Ssn6 Defines a New Level of Regulation of White-Opaque Switching in Candida albicans and Is Required For the Stochasticity of the Switch

UNLABELLED The human commensal and opportunistic pathogen Candida albicans can switch between two distinct, heritable cell types, named "white" and "opaque," which differ in morphology, mating abilities, and metabolic preferences and in their interactions with the host immune system. Previous studies revealed a highly interconnected group of transcriptional regulators that control switching between the two cell types. Here, we identify Ssn6, the C. albicans functional homolog of the Saccharomyces cerevisiae transcriptional corepressor Cyc8, as a new regulator of white-opaque switching. In A: or α mating type strains, deletion of SSN6 results in mass switching from the white to the opaque cell type. Transcriptional profiling of ssn6 deletion mutant strains reveals that Ssn6 represses part of the opaque cell transcriptional program in white cells and the majority of the white cell transcriptional program in opaque cells. Genome-wide chromatin immunoprecipitation experiments demonstrate that Ssn6 is tightly integrated into the opaque cell regulatory circuit and that the positions to which it is bound across the genome strongly overlap those bound by Wor1 and Wor2, previously identified regulators of white-opaque switching. This work reveals the next layer in the white-opaque transcriptional circuitry by integrating a transcriptional regulator that does not bind DNA directly but instead associates with specific combinations of DNA-bound transcriptional regulators. IMPORTANCE The most common fungal pathogen of humans, C. albicans, undergoes several distinct morphological transitions during interactions with its host. One such transition, between cell types named "white" and "opaque," is regulated in an epigenetic manner, in the sense that changes in gene expression are heritably maintained without any modification of the primary genomic DNA sequence. Prior studies revealed a highly interconnected network of sequence-specific DNA-binding proteins that control this switch. We report the identification of Ssn6, which defines an additional layer of transcriptional regulation that is critical for this heritable switch. Ssn6 is necessary to maintain the white cell type and to properly express the opaque cell transcriptional program. Ssn6 does not bind DNA directly but rather associates with specific combinations of DNA-bound transcriptional regulators to control the switch. This work is significant because it reveals a new level of regulation of an important epigenetic switch in the predominant fungal pathogen of humans.