Mother and Daughter Are Doing Fine: Asymmetric Cell Division in Yeast

Angelika Amon of the HO gene. Among these, Swi4p and Swi6p confer Whitehead Institute for Biomedical Research cell cycle regulation through URS2 (reviewed by Koch 9 Cambridge Center and Nasmyth, 1994) and the zinc finger transcription Cambridge, Massachusetts 02144 factor Swi5p activates HO transcription in mother cells via the URS1 element (Tebb et al., 1993, and references therein). None of these regulatory factors, however, seems to be the primary or sole determinant for the The underlying principle of every developmental pro-asymmetric expression of the HO gene. At first, Swi5p gram is that single cells divide to give rise to daughter had appeared to be a promising candidate because cells with different developmental fates. This is true not SWI5 is required for HO expression in mother cells and only for the fertilized egg that develops into an adult, because either constitutive expression of SWI5 (Lydall but also for the yeast cell that buds. In general, two et al., 1991) or the stabilization of Swi5p by mutation mechanisms can account for this phenomenon. Differ-(Tebb et al., 1993) causes a certain fraction of daughter ences between daughter cells may originate from differ-cells to switch mating type. However, Swi5p is found in ences in their environment. For example, vulva formation both mother and daughter cells (Figure 2A) and activates in Caenorhabditis elegans requires cell–cell interaction transcription of genes other than HO equally well in both and signaling between equipotent cells (reviewed by mother and daughter cells (Koch and Nasmyth, 1994). Kenyon, 1995). Alternatively, differences between-daughter cells can be generated by unequal segregation Thus, some other factor, either a mother cell–specific of cell fate determinants during cell division. Examples activator or a daughter cell–specific repressor, must ex-of this process include cell division in the bacterium ist to generate the asymmetric expression of HO. Caulobacter crescentus, whereby a nonmotile stalked In this issue of Cell, the Herskowitz (Sil and Herskow-cell divides into a motile swarmer cell and another itz, 1996) and Nasmyth (Bobola et al., 1996) groups re-stalked cell, spore formation in Bacillus subtilis (report an important advance in understanding how HO viewed by Shapiro, 1993), and sensory organ formation asymmetry is generated: they have identified a negative in Drosophila melanogaster (reviewed by Rhyu and regulator of HO expression that is daughter cell specific. Knoblich, 1995). This factor, termed Ash1p (for asymmetric synthesis of One of the best studied examples of asymmetric seg-HO), preferentially accumulates in …