A Set of Diverse Genes Influence the Frequency of White-Opaque Switching in Candida albicans

Abstract

The fungal species Candida albicans is both a member of the human microbiome and a fungal pathogen. C. albicans undergoes several different morphological transitions, including one called white-opaque switching. Here, cells reversibly switch between two states, "white" and "opaque," and each state is heritable through many cell generations. Each cell type has a distinct cellular and colony morphology and they differ in many other properties including mating, nutritional specialization, and interactions with the innate immune system. Previous genetic screens to gain insight into white-opaque switching have focused on certain classes of genes (for example transcriptional regulators or chromatin modifying enzymes). In this paper, we examined 172 deletion mutants covering a broad range of cell functions. We identified 28 deletion mutants with at least a five-fold effect on switching frequencies; these cover a wide variety of functions ranging from membrane sensors to kinases to proteins of unknown function. In agreement with previous reports, we found that components of the pheromone signaling cascade affect white-to-opaque switching; however, our results suggest that the major effect of Cek1 on white-opaque switching occurs through the cell wall damage response pathway. Most of the genes we identified have not been previously implicated in white-opaque switching and serve as entry points to understand new aspects of this morphological transition.

Farnesol and phosphorylation of the transcriptional regulator Efg1 affect Candida albicans white-opaque switching rates

“…Here, we report that the putative PKA phosphorylation of residue T208 on the transcriptional regulator Efg1 increases white-to-opaque switching rates and that exposure to submicromolar concentrations of (E,E)-farnesol decreases white-to-opaque switching rates in C. albicans. Farnesol did not affect the reverse transition, opaque-to-white switching rates (at 25˚C on glucose), a unidirectional effect that had been observed previously for other mutations and environmental conditions [23,64,65]; these results further underscore the independence of the mechanisms for the establishment versus the maintenance of the opaque cell type. Unlike the yeast-to-hyphal transition, where farnesol's negative effects are mediated by Czf1, we found that Czf1 was not required for farnesol to affect white-to-opaque switching, suggesting that the two effects are mediated, at least in part, through different signaling pathways.…”

“…Neither 0.1 μM nor 1 μM farnesol affected the reverse switching rate, that is switching of opaque cells to white cells (Table 1). The unidirectional nature of farnesol's effect is similar to that of many of the gene deletions and environmental signals that affect white-opaque switching, and is consistent with the apparent independence of the mechanisms for the establishment and maintenance of the opaque cell type [23,64,65].…”

“…Switching between the two cell types is controlled by a circuit with eight transcriptional regulators connected by interlocking feedback loops, with Efg1 and Wor1 being especially important for the establishment and maintenance of the white and opaque cell types respectively [38][39][40][41][42][43][44][45][46][47][48][49][50]. Switching rates between the two cell types are dependent on numerous signaling pathways, including the cAMP/protein kinase A pathway [23,[51][52][53], the Hog1 pathway [54][55][56], the Cek1 MAP kinase pathway [51,55], and others [51,[57][58][59][60][61][62][63][64][65]. Environmental conditions that trigger these pathways also affect this switch, including elevated temperature and exposure to N-acetylglucosamine (GlcNAc) [12,23,36,66].…”

Candida albicans Double Mutants Lacking both EFG1 and WOR1 Can Still Switch to Opaque

“…A variety of genes ( 43 ) encoding signaling proteins ( 35 , 36 , 38 ), trans -acting factors ( 12 , 39 , 40 ), and chromatin modulation factors ( 11 , 41 , 42 ) play roles in regulating switching from white to opaque. Central to many of the proposed regulatory models are the two TFs.…”