FIG. 1.

Schematic representation of the experimental system (strain OI29). Open rectangles represent the ura3 alleles on chromosomes II and V. Striped rectangles represent LYS2 sequences on chromosome II. A hatched box represents the HO cut site (HOcs); black boxes represent the inactive HOcs-inc, with the two lines representing polymorphic sites recognized by the EcoRI and BamHI restriction enzymes. Transfer of the cells to galactose-containing medium induces the production of the HO endonuclease, which recognizes the HOcs and creates a DSB. If this DSB is not repaired, it leads to cell death. We have found that the break is repaired in all of the cells of the population by a gene conversion event that transfers the HOcs-inc information to chromosome II, thus preventing further recognition by the HO endonuclease (16).