Fission yeast cells switch mating-type in a directional manner, by gene conversions of the <i>mat1</i> locus.

<p>A recombinogenic DNA end is formed at <i>mat1</i> during DNA replication (black arrow) and the broken DNA invades a donor whose genetic information is copied into <i>mat1</i>. Wild-type M cells (<i>mat1-M</i> allele) use the <i>mat2-P</i> donor, while P cells (<i>mat1-P</i> allele) use the <i>mat3-M</i> donor, as depicted in the top drawing. The recombination enhancers SRE2 and SRE3 are central to these choices. Experiments in which donors and enhancers are swapped, alone or in combination, show that SRE2 and SRE3 are recognized in a cell-type specific manner to promote use of their adjacent donor. The heterochromatic structure of the <i>mat2-P–mat3-M</i> region is required for this differential recognition.