Mother cell-specific HO expression in budding yeast depends on the unconventional myosin myo4p and other cytoplasmic proteins.

Certain cell types give rise to progeny that adopt different patterns of gene expression in the absence of any differences in their environment. Cells of budding yeast give birth to mother and daughter cells that differ in that only mother cells express the HO endonuclease gene and thereby switch mating types. We describe the identification of five genes, called SHE1-SHE5, that encode cytoplasmic proteins required for mother-specific HO expression. She1p, which is identical to the minimyosin Myo4p, and She3p are not, however, mother-specific proteins. On the contrary, they accumulate in growing buds. She proteins might be required for the transport of factors that promote HO repression from the mother cell into its bud. In an accompanying paper, we show that SHE genes are needed for the accumulation in daughter nuclei of Ash1p, a repressor of HO.