Bistable expression of WOR1 , a master regulator of white–opaque switching in Candida albicans

Abstract

Candida albicans, a commensal organism and a pathogen of humans, can switch stochastically between a white phase and an opaque phase without an intermediate phase. The white and opaque phases have distinct cell shapes and gene expression programs. Once switched, each phase is stable for many cell divisions. White-opaque switching is under a1-␣2 repression and therefore only happens in a or ␣ cells. Mechanisms that control the switching are unknown. Here, we identify Wor1 (white-opaque regulator 1) as a master regulator of white-opaque switching. The deletion of WOR1 blocks opaque cell formation. The ectopic expression of WOR1 converts all cells to stable opaque cells in a or ␣ cells. In addition, the ectopic expression of WOR1 in a͞␣ cells is sufficient to induce opaque cell formation. Importantly, WOR1 expression displays an all-or-none pattern. It is undetectable in white cells, and it is highly expressed in opaque cells. The ectopic expression of Wor1 induces the transcription of WOR1 from the WOR1 locus, which correlates with the switch to opaque phase. We present genetic evidence for feedback regulation of WOR1 transcription. The feedback regulation explains the bistable and stochastic nature of white-opaque switching.feedback regulation ͉ a1-␣2 repression C andida albicans is associated with humans both as a harmless commensal organism and a pathogen. In most humans, it is a normal part of the microflora in the gastrointestinal tract and in other parts of the body. However, when the host immune system is compromised, it can cause mucosal infections and life-threatening disseminated infections. The ability of C. albicans to undergo a yeast-to-hypha transition and high-frequency phenotypic switching provides the organism with a high degree of phenotypic diversity and the adaptability that is necessary to survive in different host environments (1). One of the high-frequency switching systems is white-opaque switching. White-opaque switching was first identified in a clinical isolate, WO-1, in which cells switched spontaneously between white and opaque phases (2). White phase cells appear round and form hemispherical, white colonies on solid agar, and opaque phase cells are elongated in cell shape with pimples on the surface and form flat, opaque colonies. Several features of white-opaque switching are particularly interesting (1, 3): (i) the transition involves only two phases, and there is not an intermediate phase, (ii) the transition is stochastic, generating heterogeneity in a cell population, and (iii) the phase is stable for many cell divisions, with daughter cells inheriting the phase of their mother cells. White-opaque switching occurs at a frequency of 10 Ϫ4 to 10 Ϫ5 per cell division, and opaque-white switching happens at a frequency of 5 ϫ 10 Ϫ4 per cell generation (4). Opaque cells are stable at 25°C. Upon shifting the temperature to 37°C, they switch en masse to white cells (5).White-opaque switching is under a1-␣2 repression (6). In C. albicans, cell type is determined by the MTL (mating type-li...

Crystal structure of the WOPR-DNA complex and implications for Wor1 function in white-opaque switching of Candida albicans

“…Crystallization experiments of CaWor1 WOPR in complex with dsDNA of different lengths (13,15,17, and 20 bp) all yielded crystals with yet varied diffraction qualities. Finally, the crystal structure of CaWor1 WOPR in complex with a 17-bp dsDNA (5′-AAGT-TAAACTTTTTTGA-3′) (WOPR-17bp dsDNA) was determined to 3.0 Å resolution, and the crystal structure of CaWor1 WOPR in complex with a 13-bp dsDNA (5′-AAGTTAAACTTTT-3′) (WOPR-13bp dsDNA) to 2.1 Å resolution (Table 1).…”

“…To verify the functional role of the WOPR-DNA interaction in vivo, we examined the activation of wild-type Wor1 or Wor1 mutants in C. albicans white-to-opaque switching using a modified method reported previously [13]. White state cells or colonies were selected on SCD (synthetic complete medium with 2% glucose) plates at 37 °C for 3 days in air.…”

“…The single-stranded oligonucleotides of different lengths (13,15,17, and 20 bases) corresponding to the template strands and their complementary strands were synthesized by Sangon Biotech (Shanghai). The dsDNAs were prepared by annealing of the template and complementary strands from 95 °C to 22 °C over a period of 6 h in the same buffer as for the protein.…”

“…C. albicans can switch between two distinct cell types, white and opaque, which have different properties including cell shape, colonial morphology, metabolic preference, mating ability, gene expression pattern, and host tissue preference [5][6][7][8][9]. White-opaque switching is controlled through expression of a master regulator, Wor1 (whiteopaque switching regulator 1), which is highly upregulated in opaque cells and is required for both the transition to and maintenance of the opaque cell type [10][11][12][13]. Wor1 can genetically interact with the other five key regulators, forming a network of positive and negative feedback loops to control white-opaque switching [14,15].…”

Uric acid levels are independent of anti-Saccharomyces cerevisiae antibodies (ASCA) in Crohn’s disease: A reappraisal of the role of S. cerevisiae in this setting

“…The same concern applies to the strains/species used as controls. These consisted of Rhodotorula aurantiaca qualified as an “emerging pathogen”, but never reported as a pathogen to date, whereas C. albicans was, for undefined reasons, a 50:50 mixture of a wild strain and mutant for WOR1, a master regulator of white-opaque switching associated with C. albicans commensalism and pathogenesis[28]. …”