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. Switching 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 preferentially in growing buds. We describe a gene, ASH1, that is necessary to repress HO in daughters. ASH1 encodes a zinc finger protein whose preferential accumulation in daughter cell nuclei at the end of anaphase depends on She1p/Myo4p. The greater abundance of Ash1p in daughter cells is responsible for restricting HO expression to mother cells.