PATTERNS OF BUD-SITE SELECTION IN THE YEAST SACCHAROMYCES-CEREVISIAE

Cells of the yeast Saccharomyces cerevisiae select bud sites in either of two distinct spatial patterns, known as axial (expressed by a and alpha cells) and bipolar (expressed by a/alpha cells). Fluorescence, t ime-lapse, and scanning electron microscopy have been used to obtain m ore precise descriptions of these patterns. From these descriptions, w e conclude that in the axial pattern, the new bud forms directly adjac ent to the division site in daughter cells and directly adjacent to th e immediately preceding division site (bud site) in mother cells, with little influence from earlier sites. Thus, the division site appears to be marked by a spatial signal(s) that specifies the location of the new bud site and is transient in that it only lasts from one budding event to the next. Consistent with this conclusion, starvation and ref eeding of axially budding cells results in the formation of new buds a t nonaxial sites. In contrast, in bipolar budding cells, both poles ar e specified persistently as potential bud sites, as shown by the obser vations that a pole remains competent for budding even after several g enerations of nonuse and that the poles continue to be used for buddin g after starvation and refeeding. It appears that the specification of the two poles as potential bud sites occurs before a daughter cell fo rms its first bud, as a daughter can form this bud near either pole. H owever, there is a bias towards use of the pole distal to the division site. The strength of this bias varies from strain to strain, is affe cted by growth conditions, and diminishes in successive cell cycles. T he first bud that forms near the distal pole appears to form at the ve ry tip of the cell, whereas the first bud that forms near the pole pro ximal to the original division site (as marked by the birth scar) is g enerally somewhat offset from the tip and adjacent to (or overlapping) the birth scar. Subsequent buds can form near either pole and appear almost always to be adjacent either to the birth scar or to a previous bud site. These observations suggest that the distal tip of the cell and each division site carry persistent signals that can direct the se lection of a bud site in any subsequent cell cycle.