Regulation of fission yeast mating-type interconversion by chromosome imprinting.

Mating types of the fission yeast Schizosaccharomyces pombe interchange nonrandomly in a cell lineage so that only one cell among four granddaughters of a cell ever switches, and the sister of the newly switched cell switches efficiently in consecutive cell divisions, thereby producing chains of recurrent switching. The programme of cellular differentiation is mediated by inheritance of parental DNA chains at the mating type locus (mat1) by progeny cells. This review summarizes recent results suggesting that two types of imprinting events at the mat1 locus are required to generate the specific pattern of switching in a cell lineage. One of those is a site- and strand- specific event that is required before the mat1 locus can be cleaved in vivo. The other is a double-stranded break at mat1 that is healed by gene conversion in the progeny cells resulting in switching the mat1 locus.