Candida albicans Double Mutants Lacking both and Can Still Switch to Opaque.

, a pervasive opportunistic pathogen, undergoes a unique phenotypic transition from a "white" phenotype to an "opaque" phenotype. The switch to opaque impacts gene expression, cell morphology, wall structure, metabolism, biofilm formation, mating, virulence, and colonization of the skin and gastrointestinal (GI) tract. Although the regulation of switching is complex, a paradigm has evolved from a number of studies, in which, in its simplest form, the transcription factors Efg1 and Wor1 play central roles. When is upregulated under physiological conditions, it represses , an activator of white-to-opaque switching, and the cell expresses the white phenotype; when is downregulated, is derepressed and activates expression of the opaque phenotype. Deletion of either or supports this yin-yang model of regulation. Here, we demonstrate that this simple model is insufficient, since strains in which and are simultaneously deleted can still be induced to switch from white to opaque. Opaque cells of double mutants ( ) are enlarged and elongate, form an enlarged vacuole, upregulate under the control of an opaque-specific promoter, form opaque cell wall pimples, express the opaque phenotype in lower GI colonization, and, if homozygous, form conjugation tubes in response to pheromone and mate. These results can be explained if the basic and simplified model is expanded to include a -independent alternative opaque pathway repressed by The switch from white to opaque in was discovered 33 years ago, but it is still unclear how it is regulated. A regulatory paradigm has emerged in which two transacting factors, Efg1 and Wor1, play central roles, Efg1 as a repressor of , which encodes an activator of the transition to the opaque phenotype. However, we show here that if both and are deleted simultaneously, bona fide opaque cells can still be induced These results are not compatible with the simple paradigm, suggesting that an alternative opaque pathway (AOP) exists, which can activate expression of opaque and, like , is repressed by .