediated induction of recombination in yeast

Diploid yeast cells heteroallelic at the HIS3 locus were transformed with a minichromosome (centromeric plasmid) carrying homology to the HIS3 region and containing the same two mutations as were present in the chromosomes. When a double-strand break (DSB) was introduced in the region of homology, an increase in the recombination frequency between heteroalleles (leading to His⁺ cells) was observed, although the plasmid was unable to donate wild- type information. This induction of recombination was dependent on the presence of homology between the plasmid sequences and the chromosomes. We show evidence for the physical involvement of the plasmid in tripartite recombination events, and we propose models that can explain the interactions between the plasmid-borne and chromosomal-borne alleles. Our results suggest that the mitotic induction of recombination by DNA damage is due to localized initiation of recombination events, and not to a general induction of recombination enzymes in the cell.