Prmted in Sweden Copyright 0 1979 by Academic Press. Inc. All right\ of reproduction in any form reserved ooI~-SX?7/7Y/0snlh7-li$O?.~W~/O Espcrimc~ntul Cell Rescwrch 120 (1979) 167-179 COMMITMENT TO GERM TUBE OR BUD FORMATION DURING RELEASE FROM STATIONARY PHASE IN CANDIDA ALBICANS LEE H. MITCHELL and DAVID R. SOLL SUMMARY Cells of the pathogenic yeast C~ndidtr ulbicuns accumulate as unbudded singlets at stationary phase in defined medium at 25°C. When released into fresh medium at 37°C and pH 6.5, these cells will synchronously form elongate pseudomycelia, and when released into fresh medium at either 25°C pH 6.5, or 37”C, pH 4.5, they will synchronously form buds. Using pH and temperature shift experiments, we have examined when cells become committed to pseudomycelium formation and bud formation under conditions conducive to each growth form respectively. It is demonstrated that in either case commitment occurs long after release from stationary phase, at approximately the same time the first evagination is visible on the cell’s surface. In addition, it is demonstrated that once a released cell has formed a bud, it and its progeny lose the capacity to form pseudomyce- lia until they re-enter stationary phase; on the other hand, elongating pseudomycelia retain the capacity to form buds. The possible relationships of the commitment events to septation and to the cell cycle are discussed. The pathogenic yeast Candida albicans is an exceptionally simple model system for studying the regulation of cell differentia- tion. Candida is dimorphic and grows either by forming buds or by elongating as a pseudomycelium [ 1, 2, 31. Recently, simple methods were developed for initiating the synchronous formation of either buds or pseudomycelia in an entire cell population [4, 5, 61. When cells enter stationary phase at 25°C in defined media, they accumulate as unbudded singlets, at a stage early in the cell cycle [6, 71. When these cells are re- leased from stationary phase by diluting them into fresh medium at 37°C and pH 6.5, they synchronously form elongate germ tubes which grow into pseudomycelia; al- ternatively, when cells are released from stationary phase by diluting them into fresh medium at 2X, they synchronously form buds. Recently, we discovered that one can also regulate the growth form of released stationary phase cells by manipulating the pH of the fresh medium into which they are diluted. If cells which have entered sta- tionary phase at 25°C are diluted into fresh medium at 37°C and pH 6.5, they synchro- nously form germ tubes, but if they are diluted into fresh medium at 37°C and pH 4.5, they synchronously form buds in the same amount of time. In this investigation, we have employed pH and temperature shift experiments to define the times at which cells become com- mitted to either germ tube or bud formation after they are released from stationary phase. Stationary phase cells were diluted into fresh medium under conditions indu-