FIG. 2.

Schematic representation of the homologous-partner choice experimental system. Open rectangles represent the ura3 alleles on chromosomes II and V. Striped rectangles represent LYS2 sequences on chromosomes II and XV. A hatched box represents the 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 HO endonuclease, which recognizes the HOcs and creates a DSB. This break is repaired by gene conversion with one of two alternative partners: the ura3 allele on chromosome V and the LYS2 allele on chromosome XV. In the latter case, the ura3-HOcs allele has to be removed.