Identification of an asymmetrically localized determinant, Ash1p, required for lineage-specific transcription of the yeast HO gene

S. cerevisiae cells exhibit asymmetric determination of cell fate. Cell division yields a mother cell, which is competent to transcribe the HO gene and switch mating type, and a daughter cell, which is not. We have isolated a mutant in which daughters transcribe HO and switch mating type. This mutation defines the ASH1 gene (asymmetric synthesis of HO). Deletion and overexpression of ASH1 cause reciprocal cell fate transformations: in ash1 Delta strains, daughters switch mating type as efficiently as mothers. Conversely, overexpression of ASH1 inhibits switching in mother cells. Ash1p has a zinc finger motif related to those of GATA transcriptional regulators. Ash1p is localized to the daughter nucleus in cells that have undergone nuclear division. Thus, Ash1p is a cell fate determinant that is asymmetrically localized to the daughter nucleus where it inhibits HO transcription.