ASYMMETRIC ACCUMULATION OF ASH1P IN POSTANAPHASE NUCLEI DEPENDS ON A MYOSIN AND RESTRICTS YEAST MATING-TYPE SWITCHING TO MOTHER CELLS

Cell division in haploid yeast gives rise to a ''mother'' cell capable of mating-type switching and a ''daughter'' cell that is not. Switchi ng is initiated by the HO endonuclease, whose gene is only transcribed in cells that have previously given birth to a bud (mother cells). HO expression depends on a minimyosin, She1p/Myo4p, which accumulates pr eferentially in growing buds. We describe a gene, ASH1, that is necess ary to repress HO in daughters. ASH1 encodes a zinc finger protein who se preferential accumulation in daughter cell nuclei at the end of ana phase depends on She1p/Myo4p. The greater abundance of Ash1p in daught er cells is responsible for restricting HO expression to mother cells.