The temporal regulation of protein synthesis during synchronous bud or mycelium formation in the dimorphic yeast Candida albicans

When stationary phase cells of the dimorphic yeast Candida albicans are diluted into fresh medium at 37°C at either pH 4.5 or pH 6.5, they evaginate at exactly the same time and with the same synchrony. However, they then grow in the budding yeast form at the former pH and in the elongate mycelium form at the latter pH. Three phases of protein synthesis are distinguished for cells forming either buds or mycelia: an initial 50-min period (phase I) during which total cell protein remains constant and the rate of incorporation of labeled amino acid into protein is virtually zero; a second period (phase II) during which there is a slow but constant increase in both total cell protein and the rate of incorporation; and a third period (phase III) during which there is a dramatic increase in both total cell protein and the rate of incorporation. The transition from phase I to phase II occurs at the same time for cells forming either buds or mycelia, but the transition from phase II to phase III occurs 20 to 30 min later in the mycelium than in the bud forming population, the same temporal difference observed for phenotypic commitment. The polypeptides synthesized during phases II and III were first analyzed by one-dimensional polyacrylamide gel electrophoresis. The patterns are similar for the two phenotypes. The major polypeptides synthesized during phase II are also synthesized during phase III, but in addition, a group of at least four new major polypeptides appear during phase III for both phenotypes. The minor polypeptides synthesized during phase III were also compared between the two phenotypes by two-dimensional polyacrylamide gel electrophoresis. The patterns, including roughly 200 distinguishable polypeptides, were similar. The similarities in the patterns of protein synthesis and the delay in the onset of phase III in mycelium forming cells are discussed in terms of phenotypic commitment. From these considerations, alternate hypotheses for the regulation of fungal dimorphism, in particular, and cell divergence, in general, are proposed.