EFG1 Null Mutants of Candida albicans Switch but Cannot Express the Complete Phenotype of White-Phase Budding Cells

The Candida albicans gene EFG1 encodes a putative trans-acting factor. In strain WO-1, which undergoes the white-opaque transition, EFG1 is transcribed as a 3.2-kb mRNA in white-phase cells and a less-abundant 2.2-kb mRNA in opaque-phase cells. cDNA sequencing and 5 * rapid amplification of cDNA ends analysis demonstrate that the major difference in molecular mass of the two transcripts is due to different transcription start sites. EFG1 null mutants form opaque-phase colonies and express the opaque-phase cell phenotype at 25°C. When shifted from 25 to 42°C, mutant opaque-phase cells undergo phenotypic commitment to the white phase, which includes deactivation of the opaque-phase-specific gene OP4 and activation of the white-phase-specific gene WH11, as do wild-type opaque-phase cells. After the commitment event, EFG1 null mutant cells form daughter cells which have the smooth (pimpleless) surface of white-phase cells but the elongate mor-phology of opaque-phase cells. Taken together, these results demonstrate that EFG1 expression is not essential for the switch event per se, but is essential for a subset of phenotypic characteristics necessary for the full expression of the phenotype of white-phase cells. These results demonstrate that EFG1 is not the site of the switch event, but is, rather, downstream of the switch event