The yeast Candida albicans is part of the microflora in most healthy people, but can become a pathogen when host defenses are compromised. The phenotypic plasticity of C. albicans, which includes switching between different morphologies, contributes to its ability to colonize and infect virtually all body locations. A particularly fascinating developmental program is white-opaque switching, a reversible transition between the normal yeast morphology (white) and an elongated cell type (opaque), which is the mating-competent form of this fungus. Although opaque cells are much less able than white cells to cause a systemic infection, they are better adapted for colonization of specific host niches, like skin. White-opaque switching is controlled by the mating type locus (MTL), which in most C. albicans strains exists in two alleles, MTLa and MTL. These strains produce a heterodimeric repressor, a1-alpha2, which suppresses switching to the opaque phase by inhibiting expression of the master regulator Wor1. Loss of MTL heterozygosity relieves this repression, a mechanism that ensures that only MTL homozygous cells can switch to the mating-competent opaque form. Several transcriptional feedback loops, including positive autoregulation of Wor1, result in bistable expression of the master regulator (low in white and high in opaque cells) and epigenetic inheritance of the two phases. White-opaque switching occurs stochastically at a low frequency, but certain environmental conditions can drive the switch from one phase to the other by affecting either the activity of the transcriptional feedback loops or accumulation of Wor1 protein in a cell. Such environmental regulation of phenotypic switching may restrict mating to suitable host niches, while allowing a C. albicans population to withstand the various challenges encountered in different tissues.
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