FIGURE 10-36 Switching of mating type in haploid yeast cells. (a) Division by budding forms a larger mother cell (M) and smaller daughter cell (D), both of which have the same mating type as the original cell (α in this example). The mother cell can switch mating type during G₁ of the next cell cycle and then divide again, producing two cells of the opposite type (a in this example). Switching depends on transcription of the HO gene, which occurs only in the absence of Ash1 protein. The smaller daughter cells, which produce Ash1 protein, cannot switch; after growing in size through interphase, they divide to form a mother cell and daughter cell. (b) Model for restriction of mating-type switching to mother cells in S. cerevisiae. Ash1 protein prevents a cell from transcribing the HO gene, whose encoded protein initiates the DNA rearrangement that results in mating-type switching from a to α or α to a. Switching occurs only in the mother cell, after it separates from a newly budded daughter cell, because the Ash1 protein is present only in the daughter cell. The molecular basis for this differential localization of Ash1 is the one-way transport of ASH1 mRNA into the bud. A linking protein, She2, binds to specific 3′ untranslated sequences in the ASH1 mRNA and also binds to She3 protein. This protein, in turn, binds to a myosin motor, Myo4, which moves along actin filaments into the bud. See S. Koon and B. J. Schnapp, 2001, Curr. Biol. 11:R166.